Sequential and selective hydrogenation of the Cα−Cβ and M−Cα double bonds of an allenylidene ligand coordinated to osmium: New reaction patterns between an allenylidene complex and alcohols

Abstract

Complex [OsH(CCCPh2)(CH3CN)2(PiPr3)2]BF4 (1) reacts with primary and secondary alcohols to give the corresponding dehydrogenated alcohols and the hydride-carbene derivative [OsH(CHCHCPh2)(CH3CN)2(PiPr3)2]BF4 (2), as a result of hydrogen transfer reactions from the alcohols to the Cα−Cβ double bond of the allenylidene ligand of 1. The reactions with phenol and t-butanol, which do not contain any β-hydrogen, afford the alkoxy-hydride-carbyne complexes [OsH(OR)(CCHCPh2)(CH3CN)(PiPr3)2]BF4 (R = Ph (3), tBu (4)), as a consequence of the 1,3-addition of the O−H bond of the alcohols to the metallic center and the Cβ atom of the allenylidene of 1. On the basis of the reactions of 1 with these tertiary alcohols, deuterium labeling experiments, and DFT calculations, the mechanism of the hydrogenation is proposed. In acetonitrile under reflux, the Os−C double bond of 2 undergoes hydrogenation to give 1,1-diphenylpropene and ¢Sq = -43.5 +/- 9.6 J mol-1 K-1. The X-ray structures of 2 and 3 are also reported.