New copper(II) complexes as efficient catalysts for olefin aziridination: the effect of ligand steric hindrance on reactivity

Abstract

Three 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 t (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 dx²--y² 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 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.