Photogenerated Quinone Methides as Useful Intermediates in the Synthesis of Chiral Binol Ligands

The photoinduced synthesis of chiral 3,3'-CH2X-disubstituted BINOL ligands (X ) NR2, SR, OH) has been achieved with excellent ee by UV-visible activation of BINOLAMs bearing L-proline ester arms. Quinone methides, detected by laser flash photolysis, are the key intermediates involved in such a synthetic protocol, which undergo reversible nucleophilic conjugate additions by a great variety of nitrogen nucleophiles (amines and R-amino acid derivatives) with complete configuration retention of the BINOL moiety.