MNDO Study of the Boulton–Katritzky Rearrangement

On the structure of 3‐acetylamino‐5‐methyl‐1,2,4‐oxadiazole and on the fully degenerate rearrangements (FDR) of its anion: a stimulating comparison between the results of ‘in‐silicon chemistry’ and ‘laboratory chemistry’

“…Semiempirical, [27,30,31] ab initio, [30] and DFT [32] calculations have been reported: so far, at our knowledge four other papers have been published concerning the FDR in 1,2,4-oxadiazoles; these include semi-empirical ab initio and DFT calculations. In the first one, by using semiempirical F o r P e e r R e v i e w calculations at the MNDO and AM1 level on 5 -La Manna and Coworkers [27] depicted an early nonsymmetrical planar transition state (TS) with the O1-N2 bond being cleaved significantly shorter than the N2-O9 being formed (2.2 and 2.8 Å, respectively; see fig.…”

“…Subsequently Andrianov and Coworkers [31] studied the FDR of the anion of 3-formylamino-1,2,4-oxadiazole (and of the isoxazole and 1,2,5-oxadiazole analogues) by using the MNDO semiempirical method. They concluded that the approach of O9 to the N2 atom to less than 2.0 Å distorts the planarity of the system leading to a folding along the N2-C3 central bond with formation of a first asymmetric transition state, which evolves into a symmetrical bicyclic intermediate having a pyramidal bridgehead nitrogen atom with a practically single C-N bond (length 1.50-1.52 Å) and a dihedral angle of 130-135° between the two heterocyclic rings.…”

“…[1][2][3][4][18][19][20][21][22] Moreover a TS having C 2V symmetry was obtained, thus confirming the occurrence of a concerted and synchronous reaction pathway; [26] nevertheless the calculated activation energies (ranging from 94.1 to 130.1 kJ mol -1 , depending on the method and the basis set used) were significantly higher than the experimental data, even if much closer with respect to previous results. [27,34] In the three papers above [27,30,31] the effect of the solvent on FDR's was not considered and the AA. ascribed the absence of a fitting between theoretical and experimental results to this fact, leaving some doubt on the description of the reaction mechanism.…”

“…In order to evaluate the possible reaction pathways for the FDR of 5we considered as references the two models proposed by La Manna and Coworkers, [30] and by Andrianov and Coworkers, [31] respectively.…”

Studies on Azole-to-Azole Interconversion − An Interesting Case of Absence of a “Primary Steric Effect” in the Ring-Degenerate Equilibration betweenortho-Substituted 3-Aroylamino-5-methyl-1,2,4-oxadiazoles and 3-Acetylamino-5-aryl-1,2,4-oxadiazoles in Methanol

“…• 3-acetylamino-5-methylisoxazole (7) could not be converted [7] into 3-acetonyl-5-methyl-1,2,4-oxadiazole (8), as was to be expected on the grounds of the higher aromaticity of 7, the stability of which is furthermore increased by the amide resonance.…”