Olefin aziridination by copper(II) complexes: effect of redox potential on catalytic activity

Abstract

A series of new copper(II) complexes of four sterically hindering linear tridentate 3N ligands N'-ethyl-N'-(pyrid-2-ylmethyl)-N,N-dimethylethylenediamine (L1), N'-benzyl-N'-(pyrid-2-ylmethyl)-N,N-dimethylethylenediamine (L2), N'-benzyl-N'-(6-methylpyrid-2-yl-methyl)-N,N-dimethylethylenediamine (L3) and N'-benzyl-N'-(quinol-2-ylmethyl)-N,N-dimethylethylenediamine (L4) have been isolated and examined as catalysts for olefin aziridination. The complexes [Cu(L1)Cl₂]·CH₃OH 1, [Cu(L2)Cl₂]·CH₃OH 2, [Cu(L3)Cl₂]·0.5 H₂O 3 and [Cu(L4)Cl₂] 4 have been structurally characterized by X-ray crystallography. In all of them copper(II) adopts a slightly distorted square pyramidal geometry as inferred from the values of trigonality index (t) for them (Γ : 1, 0.02; 2, 0.01; 3, 0.07; 4, 0.01). Electronic and EPR spectral studies reveal that the complexes retain square-based geometry in solution also. The complexes undergo quasireversible Cu(II)/Cu(I) redox behavior (E_½, -0.272 - -0.454 V) in acetonitrile solution. The ability of the complexes to mediate nitrene transfer from PhINTs and chloramine-T trihydrate to olefins to form N-tosylaziridines has been studied. The complexes 3 and 4 catalyze the aziridination of styrene very slowly yielding above 80% of the desired product. They also catalyze the aziridination of the less reactive olefins like cyclooctene and n-hexene but with lower yields (30-50%). In contrast to these two complexes, 1 and 2 fail to catalyze the aziridination of olefins in the presence of both the nitrene sources. All these observations have been rationalized based on the Cu(II)/Cu(I) redox potentials of the catalysts.