The reactions of thiophens and furans with 2,4-dinitrobenzene diazonium compounds

Abstract

The reaction of several thiophens and furans with 2, 4-dinitrobenzene diazonium sulphate in glacial acetic acid/water mixture has been studied. Thiophen and its 2-and 3-monomethyl derivatives gave arylated products with the liberation of nitrogen. Although there is a strong indication that free radicals are involved, the reaction mechanism remains uncertain. Various likely paths are discussed. 2, 4-Dimethyl, 2-t-butyl, and 2-phenyl thiophens yielded the azo compounds with the same diazonium salt. 2-Methylbenzo(b) thiophen, and 3-methylbenzo (b) thiophen also gave the azo coupled products. 2, 5-Dimethylthiophen yielded an equal proportion of the 3-azo- compound and the 2, 4-dinitrophenylhydrazone of 5-methylthiophen-2- aldehyde. Both the 2, 3, 5-trimethylthiophen and the tetramethylthiophen underwent side chain substitution in the 5-and 2-position respectively to give the corresponding 2, 4-dinitrophenylhydrazones. Dinitrophenylhydrazones of various polyalkyl thiophen aldehydes were prepared and characterised. ¹H. n. m. r. spectra of polyalkyl thiophens in deuteriotrifluoroacetic acid showed the proton exchange in methyl groups. However, acelylation of polymethyl thiophens and diazocoupling of polyalkyl benzenoid hydrocarbons failed to give any side chain substitution products. The reaction of furan, 2-methylfuran and 3-methylfuran with the 2, 4-dinitrobenzene diazonium solution in acetic acid yielded substituted N-(2, 4 dinitroanilino) - 2-oxo-2, 5-dihydropyrroles. Initial azo coupling with the furan followed by ring opening and subsequent rearrangement is thought to be the likely path for this reaction. Benzo(b) furan gave arylated products under similar conditions while 2-methylbenzo(b) furan gave a small amount of the azo coupled product in addition to the arylated product. Several primary aromatic amines including the weakly basic mono and dinitro anilines were diazotised in trifluoroacetic acid. Short reaction periods were required for the completion of the diazotisation and high yields of azo coupled products were obtained by subsequent coupling reactions. ¹H and ¹³C n.m.r. data on these benzene diazonium ions in trifluoroacetic acid was also obtained.