Highly efficient asymmetric epoxidation of alkenes with a D4-symmetric chiral dichlororuthenium(IV) porphyrin catalyst

Abstract

A dichlororuthenium(IV) complex of 5,10,15,20-tetrakis[(1S,4R,5R,8S)-1,2,3,4,5,6,7,8-octahydro-1,2: 5,8-dimethanoanthrance-9-yl] porphyrin, [Ru IV(D4-Por*)Cl 2] (1), was prepared by heating [Ru II-(D4-Por*)(CO)(MeOH)] (2) in refluxing CCl 4. Complex 1 is characterized by 1H NMR (paramagnetically shifted pyrrolic protons at δ H = -52.3 ppm), FAB-mass spectroscopies, and magnetic susceptibility measurement (μ eff = 3.1 μ B). The ruthenium complex exhibits remarkable catalytic activity toward enantioselective alkene epoxidation using 2,6-dichloropyridine N-oxide (Cl 2pyNO) as terminal oxidant. The Ru(IV)-catalyzed styrene epoxidation is achieved within 2 h (versus 48 h for the 2-catalyzed reaction), and optically active styrene oxide was obtained in 69% ee and 84% yield (875 turnovers). Likewise, substituted styrenes and some conjugated cis-disubstituted alkenes (e.g., cis-β-methylstyrene, cis-1-phenyl-3-penten-1-yne, 1,2-dihydronaphthalene, and 2,2-dimethylchromenes) are converted effectively to their organic epoxides in 50-80% ee under the Ru(IV)-catalyzed conditions, and more than 850 turnovers of epoxides have been attained. When subjecting 1 to four repetitive uses by recharging the reaction mixture with Cl 2pyNO and styrene, styrene oxide was obtained in a total of 2190 turnovers and 69% ee. UV - vis and ESI-mass spectral analysis of the final reaction mixture revealed that a ruthenium - carbonyl species could have been formed during the catalytic reaction, leading to the apparent catalyst deactivation. We prepared a heterogeneous chiral ruthenium porphyrin catalyst by immobilizing 1 into sol - gel matrix. The heterogeneous catalystis highly active toward asymmetric styrene epoxidation producing styrene oxide in 69% ee with up to 10 800 turnovers being achieved. The loss of activity of the Ru/sol - gel catalyst is ascribed to catalyst leaching and/or deactivation. On the basis of Hammett correlation (ρ + = -1.62, R = 0.99) and product analysis, a dioxoruthenium(VI) porphyrin intermediate is not favored.