13C-NMR based evaluation of the electronic and steric interactions in aromatic amines

Chemical shifts of the para carbon atoms, d(C-13-4), in a series of aromatic amines were used to calculate the sigma(p), sigma(R) and sigma(O)(R) substituent constants for different amino groups. 1-Pyrrolidino, N,N-di-n-butylamino and N, N-diethylamino groups were found to be the most strong electron-donors. ortho-Substitution decreases the donor properties of the amino group. The amino groups in 2,6-di-i-propylaniline and N, N-2,6-tetramethylaniline have very weak electron-donor properties. The nitrogen atom in benzoquinuclidine and N, N-dimethyl-2,6-di-i-propylaniline have an electron-acceptor character. The calculated substituent constants of the amino groups studied are consistent with the spectral and reactivity data available in literature. Values of delta(N-15) cannot be used as a direct measure of electronic effects of the N atom in anilines.