Photolysis and oxidation of aminotriazinone derivatives: a carbon-13 labelling study

Abstract

An Ar=C=O species trapped during the FVT of 2,3-thiophenedicarboxylic anhydride has been shown to be the open structure, the thio-3-ketene-2-carbene 1, rather than the cyclic thienocyclopropenone. Investigations to determine if the same is true for benzocyclopropenone 2 were undertaken. In 1969 Burgess and Rees independently reported the first evidence for the presence of a benzocyclopropenone intermediate in solution based on the isolation of rearranged esters during the photolysis and oxidation of aminobenzotriazinone derivatives in methanol. The evidence for the intermediacy of benzocyclopropenone in the photolysis has now been discredited. This investigation confirms the finding of Rees that both unrearranged and rearranged esters are formed in the oxidation of aminobenzotriazinone 3. The formation of the rearranged esters indicates the possible presence of benzocyclopropenone 2 in the reaction. A ring-opened form of benzocyclopropenone, the ketene carbene 4, was suggested as an intermediate by Wentrup to explain the $\sp{13}$C scrambling of (1,6-$\sp{13}{\rm C}\sb2$) benzyne from the flash vacuum pyrolyses of phthalic anhydride. The ketene carbene, proposed by Wentrup, can undergo a ring contraction to give cyclopentadienylideneketene 5. This equilibrium also could account for the rearrangement discussed above instead of the intermediacy of benzocyclopropenone 2. To distinguish these two mechanisms a doubly $\sp{13}$C labelled aminobenzotriazinone was synthesized from $\rm Ba\sp{13}CO\sb3.$ Barium carbonate-$\sp{13}$C(99%) was converted to the (1,6-$\sp{13}{\rm C}\sb2$) methyl 2-((ethoxyl carbonyl)amino) 2,5-dihydrobenzoate 6 in a one-pot reaction. The synthesis of (5,10-$\sp{13}{\rm C}\sb2$) 3-amino-6-nitrobenzotriazinone was completed in 8 steps from 6. Oxidation of the doubly $\sp{13}$C labelled compound in methanol gave the unrearranged methyl (1,2-$\sp{13}{\rm C}\sb2$) 3-nitrobenzoate 7 and the rearranged methyl (2,3-$\sp{13}{\rm C}\sb2$) 4-nitrobenzoate 8. Based on the isolation of the $\sp{13}$C unscrambled esters 7 and 8, and the prediction of semi-empirical quantum chemical calculations that benzocyclopropenone would have the lowest energy among the possible intermediates, it can be concluded that this intermediate is responsible for the rearrangement during the oxidation of amino-benzotriazinone. This conclusion is supported by the recently published $\sp1$H and $\sp{13}$C NMR spectra of benzocyclopropenone in solution at low temperature.