Half-Sandwich Ruthenium(II) Complexes as Catalysts for Stereoselective Carbene−Carbene Coupling Reactions

Abstract

Cyclopentadienyl complexes of general formula [RuX(η5-ligand)(PR3)2] have been found to catalyze the stereoselective decomposition of ethyl diazoacetate (EDA) to form diethyl maleate (DEM) in 95−99% purity with less than 1 mol % of catalyst, at temperatures depending on the bulkiness of the phosphine and the η5-ligand as well as the nature of the anionic ligand X. A detailed study of the reaction between [RuCl(η5-C5H5)(PPh3)2] and EDA suggests that dissociation of one PPh3 is a key step of the catalytic process, which proceeds via the intermediate [RuCl(η5-C5H5)(CHCO2Et)(PPh3)]. Although this electrophilic carbene was not detected in the reaction mixture, it was independently prepared in solution at low temperature starting from [Ru(η2-O2CMe)(η5-C5H5)(PPh3). The acetate reacts with EDA at −40 °C to form the cyclic ester which on treatment with Me2SiCl2 gives the metal carbene postulated in the catalytic cycle. The stoichiometric reaction of the latter compound with EDA selectively affords the olefin deriv...