Hydrophenylation of ethylene using a cationic Ru(ii) catalyst: comparison to a neutral Ru(ii) catalyst

Abstract

Charge neutral Ru(II) complexes of the type TpRu(L)(NCMe)Ph [Tp = hydridotris(pyrazolyl)borate; L = CO, PMe₃, P(OCH₂)₃CEt or P(OCH₂)₂(OCCH₃)] have been previously reported to catalyze the hydrophenylation of ethylene (Organometallics, 2012, 31, 6851–6860). However, catalyst longevity for the TpRu(L)(NCMe)Ph complexes is inhibited by competitive ethylene C–H activation. For example, ethylene C–H activation limits catalysis using TpRu(P(OCH₂)₃CEt)(NCMe)Ph to a maximum of 20 turnover numbers for conversion of benzene and ethylene to ethylbenzene. In contrast, reaction of the cationic Ru(II) complex [(HC(pz⁵)₃)Ru(P(OCH₂)₃CEt)(NCMe)Ph][BAr^′₄] [HC(pz⁵)₃ = tris(5-methyl-pyrazolyl)methane; BAr^′₄ = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate] (0.025 mol% relative to benzene) in benzene with C₂H₄ (15 psi) at 90 °C gives 565 turnover numbers of ethylbenzene after 131 hours. The production of 565 turnovers of ethylbenzene corresponds to an approximate one-pass 95% yield with ethylene is the limiting reagent and is a 28-fold improvement compared to the charge neutral catalyst TpRu(P(OCH₂)₃CEt)(NCMe)Ph. Under identical conditions, the activity of [(HC(pz⁵)₃)Ru(P(OCH₂)₃CEt)(NCMe)Ph][BAr^′₄] is only 1.3 times less than TpRu(P(OCH₂)₃CEt)(NCMe)Ph, but the increased stability of the cationic Ru(II) catalyst allows reactivity at much higher temperatures (up to 175 °C) and significantly enhanced rates.