An EPR study on the enantioselective aziridination properties of a CuNaY zeolite

A CuNaY catalyst was prepared and used to study the enantioselective azirid ination of styrene, with PhI2=NTs as the nitrogen source, in the presence o f a bis(oxazoline) chiral modifier. The chiral modifier used was a diimine ligand, (S)-(-)-2,2'-isopropylidenebis(4-phenyl-2-oxazoline). EPR spectrosc opy provides the first direct experimental evidence for the formation of a copper(II)-bis(oxazoline) complex inside the Y zeolite pores after stirring the calcined catalysts with the chiral ligand using acetonitrile as solven t. The copper complexes possess square pyramidal and square planar symmetri es, with spin Hamiltonian parameters analogous to those of the equivalent h omogeneous complex dissolved in solution. These copper(II) complexes accoun ted for at least 40% of all available copper within the ion exchanged CuNaY catalyst and represent one Cu(II)-bis(oxazoline) complex per supercage. Th e remaining uncomplexed Cu(II) ions remain solvated to the acetonitrile mol ecules. After the aziridination reaction was carried out in the presence of styrene and PhI=NTs, EPR evidenced the selective loss of the signal due to the copper(II)-bis(oxazoline) complex with square pyramidal and square pla nar symmetries but practically no loss in overall Cu(II) content. This was explained on the grounds of a changing co-ordination environment of the enc apsulated complex. However when PhI=NTs was added separately to the catalys t a dramatic loss in Cu(II) signal intensity was observed. These results ar e discussed in terms of the reaction mechanism in operation.