Facile decarboxylation of propiolic acid on a ruthenium center and related chemistry

Abstract

Spontaneous decarboxylation of RC≡CCO 2H (R = H, Ph) occurs in reactions with RuCl(PP)Cp (PP = (PPh 3) 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 CO 2 and formation of [Ru(=C=CH 2)(dppe)Cp] + most likely occurs by initial interaction of the anion HC≡CCO 2- with RuCl(dppe)Cp by coordination of carboxylate to Ru, followed by formation of an η 2-alkyne intermediate which rearranges to the η 1-ethynyl species with loss of CO 2. Protonation of the ethynyl group affords the parent vinylidene. In contrast, reactions of HC≡CCO 2R (R = Me, Et) with RuCl(PP)Cp and [NH 4]PF 6 in MeOH have given [Ru{=C(OMe)CH 2(CO 2R)}(PP)Cp] +, formed by attack of MeOH at C α of the intermediate vinylidenes [Ru{=C=CH(CO 2R)}(PP)Cp] +. Deprotonation of the carbenes affords Ru{C(OMe)=CH(CO 2R)}(PP)Cp as mixtures of cis and trans isomers. The vinylidenes, which are obtained directly from RuCl(PP)Cp and HC≡CCO 2R in the presence of [NH 4]PF 6 in Bu tOH, can be deprotonated (Na/Pr iOH) to the corresponding alkynyls. Attempted deprotonation of [Ru(=C=CH 2)(dppe)Cp] + with LiBu gave the binuclear cyclobutenylidinium complex [{Ru(dppe)Cp} 2(μ-C 4H 3)] +. The X-ray diffraction molecular structures of [{Ru(dppe)Cp} 2(μ-C 4H 3)]PF 6 (11), [Ru{=C(OMe)CH 2(CO 2Me)}(dppe)Cp]PF 6 (13), Ru{C(OMe)=CH(CO 2R)}(dppe)Cp (R = Me (15), Et (16)) and Ru(C≡CCO 2R)(dppe)Cp (R = Me (21), Et (22)) are described. © 2012 American Chemical Society.