Addition Reactions to Carbonyl Compounds
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The Barbier reaction is an old, very attractive one-step procedure for the preparation of alcohols from halides and carbonyl compounds in the presence of magnesium. This method is a variation of an even older procedure developed by Saytzeff and Wagner, who employed zinc metal. Although Grignard’s reaction is a two-stage synthetic procedure, it offers some advantages over the Barbier reaction. Two basic mechanisms have been proposed for this apparently simple reaction. It is currently assumed that an intermediate organometallic reagent is generated in situ and rapidly trapped by the carbonyl compound. Lithium- and zinc-promoted allylation of carbonyl compounds has been improved by several methods, particularly by means of ultrasonic irradiation, with which Luche and his associates have carried out important contributions. Although important degrees of asymmetric induction can be obtained from chiral donors or carbonyl acceptors, recent efforts to increase the stereoselectivity are focused mainly on the specific coordination of zinc or other metals with highly chelating agents.