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Mechanisms of the cascade synthesis of substituted 4‐amino‐1,2,4‐triazol‐3‐one from huisgen zwitterion and aldehyde hydrazone: A DFT study

The detailed reaction mechanisms of the title reaction are shed light on by using the density functional theory (DFT). The calculated results have demonstrated that the whole reaction takes place via four processes (processes (I–IV)), among which, three possible reaction mechanisms are proposed for process (II) (channels 1–3) and two for process (IV) (channels 4–5). According to our calculated results, channel 3 and channel 5 are verified to be most energetically favorable. As interpreted in the text, in process (II), the proton transfer should be performed prior to the nucleophilic attack, and the AA‐Type transfer strategy is more likely to occur. The global reactivity index (GRI) and frontier molecular orbital (FMO) analyses of the aldehyde hydrazone have further supported the AA‐Type mechanism. In process (IV), however, the titled product has been demonstrated to be formed by the synergetic elimination of two protons via a six‐membered ring transition state. Taking an integrated view, the highest energy barrier for the whole reaction along the most favorable pathway is 32.19 kcal/mol, which is consistent with the mild thermal experimental conditions. More interestingly, the qualified mechanisms in this work have given a perfect explanation to the optimal reactants molar ratio of highest yields (R1/R2/R3 = 2/1/1) employed in the experiment. © 2012 Wiley Periodicals, Inc.