Chiral Benzyllithium Compounds: High Configurative Stability of (R)- and (S)-1-Lithioindan-1-ylN,N-Diisopropylcarbamate and Unexpected Stereochemical Course of the Substitution Reactions

The title compound, 6, was generated by stereospecific deprotonation of the optically active carbamate 5 with sec-butyllithium/TMEDA and proved to be configuratively completely stable in ethereal solution at –78 °C. Compared with open-chain analogs, the trend for stereoretentive substitution is enhanced. Even the reaction with trialkyltin chlorides leads to partial racemization due to competing front face attack. Semiempirical calculations point to an increased degree of pyramidalization and to a higher barrier for planarization in the cyclic benzyllithium compound, both of which disfavor the rear face attack.