Coordination-induced skeletal rearrangements of zirconacyclobutene–silacyclobutene fused complexes

Coordination Chemistry Reviews 270–271 (2014) 2–13

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

Review

Coordination-induced skeletal rearrangements of zirconacyclobutene–silacyclobutene fused complexes Jing Zhao a , Shaoguang Zhang b , Wen-Xiong Zhang b , Zhenfeng Xi b,∗ a

Beijing National Laboratory of Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190, China Beijing National Laboratory of Molecular Sciences (BNLMS), Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, College of Chemistry, Peking University, Beijing 100871, China b

Contents 1. 2.

3.

4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nitrile-induced skeletal rearrangement and synthetic application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Coordination with less bulky nitriles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1. Formation of Zr/Si-containing three-ring fused complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2. Synthetic applications of the azazirconacyclic intermediates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Coordination with bulky nitriles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1. Formation of azasilacyclopentadiene–zirconacyclopropene complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2. Ligand-substitution-induced skeletal rearrangement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3. Ph2 CHCN-induced skeletal rearrangement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Other unsaturated substrate-induced skeletal rearrangement and synthetic application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Coordination with monoalkynes, poly-ynes and bis(alkynyl)silanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Coordination with ketones, aldehydes and isocyanates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary and future outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

a r t i c l e

i n f o

Article history: Received 21 June 2013 Accepted 20 August 2013 Available online 31 August 2013 Keywords: Zirconacycle Silacycle Metallacyclobutene Zirconacyclobutene–silacyclobutene fused complex Coordination Skeletal rearrangement

2 4 4 4 5 7 7 7 9 10 10 11 11 12 12

a b s t r a c t Coordination-induced skeletal rearrangement of transition-metal complexes is of fundamental interest both synthetically and mechanistically. Besides commonly observed ligand substitution, the coordination of ligands may greatly alter the steric and electronic environment around the metal center, thus activating the whole compound, resulting in novel rearrangement or cleavage of chemical bonds. Subsequent formations of chemical bonds would lead to synthetically useful methods for new transition-metal complexes and organic compounds. Zirconacycles have been proved to be very useful for synthetic organic and organometallic chemistry. The present review concentrates on the coordination-induced skeletal rearrangement of zirconacyclobutene–silacyclobutene fused complexes. When treated with nitriles, alkynes, ketones, or other unsaturated organic molecules, the zirconacyclobutene–silacyclobutene fused complexes underwent the coordination-induced rearrangement and new chemical bond forming process, other than the expected direct insertion reactions. Different types of rearrangements were observed depending on the coordinating unsaturated organic molecules. A variety of zirconacycles and organic products were formed efficiently from the zirconacyclobutene–silacyclobutene fused complexes and the unsaturated organic molecules through the coordination-induced rearrangement and new chemical bond forming process. © 2013 Elsevier B.V. All rights reserved.

1. Introduction

∗ Corresponding author. Tel.: +86 10 62759728. E-mail address: [email protected] (Z. Xi). 0010-8545/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ccr.2013.08.032

Cyclic organometallic compounds (metallacycles) have proved to be prominent in the field of organometallic and coordination chemistry, especially when they are considered as reactive intermediates in metal-promoted or catalyzed reactions [1,2]. Metallacyclobutenes have been proposed as intermediates in

J. Zhao et al. / Coordination Chemistry Reviews 270–271 (2014) 2–13

M

M

M X

¢°

3

¢¢ M = Co, Pt, Ti, Cr, M = Co, Cr, Mo, Ni, Ir, Mn, Mo, Re, Pt, Re, Ru, Ti, Pd, Ta, Zr, V, Rh X = C, N, O W

¢£ M = Fe

Scheme 3. Formation of the zirconacyclobutene–silacyclobutene fused complexes 2.

¢Ò M

M

M

M

M

M

X

M

¢§

¢•

¢¶

¢ß

¢®

M = Ti, Zr

M = Ti

M = Ti, Zr X = Si

M = Ir

M = Ti

¢Ú Scheme 1. Two major types of metallacyclobutenes.

Fig. 1. Molecular structure of a zirconacyclobutene–silacyclobutene fused complex 2 with (t-BuC5 H4 )2 Zr Moiety. Selected bond lengths [Å]: Zr C(1) 2.202(4), Zr C(3) 2.180(4), Si C(2) 1.883(4), Si C(4) 1.854(5).

