Cis-β-bis(carbonyl) ruthenium-salen complexes: X-ray crystal structures and remarkable catalytic properties toward asymmetric intramolecular alkene cyclopropanation

Abstract

cis-β-[Ru"(salen A)(CO) 2] (salen A = N,N-bis(3-R 1-5-R 2-salicylidene)-1,2-cyclohexenediamine di- anion; R 1 = R 2 = Bu t, 1a;R 1 = Pr t,R 2 = H, 1b;R 1 = Bu t,R 2 = H, 1c) complexes were prepared by treating Ru 3(CO) 12 with the respective H 2salen A in 1,2,4-trichlorobenzene and structurally characterized by X-ray crystallography. Complexes 1a-c catalyze intramolecular cyclopropanation of trans-allylic diazoacetates N 2CHCO 2CH 2CH=CHR (3,R = Ph, 4-ClC 6H 4, 4-BrC 6H 4, 4-MeC 6H 4, 4-MeOC 6H 4, 2-MeC 6H 4, 2-furanyl) under light irradiation to give cyclopropyl lactones 4 in up to 96% yield and up to 98% ee. DFT calculations on intramolecular cyclopropanation of 3a (R = Ph) with model catalyst cis-β-[Ru ll(salen A0)(CO) 2] (salen A0 = N, N-bis(salicylidene)-1,2-cyclohexenediamine dianion) reveal the intermediacy of both cis-β- and trans- [Ru(salen A0) (CHCO 2CH2CH=CHPh)(CO)] bearing salen A0 in a nonplanar and planar coordination mode, respectively, with the cis-β-carbene species being a major intermediate in the catalytic carbenoid transfer reaction. The intramolecular cyclopropanation from the cis-β-carbene species is the most favorable pathway and features an early transition state and an asynchronous concerted [2 + 1] addition mechanism. Enantioselectivities in the reactions involving [Ru(salen A0)(CHCO 2CH 2CH=CHPh)(CO)] were predicted to be 77% ee for the trans-carbene species and 96% ee for the cis-p-carbene species; the former dramatically increases to 98% ee, whereas the latter slightly increases to 99% ee, upon replacing salen A0 with salen A1 (R 1 = R 2 = B t). The observed variation in enantioselectivity (90-98% ee) for the conversion of 3a to 4a catalyzed by 1a-c could result from an equilibrium between cis-β (major) and tans (minor) ruthenium-carbene intermediates. © 2009 American Chemical Society.