Facile Decarboxylation of Propiolic Acid on a Ruthenium Center and Related Chemistry

Spontaneous decarboxylation of RCCCO2H (R = H, Ph) occurs in reactions with RuCl(PP)Cp (PP = (PPh3)2, dppe) to give [Ru(CCHR)(PP)Cp]+. Computational studies (DFT) of possible decarboxylation mechanisms suggest that the reaction that leads to extrusion of CO2 and formation of [Ru(CCH2)(dppe)Cp]+ most likely occurs by initial interaction of the anion HCCCO2 – with RuCl(dppe)Cp by coordination of carboxylate to Ru, followed by formation of an η2-alkyne intermediate which rearranges to the η1-ethynyl species with loss of CO2. Protonation of the ethynyl group affords the parent vinylidene. In contrast, reactions of HCCCO2R (R = Me, Et) with RuCl(PP)Cp and [NH4]PF6 in MeOH have given [Ru{C(OMe)CH2(CO2R)}(PP)Cp]+, formed by attack of MeOH at Cα of the intermediate vinylidenes [Ru{CCH(CO2R)}(PP)Cp]+. Deprotonation of the carbenes affords Ru{C(OMe)CH(CO2R)}(PP)Cp as mixtures of cis and trans isomers. The vinylidenes, which are obtained directly from RuCl(PP)Cp and HCCCO2R in the presence of [NH4]PF6 in ButOH, can be deprotonated (Na/PriOH) to the corresponding alkynyls. Attempted deprotonation of [Ru(CCH2)(dppe)Cp]+ with LiBu gave the binuclear cyclobutenylidinium complex [{Ru(dppe)Cp}2(μ-C4H3)]+. The X-ray diffraction molecular structures of [{Ru(dppe)Cp}2(μ-C4H3)]PF6 (11), [Ru{C(OMe)CH2(CO2Me)}(dppe)Cp]PF6 (13), Ru{C(OMe)CH(CO2R)}(dppe)Cp (R  Me (15), Et (16)) and Ru(CCCO2R)(dppe)Cp (R = Me (21), Et (22)) are described.