In the course of our study on effects of solvents on the configurational stability of chiral carbanions, we examined the stereochemistry in reactions of an enantioenriched a-lithiated N,N-diisopropyl carbamate 1 with various electrophiles in a variety of solvents. While the reaction of 1 with BnBr in all solvents used proceeded with inversion of stereochemistry to give 2, almost all protonation reactions resulted in retention of stereochemistry regardless of the solvent. The use of cyclopentadiene as a proton source, however, showed stereodivergence depending upon the nature of the solvent employed.
Also, the configurationally stabilizing effect of the carbamoyloxy group on chiral carbanions was successfully exploited for trapping of chiral a-nitrile carbanion by ethyl cyanoformate without complete racemization (3 to 4) and for the estimation of configurational stability of carbanions 5 next to conjugative electron-withdrawing groups.
