Ruthenium-catalyzed synthesis of alkylidenecyclobutenes via head-to-head dimerization of propargylic alcohols and cyclobutadiene-ruthenium intermediates

The reaction of propargylic alcohols with carboxylic acid, or phenol derivatives, in the presence of the precatalyst RuCl(cod)(C5Me5)] leads selectively to a variety of alkylidenecyclobutenes through head-to-head dimerization of propargylic alcohol. The first step is the formation of a cyclobutadiene-ruthenium intermediate resulting from the head-to-head coupling of two molecules of propargylic alcohol. On protonation with strong acids (HPF6, HBF4) dehydration of the cyclobutadiene complex leads to formation of an alkylidenecyclobutenyl-ruthenium complex. The X-ray structure of one such complex, RuCl(C5Me5)(eta4-R'CCH--CH--C=CR2)] (R'=cyclohexen-1-yl, CR2 = cyclohexylidene) has been determined. Carboxylate is added at the less substituted carbon of the cyclic allylic ligand. DFT/B3 LYP calculations confirm that the intermediate arising from head-to-head coupling of alkyne to the RuClCp* species yields the cyclobutadiene-ruthenium complex more easily with propargylic alcohol than with acetylene.