Secondary phosphine oxides: Versatile ligands in transition metal-catalyzed cross-coupling reactions

Coordination Chemistry Reviews 256 (2012) 771–803

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

Review

Secondary phosphine oxides: Versatile ligands in transition metal-catalyzed cross-coupling reactions Tanveer Mahamadali Shaikh, Chia-Ming Weng, Fung-E Hong ∗ Department of Chemistry, National Chung Hsing University, 250 Kuo-Kuang Road, Taichung 40227, Taiwan

Contents 1. 2.

3. 4.

5. 6. 7. 8.

9.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Catalytic reactions involving carbon carbon bond formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Heck cross-coupling reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Suzuki–Miyaura cross-coupling reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Negishi cross-coupling reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Kumada cross-coupling reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5. Stille cross-coupling reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6. Sonogashira cross-coupling reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7. Hiyama cross-coupling reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.8. Coupling of acyl chlorides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cycloaddition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Catalytic reactions involving carbon heteroatom bond formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. C N bond formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. C S bond formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Miscellaneous . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SPO in asymmetric synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis of new SPOs and ligated Pd complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electronic properties and bonding modes of SPOs and ligated Pd complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1. DFT studies on the charges of selected tri-substituted phosphines and SPOs and their corresponding Pd complexes . . . . . . . . . . . . . . . . . . . . 31 8.2. P NMR of selected phosphines and SPOs coordinated Pd complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Appendix A. Supplementary data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

a r t i c l e

i n f o

Article history: Received 31 May 2011 Accepted 8 November 2011 Available online 7 December 2011 Keywords: Secondary phosphine oxide (SPO) Cross-coupling SPO in asymmetric reactions Density functional theory (DFT) Palladium

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a b s t r a c t This review describes the development in the use of secondary phosphine oxides ligands in transitionmetal-catalyzed oxidative coupling reactions to the formation of C C, C N and C S bonds. Also the developments in the recent literature are highlighted in the areas of cycloaddition, coupling of acyl halide with organometallic reagents, oxidations and other reactions. The secondary phosphine oxides are known to exist in the tautomeric forms and are configurationally stable; this property enables a variety of useful transformations for accessing optically active organophosphorus compounds which are useful in the formation of C C bonds and asymmetric hydrogenation reactions. Finally, the mechanistic aspect employing DFT technique in these oxidative coupling reactions is described in this article. © 2011 Elsevier B.V. All rights reserved.

∗ Corresponding author. Fax: +886 4 22862547. E-mail address: [email protected] (F.-E. Hong). 0010-8545/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ccr.2011.11.007

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1. Introduction Transition metal catalyzed coupling reactions, which lead to the formation of C C, C N, C O and C S bonds, are among the most powerful transformations in modern organometallic chemistry [1]. These methods have been applied in a number of fields including academic research laboratories as well as pharmaceutical industries. Moreover, the importance of such biaryl components in drugs, herbicides and natural products fine chemicals, as well as in the field of engineering materials, conducting polymers, molecular wires and liquid crystals has attracted enormous interest in the chemistry community [2]. The cross-coupling methodologies generally consist of a coupling between an electropositive carbon fragment and an electronegative carbon moiety. Importantly, the ligand plays a significant role in activating the transition metal as an efficient catalyst in such coupling reactions. Although various kinds and functions of ligands are available, phosphines with various shapes and functions remain the most important type of ligands in transition metal catalyzed reactions [3]. Plain trialkylphosphines were prepared and characterized as early as 1870s [4]. The number of known transition-metal complexes containing phosphine ligands is truly immense and includes monodentate, bidentate, tridentate, higher chelating phosphines, as well as a great variety of diverse substituents on phosphorus [5]. Most phosphines are sensitive to air oxidation and moisture, are extremely malodorous [6], which makes them difficult to handle. To overcome these problems, in the last few years a trend has been seen towards using air and moisture stable secondary phosphine oxides (SPOs) as ligands. The first synthesis of di-n-alkylphosphine oxides was reported by Williams and Hamilton in 1952 [7]. They prepared di-n-hexyl-, n-octyl- and n-octadecylphosphine oxides via reaction of an appropriate Grignard reagent with diethylphosphite. At the same time, Kosolapoff and Watson disclosed the oxidation of phosphinate to its corresponding phosphinic acid by reaction with H2 O2 without isolating di-alkyl phosphine oxides [8]. Later, in 1968 Emmick and Letsinger reported two new methods for the synthesis of SPOs. The first method involved the displacement of alkoxide from an ester of a monosubstituted phosphinic acid with Grignard reagent. The second method was generated through the reduction of an ester of a di-substituted phosphinic acid by lithium aluminum hydride [9]. Interestingly, Mislow and coworkers disclosed a study of LiAlH4 induced stereomutation of phenyl-␣-phenylethyl phosphine oxide in 1970 [10]. The first example of phosphinous acid-phosphinito complexes of Pt (II) were reported by Roundhill [11]. Several years later in 2001, Li prepared air-stable secondary phosphine oxides ligated metal complexes [12]. A new territory of research in organometallic chemistry was opened. The present review aims to produce a clear description of the chemistry of secondary phosphine oxide ligands and their metal complexes in the last decade. We confine our discussion to those reactions which are important and would benefit the entire chemistry community and researchers involved in catalytic carbon carbon bond formation and asymmetric catalysis. The first section describes secondary phosphine oxide-based ligands involved in catalytic cross-coupling such as carbon carbon, carbon nitrogen and carbon sulfur bond-forming reactions. Also included in this section are cycloaddition reactions. Reactions which do not fall into this category are presented in the miscellaneous section. The SPOs in asymmetric catalysis are also discussed in-details according to the reaction classifications. The last section is devoted to the structural and electronic properties of SPO ligands as well as a discussion on reaction mechanism. Reaction schemes and compounds are numbered and are highlighted in bold type, enabling a facile search of each section.

Fig. 1.

The following figure shows a probable reaction path leading to the formation of stable phosphinous acids coordinated to metal complexes. Secondary phosphine oxides (SPOs) (A) are weak acids and could exist in equilibrium in solution with species (B) under ambient conditions [13]. Coordination to the metal center through the phosphorus atom affords (C), which are air-stable and resistant to moisture and high temperature [14]. Transition metal complexes (C) might function as active catalysts in various reactions. The catalytic performance of these complexes will be disclosed in the following sections (Fig. 1). 2. Catalytic reactions involving carbon carbon bond formation C C bond forming processes are fundamentally important reactions in organic synthesis. They have been extensively exploited by chemists due to their far-reaching applications using coupled building blocks [15]. The contributions of these secondary phosphine oxide/transition metal complex-catalyzed carbon carbon bond forming reactions will be discussed in the following section. Generally speaking, these ligand complexed transition metal catalysts exhibit comparable efficiencies to that of tri-substituted phosphines. 2.1. Heck cross-coupling reaction The Pd-catalyzed coupling of alkenyl or aryl (sp2 -C) halides with olefins was first reported by Heck in the late 1960s [16a,b] and further developed by Mizoroki in 1970 [16c]. Traditional Heck coupling uses an aryl iodide or bromide as the electrophilic partner and a terminal alkene as the nucleophilic counterpart. In the last few decades, several major advances have been made using this coupling reaction. One particular advance is the exploration of more efficient ligands and diverse reaction protocols to obtain higher turnover numbers (TON) affording synthetically and industrially useful processes. The first synthesis of simple, readily available, air and moisture stable secondary phosphine oxides were reported by Li in 2001 (Scheme 1) [12]. These SPOs were used as ligand precursors in the Pd-catalyzed C C bond forming reactions of aryl chlorides. The preparation of di-tert-butyl halophosphine (2a) oxide was achieved by hydrolysis of di-tert-butylhalophosphine halide (1a or b) in aq. medium. A tautomeric equilibrium is expected between the secondary phosphine oxide (2a) and its corresponding phosphinous acid (3a) in solution. Further, they have extended this method to the preparation of other substituted air-stable phosphine oxide precursors (2) (Scheme 2) [17]. Synthesis of (5) was performed from a polymer supported halide (4), which on treatment with either Grignard

Scheme 1.

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773

Scheme 2.

or organometallic reagent (RM) produces (5) via nucleophilic displacement. Subsequent hydrolysis yields SPO (2). Li developed a novel preparation of a series of SPO-coordinatedPd complexes (6)–(9) via the treatment of SPOs (2) with Pd salts at either rt or elevated temperatures (Fig. 2). Indeed, the binuclear monophosphinous acid chloride-bridged Pd(II) complex POPd2 (8) was synthesized via the treatment of (2a) with Pd(cod)Cl2 at ambient conditions. These complexes are stable and can be purified by column chromatography. The formation of these catalyst precursors can be monitored by 31 P NMR spectroscopy and were verified by X-ray crystallography. For compound POPd1 (9), the combination of one phosphinous acid ligand (RR P-OH) with its anionic ligand moiety (RR P-O− ) through hydrogen bonding form a six membered chelating ring with Pd. The authors extended the scope of these air-stable complexes, POPd (6), POPd2 (8) and POPd1 (9), and demonstrated their use as pre-catalysts in Heck cross-coupling reactions of aryl chlorides [17]. They demonstrated that the coupling of aryl chlorides (10) and tert-butyl acrylate (11) in the presence of K2 CO3 in DMF solvent could be accomplished using 1.5 mol% of POPd (6) resulting in ␣,␤-unsaturated esters (12) in good yields (Scheme 3). The other substituted aryl chlorides underwent smooth coupling to the corresponding ␣,␤-unsaturated esters in good yields. In a typical phosphine-assisted catalytic system, a slight excess amount of ligand is employed to activate the catalyst and to protect it from degradation during the catalytic process. This is also true for the SPO-assisted catalytic reaction. In the presence of air-stable Pdcomplexes POPd in cross-coupling of aryl chlorides, comparable efficiencies were observed with catalytic systems using and an excess amount of ligand. A year later, the activities of these phosphinous acid coordinated Pd catalysts were examined by Wolf and coworkers [18]. They reported the Heck coupling of heterocyclic aryl chlorides (13) and tert-butyl acrylate (11) catalyzed by POPd (6) resulted in the formation of ␣,␤-unsaturated ester (14) in 85% yield with high diastereoselectivity (E/Z = 25:1) (Scheme 4). Moreover, the authors mentioned the screening of other catalysts (8) and (9) under similar reaction conditions to afford (14) in unexpectedly low yields of 43% and 65%, respectively. It is thought that the anionic nature of Pdphosphinous acids under basic conditions may limit the solubility and therefore accessibility and activity of these catalysts, resulting in poor yields. An increase in solution homogeneity was accomplished by employing Cy2 NMe as base in DMF. This resulted in an increase in the coupling reactivity of POPd (6). The other substituted chloroquinoline derivatives also converted to the corresponding ␣,␤-unsaturated esters. Their screening of other inorganic bases such as NaOAc and Cs2 CO3 greatly affects yields because the aliphatic amines cause problems in chromatographic separation. From a synthetic standpoint, this method is advantageous over the existing methods due to availability and low cost of the heteroaryl chlorides (13). Recently, we reported the preparation of a new type of SPO (17a–b) and their applications in the Heck coupling reaction. The synthesis of ligand (17a or 17b) was accomplished by starting from amino(chloro)phosphine (15), which was prepared by a modified

