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-disubstituted-but-3-en-1-ynes (PP3=P(CH2CH2PPh2)3). In the presence of H2O or C2H5OH, the catalytic reaction with HC=CSiMe3 selectively gives but-3-en-1-ynyl-trimethylsilane. A detailed study under different experimental conditions, the detection of some intermediates, and the use of isolated complexes in independent reactions, taken altogether, permit mechanistic conclusions which account for the observed products. A key-role is played by (vinylidene)sigma-alkynyl complexes which transform into eta3-butenynyl derivatives via intramolecular C-C bond formation. The OS(II) eta3-butenynyl complexes are likely reagents in the rate determining step of the catalytic cycle, and produce free (Z)-1,4-disubstituted-but-3-en-1-ynes upon sigma-bond metathesis reaction with HC=CR. The 16-electron fragments [(PP3)OsX]+ (X=H, Cl, C=CR) are capable 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 formation 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.