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

Abstract

A CuNaY catalyst was prepared and used to study the enantioselective aziridination of styrene, with PhI=NTs as the nitrogen source, in the presence of a bis(oxazoline) chiral modifier. The chiral modifier used was a diimine ligand, (S)-(-)-2,2′-isopropylidenebis(4-phenyl-2-oxazoline). EPR spectroscopy 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 solvent. The copper complexes possess square pyramidal and square planar symmetries, with spin Hamiltonian parameters analogous to those of the equivalent homogeneous complex dissolved in solution. These copper(II) complexes accounted for at least 40% of all available copper within the ion exchanged CuNaY catalyst and represent one Cu(II)-bis(oxazoline) complex per supercage. The remaining uncomplexed Cu(II) ions remain solvated to the acetonitrile molecules. 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 planar symmetries but practically no loss in overall Cu(II) content. This was explained on the grounds of a changing co-ordination environment of the encapsulated complex. However when PhI=NTs was added separately to the catalyst a dramatic loss in Cu(II) signal intensity was observed. These results are discussed in terms of the reaction mechanism in operation.