Amino-imino-tautomerization

As was concluded (97MI84), many other factors that may affect the position of the amino-imino tautomerism of aminotetrazoles remain to be elucidated. 

The possible amino-imino tautomerism of sulfonamides 98 [76AHC(S1), p. 438] and hydrazones 99 [76AHC(S1), p. 438] has been previously discussed. 

However, for derivatives of 2,4-diaminothiazole 214 evidence exists for the amino-imino tautomeric equilibrium (Scheme 71) [76AHC(S1), p. 460]. 

A careful and NMR study of 1,5-benzoxazine and 1,5-benzo-diazepine shows that these compounds exist in the amino-thione forms 55 and 56, respectively. Compound 55 displays a solvent-dependent amino/imino tautomerism (92MRC673).Tricyclic compounds 57, analogous to the bicyclics discussed above have been described they exist in the tautomeric form shown below (87BSB399,92BSB995,96BSB345). 

The amino-imino tautomerism in 3-amino-1,2,4-triazine 2-oxides 1 was suggested as the reason for the oxidation of 3-amino-1,2,4-triazines 2, predominantly at N(2) (77JOC546). 

Amino-4-oxo-5-phenylse)enazoiine, synthesis, amino-imino tautomerism, 262... 

Amino-4-oxoselenazoJines, amino-imino tautomerism, spectroscopy studies, 120... 

A-nitrosamino derivatives. 247 2-Arylamino-4-oxo-selenazolines. amino-imino tautomerism. spectroscopic studies, 261... 

Such is the case of 2-amino or 2-(monoalkyl or arylamino)-4-oxo-selenazolines. Thus because of this aminos imino tautomerism. these compounds can be called 2-aminoselenazolines or 2-iminoselenazolidines and can be classed either as selenazolines or selenazolidines. This has been a source of controversy that has been resolved by recent spectroscopic studies (67. 68). 

For 340a and 341a, another dynamic process was also frozen out. This process must be due to nitrogen inversion in the imino function, which results in E/Z isomerization (93MI2). This inversion could occur via amino-imino tautomerism (91JOC3194). The E and Z isomers are practically equally stable since there is no noteworthy steric hindrance in either form. 

Thiol-thioxo tautomerism Amino-imino tautomerism Perimidine... 

Experimental studies as well as data on amino-imino tautomerism are in agreement with the suggested mechanism of decomposition of the tetrazole ring. It has been shown that secondary reactions significantly increase the number of products of thermal decomposition of aminotetrazoles <2002THE233, 2003MI181>. 

Figure 3-17. Scheme of amino-imino tautomerism, X=N, O R=CH3, Phenyl... 

Adenine undergoes amino imino tautomerism. Thus,... 

Thiono-mercapto and amino-imino tautomerism of azines. In the same heterocyclic systems, the stability of thiols with respect to the corresponding thione form is considerably higher than hydroxy derivatives with respect to their oxo forms. In the gas phase 2- and 4-mercaptopyridine are the major tautomers, e.g., 191. 15N NMR spectroscopy is useful for estimating the tautomeric composition of mercaptopyridines in solution. 2-Mercaptopyr-idine in acetone or methanol and 4-mercaptopyridine 191 in methanol or acetone/DMSO were estimated to be ca. 95% in the thione form, e.g., 192. This solvent effect can be attributed to the polarity of the thione tautomers <2006AHC(91)1>. 

Unlike the 0-H 0 bonds, there are no examples of strong N-H 0 hydrogen bonds. The strong and almost symmetrical N H 0 configuration which might be expected to favor the amino imino tautomerism... 

Fig. 15.4. Keto/enol and amino/imino tautomeric forms of uracil, cytosine, adenine, guanine. The keto and amino forms predominate, the enol and imino forms are only present in equilibrium at 0.01 Vo or less...

Fig. 20.6. Possible stabilization of the A C base pair as seen in the crystal structure of d(CGCGAATTAGCG) by protonation of adenine (left) is more probable than the amino-imino tautomerism (right). Atom designation as in Fig. 20.4 R = sugar residue...

Compounds capable of a solvent-dependent amino/imino tautomerism are 3-methylcytosine (14a,b) and 1-alkyladenines [69b]. It has been shown by IR and UV/Vis spectroscopy that, in all cases, the imino forms such as (14b) predominate in nonpolar media e.g. 1,4-dioxane). However, the content of the amino form (14a) increases with increasing solvent polarity, and in aqueous solution the amino form predominates [69b]. Further interesting examples of solvent-dependent tautomeric amino/imino equilibria... 

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