Diazoles pyrazoles

The difference in the molecular weights (MWs) of all parent azoles does not exceed 3 units. However, in going from 1,2-diazole (pyrazole) to tetrazole, both the nitrogen content in the heterocycle and the enthalpy of formation increase considerably (nearly twofold) (Table 11). 

The action of pyrazole and imidazole as acids is simpler. In each case the anion (114 or 117) is aromatic, symmetrical, and more stable than the pyrrole anion because of the extra nitrogen atom. The two diazoles pyrazole and imidazole are almost equal in acid strength. 

The addition of diazoles (pyrazole and imidazole) to vinyltrimethoxysilane and vinyl-silatrane, catalysed by lithium, has been studied by the B3LYP/6-311G(d,p) method, which revealed the initial formation of lithium salts of the diazoles. The latter intermediates exist in the planar and pyramidal forms in which lithium atom is bound to all atoms of the aromatic ring. ... 

Despite the greater acidities of the diazoles and triazoles (pKa = 10-14), fairly vigorous reaction conditions are still required for the alkylation of the unsubstituted systems [26] (Table 5.32) and the effectiveness of the alkylation of pyrazoles and imidazoles is enhanced under solidiliquid conditions [27-30]. Under these conditions, quaternization is avoided if no solvent is added [27],... 

Addition of nucleophiles to the cyano group of cyanothiadiazole under basic conditions takes place with unusual ease <88AG(E)434,94ACS372). Hydrolysis to the amide, for example, can be effected at 0°C in the presence of a catalytic amount of sodium hydroxide or basic ion-exchange resin. At reflux temperature, hydrazine and monosubstituted hydrazines convert 3,4-dicyano-l,2,5-thia-diazole into the l,2,5-thiadiazole[3,4-. The base-catalyzed addition of acetone to cyanothiadiazole forms an enamino ketone, used as a key intermediate for the synthesis of a number of heterocyclic ring systems, e.g. isothiazole, isoxazole, pyrazole, pyrimidine, and thiazole <77H(6)1985>. 

Pyrazole and Imidazole. These are five-membered ring heterocycles containing two nitrogens, one basic and one neutral. Two common forms are pyrazole (1,2-diazole) and imidazole (1,3-diazole) both are prone to aromatic hydroxylation. 

B. Five-Membered Rings with Two or More Heteroatoms 1. Pyrazoles (1,2-Diazoles)... 

Nitro-l,2-Diazole or 4-Nitro-pyrpzole (called 4-Pyrazol in Ger), HC—NH — N... 

TT-Electron delocalization in isoxazole seems to be more effective than in oxazole however, isothiazole is less aromatic than thiazole thus it is not a general rule that 1,2-diazoles possess higher aromaticity in comparison with 1,3-diazoles. Oxygen-containing heterocycles are always less aromatic than their sulfur and nitrogen counterparts, e.g. thiazole > imidazole > > oxazole. At the same time, the relative aromaticity of S- and N-containing heterocycles can interchange (pyrazole > isothiazole > isoxazole). 

Amino-5-pyridyl-pyrazole or 3-Amino-5-pyridyl-1,2-diazole, CtH N4, mw 160.18,... 

The ending -ole is systematic and refers to a five-membered heterocyclic ring. All the flve-membered aromatic heterocycles with nitrogen in the ring are sometimes called the azoles. Strictly speaking, pyrrole is "azole, pyrazole is 1,2-diazole", and imidazole is 1,3-diazole. These names are not used but oxazole and thiazole are used for the oxygen and sulfur analogues of imidazole. 

DIAZOLE AND DERIVATIVES 1,2-Diozole or Pyrozole (called Pyrazol In Ger), HC--NH N... 

The ability of the 1,2-diazole ring unit of pyrazolate to bridge two metal ions is well documented.99 Meyer reported some pyrazolyl-based ligands possessing additional chelating substituents in the 3- and 5-positions.100 101... 

Pyrazole, like its structural isomer imidazole, contains a pyrrole-like and a pyridine-like N-atom, but in the 1- and 2-positions (1,2-diazole). 

The conversion of 3-sulfolene to the useful sulfone (36) is itself an important procedure. A 1,3-dipolar cycloaddition of diazomethane to (36) was based on literature precedent for the formation of pyrazoles from sulfonyl alkenes and provided (37), from which benzene sulfonic acid was eliminated to give diazole (38) [17]. A mixture of products was obtained upon subsequent -substitution (Scheme 6.10). 

S-Aminopyrasole [Z-Amino-l 2-diazole. Note.—The pyrazole ring is sometimes numbered from the other nitrogen atom, in whith case the compound is b-aminopyrazde)... 

Scheme 2 Two types of nitrogen atom in 1,2- (a) and 1,3-diazoles (b), pyrazole and imidazole, respectively.

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