Additions to Aldehydes, Ketones and their Synthetic Equivalents

2-Additions to Aldehydes, Ketones and their Synthetic Equivalents 381 

It is appropriate to mention that the first reported use of In(III) salts in addition reactions was as a cocatalyst in a cycloaddition protocol. In the paper by Trost et al, the Pd-catalyzed trimethylenemethane cycloaddition reaction proceeded with an initial conjugate addition followed by cyclization to form five-membered rings. Addition of In(acac)3 swings the usual 1,4-addition preference to that of a 

Majority of 1,2-additions to carbonyl compounds using In(III) salts as promoters or catalysts are in the area of allylation reactions. For several such reactions, In(III) salts may participate mainly through transmetallation, and thus strictly not acting as Lewis acids. However, such reactions are presented here, both for the sake of completion and for illuminating pertinent coordinating characteristics of In(III) complexes in reactions. 

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The reaction is based upon the two components condensation between an aldehyde or ketone 6 (or their synthetic equivalents) and alcohol 95, which contains an allylsilane (or vinylsilane) moiety. The IMSC reaction is mediated by Lewis or Bronsted acids, which activate the carbonyl group of 6 towards nucleophilic attack. After addition of alcohol 95 on the activated carbonyl, the oxonium cation 96 is formed, which is intramolecularly captured by the pendant allylsilane function, leading to oxygen-containing rings 97 (Scheme 13.38). This process typically requires a stoichiometric (or more) amount of Lewis acid. 

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