C−H Bond Activation of Terminal Allenes: Formation of Hydride-Alkenylcarbyne-Osmium and Disubstituted Vinylidene-Ruthenium Derivatives

The reactivity of the dihydrides MH2Cl2(PiPr3)2 (M = Os (1), Ru (2)) toward allenes has been studied. Complex 1 reacts with 2 equiv of 3-methyl-1,2-butadiene and 1-methyl-1-(trimethylsilyl)allene to give 1 equiv of olefin and the π-allene derivatives OsCl2(η2-CH2CCRMe)(PiPr3)2 (R = Me (3), Me3Si (4)). The X-ray structure of 4 proves the coordination to the metal center of the carbon−carbon double bond of the allene with the lowest steric hindrance. In toluene, complexes 3 and 4 are unstable and evolve into the hydride-alkenylcarbyne derivatives OsHCl2(CCHCRMe)(PiPr3)2 (R = Me (5), Me3Si (6)). DFT calculations on the model compound OsCl2(η2-CH2CCMe2)(PMe3)2 (3t) suggest that the π-allene to hydride-alkenylcarbyne transformation involves the migration of both hydrogen atoms of the CH2 group of the allene. The first of them occurs between the terminal and central carbon atoms and takes place throught the metal center. The second one is a 1,2-hydrogen shift from the allene terminal carbon to osmium. The reactions of the ruthenium complex 2 with the previously mentioned allenes give olefins and RuCl2(η2-CH2CCRMe)(PiPr3)2 (R = Me (7), Me3Si (8)), which in dichloromethane and in the presence of allene afford the disubstituted vinylidene complexes RuCl2(CCRMe)(PiPr3)2 (R = Me (9), Me3Si (10)). The structure of 10 in the solid state has been determined by X-ray diffraction analysis. DFT calculations show that the formation of 9 and 10 can be rationalized in terms of the initial isomerization of 7 and 8 to alkenylcarbene species, which subsequently undergo metathesis reactions with a second allene molecule.