On the Question of Zwitterionic Intermediates in the [3+2] Cycloaddition Reactions between Aryl Azides and Ethyl Propiolate

Abstract

The molecular mechanism of the [3+2] cycloaddition reactions between aryl azides and ethyl propiolate was evaluated in the framework of the Molecular Electron Density Theory. It was found that independently of the nature of the substituent within the azide molecule, the cycloaddition process is realized via a polar but single-step mechanism. All attempts of localization as postulated earlier by Abu-Orabi and coworkers’ zwitterionic intermediates were not successful. At the same time, the formation of zwitterions with an “extended” conformation is possible on parallel reaction paths. The ELF analysis shows that the studied cycloaddition reaction leading to the 1,4-triazole proceeds by a two-stage one-step mechanism. It also revealed that both zwitterions are created by the donation of the nitrogen atom’s nonbonding electron densities to carbon atoms of ethyl propiolate.

Energetic Aspects and Molecular Mechanism of 3-Nitro-substituted 2-Isoxazolines Formation via Nitrile N-Oxide [3+2] Cycloaddition: An MEDT Computational Study

“…Global electronic properties of the reactants were estimated according to the equations recommended by Parr and Domingo [64,66,67,[93][94][95][96]. According to Domingo's recommendation, for this purpose, the B3LYP/6-31G(d) level of theory was used.…”

“…Independently of the energetic aspects and their influence on the regioselectivity, the main processes also require a detailed mechanistic evaluation. According to the actual state of knowledge, different types of molecular mechanisms can occur during the [3+2] cycloaddition reactions: (a) polar mechanisms (one-step synchronous mechanism, one-steptwo-stage asynchronous mechanism, and stepwise zwitterionic mechanism) [61][62][63][64], or (b) non-polar mechanisms (one-step synchronous mechanism, one-step-two-stage asynchronous mechanism, and stepwise biradical mechanism) [65,66].…”