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 tBu(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 tBu(CH3)CNBn, which reacts with B(C6F5)3 through its enamine tautomer to form an α-C
bound adduct, 5. In the N-bound imine−borane adducts 1−4, steric crowding and π-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 19F and 1H
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.