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.