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

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• 作者：Meng Zhou ; Samantha I. Johnson ; Yang Gao ; Thomas J. Emge ; Robert J. Nielsen ; William A. Goddard ; III ; Alan S. Goldman
• 刊名：Organometallics
• 出版年：2015
• 出版时间：June 22, 2015
• 年：2015
• 卷：34
• 期：12
• 页码：2879-2888
• 全文大小：680K
• ISSN：1520-6041

文摘

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