BF3.OEt2-promoted synthesis of acridines via N-aryl nitrenium-BF3 ions generated by dissociation of 2-oxo azidoarenes in benzene

2-Oxo-substituted aryl azides such as 2-azidobenzenecarbaldehyde 1, 1-(2-azidophenyl)-1- ethanone 2 and (2-azidophenyl) (phenyl)methanone 3 react with benzene in the presence of BF3•OEt2, mainly affording 9-substituted acridines via formal 2-anilino-oxobenzene-BF3 complexes rapidly followed by intramolecular cyclo-dehydration at the activated carbonyl groups. Under the same conditions, 2-azidobenzoic acid 4 gives mainly 2-anilinobenzoic acid 4b together with trace amounts of the 9(10H)-acridinone 4a. On the other hand, 2- azidobenzenecarbonitrile 5 gives the 9-amino acridine 5a via a conjugated imine, which undergoes intramolecular cyclization. The BF3•OEt2 promoted dissociation of aryl azides to aryl nitrenium ions is compared with those promoted by AlCl3 or a strong protic acid (TFA/TFSA mixture)