Imino structure

Comrie et al. (68) have thus prepared several compounds of this type by condensation of various selenoureas with a-halocarboxylic acids and have assigned them the imino structure (Table X-15a). 

The piperideine derivatives have not been studied as extensively as the analogous pyrrolines (151,152). The imino structure has been established, for example, for the alkaloid y-coniceine (146) (46). The great influence of conjugation on the structure is seen with l-(a-picolyl)-6,7-methylenedioxy-3,4-dihydroisoquinoline (47), possessing an enamine structure, whereas the analogous 1-methyl derivative (48) possesses an imine structure according to infrared spectra (152,153). 

Earlier studies of 4-aminopyridine 1-oxide were less conclusive. The solid-state infrared spectrum could be interpreted to indicate the existence of both the imino structure and/or, more probably, the amino structure. Comparison of the actual pKa value of 4-aminopyridine 1-oxide wdth the value calculated using the Hammett equation was considered to indicate that the compound existed as such or as an equilibrium mixture with l-hydroxypyrid-4-onimine, the latter possibility being considered the less likely on the basis of resonance and bond energies/ Resonance energy and ultraviolet spectral considerations have been advanced to support the 4-aminopyridine 1-oxide structure/ The presence of an infrared absorption band at... 

Amino-pyrazines and -pyridazines have been shown to exist predominantly in the amino form by infrared spectroscopic studies (cf. Table VI). Ultraviolet spectral data have been interpreted to indicate that 4-aminocinnoline exists predominantly in the imino form 256, but this conclusion, which was based on comparison of its spectrum with those of cinnolin-4-one and 4-ethoxycinnoline, is probably incorrect. Ultraviolet spectroscopic data strongly support the predominance of amino structures for 2-aminopyrazine (257) and 2-aminoquin-oxaline how ever, the former compound was at first erroneously concluded to exist in the imino form from ultraviolet spectral evidence. Isolation of two isomers of 2-amino-8-dimethylamino-3-methylphenazine, assigned the amino and imino structures 258 and 259, respectively, has been claimed, but it is very unlikely that these assignments are correct. 

Tlie amino form 84a exists in equilibrium with the imino structure 84b and not with 85 as concluded from the structure of products of deuterium exchange (87MRC757 91H329). 

The amino structure 173 was suggested for 5-aminoisoxazoles rather than the imino structure 174 on the basis of tentative chemical data and evidence from the exaltation of the molecular refractivity however, forms of type 175 were not taken into consideration in these... 

Other chemically modified amino groups have found use as precursors to furo[2,3- pyrimidines. The imino structure 309 has been successfully transformed into 4-bromo products 310 (Equation 112) <1999H(51)2723>. 

Analogous to MOT the yellow 7-mercapto compound proves to be the thioxo tautomer (59JOC779). In 5,7-dialkyl-2-amino-TPs any imino structure is excluded by IRand NMR spectra (71AP856,71T3247) aminoa-zaindolizines generally behave similarly (91H(32)329). 

The l-amino-3H-pyrrolizines are potentially tautomeric. Whereas the 1-isopropylamino derivative 227 is reported to have the 3-imino structure, the 1-benzyl derivative is said to favor the 3-amino form (228).67... 

For 4//-thiazine-4-ones (78), two maxima are generally noted one between 233 and 258 nm and the other between 278 and 298 nm [66TL3225 77LA1249 78JCS(P1)1428]. The differences in the long-wave maxima between amino and imino structures in the UV spectra are not usually substantial, with the exception of the dirnethylamino derivative (79) (82CCC3268). This compound (X max = 330 nm) shows a bathochromic shift of 60-80 nm compared to all other thiazine derivatives, probably due... 

Imino structure—for example, proline (P), whose amino group forms imino ring with the rigid conformation therefore its presence disrupts the formation of regular secondary structures. 

The double bond is kept in the exocyclic position by conjugation with the carbonyl group (the free base absorbs at 3270 cm-1). The free base of l-(a-picolyl)-6,7-methylenedioxy-3,4-dihydroisoquinoline also possesses the stabilized enamine structure, whereas the imino structure is exhibited by the 1-methyl analog, as shown by comparison of the... 

The tautomerism of 4-aminopyrazolo[3,4-J]pyrimidines as well as aminopyrazolo[4,3-d]py-rimidines has been investigated. 4-Aminopyrazolo[3,4-J]pyrimidines exist in water in two tautomeric forms l//-4-amino (85) and 2//-4-amino (86) [K (86/85) = 0.1 at 100°C AH = 9.0 kcal, mol ]. Interconversion of these two forms is catalyzed by acid or base <77JA7257, 80ZN(C)878, 82MI 712-02). Together with these two predominant species there are smaller proportions of the 7//-4-amino (87) (1 x 10-3) and the 5//-4-amino isomer (2 x 10-4). The species (87) exists only as an amino tautomer whereas the 5//-4-amino species in water has a partial imino structure (amino/imine =10) <77JA7257>. 

Neither monosubstituted 3-aminobenz[djisothiazole-1,1 -dioxides (9) nor 3-alkoxy or aryloxy derivatives (3),17 over a temperature range of 150°, gave any indication of a separation of 4H NMR signals,14,18 due to type 9b (E-form)19 structures. Spin coupling of the N-H protons with the protons of substituents R (R = Alkyl) excludes an appreciable proportion of 3-alkyl imino structures (9c)14,18 in the equilibrium. 

Aminoimidazoles are rather unstable and have not been studied extensively, but despite the absence of a specific study of their tautomerism it is possible to gather some evidence in favour of the amino tautomer of types (35 X = NH) (39 X = NH) or (47 X = NH). Imino structures (e.g. 34, 38, 44 X = NH) require the appropriate CH signals in the NMR spectrum these are absent, and the IR spectra display characteristic bands for NH2. The X-ray structure for 4-acetamido-2-bromo-5-isopropyl-1-methylimidazole confirms the structure (65) rather than the tautomer (66) (Scheme 21). On the basis of CNMR evidence, 2-arylamino-2-imidazolines prefer to exist in the arylimino forms (B-77MI40600). 

On the other hand, there is a close correlation in the spectra of the monoaminopurines with those of the corresponding iV-dimethyl derivatives. This at least does not conflict with the primary amino (as distinct from an imino) structure assigned to the aminopurines which is well characterized by, for example, X-ray crystallography and magnetic resonance studies. However, full comparison with the various imino derivatives would be required before the result could be considered meaningful. 

Although it has been suggested that some of the reactions of 2-ami-noimidazole can best be explained by its existence as the imino tautomer in dimethylformamide, this result was based on incorrectly interpreted chemical evidence. In fact, a UV study of 2-amino-l-methyl-4,5-di-phenylimidazole provides convincing evidence for the amino form with Kj = 3 X 10". Calculations of heats of combustion show that in the 4-aminoimidazole 5-aminoimidazole equilibrium there is a slight predominance of the 4-amino form [a conclusion which is inconsistent with Charton s hypothesis since (7m(NH2) = —0.161]. The compounds, however, exist as amino rather than imino structures. ... 

X-ray analysis carried out by Bryden and co-workers [59] favoured the nitrimino form. Also NMR analysis of Richards and York 60) showed that the solid nitroguanidine exists in the imino form. Owen (61] calculated the MO of several compounds containing nitrogen and among thent nitroguanidine and came to the conclusion that the calculated imino form fits better to the experimental values, as shown in Table 59, with the diagram of bond lengths. The imino structure was also confirmed by Kumler and Sah [62] measurement and calculation of dipole moment. 

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