Stereospecificity and Stereoselectivity in Electrophilic Substitution Reactions of Non-α-Heterosubstituted Organolithiums and Stannanes: A Rotationally Restricted Amide as an Internal Stereochemical Marker

The complete stereochemical course of a tin−lithium exchange/electrophilic quench sequence has been unambiguously determined by stereochemical characterization (using X-ray crystallography or NOE studies) at every step. Pairs of diastereoisomeric stannanes of known stereochemistry bearing atropisomeric amide substituents undergo tin−lithium exchange with alkyllithiums to give diastereoisomeric benzylic organolithiums whose stereochemistry can be assigned by NMR. For one atropisomer of the stannanes, the tin−lithium exchange is fully stereospecific and proceeds with retention of stereochemistry. The other atropisomer undergoes nonstereospecific tin−lithium exchange: the first reported example of a lack of stereospecificity in electrophilic substitution of tin for lithium. One of the diastereoisomeric atropisomeric organolithiums produced by the tin−lithium exchange is deuterated and alkylated with retention but stannylated with inversion of stereochemistry. The other is alkylated nonstereospecifically but stannylated with retention.