Zeolites styrene, chiral aziridination

Scheme 9.22 Chiral aziridination of styrene over a chiral copper bis-oxazoline complex supported in zeolite Y (from reference 59).

The major advantage of the use of CuHY as a catalyst for this reaction is the ease with which it can be recovered from the reaction mixture by simple filtration if used in. a batch reactor (alternatively it can be used in a continuous flow fixed bed reactor). We have carried out the heterogeneous asymmetric aziridination of styrene until completion, filtered and washed the zeolite then added fresh styrene, PhI=NTs and solvent, without further addition of chiral bis(oxazoline), for several consecutive experiments. The yield and the enantioselectivity decline slightly on reuse we have found that adsorbed water can build up within the pores of the zeolite on continued use and we believe that this is the cause of loss of activity and enantioselection. However, full enantioselectivity and yield can be recovered if the catalyst is simply dried in air prior to reuse, or alternatively the catalyst can be recalcined and fresh oxazoline ligand added. 

An approach toward the (asymmetric) aziridination of double bonds was published by ICI [61]. The company disclosed an acidic, copper-impregnated zeolitic material having a pore size large enough for the reactants to enter and for the aziridination product to leave the zeolite supercage. Asymmetric azirid-inations may be conducted by treating the catalyst with a chiral modifier, e.g., the 4,4 -disubstituted bis(oxazoline) 12, before contact with the nitrene donor (PhI=NTs). The aziridines could be isolated in moderate optical purity (30-40% ee)y and Scheme 8 shows the exemplified transformation of styrene (10) to the corresponding aziridine 11. Further details of this topic can be found in a research project, financially supported by Synetix [62]. 

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