Z-Selective, Catalytic Internal Alkyne Semihydrogenation under H_2/ CO Mixtures by a Niobium(III) Imido Complex

摘要

The discovery of a Nb(III)-mediated catalytic hydrogenation of internal alkynes to (Z)-alkenes that proceeds through an unprecedented mechanism is reported. The mechanistic proposal involves initial reduction of the alkyne by the Nb(III) complex (BDI)Nb(N~tBu)(CO)_2 to provide a Nb(V) metallacydopropene, itself capable of σ-bond metathesis reactivity with H_2. The resulting alkenyl hydride species then undergoes reductive elimination to provide the (Z)-alkene product and regenerate a metal complex in the Nb(III) oxidation state. Support for the proposed mechanism is derived from (i) the dependence of the product selectivity on the relative concentrations of CO and H_2, (ii) the isolation of complexes closely related to those proposed to be part of the catalytic cycle, (iii) H/D crossover experiments, and (iv) DFT studies of multiple possible reaction pathways.