Benzo[c]cinnoline N-imides

Abstract

Benzo[c]cinnoline N-imide (1) is formed quantitatively by mild thermal isomerisation of dibenzo[d,f][1,2,3]-triazepine (2), or directly from 2,2′-diaminobiphenyl by aprotic diazotisation without isolation of the intermediate triazepine (2). The N-imide (1) is also formed by amination of benzocinnoline, preferably with O-mesitylsulphonyl-hydroxylamine. The N-imide (1) is readily converted into N-alkyl, -aryl, -acyl, and -alkoxycarbonyl derivatives (5), and the spectroscopic and chemical properties of all the N-imides are in accord with the presence of the proposed stable 1,3-dipolar 4 π-electron azimine system. Photolysis of the N-imides gave benzocinnoline and the nitrene, R–N:. Pyrolysis of the parent N-imide (1) gave carbazole exclusively whereas the N-methylimide gave only benzocinnoline. The N-benzoyl-, N-acetyl-, and N-ethoxycarbonyl-imides gave both N-substituted carbazoles and benzocinnoline. The results of ¹⁵N-labelling and other experiments, including conversion of the N-imide (1) into carbazole, suggest that isomerisation of the triazepine (2) into the N-imide (1) is reversible, and involves the ring-opened, diazo-imine form of the triazepine as intermediate (Scheme 2).