New Copper(II) Complexes as Efficient Catalysts for Olefin Aziridination: The Effect of Ligand Steric Hindrance on Reactivity

Three new copper(II) complexes derived from the tridentate N 3 ligands 4‐methyl‐1‐(pyrid‐2‐ylmethyl)‐1,4‐diazacycloheptane (L1), 1‐(quinol‐2‐ylmethyl)‐1,4‐diazacycloheptane (L2), and 4‐methyl‐1‐(quinol‐2‐ylmethyl)‐1,4‐diazacycloheptane (L3) have been prepared and examined as copper catalysts for olefin aziridination. In the X‐ray crystal structures of the complexes [Cu(L2)Cl 2 ] ( 2 ) and [Cu(L3)Cl 2 ] ( 3 ) copper(II) adopts a trigonal‐bipyramidal distorted square‐based pyramidal geometry (TBDSBP) as seen from the values of the trigonality index τ (0.08 for 2 and 0.48 for 3 ). The enhanced trigonal distortion in 3 is due to the presence of an N‐Me group, the lone pair orbital of which is not oriented exactly along the d   x   2 – y   2orbital of copper(II). While [Cu(L1)(H 2 O)](ClO 4 ) 2 ( 1 ) assumes a tetragonal geometry in solution, complexes 2 and 3 adopt a distorted tetragonal geometry, as revealed by UV/Vis and EPR spectral studies. The complexes undergo quasi‐reversible Cu II /Cu I redox behavior in methanol solution. The ability of the complexes to mediate nitrene transfer from PhINTs to olefins to form N ‐tosylaziridines has been studied. The complexes are found to be efficient catalysts (in 5 mol‐% amounts) for the aziridination of the reactive olefin styrene, with yields varying from 80 to 90 % (with respect to PhINTs). They exhibit significant catalytic nitrene transfer reactivity (yields of 30 to 60 %) also towards the less reactive olefins cyclohexene and cyclooctene. A remarkable observation is the significantly accelerated rate of aziridination by 3 , which is ascribed to the steric crowding around copper(II) imposed by the bulky quinolyl and N‐Me groups of the tridentate ligand L3.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)