Trapping of 1,8-biradical intermediates by molecular oxygen in photocycloaddition of naphthyl-N-(naphthylcarbonyl)carboxamides; Formation of novel 1,8-epidioxides and evidence of stepwise aromatic cycloaddition

The photocycloaddition reaction of naphthyl-N-(naphthylcarhdnyl)carboxamide s was examined under argon and oxygen atmospheres. In addition to the [2 2] and [4 + 4]cycloadducts, 3 and 4, respectively, novel 1,8-epidioxides (5 ) were formed a under oxygen atmosphere. We transient absorption at lambda (max) of 360 nm with the lifetime of 360 ns was observed by laser flash pho tolysis of Ic and was interpreted as the absorption of biradical intermedia te 2. On the basis of the anti I stereochemistry of 5, which was different from that of the major [4 + 4] cycloadducts, syn-4, it was deduced that; eq uilibrium between biradical intermediates syn-2 and anti-2 ould exist. Retr o [2 + 2] cycloaddition of 3 was responsible for the efficient trapping of the biradical intermediate with molecular oxygen. The photocycloaddition of the anthryl derivatives, 9-anthryl-N-(methylethyl)-N-(naphthylcarbonyl)car boxamides (7), afforded the [4 + 4] cycloadducts (8) exclusively in a quant itative yield even under oxygen atmosphere. The absence of trapping with mo lecular oxygen was interpreted to de due to the lack of retro [4 + 4] cyclo addition of 8.