EVIDENCE FOR A STRONG ENAMINIC CHARACTER OF 3,4-DIAMINOTHIOPHENE - A FAST CARBON-CARBON COUPLING WITH 4,6-DINITROBENZOFUROXAN

While exhibiting a nitrogen basicity comparable to that of aniline, 3,4-diaminothiophene (DAT) does not show any tendency to react as a nitrogen nucleophile with a highly electrophilic compound like 4,6-dinitrobenzofuroxan (DNBF). Depending upon the experimental conditions employed, the addition of DNBF takes place at C-2 to give a carbon-bonded monoadduct (3,H) or at C-2 and C-5 to give a C,C-diadduct (4,H) which exists as a mixture of two diasteromers. Both the acid and base forms of these adducts have been characterized. A kinetic study of the monocomplexation of DAT and its 2,5-dideuteriated analog have been carried out at various pH in 50% H2O-50% Me2SO. The results show that electrophilic attack by DNBF is the rate-limiting step of the formation of the adduct 3,H. The measured rate constant for DNBF addition is very high (k1 = 9 X 10(5) L mol-1s-1), emphasizing the very strong nucleophilic character of the C(alpha) positions of DAT and supporting the idea that this compound is strongly enaminic in nature.