Imine tautomer

Tile existence of two annular tautomers in solution has been concluded from NMR spectroscopy for the cyclic A -acylbenzazoles 83 [82JPR(324)569]. Enamine and methylene imine tautomers have been described for condensed azolo-quinoxalines [93JHC782, 93JHC1463 95H2057] this type of tautomerism is discussed in detail for the [6.6]bicyclic compounds (see Section III,E,2). 

It is likely that initially the open-chain adducts 353 and 354 are formed by the addition of an amino group either to the carbonyl function or to the triple bond, whereupon these intermediates close up to the azepines 355 and their bis-imine tautomers 356. In the H NMR spectra, the methylene protons of 356 are at 2.85-2.97 ppm, whereas the methyl protons are fixed at 2.20-2.27 ppm. The IR spectra show absorption bands corresponding to the aromatic ring (1600 cm ) and to the diazepine cycle C=N double bonds (1580 cm ). However, there are no bands of... 

However, valence isomerism in some l//-azepinc-4,5-dicarboxylates, e.g. 3, which is undetectable by NMR spectral measurements, has been confirmed by trapping out the benzene imine tautomers 4, as their bisdipolar cycloadducts 5, with diazomethane.233,234 In contrast, ethyl l//-azepine-l-carboxylate and diethyl l-acetyl-l//-azcpinc-3,5-dicarboxylate, with diazomethane, yield only the C4-C5 dipolar cycloadducts. 

Another example is dimethyl 3,6-dichloro-l-tosyl-l//-azepine-4,5-dicarboxylate which, on the basis of ]H and 13C NMR spectral studies, is a 9 1 equilibrium mixture of azepine and benzene imine tautomers at —70 C(AG = 50.16 kJ mol-1) 22 however. X-ray analysis indicates that the mixture crystallizes as the 1//-azepine isomer. 

Dialkyl esters of cystine (39) and lanthionine (40) undergo a surprising thermolysis reaction at between 25 C and 80 °C to afford cis and trans methyl 2-methylthiazolidine-2,4-dicarboxylates (43) in protic solvents. A two stage process is proposed for this transformation. An initial i-elimination reaction gives the thiol (41) and the enamine (42). Thiol addition to the imine tautomer of (42) is then followed by loss of ammonia and an intramolecular cyclisation to give (43) <96CC843>. 

Tomic K, Jorg T, Marian CM (2005) Quantum chemical investigation of the electronic spectra of the keto, enol, and keto-imine tautomers of cytosine. J Phys Chem A 109 8410-8418... 

The Yao group has made use of a Ic type intramolecular Heck reaction to prepare the C2-symmetric dimeric indole core of chloptosin <06OL4919>. A solvent-free variation of the Bischler indole synthesis, electrophilic cyclization of a-arylamino imine tautomers prepared from aniline derived a-arylamino ketones, has been used by Menendez and co-workers for the preparation of 2-arylindoles <06SL91>. 

The aminomethylenemalonates (1 and 2) may exist in imine or enamine tautomeric forms of R1 or R2 is a hydrogen atom. In some early papers, the imine tautomer (see below) was given for the structure of the products, usually without any explanation or evidence (e.g., 1885CB319 37JCS867 49JIC171 88KGS931). 

Since 1960, almost all papers have provided at least some spectroscopic evidence that these compounds (1 and 2) exist as the enamine tautomer in solid phase or in solution. Imine tautomers have been detected for only those aminomethylenemalonates in which R2 and R3 formed a ring (e.g., 83IZV1687, 83MI2 88ZOR1793). This chapter will examine only certain papers in which details of spectroscopic investigations or quantum-chemical calculations are given. 

The prototropic tautomerism of 8-azaadenine has been studied theoretically in both the gas phase and aqueous solution by means of ab initio methods. It has been shown that dehydrovaline (399 R =Me, R = H, R = Me), dehydrophenylalanine (399 R = Ph, R, R = H), and dehydropipecolinic acid [399 R R = (CH2)3, R = H] hydrolyse rapidly via the imine tautomer (400) even when the corresponding esters and sodium salts exist as the enamine tautomers. The 3-methoxy-substituted deriva-... 

There is a distinct relationship between keto-enol tautomerism and the iminium-enamine interconversion it can be seen from the above scheme that enamines are actually nitrogen analogues of enols. Their chemical properties reflect this relationship. It also leads us to another reason why enamine formation is a property of secondary amines, whereas primary amines give imines with aldehydes and ketones (see Section 7.7.1). Enamines from primary amines would undergo rapid conversion into the more stable imine tautomers (compare enol and keto tautomers) this isomerization cannot occur with enamines from secondary amines, and such enamines are, therefore, stable. 

