The effect of the oxidation state of molybdenum complexes on the catalytic transformation of terminal alkynes: Cyclotrimerization vs. polymerization

Reactions of monosubstituted alkynes (PhC≡CH, tBuC≡CH, nBuC≡CH, HOCH2C≡CH, HO(CH3)2CC≡CH) in the presence of molybdenum(0) and molybdenum(II) carbonyl complexes (Mo(CO)6/hv, [Mo(CO)4(pip)2] (pip = piperidine), [Mo(CO)4(pip)2]/SnCl4, [Rpip]2[{(μ-Cl)Mo(μ-Cl)(SnCl3)(CO)3}2] (R = C3H5, H)) lead to the formation of cyclotrimerization and polymerization products, which were characterized by chromatography (GC–MS, GPC) and by 1H and 13C NMR spectroscopy. The effect of the oxidation state of the molybdenum catalyst on the transformation of the terminal alkynes was observed: cyclotrimerization vs. polymerization. Only molybdenum(II) complexes lead to the formation of polyenic polymers. Moreover, reaction of prop-2-yn-1-ol initiated by [Mo(CO)4(pip)2] in dichloromethane leads to the formation of oligomers containing the vinylidene unit. Mechanistic NMR studies show that η2-alkyne complex formation is the principal feature of all transformations of alkynes catalyzed by molybdenum complexes.