title: On the enantioselectivity of aziridination of styrene catalysed by copper triflate and copper-exchanged zeolite Y: consequences of the phase behaviour of enantiomeric mixtures of N-arene-sulfonyl-2-phenylaziridines.
abstract: By synthesising S-2-phenyl-N-(4-nitrophenyl)aziridine from S-phenylglycinol, it has been demonstrated that the aziridination of styrene by [N-(4-nitrobenzenesulfonyl)imino]phenyliodinane (nosyliminophenyliodinane, PhINNs) in the presence of S,S-2,2'-isopropylidene-bis(4-phenyl-2-oxazoline), catalysed by copper(II) triflate in CH(3)CN solution or heterogeneously by CuHY, has predominantly an R-configuration. The enantioselectivity of the aziridination of styrene by [N-arenesulfonylimino]-phenyliodinanes catalysed by copper-exchanged zeolite Y (CuHY), in conjunction with a chiral bis-oxazoline ligand, has been re-examined. In the case of PhINNs, it is shown that the product mixture of enantiomeric aziridines, on treatment with hexane, gives rise to a solid phase of low enantiomeric excess (ee) and a solution phase of high ee. Separation of the solid phase and recrystallisation afforded a true racemate (racemic compound), which has been confirmed by X-ray crystallography. The aziridine obtained from the solution phase could be recrystallised to produce the pure enantiomer originally in excess. A consequence of the new findings is that previous reports on the enantioselectivity of copper-catalysed aziridination, both in heterogeneous and homogeneous conditions, should be regarded with caution if the analytical procedure involved HPLC with injection of the enantiomeric mixture in a hexane-rich solvent. Such a method has been used in previous work from this laboratory, but has also been used elsewhere, following the procedure developed by Evans and co-workers when the (homogeneous) copper-catalysed aziridination by PhINTs was first discovered. Evidently, the change of substituent in the benzenesulfonyl group reduces the solubility in hexane, affording a solution phase of enhanced ee.
authors.affiliation: School of Chemistry, Cardiff University, Cardiff, CF10 3AT, UK.
authors.forename: Laura
authors.initials: L
authors.lastname: Jeffs
authors.forename: Damien
authors.initials: D
authors.lastname: Arquier
authors.forename: Benson
authors.initials: B
authors.lastname: Kariuki
authors.forename: Donald
authors.initials: D
authors.lastname: Bethell
authors.forename: Philip C Bulman
authors.initials: PC
authors.lastname: Page
authors.forename: Graham J
authors.initials: GJ
authors.lastname: Hutchings
mesh_terms: D001388:Aziridines / Q000737:chemistry*; D002384:Catalysis; D003300:Copper / Q000737:chemistry*; D018360:Crystallography, X-Ray; D008698:Mesylates / Q000737:chemistry*; D008958:Models, Molecular; D044367:Phase Transition; D013237:Stereoisomerism; D020058:Styrene / Q000737:chemistry*; D013457:Sulfur Compounds / Q000737:chemistry*; D017641:Zeolites / Q000737:chemistry*
chemical_list: D001388:Aziridines; D008698:Mesylates; D013457:Sulfur Compounds; C548824:copper(I) triflate; D017641:Zeolites; D020058:Styrene; D003300:Copper