Synthesis and catalytic relevance of P(III) and P(V)-functionalised calixarenes and resorcinarenes

Coordination Chemistry Reviews 279 (2014) 58–95

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Coordination Chemistry Reviews journal homepage: www.elsevier.com/locate/ccr

Review

Synthesis and catalytic relevance of P(III) and P(V)-functionalised calixarenes and resorcinarenes David Sémeril ∗ , Dominique Matt ∗ Université de Strasbourg, Laboratoire de Chimie Inorganique Moléculaire et Catalyse, UMR 7177 CNRS, 1 rue Blaise Pascal, 67008 Strasbourg Cedex, France

Contents 1. 2.

3.

4.

5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis of calixarene-derived P(III) and P(V) ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Phosphines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Phosphinites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Phosphonites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Phosphites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5. Oxy-phosphorus diamides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6. Phosphorus ylides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7. Iminophosphoranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis of resorcinarene-derived P(III) ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Phosphines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Phosphites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Catalytic applications of calixarene- and resorcinarene-derived P(III) and P(V) ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Olefin hydroformylation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Olefin hydroalkoxycarbonylation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Olefin hydrogenation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Tsuji–Trost reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5. Cross-coupling reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6. Olefin oligomerisation/polymerisation reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

a r t i c l e

i n f o

Article history: Received 10 April 2014 Accepted 23 June 2014 Available online 23 July 2014

58 59 59 64 65 65 68 68 68 69 69 71 72 72 83 84 85 88 91 94 94 94

a b s t r a c t The goal of this review is twofold: (a) to provide synthetic methodology for the preparation of P(III)- and P(V)-functionalised calix[n]arenes and resorcin[4]arenes; (b) to give an up-to-date overview of catalytic applications of these ligands. © 2014 Elsevier B.V. All rights reserved.

Keywords: Calixarenes Resorcinarenes Phosphorus (III) Phosphorus (V) Phosphine syntheses Homogeneous catalysis

1. Introduction

∗ Corresponding authors. Tel.: +33 368851719. E-mail addresses: [email protected] (D. Sémeril), [email protected] (D. Matt). http://dx.doi.org/10.1016/j.ccr.2014.06.019 0010-8545/© 2014 Elsevier B.V. All rights reserved.

Calixarenes and resorcinarenes (Fig. 1) are two well-established families of polyphenolic compounds employed in numerous fields of contemporary chemistry [1–5]. Interest in these macrocycles has

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Fig. 1. Generic calix[n]arenes, resorcin[4]arenes, and resorcinarene-cavitands.

developed continuously and in a spectacular fashion since rational methods for the preparation of their parent versions became available ca. 35 years ago. Their success as molecular synthons for the construction of sophisticated functional molecules essentially relies on two main factors: (a) their capacity to control the orientation if not convergence of a set of substituents attached to one of their circular rims; (b) their particular tridimensional structure which delineates a molecular cavity with potential receptor properties. It is worthy of note that both skeletons may adopt various flexible conformations, which can be rigidified through appropriate chemical modification. The most striking example of backbone rigidification concerns the generic resorcinarenes (Fig. 1), which after introduction of links between each pair of neighbouring resorcinol units can be converted into inflexible bowl-shaped molecules that were initially termed cavitands, although today this nomenclature is also applied to other receptor-cavities. This publication provides a review of catalytically relevant P(III) ligands built on a calixarene or resorcinarene platform. The corresponding catalytic applications are discussed in the present study. As such it is designed as a tool for synthetic chemists. Initial studies on P(III)-functionalised calixarenes (vide infra) were aimed at synthesising tetraphosphinites based on the lower rim of a calix[4]arene. These species have their P(III) binding sites in close proximity and were therefore expected to facilitate cooperative effects between coordinated metal centres. Many other studies on the coordination chemistry of calixarene and resorcinarene-P(III)derivates have appeared since, each of them exploiting the multiple specific structural and functional features of the corresponding polyphenolic backbone. Among these, over 65 reports deal with catalytic applications of such ligands, some of which being relevant to supramolecular chemistry. In the present study, P(V)-derivatives (notably P-ylides and iminophosphoranes) will also be considered. Some aspects of the chemistry reported in this paper have been considered in various reviews and books published in recent years [6–10].

Scheme 1. Introduction of P(III)-atoms on the upper rim of a calix[4]arene platform.

Scheme 2. Synthesis of phosphines 4–9 starting from a monobromo calixarene.

2. Synthesis of calixarene-derived P(III) and P(V) ligands 2.1. Phosphines The first calix[4]arene bearing trivalent phosphorus atoms grafted on to its upper rim was reported by Hamada et al. [11]. Tetra-phosphine 2 was obtained from tetrabrominated calixarene 1

[12] through halogen–lithium exchange followed by reaction with Ph2 PCl (yield 59%, Scheme 1, top). In solution compound 2 exists as a mixture of several non-identified conformers. The same compound was obtained some time later by Kalchenko et al. applying a two-step procedure: reaction of 1 with Ph2 POi Pr/NiBr2 giving phosphine oxide 3 (75%), which was then quantitatively reduced to 2

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Scheme 3. Synthesis of diphosphine 10 via a modified Arbuzov reaction.

Fig. 2. Calixarenyl-diphosphines 11–17 obtained via the lithium–bromide exchange route.

Scheme 4. Synthesis of diphosphine 18.

with PhSiH3 (Scheme 1) [13]. The same authors also applied this methodology for the preparation of phosphonites (vide infra). Inspired by Hamada’s method, Monnereau et al. prepared the monophosphines 4–8 (Scheme 2). The yield of these reactions depended on the substituent (R) attached at the lower rim [14,15]. Highest yields (75–90%) were obtained with R groups devoid of ether side chains. Reaction of 5 with AlCl3 resulted in the tetrahydroxy-calixarene 9. The 1 H NMR (CDCl3 ) spectrum of 9 is consistent with fast trans annular rotation of the phenoxy groups occurring in solution. It is worthy of note that upper-rim diphosphinated versions of the above calixarenes could be prepared either via the above modified Arbuzov procedure (notably 10; Scheme 3) [16] or by using Hamada’s method (e.g. 10–17, Fig. 2) [17–20]. For practical reasons, the air sensitive dialkylphosphines 13, 14, 16 and 17 were isolated

Scheme 5. Synthesis of diphosphines 21 and 22.

as phosphine oxides. These were then conveniently reduced with PhSiH3 into the corresponding phosphines. Diphosphine 18 was quantitatively obtained by debenzylation of 12 with AlCl3 in toluene (Scheme 4) [21]. Like its monophosphine counterpart 9, diphosphine 18 exists in solution as several

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Scheme 6. Synthesis of water-soluble diphosphines 27 and 28.

Fig. 3. P-chirogenic diphosphines 37–39. Scheme 7. Synthesis of the proximally functionalised calix[4]arene 30.

interconverting conformers. An X-ray structure determination was carried out for the corresponding phosphine oxide 19, which revealed a conical structure of the calixarene moiety in the solid state. Diphosphine 21 was synthesised by Matt et al. by reaction of the bis-chloromethyl-calixarene 20 with two equiv. of Ph2 PLi, which was itself generated in situ from Ph2 PH and n BuLi [22] (Scheme 5). The same method was applied by Kubas et al. for the synthesis of diphosphine 22 (89% yield) [23]. In 2000, Shimizu et al. prepared the water-soluble calixarenylphosphines 27 and 28, which were obtained in two steps: (a) reaction of Ph2 PLi with 23 and 24, leading to 25 (87%) and 26 (82%), respectively; (b) reaction of the resulting phosphines with orthoboric acid in H2 SO4 , then with oleum, this affording the decasulfonated derivatives 27 (83%) and 28 (88%), respectively (Scheme 6) [24]. 31 P NMR data indicate that during each synthesis several phosphines formed, together with phosphine oxides. The catalytic applications of these water-soluble ligands (vide infra) were performed in the presence of the corresponding phosphine oxides. In 2008, Harvey et al. reported the synthesis of diphosphine 30, which was prepared by reacting the proximally

bis-chloromethylated calixarene 29 with Ph2 PK in THF (Scheme 7) [25]. In 2010, Jugé, Harvey et al. achieved the synthesis of the optically pure aminophosphine-phosphinite 33 applying the wellestablished “ephedrine” methodology. Its synthesis was performed in four steps (62% overall yield) (Scheme 8) according to the following reaction sequence: (a) reaction of the lithiated calixarene 31 with (+)-oxazaphospholidine borane (obtained from (−)-ephedrine); (b) nucleophilic attack of the resulting alcoholate on Ph2 PCl; (c) protection of the phosphinite formed with BH3 and subsequent purification of the resulting adduct 32; (d) quantitative deprotection of 32 with 1,4-diazabicyclo[2,2,2]octane (DABCO) in toluene. This latter step occurred with retention of configuration at phosphorus [26]. The authors further applied the “ephedrine” methodology to the synthesis of the P-chirogenic monophosphines 35 and 36 (Scheme 9) and diphosphines 37–39 (Fig. 3), which could be synthesised in 32–69% yield [27]. The P N bond of intermediate 34 was cleaved with HCl to give a chlorophosphine, which was subsequently reacted with the appropriate organolithium reagents. The resulting optically active phosphine-boranes were then treated with DABCO to give the corresponding free phosphine or diphosphine.

Scheme 8. Synthesis of the aminophosphine–phosphinite calixarene 33 derived from (−)-ephedrine.

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Scheme 9. Synthesis of P-chirogenic phosphines 35 and 36.

Matt et al. described the synthesis of calixarene 41 with two dangling CH2 CH2 PPh2 moieties anchored at distal positions. This compound was obtained in only 7% yield after reaction of ditosylate 40 with two equiv. of Ph2 PLi (Scheme 10) [28]. The anchoring of four such groups was achieved by McKervey et al. by reaction of tetratosylate 43 (itself obtained from tetrol 42) with NaPPh2 [29]. There was no indication of the corresponding reaction yield. The calixarene thus obtained (44) was re-synthesised later by Kollár et al. using LiPPh2 as source of phosphorus (62% yield) (Scheme 11) [30]. The Matt group also reported the preparation of a series of calixarenes substituted at the lower rim by two distally positioned CH2 PPh2 groups [31]. These were obtained by alkylation of the dihydroxy-dialkylated precursors 45–49 with Ph2 P(O)CH2 OTs, and reduction of the resulting doubly-phosphorylated compounds 50–54 with PhSiH3 (Scheme 12 and Fig. 4) [28,32,33].

Scheme 10. Synthesis of diphosphine 41.

Scheme 11. Synthesis of tetraphosphine 44.

Scheme 12. General synthesis of calix[4]arenes distally-substituted by

CH2 PPh2 groups.

Scheme 13. Synthesis of diphosphine 56 based on a 1,2-disubstituted calix[4]arene skeleton.

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Fig. 4. Diphosphines 50–54.

Scheme 14. Synthesis of optically pure diphosphines 59–61.

The synthesis of calixarene 56, which contains two -CH2 PPh2 substituents attached to proximal phenolic oxygen atoms of its backbone, was achieved with the same alkylating reagent as was used for the preparation of 50–54, but with NaH (as base) which ensured selective proximal alkylation [34]. The di(phosphine oxide) intermediate 55 was obtained in 66% yield [35]. Its reduction with PhSiH3 afforded 56 quantitatively (Scheme 13). Note that monoO-acylation of 56 afforded a diphosphine (not shown) displaying a rare example of through-space J(PP ) coupling (8 Hz) [36]. The dihydroxy-calixarene 55 is a useful synthon for the preparation of chiral diphosphines. Thus, treatment of 55 with

(R)-BrCH2 C(O)NHCHMePh in the presence of 0.5 equiv. of K2 CO3 led to the two diastereoisomers 57 and 58 (actually two inherently chiral calix[4]arenes), which could be separated by column chromatography (Scheme 14). Their reduction with PhSiH3 gave the corresponding optically active diphosphines 59 and 60, respectively. As shown by a variable temperature NMR study, due to trans annular rotation of the ArOH ring, both phosphines exist in solution as a mixture of two equilibrating conformers (cone and partial cone). Alkylation of 59 with Me3 SiCl gave a cone conformer selectively, namely optically pure diphosphine 61.

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Scheme 15. Synthesis of optically pure diphosphine 63.

Scheme 16. Synthesis of monophosphines 65–71, based on a calix[6]arene skeleton.

Repeating the alkylation of 55 with two equiv. of (R)BrCH2 C(O)NHCHMePh afforded di(phosphine oxide) 62 in high yield. The latter was conveniently converted with PhSiH3 into optically pure diphosphine 63 (Scheme 15) [37]. The only reported phosphines based on a calix[6]arene that were used in homogeneous catalysis have been prepared by Tsuji et al. Thus, phosphines 65–71 were obtained in good yield applying a two-step alkylation/reduction procedure starting from pentamethoxycalix[6]arene (64) (Scheme 16) [38,39]. As revealed by NMR studies, calixarenes 69–71 exist in solution as several interconverting isomers, the major isomer adopting a 1,2,3-alternate conformation. Dynamic behaviour was also observed for calixarenes 66–68, but for these compounds the dominating species is the cone conformer. 2.2. Phosphinites In 1989, Floriani et al. reported the first P(III)-functionalised calixarenes (Note that in the same year, Ungaro et al. described the first calix[4]arene with P(V)-containing substituents grafted at

Scheme 17. Synthesis of tetraphosphinite 73.

the upper rim [40]). Among these is tetraphosphinite 73, which was obtained by deprotonation of 72 with a strong base, followed by reaction with Ph2 PCl (Scheme 17) [41]. This compound was intended to be used as a P4 -platform for forming tetranuclear complexes in which the four metal atoms lie in close proximity and may therefore facilitate synergetic interactions in catalytic reactions. Related calixarenes bearing only two phosphinito groups, notably 74–79 (Fig. 5), were reported later, independently by Roundhill

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Fig. 5. Diphosphinites 74–79 based on the calix[4]arene platform.

Scheme 19. Synthesis of the calix[6]arene hexaphosphinites 81 and 82. Scheme 18. Synthesis of tetraphosphinite 80.

