Catalytic asymmetric heterogeneous aziridination of styrene using CuHY/bis(oxazoline):comments on the factors controlling enantioselectivity

摘要

The copper-catalyzed aziridination of styrene is described using both heterogeneous,copper-exchanged zeolite HY,and homogeneous,copper(II) triflate catalysts using obht [N-(p-tolylsulfonyl)imino]phenyliodinane(PhI=NTs) and [N-(p-nosylsulfonyl)imino]phenyliodinane(PhI=NNs) as nitrene donors.The key differences observed for the catalysts when modified by chiral bis(oxazoline)ligands are discussed in detail.In particular,the heterogeneously catalyzed asymmetric reaction can give much higher enantioselection than the comparablej homogeneously catalyzed reaction.The structure of the bis(oxazoline)ligand is the critical factor,and bis(oxazoline)ligands that are ineffective with the homogeneous catalysts are highly effective for the Cu~2+ cation constrained within the zeolite micropores.The consequences of this observation for the design of chiral ligands for asymmetric heterogeneous catalysis are discussed.The effect of the degree of styrene conversion on the enantioselectivity is described in detail using PhI=NNs as a nitrene donor.The reaction shows a significant enhancement in ee with conversion at 25 degC,and the possible origin of this effect is discussed.