1,3,7-Triazapyrene: the first case of hetarylation of benzene and its analogs

Abstract

The reactions of heteroaromatic cations with neutral aromatic compounds is an important method for the introduction of heterocyclic substituents into the molecules of organic compounds. Depending on the nature of the substrate, similar processes are known as electrophilic hetarylation of arenes or nucleophilic arylation of hetarenes; such reactions are well known for various azines [1-5]. Generally azaheterocycles are activated by N-acylation, N-protonation, or N-alkylation, but π-rich arenes or hetarenes (phenols, arylamines, their O- and N-alkyl derivatives, pyrrole, indole, etc.) are used as the aromatic reagents. This process has found use for the construction of new heterocyclic systems [6, 7]. During a study of the nucleophilic substitution of hydrogen in a series of 1,3,7-triazapyrenes [8-11] we observed a reaction unusual for azines, namely, arylation by benzene and its homologs. The reaction occurred in a polyphosphoric acid (PPA) medium containing 86% P2O5 [12]. The reaction of 1,3,7-triazapyrene (1) with a large excess of benzene occurred at the boiling point of the latter, and in the case of toluene or o-xylene at 100-110°C. The process was accompanied by the development of a deep red coloration that is characteristic of the dihydrotriazapyrene structure. However, we were unable to isolate the intermediate products resulting from the nucleophilic addition of arenes – the σ-adducts 2a-c – since on alkalinization they underwent oxidative dehydroaromatization in the presence of air oxygen with the formation of 6-phenyl- (3a), 6-p-tolyl- (3b), and 6-(3,4-dimethylphenyl)-1,3,7-triazapyrene (3c), respectively.