The diastereoselective and enantioselective substitution reactions of an isoindoline–borane complex

Abstract

The alkylation of N-methylisoindoline–borane complex, using nBuLi in THF is diastereoselective, the substitution occurring predominantly syn to the borane group. Use of the sBuLi–sparteine reagent mixture in Et2O changes the diastereoselectivity observed and enables the reaction to be conducted enantioselectively, giving the chiral isoindoline–borane complexes in up to 89% ee. The relative and absolute configurations of the chiral products were established by X-ray structure determinations and NMR studies. The new asymmetric process is shown to be an enantioselective deprotonation reaction, and the intermediate organolithium is shown to be epimerisable.