3-Silylalkoxide

Kuwajima et al. developed an efficient route to (Z)-y-siloxyallylmagnesium bromides utilizing the Brook rearrangement of a-silylalkoxides derived from acylsi-lanes and vinylmagnesium bromide [594], Corey et al. recently applied this method to the generation of (Z)-y-siloxyallyllitliiums to achieve rapid syntheses of natural products [595],... 

Equation 126 involves a syn p elimination of a / -silylalkoxide as in the Peterson reaction equation 127 involves a Brook rearrangement of an a-silylalkoxide with elimination of a /Meaving group. The evidence presented concluded that 127 predominated and that 127 proceeds via an anti stereochemistry for the elimination. The... 

Reaction of a-silyl carbanions with carbonyl compounds yielding p-silylalkoxides which undergo instantaneous elimination to afford olefins ... 

In their study, Jung and co-workers proposed that metalation at the allylic position, followed by epoxide opening led to or-silylalkoxide 176. A 1,2-Brook rearrangement provided resonance-stabilized dienyl anion 177, which underwent retro-1,6-Brook rearrangement to enolate 178. The thermodynamically favored ( )- ,y -unsaturated enal 175 was formed during the aqueous workup. 

The former evidently arose from trapping by ester interchange of an intermediate silylalkoxide formed by normal addition of silyllithium to triphenylsilylformaldehyde formed in situ, whereas the latter (III) is the commonly found rearrangement product of a silylalkoxide ion. 

On the other hand, elimination under basic conditions should proceed in a syn manner. Two possible pathways have been postulated for the elimination of a silyl-oxide moiety after deprotonation of a hydroxy group with an equimolar amount of base (Scheme 2.4). One is the stepwise 1,3-migration of a silyl group from carbon to oxygen, followed by elimination of a trimethylsilyloxide moiety. The other involves the formation of a pentacoordinate 1,2-oxasiletanide 2, which is in equilibrium with the j8-silylalkoxide anion 1, and extrusion of the trimethylsilyloxide moiety therefrom. The reaction mechanism has not yet been entirely elucidated. 

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. 

Peterson reactions of a-sUyl carbanions and carbonyl compounds leading directly to the alkenes are generally not stereoselective since the j8-hydroxyalkylsilane or yff-silylalkoxide intermediates are usually formed as a mixture of syn and anti isomers, whereas Peterson elimination from a j8-hydroxyalkylsilane proceeds in an exclusively stereospecific manner. Furthermore, the addition of an a-silyl carbanion to a carbonyl compound proceeds in an irreversible manner [29, 30]. Therefore, when the 8-hydroxyalkylsilane intermediate can neither be isolated nor separated, the diastereomeric ratio of the alkene products of the Peterson reaction is determined in the addition step of the a-silyl carbanion and the carbonyl compound. Stereospecific preparation of the y8-hydroxyalkylsilane is required for the utilization of the Peterson reaction in organic synthesis. 

For the addition of an a-silyl carbanion to a carbonyl compound, some reaction models have been put forward that would indicate that the ratio of the resulting fi-silylalkoxide anions is determined by the relative stabilities of the diastereoiso-meric intermediates. This mechanism assumes that the carbanion attacks at 90° to the carbonyl framework [31]. For example, regarding the stereoselective formation of the alkene (Z)-IO by the reaction of an a-silyl carbanion derived from methyl trimethylsilylacetate with 2-alkylcydohexanone 8, the stability of the intermediate yS-silylalkoxide anion 9 is discussed (Scheme 2.9). The trimethylsilyl group occu-... 

There is another methodology for carrying out stereoselective Peterson reactions, which utilizes the difference in reactivity of the two diastereomers, syn- and anti-fi-silylalkoxides, at low temperature. This is the convergent transformation from both diastereomers of syn- and anti-jS-silylalkoxides to give one alkene (Scheme 2.21) [47]. When an a-silyl carbanion bears a bulky alkyl group on the anionic car-... 

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