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Zum Mechanismus der photochemischen Umwandlung von 2‐Alkyl‐indazolen in 1‐Alkyl‐benzimidazole. I. Struktur und Reaktivität eines Zwischenproduktes
Author(s) -
Heinzelmann Willy,
Märky Michael,
Gilgen Paul
Publication year - 1976
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19760590512
Subject(s) - chemistry , alkyl , medicinal chemistry , stereochemistry , organic chemistry
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.