Zum Mechanismus der Photochemie des Benzfurazans

✍ Scribed by Willy Heinzelmann; Paul Gilgen

• Publisher: John Wiley and Sons
• Year: 1976
• Tongue: German
• Weight: 538 KB
• Volume: 59
• Category: Article
• ISSN: 0018-019X
• DOI: 10.1002/hlca.19760590812

✦ Synopsis

Mechanistic studies on the photochemical reactions of benzfurazan.

From other works it is known that irradiation of benzfurazan (1) in methanol gives the carbaminacid‐ester 4, whereas in benzene the azepinederivative 3 is obtained (Scheme 1). The compounds 5–8 (Scheme 2) have been proposed as intermediates.

In our investigations we detected and characterized by means of UV.‐ and IR.‐spectroscopy the two species 5 and 8. Irradiation of 1 with 350 nm light at room temperature in a strongly polar solvent (e.g. H~2~O) yields exclusively 5 (Fig. 1) with a quantum yield of 0.48. In non polar solvents (e.g. hexane) 5 isomerizes in a second photochemical step to 8 (quantum yield 0.43) (Fig. 3). Thermally, 5 can be converted back to 1. The rate constant for this reaction at room temperature is 2 · 10^–5^s^–1^.

The transformation 5 → 8 was also investigated at low temperature. There was no direct evidence for any intermediates of the type oxazirene (6) or nitrene (7). However, the formation of azepine 3 upon irradiation of 5 in benzene suggests as intermediate the nitrene 7 which could be converted into 8 in a fast thermal reaction (Scheme 3).