literature procedure [19]. The treatment of (15) with lithiated phenyl acetylene and subsequent hydrolysis of the P N bond using silica and amberlyst resin yielded secondary phosphine oxides (16). Treatment of 16 with dicobalt carbonyl led to the formation of new cobalt-containing secondary phosphine oxide (17) in high yields (Scheme 5) [20]. The formation of 17a and 17b was confirmed by NMR spectroscopy and single crystal X-ray diffraction methods (Fig. 3). A typical large coupling constant (JP–H = 457.8 Hz) was observed for 17a in the 31 P NMR indicating the existence of a direct P H bond in 17a. Similarly, a large coupling constant of 483.4 Hz for 17b was also observed. The ORTEPs of 17a and 17b are depicted in Fig. 3. The P O ˚ respecbond lengths for 17a and 17b are 1.379(6) A˚ and 1.470(4) A, tively revealing the presence of a typical double bond between phosphorus and oxygen atoms. The scope of these secondary phosphine oxides, (17a) and (17b), has also been studied in the Heck coupling of bromobenzene (19) with styrene. A catalytic amount of Pd(OAc)2 /(17) in 1:2 ratio afforded coupled products (20) in excellent yields within 5 h (Table 1). Both (17a) and (17b) exhibited good catalytic activities even with other substituted aryl bromides. 2.2. Suzuki–Miyaura cross-coupling reaction Suzuki cross coupling is another versatile and probably the most employed method for C C bond forming reactions [21]. Normally, the reaction is carried out for aryl- or vinyl-boronic acids with aryl-, vinyl- or even an alkyl-halide by Pd in the presence of a base such as sodium methoxide, ethoxide, acetate, or hydroxide. This method is widely used to synthesize poly-olefins, styrenes and substituted biphenyls [22]. This reaction was first reported by Suzuki and his co-workers in 1979 [23]. Remarkable developments have been made in this field for the construction of conjugated dienes, polyene, biaryl and related systems. Recent development in the Suzuki coupling reaction involves coupling of unactivated alkyl halides [24] in C C bond-forming processes and the preparation of air and water stable organoboron compounds under mild reaction conditions [25]. Indeed, it has become one of the most reliable and widely applied

Table 1 Pd(OAc)2 /(17) catalyzed Heck reaction.

Entry

1 2 3 4 5 6 7

Aryl bromides

Yield (%)

R

Ligand (17a) (R = t Bu)

Ligand (17b) (R = Cy)

4-NO2 4-COCH3 4-COH 4-CH3 4-OMe 2-CH3 4-H

99 99 97 98 95 100 100

97 98 90 99 81 100 100

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Fig. 2.

Scheme 3.

Pd-catalyzed cross-coupling reactions. In the last decade, other major advances have been made in the newly designed phosphorus ligands, which have dramatically improved the efficiency and selectivity of such cross-coupling reactions. The ligands including bulky dialkylbiaryl- [26] and trialkylphosphines [27] remain the most widely used ligands and are commercially available. Recently, another category of highly employed ligands are the N-heterocyclic carbenes (NHCs) [28]. Nevertheless, the development of stable SPO type ligands, which offers complementary applications in the Suzuki cross-coupling, is gaining interest both from an academic and industrial operation. The first example of a SPO/Pd catalyzed Suzuki–Miyaura crosscoupling reaction was reported by Li in 2001 [12] An air-stable

phosphine oxide ligand (2b) was combined with Pd(OAc)2 or Pd2 (dba)3 generating the in situ complex POPd2 (8). The Suzuki cross-coupling reaction of aryl chlorides (10) with phenylboronic acid (21) in toluene catalyzed by P( O)H(t Bu)(Ph)/Pd2 (dba)3 gave biaryls in good yields (Scheme 6) [12]. The authors reported that even electron-donating substituents were converted to biaryls when CsF was used as base under the identical conditions. Despite tremendous progress, the Suzuki–Miyaura coupling of 2-substituted nitrogen-containing heteroarylboronic acids or esters with aryl or heteroaryl halides remains a challenge. However, other procedures for such coupling resulted in low yields of coupled products [29]. These reactions fail to give the expected yields of the desired coupling products due to side reactions

Scheme 4.

Scheme 5.

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775

Fig. 3.

Scheme 6.

such as protodeboronation and dimerization. Yang took this concept a step further by developing the Suzuki–Miyaura coupling of heteroaryl halide and boronic acid [30]. The authors reported an air-stable PXPd2 (24)-catalyzed regioselective Suzuki–Miyaura coupling of 2,6-dichloronicotinamide (23) with arylboronic acids to give the 2-aryl-6-chloronicotinamides (25a) in moderate to good yields accompanied with minor side product (25b) (Scheme 7). The PXPd2 (24)-catalyzed regioselective Suzuki–Miyaura coupling was achieved by chelation of the Pd(0) species to an ester/amide group. The authors noted that the air-stable Pd catalyst (24) gave the best results in reagent grade methanol (without degassing the reaction mixture) and in the presence of K2 CO3 within shorter reaction time and good regioselectivity. Generally, the Suzuki coupling of amides are less reactive than esters. Under the present catalytic reaction conditions, amides give higher yield. Nevertheless, the boronic acids bearing both electron-rich and electron-deficient

groups underwent coupling reactions to the corresponding heterobiaryls. The reaction of sterically hindered boronic acid such as 2-methoxyphenylboronic acid did not undergo coupling even at high temperature. As demonstrated, ␤-aryl or alkylarylidene malonate (27) derivatives possess some useful biological activities [31]. Syntheses of such derivatives have mostly relied upon Knoevenagel condensation of diethyl malonate with the corresponding ketones. However, this reaction is limited in the sense that good yields are only obtained in the case of methyl ketones and cyclohexanone derivatives [32]. Turner developed a novel Pd/phosphinous acid POPd (6)-catalyzed Suzuki coupling of ␤-chloroalkyl or arylidene malonates to the corresponding ␤-aryl or alkylarylidene malonates (27) in good yields (Scheme 8) [33]. The authors mentioned that the employment of microwave irradiation allows the reaction to be completed within 30 min to obtain the coupled product. This

Scheme 7.

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Scheme 8.

Scheme 9.

coupling procedure tolerates a wide range of arylboronic acids including electron-rich (NH2 , OH, OMe, Me, etc.), electron-deficient (NO2 , F, etc.) and sterically hindered substrates. Ackermann reported the preparation of air-stable diaminophosphine oxide (30) ligands (Scheme 9) [34]. The synthesis from diamine (28) and PCl3 gave intermediate (29) [35]. Subsequently, hydrolysis of (29) led to the formation of (30) in 51–79% yield. Further, the authors extended the scope of these ligands to Pd-catalyzed Suzuki-type cross-coupling reactions. The catalytic coupling of aryl chlorides (10) and phenylboronic acids (21) in the presence of potassium tert-butoxide and Pd2 (dba)3 /(30) in 1:2 ratio efficiently converted to the corresponding biaryls (22) (Scheme 10). The biaryls were obtained in good to excellent yields under mild reaction conditions. Both sterically hindered arylchloride and boronic acids with ortho-substituents were tolerated under these reaction conditions. The coupling of heteroaromatic chlorides, such as pyridine and isoquinoline derivatives was efficiently converted to the corresponding biaryls. The cross coupling of aryl chlorides with boronic acids using various Pd catalysts have provided chemists with access to several versatile methods using a variety of aryl chlorides [36]. The majority of reported coupling methods were carried under inert conditions. Leadbeater and co-workers reported the microwavepromoted Suzuki–Miyaura cross-coupling in aqueous medium

without ligands [37]. However, the yields related to chlorides in this method were poor to moderate as compared to those of bromides. To overcome a few of these problems, Yu drew inspiration from Turner’s work in developing the efficient procedure employing catalyst POPd2 (8) in the Suzuki cross coupling reaction (Scheme 11) [38]. This catalytic process gave good yields of diaryls (32) in mixture of DMF:H2 O solvent within 15 min. They also screened other sources of Pd catalysts such as Pd(OAc)2 and Pd(PPh3 )2 ; all failed to give good yields of the coupled products. Unexpectedly, they observed that addition of tetrabutylammonium iodide (TBAI) increased yields of the coupled product. Indeed, the authors did not mention the role of this reagent in the mechanism. Another convenient procedure for Suzuki coupling promoted by POPd (6) was reported by Wolf and co-workers [39]. The coupling of chloroquinoline (13) with allylboronic acid (33) in the presence of chloro-bridged phosphinous acid-Pd complex POPd (6) (2.5 mol%), Cs2 CO3 and 1,4-dioxane produced biaryl (34) in 74% yield (Scheme 12). The process is applicable to electron-deficient and electron-rich aryl iodides, bromides or chlorides as well as to the compatible organoboronic acids giving the corresponding biaryl products in good to excellent yields. The authors noted that the highest catalytic activity providing good yields of biaryls was observed with the mono- and dinuclear structures POPd, POPd7 , and PXPd, as compared to dimeric complexes (POPd2 , POPd6 ,

Scheme 10.

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Scheme 11.

Scheme 12.

PXPd2 , and PXPd6 ). The differences in yields obtained with the former complexes reveal that the bulky cyclohexyl groups in the phosphane ligands afford superior results. However, replacement of cyclohexyl groups of monomeric Pd–dialkyl(chloro)phosphanes by bulky tert-butyl groups increased the yields of biaryls. Another interesting observation was that the chloro ligands attached to the Pd center in POPd and POPd-Br exhibited better results as compared to the bromides. Li reported that the POPd1 (7)-catalyzed coupling of heteroarylboronic acids (35) with aryl halides (10) gave (36) with the 2-pyridyl moiety (Scheme 13) [40]. Although reports are available for this type of reaction, coupling of aryl iodides with 2-pyridyl boronic acid/esters often gives lower yields of coupled products. More often than not, bromo-benzene, rather than chloro-benzene, was chosen in this reaction to obtain reasonable yields of the

desired product (36). The authors also screened a variety of conditions such as various catalysts, bases, and solvents. Still, the best results were obtained with the combination of POPd1 (7), CsF (or Cs2 CO3 ) in i PrOH. The coupling of other substituted aryl bromides or chlorides with good yields were also reported. This approach in synthesizing 2-pyridyl moiety, a useful building block, might be incorporated into drug discovery programs. Recently, we have reported Suzuki coupling employing a newly synthesized ferrocene derived SPO ligands (40) and (41) (Scheme 14) [41]. The preparation of (40) or (41) was begun with commercially available ferrocene (37), which was treated with nbutyl lithium (1 equiv.) followed by nucleophilic displacement with PPhCl(NEt2 ) to produce (38) in excellent yield. Similarly, (39) was obtained by using two molar equivalents of n-butyl lithium. Finally, the hydrolysis of the P-N bond was achieved under acidic media to

Scheme 13.

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Scheme 14.

