Mechanistic Insights into the Formation of 1-Alkylidene/Arylidene-1,2,4-triazolinium Salts: A Combined NMR/Density Functional Theory Approach.

In a recent report on the synthetic approach to the novel substance class of 1-alkylidene/arylidene-1,2,4-triazolinium salts, a reaction mechanism suggesting a regioselective outcome was proposed. This hypothesis was tested via a combined NMR and density functional theory (DFT) approach. To this end, three experiments with 13C-labeled carbonyl reactants were monitored in situ by solution-state NMR. In one experiment, an intermediate as described in the former mechanistic proposal was observed. However, incorporation of 13C isotope labels into multiple sites of the heterocycle could not be reconciled with the "regioselective mechanism". It was found that an unproductive reaction pathway can lead to 13C scrambling, along with metathetical carbonyl exchange. According to DFT calculations, the concurring reaction pathways are connected via a thermodynamically controlled cyclic 1,3-oxazetidine intermediate. The obtained insights were applied in a synthetic study including aliphatic ketones and para-substituted benzaldehydes. The mechanistic peculiarities set the potential synthetic scope of the novel reaction type.