An NMR Study on the Ruthenium Complex-Catalyzed C-H/Olefin Coupling Reaction

A dihydridoruthenium(II) complex, [RuH2(CO)(PPh3)3], reacts with styrene to give two species: a bis(styrene)ruthenium(0) complex, [Ru(CO)(CH2=CHPh)2(PPh3)2], and a cyclometallated hydridoruthenium(II) one, [Ru(C6H4PPh2)H(CO)(PPh3)2]. The complex [RuH2(CO)(PPh3)3] reacts with isoprene to give a piano-stool type ruthenium(0) complex [Ru(η4-CH2=CMeCH=CH2)(CO)(PPh3)2]. In reactions among [RuH2(CO)(PPh3)3], styrene, and 3′-(trifluoromethyl)acetophenone, three cyclometallated hydridoruthenium(II) complexes: P,P′-cis-C,H-cis-, P,P′-trans-C,H-trans-, and P,P′-trans-C,H-cis-[Ru{C6H3(CF3)C(=O)Me}H(CO)(PPh3)2] are detected by NMR spectroscopy. The 1H NMR spectra of the reaction mixture exhibit the catalytic formation of 2′-(2-phenylethyl)- and 2′-(1-phenylethyl)-5′-(trifluoromethyl)acetophenones. On the basis of these findings, a mechanism for the C-H/olefin coupling reaction is discussed. The first of the three complexes is assigned to an active intermediate in the catalytic coupling reaction, whereas the other two are assigned as quasi-stable ruthenium(II) complexes which are in equilibrium with active species.