Scheme 15.

the corresponding secondary phosphine oxides (40) and (41) [19]. These mono- and di-phosphino-substituted ferrocene derivatives, (40) and (41), were further characterized by 31 P NMR and X-ray crystallography. We further studied the catalytic performance of these ligands in the Suzuki–Miyaura cross-coupling reactions (Scheme 15). The Pd-catalyzed coupling of bromobenzenes with phenylboronic acid in the presence of either of ligands (40) or (41) proceeded to the formation of the corresponding biaryls (22) in good to excellent yields. The reaction was completed within 2 h in aqueous medium. This catalytic process tolerates both electron-donating and electronwithdrawing aryl bromides and chlorides. In combination with these ligands, some of the commonly used Pd salts were screened. However, the best performance was with Pd(OAc)2 /(40) in 1/1 ratio, with NaOt Bu in water. 2.3. Negishi cross-coupling reaction In 1977, Negishi reported another novel method for the Pdcatalyzed cross-coupling of organozinc with aryl halides and this is now a well-known reaction in C C bond formation [42]. In recent years, the chemistry community has witnessed a growing interest in the development of organozinc mediated cross-coupling due to its improved reactivity compared to other electropositive metal species. In addition, the organozinc reagents and their preparative methods are relatively low in toxicity which makes the Negishi coupling an attractive alternative to the other cross-coupling C C bond forming reactions. So far, there are only handful reports available on SPO-assisted Pd complex-catalyzed Negishi coupling reactions as presented in this section.

Initially, Li developed a POPd (6)-catalyzed Negishi coupling of arylzinc chloride (42) with aryl chlorides to access biaryls (Scheme 16) [43]. This procedure tolerates both electron-donating and electron-withdrawing groups on aryl halides. Normally, it takes less time for the electron deficient arylchlorides (<2 h) to couple with arylzinc chloride than the electron-donating aryl halides (>14 h). In addition, the reaction is highly chemoselective. Addition of organozinc to the chloro-substituted acetophenone preferably gave the coupled product over the potential side reactions. Later, Wolf and co-workers reported another procedure employing POPd7 (43)-catalyzed Negishi coupling of aryl chlorides with organozinc compounds (Scheme 17) [44]. They prepared a variety of new SPOs to achieve good yields of biaryls 22. Among the catalysts screened, the best yields were observed using POPd7 (43). Their conditions are similar to the aforementioned Li’s procedure. Moreover, they have studied a wide range of coupling including aryl bromides and iodides to the formation of biaryls with high yields. This procedure tolerated a number of functional groups, such as ester, ketone, cyano, amino, thiophene, pyridine, quinoline including halogens in the range of 75–93% yields. The process is also applicable to neutral, sterically hindered substrates. Interestingly, the reactions proceeded faster in the case of electron-donating substituents and in excellent yields of the coupled products. 2.4. Kumada cross-coupling reaction In 1943, Kharasch reported cobalt(II)-catalyzed coupling of organomagnesium bromides with vinyl halides [45]. Later in 1972, Kumada [46] and Corriu [47] independently reported the first Pd-catalyzed cross-coupling of arylmagnesium halide with aryl

Scheme 16.

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779

Scheme 17.

Scheme 18.

halides. This method was coined as the Kumada reaction. Kumada coupling has regained interest in recent years since the often employed boronic acids in C C bond forming reactions are usually derived from Grignard reagents. This method offers a more direct approach to the synthesis of biaryls. Later, Knochel [48] reported that organomagnesium compounds tolerate the presence of ester, nitrile, nitro, and other functional groups at ambient conditions. The combination of metal catalysts such as nickel or palladium and with phosphines as ligands and employing Kumada coupling was reported early in the literature [49]. However, the application of SPOs as preligands in Kumada cross-coupling has only been reported in recent years. Li et al. firstly developed the combination of Ni(cod)2 / (t Bu)2 P(O)H as a pre-catalyst in the Kumada cross-coupling. The coupling of aryl chlorides (10) with organomagnesium compounds (44) at ambient temperature proceeded to the corresponding biaryls (22) in excellent yields (Scheme 18) [50a]. This procedure

benefits from mild conditions and the products were isolated in quantitative yields. The authors also noted that the coupling showed good results when employing phosphine sulfide as ligand instead of SPOs. The conventional sulfur mediated reactions or even organo-sulfur compounds always exhibit negative effects in catalytic performance by poisoning the transition metal presumably through a strong metal-sulfur bond. Nevertheless, the presence of phosphinothious acid (R2 P-SH), derived from tautomerization of phosphine sulfide, can be tolerated in this catalytic process. More recently, Ackermann et al. reported the use of airstable secondary phosphine sulfide ligands (29d and 29e) in nickel-catalyzed Kumada-Corriu type cross-coupling reactions (Scheme 19) [50b]. Their preparation of air-stable ligands 29d–e was achieved from the corresponding SPOs (1,3-bis(2,6-diisopropylphenyl)[1,3,2]diazaphospholane-2-oxide) (29b) (1,3-bis-(2,4,6-trimethylphenyl)[1,3,2]diazaphospholaneand 2-oxide) (29a) respectively. This was subjected to the thiation

Scheme 19.

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Scheme 20.

reaction using Lawesson’s reagent and led to the formation of 29d in 76% yield. Thus, the cross-coupling of aryl halides with arylmagnesium halide 10 in the presence of Ni(acac)2 and ligand 29d or 29e resulted in good yields of biaryls 22 in 67% and 89% respectively. The ligand 29e displayed a superior reactivity over 29d in this cross-coupling. They have studied a number of fluoro, chloro, electron–rich and electron–deficient aryl halides and arylmagnesium halides which were tolerated under these reaction conditions. Wolf et al. developed another alternative coupling, using the same ligand (t Bu)2 P(O)H (2a) discussed above [51]. In this reaction, the authors chose ortho-substituted aryl halides and organomagnesium bromides providing direct access to the formation of sterically hindered multifunctional biaryls (22). Generally, the yields in such coupling reactions are low due to strong repulsion between ortho-substituent. The authors stated that although the coupling fragments are sterically hindered, the biaryls were obtained in excellent yields. A variety of functional groups that are generally not compatible with Grignard reagents are tolerated, for example aryl bromides with electron-donating and electron-withdrawing substituents underwent coupling at ambient conditions. The procedure is an alternate method to prepare a variety of substituted BINAP derivatives. The coupling of aryl iodides required a lower temperature, i.e. −20 ◦ C under similar conditions (Scheme 20). More recently, Ackermann reported the preparation of a new sterically demanding and non-hygroscopic adamantane substituted SPO ligand (46) and its application in Pd-catalyzed

Kumada-Corriu cross-coupling reactions (Scheme 21) [52]. Electron-deficient N-heterocyclic nucleophiles i.e. 2-pyridyl organomagnesium reagents (45) with aryl halides (19) in the presence of Pd2 (dba)3 /(46) in 1:4 ratio were efficiently converted to the heterobiaryl compounds (34) in moderate yields. Other monodentate or bidentate ligand and N-heterocyclic carbene ligated Pd complexes did not deliver the desired products under these conditions. The authors also noted that a new Pd complex (47) was formed when (46) was treated with Pd(OAc)2 at 60 ◦ C (Scheme 22). Its structure was confirmed by X-ray crystallographic studies. The catalytic performance of this newly prepared catalyst (47) was further examined under the similar conditions and the yields of corresponding heterobiaryls had been improved. The electrophiles bearing functional groups, such as halo, ester, N- and S-heteroarenes substituents, were chemoselectively converted to the corresponding products in good yields. 2.5. Stille cross-coupling reaction The Pd-catalyzed cross-coupling of organotin compounds with organic electrophiles has been known as Stille coupling, which was initially reported in 1978 [53], although, the discovery was mentioned by Kosugi the previous year, he published the first report on transition-metal-catalyzed carbon carbon bond forming reactions with organotin compounds [54]. Until now, the Stille

Scheme 21.

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781

Scheme 22.

Scheme 23.

reaction remains one of the most widely applied Pd-catalyzed C C bond-forming reactions. The mild conditions, easy preparation of a wide range of coupling partners, and the tolerance of a variety of sensitive functionalities in this transformation, are the main reasons. In this section, we described SPO/Pd-catalyzed Stille coupling reactions, mostly reported by Wolf and co-workers [55]. The application of haloquinolines as synthetic intermediates in coupling reactions has been limited because of the unstable nature of bromo- and iodoquinolines and less reactive chloroquinolines. The authors developed an efficient method for the preparation of substituted chloroquinolines in good yields (Scheme 23). The preparation of quinolines has been established by an unprecedented Ziegler reaction. Thus, the reaction of chloroquinoline with alkyl or aryllithium in THF, followed by its treatment with cericammonium nitrate (CAN) resulted in the formation of substituted chloroquinolines with high regioselectivity and yields [18]. The authors also reported the coupling of organotin compounds with aryl chlorides and bromides in the presence of phosphinous acid coordinated Pd complex POPd (6) in aqueous medium to produce biaryls. Developed next were a water-soluble (6)-catalyzed Stille crosscoupling of aryl stannanes with chloroquinolines in another C C

bond forming reactions. This procedure provided aryl quinolines (50) in good to excellent yields (Scheme 24). The coupling product (50) was isolated by a simple extraction method with diethyl ether and the catalyst was recovered from the aqueous phase and recycled. Notably, the recycled catalyst (6) was further studied in the Stille cross-coupling reaction, and interestingly the corresponding biaryls (50) was obtained in excellent yields (95–96%) for the first two cycles. However, the catalytic performance of the recycled catalyst slightly diminished after two runs and the coupled products formed with low yields. The procedure is also applicable to a variety of neutral and heteroaryl bromides and chlorides. This route is substantially beneficial to the industrial operation because although it requires higher temperatures this reaction can be performed in water. Wolf and co-workers also proposed a mechanism for this catalytic reaction. Initially, it proceeds through an oxidative addition step to generate an aryl-Pd species (51a) (Scheme 25). The Pd-species (51a) formed a complex with aryl stannanes (49) in the usual transmetallation step. Subsequently, reductive elimination of (51b) generates the biaryl quinolines (50) and regenerated catalyst, which takes part further in catalytic cycle. The authors mentioned that the base plays an important role in the hydrolysis of trimethyltin chloride to trimethyltin hydroxide in the catalytic cycle. 2.6. Sonogashira cross-coupling reaction The Pd complex-catalyzed cross-coupling of alkynes with aryl halides, which was accelerated by the addition of Cu-salt as cocatalyst, is known as the Sonogashira reaction [56a,b]. This coupling

Scheme 24.

