Zum Mechanismus der photochemischen Umwandlung von 2‐Alkyl‐indazolen in 1‐Alkyl‐benzimidazole. I. Struktur und Reaktivität eines Zwischenproduktes

Mechanistic studies on the photoisomerization of 2‐alkyl‐indazoles into 1‐alkyl‐benzimidazoles. I. Structure and reactivity of an intermediate. 2‐Alkyl‐indazoles ( 1 ) undergo photochemical isomerization to 1‐alkyl‐benzimidazole via previously unknown intermediates 3 ( Scheme 1 ). In the present paper the structure and reactivity of these intermediates are discussed. Low‐temperature irradiation (−60°) of 1 b with 300 nm light gives 3 b in quantitative yield. 3 b is transformed during warm‐up to 1 b and 2 b (UV.‐evidence). The formations of 1 and 2 show the same temperature dependence but their ratio is found to be temperature‐independent. In contrast to the above behaviour, low‐temperature irradiation with 250 nm light of 3 b yields 1 b only (no 2 b ). These findings are consistent with the proposed reaction mechanism 2 c in Scheme 2 . On the basis of spectroscopic properties and the described reaction pathways, it appears that the most suitable structure for intermediate 3 is a 7,8‐diaza‐tricyclo[4.3.0.0 7,9 ]nona‐2,4,6(10)‐trien ( 9 ). In Scheme 4 the reaction pathway for the iudazole‐benzimidazole‐rearrangement is summarized.