The dihydride-osmium(IV) complex [OsH2(k2-O2CCH3)(H2O)(PiPr3)2]BF4 as a precursor for carbon-carbon coupling reactions

The dihydride - osmium(IV) complex [OsH2(kappa (2)-O2CCH3)(H2O)((PPr3)-Pr-i)(2)]BF4 (1) reacts with acetylene (1 atm) at 0 degreesC to give polyacetylene and the vinyl-carbyne derivative [Os(CH = CH2)(kappa (2)-O2CCH3)(=CCH3)((PPr3)-Pr-i)(2)]BF4 (2) Both polyacetylene and 2 can also be obtained by reaction of [OsH(kappa (2)-O2CCH3)(=CCH3)((PPr3)-Pr-i)(2)]BF4 (3) with acetylene, Under an acetylene atmosphere, complex 2 yields polyacetylene in a slow but constant manner. Complex 2 reacts with carbon monoxide to give initially the carbene derivative [Os(kappa (2)-O2CCH3){=C(CH=CH2)CH3}(CO)((PPr3)-Pr-i)(2)]BF4 (4), by migratory insertion of the carbyne ligand of 2 into the Os-vinyl bond. Under a carbon monoxide atmosphere, complex 2 is unstable and evolves into [Os(kappa (2)-O2CCH3)(CO)(2)((PPr3)-Pr-i)(2)]BF4 (5). Treatment of 2 with KOH in methanol produces the deprotonation of the carbyne ligand and the formation of the vinyl-vinylidene derivative Os(CH=CH2)(kappa (2)-O2CCH3)(=C=CH2)((PPr3)-Pr-i)(2) (6), which reacts with carbon monoxide to give the butadienyl compound Os{C(CH=CH2)=CH2}{kappa (1)-OC(O)CH3}(CO)(2)((PPr3)-Pr-i)(2) (7) by migratory insertion of the vinylidene ligand into the Os-vinyl bond. The structure of 7 has been determined by X-ray diffraction analysis. The geometry around the osmium atom can be rationalized as a distorted octahedron with the phosphine ligands occupying opposite positions. The remaining perpendicular plane is formed by the butadienyl ligand, the acetate, and the carbonyl groups mutually cis disposed. The Os-butadienyl distance is 2.195(5) Angstrom, whereas the torsion angle in the diene is 50.7(8)degrees. The butadienyl ligand of 7 acts as a diene in Diels-Alder reactions. Thus, the addition of dimethyl acetylenedicarboxylate and maleic anhydride to benzene solutions of 7 affords the corresponding cycloaddition products Os{CH=CHCH2C (CO2Me)=C(CO2Me)CH2(kappa (1)-OC(O)CH3}(CO)(2)((PPr3)-Pr-i)(2) (8) and Os{C=CHCH2CH-[C(O)OC(O)]CHCH2}(kappa (1)-OC(O)CH3}(CO)(2)((PPr3)-Pr-i)(2) (9) Treatment of 8 with HBF4. OEt2 gives 5 and dimethyl 1,4-cyclohexadiene- 1,2-dicarboxylate.