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

作者：Zhou Meng; Johnson Samantha I; Gao Yang; Emge Thomas J; Nielsen Robert J; Goddard William A III; Goldman Alan S

来源：Organometallics, 2015, 34(12): 2879-2888.

DOI：10.1021/acs.organomet.5b00200

A pincer iridium(III) complex, (Phebox)Ir(OAc)(2)OH2 (1) (Phebox = 3,5-dimethylphenyl-2,6-bis(oxazolinyl)), selectively cleaves the benzylic CH bond of mesitylene to form an isolable iridium mesityl complex, (Phebox)Ir(mesityl)(OAc) (3), in >90% yield. The trifluoroacetate analogue, (Phebox)Ir(OCOCF3)(2)OH2 (2), was synthesized to compare with complex 1 for CH activation, and (Phebox)Ir(mesityl)(OCOCF3) (4) was synthesized by ligand exchange of complex 3. Both complexes 1 and 2 catalyze H/D exchange between mesitylene and D2O at 180 degrees C, exclusively at the benzylic position; 2 gave a higher turnover number (11 TO) than 1 (6 TO) in 12 h. Using d(4)-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)(2)OH2 catalyzed the benzylic CH oxidation of mesitylene using Ag2O as a terminal oxidant at 130 degrees 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 CH activation pathway appear to be the more consistent with the experimental observations.

• 出版日期2015-6-22