The Beckmann rearrangement of some pyrazolyl oximes

Abstract

The oximes of several pyrazolyl carbonyl compounds have been prepared, in some cases both as syn- and anti-isomers. They have been subjected to the Beckmann rearrangement with phosphorus pentachloride, toluene-p-sulphonyl chloride, concentrated sulphuric acid, and polyphosphoric acid. With di-(1-phenylpyrazol-4-yl) ketoxime, 4-benzoyl-1-phenylpyrazole oximes, 4-acetyl-1-phenylpyrazole oximes, ethyl 1-phenylpyrazol-4-yl ketoxime and isopropyl 1-phenylpyrazol-4-yl ketoxime, the rearrangements were found to proceed in a normal, intramolecular fashion to yield N-substituted amides, although sulphuric acid was found to be a poor reagent. The operation of a two-stage, fragmentation–recombination mechanism for the Beckmann rearrangement of t-butyl 1-phenylpyrazol-4-yl ketoxime in polyphosphoric acid has been demonstrated by crossed experiments, in which the nitrile or the electrofuge formed by fragmentation of the oxime was trapped before recombination could occur. The recombination step between the nitrile and electrofuge has been independently demonstrated under Beckmann rearrangement conditions. Pyrazolyl aldoximes failed to undergo the Beckmann rearrangement. N.m.r. spectroscopy has been used to assign configurations to all the pyrazolyl oximes except those of 4-acetyl-1-phenylpyrazole.