Fragmentation of 2-(2-azidophenyl)pyridine and isomerisation of 5λ5σ3-pyrido[1,2-b]indazole

Abstract

The action of heat on 2-(2-azidophenyl)pyridine (1) quantitatively produced 5λ⁵σ³-pyrido[1,2-b]indazole (7) in tetralin, mesitylene, or acetophenone, and gave a nearly 1 : 1 mixture of (7) and 2-(2-aminophenyl)pyridine (3) in di-n-butylamine. The photoreaction of the azide (1) in cyclohexane was complete after 3 h, and gave quantitatively a 1 : 2 mixture of pyrido[3,2-b]indole (4) and the pyridoindazole (7) from irradiation at 254 or 300 nm and a 1 : 5 mixture from irradiation at 350 nm. Irradiation of a mixture of the azide and acetophenone (1 : 85) in cyclohexane gave the amine (3)(39%) and lesser amounts of the heterocycles (4) and (7), and irradiation in diethyl or di-n-butylamine transformed the azide into the corresponding 2-dialkylamino-3-(2-pyridyl)-3H-azepine (5)(51 and 65%). 5λ⁵σ³-Pyrido[1,2-b]indazole (7) in tetrahydrofuran was converted (40%) into the pyridoindole (4)(100%) by irradiation at 254 nm for 136 h. Solvent and sensitisation effects on the photolysis of the azide (1) and the indazole (7) are presented. Mechanisms for the thermal and photochemical reactions are discussed.