Zum Mechanismus der Photochemie des Benzfurazans

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 ).