β-Hydride Elimination and C–H Activation by an Iridium Acetate Complex, Catalyzed by Lewis Acids. Alkane Dehydrogenation Cocatalyzed by Lewis Acids and [2,6-Bis(4,4-dimethyloxazolinyl)-3,5-dimethylphenyl]iridium

Journal of the American Chemical Society

5 years ago

β-Hydride Elimination and C–H Activation by an Iridium Acetate Complex, Catalyzed by Lewis Acids. Alkane Dehydrogenation Cocatalyzed by Lewis Acids and [2,6-Bis(4,4-dimethyloxazolinyl)-3,5-dimethylphenyl]iridium

Ashley M. Wright, Karsten Krogh-Jespersen, Changjian Guan, Karen I. Goldberg, Yang Gao, Akshai Kumar, Meng Zhou, Thomas J. Emge, Alan S. Goldman

NaBAr^F₄ (sodium tetrakis[(3,5-trifluoromethyl)phenyl]borate) was found to catalyze reactions of (Phebox)Ir^III(acetate) (Phebox = 2,6-bis(4,4-dimethyloxazolinyl)-3,5-dimethylphenyl) complexes, including (i) β-H elimination of (Phebox)Ir(OAc)(n-alkyl) to give (Phebox)Ir(OAc)(H) and the microscopic reverse, alkene insertion into the Ir–H bond of (Phebox)Ir(OAc)(H), and (ii) hydrogenolysis of the Ir–alkyl bond of (Phebox)Ir(OAc)(n-alkyl) and the microscopic reverse, C–H activation by (Phebox)Ir(OAc)(H), as indicated by H/D exchange experiments. For example, β-H elimination of (Phebox)Ir(OAc)(n-octyl) (2-Oc) proceeded on a time scale of minutes at −15 °C in the presence of (0.4 mM) NaBAr^F₄ as compared with a very slow reaction at 125 °C in the absence of NaBAr^F₄. In addition to NaBAr^F₄, other Lewis acids are also effective. Density functional theory calculations capture the effect of the Na⁺ cation and indicate that it operates primarily by promoting κ²–κ¹ dechelation of the acetate anion, which opens the coordination site needed to allow the observed reaction to proceed. In accord with the effect on these individual stoichiometric reactions, NaBAr^F₄ was also found to cocatalyze, with (Phebox)Ir(OAc)(H), the acceptorless dehydrogenation of n-dodecane.

Publisher URL: http://dx.doi.org/10.1021/jacs.6b12995

DOI: 10.1021/jacs.6b12995