Haptotropic rearrangement in tricarbonylchromium complexes of 2-aminobiphenyl and 4-aminobiphenyl

摘要

The para-aminobiphenyl compound [(η~6-C_6H _5)(C_6H_4-4-NH_2)]Cr(CO)_3 (1) has an arene-phenyl dihedral angle of 38.01(6)°, as determined by single-crystal X-ray crystallography, and 34.7(11)°, as determined by DFT calculations. It undergoes haptotropic rearrangement at 140 °C in solution to form [(η~6-C_6H_4-4-NH_2)(C _6H_5)]Cr(CO)_3 (2), even though previous reports have suggested that such rearrangements should not be observed in compounds with arene-phenyl dihedral angles greater than 22°. NMR analysis gave a rate constant of k = 5.0 × 10~(-5) s~(-1) for the rearrangement of 1 to 2. The ortho-substituted analog [(η~6-C_6H _5)(C_6H_4-2-NH_2)]Cr(CO)_3 (3) has an arene-phenyl dihedral angle of 67.70(7)°, as determined by single-crystal X-ray crystallography, and 51.9(10)°, as determined by DFT calculations. Surprisingly, even though it displays a more extreme canting of arene rings, 3 rearranges to [(η~6-C_6H _4-2-NH_2)(C_6H_5)]Cr(CO)_3 (4) at 140 °C in solution with a rate constant of k = 2.6 × 10 ~(-4) s~(-1). This approximately five-fold rate enhancement likely results from the ortho-amino group providing intramolecular stabilization for intermediates formed during the rearrangement.