Styrene cyclopropanation and ethyl diazoacetate dimerization catalyzed by ruthenium complexes containing chiral tridentate phosphine ligands

Various five-coordinate ruthenium(II) complexes of the general formula RuCl₂L, where L is a chiral triphosphine ligand, have been tested as catalyst precursors for the cyclopropanation of styrene using ethyl diazoacetate (EDA) as the carbene source. Some of the complexes investigated have been found to catalyze the cyclopropanation reaction, but the conversions to cyclopropanes and the diastereoselectivities were generally poor. With all the catalyst precursors investigated, the dimerization of ethyl diazoacetate to diethyl maleate or diethyl fumarate largely prevailed over cyclopropanation. The best performance was obtained with the complex RuCl₂(ttp*) (2; ttp* = (S,S)-PhP(CH₂CHMeCH₂PPh₂)₂) which gave ca. 21% of cyclopropanes, 42% of olefins, and 1% of metathesis products. The highest enantiomeric excess was 35% for the (Z)-2-phenylcyclopropanecarboxylate. In the presence of silver triflate, 2 gave rise to a much more active and selective catalyst system, as the yield in cyclopropanation products increased up to 84%. Neither the regioselectivity nor the diastereoselectivity was appreciably improved, however. The carbene complexes RuCl₂(ttp*)(=CHCO₂Et) and RuCl₂(ttp)(=CHCO₂Et), where ttp is the achiral ligand PhP(CH₂CH₂CH₂PPh₂)₂, were isolated and characterized by multinuclear NMR spectroscopy. These carbene complexes were found to react with an excess of ethyl diazoacetate to give exclusively diethyl maleate and diethyl fumarate in a 95:5 ratio. The selective formation of cyclopropanation products was conversely observed upon reaction with styrene in the presence of silver triflate. In the absence of a chloride scavenger, cyclopropanes were still formed together with an appreciable amount of the metathesis product PhHC=CHCO₂Et.