Evaluation of the electron-withdrawing character of the 4-nitrobenzofurazanyl moiety: Ionization of 7-anilino-4-nitrobenzofurazans

The ionization of a series of 7-(4'-X anilino) 4-nitrobenzofurazans (4H) (X = NO2, CN, CO2Me, F, H, Me, OMe) to give the conjugate amide anions 4(-) has been studied by H-1 and C-13 NMR in Me2SO-d(6). Concomitantly, their acidity (pK(a)) has been measured spectrophotometrically in a H2O-Me2SO mixture with a molar fraction N-Me2SO = 0,509. Although the measured pK(a) values for the whole series of compounds 4H lie in a relatively narrow range (6.40, X = NO2/7.98, X = OMe), they reveal that each derivative is about 3 times more acidic than the similarly X-substituted 2,4,6-trinitrodiphenylamines. These results, together with an analysis of substituent effects on the proton and carbon chemical shifts (delta(Hi) et delta(Ci)) and the related deprotonation parameters (Delta delta), which are specially significant for H-6, C-4, C-6, and C-7, emphasize the very strong electron-withdrawing character of the 4-nitrobenzofurazanyl moiety. In addition, the observation of nice linear correlations of the type delta(Hi(Ci)) = f(pK(ai)) has enabled us to control and sometimes to firmly assign the various chemical shifts.