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

Article abstract of DOI:10.1039/c4sc01665c

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. This journal is