Photochemical and thermal rearrangements of some 3H-pyrazoles Academic Article uri icon

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abstract

  • Low temperature (−70 °C) irradiation of 3,3,4,5-tetramethyl-3H-pyrazole (2a) leads to the formation of the 1,2-diazabicyclo[2.1.0]pent-2-ene (3a), as has been reported previously. Irradiation of 3,3-dimethyl-4,5-diphenyl-3H-pyrazole (2b) under identical conditions did not result in formation of the analogous product but lead instead to formation of the cyclopropene via decomposition of the vinyl diazo compound. The thermal rearrangements of 2a and a series of 5-aryl-3,3-dimethyl-4-phenyl-3H-pyrazoles (2b–d) lead to formation of the 4H-pyrazoles 6a–d and the N-methylpyrazoles 5a–d, resulting from competitive methyl migrations to carbon and nitrogen. The Arrhenius parameters for the rearrangement of 2b were determined. Decomposition of the methiodides derived from the 4H-pyrazoles 6a–d leads to formation of the N-methylpyrazoJes 5a–d; the mechanism of this decomposition was examined by high temperature 1Hmr spectroscopy. The N-methylpyrazoles were synthesized by independent routes and the structures were assigned with the aid of lanthanide shift reagents, 1Hmr solvent shift studies, and 13Cmr spectroscopy. The thermodynamic equilibria of the isomeric N-methylpyrazoles were determined and found to be influenced by a small substituent effect. The photochemical and thermal behaviour of 3H-pyrazoles is discussed and compared to that of 1-pyrazolines.

publication date

  • May 15, 1979