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

2, 3-Diphenyl-2H-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-2H-isoindole-4, 7-diones 10–13 (33–43% yield) resp. 1, 3-diphenyl-2H-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-2H-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 .