Enhancer Four-centered transition state

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... 

The following mechanism is suggested for the cross-coupling of alkenylsilanes. Nucleophilic attack of a fluoride ion to the silicon atom of alkenylsilanes is first assumed to afford a pentacoordinate silicate and enhance both nucleophilicity of the silicon-substituted carbon and Lewis acidity of silicon to assist transmetalation effectively through a four-centered transition state (Scheme 2). Lewis acidity on silicon is critical as evidenced by the fact that hexacoordinate pentafluorosilicates that are fully coordinated and thus should have sufficient nucleophilicity are much less effective for the cross-coupling reaction (Eq. 2, vide supra). 

A living polymerization with the use of a similar reaction was reported on Zn porphyrins [350,351]. A unique reaction through the photochemical homolytic alkyl dissociation was observed in dirhodium diporphyrin [344,345] the enhanced reactions of bimetalloradical with hydrogen and methane were interpreted by a four-centered transition state [346-348] (Fig. 34). 

In the first step of the sequence (A in Scheme 5-15) a haloalkene or haloarene is commonly assumed to add oxidatively, generating a coordination sphere, probably by exchange for another ligand. If the alkenyl (aryl) residue and alkene ligand on palladium are in a cM-orientation, rotation of the alkene can lead to its in-plane coordination, and subsequent 5y -insertion of the C-C double bond into the a-alkenyl- or a-aryl-palladium bond occurs to yield a a-(P-alkenyl)- or a-(p-aryl)alkylpalladium complex via a four-centered transition state (B). 

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