THERMAL AND PHOTODECOMPOSITION STUDIES WITH EXO- AND ENDO-5-METHOXY-2,3-DIAZABICYCLO[2.2.1]HEPT-2-ENE SYSTEM

Decomposition of the exo and endo epimers of 5-methoxy-2,3-diazabicyclo[2.2.1]hept-2-ene (lx and In, respectively) by either heat or light leads to product which consists almost entirely of the cis and trans isomers of 2-methoxybicyclo[2.1.0]pentane (cis-Ill and trans-Ill, respectively). The two azo epimers give widely different cis-trans product ratios by gas phase thermolysis. Similarly, direct photolysis in the liquid phase gives a different product ratio for each epimer. It is of particular interest that in both of these decompositions each epimer produces product which shows a net crossover of configuration, i.e., excess trans-III from lx and excess cis-III from In. Benzophenone-sensitized photolysis leads to the same product ratio for each epimer. In contrast, direct photolysis of crystalline lx and In affords product with highly retained structure for each epimer. All of the observations can be explained readily on the basis of a mechanistic scheme involving interconverting pyramidal diradicallike intermediates which are shortlived. To account for the cases of product crossing over, it is proposed that structurally inverted diradicals arise directly upon nitrogen elimination from lx and In and that closure occurs before the intermediates fully equilibrate. This inversion is thought to be a consequence of recoil from C-N bond breaking. Significant differences in product ratios from thermolysis, direct photolysis, and sensitized photolysis provide evidence that ground-state singlet diradicals, electronically excited singlet diradicals, and electronically excited triplet diradicals are involved, respectively. © 1969, American Chemical Society. All rights reserved.