[42] and Matt [28,43,44]. Diphosphinite 74 displays conformational mobility. A calixarene O-substituted by four -CH2 CH2 OPPh2 pending arms (80) was obtained in 76% yield by Kollár et al. by reacting tetrol 42 in THF with Ph2 PCl in the presence of pyridine (Scheme 18) [30]. Starting from generic p-R-calix[6]arenes, Karakanov et al. synthesised hexaphosphinites 81 and 82 using the same method as that reported by Floriani for the preparation of 73 (Scheme 19) [45]. 2.3. Phosphonites To date, only one phosphonite-calixarene, namely 85, has been used in catalysis. Its synthesis involved treatment of

dihydroxyl-calixarene 83 with n BuLi and chlorophosphonite 84 (Scheme 20) [46].

2.4. Phosphites In 2000 Pringle et al. reported the synthesis of phosphites 88 and 89 having each their phosphorus atom capping three oxygen atoms of a calix[4]arene skeleton. They were obtained in ca. 70% yield by slow addition of trifluoroacetic acid to a solution of the hexacoordinate phosphorus(V) derivatives 86 and 87, respectively [47]. The latter were obtained according to a method previously described by Lattman, namely by reacting a generic calixarene with P(NMe2 )3 (Scheme 21) [48].

Scheme 20. Synthesis of diphosphonite 85 based on the calix[4]arene platform.

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Scheme 21. Synthesis of phosphites 88 and 89 derived from calix[4]arene.

Fig. 6. Water-soluble phosphite 90.

A water-soluble version of these phosphites, 90 (Fig. 6), was obtained applying a similar reaction sequence, but with p-(SO3 H)4 calix[4]arene as starting compound [49]. van Leeuwen et al. synthesised calixarene-phosphites 91 and 92 by acylation of 89 in the presence of KOt Bu (Scheme 22) [50]. Attempts to prepare the related compounds 93–96 by alkylation of phosphite 89 was unsuccessful, however these compounds could be prepared conveniently in two steps according to the following procedure: (a) monoalkylation of p-tert-butyl-calix[4]arene; (b) reaction of the resulting (tris-hydroxy)-(alkyl)-calix[4]arene with P(NMe2 )3 in the presence of tetrazole (Scheme 23).

Scheme 22. Synthesis of O-acylated calix[4]arene phosphites 91 and 92.

The groups of van Leeuwen and Matt independently developed an alternative synthetic route for the formation of lower-rim based calix[4]arenyl phosphites containing an additional binding function (phosphine oxide, ester, amide, ether, quinoline). Thus 97–101, which were obtained by reacting of PCl3 /NEt3 with the appropriate mono-O-alkylated precursor [28,35,51,52], were all designed as potential chelators (P, O or P, N chelation) (Fig. 7) [52,53].

Scheme 23. Synthesis of O-alkylated calix[4]arene phosphites 93–96.

Fig. 7. O-alkylated calix[4]arene phosphites 97–101.

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Scheme 24. Synthesis of diphosphite calix[4]arenes derived from BINOL (102 and 103) and TADDOL (104 and 105).

Scheme 25. Synthesis of calix[6]arene diphosphites 108 and 109.

Scheme 27. Synthesis of diphosphites 110–116 with P atoms linking proximal phenolic units.

Scheme 26. . Up–up–out/out–up–up equilibrium within the calix[6]arene backbone of syn-109.

van Leeuwen et al. extended this two-step synthetic route to the build-up of the optically active BINOL-derived phosphites 102–103 and TADDOL derivatives 104–105 (Scheme 24) [54]. van Leeuwen et al. further achieved the synthesis of bisphosphites 108 and 109 starting from generic p-R-calix[6]arenes (R = t Bu, H). These were obtained in moderate yield upon treatment

of the precursor with NaH and PCl3 (Scheme 25). Whatever the calixarene used, 106 or 107, two conformationally distinct bis-phosphites formed, adopting respectively a syn and an anti conformation [55,56]. The ligand having the syn conformation was found to display dynamic behaviour in solution. The observed motion corresponds to a continuous variation of orientation of the phenoxy rings as shown in Scheme 26. In 1999 Paciello and Röper reported the synthesis of diphosphites 110–113 that have each phosphorus atoms bridging adjacent phenolic rings (Fig. 8) [57]. These ligands were obtained in 23–65% yield by reacting p-tert-butylcalix[4]arene with two equivalents of the appropriate dichlorophosphite (ArOPCl2 ) in the presence of

Scheme 28. Synthesis of diphosphites 117 and 118.

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Fig. 8. Calix[4]arene-diphosphites 110–116.

synthesised by reaction of 2-chloro-1,3,5-trimethyl-1,3,5-triaza2␴3 ␭3 -phosphorin-4,6-dione (144) with the appropriate, fully deprotonated di- or tetrahydroxy calixarene (Scheme 30). These phosphorus ligands behave as P(III)-ligands with poor ␴-donor properties.

2.6. Phosphorus ylides Scheme 29. Synthesis of monophosphite 121.

NEt3 (Scheme 27). Krishnamurty et al. obtained the related diphosphites 114–116 (Fig. 8) using a similar synthetic method [58,59]. Krishnamurty also carried out the stepwise construction of related calixarenes having two distinct P(OR) bridging units, namely 117 and 118 (Scheme 28) [60]. The corresponding yields did not exceed 20%. Very large J(PP’) coupling constants (223–244 Hz) were observed for these unsymmetrical calixarenes, these reflecting strong through-space P,P interactions. Numerous calixarene-phosphites have been reported in which the phosphorus atoms are attached to a single oxygen atom of the lower rim. Most of them are diphosphites. A catalytically relevant monophosphite of this family is calixarene 121, which was obtained by mono-deprotonation with NaH of the proximally-dipropylated calixarene 119, followed by reaction with one equiv. of [1,1 biphenyl]-2,2 -phosphorochloridite (120) (Scheme 29) [61]. In the calixarene-diphosphite series reported to date (122–124 [44], 125–126 [46], 127 [62], 128 [61], 129 [63], 130 [64], 131–132 [65], 133 [64], 134–138 [66] and 139–141 [64], Fig. 9) the calixarenes are all distally P-substituted. The only reported calixarenes having four P(OR)2 groups connected to their lower rim were described by Schmutzler and Börner in 2001 (142 and 143, Fig. 10) [67]. 2.5. Oxy-phosphorus diamides Schmutzler and Börner also reported a series of Biuretbased ligands (145 [67], 146 [68] and 147 [67]) that were

In 2006 Matt et al. described the synthesis of keto-stabilized phosphorus ylides tethered to a calix[4]arene platform [69]. Thus, reaction of the dibromoacetylated calixarene 148 with PPh3 followed by deprotonation of the resulting phosphonium salt with NaH produced quantitatively the bis-phosphorus ylide 149 (Scheme 31). The phosphorus ylide 151, which is based on a 1,3-alternate-conformer was obtained similarly starting from precursor 150, that adopts also a 1,3-alternate conformation (Scheme 31). The only reported mono-P-ylides built on a calixarene platform are compounds 152 and 153 (Fig. 11). Their synthesis required the preparation of monobromoacetylated calixarenes [69]. It should be remembered here that keto-stabilised phosphorus ylides are classical precursors of phosphine-enolato ligands. Nickel complexes containing such ligands promote the oligomerisation/polymerisation of ethylene and are used in the SHOP process.

2.7. Iminophosphoranes Sémeril et al. reported the synthesis of the monoiminophosphoranyl-calix[4]arenes 154–158 [70] and the di-iminophosphoranyl derivatives 159–163 [71]. Iminophosphoranes are P(V) derivatives that can be obtained quantitatively by Staudinger condensation of a phosphine with an azide (Scheme 32). They can be handled in air, despite the fact that they are water-sensitive, the action of water resulting in an aniline, and a phosphine oxide. They should be regarded as N-donor ligands.

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Fig. 9. Diphosphites 122–141 based on the calix[4]arene platform.

3. Synthesis of resorcinarene-derived P(III) ligands

3.1. Phosphines

Those P(III)-containing resorcinarenes that were employed in homogeneous catalysis are all based on rigidified resorcinarenes, that is on so-called cavitands. Most of the corresponding syntheses were carried out with precursors having Br atoms fixed at the cavity wider rim [72]. This class of compounds is restricted to phosphines and phosphites.

Matt et al. reported the synthesis of a series of resorcinarenecavitands substituted by one (166 [72]), two (167 and 168 [72]) or four (169 [73]) diphenylphophinomethyl group(s). In all of them, the phosphine pending arm is attached to one of the C2-positions of the resorcinol unit (Fig. 12).

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Scheme 30. Oxy-phosphorus diamides 145–147 based on the calix[4]arene platform.

Scheme 31. Synthesis of bis-phosphorus ylides 149 and 151.

Scheme 32. Synthesis of mono- and di-iminophosphoranyl-calix[4]arenes 154–163.

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Scheme 33. Synthesis of phosphine 166.

Scheme 34. Synthesis of phosphines 170 and 172.

Fig. 10. Tetraphosphinated calix[4]arenes 142 and 143.

Fig. 12. Phosphines 166–169, based on a resorcin[4]arene platform.

Fig. 11. Calixarenes 152 (cone) and 153 (1,3-alternate) substituted by a single phosphorus ylide moiety.

The synthesis of these phosphines was performed starting from partially- or fully-brominated resorcinarenes. For example, the route leading to monophosphine 166 involved phosphorylation with Ph2 POEt of the bromomethylated intermediate 165, itself being obtained from precursor 164 [72]. The resulting phosphine oxide (not drawn) was then reduced with PhSiH3 (Scheme 33). All these phosphines were obtained in overall yields of about 50%. The monobrominated cavitand 164 was further used for the synthesis of phosphines 170 and 172 in which the phosphorus

atom is directly grafted on the resorcinol unit. Treatment of 164 with n BuLi and addition of Ph2 PCl or i Pr2 PCl led to 170 and 172, respectively (Scheme 34). Phosphine 170 was purified by column chromatography (70%) [74]. Because of its easy oxidation, the related bis(dialkyl)arylphosphine 172 could not be purified on a silica gel column. Instead, 172 was treated with BH3 to afford a mixture of compounds from which the borane adduct 171 could be separated readily. Removal of borane with NEt2 H gave then pure 172 in 43% yield. 3.2. Phosphites Resorcinarene-phosphites were used for catalytic purposes only by Rebek et al. This group reported the synthesis of cavitand 176, containing a phosphorus atom that links two neighbouring resorcinol units [75]. As such, the P atom of 176 contributes to the

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Scheme 35. Synthesis of resorcinarene-phosphite 176, obtained as a mixture of exo and endo isomers.

Scheme 36. Transition metal-catalysed hydroformylation of ˛-olefins.

rigidity of the macrocyclic structure. The synthesis of this phosphite (Scheme 35) involved the preparation of chlorophosphite 174, which was obtained by treating diol 173 with PCl3 in the presence of pyridine. In situ addition in the presence of NEt3 of the oxazoline-alcohol 175 to preformed 174 gave a 7:3 mixture of exo176 (21% isolated yield) and endo-176, which were separated by chromatography. 4. Catalytic applications of calixarene- and resorcinarene-derived P(III) and P(V) ligands 4.1. Olefin hydroformylation The hydroformylation of olefins (RCH = CH2 ) currently represents the main route for the production of linear C3 –C18 aldehydes. In this reaction, aldehydes (linear or branched) are produced from olefins, carbon monoxide and hydrogen in the presence of a transition metal catalyst (Scheme 36). Typical catalysts for this reaction are P(III)-based rhodium complexes. The formation of the two regioisomers in different proportions (linear and branched aldehydes) can be explained by the presence in the catalytic cycle (Scheme 37) of two types of

Fig. 13. Origin of the formation of the two regioisomers (linear and branched).

rhodium-alkyl intermediate, [Ln Rh–CH(Me)R] (branched isomer) and [Ln Rh–CH2 CH2 R] (linear isomer). In these complexes the metal–carbon bond is polarised, the metal and the carbon atoms bearing respectively a partial positive and a partial negative charge. When R is an aromatic or an electron-withdrawing group, the negative charge is better delocalised in the branched isomer, that is the complex that will give the branched aldehyde (note that when R = Ar, the formation of an ␩3 -complex occurs). When R is an nalkyl group (electron donor), no delocalization occurs neither in the branched, nor in the linear isomer. In that case both aldehydes are formed, although usually the linear one is the major product. This is due to the fact that the metallated carbon atom of the linear isomer undergoes electron donation from a single alkyl group, whereas the branched isomer undergoes that of two alkyl substituents. However, if the R group is bulky, steric hindrance will also strongly influence the selectivity, resulting then mainly in the linear aldehyde (Fig. 13). One efficient way to drive the hydroformylation reaction towards the formation of linear aldehydes consists in using chelating diphosphines that display a large natural bite angle. According to Casey [76] and van Leeuwen [77,78], with such ligands the trigonal bipyramidal [RhH(alkyl)(CO)(diphos)] intermediates

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Scheme 37. Catalytic cycle of olefin hydroformylation [77]. Table 1 Rhodium-catalysed hydroformylation using complexes 177–179. Entry

Rh complex

␣-Olefin

TOF (mol(olefin) mol(Rh)−1 h−1 )

l/b

1a [22] 2b [23] 3c [25]

177 178 179

Styrene Styrene 1-Hexene

31 58 250

9:91 14:86 2.5

a Catalyst activation, styrene: Rh:NEt3 = 305:1:50, toluene/CH2 Cl2 (9:1 v/v), 40 bar CO/H2 (1:1), 70 ◦ C, 12 h. b Styrene: Rh:NEt3 = 260:1:43, toluene/CH2 Cl2 (4:1 v/v), 55 bar CO/H2 (1:1), 70 ◦ C, 5 h. c Hexene: Rh = 2000:1, THF, 71 bar CO/H2 (1:1), 50 ◦ C, 48 h.

