THE REGIOSPECIFIC AND STEREOSPECIFIC SELECTIVE HYDROGENATION OF ETA-2-COORDINATED ARENES

Several η2-bound arene complexes of pentaammineosmium(II) have been selectively hydrogenated to alkene products in the presence of a heterogeneous catalyst (Pd, Rh). In all cases the coordinated osmium was shown both to activate the arene ligand toward catalytic hydrogenation and to protect the alkene product toward further reduction. Hydrogenation can be achieved both regio- and stereoselectivity and is shown to occur exclusively anti to the metal center. The structure of the 3-methoxycyclohexene complex [Os(NH3)5(η2-C7H12O)](OTf)2 has been determined. The hydrogenation of η2-bound anisole or 1,4-dimethoxybenzene is not concerted; the semihydrogenation product, a vinyl ether, rapidly hydrolyzes in acid medium to form a cyclohexenone complex. © 1990, American Chemical Society. All rights reserved.