Allylic amination and aziridination of olefins by aryl azides catalyzed by Co(II)(tpp) : a synthetic and mechanistic study

CoII(TPP) catalyzes the reaction of aromatic azides (ArN3) with non-activated olefins to yield allylic amines or aziridines in moderate to good yields. The chemoselectivity of the catalytic reaction is particularly high. Depending on the substrate employed allylic amines or aziridines can be obtained. The reaction mechanism has been investigated. The reaction proceeds through a reversible coordination of the aryl azide to the CoII-porphyrin complex. The often postulated “nitrene” complex is not an intermediate in this reaction. The kinetics for the allylic amination is first order in azide, Co(TPP) and olefin. For the aziridination the kinetics is again first order in azide and catalyst, but we observed a first order dependence of the rate on alpha-methylstyrene only up to a 6.9 M concentration of the olefin. An inhibiting role of the competitively formed 1-(4-nitrophenyl)-5-methyl-5-phenyl-1,2,3-triazoline was identified. The triazoline was shown to reversibly coordinate to Co(TPP) blocking the free coordination site necessary for the catalytic reaction to proceed and is responsible for the catalyst deactivation in the aziridination reaction of alpha-methylstyrene by 4-nitrophenyl azide.