Chiral Ruthenium(II)-Bis(2-oxazolin-2-yl)pyridine Complexes. Asymmetric Catalytic Cyclopropanation of Olefins and Diazoacetates

Article abstract of DOI:10.1246/bcsj.68.1247

A chiral ruthenium(II)-bis(2-oxazolin-2-yl)pyridine catalyst prepared in situ from optically active bis(2-oxazolin-2-yl)pyridine (Pybox-ip) (2) and 2 (1) exhibited efficient activity for the asymmetric cyclopropanation (ACP) of styrene and several diazoacetates to give the corresponding trans- and cis-2-phenylcyclopropane-1-carboxylates (3 and 4) in good yields (66-87percent).A mixture of 1 and 2 in an atmosphere of ethylene produced the trans-RuCl2 (Pybox-ip)(ethylene) complex (5), which also proved to be a poweful catalyst for ACP.The stereoselectivity of the trans- and cis-cyclopropanes were from 90:10 up to 98:2, and their enantioselectivities reached 97percent.A catalytic system with 5 could be used for several olefins and internal olefins.A concerted mechanism of ACP with the Ru-Pybox catalyst was postulated on the basis of the stereospecificity with deuterated styrene.Other substituents, such as ethyl, s-butyl, benzyl, and phenyl on the oxazoline rings of Pybox in place of the isopropyl group were also examined, and were found to exhibit a similar high trans-stereoselection and a high enantioselectivity of the cyclopropane products; ex . with Ru-Pybox-ethyl 91:9 of 3:4 and 82percent ee for 3.The non-chiral Ru-Pybox-dihydro catalyst exhibited asymmetric induction with 39percent ees of 3 with d- and l-menthyl diazoacetates, and kept a high trans- and cis-stereoselection of 97-3.