Osmium–carbon multiple bonds: Reduction and C–C coupling reactions

This paper shows that the redox equilibria hydride-alkenylcarbyne/alkenylcarbene and alkenyl-alkenylcarbyne/dienylcarbene are readily governed by the electronic properties of the ligands of the complexes. Because we have learned to control the position of these equilibria, we are able to build, step by step, osmium derivatives with cyclic alkenylcarbene ligands and osmacyclopentapyrrole complexes. The dihydride OsH2Cl2(PiPr3)2 reacts with alkynols, allenes, enynes, and dienes to give hydride–alkenylcarbyne derivatives, OsHCl2(triple bond; length of mdashCCR′double bond; length as m-dashCR2)(PiPr3)2, which can be transformed into dicationic species by replacement of chloride ligands by acetonitrile molecules. The selective deprotonation of the alkenylcarbyne ligand of [OsH(triple bond; length of mdashCCHdouble bond; length as m-dashCPh2)(CH3CN)2(PiPr3)2]2+ affords the hydride–allenylidene [OsH(double bond; length as m-dashCdouble bond; length as m-dashCdouble bond; length as m-dashCPh2)(CH3CN)2(PiPr3)2]+, which undergoes the reduction of the Cα–Cβ double bond of the cumulene in the presence of alcohols. The insertion of monosubstituted alkynes into the Os–H bond of the hydride–allenylidene complex leads to alkenyl–allenylidene derivatives, which are transformed into dienylcarbene compounds. The coordination of carbon monoxide to the osmium atom of the latter promotes the 4π-conrotation of the dienylcarbene ligand, to afford a cyclic alkenylcarbene complex via an η1-cyclopentadienyl intermediate. Through a similar cyclization, in acetonitrile under reflux, the alkenyl–allenylidene complexes are converted into osmapyrrole derivatives by means of the formation of three C–C bonds involving the three carbon atoms of the cumulene, the alkenyl ligand, and an acetonitrile molecule.