Decomposition of Vanadium(V) Alkylidenes Relevant to Olefin Metathesis

Vanadium alkylidenes can be highly active initiators for ring-opening metathesis polymerization of cyclic olefins; however, attempts to expand their use to cross metathesis have been unsuccessful due to catalyst decomposition. Detailed knowledge of the decomposition reaction is imperative to guide future catalyst design. Herein, we demonstrate that β-hydride elimination is the dominant decomposition pathway during cross metathesis. The isolated vanadium decomposition products [N-2,6-(CH3)2C6H3]­(OC6Cl5)­[P­(CH3)3]2VCl (10) and [N-2,6-(CH3)2C6H3]­V­(OC6F5)2[P­(CH3)3]2 (11), generated from two separate catalysts, agree with this pathway. Compounds 10 and 11 were shown computationally to form via bimolecular routes after β-hydride elimination from the metallocyclobutane and reductive elimination of propylene, which was itself demonstrated to be exergonic with low thermodynamic barriers to metallocyclobutane formation. Relative conversions in the self-metathesis of 1-hexene with the series of vanadium alkylidenes of various sterics and electronics (N-2,6-X2C6H3)­(OC6Y5)­[P­(CH3)3]2V­[CHSi­(CH3)3] [X = CH3, Y = Cl (1); X = CH­(CH3)2, Y = Cl (6), X = CH­(CH3)2, Y = F (7)] were determined. It was found that bulkier, yet more electron-donating ligands decreased conversions, indicating that β-hydride elimination is favored over bimolecular decomposition. Analysis of organic reaction products demonstrated that reductive elimination of propylene occurs before insertion of ethylene into the newly formed vanadium–hydrogen bond.