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.

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A mechanism that is simpler than the recently advanced is followed for the (salen)Mn‐catalyzed asymmetric epoxidation of olefins. Oxametallacycles such as B or B′ are not viable intermediates; radical species such as A, which are directly formed by attack of the olefin on the oxomanganese catalyst are more likely candidates; salen  N,N′‐bis(salicylidene)ethylenediamine dianion.

Abstract:The relationship between catalyst structure and enantioselectivity in the asymmetric epoxidation of unfunctionalized olefins by a series of chiral Mn (sa1en) complexes (1 -10) was examined. The Xray structures of 5-coordinate complexes 5, 8, of 6-coordinate 9 ([6,6 = -fBu; 4,4 = -tBu]+CIO;), and 10 (6,6 = -fBu; 4.4 = -Br ) were determined. Catalysts 1 -9 were derived from (R,R)-1 ,tdiaminocyclohexane and catalyst 10 from (S,S)-1,2-diphenylethyIenediamine. Catalysts 1-9 differ in the stereoelectronic substitution of the orfho (6,6) and para (4,4) positions of the salicylidene moiety. A comparison between structures 5, 8, and 9 reveals that the ligand geometry around the metal center and the chiral diimine backbone remains remarkably constant in both five-and six-coordinate cyclohexanediamine-derived complexes; in contrast, the salicylidene regions of the complexes display a wide range of conformations. The asymmetric epoxidation of indene and 6-cyano-2,2-dimethylchromene with NaOCl catalyzed by complexes 1 -10 was effected. Systematically increasing the steric bulk on the ortho and vtynordp asymmetric eponidations Cstalysis * manganese complexes * structure elucidation

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