Pyrazoles, amino-, tautomerism

Substituted isoxazoles, pyrazoles and isothiazoles can exist in two tautomeric forms (139, 140 Z = 0, N or S Table 37). Amino compounds exist as such as expected, and so do the hydroxy compounds under most conditions. The stability of the OH forms of these 3-hydroxy-l,2-azoles is explained by the weakened basicity of the ring nitrogen atom in the 2-position due to the adjacent heteroatom at the 1-position and the oxygen substituent at the 3-position. This concentration of electron-withdrawing groups near the basic nitrogen atom causes these compounds to exist mainly in the OH form. 

The tautomeric equilibrium between IH- and 2//-formycin (7-amino-3)8-D-ribofuranosyl-l//-pyrazolo[4,3-d]pyrimidine) has a constant ratio N(2)H/N(1)H = 0.2 and an enthalpy difference estimated at 1 kcal mol". The tautomeric interconversion is catalyzed by (3 x 10 Msec" ) and by OH" (5 X 10 M" sec" ). No other catalytic pathway such as water catalysis or tautomerization via tautomeric cations contributes significantly to the interconversion. Protonation on the pyrazole ring of formycin does not occur significantly (80JA3897). 

The tautomerism of other pyrazoles has also been studied. The examination of the tautomerism, protonation, and E/Z isomerism of a 4-nitroso-5-aminopyrazole and its salts shows that this compound exists as a mixture of two amino/nitroso isomers 50a and 50b the imino/oxime tautomer 50c was not present [66], The salt is a mixture of isomers 51a and 51b resulting from the protonation on the oxygen of the nitroso tautomers. The functional tautomerism of l,5,6,7-tetrahydro-4//-inda-zol-4-ones (a pyrazole derivative) has been studied theoretically and experimentally [67], In the case of imidazoles 2-substituted by Se, there is experimental evidence that the selenone is more stable than the selenol tautomer [68],... 

The prototropic tautomerism studies of 4-( TV-ary lamino)-2-( 1 //-pyrazol-1 -yl)pyrimidines have shown that these compounds exist in dilute CC14, CHC13 and DMSO-J6 solution exclusively as amino tautomers no imino tautomer was detected (80KG1114). 

H-Pyrazole mit Hydroxy-, Mercapto- oder Amino-Gruppen bilden weitcre tautomere Formen, So konnen 1 -substituierte 5-Hydroxy-1 H-pyrazole indrei tautomeren Formen auftreten. 

The fluorescence quantum yields of pyrene-1-carboxaldehyde in water and methanol are 0.98 and 0.07/ an effect attributed to solvent effects on 7c,n and n,n states. Cycloaddition reactions of 1-naphthonitrile to 1,2-dimethyl-cyclopentene are attributed to both and Lj, states.It is pointed out that although dual fluorescence is known, this is the first example of divergent reaction from two nearly isoenergetic singlet states. An analysis of the u.v. spectra of some acyl pyridines, including a theoretical examination of the molecular geometry, and excited states of bipyrimidine compounds have also been made. Photo tautomerism and the fluorescence of the cation of 4-amino-pyrazole[3,4-iflpyrimidine, an analogue of adenine, has been published by Wierzchowski et Intramolecular heteroexcimer formation in... 

Condensation of ethoxymethylenemalonitrile with hydrazine via deethylation, addition and cyclization gives rise to 3-amino-4-cyanopyrazole whieh upon hydrolysis in the presence of sulphuric acid yields 3-amino-4-amino pyrazole. This on heating with formamide inserts the last carbon atom to afford allopurinol which exhibits tautomerism. 

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