Asymmetric inter- and intramolecular cyclopropanation of alkenes catalyzedby chiral ruthenium porphyrins. Synthesis and crystal structure of a chiral metalloporphyrin carbene complex

Extensive investigations of asymmetric intermolecular cyclopropanation of t erminal alkenes with diazoacetates catalyzed by ruthenium porphyrin [Ru(P*) (CO)(EtOH)] (1, H2P* = 5,10,15,20-tetrakis-{[1S,4R,5R,8S)-1,2,3,4,5,6,7,8-o ctahydro-1,4:5,8-dimethanoanthracene-9-yl}porphyrin) and the application of catalyst 1 to asymmetric intramolecular cyclopropanation of allylic or hom oallylic diazoacetates are described. The intermolecular cyclopropanation o f styrene and its derivatives with ethyl diazoacetate afforded the correspo nding cyclopropyl esters in up to 98% ee with high trans/cis ratios of up t o 36 and extremely high catalyst turnovers of up to 1.1 x 10(4). Examinatio n of the effects of temperature, diazoacetate, solvent, and substituent in the intermolecular cyclopropanation reveals that (i) both enantioselectivit y and trans selectivity increase with decreasing temperature, (ii) sterical ly encumbered diazoacetates N2CHCO2R, such as R = Bu-t, and donor solvents, such as diethyl ether and tetrahydrofuran, are beneficial to the trans sel ectivity, and (iii) electron-donating para substituents on styrene accelera te the cyclopropanations, with the log(k(X)/k(H)) vs sigma (+) plot for par a-substituted styrenes p-X-C6H4CH=CH2 (X = MeO, Me, Cl, CF3) exhibiting goo d linearity with a small negative rho (+) value of -0.44 +/- 0.09. In the c ase of intramolecular cyclopropanation, complex 1 promoted the decompositio n of a series of allylic diazoacetates to form the cyclopropyl lactones in up to 85% ee, contributing the first efficient metalloporphyrin catalyst fo r an asymmetric intramolecular cyclopropanation. Both the inter- and intram olecular cyclopropanations were proposed to proceed via a reactive chiral r uthenium carbene intermediate. The enantioselectivities in these processes were rationalized on the basis of the X-ray crystal structures of closely r elated stable chiral carbene complexes [Ru(P*)(CPh2)] (2) and [Ru(P*)(C(Ph) -CO2CH2CH=CH2)] (3) obtained from reactions of complex 1 with N2CPh2 and N2 C(Ph)CO2CH2CH=CH2, respectively.