Synthesis and properties of 3-arylcyclohepta[4,5]imidazo[1,2- a]-1,3,5-triazine-2,4(3 H)-diones and related compounds: photo-induced autorecycling oxidation of some amines

Abstract

Novel 3-phenyl- and 3-(4-nitrophenyl)cyclohepta[4,5]imidazo[1,2- a]-1,3,5-triazine-2,4(3 H)-diones and the corresponding imino derivatives 5a, b and 6a, b were synthesized in modest to moderate yields by the abnormal and normal aza-Wittig reaction of 2-(1,3-diazaazulen-2-ylimino)triphenylphosphorane with aryl isocyanates and subsequent heterocyclization reaction with a second isocyanate. The related cationic compound, 1-methyl-3-phenylcyclohepta[4,5]imidazo[1,2- a]-1,3,5-triazine-2,4(3 H)-dionylium tetrafluoroborate 7a , was also prepared. The electrochemical reduction of these compounds exhibited more positive reduction potentials as compared with those of the related compounds of 3,10-disubstituted cyclohepta[4,5]pyrrolo[2,3- d]pyrimidine-2,4(1 H,3 H)-dione systems. In a search of the oxidizing ability, compounds 5a , 6a , and 7a were demonstrated to oxidize some amines to give the corresponding imines in more than 100% yield under aerobic and photo-irradiation conditions, while even benzylamine was not oxidized under aerobic and thermal conditions at 100 °C. The oxidation reactions by cation 7a are more efficient than that by 5a and 6a . Quenching of the fluorescence of 5a was observed, and thus, the oxidation reaction by 5a probably proceeds via electron-transfer from amine to the excited singlet state of 5a . In the case of cation 7a , the oxidation reaction is proposed to proceed via formation of an amine-adduct of 7a and subsequent photo-induced radical cleavage reaction.