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Scheme 25.

reaction was originally inspired by the Stephens-Castro reaction, which involves the coupling of vinyl or aryl halides with stoichiometric amounts of copper(I) acetylides and can be viewed as both an alkyne version of Heck reaction and as an application [56c]. This reaction has emerged in recent years as one of the most general, reliable, and effective methods for the synthesis of substituted alkynes. The Sonogashira reaction provides a valuable method for the synthesis of conjugated acetylenic intermediates, which could be important building blocks in natural products and pharmaceuticals [57]. Normally, this reaction requires anhydrous conditions to obtain a chemoselective coupled product. However, there are reports available using water as co-solvent. For example, Kotschy reported the Pd(0)/CuI-catalyzed coupling of aryl halides in aq. dimethylacetamide [58]. In addition, Amatore developed homogeneous reaction conditions for conducting Sonogashira coupling of aryl iodides in water-soluble sulfonated triphenylphosphine coordinated Pd(OAc)2 complex in mixed solvent systems (H2 O/CH3 CN) [59]. Dibowski and co-workers described water-soluble guanidinophosphane coordinated Pd catalysts for the coupling of aryl iodides with terminal alkynes [60]. Currently, there is only one report employing SPO as a pre-ligand in the Sonogashira coupling and is described in this section. Wolf and co-workers reported a water-soluble POPd (6)catalyzed Sonogashira coupling of alkynes (52) with heterocyclic arylhalides (53) in aqueous medium to give (54), disubstituted

alkyne derivatives (Scheme 26) [61]. The coupling procedure with the addition of TBAB/CuI as additives in aqueous medium gave good yields of products. However, pyrrolidine and 0.5 M aq. NaOH play crucial roles in this catalytic reaction. It gave good to excellent yields of coupled products within 5 h, with a number of aryl halides and terminal alkynes. As expected, the electron-rich arenes gave poor yields of coupled products. Wolf also proposed a mechanism to accommodate the reaction [61]. First, it proceeds through an oxidative addition of anionic Pd(0)/(6a) complex to give intermediate (55a) (Scheme 27). Next, the intermediate (55a) was oxidized in the presence of alkynes to generate the Pd-alkynyl complex (55b). The author mentioned that base (pyrrolidine or 0.5 M aq. NaOH) plays an indispensable role in activating catalyst (6–6a) during oxidative addition and also deprotonation of alkynes 52 during catalysis. Subsequent reductive elimination of (55c) provides alkynylated product (54) and regenerated Pd catalyst (6a) which further takes part in the catalytic cycle. 2.7. Hiyama cross-coupling reaction Another important palladium-catalyzed C C bond forming reaction involves the coupling of aryl halides with organosilane compounds, a process today known as the Hiyama coupling, was reported in 1964 [62]. This reaction has rapidly gained acceptance

Scheme 26.

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783

Scheme 27.

as a suitable alternative to other coupling reactions due to their ease of handling, stability towards air and moisture, low toxicity. Later, Denmark et al. studied the Lewis base promoted reaction with modified conditions [63]. Deshong and Mowery reported that vinyl and aryl halides, allylic benzoates, and aryl triflates perform well in the Hiyama cross-coupling with hypervalent siloxane derivatives [64]. Nolan and co-workers employed a combination of imidazolium chloride and Pd(OAc)2 (3 mol %) as pre-catalyst, which was very efficient to the activated aryl chlorides. However, the unactivated substrates gave poor yields [65]. However, the scope of the heteroaryl halide in Hiyama cross-coupling was not fully investigated. Wolf and co-workers reported the coupling of heterocyclic halides (13) with phenyltrimethylsiloxane (56) employing POPd1

(9) (7 mol%) in acetonitrile to afford biaryls (50) in good yields (Scheme 28) [66]. The authors also screened other SPO coordinated Pd complexes, POPd (6), POPd2 (8), and POPd1 (9), high yields were obtained from POPd1 (9). These conditions are compatible with a number of aryl halides, electron-rich heteroaryl halides including chloro- and bromo-substituted electrophiles. However, the bromo-substituent afforded higher yields than their chloride analogs. This procedure tolerates a variety of functional groups such as, nitrile, ketone and ester to produce coupling products with high regioselectivity. The role of tetrabutylammonium fluoride (2 equiv.) is to activate phenyltrimethoxysilane, where the fluorideion binds with Si to form a pentavalent sp3 d-hybridized siloxane, which undergoes transmetallation with the Pd complex.

Scheme 28.

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Scheme 29.

Further, the same group developed another solvent-free Hiyama type coupling reaction employing POPd1 (9) promoted by NaOH in aqueous medium (Scheme 29) [67]. The authors favored the use of the same POPd1 (9) as catalyst. Their procedure is advantageous over the existing methods in terms of solvent-free and can be performed in the open-air. They also screened a variety of hetero arylsiloxanes to investigate the feasibility of this coupling reaction. Notably, the yields were increased with the addition of aqueous NaOH at higher temperatures (>80 ◦ C). They also mentioned that addition of NaOH enhanced the rate of reaction, probably due to its coordination with siloxane, which forms a pentavalent sp3 d-hybridized siloxane undergoing transmetallation with the Pd complex. Thus, the addition of sodium hydroxide fulfills two fundamental functions, i.e., activation of the catalyst and arylsiloxane in the oxidative addition and transmetallation steps respectively.

2.8. Coupling of acyl chlorides In this section, we discuss the Pd/SPO complex catalyzed cross-coupling of acyl chlorides with organoboron, organotin, and organozinc reagents in C C bond forming reactions. The availability and high reactivity of acyl halides towards oxidative addition has been the prime choice of starting materials in these transition metal-catalyzed cross-coupling reactions. The cross-coupling of acyl chlorides with organometallic reagents provides convenient access to ketones that one might not be able to prepare otherwise, for example, via Friedel-Crafts acylation methods. The Pd-catalyzed coupling of acyl chlorides and organotin compounds to form unsymmetrical ketones was reported by Stille in 1983 [68]. The reaction gave poor yields when reactive aryl halide functionalities such as in 4-bromobenzoyl chloride were present. Falck and co-workers described alternative coupling of acyl chlorides with ␣-amino- and ␣-alkoxystannanes [69]. The other coupling reaction of acyl halides with organometallic reagents such as organostannanes [68a,b,70], organoboronic acids [71], organozinc [72], arylbismuth, and Grignard reagents have been reported. Scheme 30 describes the coupling of acyl chlorides with organotin, organoboron and organozinc compounds which was reported by Wolf and co-workers. Initially, the authors reported the phosphinous acid/Pd complex (7) catalyzed cross-coupling of acyl chlorides (58) with organostannanes (59) in refluxing acetonitrile which proceeded to the formation of ketones (60) in high yields [70d]. Notably, the reaction favored the formation of ketones rather than the competitive Stille coupling reaction. These procedures exerted the regioselective coupling of a variety of acyl halides and organostannanes derivatives, including aliphatic, electrondonating, heteroaryl, cyano and halo substituents.

Further, the authors extended the scope of this catalytic reaction to the cross-coupling of acyl halide and organoboronic reagents by employing POPd (6) [73]. Thus, the coupling of organoboronic reagent (21) with acyl chlorides (61) smoothly underwent coupling at 80 ◦ C, to the formation of the corresponding ketones (62a) in good yields. They also mentioned that the same reaction when subjected to the microwave irradiation was completed faster with good yields of ketones. This procedure is advantageous over the conventional heating and prolonged lengthy reaction times. This coupling procedure tolerates the presence of aryl chlorides and bromides on both the acyl halide substrate and the aryl boronic acid. For example, ␣,␤-unsaturated ketones can be prepared in good yields by coupling of either styrylboronic acid or cinnamoyl chloride with phenylzinc chloride. Later, the authors developed another efficient method for the preparation of ketones. The coupling of acyl halides with organozinc reagents catalyzed by POPd (6) under microwave irradiation resulted in the formation of ketones in excellent yields [44]. This process provides a pathway for the preparation of a variety of aliphatic and aromatic ketones having methoxy, cyano, ester, alkyl and halo substituents. The superior control of chemo- and regioselectivity of POPd (6)-catalyzed coupling in the formation of ketones is advantageous over the traditional Friedel-Crafts acylation reaction. The procedure is also applicable to allylic, aliphatic acyl chlorides to give corresponding ketones.

3. Cycloaddition Cycloadditions are among the most important tools in organic chemistry since these reactions are vital to the modern synthesis of natural products and biologically active substances. Transition metals play an increasingly important role in these catalytic reactions [74]. In this section, we discuss phosphinous acid-Pd catalyzed cycloaddition reactions, which have been developed by Buono and co-workers [75]. Initially, the authors prepared a new type of SPO/Pd complex (64a) (dihydrogen di-␮-acetatotetrakis(tertbutylphenylphosphinito-␬-P)dipalladate) and its derivatives (64b) and (64c). The reaction of phosphine oxide (2b–d) and Pd(OAc)2 at 50 ◦ C produced dinuclear Pd complexes (64a–c) in quantitative yields (Scheme 31) [76]. The author extended the scope of catalyst (64a) to [2 + 1] cycloadditions. First, the cycloaddition of phenyl acetylene (52) with norbornadiene (65) in the presence of 64a gave benzylidenecyclopropane (66) as a single diastereomer in low yields (17%) (Scheme 32). In order to achieve good yield and selectivity, they have chosen to perform this reaction employing POPd1 (9) as the catalyst, which did not favor [2 + 1] cycloaddition. The addition of a Lewis acid AgOAc (10 mol%) in the same pot afforded (66) in

T.M. Shaikh et al. / Coordination Chemistry Reviews 256 (2012) 771–803

Scheme 30.

Scheme 31.

Scheme 32.

785

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Scheme 33.

Scheme 34.

low yields. These results prompted them to study a variation of Lewis acid/ligand combinations in cycloadditions. Surprisingly, the bulky ligand (64c) (R = Cy) in combination with AgOAc (10 mol%) smoothly underwent [2 + 1] cycloadditions with increased yields (up to 80%). This modified procedure has been applied to [2 + 1] cycloadditions of various substituted terminal alkynes at ambient conditions to give corresponding cyclic products. The authors proposed a mechanism that accommodates the transformation that proceeds through the Pd vinylidene complex (67a) (Scheme 33). The intermediate (67a) would favor [2 + 1] cycloaddition with norbornene to give palladocyclobutane (67b), which could undergo reductive elimination to form the product (66). Their proposed mechanism was supported by a deuterium labeling study. Encouraged by these results, the authors prepared a new phosphinous acid-Pt complex (69) to broaden the scope of this reaction (Scheme 34) [77]. The preparation of (69) involves reaction between optically active SPO ligand (R)-(+)-tert-butyl phenylphosphine oxide (2b) and a Pt chloride-acetonitrile complex, which was heated at 60 ◦ C in THF for 12 h. This reaction resulted in the formation of (68) in 100% yield as the trans isomer. Subsequently, its treatment with Ag(OAc) gives (69), a structural related complex of (47), as discussed in Scheme 22. The efficiency of this catalyst was then demonstrated in [2 + 1] cycloadditions. The reaction of norbornadiene (65) and phenyl acetylene (52) in the presence of 5 mol% of Pt-complex (69), proceeded smoothly to give cyclic product (66) (Scheme 35). As

observed previously the same reaction employing other SPOcomplexes failed to undergo [2 + 1] cycloadditions. The Lewis acid Ag(OAc) plays an important role in Pd-catalyzed cycloadditions, in this case the Pt-complex (69) gave better results with AcOH. The catalytic cyclopropanation of various norbornene derivatives were obtained in good yields. Interestingly, the hetero olefins such as diaza- and oxyaza-bicyclic derivatives resulted in the formation of excellent yields of the products. Another strategy closely related to the previous one using ligand (2d) in tandem cycloaddition-ring expansion to afford bicycle[3.2.1]octadienes has been studied by the same group (Scheme 36) [78a]. Interestingly, the authors observed that the reaction of norbornene (65) and tertiary propargylic acetate (67) employing SPO ligand (2d) gives a mixture of an unusual ring expanded major product (70) and cycloadduct (71) as minor product obtained at 60 ◦ C. Though, the major product (70) exists as a diastereomeric mixture in 3.8:1 ratio. In seeking to elucidate the formation of the unusual ring expanded product (70), the authors conducted another experiment. Readily prepared (71) was subjected to the reaction employing same conditions (Pd(OAc)2 /(2d)), however the ring-expanded product (70) did not form. This indicates that the product (70) was not converted through (71). This tandem [2 + 1] cycloaddition-ring expansion was studied on various diene and tertiary propargylic acetate derivatives. In the literature most of the cycloaddition reactions has been reported with intramolecular fashion. Very recently, Buono and co-workers disclosed the platinum-catalyzed regio- and

Scheme 35.

