Imino tautomer

In principle, aminopyrazine and 2-aminoquinoxaline are capable of existing in the form of the imino tautomers (81) and (82) however, comparison of the UV spectra of the amino, methylamino and dimethylamino derivatives indicates that in both systems the amino rather than the imino tautomer is favored (60JCS242, 58JCS108). 

The energy difference between 3-aminopyrrole 163 and the corresponding imino tautomer 164 was calculated by this method as 9.2 kcal/mol in favor of the amino tautomer (99UP1). Interestingly, l-(triphenylmethyl)-3-... 

Even in tricyclic bis-imidazole 57, for which the potential imino-tautomer 57b would be stabilized by the intramolecular N-H -N bond, the only observable form is the amino tautomer 57a (Scheme 30) [73KGS807 76AHC(S1), p. 431]. 

However, the equilibrium amount of the imino tautomers 88b and 91b is significantly increased with the increase in the electron-withdrawing ability... 

According to an X-ray crystallographic study, 2-(A-phenylamino)thia-zole forms in the crystal dimers of the imino tautomers, which are linked together by the intramolecular NH—N bonds [77AX(B)106]. 

The more basic benzimidazolylformazanes 261 (A = NR R R = Aik, Ar, Het R = Aik) prefer the imino tautomeric form 261b [73KGS699 75UK1052 92MI144]. l,5-Di-(l -methylbenzimidazolyl-2)-3-methylformazan exists as amino-imino tautomer 262 [90ZSK117]. 

Tile ID and 2D NMR experiments for 5-aza-7-deaza-2 -deoxyguanosine concluded that the oxo-amino tautomer 59a is preferable in DMSO-dg whereas the oxo-imino form 59b dominates in D2O (87JOC5136). Evidence for the hydroxy-imino tautomer 59c was not found. Poor solubility of the parent compound, 5-aza-7-deazaguanine (60) did not allow study of its tautomerism, but the pK values of protonation and deprotonation for 60 are identical with those for 59. 

Comparison of the UV speetra of 5-amino-2-(2-furyl)-l,2,4-triazolo[l,5-cjquinazolines (196) and its 5-methylamino derivative (197) in neutral, aeidie, and basie media with the speetra of the two tautomerieally looked derivatives—2-(2-furyl)-5-dimethylamino-l,2,4-triazolo[l,5-c]quinazolines (198) (amino-loeked tautomers) and the imino-loeked tautomers 2-(2-furyl)-5-imino-6-methyl-l,2,4-triazolo[l,5-c]quinazolines (199)—indieated that eompounds 196 and 197 are best represented as a mixture of their amino and imino tautomers (88JMC1014) (Seheme 75). 

Reaction of 2-chloro-6,7-dihydro-4//-pyrimido[6,1 -a]isoquinolin-4-ones with liquid NH3 in a pressure bomb at 85 °C for 4h, and with primary and secondary amines in boiling CHCI3 (98MIP15), and anilines in boiling i-PrOFI (00MI17) yielded 2-amino-6,7-dihydro derivatives or their 2,3,6,7-tetrahydro-2-imino tautomers. 

Bromo-6,7,8,9-tetrahydro-l//-3-benzazepin-2-amine(6) with thiocyanate ion undergoes substitution of bromide to give the thiocyanatotetrahydro-l//-3-benzazepine 7.105 Attempts to replace bromide by azide ion failed, as did diazotization of the amine group with sodium nitrite in 6 M sulfuric acid. Oddly, treatment of the aminobromo compound with sodium borohydride in methanol results not in reduction, but in methoxy-debromination to give the 2-methoxy derivative which, on the basis of HNMR spectral data, is best represented as the 2-imino tautomer 8. 

Fig. 30. Possible mispairing schemes for adenine N6-metallo imino tautomer. Adapted from Ref. (88).

The stabilities and reactivities of monoaminoazoles (6 and 8) vary considerably and are related to the number of nitrogen atoms in the heterocyclic ring. Physical evidence indicates that members of this family exist as primary amines (6 and 8) rather than as imino tautomers (7 and 9). Nitrogen... 

It has been reported that the UV spectra of l,2,3-benzotriazin-4(3/f)-one (10, R = H), the corresponding triazinethione (39, R = H) and the 4-alkylamino-l,2,3-benzotriazines (56) formed by treatment of 39, R = H, with alkylamines are nearly identical, and this has been taken as evidence that the compounds 56 actually exist as the 3,4-dihydro-4-imino tautomers. This claim has not, however, been substantiated, and, in the absence of more definitive evidence, structure 56 is almost certainly the more accurate representation for these compounds. 

