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 of 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. Catalysts 1-9 were derived from (R,R)-1,2-diaminocyclohexane and catalyst 10 from (S,S)-1,2-diphenylethylenediamine. Catalysts 1-9 differ in the stereoelectronic substitution of the ortho (6,6') and pma (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 then the para position 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 electronically modifying the para substituents in 7 (6,6' = -tBu; 4.4' = -OMe) and 8 (6,6' = -tBu; 4,4' = -OTIPS) resulted in enhanced enantioselectivities of the desired epoxides. The conformational variations observed in the solid state are likely to reflect accessible solution conformations and may help explain the high levels of stereoinduction obtained with these catalysts in the asymmetric epoxidation of unfunctionalized olefins.