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787

Scheme 36.

Scheme 37.

diastereoselective tandem [2 + 1]/[3 + 2] intermolecular cycloaddition (Scheme 37) [78b]. Their catalytic procedure describes the reaction of norbornadiene 65 with alkynes 52 in the presence of catalyst (69, R = Cy) in a mixture of acetic acid and toluene at 55 ◦ C, which led to the formation of an unprecedented tricyclic compound 66a in 62% yield. However, the methylenecyclopropane (MCP) (66) was formed in 10% yield. Interestingly, they have found that the yield corresponds to tricyclic product (66a) increased to 71% when the reaction was heated for 72 h. Nevertheless, the other substituent on catalyst (69) complex (R = t Bu) or change in reaction conditions did not produce the expected tricyclic product (66a). The generality of this reaction was subsequently demonstrated by subjecting a variety of alkynes (52) in [2 + 1]/[3 + 2] intermolecular cycloaddition. Further studies indicated that the oxygen in propargylic ethers (52) must play an important role, hence reaction proceeded through intermediate (MCP, 66). This process was later extended to the cycloaddition of alkylidenecyclopropane 66 (R = OAc) and terminal alkyne 52 to afford tricyclic product 66a directly in good yields (Scheme 38). This catalytic reaction condition was found to be tolerant towards various substituted terminal alkynes, trimethylsilylethyne and trimethylsilyl ether including heteroatom groups such as sulfonamide, phthalamide, sulfone to provide the corresponding tricycle product 66a in moderate to good yields.

4. Catalytic reactions involving carbon heteroatom bond formation The majority of compounds produced by nature have heterocyclic rings as part of their structures [79]. Many of these compounds are shown as key components in biological activities. The syntheses of such cores have been achieved through the formation of C N, C S and C O bond by employing transition metal-catalyzed cross-coupling reactions. The Pd-catalyzed amination was initially reported in 1994 by Buchwald [80], and Hartwig [81], using bulky and electron-rich phosphine ligands, which dramatically improved the efficiency and selectivity of such cross-coupling reactions. The transition-metal complex catalyzed C S bond formation has been a less studied transformation than the corresponding C C, C N and C O bond formations. Sulfur-containing compounds require special conditions since this functionality is known to be reactive and may act as a poison for metal-based catalysts due to its strong coordinative properties, often making the catalytic reaction ineffective. In the last few decades, transition-metal complex catalyzed organosulfur chemistry received particular interest, which brought about important progress in this field. Initially, Migita reported the first Pd(PPh3 )4 catalyzed coupling of aryl halides with thiols [82]. Other efficient Pd catalysts are based on bidentate phosphines or diverse

Scheme 38.

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Scheme 39.

Scheme 40.

Scheme 41.

organophosphane derivatives. Recently, various metals such as Ni, Co, and Cu in combination with effective ligands have emerged as appealing catalysts for these coupling reactions [83]. However, there are only few reports employing SPOs as pre-ligands in the C S bond forming reaction. In this section we described SPO/Pd complexes catalyzed C N and C S bond forming reactions. 4.1. C N bond formation The first application of a phosphinous acid-Pd-catalyzed amination was reported by Li et al. in 2001 [12]. The process involved coupling of aryl chlorides with cyclic amines in the presence of Pd2 (dba)3 /(t Bu)2 P(O)H (2a) to afford the aminated product (73) in moderate yields (Scheme 39). However, the authors demonstrated this reaction with piperidine as the only amine source. Later, Wolf et al. developed an amination reaction by exploring POPd (6) as a catalyst. The authors chose chloroquinolines (50) as the coupling fragment in the amination in the presence of Cy2 NMe or t-BuOK in tert-butanol at 100 ◦ C (Scheme 40) [18]. This catalytic procedure to obtain coupled products resulted in good yields compared to Li’s method [12]. Among the catalysts studied, the same reaction with catalyst precursor POPd2 (8) gave similar yields of products. The scope of this reaction was studied in other chloroquinoline derivatives by the same research group [18]. Ackermann and co-workers developed the combination of a pre-SPO ligand (29c) with Pd source in the Buchwald-Hartwig type amination (Scheme 41) [34]. Initially, authors utilized pre-catalyst (30) in the coupling of arylchloride (10) with amine (75), which gave unsatisfactory results presumably due to the formation of

corresponding, hydrodehalogenated arenes. However, the phosphine chloride (29c) (R = t Bu) in combination with Pd2 (dba)3 resulted in efficient amination. The amination was then studied with various amines and diversely substituted aryl chlorides with electron donating groups. Similarly, aryl bromides were also converted to aminated products (76) with slightly improved yields. The authors noted that the low reactivity observed in some haloarenes was due to the reduction of the aryl halide. 4.2. C S bond formation The first Pd-phosphinous acid catalyzed C S bond forming reaction was reported by Li et al. [12]. The procedure allowed the thiolation of aryl halides (10), such as chlorides and bromides, with thiols employing a combination of (t Bu)2 PCl/[Pd2 (dba)3 ] in aqueous medium. The catalytic combination is generated in situ POPd (6), which is responsible for thiolation giving (78) in moderate yields (Scheme 42). Nevertheless, this reaction had been studied only with few substrates.

Scheme 42.

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789

Scheme 43.

Scheme 44.

Inspired by this method, Wolf reported another process for the C S bond forming reaction employing POPd (6) as catalyst [18]. They again selected challenging haloquinolines (50) as substrates to study this reaction. Although this reaction was performed at higher temperature, the yields of the coupled product (80) were good (85%) (Scheme 43). 5. Miscellaneous The preparation of amides is one of the most important synthetic transformations in organic synthesis [84]. Generally, most of the syntheses rely on the reaction between an activated carboxylic acid and an amine or ammonia. Enzymes and other heterogeneous metal-catalyzed reactions are also known in the literature [85]. Parkins reported a homogeneous phosphinito-Pt complex (82)catalyzed hydrolysis of nitriles (81) to amides (83) (Scheme 44) [86]. This hydrolysis procedure was selective in the formation of amides. The catalytic oxidation was also advantageous over the other known methods, because it stops at the amide stage and does not hydrolyze to the corresponding acid. In addition, the catalyst (82) produced up to 50,000 TON. This process was later extended by the same group. They reported the hydration of nitriles to amides employing a new homogeneous catalyst (84), which was derived from dimethylphosphine oxide by simple coordination with Pt(II) (Scheme 45) [87]. This catalytic hydrolysis procedure gave amide (83) as the sole product and there was no formation of acid by over-hydrolysis. The authors observed that variations in ligand substituents on phosphorus did not produce better yields. The most active catalyst (84), gave high TON of up to 77,000 for the hydration of acrylonitrile. This process on a number of aromatic and

Scheme 45.

aliphatic nitriles has been tested in order to obtain optimized yields of amides. de Vries and co-workers utilized Pt(II) complex (84) [(Me2 PO-HOPMe2 )PtH(PMe2 OH)] to mediate the direct conversion of nitriles (81) to N-substituted amides (86) (Scheme 46) [88]. Although the preparation of N-substituted amides from nitriles and amines has been reported in literature [89], those methods are associated with certain drawbacks. For example, certain catalysts are sensitive to severe reaction conditions such as extremely high temperatures and are vulnerable to either strong acids or strong bases. The authors drew inspiration from the Parkins’ work by employing amines as nucleophiles instead of water. Thus, their process involves the reaction of acetonitrile with propylamine in the presence of mixed water/DME at 160 ◦ C employing catalyst (84) (0.1 mol%) to give the amide (86) in 57% isolated yield. Under these catalytic conditions, low catalyst loading (0.02 mol%) is sufficient to achieve excellent yields of amides (86). However, the presence of ammonia, which was created during the reaction, retarded the catalytic performance. This process works well for both primary and secondary amines. The reaction of unactivated alkylnitriles such as benzonitrile gave only benzamide, while in the case of succinonitrile, the use of two equivalents of primary amine resulted in the formation of bis-amides. Further, the authors developed another method employing catalysts (84) as well as (89) and (90) in the selective hydration of hindered nitriles [90]. Catalyst (89) or (90) was prepared from PtCl2 with Ph2 P(O)H (2c) in toluene. Both compounds were crystallized in DCM/Et2 O and structures were determined by single crystal X-ray diffraction methods. Initially, the authors examined the hydrolysis in acetonitrile and ␣-methylbenzyl cyanide (Scheme 47). The catalysts (84) as well as (89) and (90) were rather efficient in the formation of the amide as the sole product. Interestingly, (84) is more reactive than (89) and (90) due to Pt-H and Pt–Cl effect. While the conversion of (87) to (88) by employing catalyst (84) (0.5 mol%), the reaction was completed within 3 h with TOF = 67 h−1 . Conversely, it takes 18 h and 20 h respectively with TOF = 3 h−1 and TOF = 10 h−1 when using the catalyst precursor (89) (2 mol%) and (90) (0.5 mol%). Based on these observations, the authors claimed that the catalytic activities decrease as follows: (84) > (90) > (89). Moreover, a number of sterically hindered tertiary nitriles and acid- or base-sensitive nitriles groups were smoothly converted to the corresponding amides under these conditions. Ackermann et al. developed a new methodology which was a directed ortho-arylation of benzene derivatives by employing a catalyst that is a combination of (46) with a Ru complex (Scheme 48) [91]. The direct cross-coupling of organic compounds via C H bond functionalization has been a less studied field in synthetic chemistry. However, the authors were able to manage the directed C H activation for both the electron-poor and electron-rich aryl

Scheme 46.

