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