cis-beta-Bis(carbonyl) Ruthenium-Salen Complexes: X-ray Crystal Structures and Remarkable Catalytic Properties toward Asymmetric Intramolecular Alkene Cyclopropanation

cis-beta-[Ru-II(salen(A))(CO)(2)] (salen(A) = N,N'-bis(3-R-1-5-R-2-salicylidene)-1,2-cyclohexenediamine dianion; R-1 = R-2 = Bu-1, 1a; R-1 = Pr-i, R-2 = H, 1b; R-1 = Bu-1, R-2 = H, 1c) complexes were prepared by treating Ru-3(CO)(12) with the respective H(2)salen(A) in 1,2,4-trichlorobenzene and structurally characterized by X-ray crystallography. Complexes 1a-c catalyze intramolecular cyclopropanation of trans-allylic diazoacetates N2CHCO2CH2CH=CHR (3, R = Ph, 4-CIC6H4, 4-BrC6H4, 4-MeC6H4, 4-MeOC6H4, 2-MeC6H4, 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-beta-[Ru-II(salen(A0)) (CO)(2)] (salen(A0) = N,N'-bis(salicylidene)-1,2-cyclohexenediamine dianion) reveal the intermediacy of both cis-beta- and trans[Ru(salen (A0))(CHCO2CH2CH=CHPh)(CO)] bearing salen(A0) in a nonplanar and planar coordination mode, respectively, with the cis-beta-carbene species being a major intermediate in the catalytic carbenoid transfer reaction. The intramolecular cyclopropanation from the cis-beta-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))(CHCO2CH2CH=CHPh)(CO)] were predicted to be 77% ee for the trans-carbene species and 96% ee for the cis-beta-carbene species; the former dramatically increases to 98% ee, whereas the latter slightly increases to 99% ee, upon replacing salen(A0) with salen(A1) (RI = R 2 = B). 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-beta (major) and trans (minor) ruthenium-carbene intermediates.