Effiziente Photoreduktion von Triazinen mit aliphatischen Aminen
Aliphatic amines are used as powerful electron donators in electrontransfer reactions of electronically excited aromatic amines. In triazines the LUMO energy is decreased compared to benzene, therefore these electron-poor compounds are easily reduced and the corresponding radical anions are stabilized. Aromatic 1,2,4-triazines and 1,2,3-triazines were irradiated in aprotic triethylamine and protic diethylamine as well as in dimethylammonium dimethylcarbamate (Dimcarb). The reduction products of 1,2,4-triazines are mostly dihydroderivates (resembling the products of Birch-reduction) and products of ring contraction. Ringcontractions in turn require the transfer of four reduction equivalents and may involve the loss of ammonia. The photolysis of the two investigated 1,2,3-triazines causes loss of dinitrogen. In one case the irradiation furnishes the product after loss of dinitrogen, in an other case the irradiation amount three products of fragmentation, however also a pyrazol. This demonstrates that the strong reducing conditions allow to override this fragmentation to a small extent. All trazine conversions depend on the nature of the amine, on amine concentration an on light intensity. The reactive state of the investigated 1,2,4-triazines may be the lowest triplet, as confirmed by phospherescence and T-T absorption spectroscopy.