Fig. 14. Bis-rhodium complexes 177–185 used in ␣-olefin hydroformylation.

formed during the catalytic cycle adopt preferentially an ee conformation, that is one in which both phosphorus atoms occupy equatorial positions (in that case the P–Rh–P angle approaches the ideal value of 120◦ ), rather than a so-called ae conformation with one P occupying an axial and the other one an equatorial site (i.e. with P–Rh–P near 90◦ ). The large PRhP angle of the ee intermediate forces the phosphorus substituents to be bent towards the metal centre, hence forming a tight molecular pocket. The resulting steric pressure exerted on the metal ion by the substituents then favours the transfer of the hydrido atom on the C2 carbon atom of the coordinated olefin, rather than to the terminal atom. The linear alkyl intermediate is the one that eventually leads to the linear aldehyde. The dinuclear rhodium complex 177 (Fig. 14) catalyses the hydroformylation of styrene in the presence of NEt3 [22]. The catalyst activity was weak (turnover frequency (TOF) = 31 mol(styrene) mol(Rh)−1 h−1 ), showing that this catalyst

Fig. 15. Phosphines 15 and 30.

functions like a classical monophosphine complex, and suggesting that here no supramolecular effect involving the cavity is operative. In keeping with monophosphine rhodium complexes, 177 produced the branched aldehyde as the major isomer (l/b = 9/91) (Table 1, entry 1). Adding 1 equiv. of diphosphine 21 to the complex improved the activity without modifying the regioselectivity. The added diphosphine probably simply increases the life-time of the catalyst by stabilising some catalytic intermediates. The less sterically hindered dirhodium complex 178 led to comparable activity (TOF = 58 mol(styrene) mol(Rh)−1 h−1 ; Table 1, entry 2) and regioselectivity (l/b = 14/86) [23]. Reaction between the proximally functionalised calixarene 30 and [Rh(cod)Cl]2 in the presence of TlPF6 produced the dimeric complex [Rh(cod)(30)]2 (PF6 )2 (179) (Fig. 15) [25]. Using this

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Scheme 38. Equilibrating dimers and oligomers derived from diphosphine 30. Table 2 Rhodium-catalysed hydroformylation using complexes 180–185 [20]. Entry

1 2 3 4 5 6

Rh complex

180 (R = Ph) 181 (R = Me) 182 (R = i Pr) 183 (R = Ph) 184 (R = Me) 185 (R = i Pr)

1-Hexene

Table 3 Rhodium-catalysed biphasic hydroformylation of 1-octene using water-soluble phosphines 27 and 28.

Styrene

TOF

l/b

TOF

l/b

250 106 211 141 51 97

1.7 1.7 1.8 1.7 1.8 1.9

11 11 65 80 35 82

6:94 5:95 11:89 8:92 6:94 10:90

Conditions: THF, 71 bar CO/H2 (1:1), 50 ◦ C. The TOFs were expressed in mol(olefin) mol(Rh)−1 h−1 .

Entry

Catalytic system

Conversion (%)

Yield (aldehydes) (%)

l/b

1 [24] 2 [24] 3 [24] 4 [24] 5 [81]

27/[Rh(acac)(CO)2 ] 28/[Rh(acac)(CO)2 ] (1st use) 28/[Rh(acac)(CO)2 ] (2nd use) 28/[Rh(acac)(CO)2 ] (3rd use) TPPTS/[Rh(acac)(CO)2 ]/DMCD

55 95 97 98 26

40 73 84 86 21

3.0 1.7 1.7 1.9 2.4

Conditions: 1-octene:P:Rh = 250:4:1, water, 40 bar CO/H2 (1:1), 100 ◦ C, 12 h.

Table 4 Rhodium-catalysed hydroformylation of 1-hexene using calix[6]arenyl-phosphines 65–71.

Fig. 16. Water-soluble phosphines 27 and 28.

complex in the hydroformylation of hexene gave activities of 250 mol(hexene) mol(Rh)−1 h−1 (Table 1, entry 3). The observed selectivities (l/b = 2.5) were close to those observed for PPh3 . With complex 180, which has its P atoms directly attached to the upper rim, the linear aldehyde selectivity slightly decreased (l/b = 1.7) [20]. Interestingly, the induction period was significantly longer for 179 than for 180. By means of 31 P NMR spin-lattice experiments, Harvey showed that both ligands readily form complexes that exist as dimers in exchange with oligomers (Scheme 38). The rhodium atoms of the dimers being less accessible than those of the oligomers, these species are likely to be less active than their oligomeric counterpart. Molecular models unambiguously showed that the dimers formed from ligand 30 have a less strained structure than those obtained from 15 (Fig. 15), this explaining why 15 was more active than 30. The dimeric rhodium complexes 180–185 were tested in the hydroformylation of 1-hexene, styrene, vinyl acetate, vinyl benzoate and vinyl p-tert-butylbenzoate [20]. For 1-hexene, the complexes exhibited TOF’s varying in the order R = Ph > R = i Pr > R = Me, while the (weak) linear aldehyde selectivities where practically independent of the R groups (l/b = 1.7–1.9) (Table 2). In the case of styrene, the TOFs decreased with the size of the phosphorus substituent R: i Pr > Ph ≥ Me. The highest regioselectivities towards branched aldehydes were obtained with the smallest R group, namely Me, the corresponding l/b ratios being 5:95 and 6:94 for 181 and 184, respectively (Table 2, entries 2 and 5). The water-soluble phosphines 27 and 28 (Fig. 16), which were both employed in a non-purified form (see above, paragraph

Entry

Phosphine

Yield(aldehydes) (%)

l/b

1 [38] 2 [39] 3 [39] 4 [39] 5 [39] 6 [39] 7 [39] 8 [39]

65 66 67 68 69 70 71 PPh3

79 94 70 85 81 89 82 46

1.9 1.7 2.6 2.4 2.0 2.3 1.5 2.7

Conditions: 1-hexene:P:[Rh(cod)2 ]BF4 = 200:4:1, benzene, 10 bar CO/H2 (1:1), 70 ◦ C, 16 h.

2.1), were tested in the hydroformylation of 1-octene. The corresponding catalytic systems were generated by mixing the ligand and [Rh(acac)(CO)2 ] in water [24,79]. A two-phase system was obtained when reacting the olefin with an aqueous solution of the catalyst, which itself behaved as an inverse phase-transfer agent. After 12 h, 1-octene was converted into aldehydes in respectively 40% (27) and 73% (28) yield, (Table 3, entries 1 and 2). The best linear aldehyde selectivity was observed with calixarene 27, i.e. the one with the shortest pending phosphine substituents (ligand 27; l/b = 3.0). The catalysts still remained active after three catalytic cycles, the regioselectivity remaining unchanged (Table 3, entries 2–4). During the catalytic runs, internal olefins also formed. Isomerisation also occurred when using cis- or trans-4-octene and trans-2-octene as substrate, this producing 1-octene, part of which was converted to nonanal [80]. In these experiments the observed l/b ratios were close to 0.3. The above water-soluble catalysts were more efficient than Monflier’s system based on tris-(3-sulfophenyl)phosphine trisodium salt (TPPTS) and (2,6-diO-methyl)-␤-cyclodextrin (DMCD) (Table 3, entry 5) [81]. The calix[6]arene-derived phosphines 65–71 (Fig. 17) were tested in the hydroformylation of several olefins. The corresponding catalytic systems were obtained by mixing the ligands with [Rh(cod)2 ]BF4 . The activity of the catalyst was dependant on the nature of the phosphine arm and on the substrate, but a rationale for the observed differences could not be established: for example, for

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Fig. 17. Phosphines 65–71.

Table 5 Rhodium-catalysed hydroformylation of 1-octene using phosphites 91–96 [50]. Entry

Phosphite

Conversion (%)

Yield(aldehydes) (%)

TOF

l/b

1 2 3 4 5 6

91 92 93 94 95 96

97 97 78 92 77 96

79 84 53 84 60 90

4700 5000 1900 7200 1800 7300

1.6 1.2 2.0 1.3 1.4 1.4

Conditions: catalyst activation, 1-octene:P:[Rh(acac)(CO)2 ] = 6370:20:1, toluene, 20 bar CO/H2 (1:1), 100 ◦ C, 12 h. The TOFs were expressed in mol(olefin) mol(Rh)−1 h−1 .

Fig. 18. Phosphites 91–96.

Fig. 20. Phosphites 88 and 89.

Fig. 19. Up–up–out–up (left) and up–out–up–up (right) conformation of calix[4]arenyl-phosphites 91–96.

1-hexene (Table 4, entries 1 and 2) and styrene, the system based on phosphine 66 resulted in higher reaction rates than those with 65. For vinyl acetate and vinylbenzoate, the system based on phosphine 65 displayed higher activities than that based on 68 [38]. Furthermore, in the hydroformylation of 1-hexene, the cone-shaped calix[6]arenes 66–68 resulted in only slightly higher regioselectivities than their 1,2,3-alternate counterparts 69–71 (Table 4, entries 2–7). It should be mentioned that the above calix[6]arene-derived phosphines all led to higher activities than the less bulky PPh3 ligand (Table 4, entry 8) [39]. van Leeuwen et al. studied the conformational influence of the calixarene moiety of phosphites 91–96 (Fig. 18) in the hydroformylation of 1-octene [50]. All these ligands led to active catalytic systems, with the l/b ratios ranging from 1.3 to 2.0. The authors observed that the activities were strongly dependant on the calix[4]arene conformation. In calix[4]arenes 93 and 95 (up–out–up–up conformation) (Fig. 19) the phosphorus lone pairs are embedded in the calixarene cavity, thus hampering metal binding and impeding olefin coordination. As expected, these effects slowed down the reaction rate with respect to less crowded ligands. Thus for example, phosphites 94 and 96 (up–up–out–up), which possess a much more accessible phosphorus lone pair, and in which the R group grafted at the fourth oxygen atom can easily move away from the P atom (TOFs = 1900, 1800, 7200 and 7300 mol(olefin) mol(Rh)−1 h−1 , respectively for 93, 95, 94 and 96; Table 5, entries 3–6). Ligands 91 and 92 displayed activities in between those of 93/95 and 94/96, owing to the presence of

auxiliary ester functions, which are able to transiently bind the metal and therefore to slow down somewhat approach of the substrate (TOF = 4700 and 5000 mol(olefin) mol(Rh)−1 h−1 for 91 and 92, respectively; Table 5, entries 1 and 2). Concomitantly with van Leeuwen’s studies on phosphites 91–96, the group of Pringle studied the hydroformylation of 1hexene with [RhL(CO)(acac*)] complexes (acac* = t BuCOCHCOt Bu; L = phosphite 88 or 89; Fig. 20). The two calixarenes used for this study adopt both an up–up–out–down conformation [82]. Hydroformylation at 160 ◦ C in toluene under 61 bar CO/H2 (1:1) resulted in full olefin conversion after 3 h. The observed l/b ratio was 1.2. The two phosphites, probably because of their bulkiness, produced highly unsaturated rhodium species, which therefore resulted in high activities and poor linear aldehyde selectivity. Addition of a large excess of ligand had an insignificant effect on the catalytic outcome, thus reinforcing the idea that the active species is a rhodium–monophosphite complex. The influence of an oxygen-containing side group in phosphite analogues of 94 was investigated in the hydroformylation of 1octene [53]. Phosphites 97–100 (Fig. 21) adopt an up–up–out–up conformation, in other words the phosphorus atom is in a relatively open environment and thus suitable for metal coordination. The presence of substituents bearing phosphoryl, amide, or ester groups, which all possess good donor properties, had a positive effect on the regioselectivity. Thus, for example, a l/b ratio of 3.6 was observed with ester 98 (Table 6, entry 2), while with ether-phosphine 100 the l/b ratio was only 1.4. Interestingly, the activities decreased with increasing donor strength of the oxo side group: 100 (R = CH2 CH2 OCH3 ) > 97 (R = CH2 P(O)Ph2 ) > 98 (R = CH2 CO2 Et) > 99 (R = CH2 CONEt2 ) (Table 6), suggesting that

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Fig. 21. Phosphites 97–101.

Table 6 Rhodium-catalysed hydroformylation of 1-octene using phosphites 97–100 [53]. Entry

Phosphite

Yield(aldehydes) (%)

TOF

l/b

1 2 3 4

97 (R = CH2 P(O)Ph2 ) 98 (R = CH2 CO2 Et) 99 (R = CH2 CONEt2 ) 100 (R = CH2 CH2 OCH3 )

89 90 86 96

2450 1300 950 4400

2.4 3.6 2.7 1.4

Conditions: catalyst activation, 1-octene:P:[Rh(acac)(CO)2 ] = 5000:10:1, toluene, 22 bar CO/H2 (1:1), 80 ◦ C. The TOFs were expressed in mol(olefin) mol(Rh)−1 h−1 .

Scheme 39. Hydroformylation of 2-methylpentnoate using [Rh(L)(CO)(acac*)] (L = 88 and 89) and [Rh(90)(CO)(acac*)][NH2 Me2 ]4 complexes.

Fig. 22. Water-soluble phosphite 90.

during catalysis, P,O-chelates are formed that hamper olefin binding. However, evidence for hemilabile behaviour of these ligands was not provided. Neither could van Leeuwen formally prove that pyridinyl-phosphite 101 (Fig. 21) behaves as a hemilabile ligand in hydroformylation experiments. Nevertheless, this author proposed that the low linear aldehyde selectivities observed with 101 in the hydroformylation of 1-octene is due to the simultaneous formation of several rhodium species, as a consequence of two possible binding modes of the ligand, monodentate or bidentate [52]. The hydroformylation at 160 ◦ C of 2-methylpentanoate with rhodium complexes based on phosphites 88–90 (Figs. 20 and 22) was studied by Pringle et al. These reactions led to mixtures containing the aldehydes 186–188 and the lactones 189 and 190 (Scheme 39). The reactions were carried out in toluene with the neutral complexes [Rh(L)(CO)(acac*)] (L = 88 and 89) and in water/toluene mixtures with the water-soluble complex [Rh(90)(CO)(acac*)][NH2 Me2 ]4 . With this latter complex the reaction was also performed at a lower temperature, namely 60 ◦ C. In this case, only formation of the branched aldehyde 187 and of the lactone 189 was observed (187:189 ratio ca. 1:1), in other words the reaction took place without double bond isomerisation (which would lead to 188) [49]. The functional diphosphines 41 and 50–52 (Fig. 23) were assessed in styrene hydroformylation. The corresponding runs were carried out at 40 ◦ C under 40 bar CO/H2 with preformed complexes, notably [Rh(51)(CO)]BF4 (191) (Fig. 24) and [Rh(L)(nbd)]BF4 (L = 41, 50 and 52) [28,32,33]. These complexes showed low activities, however gave high branched aldehyde selectivities (95%) (Table 7). The low reaction rates may be attributed to partial encapsulation of the catalytic centre and/or competitive binding of the

Fig. 23. Diphosphines 41 and 50–52.

Fig. 24. Complex 191.

