Lanthanide Pybox Complexes as Catalysts for Enantioselective Silylcyanation of Aldehydes

Complexes of lanthanide chlorides with pybox ligands catalyze the enantioselective
silylcyanation of aromatic and aliphatic aldehydes with ee's of up to 91% under mild and
convenient conditions. Studies of catalysts prepared with pybox ligands of varying enantiomeric purity show no marked nonlinear effects, indicating a monometallic transition state.
A mechanism for the catalytic reaction is proposed. Crystal structures of [LnCl2(S-Pri-pybox)2][LnCl4(S-Pri-pybox)] (Ln = Eu, Yb) are reported. The structure of the related triflate complex
[La(OTf)3(S-Pri-pybox)2] is also reported. Diastereoselectivity has been observed in the
reactions of Ln(OTf)3 with racemic Pri-pybox:  for Ln = Eu, a heterochiral product containing
one R- and one S-pybox ligand is observed, whereas for Ln = Yb a racemic mixture of
homochiral bis(pybox) complexes is formed.