Synthese von 2 H ‐Isoindol‐4,7 ‐dionen durch photochemische Cycloaddition von 2,3‐Diphenyl‐2 H ‐azirin an 1,4‐Chinone. 38. Mitteilung über Photoreaktionen

2, 3‐Diphenyl‐2 H ‐azirine ( 1 ) reacts on irradiation with light of wavelength 290–350 nm with 1,4‐benzoquinones 3–6 or with 1,4‐naphthoquinones 7–9 forming the yellow to red coloured 1,3‐diphenyl‐2 H ‐isoindole‐4, 7‐diones 10–13 (33–43% yield) resp. 1, 3‐diphenyl‐2 H ‐benzo[ f ]isoindole‐4,9‐diones 14–16 (33–36% yield) ( Scheme 1 ). The structures of these hitherto unknown products follow from the analytical and spectral data. The probable formation of the isoindole‐diones is depicted in Scheme 2 . The intermediate benzonitrile‐benzylide ( 2 ), which most certainly arises, adds onto the unsubstituted C, C‐double bond of the quinones and not onto the C,O‐double bonds. On exclusion of atmospheric oxygen there results from 1 and 2‐methyl‐1, 4‐benzoquinone ( 4 ) a product (probably b ) which hardly absorbs in the region 350–450 nm. The latter, with the agency of atmospheric oxygen (but not 4 ), is converted into 5‐methyl‐1, 3‐diphenyl‐2 H ‐isoindole‐4, 7‐dione ( 11 ). The relative slowness of this oxidation (see Fig. 2 ) enables an almost complete photochemical transformation of the azirine 1 , which only weakly absorbs above 290 nm. Otherwise 11 , which strongly absorbs above 290 nm, would hinder the photolysis of 1 .