D. Sémeril, D. Matt / Coordination Chemistry Reviews 279 (2014) 58–95 Table 7 Rhodium-catalysed hydroformylation of styrene using phosphines 41 and 50–52. Entry

Rh complex

Styrene/Rh

TOF (mol(olefin) mol(Rh)−1 h−1 )

l/b

1 [28] 2 [28] 3 [33] 4 [32]

[Rh(41)(nbd)]BF4 [Rh(50)(nbd)]BF4 [Rh(51)(CO)]BF4 (191) [Rh (52)(nbd)]BF4

870:1 600:1 500:1 350:1

7 7 1 7

5:95 5:95 5:95 5:95

Table 9 Rhodium-catalysed hydroformylation of 1-octene and styrene using phosphinites 75, 76 and 79 [44]. Entry

Conditions: catalyst activation, CH2 Cl2 , benzene or toluene, 40 bar CO/H2 (1:1), 40 ◦ C.

77

1 2 3

Ligand

75 76 79

1-Octene

Styrene

TOF

l/b

TOF

l/b

2625 1032 1219

1.6 2.5 2.2

854 891 210

34:66 36:64 41:59

Conditions: catalyst activation, olefin:P:[Rh(acac)(CO)2 ] = 2500:1:1, toluene, 20 bar CO/H2 (1:1), 80 ◦ C. The TOFs were determined at ca. 30% conversion and were expressed in mol(olefin) mol(Rh)−1 h−1 .

Fig. 25. Oxy-phosphorus diamides 145 and 192. Table 8 Rhodium-catalysed hydroformylation of 1-octene using oxy-phosphorus diamides 145 and 192 [67]. Entry

Ligand

L/Rh

Solvent

P(CO/H2 ) (bar)

Yield (aldehydes) (%)

l/b

1 2 3

145 145 192

2:1 10:1 4:1

THF THF CH2 Cl2

50 50 40

88.4 86.6 95.9

1.0 1.4 0.8 ◦

Conditions: 1-octene:[Rh(acac)(cod)] = 15,700:1, CO/H2 (1:1), 100 C, 3 h.

Fig. 27. Phosphonite 85.

Table 10 Rhodium-catalysed hydroformylation of 1-octene using phosphites 125 and 126 [46]. Entry

Ligand

L:Rh

Yield(aldehydes) (%)

l/b

1 2 3 4

125 125 126 126

1:1 10:1 1:1 10:1

83.3 46.0 58.7 89.2

1.1 1.4 1.6 1.4

Conditions: 1-octene:[Rh(acac)(cod)] = 15,700:1, THF, 40 bar CO/H2 (1:1), 100 ◦ C, 3 h.

Fig. 26. Diphosphinites 74–76 and 79.

functional side groups, both phenomena hindering approach of the olefin. The bis-phosphorus ligand 145 (Fig. 25) was tested in the hydroformylation of 1-octene [67]. The corresponding catalytic system was generated by mixing 145 with [Rh(acac)(cod)]. This system gave high activities, but led to very low selectivities towards the linear aldehyde even in the presence of large excess of ligand (l/b ranging from 1.0 and 1.4) (Table 8, entries 1 and 2). A possible explanation for these findings is that the phosphorus atoms bind weakly to the metal, thereby readily affording unsaturated rhodium intermediates. That this effect is not related to the presence of the calixarene backbone was suggested by the results obtained with the monodentate ligand 192 (Fig. 25), which also resulted in very low l/b ratios (0.8 !). Combining diphosphinites 75, 76 or 79 (Fig. 26) with stoichiometric amounts of [Rh(acac)(CO)2 ] produced effective systems for 1-octene hydroformylation [44]. A TOF of

2625 mol(octene) mol(Rh)−1 h−1 and a l/b ratio of 1.6 was obtained with ligand 75 in which the calixarene skeleton is substituted by two propyl groups (Table 9, entry 1). Replacement of the propyl substituents with CH2 C(O)OEt groups (i.e. using ligand 76) led to lower activities but increased the regioselectivities (l/b = 2.5) (Table 9, entry 2). Furthermore, the l/b ratio raised to 3.1 when 2 equiv. of 76 were used instead of 1. In the hydroformylation of styrene, diphosphinite-diester 76 led to good activities and branched aldehyde selectivities lying in the normal range (TOF = 891 mol(octene) mol(Rh)−1 h−1 and l/b = 36:64; Table 9, entry 2). Replacing the CO2 Et auxiliary functions with the bulkier CO2 cholesteryl groups (ligand 79) increased the proportion of linear aldehyde (l/b = 41:59) (Table 9, entry 3). The observation that bulkier auxiliary side groups increase the 3phenylpropanal selectivity was also made when comparing ligands 74 and 75 (l/b = 10:90 and 34:66, respectively for 74 and 75) [83]. Poor selectivity (l/b = ca. 1), but good activity, was observed in the hydroformylation of 1-octene (octene:Rh = 15,700, 40 bar CO/H2 (1:1), 100 ◦ C), with the system obtained by mixing phosphonite 85 (Fig. 27) and [Rh(acac)(cod)] (1:1) [46]. The linear aldehyde selectivity could be improved by adding an excess of phosphonite. For example, using 10 equivalents of 85 raised the l/b ratio up to 1.7, but this was detrimental to the reaction rate. The catalytic system obtained by combining diphosphite 125 (Fig. 28) with a stoichiometric amount of [Rh(acac)(cod)] produced an active catalyst for the conversion of 1-octene into aldehydes, but the linear aldehyde selectivity was here again disappointing (l/b = 1.1) (Table 10, entry 1) [46]. As expected, an increase of the ligand:rhodium ratio led to a decrease of the reaction rate while favouring the formation of the linear product (l/b = 1.4) (Table 10,

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Fig. 28. Diphosphites 125, 126 and 129.

Table 11 Rhodium-catalysed hydroformylation of 1-octene and styrene using phosphites 122–124 [44]. Entry

1 2 3 4 5 6

Ligand

122 122 123 123 124 124

L/Rh

1:1 2:1 1:1 2:1 1:1 2:1

1-Octene

Entry

Styrene

TOF

l/b

1204 156 629 247 1045 65

2.3 5.0 3.2 9.6 2.7 10.6

TOF 2033 1975 140 28 789 587

Table 12 Rhodium-catalysed hydroformylation of 1-octene using phosphites 131, 132, 134, 135, 137 and 138 [65,66].

l/b 33:67 37:63 34:66 12:88 27:73 25:75

Conditions: catalyst activation, olefin:P:[Rh(acac)(CO)2 ] = 2500:1:1, toluene, 20 bar CO/H2 (1:1), 80 ◦ C. The TOFs were determined at ca. 30% conversion and were expressed in mol(olefin) mol(Rh)−1 h−1 .

entry 2). These observations are in contrast with those made for the dihydroxy-diphosphite 126 (Fig. 28), for which higher reaction rates and lower l/b ratios were obtained when applying L:Rh ratios higher than 1 (Table 10, entries 3 and 4). A 3:1 mixture of N,N-diethylacetamide-diphosphite 129 (Fig. 28) and [Rh(acac)(CO)2 ] was tested for the hydroformylation of propylene. The runs were performed at 4 bar under a 1:1:1 CO/H2 /propylene atmosphere in the presence of large amounts of triphenylphosphine (PPh3 :129:Rh = 300:3:1). The reactions gave high proportions of n-butanal (l/b = 18.5). The initial reaction rate increased by a factor 17 when the amount of PPh3 was reduced to 150 equiv. per rhodium; concomitantly, the l/b ratio dropped to 13.3. Even higher regioselectivities were observed in the case of 1-butene (l/b = 24.5 under the following conditions: PPh3 :129:Rh = 150:3:1, 2.7 bar CO/H2 at 100 ◦ C in toluene) [84]. Repeating the 1-butene hydroformylation in the absence of triphenylphosphine led, after optimisation, to a l/b ratio of 51 (129:Rh = 2:1, 1.6 bar CO/H2 , 70 ◦ C in tetraglyme) [85]. In the case of 1-octene, the linear aldehyde selectivity was again remarkably high (l/b = 31), this performance being reached under the following conditions: 60 ◦ C, 10 bar CO/H2 (1:2), 129:Rh = 2:1, tetraglyme. The same catalytic system was also tested in the hydroformylation of butene raffinate feed stream. Thus, at 75 ◦ C in tetraglyme and under a total pressure of 6.5 bar CO/H2 /butene, n-valeraldehyde (i.e. pentanal) was obtained as the major aldehyde (l/b = 6.3) [63]. Diphosphites 122–124 were assessed in combination with [Rh(acac)(CO)2 ] (L/Rh = 1:1) in the hydroformylation of 1-octene. TOFs up to 1204 mol(octene) mol(Rh)−1 h−1 were obtained with the propyl-substituted ligand 122 (Table 11, entry 1; Fig. 29) [44]. Doubling the ligand stoichiometry decreased the activity while increasing the linear aldehyde selectivity to an optimal value (l/b = 5.0) (Table 11, entry 2). In fact, applying a ligand/Rh ratio of 2:1 also led to optimal selectivities with the other two ligands, 123 (l/b = 9.6) and 124 (l/b = 10.6) (Table 11, entries 4 and 6; Fig. 29). As far as styrene hydroformylation is concerned, the highest activity was obtained with ligand 122 (TOF = 1975 mol(styrene) mol(Rh)−1 h−1 ), which resulted in a l/b

1 2 3 4 5 6 7

Ligand n

131 (R = Pr) 131 (R = n Pr) 132 (R = Bn) 134 (R = CH2 naphthyl) 135 (R = fluorenyl) 137 (R = Bn) 138 (R = n Pr)

L/Rh

Time (h)

Conversion (%)

l/b

1/1 10/1 10/1 10/1 10/1 10/1 10/1

1 1 1 4 4 1 1

13.2 (TOF = 660) 34.5 (TOF = 1440) 23.9 (TOF = 1190) 48.5 (TOF = 720) 28.8 (TOF = 360) 24.2 (TOF = 1200) 19.7 (TOF = 980)

3.9 58.0 80.1a 101.6 3.3 71.0a 91.6

Conditions: catalyst activation, 1-octene:[Rh(acac)(CO)2 ] = 5000:1, toluene, 20 bar CO/H2 (1:1), 80 ◦ C. The TOFs were determined at ca. 30% conversion and were expressed in mol(octene) mol(Rh)−1 h−1 . a The l/b ratios were determined at quasi-full conversion because of the very low amount of branched aldehyde formed after 1 h.

ratio of 37:63 (Table 11, entry 2). The relatively high linear aldehyde selectivities observed in the hydroformylation of both 1-octene and styrene are likely to arise from steric effects induced by the chelator, which because of its natural bite angle greater than 90◦ (vide supra) favours the formation of ee intermediates over that of ae ones. The net result is that in the ee intermediates the P substituents are bent towards the metal so as to form a tight pocket around it. The resulting metal confinement influences the hydride migration, that favours then (for steric reasons, as shown by theoretical calculations) formation of a “Rh-n-alkyl” rather than a “Rh-i-alkyl” (see Scheme 37). Inspired by these observations, Sémeril et al. decided to increase the ligand bulk of such diphosphites by replacing the two OPh substituents of each phosphorus atom with the binol derived (R or S)- (1,1 -binaphthalene-2,2 -diyl) group [65,66]. The diphosphites thus obtained, 131, 132 and 134–138 (Fig. 30), were tested in the hydroformylation of 1-octene and their performance compared with that of 122. The catalytic systems were generated by mixing the ligand with [Rh(acac)(CO)2 ] in toluene, followed by activation with CO/H2 overnight at 80 ◦ C. Again, excess ligand was necessary to prevent the formation of “naked” rhodium, which behaves as an unselective catalyst leading in fine to lower regioselectivities (Table 12, entries 1 and 2). Under optimised conditions, all the binol-derived ligands resulted in increased linear aldehyde selectivity in comparison with the reference ligand. Thus for example, on going from diphosphite 122 to diphosphite 131, the l/b ratio increased from l/b = 5.0 to l/b = 58.0 (Table 12, entry 2). Furthermore, modifying the steric encumbrance of the two alkyl auxiliary groups grafted at the lower rim also induced considerable variations in both reactivity and regioselectivity. As a general trend, within the binol derived ligands, the activities of the catalytic system decreased with the bulkiness of the two auxiliary substituents. For example, catalyst 131, bearing the two propyl substituents, led to a TOF of 1440 mol(octene) mol(Rh)−1 h−1 , while with that having two fluorenyl groups (135) the TOF dropped to 360 mol(octene) mol(Rh)−1 h−1 (Table 12, entries 2 and 5). More

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Fig. 29. Diphosphites 122–124.

Fig. 30. Diphosphites 131, 132 and 134–138.

importantly, the l/b ratio increased with the size of the auxiliary substituents, except for the fluorenyl group, which led to very poor linear aldehyde selectivity (l/b = 3.3) (vide infra). Thus, on going from the propylated catalyst 131 to that with the methynaphthalenyl’s (134), the l/b ratio raised from l/b = 58.0 to l/b = 101.6 (Table 12, entries 2–4). As expected, no significant differences in terms of activity and selectivity were observed between the two diastereoisomeric diphosphites (S,S)-132 and (R,R)-137 (Table 12, entries 3 and 6). It should be mentioned that with the above diphosphites, internal octenes also formed (isomerisation). Their capacity to isomerise olefins was exploited in the hydroformylation of trans-2-octene. Thus, at 120 ◦ C under 20 bar CO/H2 (1:1), high proportions of linear aldehyde could be produced with the benzyl-substituted ligands 132 (l/b = 21.7) and 134 (l/b = 25.3) (b representing all branched aldehydes formed) (Table 13, entries 1 and 2). Remarkably, in the hydroformylation of styrene, the above binol derived-diphosphites led mainly to the linear aldehyde. For example, with the methylnaphthalenyl substituted ligand 134, product analysis revealed a high regioselectivity towards 3-phenylpropanal 76.8%, 2-phenylpropanal representing only 23.2% of the product formed (Table 13, entry 3). The proportion of linear aldehyde could even be brought to 88.7% by employing the tert-butyl-free ligand version 138 (Table 13, entry 4). These observations suggest that with these ligands, the shape of the pocket nesting the metal centre no longer allows the formation of an 3 -complex

Table 13 Rhodium-catalysed hydroformylation of olefins using phosphites 132, 134 and 138 [66]. Entry

Olefin

Ligand

TOF

l/b

1 2 3 4 5

trans-2-Octenea trans-2-Octenea Styreneb Styreneb Allyl benzyl etherc

132 (R = Bn) 134 (R = CH2 naphthyl) 134 (R = CH2 naphthyl) 138 (R = n Pr) 138 (R = n Pr)

120 70 320 700 Full conversion

21.7 25.3 76.8:23.2 88.7:11.3 19.8

Conditions: catalyst activation, L:[Rh(acac)(CO)2 ] = 10:1, toluene, 20 bar CO/H2 (1:1). The TOFs were determined at ca. 30% conversion and were expressed in mol(olefin) mol(Rh)−1 h−1 . a trans-2-Octene:[Rh(acac)(CO)2 ] = 800:1, 120 ◦ C. b Styrene:[Rh(acac)(CO)2 ] = 5000:1, 80 ◦ C. c Allyl benzyl ether:[Rh(acac)(CO)2 ] = 400:1, 120 ◦ C, 24 h.

