Abstract
Complex OsH(6)(P(i)Pr(3))(2) releases H(2) at 50 °C. The resulting tetrahydride OsH(4)(P(i)Pr(3))(2) promotes head-to-head reductive dimerization of phenylacetylenes to give the 1,4-dibranched-butenediyl derivatives OsH(2){η(4)-[C(4)H(4)R(2)]}-(P(i)Pr(3))(2) (R = C(6)H(5), C(6)H(4)-CF(3), C(6)H(4)-NMe(2)). DFT calculations suggest that the formation of these compounds proceeds via five-coordinate unsaturated bis-(alkenyl)-osmium-(II)-(Kubas-type dihydrogen) intermediates, which evolve by alkenyl coupling and H-H cleavage of the dihydrogen. The reactions are sensitive to temperature and the amount of alkyne used. At higher temperatures and excess alkyne, the reductive coupling is accompanied by two dehydrogenation reactions, one at the metal center and the other involving an isopropyl substituent of a phosphine. As a result, mixtures of the dihydrides and Os-{η(4)-[C(4)H(4)R(2)]}-(P(i)Pr(3))-{η(2)-C,C;κ(1)-P-[(CH(2)CMe)-P(i)Pr(2)]}-(P(i)Pr(3)) (R = H, CF(3), NMe(2)) are formed. Both families react with H(2) to regenerate OsH(6)(P(i)Pr(3))(2) and release the corresponding 1,4-diarylbutane. According to these reactions, 1,4-diarylbutanes have been obtained in approximately 20% yield, by stirring phenylacetylenes with 5 mol % of OsH(6)(P(i)Pr(3))(2), in toluene, under 1 atm of H(2).