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

Abstract

Cationic platinum(II) complexes [( t bpy)Pt(Ph)(L)]+ [ t bpy =4,4′-di-tert-butyl-2,2′-bipyridyl; L = THF, NC5F5, 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 °C. 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 (C6H6 or C6D6) C–H/C–D activation by [( t bpy)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 C6H6 and C6D6 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 CD2Cl2, [( t bpy)Pt(Ph)(THF)][BAr′4] [Ar′ = 3,5-bis(trifluoromethyl)phenyl] reacts with ethylene to generate [( t bpy)Pt(CH2CH2Ph)(η2-C2H4)][BAr′4] with k obs = 1.05(4) × 10–3 s–1 (23 °C, [C2H4] = 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.

Olefin Hydroarylation Catalyzed by (pyridyl-indolate)Pt(II) Complexes: Catalytic Efficiencies and Mechanistic Aspects

“…Given the observations above, a mechanism for ethylene hydroarylation similar to those reported by Gunnoe 14 and Goldberg 15 is proposed ( Figure 6). An equilibrium between the dimethylsulfide precatalyst 2a and the active species 3 occurs via a rapid ligand exchange with ethylene.…”

“…Hydride migration to the bound ethylene can occur followed by coordination of an L-type ligand to generate the Pt(II)-ethyl complex 7. Similar Pt-ethyl complexes have been identified as off-cycle organometallic products by the Gunnoe group, 14 for cationic ( t bpy)Pt complexes. Finally, complex 2a can also undergo ligand substitution with any styrene generated during the course of decomposition to afford the styrene-bound complex 8.…”

“…In general, complexes of the type (N-N)PtPh(SR2) exhibited reasonable activity compared to other reported Pt complexes. 14,15 Additionally, reduced over-alkylation was observed using the current catalysts.…”

“…On the other hand, the isotopologue distribution as measured in the intermolecular competition experiment provides direct mechanistic information about the C-H activation step. 20 14,19 To elucidate the role of such species during catalysis, several stoichiometric reactions with ethylene were performed to generate potential catalytic intermediates. Using a low-pressure J-Young NMR tube (3 mL volume) as a reaction vessel, complex 2a was treated with ethylene (1 atm) in benzene-d6 at ambient temperatures over a 4 day period and the reaction was monitored by 1 H NMR spectroscopy.…”

“…12,13 In the latter context, it has been suggested that modification of the ancillary ligand donating ability could potentially improve the activity of Pt-based catalysts. 14 However, there has not been a thorough study of ligand electronic effects for neutral Pt catalysts, even though these systems appear to exhibit higher selectivities for mono-(vs. poly-) alkylation, relative to cationic catalysts. 15 Moreover, detailed investigations into complex degredation pathways have not been performed for the neutral Pt(II) systems.…”