Pyrazoline formation mechanism

There is much evidence that the mechanism" of the 1-pyrazoline reactions generally involves diradicals, though the mode of formation and detailed structure (e.g singlet vs. triplet) of these radicals may vary with the substrate and reaction conditions. The reactions of the 3 f-pyrazoles have been postulated to proceed through a diazo compound that loses N2 to give a vinylic carbene." ... 

It was demonstrated (83) that the reaction of dinitrostyrenes (28) with aryl diazo compounds RR CN2 afford nitronates (24 g) in good yields. These products contain the nitro group at the C-4 atom in the trans position with respect to the substituent at C-5 (if R =H). Since the reaction mechanism remains unknown, the direct formation of cyclic nitronates (24 g) from pyrazolines A without the intermediate formation of cyclopropanes also cannot be ruled out. 

The formation of pyrazoline derivatives 175 (but not dihydrotetrazolotria-zepines 173, as assumed earlier [53]) is brought about by the condensation of chalcones with diaminotetrazole 172 [128,129]. The structure of the compounds 175 is convincingly verified using X-rays. The mechanism of their formation implies a Dimrothe rearrangement for either the initial diamine or for one of the cyclocondensation intermediates (Scheme 4.51). 

Reaction of 4-chloro-2-pyrazolin-5-ones with potassium carbonate proceeds along two pathways, leading to diketo derivatives of pyrazoIo[l,2-a]pyrazoIe (syn- and anti- dioxabimanes )- A mechanism involving the intermediate formation of diazacyclopen-tadienone (185) which reacts with its potential extended diazoalkane functionality in dipolar cycloaddition fashion to give syn-dioxabimanes (major product) and anri-dioxabimanes (minor product) has been proposed (Scheme 6) (80JA4983). 

The new pyrazolo[l,5-tf][l,3,5]benzoxadiazocine heterocyclic ring system 123 was prepared by cyclization of 4,5-dihydro-3-methyl-5-(2-hydroxyphenyl)-17/-pyrazole-l-carboximidamide with triethyl orthoformate. A reaction mechanism involving re-esterification of triethyl orthoformate with phenolic hydroxyl of the additional equivalent of hydroxyphenyl pyrazoline was proposed to explain the formation of the product with an additional guanidine moiety (Equation 17 <2002J(P1)1260>). 

We will now briefly examine y-elimination and extrusion reactions. An example of the former is the formation of a cyclopropane from 1-pyrazoline initiated either by heat or light. Suggest a mechanism for this reaction. 

A similar process is observed in the formation of pyrazolines from Mannich bases. The process apparently involves an elimination-cyclization mechanism (Scheme 6) and not intramolecular displacement of the dimethylamino group in (95). 

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