Évaluation de l'effet électroattracteur du motif 4-nitrobenzofurazanyle : ionisation de 7-anilino 4-nitrobenzofurazanes

Abstract

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 1H and 13C NMR in Me2SO-d6. Concomitantly, their acidity (pKa) has been measured spectrophotometrically in a H2O–Me2SO mixture with a molar fraction [Formula: see text] Although the measured pKa 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 [Formula: see text] and the related deprotonation parameters (Δδ), which are specially significant for H6, C4, C6, and C7, emphasize the very strong electron-withdrawing character of the 4-nitrobenzofurazanyl moiety. In addition, the observation of nice linear correlations of the type [Formula: see text] has enabled us to control and sometimes to firmly assign the various chemical shifts. Keywords: 7-anilino 4-nitrobenzofurazans, acidities, 1H and 13C NMR deprotonation shifts, water – dimethyl sulfoxide mixtures.