The Mechanistic Basis for Electronic Effects on Enantioselectivity in the (salen)Mn(III)-Catalyzed Epoxidation Reaction

Abstract

Enantioselectivity in the (salen)Mn-catalyzed asymmetric epoxidation reaction correlates directly with the electronic properties of the ligand substituents, with complexes bearing electron-donating substituents affording highest ee's. Several lines of evidence point to a single factorcontrol of the position of the transition state along the reaction coordinateas being responsible for the electronic effects on enantioselectivity. Analysis of the epoxidation of cis-β-deuteriostyrene reveals that electron-rich catalysts display a more pronounced secondary inverse isotope effect than electron-deficient catalysts. A strong correlation between ΔΔH ⧧ and the electronic character of the catalyst is also observed. The conclusion that enantioselectivity is tied to the position of a transition state along the reaction coordinate may hold general implications for the design of asymmetric catalysts, particularly those that effect reactions without substrate precoordination.

Epoxidation of unfunctionalized olefins by Mn(salen) catalyst using organic peracids as oxygen source: A theoretical study

“…To verify this prediction, experimental kinetics data on the epoxidation rates would be indispensable. Unfortunately, recent experimental mechanistic studies of the Mn III (salen)-catalyzed epoxidation by peracids lack solid kinetics data due to very fast (within 1 min) epoxidation rates even at Ϫ78°C (14,(16)(17)(18)(19)35). Nevertheless, for closely related Mn III (porphyrin) catalysts the rates of the Mn V oxo species formation and͞or subsequent epoxidation have been estimated in water and acetonitrile at room temperature and agree with our predictions (51,52).…”

“…The latter is tentatively attributed to the direct epoxidation by acylperoxo complexes of Mn III (salen) (30)(31)(32). Despite recent experimental mechanistic studies of the Mn III (salen)-catalyzed epoxidation by peracids, the nature of oxygenating species remains uncertain (14,(16)(17)(18)(19)35).…”

Polymer-supported salen complexes for heterogeneous asymmetric synthesis: Stability and selectivity

“…30 In addition, the enantioselectivity for these reactions has been demonstrated to be sensitive to changes in the ligand structure. 31,32 Earlier work on heterogenizing asymmetric homogeneous catalysts for oxidation involved the incorporation of chiral moieties such as the tartrate-titanium isopropoxide system of Sharpless for epoxidation 33,34 and various cinchona alkaloid derivatives for dihydroxylation onto polymeric supports. 1,[35][36][37] While the enantioselectivities achieved with these catalysts have approached those of the homogeneous catalysts, they suffered upon recycle due to leaching or a lack of stability.…”

Efficient Biomimetic Oxidative Decarboxylation of Some Carboxylic Acids Catalyzed by a Manganese (III) Schiff Base Complex

“…[1][2][3] Several oxidants such as iodosylbenzene, hypochlorite, m-chloroperbenzoic acid, hydrogen peroxide, and periodates have been have been applied as oxygen source for performance of catalytic role of those compounds to understand the mechanism of cytochrome P-450 monooxygenase enzyme. [4][5][6][7][8] Among the various coordination compounds, especially, metalloporphyrins and the metal Schiff base complexes including salen and salophen ligands aroused the interest of synthetic chemists as model compounds for the active site of cytochrome P-450 due to their electronic structure and catalytic activity.…”