The additions of allylmagnesium reagents to carbonyl compounds are important methods in synthetic organic chemistry, but the mechanisms of these reactions are likely to be distinct from mechanisms followed by other organomagnesium reagents. Additions to alkyl aldehydes and ketones are likely to be concerted, proceeding through six-membered-ring transition states. These highly reactive reagents appear to react at rates that approach the diffusion limit, so chemoselectivity is generally low. Furthermore, reactions of allylmagnesium halides with carbonyl compounds are unlikely to follow stereochemical models that require differentiation between competing transition states. This Short Review discusses the current state of understanding of these reactions, including the structure of the reagent and unique aspects of the reactivity of allylmagnesium reagents.1 Introduction2 Reactions with Carbonyl Compounds2.1 Reactivity of Allylmagnesium Halides2.2 Selectivity of Addition3 Structure of Allylmagnesium Reagents3.1 Schlenk Equilibrium and Aggregation3.2 Spectroscopic Studies3.3 X-ray Crystallographic Studies3.4 Computational Studies of Structure4 Reaction Mechanism4.1 Substrate-Dependent Mechanisms4.2 Concerted Mechanisms4.3 Single-Electron Transfer Mechanisms4.4 Open, SE2′-Like Transition State4.5 Computational Studies of Mechanism5 Conclusion
