The Photochemistry of Benzotriazole Derivatives. Part 2: Photolysis of 1-Substituted Benzotriazole Arylhydrazones: New Route to Phenanthridin-6-yl-2-phenyldiazines

Irradiation of 1-substituted benzotriazole arylhydrazones 3a–c, 4a,b and 5a,b with a 16 W low pressure mercury arc-lamp (254 nm) for 24 h gave phenanthridin-6-yl-2-phenyldiazines 9a–c, phenanthridin-6(5H)-ones 10a–c, 1-anilinobenzimidazoles 11a–c, 2-aryl-1H-benzimidazoles 12a–c, 1-arylamino-1H-benzimidazol-2-carboxylic acid ethyl esters 14a,b, 1-aryl-1H, 9H-benzo [4,5][1,2,3] triazolo[1,2-a]tetrazole-3-carboxylic acid ethyl esters 16a,b, 1-arylamino-2-benzoylbenzimidazoles 18a,b and 2-benzoylbenzoxazole 21.


Results and Discussion
Compounds 3a-c and 5a,b were obtained by the procedures described earlier and were fully characterized [12][13][14]. The UV spectra of these compounds display two absorption maxima in the region 243-392 nm wavelength regions.

Scheme 2. Mechanism of photolysis of N-(benzotriazol-1-yl-phenylmethylene)-N′-
followed by cyclization to yield 18a,b. On the other hand, photooxidation of 5a,b afforded 1-benzotriazole-2-phenylethan-1,2-dione 19, which upon elimination of N 2 formed diradical 20, which either cyclizes to yield 2-benzoylbenzoxazole 21 in (24-27%) or cyclizes to 22, which spontaneously loses CO through intermediate 23 to produce phenanthradin-6(5H)-one 10a in 15-17% yield. The structure of all new compounds were assigned by spectroscopic and analytical methods. The structure of 21 is readily assigned based on 2D-NMR results. The 1 H-and 13 C-NMR signal assignments and the H-C correlation from the HMBC 2-D experiments are displayed in Figure 5. Table 3 summarizes the absorption maxima (λ max ) and the photoproducts of substrates 3a-c, 4a,b and 5a,b. The fact that the substituents R affected the nature of the products much more than the substituted Ar may be attributed to the strong influence of the substituents on the formed biradicals.

General
All melting points were recorded on a Gallenkamp apparatus. IR spectra were recorded using KBr pellets on a Perkin-Elmer System 2000 FT-IR spectrophotometer. 1 H-and 13 C-NMR spectra were recorded on Bruker DPX 400 MHz or Avance II 600 MHz super-conducting NMR spectrometers with proton spectra measured at 400, 600 MHz and carbon spectra at 100 and 150 MHz, respectively. Mass spectra were measured on a VG Auto-spec-Q (high resolution, high performance, tri-sector GC/MS/MS) and with LCMS using Agilent 1100 series LC/MSD with an API-ES/APCI ionization mode. Microanalyses were performed on LECO CH NS-932 Elemental Analyzer. The UV/VIS absorption spectra were recorded using a Varian Cary 5 instrument in the wave length range 200-450 nm using dry clean quartz cuvette of 1.0 cm path length. X-Ray analysis were performed using a Rigaku Rapid II diffractometer.

Irradiation Using a Low Pressure Mercury Arc-Lamp
Each of the substrates 3a-c, 4a,b and 5a,b (10.0 mmol) was dissolved in acetonitrile (250 mL) in a number of quartz tubes (10 × 25 mL) and introduced to irradiate for 24 hours at room temperature (RT). The progress of each reaction was monitored by using TLC. The solvent was removed in vacuo and the resulting residue was subjected to column chromatography on silica gel using ethyl acetate/ petroleum ether (b.p. 60-80 °C) as the eluent to give the corresponding products.    1-Phenyl-1H,9H-benzo [4,5] [1,2,3]triazolo [1,2-a] 13 13

Conclusions
The present study offers a new route for the synthesis of some new heterocyclic phenanthridin-6-yl-2-phenyldiazines. Some of these photoproducts 10a-c have been shown to have efficient compelexation properties with transition metals and exhibit interesting photo-emission and fluorescence properties [18,19]. It also shows that 1-substituted benzotriazole arylhydrazones behave photochemically in a different manner than in flash vacuum pyrolysis (FVP) or static pyrolysis (STP) reactions [12][13][14].