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Scheme 47.

chlorides which resulted in excellent isolated yields of products. The catalytic regioselective arylation of heterocyclic compounds smoothly underwent reaction with aryl iodides and bromides. Importantly, a wide variety of functional groups, such as ester, cyano and ketone, were tolerated under these conditions. In the case of aryl chloride with nitro substituents the reaction fails to undergo arylation due to steric effect. The authors extended this methodology using ketimines as one of the coupling fragments. Thus, under similar conditions the ketimines (93) and aryl chlorides (10) were converted selectively to the monoarylated product (94) in good yields (Scheme 49). Their catalytic procedure proceeded through coupling and direct hydrolysis of the imines to obtain the corresponding ketones. A variety of substrates with electron-poor as well as electron-rich aryl chlorides have been studied including substituted aryl chlorides which were efficiently converted to products. More recently, Ackermann and co-workers developed an intramolecular arylation of amides with unactivated

ortho-substituted aryl chlorides as electrophiles in the synthesis of substituted oxindoles and aza-oxindoles (Scheme 50) [92]. The combination of sterically hindered alkyl-substituted ligand (46) and Pd(OAc)2 generated a complex in situ in the intramolecular C C bond forming reaction. This reaction tolerates a variety of substituted amides including heterocyclic amides that undergo ␣-arylation. The authors noted that the synthesis of 3-alkoxysubstituted oxindole was the first example of a intramolecular ␣-arylation of aryl chlorides. A direct oxidative esterification of aldehydes with siloxanes catalyzed by (6) was reported by Wolf and co-workers (Scheme 51) [93]. The reaction of benzaldehydes and phenyl siloxanes in the presence of the catalyst precursor (6) (2.5 mol%) and the common fluoride source tetra-n-butylammonium fluoride (TBAF) directly converted to the methyl ester under mild conditions in high yields. Other aldehydes, such as phenylacetaldehyde, 3-phenylpropanal, cinnamaldehyde, phenylglyoxal, and cyclohexanecarboxaldehyde were also converted to the corresponding esters.

Scheme 48.

Scheme 49.

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791

Scheme 50.

The mechanistic aspect for one-pot oxidative esterification of aldehydes is presented in Scheme 52. The reaction of TBAF and tetramethyl orthosilicate acts as a Lewis acid and coordinates to the aldehyde, generating species (99a). It likely forms an oxophilic pentavalent silicate and facilitates the transfer of methoxide group due to its electrophilicity. Transmetallation of Si to Pd is followed by reductive elimination to produce ester (98) and regeneration of catalyst precursor (6) at the end of the catalytic cycle. This proposed mechanism is supported by crystallographic studies on several relevant structures and the formation of the HSiF(OMe)3 species was confirmed by NMR studies.

Scheme 51.

6. SPO in asymmetric synthesis In asymmetric catalysis, chiral diphosphines are the most frequently used ligands to access high enantioselectivity to the target molecules [94]. In contrast, chiral secondary phosphine oxides are little explored as ligands. While their chemical and physical

Scheme 52.

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Scheme 53.

Scheme 54.

properties are well known, however, application of these ligands in asymmetric catalysis is still in its infancy. The secondary phosphine oxides exist in the tautomeric forms and are configurationally stable; this property enables a variety of useful transformations for accessing optically active organophosphorus compounds. Among many substrates for the synthesis of a variety of enantiomerically pure phosphine oxides and other structurally related compounds, the enantiomeric forms of secondary tert-butylphenylphosphine oxide (2b) are the most useful and promising ones. Initially, Aron and co-workers reported the preparation of optically active hydrogen phosphinothiolate ligands [95]. Later, Michalski et al. developed Raney Ni-catalyzed reduction of optically active tert-butylphenylphosphinoselenoic acid (100a) or tert-butylphenylphosphinothioic acid (100b) to the corresponding phosphine oxide (101) in high yields and optical purity (Scheme 53) [96]. Fiaud and co-workers drew inspiration from Michalski’s work and developed another method for the synthesis of optically active trans-(2,5)-diphenylphospholanic acid (107) [97]. The synthesis commenced from the commercially available 1,4-diphenylbuta-1,3-diene (102). The treatment of it with (N,Ndimethylamino)dichlorophosphine gave intermediate 1-(N,Ndimethylamino)-r-1-oxo-t-2,t-5-triphenyl-phosphol-3-ene. Subsequently, its hydrogenation on 5% Pd/C gives phospholane (103) in 65% yield. The isomerization of (103) was carried out using NaOCH3 in methanol to afford the more stable trans isomer (104). Phospholanic acid (105) was obtained via two steps reaction sequence, acid-promoted hydrolysis of amide (104) and then resolution of the diastereomeric quinine salts. The phospholanic acid (105) was then converted to the corresponding chloride (106) in almost quantitative yield. Subsequently, its reduction using DIBAL-H resulted in the formation of secondary phosphine oxide (107) in 63% yield (Scheme 54). Drabowicz and Mikolajczyk developed a new procedure for the resolution of (±)-phosphine oxide (101) to prepare chiral phosphine oxide [98]. Thus, the equimolar mixture of phosphine oxide

(±)-(101) and (+)-(S)-mandelic acid (108) were dissolved in ethyl ether and stirred for 70 h to yield precipitate (101a) and soluble (101b) (Scheme 55). The crystalline (101a) obtained as a single diastereomeric form and separated by filtration, which was further characterized by 1 H NMR spectra. The solution, which contains (101b) shows mixture of diastereomers in a 7:3 ratio. Finally, the treatment of (101a) with 5% aqueous solution of potassium carbonate followed by extraction with chloroform gives enantiomerically pure (−)-(S)-(101) in 28.4% yield and 98.7% ee. Although the yields of isolated isomers were moderate, this process to prepare such types of compound is worthy of pursuing since the enantiopurity was excellent. Later, Polavarapu and co-workers reported another process for the separation of racemic phosphine oxides (101) by employing

Scheme 55.

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resolution technique using hand-made HPLC column and analytical gradient [99]. The authors also studied its configuration by using density functional theory (DFT). de Vries, Minnaard and Feringa together reported the synthesis of enantiopure ligands (101a–e) (Scheme 56) [100]. The preparation of these ligands had been achieved by reverse Grignard addition with t BuMgBr onto PhPCl2 followed by hydrolysis. The resulting mixture was then separated by preparative chromatography to obtain enantiopure (101). The authors have prepared a number of such derivatives (101a–e) with good yields. Nevertheless, the synthesis of (109) was achieved from chiral Taddol and PCl3 . Further, the application of these chiral SPO-ligands (101) in Ircatalyzed hydrogenation of imines (110) has been studied by the same group (Scheme 57). Hydrogenation of acetophenone-based imines (110) and N-diphenylphosphinyl-imine was investigated with a variety of conditions. Initially, the ligand (101) in combination with [Ir(COD)Cl]2 proceeded with low yields and ee’s of amine (111). Base plays a key role in such reactions. The authors screened various bases such as K2 CO3 , t BuOK, Na2 CO3 , phthalamide and TBAI as additives which did not improve the yields of product. Later, they employed Crabtree’s catalyst [Ir(COD)(PCy3 )py]PF6 in combination with ligand (101) and pyridine as additive to achieve best yields. The authors do not exclude the possibility that pyridine may act as ligand in this hydrogenation reaction. The screening of other ligands (101a–e) and (109) was pursued; none of them gave better enantiomeric excess (ee) excluding (101). This procedure was applicable to a variety of imines, which produce corresponding amines in good yields. The authors reported that enantioselectivities obtained are comparable to the best results obtained so far in iridium catalyzed hydrogenation of similar imines. They have later extended this method to the asymmetric hydrogenation of N-acyl dehydroamino acids and esters (Fig. 4) [101]. Again, the same ligand (101) was chosen because of its excellent performance in the imine hydrogenations. However, the reaction gave very low enantiomeric excess with cationic iridium complex. Thereby, the employment of cationic Rh(COD)2 BF4 − as catalyst in an Endeavour reaction system was preferred to carry out this reaction. Interestingly, the authors observed that the hydrogenation of (112a–c) in EtOAc gave the opposite enantiomer of the product compared to the same reaction in CH2 Cl2 . They have also studied asymmetric hydrogenation of a series of ␤-branched dehydroamino esters (113a–b), which were prepared by condensation of the corresponding keto esters with acetamide. Notably, the hydrogenations went to completion using a cationic rhodium precursor, whereas the ee was low to moderate. Nevertheless, complete conversion and high ee’s (85%) was obtained while employing iridium precursor [Ir(COD)Cl]2 in hydrogenation of ␤-branched dehydroamino esters. The authors has also studied the catalytic

793

Fig. 4.

hydrogenation reaction of N-acyl enamide (114) by employing Rh(COD)2 BF4 as catalyst, which led to the complete conversion of hydrogenated products; nevertheless, with low enantioselectivities. However, the same reaction in presence of cationic iridium precursor afforded racemic reduced product of (114). A year later, Dai et al. developed asymmetric allylic alkylation reaction employing the same ligand (101) (Scheme 58) [102]. The reaction of 1,3-diphenylprop-2-enyl acetate (115) with dimethyl malonate (116) in the presence of pre-catalyst ([PdCl(␩3 C3 H5 )]2 /(101) = 1:2), a catalytic amount of NaOAc as base and N,O-bis(trimethylsilyl) acetamide (BSA) as additive afforded product (117) at ambient conditions. Although the study was limited to the same substrates, however, the authors had investigated the solvent effect for this catalytic reaction of P-stereogenic SPOs. Interestingly, the strongly coordinating nitrile solvent, acetonitrile, caused a (R)-(117) reversal in stereochemistry of the product in contrast to the same reaction carried out in CH2 Cl2 , which induced (S)-(117) isomer. The other nitrile solvent, phenylacetonitrile, gave racemic products. The highly polar aprotic solvents, DMF, NMP, and DMSO, gave both low chemical yields and low enantiomeric excess. Presumably, the highly polar solvents weaken the coordination of SPO towards Pd due to strong solvation of the metal ion. The other

Scheme 56.

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Scheme 57.

Scheme 58.

non-coordinating solvents, toluene and CH2 Cl2 , resulted with low ee’s. The authors stated that highly polar and strongly coordinating solvents should be avoided in SPOs asymmetric catalysis. Leung reported the synthesis of some novel SPOs coordinated transition metal complexes (118–120) [103]. The preparation of complex (118) began with enantiopure (101). The treatment of (101) with Pd complex [PdCl2 (MeCN)2 ] and Et3 N in CH2 Cl2 at ambient condition for overnight afforded (118) in 72% yield (Scheme 59). The formation of (118) was confirmed by IR and 31 P NMR spectroscopic methods and showed that it is a single diastereomer. Complex (118) was then dissolved in diethyl ether and cooled to 0 ◦ C followed by the addition of HBF4 − and resulted in the formation of (119) in 43% yield. The presence of a BF2 group in the BF2 capped complex (119) was indicated by 19 F NMR. Further, a tungsten complex (120) was prepared from (119) via a succession of reaction steps to give complex (120) in 59% yield: chloride abstraction with AgBF4 in acetone/water followed by treatment with tungsten sulfide salt. They also prepared copper and vanadium complexes, (123) and (124), from (118) by means of exchanging metals (Scheme 60). Firstly, the reaction of (118) with either [Na(S2 CNEt2 )·3H2 O] or K[N(PPh2 S)2 ] in acetone led to the formation of complex (121) or (122), respectively. Then, the formation of (123) or (124) was

observed from the reaction of (121) or (122) with CuSO4 .5H2 O/Et3 N or VO(acac)2 . Hamada et al. reported the preparation of a novel P-chirogenic diaminophosphine oxide complex (128) (Scheme 61) [104a,b]. The synthesis began from known enantiopure anhydride (125). The nucleophilic ring-opening of (125), followed by its condensation with aniline, yielded the corresponding dianilide. It was then transformed into triamide (126) in 83% overall yield. Reducing the functionality of amide in (126) led to the formation of the corresponding triamine (127). Subsequently, its treatment with PCl3 /Et3 N yielded the corresponding triaminophosphine. Then, further treatment with SiO2 in wet AcOEt led to the final conversion to diaminophosphine oxide (128). The authors extended the scope of P-chirogenic diaminophosphine oxide ligand (128) in catalytic asymmetric allylic substitutions to generate a chiral quaternary carbon center (Scheme 62). Thus, the reaction of cinnamyl acetate (129) with ethyl 2-oxocyclohexane carboxylate (130) in the presence of [(␩3 -C3 H3 )PdCl]2 (2.5 mol%), ligand (128) (10 mol%), BSA and Zn(OAc)2 as additive afforded (131) in 99% yield and 93% ee. This procedure of asymmetric allylic substitution is applicable to both ␥-aryl and -alkyl acetates. Interestingly, the reaction is also favored by five, six, seven and eight-membered ring systems to give the corresponding allylic

Scheme 59.