Scheme 2. Metallacyclobutenes in metathesis and polymerization reactions.

numerous metal-catalyzed and promoted reactions [3,4], and have been synthesized based on various pathways, including the [2 + 2] cycloaddition reactions of metal carbene complexes with alkyne, ring-contraction reactions of five-membered metallacycles, oxidative addition reactions of metal complexes into cyclopropene, and radical reactions [5]. Metallacyclobutenes can be divided into two major categories: mono metallacycles (Type I) and fused metallacyclobutenes (Type II). Selected examples are shown in Scheme 1. These two classes of four-membered metallacycles are well investigated and used in a growing number of stoichiometric and catalytic reactions [3,4]. Metallacyclobutenes are involved in processes of metathesis and polymer synthesis (Scheme 2) [6], four-membered heterocyclic synthesis [3c], and ring expansion reactions [3d]. Skeletal rearrangement of metallacyclobutenes may occur upon adding or removing coordinative ligands to/from the metal center, but related reports are rare [3h,4e,4j,4k]. In 1995, Takahashi and co-workers reported the generation of zirconacyclobutene–silacyclobutene fused complexes 2, from silicon-tethered diynes 1 and low-valent Cp2 Zr(II) species (Scheme 3) [7]. In this process, unexpected skeletal rearrangements occurred to generate the structures which were unambiguously confirmed by single crystal X-ray structural analysis (Fig. 1). Complex 2 is unique for the concomitant presence of two Zr C bonds and two Si C bonds in the structure. Rosenthal and co-workers reported the synthesis of spirocomplexes 4 from the reaction of tetraalkynylsilanes 3 with Cp2 M(L)(2 -Me3 SiCCSiMe3 )(M = Ti, without L, M = Zr, L = THF) (Scheme 4) [8a]. The zirconacyclobutene–silacyclobutene fused

Scheme 4. Reactions of tetraalkynylsilanes with titanocene or zirconocene complexes.

complex 2j could be generated by reductive cleavage and rearrangement of the zirconacyclopentadiene 5 (Scheme 5) [8b]. This research group has been studying the reaction chemistry of those zirconacyclobutene–silacyclobutene fused–ring complexes 2a–i shown in Scheme 6 since 2001 [9]. Initially, as our continued interest in the synthetic applications of zirconacycles, we treated complexes 2 with various unsaturated compounds, expecting direct insertion reactions followed by ring-expansion.

Scheme 5. Reductive cleavage and rearrangement of complex 5 to form 2j.

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Scheme 6. Complexes 2 discussed in this review.

Scheme 10. Three classes of unsaturated substrates inducing the rearrangement reactions of complex 2.

Scheme 7. A general scheme for the reaction between complex 2 and unsaturated compounds.

coordination ability to become an 18e complex. However, because the spatial environment around the Cp2 Zr center in the zirconacyclobutene moiety becomes more crowded being coordinated with an additional ligand (L), the L-coordinated complex 2 would undergo skeletal rearrangements to release the high strain. Three major classes of unsaturated compounds toward complexes 2 are summarized in Scheme 10. Details of their corresponding reactions are discussed in this review. 2. Nitrile-induced skeletal rearrangement and synthetic application

Scheme 8. Different types of skeletal rearrangements involving complex 2.

However, instead, unexpected skeletal rearrangements of complexes 2 and their corresponding zirconacycles were observed (Scheme 7). Different types of zirconacycles were formed via the coordination-induced skeletal rearrangements, mainly depending on the coordination-ability and bulkiness of the unsaturated compounds (Scheme 8). To describe the coordination-induced skeletal rearrangements, we proposed an initiating-braking-releasing process model (Scheme 9) [10]. Depending on its steric or electronic property, the L may behave as a brake handle to stabilize the Lcoordinated complexes. When the coordinating L is substituted by a different substrate (S), the whole complex will become reactive again. The complex 2 is coordinatively unsaturated with 16e. Thus the Zr center may be readily coordinated with a substrate of good

Scheme 9. An Initiating-braking-releasing process model: an incoming ligand (L) behaves as both an initiator and a brake/release handle. The different shapes around the metal center indicate different structure and bonding.

