Brook rearrangement silyl group

In the well-known Brook rearrangment, silyl groups migrate from oxygen to carbon, but the following example is less obvious and not necessarily predictable ... 

Small groups on silicon favor the formation of the corresponding silyl dienol ether, formed by a Brook rearrangement of 81, and this reaction pathway is minimized by employment of P(OPh)3. 

A recent paper [44] shows that the treatment of silyl thioketones 68 with lithium diethylphosphite proceeds via a thiophiUc attack followed by a thio-phosphate mercaptophosphonate (69 70) carbanionic rearrangement and the migration of the silyl group from the carbon to the sulfur atom leading to the S-silylated sulfanylphosphonate carbanion 71. The last step represents the first example of the thia-Brook rearrangement (Scheme 18). 

E)-Enol silyl ethers.1 A new highly stereoselective route to (E)-enol silyl ethers involves addition of CH,Li to silyl ketones substituted at the a -position by a SC6H5 group such as 1. The adduct (a) undergoes a Brook rearrangement and... 

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 . 

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. 

The overall reaction is the insertion of an oxygen atom between the silicon and the alkene and the product is a useful silyl enol ether (Chapter 21). The Brook rearrangement takes place first but the carbanion has a leaving group (OH) on the neighbouring carbon atom so an ElcB reaction (Chapter 19) occurs next. 

Anionic migration of an alkylsilyl group from carbon to oxygen to afford silyl ether a-oxycarbanion is known as a Brook rearrangement, and the reverse reaction is called... 

A proposed reaction mechanism is shown in Scheme 130. It involves the addition of a neutral carbene/zwitterionic species 514 (generated in situ from the exposure of thiazolium salt to DBU) to an acylsilane <2001JOC5124, 1996TL8241>. This nucleophilic addition initiates a 1,2-silyl group migration (Brook rearrangement)... 

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