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795

Scheme 60.

Scheme 61.

Scheme 62.

substitution products in excellent yields and enantioselectivity. They also noted that the BSA (3 equiv.) plays an important role in tautomerizing SPO ligand (128), which helps to coordinate with metal catalyst. In addition with to these processes, the authors extended the scope of this catalytic method in Pd-catalyzed asymmetric allylic amination reactions using P-chirogenic diaminophosphine oxides (128) which was derived from aspartic acid (Scheme 63) [104c]. Thus, the reaction of 1,3-diphenylallyl acetate (115a) with benzylamine employing catalyst, [(␩3 -C3 H5 )PdCl]2 /(128) = 1:2, in CH2 Cl2 solvent, yielded the corresponding allyl amine (111a) in high yield and enantiomeric excess (ee). They also tested various amine sources, which were used as nucleophiles in asymmetric allylic amination to produce (111a) in high yield and ee (>99%). However, the aniline derivatives did not act as nucleophiles in this catalytic amination. The asymmetric allylic amination of 1,3diakyl-substituted allyl carbonate (115a, X = OCO2 Me) was carried out using CH3 CN solvent, however, the corresponding aminated product was obtained in moderate yield and low ee. Further, the authors demonstrated the asymmetric allylic amination of 2-aryl cyclic carbonates (115b) under optimized reaction conditions (Scheme 64). The combination of palladium salt

with ligand (128) (1:2 ratio) as catalyst precursor provided the corresponding allyl amine (111b) in good yield and high enantioselectivity (up to 95–97% ee). The same amination source (NHR1 R2 ) was used as we have discussed in Scheme 63. The other cyclic carbonates having five-membered and seven-membered rings also

Scheme 63.

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Scheme 64.

afforded the chiral allylic amines (111b) with good to excellent enantioselectivity. The reaction tolerates both electron-donating and electron-withdrawing groups on the 2-substituted cyclic carbonates (115b) proceeding to form the corresponding products (111b) in high enantioselectivity. However, the aliphatic substrates (2-substituted cyclic carbonates 115b) with alkyl, alkenyl and alkynyl substituents gave the corresponding products in only moderate to good ee. In addition to these processes involving allylic amination, the authors extended the application of P-chirogenic diaminophosphine oxide ligand (128a) in palladium-catalyzed asymmetric aza-Morita-Baylis-Hillman reaction (Scheme 65) [104d]. The authors noted that the asymmetric allylic amination of carbonate (115c) employing benzyl amine as aminating source in the presence of palladium catalyst and ligand (128a) proceeded to form (111a) in 91% yield and 87% ee. Therefore in order to achieve high enantioselectivity, they found that electron-donating groups on the aromatic rings attached to the nitrogen atoms increased the enantioselectivity. The authors extended this procedure to a variety of cyclic carbonates with five-membered and seven-membered rings which were compatible with the system, including primary-, secondaryand aromatic-amine nucleophiles. The authors commented that the catalytic reaction could be performed on a gram scale without any

decrease in the yield and ee. Their catalytic procedure described in the above schemes could be synthetically important, since the product obtained with high ee could be used as a synthon for the cyclic ␤-amino acids and other natural products. Buono and coworkers later developed the asymmetric version of the [2 + 1] cycloaddition reaction [105a], employing the same catalyst (64a), which was discussed in previous section. Thus, the reaction of norbornadiene (65) and phenylethyne (52) in the presence of Pd(OAc)2 and ligand (101), which in situ was converted to the chiral catalyst (64a) led to the formation of (66) in 42% yield and 59% ee (Scheme 66). The authors noted that AcOH was generated in reaction, which enhanced the rate of cycloaddition reaction. However, the addition of additives, such as (S)-(+)-mandelic acid (10 mol%) increased enantioselectivity (76% ee) to the formation of (66). The authors noted that other palladium sources such as, Pd2 (dba)3 instead of Pd(OAc)2 also formed complex (64a). They undertook the detailed investigation of other palladium complexes and the influence of additives to this catalytic cycloaddition. Notably, the reaction of 1-ethynylcyclohex-1-ene (52a) with norbornadiene (65) in the presence of catalyst (68) (R = t Bu), Lewis acid AgOAc and addition of additive (S)-(+)-mandelic acid (10 mol%) resulted in excellent enantiomeric excess (95%) and low yields (21%) of (66). The proposed absolute E- and Z-configuration of the chiral cycloadduct was determined by Vibrational Circular Dichroism (VCD) spectra. Substituted alkynes and norbornadiene derivatives were compatible under these reaction conditions. Buono and Giordano developed a new method for the asymmetric syntheses of secondary phosphine oxides (101)–(101a–i) by a ex-chiral pool approach (Scheme 67) [105b]. It began with (Rp )-(−)-methyl hydrogenophenylphosphinate (132), which was derived from commercially available (−)-menthol. The nucleophilic addition of t BuMgBr on (132) produced (101) in 86% yield, yet it did not exhibit any enantiomeric excess. However, the addition of other alkyl Grignard reagents, i PrMgBr and EtMgBr, gave (101a) and (101f) in moderate ee (56% and 82% ee, respectively) of the corresponding isomer. Interestingly, the addition of o-tolylMgBr also produced chiral SPO (101 g) in good ee’s. The authors also studied other sterically hindered nucleophiles, which produced the corresponding SPOs (101h–i) in good yields with low ee’s. The reaction appears to require excess of Grignard reagent, one molar equivalent to abstract proton from (132) while the

Scheme 65.

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797

Scheme 66.

Scheme 67.

other equivalents are employed in nucleophilic substitution. The authors noted that the low enantioselectivity was due to steric hindrance of the nucleophilic fragment. The use of alternative alkyl organometallic reagents, such as organolithium species were then studied by them. Thus, the reaction of (132) with tertbutyllithium (2 equiv) in THF at −78 ◦ C yielded the corresponding SPO (S)-(101) with good enantioselectivity (86% ee), higher than that with t BuMgBr. Other lithiated agents, n-BuLi and MeLi, were also tested in this reaction, which resulted in good yields and ee’s. Surprisingly, the reaction with sterically hindered lithiated biphenyl produced (101 h) with high ee 96%. More recently, Han and Zhao reported the method for the preparation of optically active organophosphorus (133) derivatives by employing a stereospecific reaction of H-phosphinates and secondary phosphine oxides with amines and alcohols (Scheme 68) [106]. This synthesis was started with the optically active (RP )l-menthyl phenylphosphinate (132) and Et3 N in mixed solvent (CCl4 /CH3 CN = 1:5) in the presence of ammonia solution (28% in water, 5 mL) at 0 ◦ C to produce amidophosphinate (133) in excellent yield. This methodology is highly stereospecific and efficient for the preparation of a variety of optically pure organophosphorus acid derivatives (133a–i). They have also examined other amines, such as n-butylamine, aniline and benzylamine. All of these amines reacted with (132) to give the corresponding aminophosphinates (133a–d) in high yields. The reactivity in the case of primary amine is faster when compared to the secondary amine. Furthermore, the authors have expanded the scope of this reaction to other nucleophiles such as alcohols and thiophenol. The reactions were converted smoothly to the corresponding coupled phosphinates (133e–i) stereospecifically. However, aliphatic thiols such as n-BuSH and n-C8 H17 SH did not give the corresponding thiophosphinates. Lately, Pugin and Pfaltz reported a new chiral ferrocenyl (137) and menthyl (142) type of SPO ligand named as JoSPOphos and TerSPOphos, respectively (Scheme 69 and 70) [107]. The synthesis of ferrocenyl type ligands (137) has been achieved by two routes (Scheme 69). Firstly, lithiation and bromination of (R)-Ugi amine (134), which gave (R)-N,N-dimethyl-1-[(S)2-bromoferrocenyl]ethylamine (135). Displacement of the

dimethylamino group with desired PR2 group resulted in the formation of ferrocenyl phosphine bromides (136a–b) with retention in configuration. Treatment of ligand (136a) or (136b) with n-BuLi at low temperature followed by the addition of dichlorophosphine and hydrolysis in aqueous medium led to the formation of chiral ferrocenyl ligands (137). Secondly, the introduction of the SPO group to (134) resulted in the formation of (138a) or (138b). Subsequently, displacement of the –NMe2 group with –PR2 in the presence of acetic acid yielded ligand (139a–b). The TerSPOphos ligands (142) were prepared starting from commercially available 2-bromoiodobenzene (140) (Scheme 70). Firstly, the latter was treated with Grignard reagent for metallation. Subsequently, its treatment with PR2 Cl gave the corresponding bromophosphine intermediate (141). Lithiation of (141) followed by addition of dichloro[(−)-menthylphosphine] and hydrolysis with 0.1 M NaOH resulted in the formation of SPOP ligands (142) in good yields and 10:1 diastereomeric ratios. The ligands were then purified by recrystallization or column chromatography. The authors further extended the scope of these ligands, JoSPOphos (137) and TerSPOphos (142), in the asymmetric hydrogenation of olefins and ketones (143)-(148) respectively (Fig. 5). Both ligands show excellent performance in combination with rhodium as catalysts under ambient reaction conditions within

Fig. 5.

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Scheme 68.

Scheme 69.

Scheme 70.

2 h. The enantiomeric purity obtained was 90–99% ee with almost all substrates. The reactions were completed rather faster with 2000–20000 h−1 turnover frequencies (TOF).

to the corresponding biaryls. It is proposed that an active mononuclear Pd(0) species is responsible for the catalytic performance through the disproportionation of (151a) or (151b) in solution.

7. Synthesis of new SPOs and ligated Pd complexes Recently, we reported the synthesis of two new secondary phosphine oxides (149a) and (149b), an aryl-1H-imidazole substituted ligands (Scheme 71) [108a]. Treatment of these ligands with Pd chloride or bromide generated complexes (150a–b) and (151a–b). Their identities were characterized by spectroscopic methods as well as X-ray crystallography. Compounds (150a) and (150b) represent commonly observed SPO coordinated Pd complexes. Whereas, the formation of the di-Pd complex (151a) or (151b) with direct Pd(I)-Pd(I) bond is noteworthy. These ligands have been studied in Pd-catalyzed Suzuki–Miyaura cross-coupling reactions (Ackermann et al. reported application of such ligands in Kumada-Corriu cross coupling reaction) [108b]. The coupling of aryl bromides with phenylboronic acid were effectively converted

8. Electronic properties and bonding modes of SPOs and ligated Pd complexes Basically, trialkyl- or triaryl-phosphines are electron-rich ligands which donate electron densities towards metals during the coordination processes. Nevertheless, the tautomeric form of the secondary phosphine oxide (phosphinous acid) is less electron-rich than the conventional tri-substituted phosphine presumably due to one of the electron-donating R being replaced by a hydroxyl group. The coordination of ligand towards metal also causes the release of electron density from the phosphorus atom. A thoughtful examination of the change of electron-density in the phosphorus atom before and after the coordination will provide us with a direct insight into the extent of the P M bonding.

