Aj3,-Unsaturated carbonyl compounds

Most organic free radicals are nucleophilic and will react with electrophilic centers. Lewis acids have been used to activate aj3-unsaturated carbonyl compounds towards addition of free radicals and also to stabilize a-keto radicals [67]. The first report of the use of a chiral Lewis acid to effect an asymmetric free-radical reaction was that of Urabe, Yamashita, Suzuki, Kobayashi, and Sato in 1995 [68]. They found that if the BINOL aluminum catalyst 313 is stoichiometrically complexed with lactone 323 and then treated with butyl iodide and tributylstannane in the presence of triethylborane the alkylated lactone 324 can be isolated in 47 % yield with 23 % ee (Sch. 40). 

It is most striking that the hydrosilylation of aj3-unsaturated carbonyl compounds using wono-hydrosilanes was found to proceed in a manner of 1,4-addition, while c i-hydrosilanes very specifically underwent 1,2-addition to carbonyl functionalities [35]. Since the resulting silyl enol ethers and allylic silyl ethers can readily be converted by hydrolysis to saturated carbonyl compounds and a,j3-unsaturated alcohols, respectively, these reactions may furnish a unique method for selective reduction of carbonyl compounds, equation (13). The results are summarized in Table 4. 

Protected cyanohydrin anions also behave as acyl carbanion equivalents, and this method can be extended to aj3-unsaturated derivatives (5). The reaction of yet another type of acyl carbanion equivalent (6) with esters gives products of type (7), which are useful for the synthesis of novel heterocyclic compounds. Species of type (8 Y = 0,S), are further modified acyl carbanion equivalents, and behave towards carbonyl compounds as synthetic equivalents of the Wittig reagents PhaP = C(YR)CHO. Compounds (9) are j8-acylvinyl anion equivalents. ... 

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