Siloxy carbanions rearrangement

The retro 1,4-Brook rearrangement in cyclic systems was unfavorable when the pertinent siloxy groups were in an anti position to the carbanion centers (equations 163 and 164). When a 2 1 mixture of cis- and trans-251 was treated with t-BuLi at —78°C and then warmed, and stood at +20 °C for 1 h followed by hydrolysis, trans-251 was completely transformed to trans-259, while 85% of cis-257 was converted to cis-258, indicating that the 1,4-silyl migration was confined to cis-257409. In some other cyclic systems, a retro 1,4-Brook rearrangement was observed even in the anti isomers410. 

Addition of cif-lithio ethyl diazoacetate 54 to acyl silanes such as 53 and subsequent 1,2-Brook rearrangement to carbanion 56 provided lithiated silyl enol ether 57, apparently in equilibrium with the corresponding y siloxy allenolate 58. Sequential alkylation was possible, providing fully substituted yS-ketoester product 55. ... 

Addition of ketone enolates to acyl silanes, followed by Brook rearrangement and closure of the resulting carbanion onto the pendant ketone carbonyl leads to 1,2-cyclopropanediol products such as 67. In the presence of an appropriate electrophile and using a sodium amide base, 67 serves as a homoenolate equivalent, generating y siloxy- hydroxyketone 68 as a mixture of diastereomers whose stereochemistry was not determined. ... 

Chirality transfer from an a-silylalcohol to a-carbamoyloxy- and a-siloxyallyl-carbanions has been investigated using a Brook rearrangement-mediated 5 2 protonation in y-carbamoyloxy- and y-siloxy-ct-silylallyl alcohols. A reaction mechanism that proceeds along one of two pathways that involves a concerted protonation of a silicate intermediate and a concerted lithiation of the intermediate followed by protonation with retention or by protonation after racemization has been proposed. 

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