Cr-bond metathesis mechanisms

Besides the cr-bond metathesis mechanism proposed by Tilley23 for the dehydrogenative coupling of silanes, a Zr(II) pathway25 and a silylene mechanism26 have been proposed based on the nature of the products. The dehydrogenative polymerization of 1,2,3-trimethyltrisilane or of a mixture of diastereomers of 1,2,3,4-tetramethyltetrasilane showed evidence that, besides Tilley s mechanism, a further mechanism is present. The product formation can be explained by a silylene mechanism where the silylenes are formed by a-elimination from the silyl complexes by a new type of /J-elimination which involves Si—Si bond cleavage (/F-bond elimination) as described in Scheme 727. 

T. D. Tilley, Acc. Chem. Res., 26, 22 (1993). The Coordination Polymerization of Silanes to Polysilanes by a cr-Bond Metathesis Mechanism. Implications for Linear Chain Growth. 

In contrast to this report, a surprising result was obtained by Chen and co-workers by studying with electrospray tandem mass spectrometry the C-H activation in -pentane, cyclohexane, and benzene by Bergman s catalyst [Cp Ir(PMe3)Me]. Neither the oxidative addition/reductive elimination nor the concerted cr-bond metathesis mechanism was found to be operative. Rather, an elimination/addition mechanism through Ir(m) intermediates was proposed and confirmed by isotopic labeling. 

Scheme 13 Generalized mechanism for the alkene insertion and cr-bond metathesis reactions catalyzed by lanthanocenes...

A proposed mechanism of the bis(allene) cyclization involves the formation of the allyl(stannyl)palladium species 6, which undergoes carbocyclization to give vinyl(stannyl)palladium intermediate 7 (Scheme 36). Reductive elimination and cr-bond metathesis may lead to the formation of the m-pentane derivative and the bicyclic product, respectively. The cyclization of allenic aldehydes catalyzed by a palladium complex was also reported.163... 

The reactions catalyzed by cationic palladium complexes are believed to proceed via a different mechanism (Scheme 67).273 Initially, a cationic silylpalladium(n) species is generated by cr-bond metathesis of the Br-Pd+ with a silylstannane. Subsequently, the alkyne and alkene moieties of the 1,6-diyne successively insert into the Pd-Si bond to form a cationic alkylpalladium(n), which then undergoes bond metathesis with silylstannane to liberate the product and regenerate the active catalyst species, S/-Pd+. 

In the end, while computations on these alternative pathways, oxidative addition and cr-bond metathesis, have provided some insight, the question by which way does a particular reaction proceed cannot yet be answered definitively. It is very interesting, however, that both mechanisms involve the same Pt(II) intermediate in which the hydrocarbon binds in the square-planar coordination sphere of the metal. 

Yamamoto has proposed a mechanism for the palladium-catalyzed cyclization/hydrosilylation of enynes that accounts for the selective delivery of the silane to the more substituted C=C bond. Initial conversion of [(77 -C3H5)Pd(GOD)] [PF6] to a cationic palladium hydride species followed by complexation of the diyne could form the cationic diynylpalladium hydride intermediate Ib (Scheme 2). Hydrometallation of the less-substituted alkyne would form the palladium alkenyl alkyne complex Ilb that could undergo intramolecular carbometallation to form the palladium dienyl complex Illb. Silylative cleavage of the Pd-G bond, perhaps via cr-bond metathesis, would then release the silylated diene with regeneration of a palladium hydride species (Scheme 2). 

Diyne cyclization/hydrosilylation catalyzed by 4 was proposed to occur via a mechanism analogous to that proposed for nickel-catalyzed diyne cyclization/hydrosilylation (Scheme 4). It was worth noting that experimental evidence pointed to a silane-promoted reductive elimination pathway. In particular, reaction of dimethyl dipropargylmalonate with HSiMc2Et (3 equiv.) catalyzed by 4 led to predominant formation of the disilylated uncyclized compound 5 in 51% yield, whereas slow addition of HSiMe2Et to a mixture of the diyne and 4 led to predominant formation of silylated 1,2-dialkylidene cyclopentane 6 (Scheme 5). This and related observations were consistent with a mechanism involving silane-promoted G-H reductive elimination from alkenylrhodium hydride species Id to form silylated uncyclized products in competition with intramolecular carbometallation of Id to form cyclization/hydrosilylation products (Scheme 4). Silane-promoted reductive elimination could occur either via an oxidative addition/reductive elimination sequence involving an Rh(v) intermediate, or via a cr-bond metathesis pathway. 

