Expedient Routes to 1,2,4-Triazolinium Salts

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

“…To this solution, pivalic acid/ N , N -diisopropyl- N -ethylamine buffer was added. 19 An aliquot of this solution was transferred into a standard 5 mm NMR tube. Then, the educt was characterized by 1 H and 13 C NMR spectroscopy at 50 °C.…”

“…The transazination was earlier only observed in different experiments at prolonged reaction times and refluxing temperatures, thus hinting at a presence of a higher lying activation barrier that slows down the kinetics of this reaction. 19 As we could not find any NMR experimental evidence for a key intermediate to connect pathway A and B and to introduce 13 C isotope scrambling, we resorted to DFT calculations. Carbonyl exchange reactions of different N-nucleophiles are long known.…”

“… 20 Notably, the respective structure motif was unambiguously confirmed by X-ray crystallography. 19 Although the combination of a highly reactive iminium functionality embedded in a somewhat unstable 1,2,4-triazoline ring might suggest a rapid refragmentation of the heterocycle, the six isolated substances proved to be robust and easy to handle. 21 , 22 Even after more than one year of storage in closed, light-protected containers under ambient conditions, no signs of degradation were observable for the iodide salts.…”

“… 21 , 22 Even after more than one year of storage in closed, light-protected containers under ambient conditions, no signs of degradation were observable for the iodide salts. 19 We were tempted to pursue further research in this area, not only because of the unprecedented structure with respect to heterocyclic chemistry. Potential applications in pharmaceutical chemistry and in materials science are known for other 1,2,4-triazoline derivatives 23 , 24 and especially for their oxidized 1,2,4-triazole congeners.…”

“…Furthermore, we were interested in a better understanding of the observed side reaction of educts IV leading to a metathetical carbonyl exchange, which was recently termed as transazination or transalkylidation. 19 , 28 The mechanistic details of the formation of V were probed by in situ NMR spectroscopy 29 − 31 using stable 13 C isotope-labeled carbonyl components: 2- 13 C-acetone and benzaldehyde-α- 13 C. Complementarily, DFT calculations were performed on the NMR-detected reaction intermediates. 32 , 33 In the following sections, the originally proposed reaction mechanism was challenged by new findings from NMR and DFT.…”