CHIRAL ORGANOMETALLIC REAGENTS .12. ON THE RACEMIZATION OF ALPHA-HETERO-SUBSTITUTED BENZYLLITHIUM COMPOUNDS

Benzyllithium compounds carrying a benzylthio substituent (6), a benzy lseleno substituent (8), and an isopropyl(methyl)amino substituent (14 ) have been generated. Racemization barriers have been determined in T HF by monitoring the coalescence of signals of diastereotopic groups i n the H-1-NMR spectra. The barrier for 6 amounts to a Delta H-double d agger of 7.5 kcal mol(-1) and Delta S-double dagger of -12 cal mol(-1) K-1 corresponding to a Delta G(213)(double dagger) of 9.9 kcal mol(-1 ). The barrier for the other compounds investigated varies from 9 to 1 0 kcal mol(-1). The rate-determining step for the racemization is disc ussed to be the transformation of the contact ion pair to the solvent- separated ion pair.