Mechanistic Studies of Ethylene Hydrophenylation Catalyzed by Bipyridyl Pt(ll) Complexes

摘要

Cationic platinum(II) complexes [(~tbpy)Pt(Ph)-(L)]~+ [~tbpy =4,4'-di-tert-butyl-2,2'-bipyridyl; L = THF, NC_5F_5, or NCMe] catalyze the hydrophenylation of ethylene to generate ethylbenzene and isomers of diethylbenzene. Using ethylene as the limiting reagent, an 89% yield of alkyl arene products is achieved after 4 h at 120 ℃. Catalyst efficiency for ethylene hydrophenylation is diminished only slightly under aerobic conditions. Mechanistic studies support a reaction pathway that involves ethylene coordination to Pt(II), insertion of ethylene into the Pt-phenyl bond, and subsequent metal-mediated benzene C-H activation. Studies of stoichiometric benzene (C_6H_6 or C_6D_6) C-H/C-D activation by [(~tbpy)Pt(Ph-d_n)-(THF)]~+ (n = 0 or 5) indicate a k_H/k_D = 1.4(1), while comparative rates of ethylene hydrophenylation using C_6H_6 and C_6D_6 reveal k_H/k_D = 1.8(4) for the overall catalytic reaction. DFT calculations suggest that the transition state for benzene C-H activation is the highest energy species along the catalytic cycle. In CD_2Cl_2, [(~tbpy)Pt(Ph)(THF)][BAr'_4] [Ar' = 3,5-bis(trifluoromethyl)phenyl] reacts with ethylene to generate [(~tbpy)Pt(CH_2CH_2Ph)(η~2-C_2H_4)][BAr'_4] with k_(obs) = 1.05(4) × 10~(-3) s~(-1) (23 ℃, [C_2H_4] = 0.10(1) M). In the catalytic hydrophenylation of ethylene, substantial amounts of diethylbenzenes are produced, and experimental studies suggest that the selectivity for the monoalkylated arene is diminished due to a second aromatic C-H activation competing with ethylbenzene dissociation.