<i>cis</i>-β-Bis(carbonyl) Ruthenium−Salen Complexes: X-ray Crystal Structures and Remarkable Catalytic Properties toward Asymmetric Intramolecular Alkene Cyclopropanation

cis-β-[RuII(salenA)(CO)2] (salenA = N,N′-bis(3-R1-5-R2-salicylidene)-1,2-cyclohexenediamine dianion; R1 = R2 = But, 1a; R1 = Pri, R2 = H, 1b; R1 = But, R2 = H, 1c) complexes were prepared by treating Ru3(CO)12 with the respective H2salenA 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-ClC6H4, 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-β-[RuII(salenA0)(CO)2] (salenA0 = N,N′-bis(salicylidene)-1,2-cyclohexenediamine dianion) reveal the intermediacy of both cis-β- and trans-[Ru(salenA0)(CHCO2CH2CHCHPh)(CO)] bearing salenA0 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(salenA0)(CHCO2CH2CHCHPh)(CO)] were predicted to be 77% ee for the trans-carbene species and 96% ee for the cis-β-carbene species; the former dramatically increases to 98% ee, whereas the latter slightly increases to 99% ee, upon replacing salenA0 with salenA1 (R1 = R2 = Bt). 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 trans (minor) ruthenium−carbene intermediates.