Structure, Chemical and Physicochemical Properties of Pyrazolines

Most chemical and physicochemical properties of aromatic substituted pyrazolines as well as isoxazolines are formed by the 7i,/ ,7i-system of the diarylhydrazone (oxime) moiety. 

Some results of the 7r-electron structure analysis of pyrazolines were systematized in [102]. According these data, several main conclusions can be drawn  

Ph-N—N = CH2 7r-system, simulating pyrazolines unsubstituted at position 3, another situation is observed—although the 7r-electron density on the ortho and para atoms of the phenyl is greater than on C3, the boundary electron density has a maximum value exactly at position 3. 

Pyrazolines are the intermediates in one of the synthetic pathways to cyclo-propanone and this fact has defined the development of pyrazoline chemistry for a long period. This aspect of their synthetic application is well described in the literature, including textbooks on general organic chemistry, and therefore we do not consider it necessary to cover it in this book. 

Oxidative processes are very characteristic for pyrazoline derivatives such as dihydroheteroaromatic compounds. However, oxidation of pyrazolines is not a widely used preparative method for the synthesis of the appropriate pyrazoles owing to numerous side reactions following heteroaromatization, and other pathways are usually applied to obtain pyrazoles [66]. It should be noted that pyrazolines unsubstituted at positions 1 and 3 are very unstable compounds and easily decompose in air, with nitrogen elimination. Introducing alkyls or aryls in these positions leads to an increase of their stability but oxidative destruction is also possible, for example, under the action of nitrous acid [116, 117]. 

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