Understanding the high diastereofacial discrimination in nucleophilic additions to nitrones: the first ab initio study on the nucleophilic addition reactions of chiral nitrones with Grignard reagents

The nucleophilic addition of methyl magnesium bromide to a C-(2-pyrrolidinyl) nitrone has been studied by ab initio molecular orbital (HF/6-31+G∗) and density functional theory (B3LYP/6-31+G∗//HF/6-31+G∗) calculations via location of the two diastereomeric transition states. The overall reaction is exothermic and proceeds via precomplexation of the nitrone with the organometallic reagent. The calculations confirm that chelation is the main factor governing the experimentally confirmed preference for the Si attack leading to syn adducts.