Nitriles are often used as unsaturated substrates [11], which take an important part in group 4 metal chemistry due to their rich C N insertion reaction [12]. Various kinds of nitriles were attempted to react with the complex 2, including t-BuCN, i-PrCN, and Ph2 CHCN. Initially, it was expected that the reactive organometallic complex 2 would undergo nitrile insertion reaction into the Zr C bonds to afford six-membered azazirconacycles. However, experimental results showed that a nitrile-induced skeletal rearrangement of the complex 2 took place, which was confirmed by the single crystal X-ray structural analysis of thus generated organometallic intermediates (zirconacycles) and organic compounds. The reaction pattern is strongly influenced by the types of nitriles, e.g. less bulky nitriles (such as PhCN), bulky nitriles (such as t-BuCN), and Ph2 CHCN. 2.1. Coordination with less bulky nitriles 2.1.1. Formation of Zr/Si-containing three-ring fused complexes When complexes 2 were treated with three identical less bulky nitriles, complexes 6 were generated, which gave 5-azaindoles 7 after hydrolysis (Scheme 11) [13]. The crystal structure of complex 6a was confirmed by single crystal X-ray structural analysis, which reveals that it consists of three fused rings: one six-membered ring

Scheme 11. Formation of three-ring-fused complex 6.

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5

Fig. 2. Molecular structure of 6a. Selected bond lengths [Å]: Zr1–N1 1.953(6), Zr1–N2 2.205(5), Si1–N3 1.752(8), Si1–C5 1.854(14). Scheme 13. Proposed mechanism for the formation of complex 6.

containing nitrogen and silicon atoms, one five-membered pyrrolo ring and one six-membered azazirconacycle (Fig. 2) [13b,14]. By decreasing the amount of i-PrCN to 1.5 equiv, a green solid 8 with incorporation of two molecules of the nitrile was obtained in high yield (Scheme 12) [13,15]. After hydrolysis, a ketone derivative 9 was obtained. Although the single crystals of complex 8 suitable for X-ray analysis were not obtained, its NMR data and the product of hydrolysis support this structure of complex 8. It is important to investigate what is going on in this one-pot reaction generating complexes 6 and 8, which involved the cleavage of C N triple bond and cleavage of Si C bond [16]. A mechanism is proposed for the formation of complex 6 from complex 2 and i-PrCN (Scheme 13). In the first pathway (pathway a), insertion of C N bonds of two nitriles into the Zr C [12] and Si C [17] bonds of complex 2 generated intermediate 10, which is unstable and would undergo skeletal rearrangement to afford complex 8. In the other pathway (pathway b), complex 2 first underwent skeletal rearrangement due to the coordinative effect of nitrile to the zirconium center to form intermediate 11 (type B in Scheme 8). In the following step, insertion reaction of C N triple bonds into Zr C and Si C bonds would afford intermediate 12, which would undergo skeletal rearrangement to generate complex 8. The insertion reaction of the third nitrile to complex 8 leads to the formation of complex 6. The steric effect also played an important role in this reaction. For example, changing the substituents on the silicon center

Scheme 12. Formation of complex 8 from complex 2 and two molecules of nitriles.

from methyl to ethyl obviously decreased the reaction rate and yields. Meanwhile, when the methyl group on the silicon center was changed to a phenyl group, the corresponding product could not be detected [13]. 2.1.2. Synthetic applications of the azazirconacyclic intermediates It is expected that the intermediate 8 could undergo further reactions because it has a reactive Zr C bond. By adding a different nitrile into the solution of complex 8a, a complex similar to complex 6a was isolated with high yield (Scheme 14) [15]. The structure of complex 6b is confirmed by single crystal X-ray structural analysis. Besides nitriles, many other unsaturated compounds have been utilized to react with complex 2, affording a series of new organometallic complexes and organic compounds. When isocyanides were added into the reaction mixture, different intermediates were obtained depending on isocyanides used (Scheme 15) [18]. Treatment of complex 8a with one equiv of the bulky aliphatic isocyanide t-BuNC at room temperature for 1 h resulted in the formation of the mono-insertion complex 13a in 91% isolated yield. No double-insertion products were obtained when a large excess of t-BuNC were added, probably owing to the steric effect of the t-Bu group. Interestingly, in the presence of a less bulky isocyanide such as CyNC, the double-insertion product 14 was obtained, which was confirmed by single crystal X-ray structural analysis (Fig. 3). Similarly, the double-insertion product 15 could also be generated when 2.4 equiv of 2,6-dimethylphenyl isocyanides were treated with 8 [18]. As stated above, the isolation and characterization of these 2 -iminoacyl–Zr complexes showed that the steric effect of isocyanides strongly affected the insertion reactions toward the Zr C bond and represented an efficient way to prepare 2 -iminoacyl–Zr complexes. When hydrolyzed with water, these mono- and double-insertion intermediates could afford 5azaindoles or dihydropyrrolo[3,2-c]azepines in high yields [18].

