Four-center transition state metathesis

High enantioselectivity (ca. 95-99% ee) is observed in this system, better than that revealed in previous reports of the hydrosilylation of imines. The mechanism is as yet unclear however, the authors propose that an active catalyst may be formed by cleavage of the Ti-F bond and generation of a Ti(III) hydride species. Insertion of an imine into the Ti-H bond, followed by a (r-bond metathesis with the silane in a four-centered transition state, may lead to the observed products. Another report on the activity of titanocene complexes as catalysts for the hydrosilylation of aid- and keti-mines also indicates formation of a Ti-H species as catalyst.188 Hydrosilylation proceeds to yield silylamines, with dependence on substitution at nitrogen and on the nature of the ligand bound to the metallocene precursor. 

Neutral dimethyl metallocenes of group 4, Cp2MMe2, are effective catalysts for the dehydropolymerization of primary silanes, via a a bond metathesis pathway.234 Given the topological similarities between the four centered transition states for olefin insertion into M-C bonds and a bond metathesis reactions, it was postulated that activation of these metallocenes by B(C6F5)3 might enhance dehydropolymerization of silanes in... 

SCHEME 18.4 Mechanism suggested by Don Tilley et al. for the dehydrocoupling of RSiHj using CpCp HfHCl (Cp = T15-C5H5 Cp = Ti -CjMej ) as a catalyst. The catalytic cycle involves two o-bond metathesis reactions that pass through four-center transition states. " ... 

A wide variety of neutral, unsaturated, d° and d°f metal alkyls undergo bond metathesis reactions in which H2 or substrate C—H bonds are activated 22,23,73,87,105-107). These reactions are believed to proceed via four-center transition states that are accessed by initial coordination of the H—H or C—H bonds to the electrophilic metal center [Eq. (34)]. The... 

Transmetalation may proceed via a concerted cr bond metathesis, involving a four-center transition state (38) (or even a metallacyclobutane intermediate) that leads to transfer of the organic group R to M with retention of configuration. 

Presumably, the oxidative cyclization of 1 commences with direct palladation at the orfAo-position, forming o-arylpalladium(II) complex 3 in a fashion analogous to a typical electrophilic aromatic substitution (this notion is useful in predicting the regiochemistry of oxidative cyclizations). The mechanism of the second formal C—H bond functionalization step is not fully elucidated, but may occur either via (a) an intramolecular carbopalladation reaction (migratory insertion) followed by czHft-P-hydride elimination from 4 (Path A) (b) by o-bond metathesis (through a four-centered transition state) followed by reductive elimination (Path B) (c) by electrophilic aromatic substitution followed by C—C bond-forming reductive elimination (PathC) [9]. 

The activation of C—H bonds by a o-bond metathesis pathway involves coordination of the C—H group followed by concerted C—H bond breaking of the activated substrate and C—H bond formation with a hydrocarbyl ligand. Thus, the reaction proceeds through a four-centered transition state (Scheme 11.8). 

In this section we reviewed the latest theoretical studies on the metathesis reaction. In general, it has been shown that the metathesis reaction at first yields adduct complex then activation of the H-R bond takes place with a four-center transition state, which leads to the second adduct. It occurs more easily for scandium complexes than for lutetium analogues. The a-metathesis reactivity between R-H and M-R bonds decreases in the order of R C2H... 

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. 

Si-H-M intermediates have also been assumed to be the immediate precursors to the four-center transition state proposed in 0 bond metathesis reactions [89]. 

In this reaction, a C—H bond in methane is made, and a C—H bond of benzene is cleaved, but the oxidation state of the scandium center remains +3. This class of reaction, which is not hmited to early transition metals, is called sigma-bond metathesis. In this mechanism, the metal is first postulated to coordinate the bond to be activated in an rf- fashion, followed by formation of a four-centered transition state that leads to an exchange of ligands at the metal (Figure 14.8). 

The proposed mechanism for the formation of the polystannanes involves <7-bond metathesis and is thought to be analogous to that for the catalytic dehydrocoupling route to polysilanes from primary silanes RSiH3. The Zr precatalyst is believed to generate a metal hydride species that participates in a step-growth process that involves four-centered transition states (Scheme 6.1) [17]. 

Four-centered transition state for (T bond metathesis... 

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