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.