Atropisomeric amides as chiral ligands: Using (-)-sparteine-directed enantioselective silylation to control the conformation of a stereogenic axis

An enantiomerically pure (1-trimethylsilyl)ethyl group, constructed by a (- )-sparteine-directed enantioselective quench of a laterally lithiated terti ary aromatic amide, exerts powerful thermodynamic control over the conforma tion of the adjacent tertiary amide substituent. Ortholithiation and functi onalization of the amide in the 6-position allows the single amide conforme r to be trapped as an enantiomerically and diastereoisomerically pure amide atropisomer. Protodesilylation of the amide gives functionalized atropisom eric amides with a stereogenic axis of single absolute configuration, whose barriers to racemization have been determined by polarimetry. Enantiomeric ally pure amides bearing phosphine substituents are effective ligands in a Pd-catalyzed allylic substitution reaction-the first use of a nonbiaryl atr opisomer as a chiral ligand-and give products with 90% ee. The rate of race mization of the phosphine-substituted amide is powerfully influenced by the presence of palladium.