204761-54-6Relevant articles and documents
Half-Sandwich Ruthenium(II) Complexes as Catalysts for Stereoselective Carbene-Carbene Coupling Reactions
Baratta, Walter,Del Zotto, Alessandro,Rigo, Pierluigi
, p. 5091 - 5096 (2008/10/08)
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)(=CHCO 2Et)(PPh3)]. Although this electrophilic carbene was not detected in the reaction mixture, it was independently prepared in solution at low temperature starting from [Ru(η2-02CMe)(η5-C5H 5)(PPh3). The acetate reacts with EDA at -40°C to form the cyclic ester [Ru(CH(CO2Et)OC(Me)O)(η5-C5H 5)(PPh3), 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 derivative [RuCl-(η5-C5H5)(η 2-DEM)(PPh3)], which was also detected in the catalytic cycle. The new complexes [RuCl(η5-C5H5)(PR3)2] (PR3 = PPh2(2-MeC6H4), PPh2Cy, P(3-MeC6H4)3), bearing phosphines bulkier than PPh3, have been prepared in high yield starting from ruthenium trichloride hydrate, cyclopentadiene, and phosphine in refluxing ethanol.