Carbanions retro-Brook-1,2-rearrangement

Retro-Brook rearrangement of the [l,3]-variant will readily take place in sp and sp carbanion systems. Kuwajima and Takeda and Corey and Rticker have developed the [l,3]-retro-Brook rearrangement of silyl enol ether anions which provide a-silyl ketones (equation 100 and 101). 

The sulfur analogue of retro-Brook rearrangement is reported by Wright and West . Furthermore, silyl groups will migrate from carbon to the adjacent carbanion center . The driving force for these rearrangements is the formation of a more stable carbanion. Representative examples are shown in equations 113 and 114 . 

E. Silylation of an a-nitrogen carbanion through a retro-Brook 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. 

Hoffmann, R., Brueckner, R. Asymmetric induction in reductively initiated [2,3]-Wittig and retro-[1,4]-Brook rearrangements of secondary carbanions. Chemische Berichte 1992, 125, 1471-1484. 

A retro-1,5-Brook rearrangement may have been the source of the unusual silane 173, formed unexpectedly in the course of Evans s studies on phosphonamide-stabilized carbanions. Metalation at the A-methyl position would lead to the rearrangement of 172 across a unique heteroatom-containing tether. ... 

---