Amine-imine tautomerism in 3-acyl-substituted aminopyrazines has been examined by H, C, and N spectral analysis as well as X-ray crystallography <2005JST67>. In the same way as the parent aminopyrazine, those aminopyrazines have been shown to exist in the amino form 11 (R = H, Me, Ph) (Scheme 2) in contrast to an expectation that the electron-withdrawing acyl groups adjacent to the amino substituent would stabilize the imino tautomers 12 and 13. Thus, all NMR spectra showed only existence of the amino tautomer 11, and additionally the... 

An unprecedented nitrogen elimination reaction of 4-amino-7-benzylpyrrolo [23- f][133]triazine-5-caibonitrile (72) to give the pyrrole derivative 74 has been described. The following mechanism, presumably via a a retro Diels-Alder reaction of the imino tautomer 73, has been proposed <990L537>. 

The observations that only the 3-cyano group is attacked, that only primary amine monosubstitution products undergo exchange, and that aromatic amines give only the substitution/addition products suggest that they are formed by reversible attack on an imino tautomer such as 107, which can undergo intramolecular transfer of amine, as shown in Scheme 38. 

The frequently observed bioisosteric relation of benzene and thiophene applies to the clonidine series as well. Reaction of the thiophenyl thiourea (85-1), in which the amine group is flanked by substituents as in the prototype, with methyl iodide and a base gives the corresponding methyl thioether (85-2). Treatment of that intermediate with ethylene diamine leads to the formation of an imidazoline ring and the antihypertensive agent, tiamenidine (85-3) [90], shown as its imino tautomer. 

MO calculations predict that 2- and 3-aminopyrroles should prefer the amino rather than imino tautomer, although experimental data are scarce (70JA2929). In support of this prediction the IR spectrum of the pyrrole derivative (74) indicates that it possesses an... 

Other classical synthetic approaches to 2-furanamine have failed, including the Curtius method and Beckmann rearrangement of 2-benzoylfuran oxime. However, hydrazinolysis of AT-(2-furyl)phthalimide, obtained from phthalimide and 2,5-dimethoxy-2,5-dihy-drofuran, gives 2-furanamine which was not isolated but detected by GLC-MS and H NMR spectroscopy. The latter reveals the absence of imino tautomers (75AP713). The chemistry of 2-dialkylamino-5-phenylfurans is typical of enamines protonation occurs on carbon to produce iminium salts. They are stable to base but afford 5-phenylfuran-2(3//)-one on hydrolysis with dilute acid. 2-Morpholino-5-phenylfuran couples with diazonium salts and affords Diels-Alder adducts with maleic anhydride and IV-phenylmaleimide (73JCS(P1)2523). 

Two C-alkylfuran-3-amines are known they both resinify very rapidly in air. They are sensitive to hot acid and alkali and ammonia is liberated. It is reported that they react with nitrous acid and that coupling products are obtained with 2-naphthol. It has been suggested that they exist as imino tautomers, but there is no spectroscopic information on this point (B-76MI31101). 5-Methyl-2,4-diphenylfuran-3-amine has been prepared by the Curtius method but its chemistry remains unexplored (73TL3353). 3-Morpholino-2,5-diphenylfuran (389) forms a with bromine. When treated with boiling ethanol, (390) yields the furanone (391 Scheme 107) (70JHC569). 3-Azidofuran-2-carbaldehyde and... 

FeClJ X = S) used to model the active sites of the nonporphyrine iron proteins with the N,S-ligand environment [96MI8 97JCS(CC)1711]. Additional metal cyclic structures are formed upon introduction of an azole framework to the hydrazone systems (291) (92MI8), in which the imino tautomer of the benzothiazole is fixed. 

Amino-4-methyl-l,2,4-triazin-3-ones (43a) were shown to exist predominantly in the 5-imino forms (43b), due to the destabilization of the amino tautomer (43a) by the formal N=N double bond (64CCC1394, 65JCS5230). On the other hand, for 3-amino-4,6-dimethyl-l,2,4-triazin-5-one (44a) the predominance of the amino tautomer was proven and no imino tautomer (44b) could be detected. 

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