The origin of diastereoselectivity in cyclopropanation reactions of the iron carbene complex [(eta(5)-C5H5)(CO)(2)Fe=CH[(eta(6)-o-CH3OC6H4)Cr(CO)(3)]](+)

The origin of diastereoselectivity in cyclopropanation reactions of the iro n car:bene complex [(eta (5)-C5H5)(CO)(2)Fe=CH[(eta (6) -0-CH3OC6H4)Cr(CO)( 3)](+) has been investigated. The chromium participation has been proved by the upfield shifts of both H-1- and C-13-NMR for C-alpha-H and C-alpha, wh ich stabilized the iron carbene and may result in a late transition-state f or the cyclopropanation reaction via tricarbonylchromium-complexed iron car bene. Hence, trans selectivity is expected. However, when using aromatic al kenes, a pi stacking effect may exist during the transition-state, resultin g in the cis selectivity for cyclopropanation. (C) 2001 Elsevier Science B. V. All rights reserved.