Unraveling the Mechanism of 1,3-Diyne Cross-Metathesis Catalyzed by Silanolate-Supported Tungsten Alkylidyne Complexes

The benzylidyne complex [PhC W{OSi(OtBu)(3)}(3)] (1) catalyzed the cross-metathesis between 1,4-bis(trimethylsilyl)-1,3-butadiyne (2) and symmetrical 1,3-diynes (3) efficiently, which gave access to TMS-capped 1,3-diynes RC=C C=CSiMe3 (4). Diyne cross-metathesis (DYCM) studies with C-13-labeled diyne PhC13 C-C-13 CPh (3*) revealed that this reaction proceeds through reversible carbon-carbon triple-bond cleavage and formation according to the conventional mechanism of alkyne metathesis. The reaction between 1 and 3* afforded the 3-phenylpropynylidyne complex PhC13 C-C-13 W{OSi(OtBu)(3)}(3)] (5*), indicating that alkynylalkylidyne complexes are likely to act as catalytically active species. Attempts to isolate 5* from mixtures of 1 and 3* afforded crystals of the ditungsten 2-butyne-1,4-diylidyne complex [(tBuO)(3)SiO}(3)W C-13-C-13 C-13-C-13 W{OSi(OtBu)(3)}(3)] (6*), which was additionally characterized by X-ray diffraction analysis. Depolymerization-macrocyclization of a carbazole-butadiyne polymer, obtained from 3,6-diethynyl-9-dodecylcarbazole (7) under copper-catalyzed Hay coupling conditions, was also efficiently catalyzed by 1 and afforded a mixture of mono-, diyne- and triyne-containing tetrameric macrocycles, revealing that diyne disproportionation into monoynes and triynes occurs as a slow side reaction that interferes with a high diyne metathesis selectivity. Potential catalytic pathways were studied by means of quantum-chemical calculations, and kinetic studies were performed to substantiate an ,-mechanism for the catalytic diyne metathesis reaction, which involves intermediate alkynylalkylidyne and alpha,alpha'-dialkynylmetallacyclobutadiene intermediates.