Reactions of Allylmagnesium Halides with Carbonyl Compounds: Reactivity, Structure, and Mechanism

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 Introduction 2 Reactions with Carbonyl Compounds 2.1 Reactivity of Allylmagnesium Halides 2.2 Selectivity of Addition 3 Structure of Allylmagnesium Reagents 3.1 Schlenk Equilibrium and Aggregation 3.2 Spectroscopic Studies 3.3 X-ray Crystallographic Studies 3.4 Computational Studies of Structure 4 Reaction Mechanism 4.1 Substrate-Dependent Mechanisms 4.2 Concerted Mechanisms 4.3 Single-Electron Transfer Mechanisms 4.4 Open, S _E 2′-Like Transition State 4.5 Computational Studies of Mechanism 5 Conclusion.