Addition reactions of conjugated double bonds. Part III. The kinetics of pyrazoline ring formation from αβ-unsaturated phenylhydrazones

Abstract

The kinetics of cyclisation of a series of 4-arylbut-3-en-2-one arylhydrazones with differing substituents in the aryl rings have been studied in acetic acid solution. Values of log k56° are discussed in terms of Hammett σ+ values and the F and R constants of Swain and Lupton. Rearrangements of 4-phenylbut-3-en-2-one phenyl-hydrazone in deuterioacetic acid showed no significant kinetic isotope effects. The ratio kHOAc/kDOAc varied from 1·036 to 1·053 for kinetic runs at three temperatures. The rates of cyclisation of 4-phenylbut-3-en-2-one phenylhydrazone have been determined in a series of water–acetic acid mixtures. A mechanism for the reaction is proposed.The work of Raiford and his co-workers on the acid-catalysed cyclisation of phenylhydrazones from unsymmetrically substituted dibenzylideneacetones has been confirmed and the products have been shown to be thermodynamically controlled.