By synthesising S-2-phenyl-Af-(4-nitrophenyl)aziridine from S-phenylglycinol, it has been demonstrated that the aziridination of styrene by N-(4- nitrobenzenesulfonyl)imino phenyliodinane, (nosyliminophenyliodinane, PhlNNs) in the presence of tS',AS'-2,2,-isopropylidene-bis(4-phenyl-2-oxazoline), catalysed by copper(II) triflate in CH3CN solution or heterogeneously by CuHY, has predominantly an /-configuration. The enantioselectivity of the aziridination of styrene by Af-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 PhlNNs, 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 PhlNTs was first discovered. Evidently, the change of substituent in the benzenesulfonyl group reduces the solubility in hexane, affording a solution phase of enhanced ee.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:584682 |
| Date | January 2009 |
| Creators | Jeffs, Laura Ellen |
| Publisher | Cardiff University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://orca.cf.ac.uk/55128/ |
Page generated in 0.0014 seconds