Catalytic asymmetric heterogeneous aziridination of styrene using Cu2+-exchanged zeolite Y: effect of the counter-cation on enantioselectivity and on the reaction profile

Abstract

Effective catalysts have Cu²⁺-exchange levels of ca. 40–60% of the maximum concentration for electroneutrality and, consequently, Cu-zeolite Y catalysts contain an additional counter-cation (H⁺, Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺) whose effect is explored. With Li⁺, Na⁺, K⁺ counter-cations, the zeolite structure is not markedly affected but, with Rb⁺ and Cs⁺, there is some loss of crystallinity. Replacement of H⁺ by group I cations does not markedly influence the overall ee observed for aziridine indicating that the presence of protons in the Cu-HY catalysts are not detrimental to the reaction. Catalysts containing group I cations typically give decreased leaching of Cu²⁺ during the reaction. At low nitrene donor to styrene molar ratios (1∶1), replacement of H⁺ by group I cations leads to a small enhancement in the ee of aziridine, although the yield of aziridine formed is decreased under all reaction condition. At higher molar ratios of nitrene donor to styrene, the ee is suppressed, particularly with Rb and Cs. The effect of reaction time on aziridine yield reveals a reaction profile in which the reaction initially proceeds rapidly, then slows down prior to accelerating again in the latter part of the reaction. This effect is accentuated by increasing the size of the counter-cation. This reaction profile is also observed for the homogeneously catalysed pathway and, consequently, it cannot be due solely to a confinement effect within the zeolite pores. Over addition of reaction by-products (NsNH₂, PhI) accentuates the shape of this reaction profile and the effect is discussed in terms of the interactions of such molecules at the active site.