PHOTOOXIDATION OF N,N'-DIACYLINDIGO DYES

Irradiation of N,N'-diacylindigo dyes in the presence of oxygen result ed in an unusual oxygen insertion/acyl transfer reaction. While this p rocess occurred over many weeks at room temperature, at 100 degrees C it was complete in a few hours. In some cases the yield-of photooxidiz ed product was virtually quantitative. The product structures were pro ved by spectroscopic methods and in one example by X-ray analysis. A s eries of mechanistic experiments led to the conclusion that at least t he oxygen insertion part of the photooxidation process proceeded via a radical mechanism. The evidence included the following observations: (1) When the photooxidation was conducted in toluene, large amounts of benzyl hydroperoxide were produced; (2) the photooxidation could be a chieved in the dark with a radical initiator; (3) attempts to generate the photooxidized product using singlet oxygen chemistry failed. The postulated radical mechanism is described.