Studies of "formal" [1,5]-sigmatropic thermal rearrangements of dimethyl 3-alkyl-3-methyl-3H-pyrazole-4,5-dicarboxylates and dimethyl 4-alkyl-5-methyl-4H-pyrazole-3,4-dicarboxylates

Article abstract of DOI:10.1021/ja00042a007

Studies of the thermal rearrangements of 3H-pyrazoles, 10a - e, in benzene/toluene, chloroform, and methanol solvents are described. The mechanism for rearrangement appears to be two-step, involving discrete ion-pair intermediates. Large rate enhancements for these rearrangements in methanol and cation trapping by methanol are consistent with the proposed mechanism. Observed rate constants for the rearrangements of 3H-pyrazoles 10c, 10d, and 10e in benzene-d₆ were determined. Insight into the sense of rearrangement by an ion-pair mechanism is also provided by analysis of the ¹H NMR spectra of the product mixtures. 3H-Pyrazoles 10b and 10e rearrange to afford 4H-pyrazoles 13b and 13e, respectively, and the thermal rearrangements of these pyrazoles in benzene-d₆ and methanol are described. Rate constants for the rearrangements of these more stable pyrazole isomers in benzene-d₆ were also determined. Cation-trapping experiments with methanol suggest that the 4H-pyrazoles also rearrange by a two-step mechanism, involving discrete ion-pair intermediates. Furthermore, it appears that different ion-pair intermediates are formed from the 3H-pyrazoles 10b and 10e, compared to the 4H-pyrazoles 13b and 13c, on the basis of cation-trapping experiments.