Scheme 14. Nitrile-insertion into 8a forming complex 6b.

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Scheme 16. Further application of complex 8a.

Scheme 15. Formation of iminoacyl–zr complexes by insertion of isocyanides into the Zr-C(sp3 ) bond.

In addition to isocyanides, other unsaturated compounds such as acid chlorides and formamides were used to react with complex 8a (Scheme 16) [13b,19]. Reaction of EtCOCl with 8a afforded the corresponding 5-azaindole 16 after hydrolysis. When formamide Me2 NCHO was treated with complex 8a, the corresponding insertion complex 17 was obtained in 86% isolated yield and was structurally characterized by single-crystal structural analysis (Fig. 4). Quench of the complex 17 afforded a different type of 5-azaindole 18 in 70% isolated yield. Azide compounds were also utilized to react with the intermediate 8 [19]. Hydrolysis of the reaction mixture afforded pyrrolo[3,2-d]pyridazine 19 (Scheme 17). In this process, the

Fig. 3. Molecular structure of 14. Selected bond lengths [Å]: Zr1 N1 2.347(2), Zr1 N4 2.245(2), Zr1 C18 2.175(3), Si1 C15 1.924(3), Si1 N2 1.744(3).

Fig. 4. Molecular structure of 17. Selected bond lengths [Å]: Zr1 O1 1.960(2), Zr1 N1 2.220(3), Si1 N2 1.754(3), Si1 C15 1.926(3).

insertion reaction of an azide into the Zr C bond of complex 8 may take place to afford the reactive intermediate 20 or 21, which has been isolated and characterized by NMR spectroscopy [20]. 1,1-Insertion of an azide into the Zr C bond of complex 8 would lead to the formation of the intermediate 20. While 1,3-insertion of an azide into the Zr C bond of complex 8 could also be possible

Scheme 17. Azide-insertion into complex 8.

J. Zhao et al. / Coordination Chemistry Reviews 270–271 (2014) 2–13

Scheme 18. t-BuCN-induced formation azasilacyclopentadiene complex 22.

of

the

7

zirconacyclopropene–

to generate the intermediate 21. It is unclear yet which one is the intermediate, but there is only one intermediate formed as demonstrated by their NMR spectroscopy.

2.2. Coordination with bulky nitriles 2.2.1. Formation of azasilacyclopentadiene–zirconacyclopropene complexes Given in Scheme 13 is a proposed mechanism for the formation of complex 6 from complex 2 and nitriles. However, it was not clear how the complex 8 was formed from 2. It was observed that steric effect played an important role in the reactions of complex 2 with nitriles. Both the substituents on the silicon atom and the groups of the nitriles remarkably affected the reaction processes. Thus, bulky t-BuCN was applied, aiming at understanding how the complex 8 was formed from 2. When one equivalent of t-BuCN was used, a mixture of products was obtained. When 2.2 equiv of t-BuCN were used, clean yellow solids were obtained in high yields (Scheme 18) [10]. An X-ray analysis of complex 22b unambiguously revealed a zirconacyclopropene–azasilacyclopentadiene skeleton (Fig. 5). In this structure, only one t-BuCN is coupled with complex 2 and the other one is coordinated to the zirconium center, which might deactivate the zirconacyclopropene moiety and stabilize the whole structure [21]. In the process forming complex 22, t-BuCN behaves both as initiator to open the four-membered zirconacycle 2 and as a brake in the three-membered zirconacycles 23 and 22 (Scheme 19). In the proposed mechanism, first, t-BuCN would be coordinated to the zirconium center and induce the skeletal rearrangement of complex 2 to generate the reactive intermediate 23 (type C in Scheme 8), which then underwent an insertion reaction with the second t-BuCN to form the complex 22.

