Substituted salen-Ru(II) complexes as catalysts in the asymmetric cyclopropanation of styrene by ethyl diazoacetate: the influence of substituents and achiral additives on activity and enantioselectivity

A series of alkyl-, halogen- and nitro-substituted salen ligands, 1, have been employed in the asymmetric cyclopropanation of styrene with ethyl diazoacetate by its ruthenium(II) complex with [RuCl2(p-cymene)](2) or RuCl2(PPh3)(3) as precursors. The introduction of appropriate electron withdrawing groups in the salen ligands benefited the enantioselectivity of the reaction. Some additives, including O-donor, N-donor and P-donor ligands, were added to the reaction to improve the enantioselectivity and activity, and e.e.s of up to 80% were achieved. In the salen/[RuCl2(p-cymene)](2) system, the (1R,2S)-isomer was obtained in 80.2% e.e. by using the salen ligand If derived from 3,5-dibrominated salicylaldehyde with Et3N as additive. E.e.s of up to 81.3% for (1S,2R)-isomers were achieved by using the complex 2 synthesized from the nitro-substituted ligand Im and RuCl2(PPh3)(3). A possible mechanism was also discussed. (C) 2001 Elsevier Science Ltd. All rights reserved