(i-isomer, Scheme 37) with styrene, but favours a conventional alkyl complex (n-isomer, Scheme 37) (Fig. 31). Carrying out the hydroformylation runs with preformed complexes instead of mixing the free ligand with [Rh(acac)(CO)2 ] (in the absence of any additional ligand) led to similar high regioselectivities (Table 14) [87]. In the hydroformylation of allyl benzyl ether the debutylated diphosphite 138 led to an excellent chemoselectivity towards aldehydes (only traces of isomerisation and hydrogenation products)

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Scheme 40. Hydroformylation of norbornene.

Fig. 31. Formation of an 3 -complex vs. -alkyl complex in hydroformylation of styrene.

Table 14 Rhodium-catalysed hydroformylation of styrene using complexes 193 and 194 [87]. Entry

Rhodium complex

Conversion (%)

l/b

1 2

[Rh(acac)(132)] (193) (R = Bn) [Rh(acac)(135)] (194) (R = fluorenyl)

23.1 8.1

92.5:7.5 65.1:34.9

Conditions: Styrene:[Rh(acac)(L)] = 2500:1, toluene, 21 bar CO/H2 (1:2), 50 ◦ C, 2.5 h.

with a high regioselectivity towards the linear one (l/b = 19.8) (Table 13, entry 5) [66]. The optically pure diphosphites 131, 132, 135 and 138 were further used in the asymmetric hydroformylation of norbornene (Scheme 40) [66]. The tests were carried out by mixing the ligands with [Rh(acac)(CO)2 ] in toluene at 80 ◦ C and 20 bar CO/H2 (1:1); all the ligands displayed a very high selectivity for the exo aldehyde, but the observed ee’s were low. The best ee, for a reaction carried

out at 80 ◦ C, was obtained with the fluorenyl-ligand 135 (ee = 52%). Lowering the temperature to 55 ◦ C improved the ee up to 61% with this ligand. Several crystal structures have been reported that illustrate the high degree of confinement of the metal in complexes derived from these diphosphites [86,87]. Thus for example, in the propylated rhodium complex [Rh(acac)(131)] (195), the rhodium atom sits deeply inside the pocket defined by the two bino groups and the two propyl substituents fixed at the lower rim (Fig. 32). Clearly, the alkyl group that will form must adapt its shape to that of the somewhat elongated pocket, and unsurprisingly, a linear alkyl group fits better inside the pocket than a branched one. In order to get some insight into the intermediates involved in catalysis, high-pressure NMR and IR studies were carried out with the rhodium diphosphite complexes [Rh(acac)(132)] (193, R = Bn) and [Rh(acac)(135)] (194, R = fluorenyl) [87]. The spectroscopic data revealed that treatment of 192 with CO (7 bar) resulted in [Rh(acac)(CO)(1 -P-132)] (196), in which the diphosphite behaves as a P-monodentate ligand, one of the phosphorus atom being pending. After addition of H2 to the CO-saturated solution, the chelate complex [RhH(CO)2 (132)] (197) was formed as the sole product (Fig. 33 left and Scheme 41a). The behaviour of 194 towards CO was different, treatment of this complex with CO leading to full ligand dissociation and formation of [Rh(acac)(CO)2 ]. However,

Fig. 32. Molecular structure of rhodium complex 195 showing the high degree of confinement of the metal ion (purple) [86]. Hydrogen atoms have been omitted in the left view, but are shown in the CPK representation on the right. The ligand bite angle in this complex is 101.5◦ .

Fig. 33. High-pressure NMR studies with complexes 193 (left) and 194 (right) [87].

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Scheme 41. Activation pathways of complexes 193 (a) and 194 (b). Fig. 35. Diphosphites 110–113.

Table 16 Rhodium-catalysed hydroformylation of 1-octene using diphosphites 110–113 [57]. Entry

1 2 3 4a

Fig. 34. Sulfonated-calix[4]arenes 199–202. Table 15 Rhodium-catalysed hydroformylation of olefins using complex [Rh(acac)(132)] in water [86]. Entry

Olefin

Sulfonated-calixarene

TOF

l/b

1 2 3 4 5 6 7 8

1-Octene 1-Octene 1-Octene 1-Octene 1-Hexene 1-Hexene Styrene Styrene

– 199 (R = H) 200 (R = n Bu) 201 (R = n Hex) – 200 (R = n Bu) – 202 (R = n Oct)

360 550 630 600 490 750 60 230

24.1 14.2 61.8 21.2 10.1 15.7 78.7:21.3 66.7:33.3

Conditions: olefin:132:193 = 2500:10:1, water, 20 bar CO/H2 (1:2), 50 ◦ C. The TOFs were determined at ca. 30% conversion and were expressed in mol(olefin) mol(Rh)−1 h−1 .

upon addition of H2 , slow re-coordination of the diphosphite occurred, resulting then selectively in [RhH(CO)2 (135)] (198), an analogue of 197 (Fig. 33 right and Scheme 41b). In both compounds, 197 and 198, the diphosphite adopts an equatorial–equatorial coordination mode, as unambiguously deduced from the 1 H NMR spectra (J(PH) = 5.8 Hz (197) and < 2 Hz (198)). This is precisely the configuration that induces formation of linear aldehydes [76]. Diphosphites 131 and 132 as well as their rhodium complexes are themselves not water-soluble. Nevertheless, they could be used for hydroformylation reactions in water, provided a surfactant was added. With the sulfonated surfactants 199–202 (Fig. 34) for example, the systems Rh/131 and Rh/132 not only still displayed medium-to-high linear aldehyde selectivities, but in all cases the activity was found superior than that observed in a surfactant-free medium [86]. This latter finding probably relies on the presence of micelles, which create microenvironments characterised by a high catalyst:olefin ratio. The most remarkable results were obtained with surfactant 200, which resulted in l/b ratios of 61.8 and 15.7, for 1-octene and 1-hexene, respectively, the corresponding TOFs being 630 and 750 mol(olefin) mol(Rh)−1 h−1 , respectively (Table 15, entries 3 and 6). Monnereau et al. found that owing to their high solubility, the complexes [Rh(acac)(L)] (L = 131 or 132) were also suitable for solvent-free hydroformylation [88]. As expected, in the absence of solvent, the reaction rates for the conversion of 1-octene, 1-hexene, allyl benzyl ether, styrene and vinylnaphthalene were considerably higher than those performed in toluene. For example, hydroformylation of 1-octene carried out in the absence of solvent, at 80 ◦ C under 20 bar of CO/H2 (1:2), gave TOFs of 17,290 (l/b = 46.6) and

Ligand

110 111 112 113

Conversion (%)

90 (4 h) 53 (4 h) 65 (4 h) 63 (8 h)

Product distribution Octenes (%)

Octane (%)

Aldehydes (%)

82 16 13 12

6 13 12 27

12 (l/b = 2.3) 71 (l/b = 12.2) 75 (l/b = 26.7) 61 (l/b = 199.0)

Conditions: olefin:L:[Rh(acac)(CO)2 ] = 4000:5:1, texanol, 5 bar CO/H2 (1:1), 80 ◦ C. a 20 bar CO/H2 (1:1), 100 ◦ C.

15,640 (l/b = 58.6) mol(octene) mol(Rh)−1 h−1 with the complexes [Rh(acac)(131)] and [Rh(acac)(132)], respectively. The same systems were about 8 times slower when operating in toluene as solvent. Olefin hydroaminovinylation is a further reaction that can be carried out under solvent-free conditions with these ligands [89]. Hydroaminovinylation is a domino reaction which combines olefin hydroformylation with a subsequent reaction between the aldehyde formed and a primary or a secondary amine, leading then either to an enamine (linear and branched) or an imine (linear and branched). Enamine formation is often followed by hydrogenation with amine formation (Scheme 42). The linear selectivity depends upon the regioselectivity of the hydroformylation step (i.e. the first step). As expected, high regioselectivities were observed when calixarenyl-diphosphites 131 or 132 were employed. For example, in the hydroaminovinylation of 1-octene (olefin:Rh = 5000:1) with dibutylamine at 130 ◦ C under 20 bar CO/H2 (1:1), nearly full conversion was obtained after 4 h using [Rh(acac)(132)] (193). The proportion of enamines was 57.9% (l/b = 28.9), that of aldehydes 3% (l/b = 4.6), and that of amines 17.8% (l/b = 29.5). Internal octenes (21.3%) also formed. In the case of benzylamine, an excellent chemoselectivity towards the formation of imine was obtained. For example, when using complex 193 at 130 ◦ C and 10 bar CO/H2 (1:1), the reaction of styrene with benzylamine gave exclusively the corresponding imines (l/b = 85.1:14.9), no follow-reaction being observed. Under the above solvent-free conditions, both catalysts converted the substrates about 15 times faster than when operating in toluene [66]. Under milder hydroformylation conditions (5 bar CO/H2 (1:1), 80 ◦ C), the combination of diphosphite 110 (Fig. 35) and [Rh(acac)(CO)2 ] led to low amounts of aldehyde, the main products being internal octenes (Table 16, entry 1) [57]. Substitution of the phenoxy substituents of 110 by 2,6-dimethylphenyloxy or 2,6diisopropylphenyloxy groups (leading respectively to ligands 111 and 112; Fig. 35) significantly enhanced the proportion of aldehydes, these being mainly linear (l/b ratios of 12.2 and 26.7 for 111 and 112, respectively; Table 16, entries 2 and 3). The highest l/b ratio (199) was obtained with the related ligand 113 (Fig. 35; Table 16, entry 4), although with this ligand the reaction rate was roughly 2 times lower than that observed for the diphosphites 111

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Scheme 42. Hydroaminovinylation of olefins with secondary (top) or primary (below) amines.

Fig. 36. Diphosphites 108 and 109.

Fig. 37. Tetraphosphine 44 and tetraphosphinite 80.

and 112. The catalytic behaviour of this ligand was rationalised by means of molecular modelling calculations, which showed that the 1-octyl-rhodium intermediate (n-isomer in Scheme 37) forms preferentially over the i-isomer as a consequence of the high steric crowding about the rhodium atom, which is sandwiched between the two 2,6-di-tert-butylphenoxy rings. The calix[6]arenyl-diphosphites 108 and 109 (Fig. 36) were tested in the hydroformylation of several linear ␣-olefins (C7 –C12 ) in combination with [Rh(acac)(CO)2 ] (L/Rh = 0.5) under 5 bar CO/H2 (1:1) at 50 ◦ C [90,91]. Low activities and poor linear aldehyde selectivities were measured whatever the olefin used (l/b near 2). For styrene, the l/b ratios were close to 30/70 with both diphosphites, which is not an unusual ratio. When the reactions were performed in the presence of more than 4 equiv. of ligand, the unexpected formation of [RhL2 ] complexes was observed. These complexes were inactive. Using tetraphosphine 44 and tetraphosphinite 80 (Fig. 37), Kollár et al. performed styrene hydroformylation experiments with platinum and rhodium [30]. For both metals and both ligands, an increase of the reactivity as well as of the l/b ratio was observed when increasing the temperature (Table 17, entries 1 and 2; 4 and 5). Tetraphosphinite 80 showed higher activities than tetraphosphine 44. Ligand 80 led to a branched aldehyde selectivity (l/b ca. 10/90) (Table 17, entry 4) that is similar to that of PPh3 [92]. Karakhanov et al. studied tetraphosphinite 73 (Fig. 38) in the rhodium catalysed hydroformylation of ␣-olefins (5 bar, 70 ◦ C),

Table 17 Platinum- and rhodium-catalysed hydroformylation of styrene using tetraphosphine 44 and tetraphosphinite 80 [30]. Entry a

1 2a 3a 4b 5b 6b a b

Catalytic system

T (◦ C)

Time (h)

Conversion (%)

l/b

[(PtCl2 )2 (80)] [(PtCl2 )2 (80)] [PtCl2 (PhCN)2 ] + 44 [Rh(nbd)Cl]2 + 80 [Rh(nbd)Cl]2 + 80 [Rh(nbd)Cl]2 + 44

50 105 105 55 130 135

135 17 28 22 4 6

57 54 85 78 100 99

53.3:46.7 60.2:39.8 53.4:46.3 7.7:92.3 47.9:52.1 43:57

Styrene:L:Pt:SnCl2 = 2000:0.5:1:1, toluene, 40 bar CO/H2 (1:1). Styrene:L:Rh = 4000:1:1, toluene, 40 bar CO/H2 (1:1).

and found that the resulting linear aldehyde selectivities were moderate. For example, in the case of 1-octene, the nonanal/2methyl-octanal ratio was 2.3 [83]. In the hydroformylation of 1-octene, the tetraphosphites 142 and 143 as well as the tetra-oxy-phosphorus diamides 146 and 147 (Fig. 39) led to lower activities than the corresponding reference compounds 203 and 192, respectively (Fig. 40; Table 18). While the latter gave l/b ratios of ca. 1.5, those obtained with 142, 143, 146 and 147 were significantly higher (2.3–2.6), this reflecting probably the steric bulk of the calixarene moieties [67]. The combination of hexaphosphinites 81 or 82 (Fig. 41) with [Rh(acac)(CO)2 ] (P:Rh = 3:1) was assessed in the hydroformylation of ␣-alkenes (C7 –C12 ) and aryl alkenes (styrene, substituted

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Scheme 43. Transition metal-catalysed alkoxycarbonylation of ␣-olefins.

Fig. 38. Tetraphosphinite 73.

Fig. 42. Tetraphosphine 44 and tetraphosphinite 80. Table 19 Palladium-catalysed alkoxycarbonylation of styrene using tetraphosphine 44 and tetraphosphinite 80 [30].

Fig. 39. Tetraphosphites 142 and 143 and tetra-oxy-phosphorus diamides 146 and 147.