Fig. 5. Molecular structure of 22b. Selected bond lengths [Å]: Zr1 C11 2.197(6), Zr1 C12 2.289(7), Si1 C15 1.875(7), Si1 N2 1.760(6).

Scheme 19. Proposed mechanism for the formation of complex 22.

2.2.2. Ligand-substitution-induced skeletal rearrangement Complex 22 has two major moieties, including a five-membered azasilacyclopentadiene and a three-membered zirconacyclopropene stabilized by t-BuCN. Zirconacyclopropene species are known to be very reactive [21]. However, in this case, the bulky t-BuCN is coordinated to the zirconium center and thus deactivates the zirconacyclopropene moiety. Whenever the coordinating t-BuCN is substituted by a smaller substrate, the stabilized zirconacyclopropene moiety will become reactive again, thus generating diversified zirconacycles or even initiating further reactions of the whole molecule including the azasilacyclopentadiene [10,21]. As coordinating ligands, ketones, carbodiimides and alkynes would substitute the bulky t-BuCN ligand in 22. In this way, the zirconacyclopropene moiety will react with those unsaturated organic compounds, affording the five-membered zirconacycles while the azasilacyclopendiene moiety in complex 22 does not take part in the reaction (Scheme 20a–c). When ketones were used, insertion of the C O double bond into the three-membered zirconacycle gave the corresponding oxazirconacyclopentene 24 with high yields [22]. Similarly, C N double bond insertion occurred when N,N diisopropylcarbodiimide was used [23]. In addition to C O and C N double bonds, the C C triple bond of alkynes reacted also with the three-membered zirconacycle to generate the corresponding zirconacyclopentadiene 26 [24]. Demetalation took place when elemental sulfur (S8 ) was used to react with complex 22, generating the alkynylazasilacyclopentadiene 27 (Scheme 20d). Compound 27 could also be generated by using CO or I2 . When complex 22 was treated with acid chloride, the whole molecule took part in the rearrangement process (Scheme 20e). Both aromatic and aliphatic acid chlorides could undergo this reaction to generate the silacycle ring expansion product azasilacyclohexadienes 28 (Fig. 6). In this reaction, the acid chloride RCOCl would replace the coordinating t-BuCN and then react with the zirconacyclopropene moiety. The azasilacyclopentadiene moiety participates in the following skeletal rearrangement to afford the complex 28 (Scheme 21).

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Scheme 21. Proposed mechanism for the formation of complex 28.

Scheme 22. Formation of complex 29 from complex 22 with less bulky nitrile.

Scheme 20. Reactions of complex 22.

When the bulky t-BuCN in the complex 22 was substituted by a less bulky nitrile, skeletal rearrangement of 22 took place again, forming the complex 29 (Scheme 22), which was confirmed by single crystal X-ray structural analysis (Fig. 7) [10]. The unprecedented

Fig. 6. Molecular structure of 28a. Selected bond lengths [Å]: Zr1 O1 1.938(2), Zr1 Cl1 2.4687(12), Si1 C1 1.919(4), Si1 N1 1.744(3).

product possesses a five-membered azasilacyclopentene-fused seven-membered azazirconacycle possessing an allenyl moiety [25]. The incoming nitriles could be a variety of aromatic and heteroaromatic nitriles. A proposed mechanism is shown in Scheme 23. In the first step, the incoming nitrile RCN replaces

Fig. 7. Molecular structure of 29a. Selected bond lengths [Å] and angles (◦ ): Zr1 C1 2.379(13), Zr1 N2 2.032(10), C1 C2 1.331(17), C2 C3 1.365(18), C1 C2 C3 175.7(13).

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Scheme 23. Proposed mechanism for the formation of complex 29. Scheme 26. Formation of complex 34 from complex 29 and Ph2 CHCN.

