Stannylated silyl enol ethers, alkylation

The presence of two substituents (generally both at the /3-carbon) still allows the reaction to occur in good yields [25,26], and the same is true when three substituents are present. Thus Verlhac et al. described the alkylation of stannylated silyl enol ethers and, after workup, obtained aryl ketones in moderate to good yields (Scheme 4-4) [27], Booth et al. [28] (Scheme 4-5) and Casson and Kocienski [29] reported couplings of heterofunctionalized vinyltins, Takeda et al. [30] prepared a series of highly substituted ajS-unsaturated ketones (Scheme 4-6) and Shi et al. [31] a number of /3-fluoro-a-keto acid derivatives. 

The enhanced reactivity of the silyl- and stannyl-substituted alkenes is also favorable to the synthetic utility of carbocation-alkene reactions. Silyl enol ethers also show enhanced reactivity. Electrophilic attack is followed by desilylation to give an a-substituted carbonyl compound. The carbocations can be generated from tertiary chlorides and a Lewis acid, such as TiCU. This reaction provides a method for introducing tertiary alkyl groups alpha to a carbonyl. This transformation cannot be achieved by base-catalyzed alkylation because of the strong tendency for tertiary halides to undergo elimination. 

This chapter is concerned with the reactions of enol ethers with carbonyl compounds as illustrated in Scheme 1. The enol ethers considered include alkyl, silyl, germyl and stannyl ethers, and to a small extent enol esters. The carbonyl compounds encompass aldehydes, ketones, esters and their functional equivalents. Overall, the reaction depicted in Scheme 1 is similar to the classical aldol and related condensations discussed in Part 1 of this volume. However, in contrast to the basic conditions inherent in... 

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