Chiral organometallic reagents, XII. On the racemization of α‐hetero‐substituted benzyllithium compounds

Abstract

Abstract Benzyllithium compounds carrying a benzylthio substituent ( 6 ), a benzylseleno substituent ( 8 ), and an isopropyl(methyl)amino substituent ( 14 ) have been generated. Racemization barriers have been determined in THF by monitoring the coalescence of signals of diastereotopic groups in the 1 H‐NMR spectra. The barrier for 6 amounts to a Δ H ≠ of 7.5 kcal mol −1 and Δ S ≠ of −12 cal mol −1 K −1 corresponding to a Δ G of 9.9 kcal mol −1 . The barrier for the other compounds investigated varies from 9 to 10 kcal mol −1 . The rate‐determining step for the racemization is discussed to be the transformation of the contact ion pair to the solvent‐separated ion pair.