Entry

Catalytic system

Alcohol

Conversion (%)

l/b

1 2 3 4

[(PdCl2 )2 (80)] [(PdCl2 )2 (80)] [PdCl2 (PhCN)2 ] + 44 [PdCl2 (PhCN)2 ] + 44

MeOH t BuOH MeOH t BuOH

– 3 39 42

– 25.0:75.0 37.7:62.3 51.2:48.6

Conditions: styrene:ROH:L:Pd = 2000:4000:1:2, toluene, 140 bar CO, 130 ◦ C, 48 h.

Fig. 40. Oxy-phosphorus diamide 192 and phosphite 203.

Table 18 Rhodium-catalysed hydroformylation of 1-octene using phosphites 142, 143 and 203 and oxy-phosphorus diamides 146, 147 and 192 [67]. Entry

1 2 3 4 5 6

Ligand

142 143 203 146 147 192

L:Rh

1:1 1:1 1:4 10:1 10:1 40:1

Solvent

THF CH2 Cl2 CH2 Cl2 THF CH2 Cl2 CH2 Cl2

T (◦ C)

120 100 100 120 100 100

P(CO/H2 ) (bar)

Yield (aldehydes) (%)

l/b

40 40 40 50 40 40

39.6 27.4 92.4 89.4 4.0 99.0

2.2 2.6 1.5 1.6 2.3 1.4

Conditions: 1-octene:[Rh(acac)(cod)] = 15,700:1, CO/H2 (1:1), 3 h.

Fig. 41. Hexaphosphinites 81 and 82.

styrenes, vinylnaphthalene, allyl benzene, allyl phenyl ether and arylprop-2-enes) under 25 bar CO/H2 (1:1) in toluene at 50 ◦ C [91]. The system based on the de-tert-butylated calix[6]arene 82 showed higher activities than that based on hexaphosphinite 81. Thus, for example, in the hydroformylation of styrene (that led to 2-phenylpropanal as the major product), TOFs of 36 and 48 mol(styrene) mol(Rh)−1 h−1 were obtained with ligands 81 and 82, respectively. The high solubility of phosphinite 81 in aromatic substrates enabled hydroformylation runs to be carried out under solvent-free conditions, this increasing significantly the efficiency of the catalyst. For example, in the hydroformylation of styrene, the TOF increased by a factor of 5 on going from a reaction carried out in toluene to one performed under solvent-free conditions. 4.2. Olefin hydroalkoxycarbonylation Alkoxycarbonylation is an easy way to generate esters from olefins, carbon monoxide and alcohols (Scheme 43). This reaction is attracting much attention from both research and commercial points of view. However, to date its success remained rather limited in comparison with olefin hydroformylation, this being related to the difficulty to induce high regioselectivities [93]. Note that high branched ester selectivities would be relevant to the synthesis of optically active compounds (this requiring the use of chiral ligands). The group of Kollár reported the alkoxycarbonylation of styrene with methanol and tert-butanol, catalysed by systems combining a palladium precursor and tetraphosphine 44 or tetraphosphinite 80 (Fig. 42) [30]. Under hard catalytic conditions (140 bar of CO, 48 h, 130 ◦ C), almost no conversion was observed with tetraphosphinite 80, probably because of ligand degradation (Table 19, entries 1 and 2). In contrast, under similar conditions, reasonable yields were observed with tetraphosphine 44, which produced mainly the branched ester with methanol (l/b = 37.7:62.3), and nearly equimolar amounts of the branched and linear ester with tert-butanol (Table 19, entries 3 and 4). In fact, these ligands gave rise neither

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Fig. 43. Diphosphine 18.

to the high branched ester selectivity which is generally observed with triaryl phosphines, nor to the linear ester selectivity classically obtained with bidentate phosphines. This observation suggests that several active species are operative, with one and with two coordinated phosphorus atoms. 4.3. Olefin hydrogenation Olefin hydrogenation is the addition of dihydrogen to an olefinic carbon–carbon double bond to generate a saturated compound. It is a very common reaction in organic synthesis, and is especially valuable in its asymmetric version for the production of fine chemicals [94,95]. The di-rhodium complex [(RhCl(cod))2 (18)] (Fig. 43) was assessed by the group of Matt in the hydrogenation of alkenes and alkynes [21]. Under mild conditions (5 bar H2 ) and in the presence of 1 mol % of rhodium, full conversions were observed after 1 h for 1-octene and allylbenzyl ether. In contrast, under similar conditions, hydrogenation of styrene and cyclooctene required 4 h, that of pentyne 24 h. Jugé and Harvey reported the use of the P-chirogenic aminophosphine-phosphinite 33 (AMP*P; Fig. 44) in the hydrogenation of prochiral olefins [26]. Ee’s up to 95% were obtained in the hydrogenation of methyl-2-acetamidocinnamate in benzene under 15 bar H2 . Comparisons were made with related ligands in which the calixarene moiety had been replaced with simple aryl substituents. These investigations unambiguously showed that the calixarene unit plays a key role in the better asymmetric induction obtained with 33. Molecular modelling revealed that two conformers for the active AMP*P rhodium species can be envisaged, one with the rhodium metal turned away from the cavity (outer conformation), the other with the metal located above the cavity entrance (inner conformation) (Fig. 44). Complexation of the substrate forces the complex to fully adopt an outer conformation. It is likely that for steric reasons this conformation facilitates enantiodiscrimination of the olefinic substrate. In the hydrogenation of dimethyl itaconate, calixarenes having two proximal CH2 PPh2 groups attached to their lower rim (diphosphines 59, 61 and 63; Fig. 45) were superior in terms of

Fig. 45. Diphosphines 52, 59, 61 and 63.

Table 20 Rhodium-catalysed hydrogenation of dimethyl itaconate using diphosphines 52, 59, 61 and 63. Entry

Complex

Time (h)

ee (%)

TOF (mol(olefin) mol(Rh)−1 h−1 )

1 [37] 2 [37] 3 [37] 4 [33]

[Rh(cod)(59)]BF4 [Rh(cod)(61)]BF4 [Rh(cod)(63)]BF4 [Rh(cod)(52)]BF4

0.10 0.17 0.75 20

48 25 0 0

2000 1176 267 10

Conditions: olefin:Rh = 200:1, P(H2 ) 20 bar, MeOH.

activity to those having the phosphorus atoms appended to distal O atoms (diphosphine 52; Fig. 45) (Table 20). Clearly, partial encapsulation of the metal combined with possible amide coordination in chelate complexes obtained from 52 restricts access of the incoming olefin and therefore leads to low reaction rates [33]. In contrast, chelate complexes derived from 59, 61 and 63 possess a much more open metal environment; one reason for that is that in such complexes the metal atom is pushed away from the cavity, thereby impeding coordination of the amide functions, and hence facilitating substrate approach [37]. More importantly, the inherently chiral calixarenes 59 and 61 resulted in enantioselectivities (ee’s of 48 (59) and 25% (61); Table 20, entries 1 and 2) significantly higher than those induced by ligands 52 and 63 (ee’s ∼0) (Table 20, entries 3 and 4). These results demonstrate that an inherently chiral calixarene skeleton is able to transfer chiral information to the catalytic centre. In the asymmetric hydrogenation of dimethyl itaconate and ␣-(acylamino)acrylate performed with the in situ generated catalytic systems [Rh(nbd)2 ]BF4 /102–105 (Fig. 46), ee’s up to 94% were reached [54]. Even higher enantioselectivities could be obtained with the binol-derived “1,3-calix-diphosphites” 130–133 and 139–141 (Fig. 47) [64]. For example, using diphosphite 131 in the hydrogenation of methyl-(Z)-2-(acetamido)acrylate and

Fig. 44. Outer/inner equilibrium in the [Rh(33)(cod)]BF4 complex.

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Fig. 46. Diphosphites 102–105.

Fig. 48. Diphosphines 41, 50, 52, 54, 59, 61and 63 and diphosphinite 77.

Scheme 44. Asymmetric hydrogenation of methyl-(Z)-2-(acetamido)acrylate (a) and methyl-(Z)-2-(acetamido)cinnamate (b).

methyl-(Z)-2-(acetamido)cinnamate led to ee’s of 98 and 96%, respectively (Scheme 44). 4.4. Tsuji–Trost reaction The Tsuji–Trost reaction, also referred to as allylic alkylation, is a palladium-catalysed substitution reaction involving a substrate containing a leaving group in an allylic position. When starting from substrates of the type RCH = CH-CH2 X (X = leaving group), two regioisomers are formed, a linear (l) and a branched (b) one, the relative proportion of each isomer being strongly dependent on the L ligands of the catalyst (Scheme 45, left). Furthermore, if the catalyst is optically active, chiral induction may occur in the reaction

leading to the branched product. When starting from substrates of the type RCH = CH-CHRX, the product that forms is branched, and therefore contains an asymmetric carbon atom (Scheme 45, right) [96]. The group of Matt studied the allylic alkylation of 1,3diphenylprop-2-enyl acetate with dimethyl malonate catalysed by [Pd(␩3 -MeC3 H4 )(L)]BF4 complexes (L = 41, 50, 52, 54, 59, 61, 63 and 77; Fig. 48) (Scheme 46) [28,37]. For all the catalysts tested, full conversion was observed within 3–5 h using 1 mol % of palladium (Table 21). Only low asymmetric induction occurred (ee’s = 0–16%; Table 21, entries 3–5 and 8) with the chelating ligands 52, 54, 63 and 77, probably because in the corresponding active species the chiral centres are remote from the -allyl moieties. Significantly higher inductions were obtained with the inherently chiral calix[4]arenes 61 (ee = 45%) and 59 (ee = 67%) (Table 21, entries 6 and 7). The higher efficiency of the corresponding palladium complexes is likely to arise from a better chirality transfer from the calix backbone to the metal, this becoming particularly efficient as a consequence of the strong size difference between the R3 and R4 substituents. Note that, since in the corresponding Pd(allyl)-complexes the metal is

Fig. 47. Diphosphites 130–133 and 139–141.

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Scheme 45. Palladium-catalysed allylic alkylation with formation of non symmetric (left) and symmetric (right) ␩3 -allyl palladium intermediates. Table 21 Palladium-catalysed allylic alkylation of 1,3-diphenylprop-2-enyl acetate using diphosphinite 77 and diphosphines 41, 50, 52, 54, 59, 61 and 63. Entry

Complex

Time (h)

TOF (mol(malonate) mol(Pd)−1 h−1 )

ee (%)

1a [28] 2a [28] 3a [28] 4a [28] 5a [28] 6b [37] 7b [37] 8b [37]

[Pd(3 -MeC3 H4 )(41)]BF4 [Pd(3 -MeC3 H4 )(50)]BF4 [Pd(3 -MeC3 H4 )(77)]BF4 [Pd(3 -MeC3 H4 )(52)]BF4 [Pd(3 -MeC3 H4 )(54)]BF4 [Pd(3 -MeC3 H4 )(59)]BF4 [Pd(3 -MeC3 H4 )(61)]BF4 [Pd(3 -MeC3 H4 )(63)]BF4

5 4 4 3 5 3.3 3.3 3.8

20 25 25 33 20 30 30 26

– – 8 16 6 67 45 0

a b

Allyl acetate:malonate:NaH:Pd = 100:200:200:1, THF, 67 ◦ C. Allyl acetate:malonate:BSA:KOAc:Pd = 100:200:200:20:1, CH2 Cl2 , 0 ◦ C.

Fig. 49. Chirality transfer to the metal.

Scheme 46. Allylic alkylation of 1,3-diphenylprop-2-enyl acetate.

pushed away from the cavity (as revealed by several X-ray structures), the transfer of the chiral information is likely to involve the PPh2 groups which act as a relay (Fig. 49). Jugé et al. tested the P-chirogenic calixarenyl phosphines 35–39 (Fig. 50) in the palladium catalysed allylic substitution of (E)1,3-diphenylprop-2-en-1-yl acetate by dimethyl malonate and benzylamine [27]. With diphosphine 37, the allylic products were formed at yields of 82% (malonate) and 79% (benzylamine) when applying optimised conditions. Using two equiv. of monophosphine 35 led to lower ee’s. Theoretical calculations suggest the formation of an (1 -allyl) palladium intermediate during catalysis. Sémeril et al. studied the allylic alkylation of (E)-3-phenyl allyl acetate with dimethylmalonate catalysed by the system obtained

Fig. 50. Phosphines 35–39.

by mixing [Pd(3 -C3 H5 )Cl]2 with diphosphites 131 or 132 (Fig. 51) [65]. The remarkably high regioselectivities observed (l/b’s better than 98:2; cf. 82:8 with 1,2-bis(diphenylphosphino)ethane) can again be attributed to the high degree of confinement of the metal centre after complexation (this arising notably from the large ligand bite angles). Whatever the transition state during alkylation, early or late, metal embrace should favour attack on the non-substituted (C-1) carbon atom. Indeed, in the case of an early transition state, the embracing effect of the ligand makes nucleophilic attack on C-1 logically easier. In a late transition state, nucleophilic attack on C-3 would result in a sp3 carbon atom, and therefore cause the substituent to bend towards the ligand, this resulting in an increase of steric hindrance which hampers the formation of the branched product. The formation of tight pockets with these ligands was confirmed by several X-ray studies [97]. That the catalytic outcome depends on the degree of metal confinement was corroborated by the results obtained with [Pd(␩3 -MeC3 H4 )(127)PF6 ] (Fig. 51), in which the metal lies in a much less crowded environment. In this case, only 80% of linear alkylation product were obtained [62]. Finally, it should be mentioned that the use of the chiral ligand 130 (Fig. 51) for the same reaction resulted in very low asymmetric induction [62]. van Leeuwen et al. employed the catalyst obtained by mixing [Pd(␩3 -C3 H5 )Cl]2 and the P,N-ligand 101 (Fig. 52) in the allylic alkylation of cinnamyl acetate with diethyl 2-methylmalonate [52]. The reaction resulted in a l/b ratio of 56:44. The authors did not provide any explanation for the observed rather low linear product selectivity. Poor regioselectivity was also observed in the allylic alkylation of crotyl acetate (Scheme 47) with the preformed dipalladium complex [{Pd(3 -MeC3 H4 )Cl}2 (128)] (Fig. 53; Table 22, entry 1;

Scheme 47. Alkylation of crotyl acetate with dimethylmalonate.

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Fig. 51. Diphosphites 127 and 130–132.

