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 PhINTs 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.