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

✍ Scribed by Willy Heinzelmann; Michael Märky; Paul Gilgen

• Publisher: John Wiley and Sons
• Year: 1976
• Tongue: German
• Weight: 778 KB
• Volume: 59
• Category: Article
• ISSN: 0018-019X
• DOI: 10.1002/hlca.19760590512

✦ Synopsis

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.