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 OsH6(PiPr3)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-d8 and benzene-d6 solutions of 2. Under hydrogen atmosphere, 2 reacts with HBF4·OEt2 to afford [HNC5H4-o-Et]BF4 and regenerate 1. Under argon atmosphere, the addition of HBF4·OEt2 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-d8 and benzene-d6 solutions of 6 produces the quantitative and selective deuteration of the β- and γ-positions of the pyridine ring. The addition of HBF4·OEt2 to diethyl ether solutions of 5 leads to (8). Isotope labeling experiments suggest that the reduction of the CC double bond of 2-vinylpyridine takes place by concerted addition of a dihydrogen ligand, while the reduction of the CN 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 1H NMR spectroscopy. Complexes 2, 3, and 5 have been characterized by X-ray diffraction analyses.