Selective Reactions Using Allylic Metals

Over the last 15 years, many approaches to the stereoselective C—C bond formation via the reaction of allylic metals with C=X electrophiles have been investigated. A variety of very successful methods have been developed for diastereoselective C—C bond formation between allylic metals and carbonyl compounds, imines, and alkenes. In many instances, high enantioselectivities have been achieved via the reactions of allylic boranes and stannanes. More recently, asymmetric allylation with catalytic amounts of certain Lewis acids has been reported. Main-group allylmetals always act as nucleophilic allyl transfer reagents, but transitionmetal allyl reagents exhibit both nucleophilic and electrophilic property depending on the metal (this review does not cover the latter reactions via π-allylpalladium complexes). New selectivities and reactivities have been observed in the reactions of transitionmetal allyl reagents including group 3 and Zr. Perfect regioselectivity has been accomplished with Ba reagents. Accordingly, in addition to the useful and common allyl reagents (B, Si, Sn, Ti, Cr), research on new types of allylmetals has just begun. Further investigations of the chemistry of allylmetals will provide us with a number of synthetically useful and selective reactions.