Regiospecific substitution of the 4-nitro group in 3-amino-4,6-dinitrobenzo[b]thiophene-2-carboxylates: unexpected activating effect of the amino group

The cyclocondensation of 2,4,6-trinitrobenzonitrile with the esters of thioglycolic acid results in the formation of 3-amino-4,6-dinitrobenzo[b]thiophene-2-carboxylates 3. The interaction of 3 with anionic nucleophiles RO− (R=CF3CH2, CHCCH2), RS− (R=Ph, PhCH2, (CH3)2CHCH2), N3− in NMP or DMF leads to the substitution of only the 4-NO2 group. The replacement of the electron-donating NH2 group in 3 with hydrogen unexpectedly and significantly hampers nucleophilic substitution of the nitro group. It is assumed that increased reactivity of the 4-NO2 in 3 is connected with the twist of this group with respect to the plane of the aromatic ring under the influence of the NH2 group as indicated by semi-empirical quantum chemical calculations.