Compounds not in tautomeric equilibrium with aromatic derivatives

1 Compounds not in tautomeric equilibrium with aromatic derivatives 

The 3H- and 4//-pyrazoles and 2H- and 4//-imidazoles (83AHC(34)2, 83AHC(34)54, 84AHC(35)376, 84AHC(35)414) contain two double bonds in the heterocyclic ring, but in each case the conjugation does not include all the ring atoms hence the compounds are not aromatic. 

Reactivity of Five-membered Rings with Two or More Heteroatoms 

3//-Pyrazoles are photochemically converted into cyclopropenes, and 3//-indazoles react similarly, e.g. (404— 405) (70AHC(ll)l). If a 3-aryl group is present, an indene can be formed, e.g. (406— 407) (83AHC(34)2). 

Peracids can oxidize 2//-imidazoles to A -oxides and N,N -dioxides, and sometimes to imidazoli-nones. Lead dioxide in methanol converts 2//-imidazole 1,3-dioxides into stable nitroxide radicals (96CHEC-II(3)146). 

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