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

Cyclopentadienyl complexes of general formula [RuX(η 5 -ligand)(PR 3 ) 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 η 5 -ligand as well as the nature of the anionic ligand X. A detailed study of the reaction between [RuCl(η 5 -C 5 H 5 )(PPh 3 ) 2 ] and EDA suggests that dissociation of one PPh 3 is a key step of the catalytic process, which proceeds via the intermediate [RuCl(η 5 -C 5 H 5 )( CHCO 2 Et)(PPh 3 )]. Although this electrophilic carbene was not detected in the reaction mixture, it was independently prepared in solution at low temperature starting from [Ru(η 2 -O 2 CMe)(η 5 -C 5 H 5 )(PPh 3 ). The acetate reacts with EDA at −40 °C to form the cyclic ester which on treatment with Me 2 SiCl 2 gives the metal carbene postulated in the catalytic cycle. The stoichiometric reaction of the latter compound with EDA selectively affords the olefin derivative [RuCl(η 5 -C 5 H 5 )(η 2 -DEM)(PPh 3 )], which was also detected in the catalytic cycle. The new complexes [RuCl(η 5 -C 5 H 5 )(PR 3 ) 2 ] (PR 3 = PPh 2 (2-MeC 6 H 4 ), PPh 2 Cy, P(3-MeC 6 H 4 ) 3 ), bearing phosphines bulkier than PPh 3, have been prepared in high yield starting from ruthenium trichloride hydrate, cyclopentadiene, and phosphine in refluxing ethanol.