Table 22 Palladium-catalysed allylic alkylation of crotyl acetate using diphosphite 128 [61]. Entry 1 2

Catalytic system 3

[(Pd( -MeC3 H4 )Cl)2 (128)] [Pd(3 -C3 H5 )Cl]2 + 128

Reagents

Yield (%)

l/b

BSA/KOAc/MeO2 CCH2 CO2 Me [MeO2 CCHCO2 Me]Na

30 60

49:51 62:38

Conditions: crotyl acetate:Pd = 2000:1, 24 h, 25 ◦ C.

Fig. 52. Diphosphite 101.

Fig. 53. Diphosphite 128. Scheme 48. Allylic alkylation using [Pd(R-allyl)176]+ .

l:b = 49:51) [61]. The selectivity in linear products significantly increased (l/b = 62:38) when using the in situ generated catalyst obtained upon reacting one equiv. of [Pd(␩3 -MeC3 H4 )Cl]2 with 128. Furthermore, in that case, the conversion doubled (Table 22, entry 2). It is likely that under these conditions two complexes formed, namely [{Pd(␩3 -MeC3 H4 )Cl}2 (128)] and the chelate complex [{Pd(␩3 -MeC3 H4 )}(128)]Cl, the latter reacting faster than the former. The unique report on the use of resorcinarene-derived complexes in allylic alkylation was by Rebek and Gibson in 2002 [75]. The phosphite-oxazoline chelator 176 (Fig. 54) is based on an expanded resorcinarene cavitand unit, which is able to bind

size- and shape-complementary molecules such as adamantanes. In the presence of palladium and allyl acetate, this ligand produces an allyl palladium ␩3 -complex, in which the allyl fragment is oriented towards the cavity axis and enters the cavity. Two ways of coordinating the allylic fragment (H2 C1 –C2 H = C3 HR )− are possible, depending on whether the carbon trans to phosphorus is the terminal carbon atom C1 or carbon atom C3 of the allyl ligand (complexes 204 and 205, respectively; Scheme 48). In fact, the left pathway dominates as this minimises the steric repulsions. As expected both for steric and electronic reasons, nucleophilic attack, for example by a malonate group, takes place selectively on carbon

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Fig. 54. Phosphite-oxazoline 176 and Pfaltz’s ligand 206.

Table 23 Nickel-catalysed Kumada–Tamao–Corriu of aryl chlorides at room temperature using phosphine 5 and 8 [15]. Entry

ArCl

Time (h)

Phosphine 5

8

1

Conversion (%)

6

72.2

88.2

2

Conversion (%)

6

66.7

92.3

3

Conversion (%)

24

62.5

80.9

4

Conversion (%)

24

45.2

64.4

Fig. 55. Phosphines 4–9 and diphosphine 12.

C1 , thereby producing the linear product. The authors studied alkylation reactions with various substrates and found that unlike for the reference ligand 206 (Pfaltz’s ligand; Fig. 54), the reaction rate is substrate dependent. Combining mass spectrometric measurements and competitive alkylation experiments (i.e. by alkylating mixtures of different allylic substrates), the authors found that the alkylation reactions with 176 depended on both the ease or forming the active allylic complex and the reactivity of the latter towards an incoming nucleophile, both steps not necessarily being correlated. Thus a given allylic substrate may form an allylic complex faster than a second one, but the resulting complex may be totally unreactive in contrast to the other allylic complex. Overall, Reebek’s study proved that the whole catalytic process with 176 relies on the molecular recognition and steric properties of the cavity. It underscores the potential of cavitand-containing catalysts in organic synthesis. 4.5. Cross-coupling reactions Cross-coupling is a catch-all term for organic reactions in which two organic fragments are coupled together by means of a transition metal catalyst [98]. Only three types of cross-coupling reactions were studied with catalysts containing a phosphinated calixarene or resorcinarene, namely Suzuki–Miyaura (boronic acid/ArX) [99,100], Kumada–Tamao–Corriu (Grignard/ArX) (KTC) [101–103] and Mizoroki–Heck (carbon–carbon double bond/ArX) [104,105] cross-couplings. The calixarenyl-phosphines 4–7 and 9 (Fig. 55) were studied in the palladium-catalysed Suzuki–Miyaura cross-coupling of phenylboronic acid with aryl halides [89,106]. Under optimised conditions (2 equiv. of ligand per [Pd(OAc)2 ]; NaH, 100 ◦ C, dioxane) high TOFs were obtained whatever the aryl bromide tested. The highest activity, 321,000 mol(ArBr) mol(Pd)−1 h−1 , was obtained in the arylation of 4-bromotoluene with ligand 6. This activity is considerably higher than that of other triarylphosphines. Thus,

Conditions: ArCl:PhMgBr:P:Pd = 100:200:6:3, dioxane, 25 ◦ C.

for example, the activity was only 65,000 mol(ArBr) mol(Pd)−1 h−1 with PPh3 , and 103,000 mol(ArBr) mol(Pd)−1 h−1 with the bulkier phosphine P(o-tolyl)3 . Most importantly, these catalytic systems were stable over several days, some turnover numbers (TON) reaching 14.5 × 106 mol(ArBr) mol(Pd)−1 . Finally, it should be mentioned that the activities compare with those obtained using a Buchwald-type triarylphosphine, namely 2-diphenylphosphanyl2 -methylbiphenyl. Buchwald phosphines are among the best performing phosphines for this kind of coupling. The same phosphines were further tested in nickel-catalysed Kumada–Tamao–Corriu cross-couplings [15]. Experiments carried out at 100 ◦ C in dioxane with 2 equiv. of ligand per Ni (precursor used: [Ni(cod)2 ]), led again to remarkable TOF’s. For example, with 6, the reaction between 1-bromonaphthalene and PhMgBr, resulted in a TOF of 439,000 mol(ArBr) mol(Ni)−1 h−1 . Note that the activities obtained with these monophosphines were considerably higher than those observed for the related diphosphine 12 (Fig. 55; vide infra), which, unlike 6, readily forms chelate complexes and therefore produces another reaction mechanism. The high reactivity of the monophosphines also allowed cross-coupling reactions with aryl chlorides at room temperature, but, as expected, the conversions were lower than in the case of aryl bromides (see performance of 5, Table 23). Nevertheless, the conversions could be improved by using a more basic and more bulky phosphine, namely 8 (Fig. 55; Table 23). A likely explanation for the high activity of the above monophosphine ligands in Suzuki–Miyaura and KTC cross coupling is the formation of transient [M(␲-ArX)(calix-phosphine)] intermediates

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Fig. 57. Diphosphines 10 and 11. Fig. 56. Proposed [M(0) (ArX)L] intermediate formed before the oxidative addition step. The dotted lines represent ␲–␲ interactions.

Scheme 50. Fanning motion of the metal plane of the catalytic intermediate [Ni(Ph)(Ar)(12)].

Scheme 49. Catalytic cycle of palladium-catalysed Suzuki–Miyaura or nickelcatalysed Kumada–Tamao–Corriu reactions using bulky monophosphines.

(M = Pd or Ni) having the coordinated ArX entrapped in a supramolecular fashion in the cavity (Fig. 56). Note that such entrapments, which involve ␲–␲ staking interactions, have been authenticated in [RuCl2 (p-cymene)(calix-phosphine)] complexes, for which several solid state structures have been determined [107]. The resulting orientation of the P M bond in these intermediates, turned towards the calixarene axis (and not outwards), confers to the corresponding ligand its maximal steric encumbrance, and consequently favours the formation of mono-ligand Pd(0) or Ni(0) intermediates. Efficient monoligand pathways have been proposed for both catalytic processes (Scheme 49) [108,109]. complex [NiCp(12)]BF4 was assessed in The Kumada–Tamao–Corriu cross-coupling by Matt et al. [110]. When using only 0.02 mol% of this complex, conversions of 92.9 and 95.6% were obtained after 1 h in the reaction of PhMgBr with 1-bromonaphthalene and 4-bromotoluene, respectively. Even at a loading of 0.002 mol %, did the catalyst remain very active. For example, with 1-bromonaphthalene, the conversion was 42.5% after 1 h (TOF 21,250 mol(ArBr) mol(Ni)−1 h−1 ). Note that the remarkable reaction rates observed with this complex were found even better than those obtained with one of the most efficient catalyst used for this reaction, namely [NiCl2 (dppp)] (dppp = 1,3bis(diphenylphosphino)propane). The catalyst was also efficient in the coupling of aryl chlorides. In marked contrast with these results, catalytic runs performed with diphosphines 10 or 11 (Fig. 57) in palladium-catalysed Mizoroki–Heck, Suzuki–Miyaura and KTC cross-coupling reactions revealed that these ligands were only slightly superior to PPh3 [111].

To understand the origin of the high activity of 12 (Fig. 55) in the above KTC cross-coupling reaction, variable temperature 31 P and 1 H NMR studies were carried out on [NiCp(12)]BF . The 4 NMR data are consistent with a fanning motion of the P–M–P unit, which displaces the nickel atom from one side to the other of the calixarene axis. During this motion, the P–Ni–P bite angle periodically increases and decreases, reaching its maximal value when the metal crosses the calixarene axis. When the PMP angle is maximal the phosphorus substituents are strongly bent towards the positions trans to the P atoms. Exactly the same phenomenon is likely to occur in the [NiArAr (12)] intermediates during cross coupling catalysis, the steric pressure caused by the substituent bending favouring then the reductive elimination step (Scheme 50). The cyclometalated phosphite complexes 207–211 (Scheme 51) were tested in Mizoroki–Heck reactions by Krishnamurthy et al. [59]. The reactions were performed at 110 ◦ C in dioxane with NBu4 Br as cocatalyst and K2 CO3 as base. The reported activities were moderate, the best activity being obtained with complex 207 (TOF of 150 mol(PhX) mol(Pd)−1 h−1 in the reaction of PhI and styrene). The same phosphites were further assessed in Suzuki–Miyaura cross coupling of aryl halides. The runs were performed at 100 ◦ C in toluene using 0.03 mol % of palladium and K3 PO4 as base. The activities were not exceptional. Thus for PhI, PhBr, and PhCl, TOF’s of respectively 148, 142 and 24 mol(PhX) mol(Pd)−1 h−1 were obtained (Table 24, entries 1, 2 and 8). The activity depends on the presence of substituents at the upper rim of the calixarene. Thus, removing the electron donating p-tert-butyl groups of the calixarene (leading to complex 208), reduced the TOF from 142 to 90 mol(ArBr) mol(Pd)−1 (Table 24, entries 2 and 3). Minor changes in activity were also observed when changing the R2 -R7 substituents of the POAr groups (complexes 209–211; Table 24, entries 4–6). Replacing the palladium atom of 207 with platinum (complex 212, Fig. 58) did not significantly increase the TOF (Table 24, entry 2 and 7). The five bis-iminophosphoranes 159–163 (Fig. 59), all obtained from diphosphine 12, were used in the palladium-catalysed Suzuki–Miyaura cross-coupling of aryl bromides and aryl chlorides [71]. The activities were high. For example, using 163 and operating at 100 ◦ C in dioxane, the reaction between

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Scheme 51. Synthesis of the palladium-cyclometalated complexes 207–211.

Table 24 Palladium- or platinum-catalysed Suzuki–Miyaura of aryl halides using the complexes 207–212 [59]. Complex

TOF (mol(ArX) mol(Pd)−1 h−1 )

1

207

148

2 3 4 5 6 7

207 208 209 210 211 212

142 90 110 118 105 147

8

207

24

Entry

ArX

Fig. 60. PPh3 -Derived iminophosphorane 213.

Conditions: M:ArBr:PhB(OH)2 :K3 PO4 = 1:3333:4500:6666, toluene, 100 ◦ C.

Fig. 61. Proposed endo-oriented [M(0) (ArX)L] intermediate formed before the oxidative addition step (the dotted line represents a ␲–␲ interaction).

Fig. 58. Cyclometallated platinum complex 212.

Fig. 59. Mono-iminophosphoranes 154–158 and bis-iminophosphoranes 159–163.

4-bromoanisole and phenylboronic acid resulted in a TOF of 35,100 mol(ArBr) mol(Pd)−1 h−1 . All these di-iminophosphoranes showed higher activities than diphosphine 12. The origin of the high performance of these ligands likely arises from the steric crowding created around the metal centre upon chelation, and this has a positive impact on the reductive elimination step. Interestingly, systems based on the mono-iminophosphoranes 154–158 (Fig. 59) showed much higher reaction rates than the related bis-iminophosphoranes 159–163, both in palladium-catalysed Suzuki–Miyaura and nickel-catalysed Kumada–Tamao–Corriu cross-couplings [70]. For both reaction types, activities of up to 400,000 mol(ArBr) mol(M)−1 h−1 were observed (Table 25, entries 1 and 5). These remarkable activities are roughly an order of magnitude higher than those observed for the calixarenyl-free iminophosphorane Ph3 P = N(o-anisyl) 213 (Fig. 60) (Table 25, entries 3 and 6). The unusual performance of these ligands was rationalised in terms of their ability to form supramolecular complexes having cavity-entrapped “M(ArX)” units (Fig. 61). Formation of such intermediates, which position the metal at the cavity entrance rather than outside, creates a highly crowded metal environment, which favours the formation of (vide supra) mono-ligated complexes over that of bis-ligated ones. It must be born in mind that bulky ligands promote the formation of mono-ligated complexes and therefore favour the oxidative addition step in Suzuki–Miyaura cross coupling, while in

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Table 25 Cross-coupling reactions of aryl bromides using ligands 157, 158 and 213 [70]. Entry

Ligand

Metal precursor (mol%)

TOF (mol(ArBr) mol(M)−1 h−1 )

ArBr

a

Palladium-catalysed Suzuki–Miyaura

1

157

1 × 10−4

400,000

2 3

157 213

1 × 10−4 1 × 10−3

270,000 24,100

Nickel-catalysed Kumada–Tamao–Corriub 4 157 1 × 10−4 158 1 × 10−4 5 213 1 × 10−4 6

381,000 390,000 56,000

a b

[Pd(OAc)2 ]:L = 1:2, ArBr:PhB(OH)2 :NaH = 1:2:2, dioxane, 100 ◦ C, 1 h. [Ni(cod)2 ]:L = 1:2, ArBr:PhMgBr = 1:2, 100 ◦ C, 1 h. Table 26 Palladium-catalysed Suzuki–Miyaura of aryl bromides using phosphines 166–168 [72]. Entry

ArBr

Resorcinarenyl-phosphine 166

167

168

1

TOF

26,680

25,030

28,740

2

TOF

27,120

29,500

34,570

3

TOF

16,690

28,780

22,770

4

TOF

27,030

29,360

32,130

Fig. 62. Phosphines 166–168. Conditions: P:Pd = 2:1, ArBr:PhB(OH)2 :NaH = 1:2:2, dioxane, 100 ◦ C, 1 h. The TOF were expressed in mol(ArBr) mol(Pd)−1 h−1 .

