Activation and Oxidation of Mesitylene C−H Bonds by (Phebox)Iridium(III) Complexes

Abstract

A pincer iridium(III) complex, (Phebox)Ir(OAc)_2OH_2 (1) (Phebox = 3,5-dimethylphenyl-2,6-bis(oxazolinyl)), selectively cleaves the benzylic C–H bond of mesitylene to form an isolable iridium mesityl complex, (Phebox)Ir(mesityl)(OAc) (3), in >90% yield. The trifluoroacetate analogue, (Phebox)Ir(OCOCF_3)_2OH_2 (2), was synthesized to compare with complex 1 for C–H activation, and (Phebox)Ir(mesityl)(OCOCF_3) (4) was synthesized by ligand exchange of complex 3. Both complexes 1 and 2 catalyze H/D exchange between mesitylene and D2O at 180 °C, exclusively at the benzylic position; 2 gave a higher turnover number (11 TO) than 1 (6 TO) in 12 h. Using d4-acetic acid as the source of deuterium, up to 92 turnovers of benzylic H/D exchange of mesitylene were obtained with complex 1. (Phebox)Ir(OCOCF3)_2OH_2 catalyzed the benzylic C–H oxidation of mesitylene using Ag_2O as a terminal oxidant at 130 °C, to form 3,5-dimethylbenzaldehyde and 3,5-dimethylbenzoic acid in 35% ± 4% yield (5.1 ± 0.6 TO). DFT calculations were used to investigate two possible pathways for the catalytic oxidation of mesitylene: (1) C–H activation followed by oxy-functionalization and (2) Ir-oxo formation followed by outer-sphere C–H hydroxylation. Results of calculations of the C–H activation pathway appear to be the more consistent with the experimental observations.

Group Members

William A. Goddard III

Charles and Mary Ferkel Professor