Catalytic epoxidation of unfunctionalized alkenes by dinuclear nickel(II) complexes

Abstract

The synthesis, crystal and molecular structure and catalytic activity in epoxidation reactions of new dinuclear nickel(II)-complexes, octahedral μ-diacetato- μ-[2,6-bis[ N-2-(2′-pyridylethyl) formimidoyl]phenolato]bisnickel(II)·perchlorate·methanol ( 6) and square planar μ-hydroxo- μ-[2,6-bis[ N-(( S)-1-benzyl-2-yl-pyrrolidine)formimidoyl]phenolato]bisnickel(II)·bisperchlorate (7), are described. For the preparation of 7 a new 5-step route for homochiral bisamine ( S)-benzyl-2-aminomethyl-pyrrolidine ( 19) was developed starting from ( S)-proline. Epoxidation of unfunctionalized alkenes with sodium hypochlorite and tert-butyl hydroperoxide as terminal oxidants was effectively catalyzed with bisnickel(II)-complexes 6 and 7, and a turnover of 165 was reached using trans-β-methylstyrene ( 34). The epoxidations probably proceed via a radical intermediate (such as OCl·) and no enantioselectivity is obtained under phase transfer conditions. In epoxidation reactions employing tert-butyl hydroperoxide as terminal oxidant a turnover of 43 was obtained with trans-stilbene ( 30) as substrate. Unexpectedly in the case of styrene ( 29) 1,2-bis-( tert-butylperoxy)ethylbenzene ( 59) was isolated as the major product.