Fig. 63. o-Anisylmethyldiphenyl phosphine (214).

KTC cross-coupling, for which oxidative addition is not a key step, they facilitate decoordination of the coupling product [109]. The phosphinomethyl-substituted resorcinarenes 166–168 (Fig. 62) showed high activities in Suzuki–Miyaura cross-coupling of aryl bromide using [Pd(OAc)2 ] as palladium precursor and NaH as base [112]. Under optimised conditions (metal/phosphorus ratio = 1:2) the performances varied in the following order: 166 < distally substituted resorcinarene monophosphine 167 < proximally substituted resorcinarene 168 (Table 26). The highest activity (TOF = 34,570 mol(ArBr) mol(Pd)−1 h−1 ; Table 26, entry 2), was obtained in the arylation of 4-bromotoluene with diphosphine 168. In terms of activity, all three phosphines were 5–7 times better than PPh3 . Interestingly, the activity of monophosphine 166 compares with that of o-anisylmethyldiphenyl phosphine (214; Fig. 63), suggesting that 166 (and also 167 and 168) operates as a P,O-chelating ligand. Chelation should increase both the electronic density of the metal as well as its crowding, two factors which facilitate the oxidative addition step in Suzuki–Miyaura reactions.

Tetraphosphine 169 (Fig. 62) was used in Mizoroki–Heck coupling of aryl bromides with styrene [73]. The catalytic runs were carried out in DMF at 130 ◦ C with [Pd(OAc)2 ] as palladium precursor (1 equiv. per tetraphosphine) and Cs2 CO3 as base. Only moderate activities were obtained under these conditions (TOF = 2000 mol(ArBr) mol(Pd)−1 h−1 in the case of 4bromoanisole). Repeating the runs with less than 4 P atoms per Pd led to a drastic activity decrease. This suggests that free phosphine is necessary for stabilisation of the active species. 4.6. Olefin oligomerisation/polymerisation reactions Each year, millions of tonnes of polymeric materials, including plastics, fibres and elastomers, are produced by polymerisation of simple olefins. The mechanism of polymerisation follows a general sequence: initiation of the reaction, chain growth, termination step and regeneration of the catalyst. This is notably what occurs in the case of transition metal catalysed ethylene oligomerisation and polymerisation (Scheme 52). The three calixarene complexes [NiBr2 (10)], [NiBr2 (11)] and [NiCp(11)]BF4 (Fig. 64) were assessed in ethylene [113,114] and propene [115] dimerisation. With ethylene (20 bar, 25 ◦ C), the catalytic systems obtained by treatment of [NiBr2 (10)] and [NiBr2 (11)] with 400 equiv. of MAO reacted with ethylene to produce 95 wt.% of butenes (mainly trans-2-butene). The corresponding activities

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Fig. 65. Diphosphine 56.

Scheme 52. Classical catalytic cycle of ethylene oligomerisation/polymerisation.

Fig. 64. Diphosphines 10 and 11. Table 27 Nickel-catalysed olefins dimerisation.a n(Ni) (␮mol)

P(olefin) (bar)

T (◦ C)

TOF

Ethylene [113,114] 1 [NiBr2 (10)] [NiBr2 (10)] 2 3 [NiBr2 (11)] [NiBr2 (11)] 4 b 5 [NiCp(11)]BF4 [NiBr2 (dppe)] 6

4.5 0.09 4.5 0.09 4.5 4.5

20 20 20 20 30 30

25 25 25 25 40 40

117,400 835,300 60,300 1,205,400 23,100 33,900

Propene [115] 7 [NiBr2 (10)] [NiBr2 (10)] 8 [NiBr2 (11)] 9 [NiBr2 (11)] 10 [NiBr2 (dppe)] 11 [NiCl2 (PCy3 )2 ] 12

4.5 0.09 4.5 0.09 4.5 4.5

5 5 5 5 5 5

25 25 25 25 25 25

219,000 1,928,000 237,000 2,599,000 115,000 282,000

Entry

a b

Complex

MAO:Ni = 400:1, toluene for ethylene and PhCl for propene, 0.25 h. MAO:Ni = 1000:1, 1 h. The TOF were expressed in mol(olefin) mol(Ni)−1 h−1 .

were 835,300 and 1.2 × 106 mol(olefin) mol(Ni)−1 h−1 , respectively (Table 27, entries 2 and 4). For comparison, with [NiBr2 (dppe)], the TOF was not higher than 33,900 mol(olefin) mol(Ni)−1 h−1 (Table 27, entry 6). A lower TOF was observed with the precatalyst [NiCp(11)]BF4 (Table 27, entry 5) owing to more difficult catalyst activation. Combining the above [NiBr2 (11)]/MAO catalyst with a [ZrCp2 Cl2 ]/MAO catalyst produced ethylene–butene copolymers. Polymer analysis revealed that the reaction produced linear lowdensity polyethylene (LLDPE) characterised by ethyl branches as sole ramifications (up to 2.7%), and with a narrow polydispersity [114,116]. In the dimerisation of propene (5 bar, 25 ◦ C, MAO/Ni ratio = 400:1) [NiBr2 (10)] and [NiBr2 (11)] displayed activities

about twice as high as those obtained with the precursor [NiBr2 (dppe)] (Table 27, entries 7, 9 and 11). When operating at catalyst loadings of 0.09 ␮mol/l the TOFs reached values of ca. 1.9 × 106 and 2.6 × 106 mol(olefin) mol(Ni)−1 h−1 with both precatalysts (Table 27, entries 8 and 10). These experiments resulted in a C6 selectivity (mainly methylpentenes) of ca. 95 wt%. For comparison, the well known complex [NiCl2 (PCy3 )2 ] gives only ca. 30 wt% of C6 products (beside higher olefins). The remarkable performance of [NiBr2 (10)] and [NiBr2 (11)] in ethylene or propene dimerisation is a consequence of the intrinsic dynamics of these complexes. As revealed by NMR studies carried out on [NiCp(11)]BF4 , the P–Ni–P moiety of this complex undergoes a fast fanning motion that displaces the metal from one side of the calixarene axis to the other (vide supra, dynamics of [NiCp(12)]+). During this movement the ligand bite angle varies periodically, reaching its largest value when the metal crosses the calix axis. The same motion is likely to occur in the catalytic intermediates of these reactions, notably in [Ni(11)(CH2 = CH2 )(ethyl)]+ and [Ni(11)(CH3 CH = CH2 )(propyl)]+ . Increasing the bite angle produces a shrinking of the ligands trans positioned to the P atom, as a result of an increased steric pressure exerted by the P substituents on the metal centre. This should then facilitate the insertion step. The complex [NiBr2 (11)] was further tested in living norbornene polymerisation. After activation of the complex with MAO, and by applying high dilution conditions, TOFs up to 750,000 mol(olefin) mol(Ni)−1 h−1 were obtained [117]. The reference complex [NiBr2 (dppe)] led to lower activities. The authors invoked reasons similar to those discussed in ethylene polymerisation (see above) for rationalising the high catalytic performance of [NiBr2 (11)]. The chelate complex [NiCl2 (56)] (Fig. 65) was assessed in ethylene oligomerisation [66,114,118]. After activation with MAO, and by applying 30 bar of ethylene, the resulting nickel catalyst produced C4 –C12 oligomers following a Schulz–Flory distribution (˛ = 0.2) with a TOF of 9800 mol(olefin) mol(Ni)−1 h−1 . The formation of rather short oligomers suggests that during catalysis, the nickel environment is somewhat encumbered, hence elimination is favoured over propagation. This catalytic behaviour is reminiscent of that observed for the binuclear complex Ni2 Cl4 [cis,trans,cis1,2,3,4-tetrakis-(diphenylphosphino)cyclobutane] (not drawn), in which the catalytic centre lies within a pocket defined by the phenyl substituents of the phosphorus atoms [119]. Kuhn et al. reported on the use of the phosphorus ylides 149 and 151–153 (Fig. 66) in the nickel-catalysed oligomerisation/polymerisation of ethylene. The catalytic system was generated by mixing the ylides with [Ni(cod)2 ]. By adding PPh3 to the mixture, the system behaved as a an oligomerisation catalyst, otherwise polyethylene was formed (Scheme 53) [66,69]. The catalysts obtained from mono-ylides 152 and 153 displayed activities in ethylene oligomerisation and polymerisation that were equal or higher than Keim’s catalyst [{Ni(Ph)(Ph2 PCH = C(O)Ph)}(PPh3 )], a complex which forms upon reaction of ylide 215 (Fig. 67) with [Ni(cod)2 ] and PPh3 . For

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Scheme 53. Oligomerisation/polymerisation catalysts obtained from a phosphorus ylide.

Scheme 54. Typical deactivation pathway of Keim-type catalysts.

Fig. 68. Inactive nickel complex 216 resulting from intramolecular deactivation of a catalytic mixture containing 151.

Fig. 66. Phosphorus ylides 149 and 151–153.

Fig. 67. Ylide 215, precursor of Keim’s catalyst. Table 28 Ethylene oligomerisation/polymerisation using ylides 149, 151–153 and 215 [120]. Entry

Ylide

TOF (mol(olefin) mol(Ni)−1 h−1 )

Ethylene polymerisationa 1 149 2 151 152 3 153 4 215 5

23,500 39,600 42,100 85,500 41,400

Ethylene oligomerisationb 6 149 151 7 152 8 153 9 215 10

28,700 34,700 55,800 54,000 41,800

a b

Ylide:Ni = 1:10, P(ethylene) = 10 bar, toluene, 70 ◦ C, 0.25 h. Ylide:Ni:PPh3 = 1:1:1, P(ethylene) = 5 bar, toluene,70 ◦ C, 1 h.

example, under oligomerisation conditions, TOFs of 55,800, 54,000, and 41,800 mol(olefin) mol(Ni)−1 h−1 were observed with the three ylides 152, 153, and 215, respectively (Table 28, entries 3–5). The results obtained with 152 and 153 illustrate the beneficial effect of the calixarene moiety on the stabilisation of the catalyst. In fact, it is well known that in oligomerisation/polymerisation with Keimtype catalysts, a possible deactivation pathway is the formation of a bis-phosphino-enolato-chelate complex according to the pathway shown in Scheme 54. The formation of such a bis-P,O complex

Fig. 69. Phosphite 101.

is seemingly prevented by appending a bulky substituent on the P,O-backbone [120]. In terms of activity, bis-ylides 149 and 151 were inferior to 215. Thus, for example, in ethylene oligomerisation, TOFs of 28,700 (149), 34,700 (151) and 41,800 (215) mol(olefin) mol(Ni)−1 h−1 were respectively obtained with these ylides (Table 28, entries 6, 7 and 10). The lower performance of 149 and 151 was explained by a possible conversion of these complexes into inactive complexes such as 216 (Fig. 68). van Leeuwen et al. investigated the behaviour of the phosphite–quinoline complex [Pd(Me)(MeCN)(101)]PF6 (Fig. 69) in ethylene/CO copolymerisation [52]. The corresponding runs were performed in dichloromethane in the presence of 1,4benzoquinone under 30 bar of CO/C2 H4 (1:1). To be operative, the system required a pressure of at least 30 bar. Under these conditions, appreciable amounts of copolymer were produced, but the activity of the system was low. Another ligand tested by these authors for the same reaction is the calix[6]arene derived diphosphite syn-109 (Fig. 70) [56]. Using [Pd(Me)(MeCN)(syn-109)]OTf in the presence of TsOH (TOFs up to 300 mol(monomer) mol(Pd)−1 ) produced copolymers having mainly a carboxylic acid as an end group rather than a vinyl

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References

Fig. 70. Diphosphite syn-109.

function. In other words, chain termination preferentially occurs by a reaction with water rather than through a ␤-hydrogen elimination step. This behaviour indicates the presence of a chelating ligand with a bite angle larger than 90◦ .

5. Conclusion The purpose of this review was twofold: (a) to provide a collection of synthetic methods for the preparation of P(III)- and P(V)-containing calixarene and resorcinarene ligands; (b) to examine the metal-catalysed reactions in which these compounds have been successfully applied. These reactions include olefin hydroformylation, hydroaminovinylation, hydroalkoxycarbonylation, asymmetric hydrogenation, allylic alkylation and olefin polymerisation, as well as Suzuki–Miyaura, Kumada–Tamao–Corriu and Mizoroki–Heck cross-couplings. While in a number of examples, phosphines of the type referred to under (a) behave merely as bulky ligands that confer improved stability on catalytic intermediates and thus increase the catalyst’s lifetime, many others display catalytic properties which exploit the particular structural and functional features of the macrocyclic cores. Typically, calixarene and resorcinarene platforms, because of their rigid or semi-rigid structure, have been helpful for introducing strain in the ligand generated from them, a parameter that may drastically influence the catalytic outcome, be it in terms of activity or selectivity; such effects are particularly relevant in hydroformylation and carbon–carbon bond-forming reactions. Further, calixarene and resorcinarene backbones are valuable templates for creating pocket-shaped phosphines that enable positioning of a metal ion in a highly confining environment, and which are therefore suitable for inducing shape and/or substrate selectivity. In addition, with some calixarene-derived phosphines and iminophosphoranes, the receptor properties of the macrocyclic unit induce endo-binding of incoming substrates. This supramolecular feature had a direct effect on the nature of some catalytic intermediates in cross-coupling reactions, lowering the energy barrier of key steps in the catalytic cycles. Overall, coordination chemists now have access to a wide range of calix- and resorcinarene-derived P(III) and P(V) ligands that can be applied in a number of reactions of practical interest. Most of these ligands are remarkably robust and their performance in catalysis is often superior to that of well-established phosphorus ligands. It can reasonably be anticipated that future research in this field will focus on exploiting the many opportunities offered by the calixarene and resorcinarene cores as a source of selective receptors, of multiply functionalised ligands, and of chiral cavities suitable for selective intra-cavity catalysis.

Acknowledgements The French Agence Nationale de la Recherche (RESICAT Programme, ANR-12-BS07-0001-01) is gratefully acknowledged for financial support. We warmly thank Prof. J. Harrowfield (UDS) and Prof. M. Chetcuti (UDS) for the many interesting discussions on this topic.

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