X-RAY STRUCTURAL STUDIES OF HIGHLY ENANTIOSELECTIVE MN(SALEN) EPOXIDATION CATALYSTS

The relationship between catalyst structure and enantioselectivity in the asymmetric epoxidation of unfunctionalized olefins by a series of chiral Mn(salen) complexes (1-10) was examined. The X-ray structures o f 5-coordinate complexes 5, 8, of 6-coordinate 9 ([6,6' = -tBu; 4,4' = -tBu](+) ClO4-), and 10 (6,6' = -tBu; 4,4' = -Br) were determined. Ca talysts 1-9 were derived from (R,R)-1,2-diaminocyclohexane and catalys t 10 from (S,S)-1,2-diphenylethylenediamine. Catalysts 1-9 differ in t he stereoelectronic substitution of the ortho (6,6') and pma (4,4') po sitions 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 con formations. The asymmetric epoxidation of indene and 6-cyano-2,2-dimet hylchromene with NaOCl catalyzed by complexes 1-10 was effected. Syste matically increasing the steric bulk on the ortho and then the para po sition in the order 1 (6,6' = -H; 4,4' = -H), 2 (6,6' = -CH3; 4,4' = - CH,),3 (6,6' = -tBu; 4,4' = -H), 4 (6,6' = -tBu; 4,4' = -CH3), 5 (6,6' = -tBu, 4,4' = -tBu), and 6 (6,6' = -tBu; 4,4' = -trityl), and electr onically modifying the para substituents in 7 (6,6' = -tBu; 4.4' = -OM e) and 8 (6,6' = -tBu; 4,4' = -OTIPS) resulted in enhanced enantiosele ctivities of the desired epoxides. The conformational variations obser ved in the solid state are likely to reflect accessible solution confo rmations and may help explain the high levels of stereoinduction obtai ned with these catalysts in the asymmetric epoxidation of unfunctional ized olefins.