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(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(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 N(2)CHCO(2)CH(2)CH=CHR (3, R = Ph, 4-ClC(6)H(4), 4-BrC(6)H(4), 4-MeC(6)H(4), 4-MeOC(6)H(4), 2-MeC(6)H(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-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))(CHCO(2)CH(2)CH=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))(CHCO(2)CH(2)CH=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) (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-beta (major) and trans (minor) ruthenium-carbene intermediates.