Carbanions Brook rearrangement

The [4 + 2] cycloaddition of a-phenylselenopropenoyl trimethylsilane (75) with 2,3-dimethylbuta-1,3-diene is unusual in that a significant portion of product mixture consists of the hetero-Diels-Alder dihydropyran adduct 76. The phenylselenenyl substituent appears to be responsible for this unusual pattern of reactivity, since propenoyl trimethylsilane gives only the expected regioisomer (77, X = H) (Scheme 116)14. a-Selenenyl substituted a,/l-unsaturated acyl silanes such as 75 were used to prepare a series of substituted dienes in excellent yields through the addition of a-sulphinyl carbanions, Brook rearrangement and expulsion of sulphinate, in a reaction pathway recognisably more typical of acyl silanes (Scheme 117). 

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). 

Nucleophilic addition to an acylsilane followed by a 1,2-Brook rearrangement and final trapping of the resulting carbanion in either an acylation or intramolecular... 

Comparison of rates of Brook rearrangement of jS-substituted a-silyl allyl alcohols as a means of estimating the a-carbanion stabilizing ability of heteroatom substituents suggests that PhS is much more stabilizing than MesSi. ... 

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. 

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 . 

Ab initio through-space/bond interaction analysis was applied to 3 + 2-anulation based on Brook rearrangement using /i-phcnylthioacryloylsilanes with alkyl methyl ketone enolates (Scheme 103).150 The major product has the large substituents on the same side of the five-membered ring. Orbital interactions related to the carbanion... 

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. 

By itself, the Brook rearrangement is not very useful but, if the carbanion can do something else other than just get protonated, something useful may happen. We have seen what happens to the epoxides of vinyl silanes. Dihydroxylation of the same alkenes also gives interesting chemistry when the diols are treated with base. 

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. 

The reaction of acylsilanes with appropriate carbon nucleophiles, followed by the Brook rearrangement, provides a novel source of carbanions which can be further elaborated on treatment with a reactive electrophile, allowing the formation of two carbon-carbon bonds in a one-pot reaction. 

Takeda, K., Ubayama, H., Sano, A., Yoshii, E., Koizumi, T. Comparing a-carbanion-stabilizing ability of substituents using the Brook rearrangement. Tetrahedron Lett. 1998, 39, 5243-5246. 

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. 

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