Radical ions and photochemical charge-transfer phenomena. 39. Cycloadditions. 42. Radical cation [3 + 2] cycloadditions of 2H-azirines. Mechanistic studies concerning the intermediate radical cation

Abstract

The product of the ring opening of 3-aryl-substituted azirines under conditions of photoinduced electron transfer (PET) is shown to be not a nitrile ylide but a short-lived 2-azaallenyl radical cation. This species has been investigated by pulse radiolysis and gamma-radiolysis techniques. The radical cation has an absorption band with lambda(max) at 485 nm. The lifetime of the radical cation is tau = 1.43 mus in n-butyl chloride. The rate constant for the reaction of the radical cation with the imine 9 is 7.8 X 10(9) M-1 s-1. Gamma-Radiolysis in n-butyl chloride leads to the same products, i.e., an N-alkylated imidazole as in the case of PET.