Experimental and computational studies of ruthenium(II)-catalyzed addition of arene C-H bonds to olefins

Hydroarylation reactions of olefins are catalyzed by the octahedral Ru(II) complex TpRu(CO)(NCMe)(Ph) (1) (Tp = hydridotris(pyrazolyl)borate). Experimental studies and density functional theory calculations support a reaction pathway that involves initial acetonitrile/olefin ligand exchange and subsequent olefin insertion into the ruthenium-phenyl bond. Metal-mediated C-H activation of arene to form a Ru-aryl bond with release of alkyl arene completes the proposed catalytic cycle. The cyclopentadienyl complex CpRu(PPh3)(2)(Ph) produces ethylbenzene and styrene from a benzene/ethylene solution at 90 degreesC; however, the transformation is not catalytic. A benzene solution of (PCP)Ru(CO)(Ph) (PCP = 2,6-((CH2PBu2)-Bu-t)(2)C6H3) and ethylene at 90 degreesC produces styrene in 12% yield without observation of ethylbenzene. Computational studies (DFT) suggest that the C-H activation step does not proceed through the formation of a Ru(IV) oxidative addition intermediate but rather occurs by a concerted pathway.