Scheme 24. Formation of complex 32 from complex 29 and a third nitrile.

the coordinating t-BuCN and reactivates the zirconacyclopropene. The smaller nitrile RCN could undergo the insertion reaction to afford its corresponding azazirconacyclopentadiene 31 [14]. Skeletal rearrangement of 30 or 31 would then give the complex 29. Further rearrangement of the complexes 29 was induced by the addition of another nitrile [26]. With a third nitrile reacting with complex 29 at a higher temperature, a three ring-fused Zr/Si-containing complex 32 was isolated, which was also characterized by single crystal X-ray structural analysis (Scheme 24). This structure clearly showed that the t-Bu group from the first nitrile was fixed at the 4-position while the second and the third nitriles were fixed at the 2-position and 6-position on the pyridine ring, respectively. A proposed mechanism for the formation of complex 32 from complex 29 is given in Scheme 25. The third RCN unit could insert into the Zr C bond of the complex 29 to afford the nine-membered azazirconacycle 33. This intermediate 33 would undergo intramolecular nucleophilic attack or a 1,3-silyl shift to generate the final three-ring fused complex 32, which afforded 5-azaindoles upon hydrolysis [26].

(Scheme 26) [27]. In this reaction, the above-mentioned direct insertion of the C N triple bond into the Zr C bond in 29 was not observed. The unexpected complex 34 was obtained through proton transfer of Ph2 CHCN and intramolecular coupling reaction. X-ray analysis of complex 34 reveals that it consists of three fused rings: one five-membered azasilacyclopentene, one fivemembered azacyclopentene, and one four-membered ring which contains a neutral N-donor ligand (Fig. 8). There are two possible mechanisms for the formation of complex 34 (Scheme 27). First, the CN group of Ph2 CHCN is coordinated to the zirconium center to generate intermediate 35. 35 undergoes intramolecular proton transfer to yield the intermediate 36 [28]. Then, intramolecular hydroamination of the allenyl moiety takes place to form the zwitterionic complex 37 [29]. Through further intramolecular proton transfer, the final product 34 was generated (pathway a). In another mechanism, the zwitterionic intermediate 39 might be formed [28]. Then intramolecular rearrangement of 39 would result in the formation of complex 34 [29]. The reactivity of complex 34 toward propargyl bromide and propionyl chloride was investigated (Scheme 28). The keteniminate ligand of complex 34 was replaced by halogen atoms, yielding complexes 40 and 41. Inspired by the above results [27], reaction of complex 2 with Ph2 CHCN was also explored [30]. Zirconocene complexes 44 were generated in high yields (Scheme 29) [30,31]. An X-ray analysis of 44 revealed that the structure includes the ketenimine unit and one chelating vinyl-imine fragment (Fig. 9). A proposed mechanism is shown in Scheme 30. First, Ph2 CHCN behaved normally as a strong coordination ligand to induce the skeletal rearrangement of zirconacyclobutene (type D in Scheme 8), generating azazirconacyclopentadiene 45 in situ [32]. The successive coordination of the second nitrile to the intermediate 45 and intramolecular proton transfer would afford the final complex [28].

2.2.3. Ph2 CHCN-induced skeletal rearrangement Ph2 CHCN, which has an ˛-acidic hydrogen, was found to react with the complex 29 in a different way from other nitriles

Scheme 25. Proposed mechanism for the formation of the three-ring fused complex 32.

Fig. 8. Molecular structure of 34. Selected bond lengths [Å] and angles (◦ ): Zr1 N1 2.222(5), N1 C41 1.175(6), C41 C42 1.395(7), Zr1 C11 2.339(5), Zr1 N2 2.465(4), Si1 N3 1.738(5) N1 C41 C42 177.2(7).

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Fig. 9. Molecular structure of 44a. Selected bond lengths [Å] and angles (◦ ): Zr1 C25 2.407(4), Zr1 N2 2.265(4), Zr1 N1 2.290(4), N1 C11 1.159(5), C11 C12 1.386(6), N1 C11 C12 177.7(5).

3. Other unsaturated substrate-induced skeletal rearrangement and synthetic application 3.1. Coordination with monoalkynes, poly-ynes and bis(alkynyl)silanes Coordination of an alkyne to complex 2 was found to induce a different skeletal rearrangement from those observed with nitriles (Scheme 31). This reaction afforded the zirconacyclohexadiene–silacyclobutene fused complexes 46, seemingly formed via a direct insertion reaction of the alkyne into the Zr C bond in 2. However, it was found out that the formation of the complex 46 was caused by the alkyne-induced skeletal rearrangement via the intermediate of alkynylsilyl-substituted zirconacyclopentadienes 47 (Scheme 31). The complex 47 was generated at 50 ◦ C and quantitatively transformed into the six-membered complex 46 at a higher temperature [33,34]. Scheme 27. Proposed mechanisms for the formation of complex 34.

