Stereo- and enantioselective alkene epoxidations: a comparative study of D4- and D2-symmetric homochiral trans-dioxoruthenium(VI) porphyrins.
The mechanism of stoichiometric enantioselective alkene epoxidations by the D4- and D2-symmetric homochiral trans-dioxoruthenium(VI) porphyrins, [RuVI(D4-Por*)O2] (1) and [RuVI(D2-Por*)O2] (2a), in the presence of pyrazole (Hpz) was studied by UV/Vis spectrophotometry and analysis of the organic products. The enantioselectivity of styrene oxidations is more susceptible to steric effects than to substituent electronic effects. Up to 72% ee was achieved for epoxidation of 3-substituted and cis-disubstituted styrenes by employing 1 as the oxidant, whereas entantioselectivities of only 20-40% were obtained in the reactions with 2-substituted and trans-disubstituted styrenes. Complex 2a oxidized 2-substituted styrenes to their epoxides in up to 88% ee. Its reactions with trans-alkenes are more enantioselective (67% ee) than with the cis-alkenes (40% ee). Based on a two-dimensional NOESY NMR study, 2a was found to adopt a more open conformation in benzene than in dichloromethane, which explains the observed solvent-dependent enantioselectivity of its reactions with alkenes. The oxidation of aromatic alkenes by the chiral dioxoruthenium(VI) porphyrins proceeds through the rate-limiting formation of a benzylic radical intermediate; the observed enantioselectivity (eeobs) depends on both the facial selectivity of the first C-O bond formation step and the diastereoselectivity of the subsequent epoxide ring closure. To account for the observed facial selection, "side-on" and "top-on" approach transition state models are examined (see: B. D. Brandes, E. N. Jacobsen, Tetrahedron Lett. 1995, 36, 5123).
Department of Chemistry, University of Hong Kong, Pokfulam Road, Hong Kong.
Rui
R
Zhang
Wing-Yiu
WY
Yu
Hong-Zhe
HZ
Sun
Wei-Sheng
WS
Liu
Chi-Ming
CM
Che
D000475:Alkenes / Q000737:chemistry; D003577:Cytochrome P-450 Enzyme System; D004852:Epoxy Compounds / Q000138:chemical synthesis* / Q000737:chemistry; D008958:Models, Molecular; D009942:Organometallic Compounds / Q000737:chemistry; D011166:Porphyrins / Q000737:chemistry; D012428:Ruthenium / Q000737:chemistry; D013237:Stereoisomerism
D000475:Alkenes; D004852:Epoxy Compounds; D009942:Organometallic Compounds; D011166:Porphyrins; D012428:Ruthenium; D003577:Cytochrome P-450 Enzyme System