Flash Vacuum Pyrolysis of Pyrazoles as an Alternative Way to Study Vinylcarbenes

Abstract

Flash vacuum pyrolysis (FVP) reactions of 3,5-diphenylpyrazole (1) and 3(5)-methyl-5(3)-phenylpyrazole (2) were carried out. The reaction products expected for nitrogen extrusion were formed through different rearrangements in the vinylcarbene intermediate. Kinetic parameters for nitrogen extrusion from 1 are reported. To show that FVP reactions of pyrazoles are useful to obtain vinylcarbenes, the reactions of other pyrazoles previously studied are also discussed.

Formation of Cumulenes, Triple-Bonded, and Related Compounds by Flash Vacuum Thermolysis of Five-Membered Heterocycles

“…50 kcal/mol in 3,5-diphenylpyrazole (46). [34] The reaction product from pyrazole 41 itself is methylacetylene, while from 3,5-dimethylpyrazole (42a) it is 1,3-pentadiene. It has been shown by deuteration (42b) that the label appears exclusively in the 2-position in 43; thus implying a 1,4-shift of H(D).…”

“…[35] Some useful fluorinated alkynes and dienes have been obtained in FVT reactions of 3,5-bis(trifluoromethyl)pyrazole (44) and 3(5)-methyl-5(3)-trifluoromethylpyrazole, through vinylcarbene intermediates such as 45. [34] Cyano and methyl groups undergo 1,5-sigmatropic shifts on FVT of 1-substituted pyrazoles. [36] Thus, the 1-cyanodimethylpyrazole (47) affords 2-methylpentadienenitrile (51), presumably through the intermediates 48Ϫ50.…”

“…3,5-Diphenylpyrazole also afforded benzonitrile and phenylacetonitrile by [3ϩ2] cycloreversion. [42,43] When hydrotalcytes were used in FVT reactions of 3,5-dimethylpyrazole 42 and 3,5-diphenylpyrazole 46, only the nitriles arising from cycloreversion reactions were observed, showing pronounced selectivity for this reaction. [42] 4.…”

Flash vacuum pyrolysis of azolyl-malonamates. Synthesis of 5-hydroxy-azolopyrimidin-7-ones

“…For several years our group has studied the flash vacuum pyrolysis of nitrogen heterocycles in homogeneous and heterogeneous systems. 1 Most of these studies were performed on relatively simple heterocycles in order to obtain ring reactions and to determine the influence of substituents on their reactivity. To complete these studies, more complex molecules were selected in order to establish the most reactive zone in the molecule in cases where there are several.…”

“…To complete these studies, more complex molecules were selected in order to establish the most reactive zone in the molecule in cases where there are several. Thus, ethyl 3-oxo-3-(1,3-thiazol-2-ylamino)propanoate (1) and ethyl 3-oxo-3-(4H-1,2,4-triazol-3-ylamino)propanoate (2) (Figure 1) were selected for our studies. As it can be seen in Figure 1, these molecules have different reactive sites, a heterocyclic ring, an amide group, and an ester group.…”