Activation of C(sp²)−H and Reduction of CE (E = CH, N) Bonds with an Osmium-Hexahydride Complex:  Influence of E on the Behavior of RCHE-py Substrates

Abstract

The hexahydride complex OsH 6 (P i Pr 3 ) 2 ( 1 ) reacts with 2-vinylpyridine to give the trihydride derivative ( 2 ). The β- and γ-positions of the pyridine ring are quantitatively and selectively deuterated by addition of 1 (5%) to toluene- d 8 and benzene- d 6 solutions of 2 . Under hydrogen atmosphere, 2 reacts with HBF 4 ·OEt 2 to afford [HNC 5 H 4 - o -Et]BF 4 and regenerate 1 . Under argon atmosphere, the addition of HBF 4 ·OEt 2 to dichloromethane solutions of 2 leads to the hydride-dihydrogen ( 3 ), which catalyzes the hydrogenation of 2-vinylpyridine to 2-ethylpyridine. In dichloromethane 3 evolves into the neutral chloro-dihydrogen compound ( 4 ). Complex 1 also reacts with N -methylene-2-pyridinamine and ( E )- N -(phenylmethylene)-2-pyridinamine. The reaction with the first pyridinamine leads to a 3:1 mixture of the trihydride complexes ( 5 ) and ( 6 ), while the reaction with the second one selectively affords ( 7 ). Similarly to 2, the addition of 1 (5%) to toluene- d 8 and benzene- d 6 solutions of 6 produces the quantitative and selective deuteration of the β- and γ-positions of the pyridine ring. The addition of HBF 4 ·OEt 2 to diethyl ether solutions of 5 leads to ( 8 ). Isotope labeling experiments suggest that the reduction of the C C double bond of 2-vinylpyridine takes place by concerted addition of a dihydrogen ligand, while the reduction of the C N double bond of N -methylene-2-pyridinamine occurs by sequential addition of H - and H + . In solution the hydride ligands of these compounds undergo two different site exchange processes. Their activation parameters have been calculated by 1 H NMR spectroscopy. Complexes 2, 3, and 5 have been characterized by X-ray diffraction analyses.