Quinolines are reduced in alkaline solution at a lead cathode or by sodium and alcohol to another cla.s.s of dimeric product [85,86]. The meso-4,4 -hydrodimer is again an intermediate in the process. It undergoes slower conversion to the imine tautomer under alkaline conditions and the intramolecular aldol condensation between one imine function and the remaining enamine function leads to the product 23 [86]. 

There are three dihydro forms of pyrrole 2,3-dihydro-1//-pyrrole (1-pyrroline) exists in equilibrium with its imine tautomer (Scheme 6.17). These isomers are more stable than 2,5-dihydro-1//-pyrrole (3-pyrroline), where no conjugation is possible between the N lone pair electrons and the C=C double bond. 

Table 11 summarizes the main results on the tautomerism of mono-hydroxy-, -mercapto-, -amino- and -methyl-azines and their benzo derivatives, in water. At first sight the equilibrium between 2-hydroxypyridine (71) and pyridin-2-one (72) is one between a benzenoid and a non-benzenoid molecule respectively (71a 72a). However, the pyridinone evidently has a continuous cyclic p- orbital system, containing six it- electrons, the usual aromatic count, if the carbonyl group contributes none. This assumption implies the formula (72b), from which by redistribution of electrons we arrive at (72c), which has the same benzenoid system as (71a). Further canonical forms (71b, 71c) can be drawn of (71) which correspond to the non-benzenoid forms of (72). The elusive property of aromaticity is therefore possessed by both tautomers, although not necessarily by both equally. When the carbonyl oxygen of (72) is replaced by less electronegative atoms, as in the imine tautomers of amino heterocycles, or the methylene tautomers of methyl derivatives, the tendency towards polarization in forms corresponding to (72b) and (72c) is considerably less, and the amino and methyl tautomers are therefore favoured in most instances. 

It is well established that for most aminopyridines and their benzo derivatives the amino form greatly predominates over the pyridone-imine tautomer <76AHC(S1)152, 63AHC(l)404>. 

The tautomeric studies of azacytosines are not so complete as those of cytosine itself. The contribution of other tautomeric forms of azacytosines to the tautomeric equilibrium has not been evaluated (because of lack of model tautomeric compounds). It appears that 6-aza-substi-tution causes a tautomeric shift from form 3 of cytosine toward its imine form, 6. Thus, upon 6-aza-substitution the contribution of the imine tautomers to the tautomeric equilibrium of cytosine increases, while that of tautomers 3 decreases. 

It is interesting that lactim-imine tautomers 5 and 4 appear to have considerably higher energies than tautomers 2 or even tautomer 6.156... 

IJC4). It has been suggested that protonation at C-3 with formation of (388) may be responsible for such facile decarboxylation (Scheme 125). The acid (389) also undergoes base-catalyzed decarboxylation the imine tautomer (390) may be an intermediate in this process (Scheme 125). 

The UV spectra of 9-(phenylamino)tetrahydro-4f/-pyrido[ 1,2-a] pyrimidin-4-ones 23 indicated the presence of an imine-enamine type of tautomerism in solution. A carboxyl or ester group in position 3 (R1 = COOR3 R3 = H, Et) and a methyl group in position 6 (R = Me) shifted the equilibrium toward the enamine form in a polar solvent [85JCS(P1) 1015]. For the imine tautomer, UV maxima at 235-250 nm and 280-300 nm and for the enamine absorbances at 260-265 nm and 355— 365 nm are characteristic. 

R2 = COOEt) exists as about a 9 1 mixture of formyl-enamine and enol-imine tautomers. 

Protonation of 9-formyltetrahydropyrido[l,2-a]pyrimidinones takes place at the formyl oxygen, and the protonated enol-imine tautomers on the N(l) atom became the predominant tautomer forms [86JCS)P2)1911]. Similar phenomena were observed for the 9-formyltetrahydro-2// pyrido[l,2-a]pyrimidin-2-ones and their homologs [85JCS)P2)1873 88MI7]. 

Thiatriazole-5-thiol 4b was described as being in equilibrium with the thione tautomer 4a (Scheme 2). The thione tautomer is predominant in acetone solution, as is apparent from the 13C NMR spectmm <1977JHC1417>. However, methylation and acylation under basic conditions occur at the sulfur atom. The analogous amino compounds do not occur as the imine tautomers. More data can be found in the earlier reviews <1964AHC263, 1976AHC145>. 

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