REGIOSELECTIVE AND STEREOSELECTIVE DIMERIZATION OF 1-ALKYNES CATALYZED BY AN OS(II) COMPLEX

The complex [(PP3)OsH(N2)]BPh4 is a catalyst precursor for the regio- and stereoselective dimerization of HC=CR (R=Ph, SiMe3) to (Z)-1,4-dis ubstituted-but-3-en-1-ynes (PP3=P(CH2CH2PPh2)3). In the presence of H2 O or C2H5OH, the catalytic reaction with HC=CSiMe3 selectively gives b ut-3-en-1-ynyl-trimethylsilane. A detailed study under different exper imental conditions, the detection of some intermediates, and the use o f isolated complexes in independent reactions, taken altogether, permi t mechanistic conclusions which account for the observed products. A k ey-role is played by (vinylidene)sigma-alkynyl complexes which transfo rm into eta3-butenynyl derivatives via intramolecular C-C bond formati on. The OS(II) eta3-butenynyl complexes are likely reagents in the rat e determining step of the catalytic cycle, and produce free (Z)-1,4-di substituted-but-3-en-1-ynes upon sigma-bond metathesis reaction with H C=CR. The 16-electron fragments [(PP3)OsX]+ (X=H, Cl, C=CR) are capabl e of promoting the 1-alkyne to vinylidene tautomerism. In particular, the (vinylidene)hydride [(PP3)OsH{C=C(H)-SiMe3}]BPh4 has been isolated and properly characterized. Since the stoichiometric reaction of the latter compound with HC=CSiMe3 gives vinyltrimethylsilane, the formati on of (vinylidene)hydride species is suggested to be an effective step , alternative to 1-alkyne insertion, in the reduction of 1-alkynes to alkenes assisted by hydrido metal complexes.