In the hydrosilylation of alkenes catalyzed by Group IV metallocene complexes, Cp2MCl2/2BuLi(M = Zr, Hf, Ti)37,38 (vide supra), the olefin-first mechanism including cr-bond metathesis of r/2-alkene-MCp2 and HSiR3 is proposed by Kesti and Waymouth (Scheme 9)37. After the formation of /J-silylalkyl—M—H species 56 via cr-bond metathesis... 

Metallacyclobutene complexes of both early and late transition metals can, in some cases, be prepared by intramolecular 7-hydrogen elimination, although the intimate mechanism of the reaction varies across the transition series. For low-valent late metals, the reaction is generally assumed to proceed via the oxidative addition of an accessible 7-C-H bond (Scheme 28, path A), but for early metals and, presumably, any metal in a relatively high oxidation state, a concerted cr-bond metathesis is considered most probable (path B). In this process, the 7-C-H bond interacts directly with an M-X fragment (typically a second hydrocarbyl residue) to produce the metallacycle with the extrusion of H-X (i.e., a hydrocarbon). Either sp3- or spz-hybridized C-H bonds can participate in the 7-hydrogen elimination. 

Computational studies performed on model complexes in collaboration with Hall and coworkers suggest that alkane borylation may occur by a ej-bond metathesis pathway (Scheme 3) [48]. The proposed mechanism for the borylation of alkanes by 1 begins with elimination of HBpin to generate the 16-electron complex Cp Rh(Bpin)2. This complex then forms a <7-complex (3) with the alkane. The vacant p-orbital on boron then enables cr-bond metathesis to generate a o-borane complex (4). Reductive elimination of the alkylboronate ester product and oxidative addition of B2pin2 then regenerate 1. 

Almost all of the reactions of metals can be classified into just a few typical reactions, and the reactions that metals promote in organic chemistry are simple combinations of these typical reactions. If you learn these typical reactions, you will have no trouble drawing metal-mediated mechanisms. The typical reactions of metal complexes are ligand addition/ligand dissociation/ligand substitution, oxidative addition/reductive elimination, insertion/j8-elimination, a-insertion/ a-elimination, cr-bond metathesis (including transmetallations and abstraction reactions), [2 + 2] cycloaddition, and electron transfer. 

There are several mechanisms for termination of the growing polymer chain. One common chain termination step is jS-hydride elimination to give Cp2ZrH+ and the polymer with a terminal double bond. When the polymerization is carried out under H2, a cr-bond metathesis can take place to give the saturated polymer and Cp2ZrH +. ... 

Intramolecular heterolysis of H2 is closely related to the well-known cr-bond metathesis processes (Scheme 20.4) that generally occur on less electrophilic centers, especially d systems [2, 5a]. Although the heterolytic process here is formally a concerted ionic splitting of H2, as often illustrated by a four-center intermediate with partial charges, the mechanism does not have to involve such charge localization. In other words the two electrons originally present in the H-H bond do not necessarily both go into the newly-formed M-H bond while a bare proton transfers onto L or, at the opposite extreme, an external base. The term cr-bond metathesis is thus actually a better description and may comprise more transition states than the simple four-center intermediate shown in Scheme 20.4, for example initial transient coordination of H2 to the metal cis to L and dissociation of transiently bound H—L as the final step. 

SCHEME 11.27 Proposed mechanism for Lu-mediated C—H activation, termed cr-bond metathesis. 

Scheme 4 Early metal hydrosilation mechanism Involving cr-bond metathesis...

The proposed mechanism for the formation of the polystannanes involves cr-bond metathesis and is analogous to that believed to operate for the dehydrocoupling route to polysilanes from primary silanes RSiH3 by means of... 

An unsaturated metal hydride derivative which is formed during an induction period is postulated as the catalytically active species. The mechanism includes two cr-bond metathesis steps, each passing through a four-centered transition state. 

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