Siloxy carbanions

The reaction of BTSP with lithium compounds 31 at the carbanionic center a to a heteroatom affords the corresponding trimethylsilyl derivatives 32 in yield from 40 to 70%. No trace of the expected 2-siloxy or 2-hydroxy derivatives was detected in the reaction mixture (equation 48). 

Silyl-substituted carbanions stabilized by heteroatom substituents reacted with epoxides providing the corresponding 3-siloxy carbanions via the anionic 1,4-silyl migrations, which reacted with various electrophiles (equation 149)361-370. 

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. 

Other types of heterofunctionalized cyclopropane derivatives used as starting materials in transition metal mediated conversions are acylcyclopropanes and similar systems. These systems, being electrophilic cyclopropanes, are opened by carbanion nucleophiles (e.g. cuprates, see Section l.B.2.1.3.) or nucleophilic transition-metal centers. In combination with siloxy groups, push-pull (capto-dative) substituted cyclopropanes are available as homoenolate precursors via Lewis acid mediated ring opening (see Section 1. B.2.1.4.).. This latter procedure has been used in tetrahydrofuran synthesis. 

The equilibrium between a-silyl alkoxides and silojq carbanions can be shifted toward the carbanion side by introduction of a conjugating group into either or both the acylsilane and the nucleophile. In 1980, Reich et al. reported that treatment of all l-substituted acylsilanes with vinyllithium followed by a variety of electrophiles affords a-substituted enol silyl ethers 22 via a siloxy allyllithium intermediate 21 fScheme 8.1Similar reactions using phenyllithium give products in which electrophilic quenching occurred at the benzylic position. 

A mixture of benzaldehyde and trifluoromethyltrimethylsilane (as trifluoromethyl carbanion equivalent) in THF at 0° treated with tetra- -butylammonium fluoride, warmed to room temp., stirred for 1 h, and the siloxy deriv. hydrolyzed with 0.5 N HCl at room temp, for 1 h - trifluoromethyl(phenyl)carbinol. Y 85%. The method is generally applicable to aldehydes and ketones (inch enones), but hindered ketones required longer reaction times. F.e.s. G.K.S. Prakash et al., J. Am. Chem. Soc. 777, 393-5 (1989). 

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. 

In the reactions of a-silyl carbanions bearing an ester function at the a-position with carbonyl compounds, the ester enolates first attack to form the ) -silylalkoxide anions 4, which give the corresponding siloxy ester enolates 5 by 1,3-silyl migration (Scheme 2.6). When the elimination of the trimethylsUyloxide anion is not so fast. 

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