Solution and solid-state characteristics of imine adducts with tris(pentafluorophenyl)borane

Adducts of the N-benzyl imines Ph(R)C NBn (R = Ph, CH3, H) and Bu-t(CH3)C=NBn and the N-phenyl imine Ph(H)C=NPh with the Lewis acid tris(pentafluorophenyl)borane, B(C6F5)(3), have been prepared and characterized in solution and in the solid state. For each imine, the Lewis acid is N-bound, with the exception of the sterically demanding base Bu-t(CH3)C NBn, which reacts with B(C6F5)(3) through its enamine tautomer to form an alpha-C bound adduct, 5. In the N-bound imine-borane adducts 1-4, steric crowding and pi-stacking between C6F5 and C6H5 rings results in restricted rotation about the B-N and B-C bonds; the dynamic behavior which results can be studied using variable-temperature F-19 and H-1 NMR spectroscopy. For the unsymmetrical ketimines and aldimines, kinetic adducts of the thermodynamically dominant free imine are observed to form initially upon treatment with B(C6F5)(3); these adducts slowly convert thermally to the thermodynamic adducts of the less stable imine. For the ketimine Ph(CH3)C=NBn, both the kinetic adduct 2-k and the thermodynamic adduct 2-t were fully characterized. In the solid-state structures of all the adducts except 5, one or two C6F5/C6H5 stacking interactions are present; in one case, the adduct 3-t between B(C6F5)(3) and Ph(H)C=NBn, intermolecular stacking interactions are apparent in the crystal-packing diagram. The implications of the structures of these adducts for Lewis acid mediated addition of nucleophiles to imines is discussed.