Metal-centered radical-pair mechanism for alkyne hydrogenation with a binuclear rhodium hydride complex. CIDNP without organic radicals

Abstract

Abstract : The binuclear complex Rh2H2(CO)2(dppm)2, 1, reacts with alkynes under H2 to form the corresponding olefins with predominantly cis addition, and an alkyne-bridged A-frame complex. Specific alkynes studied include PhC triple bond CH, MeC triple bond CH, t-Buc triple bond CH, EtOC triple bond CH, HC triple bond CCOOMe, HC triple bond CCH2OH, and CH3C triple bond CCH2OH. In all of the hydrogenation reactions, CIDNP is observed with major polarizations occurring as multiplet effects in thr trans and geminal proton resonnances of the product olefins. Specific labelling studies using 1 + PhC triple bond 13CH, Ph13C triple bond CH, and PhC triple bond CD, and 1-d sub 2 + PhC triple bond CH show no polarization due to 13C hyperfine coupling, and major polarizations only in those protons originally on the binuclear complex 1. These studies, together with radical trapping experiments, lead to the conclusion that the radical pair responsible for the observed CIDNP is metal-based and does not involve alkenyl radicals. The basis of the observed CIDNP thus differs from the previously described mechanism of H-atom transfer between a transition metal hydride and an organic substrate. The present results are consistent with a new mechanism in which a metal-centered biradical acts as the radical pair leading to the observed polarization effects.