Scheme 28. Formation of zirconocene complex 40 and 41 by reactions of 34 with propargyl bromide or propionyl chloride.

Scheme 29. Ph2 CHCN-induced skeletal rearrangement of complexes 2.

Scheme 30. Proposed mechanism for the reactions of complex 2 and Ph2 CHCN.

J. Zhao et al. / Coordination Chemistry Reviews 270–271 (2014) 2–13

11

Scheme 31. Alkyne-induced skeletal rearrangement.

Two pathways were proposed for the formation of complex 47 from complex 2 with alkynes (Scheme 32). One is the associative path (pathway a), the other is a dissociative path (pathway b) (type A and D in Scheme 8). In pathway b, the coordination of the alkyne to the zirconium center is essential for inducing the rearrangement of complex 2. Based on reactions described above, a series of -conjugated organic systems containing two or three silacyclobutene units were efficiently synthesized (Scheme 33a). After hydrolysis of complex 49, star-shaped tris(silacyclobutene)-containing benzenes were obtained, which are potentially useful for electronic and optoelectronic applications [35]. When the Si-tethered diyne was treated with complex 2, similar products were generated (Scheme 33b). Formation of the 2,5bis(alkynylsilyl)zirconacyclopentadiene 50 was observed when the reaction was carried out at 50 ◦ C. When the reaction temperature was increased to 90 ◦ C, the zirconacyclohexadiene-silacyclobutene fused complex 51 was formed from the alkyne-coordinationinduced skeletal rearrangement of the complex 50 [36]. Further insertion reaction of the remaining C C triple bond into the Zr C bond in complex 51 was not observed even at higher temperatures and longer reaction time.

Scheme 33. Reactions of complex 2 with poly-ynes, and bis(alkynyl)silanes.

Scheme 34. Reactions of complex 2 with ketones, aldehydes and isocyanates.

3.2. Coordination with ketones, aldehydes and isocyanates Similar to the above alkyne-induced rearrangement, carbonyl compounds such as ketones, aldehydes and isocyanates were also found to induce rearrangement of complex 2 (Scheme 34) [37]. When ketones and aldehydes were treated with complex 2, the corresponding five-membered oxazirconacyclopentenes 52 were formed. When isocyanates were used, the five-membered zirconacycles 54 or 55 were generated. Unlike the above alkyne-induced skeletal rearrangement, the alkynylsilyl-substituted oxa- or azazirconacyclopentenes were very stable and did not undergo further rearrangement. 4. Summary and future outlook

Scheme 32. Proposed mechanisms for the formation of complex 47.

In this review, we summarized coordination-induced skeletal rearrangements of zirconacyclobutene–silacyclobutene complexes

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2. Compared with five-membered metallacycles such as zirconacyclopentadienes [2c,2l,24], four-membered zirconacyclobutenes should have more crowded spatial environment around the metal center. Although both the five-membered and the four-membered metallacycles are coordinatively unsaturated, upon coordination with a ligand, the four-membered metallacycles would become more crowded and would undergo skeletal rearrangements or insertion reactions to release the high strain. In the case of zirconacyclobutene–silacyclobutene fused complexes 2, skeletal rearrangements were found to be the major reaction, probably as the result of large ring strain of the fused four-membered ring. The presence of the bulky cyclopentadienyl moieties could also exert additional strain that would contribute to the higher reactivity of the Zr C bonds. It is not clear yet why the strained silacyclobutene moiety in complex 2 does not undergo insertion reactions. Perhaps low reactivity (and polarity) of the Si C bond might be one of the reasons. However, adding different structural features to it could electronically or sterically (or both) activate the Si C bond and thus enable its participation in further reactions. If such an insertion reaction could be initiated, new methods for the synthesis of silacycles could be expected [38,39].

Acknowledgements We thank past and present co-workers and students involved in this research group’s long-term study on metallacyclic reagents and mechanism-based synthetic organometallic chemistry and organic chemistry. Financial support from the Natural Science Foundation of China, the 973 Program (2011CB808700), the State Key Laboratory of Organometallic Chemistry, and Peking University is gratefully acknowledged.

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