Selective Thermolysis Reactions of Bromo-1-nitro-1H-pyrazoles. Formation of 3-Nitro-1H-vs. 4-Nitro-1H-pyrazoles

Article abstract of DOI:10.1021/jo00374a029

Refluxing 3,4,5,-tribromo-1-nitro-1H-pyrazole (1a) in benzene results in the evolution of bromine and NO2 and gives the 4-nitro-1H-pyrazole 2 and the 1-phenyl-1H-pyrazoles 4 and 5, while heating 1a in toluene gives 2 and benzyl bromide.Thermolysis of 1a in refluxing acetonitrile affords both 2 and the isomeric 5-nitro-1H-pyrazole 6a.Refluxing 1a mixed with the electron-rich 3,5-dimethyl-1H-pyrazole (7) in all three solvents gives 6a and 4-bromo-3,5-dimethyl-1H-pyrazole (8), whereas refluxing 1a mixed with anisole in benzene solution gives 2 and bromoanisoles.3,5-Dibromo-1-nitro-1H-pyrazole (1b) in refluxing acetonitrile gives mainly 3,4-dibromo-5-nitro-1H-pyrazole (6a) and 3,5-dibromo-1H-pyrazole (3b), but refluxing 1b mixed with 7 affords 3-bromo-5-nitro-1H-pyrazole (6b).Possible mechanisms are discussed involving intramolecular rearrangements to intermediates 3-bromo-3-nitro-3H-pyrazoles 9a,b and 4-bromo-4-nitro-4H-pyrazole 10 responsible both for the loss of bromine and NO2 as well as for the electrophilic bromination of 7 and anisole.