Pentacoordinate silicon intermediate

Rearrangement of a-silyl oxyanions to a-silyloxy carbanions via a reversible process involving a pentacoordinate silicon intermediate is known as the [l,2]-Brook rearrangement, or [l,2]-silyl migration. 

Reaction of DMSB with triphenylsilyl-substituted oxiranyllithium leads to the formation of an olefinic silanol via sequential (1) coordination to the silicon, (2) Si-C bond migration, and (3) Peterson-type Si-O elimination to furnish the alkene. A pentacoordinate siliconate intermediate is presumably involved in this transformation. Therefore, it was reasonable to expect that addition of a nucleophile (methyllithium or lithium t>-propoxide) to an oxiranyl-substi-tuted SCB, which could generate a similar intermediate, would induce the C-Si bond migration to form the same silacyclopentane. Indeed, this alternative order of addition sequence provides the corresponding silanol with better efficiency (84% yield vs. 44%, Scheme 36). 

In essence, this reaction proceeds by a one-stage nucleophilic mechanism through the formation of a pentacoordinate silicon intermediate. 

Several mechanisms were proposed in the scientific literature for the nuclophiles-mediated GTP. Webster and Sogah proposed a concerted associative mechanism based on the direct transfer of a pentacoordinated siliconate intermediate (49) from a chain to an incoming monomer (equation 47) . 

The data are interpreted by a reversible formation of a pentacoordinate silicon intermediate followed by attack of a second molecule of nucleophile in the rate-determining step, involving either a symmetrical octahedral intermediate 208 or a pentacoordinate siliconium ion 209 (Scheme 62). In either case, racemization takes place. 

TBAF-assisted Trifluoromethylation, Perfluoroalkylation, and Polyfluoroalkoxylation. TBAF combines with TMSCF3 to form nucleophilic trifluoromethyl anion equivalents. The precise structure of this nucleophile is unknown, although it is likely that a pentacoordinate silicon intermediate is involved. The TBAF/TMSCF3 combination leads to smooth trifluoromethylation of aldehydes,ketones, esters, imines, nitroso compounds, a-ketoesters, Q -ketoamides (eq and... 

In this study, Uang proposed a plausible mechanism involving a hexa-coordinated vanadium species (Scheme 9.7) generated from a mixture of complex 15, an aldehyde substrate, and a pentacoordinated silicon intermediate that was formed by the reaction of trimethylsilyl cyanide with tetrabutylammonium fluoride. The free aldehyde group on the ligand 3-tert-hutylsalieylaldehyde) of the vanadium species derived from complex 15 did not reaet with trimethylsilyl cyanide, presumably due to deactivation by the ortho-o yanion. 

Brook addressed this important mechanistic issue early, developing evidence for retention of configuration at silicon in the rearrangement. Based on the precedented assumption of inversion at asymmetric silicon in chloride displacement with either an organolithium or an alcohol, Brook and coworkers showed a stereochemical Walden cycle that implicated retention at silicon in the rearrangement step. Mechanistically, retention at silicon corresponds to a frontside-type displacement, consistent with the mechanism shown above. The displacement might implicate 10 as a pentacoordinate silicon intermediate rather than a transition state. 

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