Silyl group transfers, Brook rearrangement

Brook rearrangement. a-Trimethylsilylpropargyl alcohols undergo Brook rearrangement to afford allenyl silyl ethers, which can be used to condense with aldehydes. Silyl group transfer from the ether four bonds apart is preferred after the Sn/Li exchange from silyl ethers of l-tributylstannyl-l,3-alkanediols. °... 

C-to-O silyl-group transfer without alkene formation is relatively uncommon. However, Oshima, Utimoto, and co-workers reported that metalation of dichloromethylsilane 96 and addition to aldehydes provided fi-alkoxysilanes (e.g., 97) that imderwent 1,3-rearrangement and alkylation to 98. Notably, the addition of HMPA promoted the 1,3-Brook rearrangement, presumably by increasing the reactivity of the lithium alkoxide intermediate. As the authors pointed out, this sequence demonstrated 96 as a methylene chloride dianion equivalent. ... 

Reverting to the simple phenylated substrate of Scheme 33, if the styryl moiety is orf/jo-substituted by a phenyldimethylsilyloxy group, the 5-exo-trig cych-zation is now followed by a retro-[ 1,4]-Brook rearrangement. Whatever the ( ) or (Z) configuration of the styryl group, the intermediate benzyllithium epimer-izes, and a pentacoordinated silicon allows transfer of the silicon moiety diaster-eoselectively to form a unique C-silylated adduct (er>98 2, dr>99 l) [59] (Scheme 35). 

We now propose that the proton transfer process could be occurring via an intramolecular 4-centered transition state between the newly formed silyl ketal (Figure 10). A similar 4-centered transition state has been previously proposed by Sommer and Fujimoto to explain retention of the stereochemistry at an asymmetric silicon center during the exchange of alkoxide groups (Scheme 37).34 Brook and co-workers also proposed a 4-centered transition state for the thermal rearrangement of (3-ketosilanes (Scheme 38).35... 

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