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)Cl 2 ]·CH 3 OH 1 , [Cu(L2)Cl 2 ]·CH 3 OH 2 , [Cu(L3)Cl 2 ]·0.5 H 2 O 3 and [Cu(L4)Cl 2 ] 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 ( τ ) for them ( τ : 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 1/2 , −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.