FORMATION OF TUNGSTEN CARBYNE COMPLEXES FROM VINYLSILANES AND ALLYLSILANES AND ALLYLSTANNANES

WX(2)(PMe(3))(4) [X = Cl (1a), Br (1b)] react with the vinyl reagents CH2=CHR [R = SiMe(3), Si(OEt)Me(2), Si(OMe)(3) and Sn(n)Bu(3)] to give the tungsten methylcarbyne plexes W(=CCH3)X(PMe(3))(4) [X = Cl (2a), Br (2b)] and XSiR(3) and XSnR(3). The connectivity of 2a has been confirmed by NMR and an X-ray crystal structure. Similarly, the allyl reagents CH2=CHCH(2)R [R = SiMe(3), SiClMe(2), Si(OMe)(3) and SnMe(3)] react with la and 1b to give the ethyl carbynes W(=CCH2CH3)X(PMe(3))(4) [X = Cl (6a), Br (6b)]. Reactions of (CH2)-C-13=CHSiMe(3) and CH2-CDSiMe(3) show that the methylcarbyne ligand is formed by net migration of the methyne hydrogen to the methylene carbon, but a crossover experiment with both of these substrates shows that the rearrangement is not intramolecular. In some of the reactions, pre-equilibrium formation of a tungsten(II) vinylsilane complex is observed. The proposed mechanism for carbyne formation, based on kinetic and labeling studies, involves loss of XSiR, from the vinylsilane intermediate to give a tungsten vinyl species. The vinyl complex is deprotonated to a vinylidene which is reprotonated to the carbyne. The relative rates of carbyne formation span more than three orders of magnitude, with reactions of 1a faster than 1b and CH2=CHSn(n)Bu(3) > CH2=CH CH(2)SnMe(3) > CH2=CHSi(OMe)(3) > CH2=CHSiMe(3) > CH2=CHCH2Si(OMe)(3) > CH2=CHCH(2)SiMe(3). A primary determinant of relative reactivity is the initial binding of the olefinic substrate to tungsten(II).