4H-Pyrazoles

As is the case with 2H- and 3//-pyrroles, and with 3//- and 4H-pyrazoles, the 2//-imidazoles undergo both thermal and acid-catalyzed rearrangements. The products may be 1ft- or 4f/-imidazoies, depending on the ring substituents. 

The quatemization of 4//-imidazoles occurs at the 3-position (Scheme 65) (64AHC(3)i). 4H-pyrazoles are also readily monoquatemized. 

Fig. 11. Solid-supported reagents in synthesis (a) poly(4-vinylpyridinium dichromate), cyclohexane, 65 °C, 12h (b)perbromideonAmberlyst A-26, cyclohexane, 65 °C, 12h (c) AmberliteIRA-900(4-chloro-l-methyl-5-(trifluoromethyl)4H-pyrazol-3-ol), cyclohexane, 65 °C, 12 h (d) reagents a, b, c, 65 °C, 16 h.

Tetramethyl-E17b, 1108 (subst. 4H-Pyrazol + Cyclobuta-dien)... 

The 3H- and 4H-pyrazoles and 2H- and 4/7-imidazoles (83UP40200) contain two doable bonds in the heterocyclic ring, but in each case the conjugation does not include all the ring atoms hence the compounds are not aromatic. 

Two double bonds. This is the most important class which includes the aromatic compounds pyrazole (3), indazole (4) and isoindazole (5), their non-aromatic isomers, pyrazolenines (or 3//-pyrazoles 6), isopyrazoles (or 4H-pyrazoles 7) and 3Lf-indazoles (8), and the carbonyl derivatives of pyrazolines with the endocyclic double bond in positions 1, 2 or 3, i.e. (9), (10) and (11), respectively. The indazolones (12) and the pyrazolidinediones (13) and (14) also belong to this group. 

---