Stereospecificity and stereoselectivity in electrophilic substitution reactions of non-alpha-heterosubstituted organolithiums and stannanes: a rotationally restricted amide as an internal stereochemical marker. |
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Authors: | J Clayden M Helliwell J H Pink N Westlund |
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Institution: | Department of Chemistry, University of Manchester, Oxford Road, Manchester M13 9PL, UK. |
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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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