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

The alkylation of N-methylisoindoline-borane complex, using (BuLi)-Bu-n in THF is diastereoselective, the substitution occurring predominantly syn to the borane group. Use of the (BuLi)-Bu-s-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.