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799

Scheme 71.

8.1. DFT studies on the charges of selected tri-substituted phosphines and SPOs and their corresponding Pd complexes Diverse forms of phosphinous acids (PAs) coordinated Pd or Pt complexes had been reported as pre-catalysts in various crosscoupling reactions (Fig. 6) [12,17,40,43,44,50,67,77,78,109]. The state-of-the-art density functional theory (DFT) methods with various functionals were utilized to examine the electronic properties of several selected tri-substituted phosphines and SPOs as well as their corresponding Pd complexes. Throughout the calculations, the abbreviations of the species involved in the reaction are as follows: P stands for Phosphine ligand, S for SPO ligand, M for Mononuclear, B for Binuclear, A for Acetate, a and b for t Bu and Ph, t and c for trans- and cis-, respectively. Obviously, the SPO ligand exhibits more diverse bonding modes than that of commonly used trialkyl- or triaryl-phosphines. Several DFT methods were employed with various functionals in the calculations. The results were compared with the experimental data for the purpose of finding the best fit to the selected model compound. As shown, the structural parameters of two known compounds, A and B, are compared with the results from the calculations via different levels of theory. All calculations were carried out using the Gaussian 09 package. Various kinds of functionals (B3LYP) [110], CAM-B3LYP [111], B3P86 [112], B3PW91 [113] and M06 [114] were employed with the basis set LACVP(d), or the LANL2DZ [115] including the double-␨ basis sets for the valence and outermost electrons. An effective core potential for core electrons was used for Pd. Basis 6−31G(d) was employed for the rest of the atoms, to evaluate the adequacy of the computation level. During the process of geometric optimization, the designated cis-form of A was eventually converted to its corresponding trans-form. Therefore, only trans-form of bis-phosphine coordinated mononuclear complex such as A was taken into consideration in the computational process. As revealed in Table 2, B3P86 is the most suitable functional used in this system. Therefore, B3P86 was employed in all calculations hereafter. All molecules were fully optimized at the B3P86 level with the basis set LANL2DZ(f) including ECP for Pd and the 6−31G(d,p) for the C, H, O, P and Cl. Subsequently, the NPA (Natural

Population Analysis) was calculated using the extended basis sets of LANL2TZ(f) and 6−311+G(d,p) for Pd and other atoms, respectively. The Natural Population Analysis (NPA) of selected trisubstituted phosphines and SPOs as well as their corresponding Pd complexes is listed in Table 3. There are several interesting phenomena to be noted here by comparison with known experimental observations. To begin with, the chemical shift of P(t Bu)3 in 31 P NMR is shifted much more down field than that of PPh3 [116]. This is consistent with the more positive charge of the former than the latter judging from the NPA values (0.864 vs. 0.839). It is also true for some other SPOs such as P(t Bu)2 (OH) relative to that of PPh2 (OH). Next, the coupling constant (JP-H ) is larger in the case of SPO with the more up-field shifts in their 31 P NMR, i.e. less positive NPA charge, indicating that more electron density surrounds the phosphorus atom. Thirdly, the ligand P(t Bu)3 donates more electron density than that of PPh3 towards Pd metal. This observation is also true for the matching SPOs P(t Bu)2 (OH) and PPh2 (OH), the former donates more electron density than the latter towards the Pd. Thereby, the NPA charge of the Pd in the complex is lower for that with the P(t Bu)3 ligand rather than with PPh3 and with P(t Bu)2 (OH) rather than PPh2 (OH). When comparing the cis-form SaMPdc with its corresponding trans-form SaMPdt, the charges for both palladium atoms are not much different. Therefore, the readiness of the oxidative addition process for the former is mostly due to the conformation rather than electronic effect. Furthermore, the charge of the palladium metal in a di-nuclear palladium complex is normally higher than that of a mono-nuclear. 8.2. 31 P NMR of selected phosphines and SPOs coordinated Pd complexes The location of the chemical shift of phosphorus atom in 31 P NMR normally reveals the electronic status surrounding its proximity. 31 P NMR of selected SPOs and their corresponding Pd complexes as well as tri-substituted phosphines and phosphine oxides are depicted in Fig. 7. Normally, the chemical shifts for SPOs in the range from 0 to 50 ppm. It is obvious that SPO is less electron-rich than the commonly used tri-alkyl or aryl phosphines. A significant down-field shift to 90–120 is observed while coordinates to

800

T.M. Shaikh et al. / Coordination Chemistry Reviews 256 (2012) 771–803

Fig. 6. Nomenclature of selected model compounds of tri-substituted phosphines and SPOs as well as their corresponding Pd complexes.

Table 2 Structural parameters obtained from crystal data and calculations.

B3LYP/LACVP(d)

CAM-B3LYP/LACVP(d)

B3P86/LACVP(d,p)

B3PW91/LACVP(d)

M06/LACVP(d)

A Pd–P1 Pd–P2 Pd–Cl1 Pd–Cl2 ∠ Cl1–Pd–P1 ∠ Cl1–Pd–P2 ∠ Cl2–Pd–P1 ∠ Cl2–Pd–P2 Pd–P1 Pd–P2 Pd–Cl1 Pd–Cl2 P1–O1 P2–O2 ∠ Cl1–Pd–P1 ∠ Cl1–Pd–P2 ∠ Cl2–Pd–P1 ∠ Cl2–Pd–P2

2.372 2.372 2.311 2.311 92.37 82.63 82.63 92.37 B 2.345 2.345 2.313 2.313 1.613 1.613 88.98 91.02 91.02 88.98 180.0

2.411 (0.039) 2.411 (0.039) 2.375 (0.064) 2.375 (0.064) 92.26 (0.11) 87.74 (5.11) 87.73 (5.10) 92.27 (0.10)

2.384 (0.012) 2.383 (0.011) 2.347 (0.036) 2.347 (0.036) 92.30 (0.07) 87.70 (5.07) 87.69 (5.06) 92.31 (0.06)

2.370 (0.002) 2.370 (0.002) 2.344 (0.033) 2.344 (0.033) 92.34 (0.03) 87.66 (5.03) 87.65 (5.03) 92.34 (0.03)

2.378 (0.006) 2.378 (0.006) 2.349 (0.038) 2.349 (0.038) 92.43 (0.05) 87.55 (4.92) 87.57 (4.94) 92.46 (0.09)

2.371 (0.001) 2.380 (0.008) 2.362 (0.051) 2.365 (0.054) 92.19 (0.18) 87.57 (4.94) 86.48 (3.85) 93.51 (1.14)

2.417 (0.072) 2.417 (0.072) 2.396 (0.083) 2.396 (0.083) 1.644 (0.031) 1.644 (0.031) 88.41 (0.57) 91.59 (0.57) 91.59 (0.57) 88.41 (0.57) 180.0 (0.00)

2.389 (0.044) 2.388 (0.043) 2.367 (0.054) 2.367 (0.054) 1.634 (0.021) 1.633 (0.020) 88.32 (0.66) 91.65 (0.63) 91.71 (0.69) 88.32 (0.66) 179.96 (0.04)

2.380 (0.035) 2.380 (0.035) 2.364 (0.051) 2.364 (0.051) 1.636 (0.023) 1.636 (0.023) 88.36 (0.62) 91.64 (0.62) 91.64 (0.62) 88.36 (0.62) 180.0 (0.00)

2.391 (0.046) 2.391 (0.046) 2.370 (0.057) 2.370 (0.057) 1.639 (0.026) 1.639 (0.026) 88.34 (0.64) 91.66 (0.64) 91.66 (0.64) 88.34 (0.64) 180.0 (0.00)

2.392 (0.047) 2.392 (0.047) 2.382 (0.069) 2.382 (0.069) 1.631 (0.018) 1.631 (0.018) 88.40 (0.58) 91.60 (0.58) 91.60 (0.58) 88.40 (0.58) 180.0 (0.00)

The number in parentheses represents the difference between bond length or angle from the crystal structure and calculated value.

T.M. Shaikh et al. / Coordination Chemistry Reviews 256 (2012) 771–803

801

Table 3 The Natural Population Analysis (NPA)a charges of the atoms of selected tri-substituted phosphines and SPOs and their corresponding palladium complexes. Pa PaMPdt PaBPd Sa SaMPdc SaMPdt SaBPd SaBPd’ SaMAPd

P(0.864) P(1.123), Pd(0.455), Cl(−0.552) P(1.213), Pd(0.480), Cl1(−0.506) C1-␮(−0.430) P(1.083) P1(1.480), P2(1.433), Pd(0.317), Cl(−0.556) P(1.410), Pd(0.322), Cl(−0.550) P(1.554), Pd(0.410), Cl (−0.507), Cl-␮(−0.453) P(1.556), Pd(0.253), Cl(−0.585) P1(1.526), P2(1.578), Pd(0.306)

Pb PbMPdt PbBPd Sb SbMPdc SbMPdt SbBPd SbBPd SbMAPd

P(0.839) P(1.086), Pd(0.331), Cl(−0.543) P(1.179), Pd(0.431), Cl1(−0.504) C1-␮(−0.427) P(1.061) P1(1.415), P2(1.362), Pd(0.279), Cl(−0.550) P(1.358), Pd(0.290), Cl(−0.556) P(1.491), Pd(0.398), Cl (−0.518), Cl-␮(−0.446) P(1.488), Pd(0.224), Cl(−0.573) P1(1.463), P2(1.525), Pd(0.291)

Calculated using Gaussian G09 B.01

Fig. 7.

31

P NMR of selected phosphorus containing compounds.

Pd complex. The formation of tautomeric form (phosphinous acid) during coordination could be observed by the disappearance of the distinct coupling constant JP-H , ranging from 400 to 700 Hz, for phosphorus and hydrogen atoms of the secondary phosphine oxide.

was mostly provided by the National Center for High-Performance Computing (NCHC). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ccr.2011.11.007.

9. Conclusion References The secondary phosphine oxides (SPOs) represent a new category of promising phosphine ligands which has been employed in various metal-catalyzed cross-coupling reactions. Although the catalytic performance for most SPOs known so far do not surpass their tri-alkyl or aryl counterparts, the advantage of the former in terms of stability towards air and moisture makes them practical in use for either academic or industrial as authentic phosphine ligands. Furthermore, some of the SPOs, apart from the conventional tri-substituted phosphines, exhibit unique bonding modes towards coordination with transition metals due to its hydroxyl group which makes it a subject worthy of further exploration.

Acknowledgments We thank the National Science Council of the ROC (Grant NSC 95-2113-M-005-015-MY3) for the financial support. The CPU time

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