CHIRAL RUTHENIUM(II)-BIS(2-OXAZOLIN-2-YL) PYRIDINE COMPLEXES - ASYMMETRIC CATALYTIC CYCLOPROPANATION OF OLEFINS AND DIAZOACETATES

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 [RuCl2(p-cymene)](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 (66-87%). 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 powerful catalyst for ACP. The stereoselectivity of the trans- and cis-cyclopropanes were from 90:10 up to 98:2, and their enantioselectivities reached 97%. 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 82% ee for 3. The non-chiral Ru-Pybox-dihydro catalyst exhibited asymmetric induction with 39% ees of 3 with d- and l-menthyl diazoacetates, and kept a high trans- and cis-stereoselection of 97:3.