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

Dalton Transactions Pub Date: 2011-08-16 DOI: 10.1039/C1DT10994D

Abstract

The para-aminobiphenyl compound [(η6-C6H5)(C6H4-4-NH2)]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 [(η6-C6H4-4-NH2)(C6H5)]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 [(η6-C6H5)(C6H4-2-NH2)]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 [(η6-C6H4-2-NH2)(C6H5)]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.

Graphical abstract: Haptotropic rearrangement in tricarbonylchromium complexes of 2-aminobiphenyl and 4-aminobiphenyl
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