CARBON-CARBON COUPLING OF 4,6-DINITROBENZOFUROXAN WITH 3-AMINOTHIOPHENES - A KINETIC AND STRUCTURAL STUDY

Potentiometric measurements carried out in a 50-50 (v/v) H2O-Me2SO mix ture together with H-1 and C-13 NMR studies carried out in pure Me2SO- d(6) reveal that 3-aminothiophene (4a), 3-N-(methylamino)thiophene (4b ), and 3-N,N-dimethylaminothiophene (4c) undergo protonation exclusive ly at the amino groups in dilute acid solutions. The pK(a) values of 4 a, 4b, and 4c are equal to 3.38, 3.65, and 3.53, respectively, at 25 d egrees C, comparing well with those for related anilines in the same a queous Me,SO mixtures. However, despite this similarity of the nitroge n basicities, compounds 4a, 4b, and 4c are found to behave differently from anilines upon treatment with super-electrophilic 4,6-dinitrobenz ofuroxan (DNBF). While anilines react to give first an N-bonded sigma- adduct that subsequently decomposes to afford the thermodynamically mo re stable C-bonded adduct, the 3-aminothiophenes react only as carbon nucleophiles, giving rise directly to the corresponding C-adducts. Bas ed on H-1 and C-13 NMR data as well as measurements of the rate consta nts (k(1)(DNBF)) for DNBF addition to 4a, 4b, and 4c, it is demonstrat ed that 3-aminothiophenes possess such high negative charge density at C-2 that they can be regarded as strongly enaminic in nature. Estimat es of the pK(a) values for protonation at C-2 of 4a, 4b, and 4c could be made: pK(a)(4a) approximate to -0.4; pK(a)(4b) approximate to 0.2; pK(a)(4c) approximate to 0.8. Thus, these compounds are more susceptib le to electrophilic addition at this position than most of the five-me mbered ring heterocycles studied so far. Altogether, the results demon strate the importance of the hyper-ortho relationship in the thiophene systems, accounting for the ease of both electrophilic and nucleophil ic reactivity of 3-X-thiophenes in sigma-complex formation processes.