A structural and mechanistic understanding of copper(II)-bis(oxazoline) catalysed asymmetric aziridination : an EPR and ENDOR investigation

• Main Author: Owen, Mari Elena
• Published: Cardiff University 2013
• Subjects: 546; QD Chemistry
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.590359

X- and Q-band EPR/ENDOR spectroscopy was used to study the structure of a series of heteroleptic and homoleptic copper(II)-bis(oxazoline) complexes, based on the (−)-2,2′-isopropylidenebis[(4S)-4-phenyl-2-oxazoline] (1) ligand and bearing different counterions (chloride versus triflate); labelled [Cu(II)(1a-c)]. The geometry of the two heteroleptic complexes, [Cu(II)(1a)] and [Cu(II)(1c)], was found to depend on the choice of counterion. Formation of the homoleptic complex was only evident when the Cu(II)(OTf)2 salt was used whereas Cu(II)Cl2 inhibited the transformation from heteroleptic to homoleptic complex. The hyperfine and quadrupole parameters for the surrounding ligand nuclei were determined by ENDOR. Well resolved 19F and 1H couplings confirmed the presence of both coordinated water and TfO- counterions in the [Cu(II)(1a)] complex. The structure of other copper(II)-bis(oxazoline) derivatives, based on 2,2’- methylenebis[(4S)-4-phenyl-2-oxazoline] (2), and 2,2’-isopropylidenebis[(4S)-4-tertbutyl- 2-oxazoline (3) were then investigated using X- and Q-band EPR/ENDOR spectroscopy. Variations in the structures of the complexes as a function of different counterions were also investigated; for ligand 2, X = TfO-, Cl- and for ligand 3, X = TfO-, Cl-, SbF6 - (where X = counterion). Formation of the homoleptic complex was evident when the Cu(II)(OTf)2 and Cu(II)Cl2 salts were used with ligand 2, but only when using the Cu(II)(OTf)2 salt with ligand 3. The substituents on the chiral carbons of the oxazoline rings and on the bridging carbon backbone were found to affect the electron spin density of the structures. An X-band EPR investigation of the interaction of substrates (pyridine, iodobenzene, (diacetoxyiodo)benzene, styrene and PhI=NTs) with [Cu(II)(1a)] was also performed, before probing the [Cu(II)(1a)] catalysed asymmetric aziridination of styrene, with PhI=NTs as the nitrene source. Using EPR to monitor the course of the reaction after the addition of styrene + PhI=NTs revealed the formation of an additional paramagnetic species, for which the g/CuA parameters indicated a change to the equatorial environment of the Cu(II) centre. This was discussed within the context of the proposed reaction mechanism which suggests the involvement of a copper-nitrene intermediate.