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

Article abstract of DOI:10.1016/0040-4020(96)00030-0

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]bis nickel(II)·perchlorate·methanol (6) and square planar (μ-hydroxo-μ-[2,6-bis[N-((S)-1-benzyl-2-yl-pyrrolidine)formimidoyl]p henolato]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.