Whereas 2H-benztriazoles are photochemically stable, 1 H-benztriazoles yield biradicals after splitting ofT nitrogen. These biradicals attack aromatic substratessuch as anisole, toluene, fluorobenzene or benzonitrile preferentially in the o-and p-positions with formation of the corresponding o-amino biphenyls (cf. schemes 2-5). With a deficiency of aromatic substrate, attack at the o-

2,2,3-Triphenyl-ZH-azirine (4a) in a matrix of 2,2-dimethylbutane/pentane 8 : 3(DMBP) at -185' gave rise on irradiation with light of 250-350 nm to a new UV.-maximum at 350 nm (Fig. 1). We assign the dipole benzonitrildiphenylmethylide (la) to this new maximum. Irradiation with monochromatic light of 366 nm destroyed this maximum and the initial absorption curve reappeared (Fig. 2). When the azirine 4a was photolysed in DMBP at -185" in the presence of methyl trifluoracetate (TFEM), the maximum at 350 nm was obtained again. This maximum vanished upon increasing the temperature to -160". Through gas chromatography we were able to show that 5-methoxy-5-trifluormethyl-2,2,4-triphenyl-3-oxazoline (6a) was produced. 6a was also obtained upon irradiation of 4a a t room temperature in the presence of TFEM (scheme 1 and table 1). Modification of the previously described experiment, in which the maximum a t 350 nm was extinguished in the matrix due to irradiation at 366 nm gave, after warm up, almost no dipole adduct 6a (table 1). From these experiments, an extinction coefficient of 17,000 for the 350 nm maximum of la, was calculated. These experiments have shown that irradiation of triphenylazirine 4a leads to the dipole la, which can be reversed photochemicallybut not thermallyinto azirine 4a. l a reacts a t less than -160" with TFEM to give adduct 6a. The results which were obtained with triphenylazirine 4a could be correspondingly obtained with 2,J-diphenyl-2H-azirine 4b (Fig. 3, scheme 2 and table 2 ) . The dipole l b showed two UV.maxima at 330 nm (e = 17,500) and 343 nm (E = 21,000). Later experiments established. that the two maxima belonged to a single dipole species.The dipole lc obtained upon irradiation of 2,2-dirnethyl-3-phenyl-ZH-azirine (4c) in DMBP at -190°, appears to absorb in the same region as the azirinc 4c. The presence of the dipole lc was univocally established by low temperature trapping experiments with TFEM. The dipole l a showed no ESR.-spectrum characteristic for a triplet state. We assume therefore, that la is in a singlet state. Photolysis of oxazolinone 7 at -190" in DMBP led to the dipole l a with loss of CO,. la recombines apparently in considerable amount with the CO, trapped in the matrix to l ) %) 27. Mitt. iiber Photoreaktionen; 26. Mitt. [l] ; 25. Mitt. [Z].

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. H2O) 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 · 10–5s–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).

Sttrmzury. 2,3-T)iplienyl-2H-azirinc (1) reacts on irradiation with light of wavelength 290-350 nni with 1,4-benzoquinoncs 3-6 or with 1,4-naphthoquinoncs 7-9 forming the yellow to red colourcd 1,3-diphenyl-2H-isoindolc-4,7-d1oiics 10 13 (33-430/, yield) rcsp. 1,3-diphenyl-2H-bcnzo~fisoindolc-4,9-diones 14-16 (33-36% yield) (Scheme I ) . The structures of these hitherto unknown products follow from the analytical and spectral data.The probable formation of the isoindole-tlioncs is depicted in Scheinr 2. The intcrmediatc bc,enzonitrile-bcnzylide ( 2 ) , which most certainly arises, adds onto the unsubstituted C, C-double bond of the quinoncs and not onto the C, 0-double bonds. On exclusion of atmospheric oxygen therc results from 1 and 2-1iiethyl-l,4-bcnzocluinone (4) product (probably b) which hardly absorbs in t h e region 350450 nni. The latter, with the agcncy of atmospheric oxygen (but not 4), IS converted into 5-methyl-l,3-diplien~1-2H-isoIndolc-4,7-dionc (11). The relative slowness of this oxidation (see Fig. 2) enables an almost coniplcte photochcniical transformation of the azirine 1, which only weakly absorbs a h v c 290 n m . Otherwisc 11, which strongly absorbs above 290 nm, would hinder the photolysis of 1.In Fortfiihrung unserer Arbeiten iiber die 1,3-dipolaren Cycloadditionen von aus 2 H-Azirinen des Typs 1 photochemisch crzeugten Renzonitril-methyliden des Typs 2 (Schema I ) , die unter anderem mit Ketonen und aktivierten Olefinen zu 5gliedrigen Heterocycleii reagiercn (vgl. r2-61 und dort zitierte Literatur), interessierte deren Verhalten gegeniiber 1,4-Chinonen. Es stcllte sic11 aucli die Frage, ob die Addition bevorzugt an die C, C-oder C, 0-Doppelbindungen des Chinonsystems erfolgt (vgl. [7a] Fig. 1 abgebildet. In 0,05ni l)37. h l i t t . siehe [l].2)