Theoretical study of the stereochemistry of crotylmagnesium chloride's addition on a series of cyclic and acyclic enones

Semi-empirical methods AMI and PM3 have been used to study the stereochemistry of the addition reaction of crotylmagnesium chloride on a series of cyclic and acyclic enones. The mechanism of this reaction has been studied by localizing the different possible transition states. The stereochemistry of the diastereoisomers obtained can be explained by comparison of the values of the energy of activation. We show in this paper that the major diastereoisomer for acyclic enones is formed via a boat-shaped transition state where the magnesium reacts with the trans conformation. Whereas for cyclic enones such as pulegone or methyl pulegone, a chair transition state where the magnesium adopts a cis conformation explains the final stereochemistry. Moreover, we demonstrate that the stereoselectivity depends of both steric and electronic factors. (C) 2002 Elsevier Science B.V. All rights reserved.