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

Abstract

AbstractThree new copper(II) complexes derived from the tridentate N3 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)Cl2] (2) and [Cu(L3)Cl2] (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 orbital of copper(II). While [Cu(L1)(H2O)](ClO4)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 CuII/CuI 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)