Chiral Organometallic Reagents, XIV. Theoretical Investigations into the Mechanism of Inversion of Configuration at the Carbon Center of α‐Sulfur‐Substituted Carbanions

Ab initio calculations showed that the two elementary steps in the racemization of the α‐methylthioethyl anion (7), inversion of the pyramidal anionic center, and rotation around the C−S bond, occur in sequence and are not concerted. The former process passes over a barrier of 1.1 kcal/mol, the latter over one of 10.9 kcal/mol. MNDO calculations on the α‐phenylthioethyl anion (8) revealed a preferred conformation of the S‐aryl bond, allowing maximum delocalization of the sulfur lone pairs into the π* orbital of the aryl group. This orientation is maintained during rotation around the C−S bond and is the origin of steric hindrance in the racemization of certain α‐arylthio‐, α‐arylseleno‐, and α‐aryltelluroalkyllithium compounds 1 to 6.