Synthese von 2H-Isoindol-4,7-dionen durch photochemische Cycloaddition von 2,3-Diphenyl-2H-azirin an 1,4-Chinone

Abstract

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.