First-principles study of the reaction mechanism governing the SNAr of the dimethylamine on 2-methoxy-5-nitrothiophenes

摘要: The SNAr mechanism invoked in this work concerned the substitution reaction of methoxy group by a secondary amine on 2-methoxy-5-nitrothiophene and 2-methoxy-3,5-dinitrothiophene molecules. We performed theoretical DFT TD-DFT calculations. It was examined hypothesis zwitterion formation during kinetically determining step has been confirmed. However, our calculations have shown that structure intermediate is much more diffuse than commonly accepted. Its geometry can be described as one situation which nitrogen nucleophilic entity not totally bound to carbon, whereas oxygen leaving prepares separated from substrate. transition state corresponding barrier 17.78 kcal mol−1 methanol. Three pathways studied describe second step, leading methanol or methoxyammonium. most probable pathway uses, first sequence, molecule convert zwitterion, dipolar compound, into hydrogen-bonding association carbanion located carbon C5 carrying nitro an ammonium with weak activation 2.25 kcal mol−1. electron density supplement provided nucleophile stored at atoms 5-nitro position α sulfur atom. presence β modifies locating negative charge C3 nitro. To visualize mechanism, we propose innovative technique called reactive internal coordinate traces 3D figure variation energy according active coordinates transition-state zone influence.