The dihydride−osmium(IV) complex [OsH2(κ2-O2CCH3)(H2O)(PiPr3)2]BF4 as a precursor for Carbon−Carbon coupling reactions

Abstract

The dihydride-osmium(IV) complex [OsH2(/c2-O2CCH3)(H2O)(PiPr3)2]BF4 (1) reacts with acetylene (1 atm) at 0 °C to give polyacetylene and the vinyl-carbyne derivative [Os(CH= CH2)(2-02CCH3)(=CCH3)(PiPr3)2]BF4 (2). Both polyacetylene and 2 can also be obtained by reaction of [OsH(K2-O2CCH3XCCH3XPiPr3)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(/t2-O2CCH3){=C(CH= CH2)CH3}(CO)(PiPr3)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(K2-O2CCH3)(CO)2(PiPr3)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)(/c2-O2CCH3)(=C=CH2)(PiPr3)2 (6), which reacts with carbon monoxide to give the butadienyl compound Os{C(CH=CH2)=CH2}{1-OC(O)CH3}(CO)2(PiPr3)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) A, whereas the torsion angle in the diene is 50.7(8)°. 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-{C=CHCH2C(CO2Me)=C(C02Me)CH2{1-OC(O)CH3}(CO)2(PiPr3)2 (8) and Os{C=CHCH2CH-[C(O)OC(0)]CHCH2}{1-OC(O)CH3}(CO)2(PiPr3)2 (9). Treatment of 8 with HBF4-OEt2 gives i 5 and dimethyl l,4-cyclohexadiene-l,2-dicarboxylate. © 2000 American Chemical Society.