Carbon and nitrogen basicity of aminothiophenes and anilines

The protonation site, aromaticity, charge distribution, and NMR properties of 3-aminothiophene, 3,4-diaminothiophene, aniline, and 1,2-benzenediamine have been investigated by means of quantum chemical calculations both for the isolated and solvated species tin water and DMSO). For the isolated species (G3-(MP2) level), the C-protonated form of aminothiophenes is more stable than the N-protonated form (by 5-9 kcal/mol), whereas the stability order of the protonated forms of anilines is reversed, with a closer energy balance (2-5 kcal/mol). In water or DMSO the stability of the C- and N-protonated forms of aminothiophenes is essentially the same las obtained by a combination of G3(MP2) and DFT-IPCM solution data), whereas for anilines a strong preference for N-protonation is borne out. However, a comparison of experimental and calculated C-13 NMR chemical shifts shows N-protonation to be the major process in solution. While the aromaticity of the two ring types (as probed by nucleus-independent chemical shifts) is very similar, the larger nucleophilicity of the C-2 atom of aminothiophenes as compared to anilines is shown to arise from a strong polarization of the C-2-C-3 bond.