3- Pyrazolin-5-ones pyrolysis

In (39), we have an example of a stereospecific formation of a pyrazo-line. When pyrazolines are photolyzed, the elimination of nitrogen, if concerted, should be syn. This stereospecific product has been observed (McGreer and Wu, 1967). The pyrolysis of pyrazolines, if concerted, should be anti. It is not clear why this process turns out to be complex in (39), one alkene is formed stereospecifically by an anti hydrogen migration, but two isomeric cyclopropanes are formed. Crawford and Ali (1967), find rather different product patterns in the pyrolysis of 3-methyl and 3,4-dimethylpyrazolines, and give evidence for diradical intermediates. The general problem of unstable intermediates in a step-wise process will be discussed in a later section. 

The decarboxylation of 5-oxo-2-pyrazolin-3- and 4-carboxylic acids has been used frequently to prepare 2-pyrazolin-5-ones. In the case of the 4-carboxylic acids decarboxylation usually occurs in boiling water.308,532,1228,1230 The 3-carboxylic acids decarboxylate by pyrolysis, at somewhat higher temperatures, or by heating with sodium carbonate or calcium carbonate.508,1544,1546... 

The 3,3 -bis(2-pyrazolin-5-ones) in which the rings are connected by carbon chains (Table VIII) are usually prepared by condensation of a hydrazine with an appropriate / ,/ -dioxodiester (eq. 84).419,462,1222,1225 Such bispyrazolinones linked by a carbonyl group have been prepared by pyrolysis of the calcium salt of 5-oxo-2-pyrazolin-5-one.1549... 

Aminoethyl)-l,2,4-oxadiazoles (606 R = alkyl or aryl) readily isomerize to 3-acylamino-2-pyrazolines (607) on heating. Treatment of the salt (608) with triethylamine yields the transient ylide (609), which decomposes to methyl-phenylcarbodi-imide (610). 2,3-Diphenyl-l,2,4-oxadiazolin-5-one (611) rearranges photochemically to a mixture of the oxadiazolinones (612) and (613) pyrolysis gives 2-phenylbenzimidazole with extrusion of carbon dioxide. The oxadiazole (614) rearranges thermally to the indazole (615). ... 

The stability of the pyrazolinone tautomers was related to their scale of aromaticity.Experimental values for the tautomerism of pyrazolinone derivatives were obtained by flash vacuum pyrolysis (FVP), which allowed the calculation of the effect of the temperature on the equilibrium between tautomers. The variation of Kj with the temperature was determined by H NMR in solution for the model compound l-phenyl-3-methyl-2-pyrazolin-5-one (Scheme 25), showing that at 0 K the only tautomers present are either the NH or the OH ones. Temperature and solvent effects on the prototropic equilibrium of the model compound in solution showed that... 

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