Cis-β-bis(carbonyl) Ruthenium-salen Complexes: X-raycrystal Structures And Remarkable Catalytic Propertiesrntoward Asymmetric Intramolecular Alkene Cyclopropanation

摘要

cis-β-[Ru~(II)(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~i, 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-CIC_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~(II)(salen~(AO))(CO)_2] (salen~(AO) = N,N'-bis(salicylidene)-1,2-cyclohexenediamine dianion) reveal the intermediacy of both cis-β- and trans-[Ru(salen~(AO))(CHCO_2CH_2CH=CHPh)(CO)] bearing salen~(AO) 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~(AO))(CHCO_2CH_2CH=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 salen~(AO) 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 trans (minor) ruthenium-carbene intermediates.