Tautomer naphthalene

Oxadiazole (1 R = R = H) is calculated to be 8.9 kcal moU less stable than its open chain tautomer (2) (85AG(E)713>. On the other hand, 1,2,3-benzoxadiazole is about 1 kcal mol more stable than 2-diazocyclohexadienone (91JST(247)135>. The position of the equilibrium is markedly affected by substituents. The benzoxadiazole (4) is more stable than its tautomer (5) by about 1.5 kcal mol but tetrafluorobenzoxadiazole is substantially less stable than the corresponding diazocarbonyl isomer. The open-chain forms are also favoured by polar solvents and by hydrogen bonding. In the naphthalene series, the stabilization of the fused oxadiazole (6) relative to its open chain tautomer can be ascribed to partial bond fixation in the naphthalene system, which disfavours... 

In the aromatic-ring-annelated oxepin series the resonance effect is clearly the major influence dominating other factors (e.g. temperature, solvent, etc.) which affect the oxepin-arene oxide equilibrium. It is however very difficult to exclude the presence of a minor (spectroscopically undetectable) contribution from either tautomer at equilibrium. This problem has been investigated by the synthesis of chiral arene oxides from polycyclic aromatic hydrocarbons (PAHs). The presence of oxepin (26) in equilibrium with naphthalene 1,2-oxide has been excluded by the synthesis of the optically active arene oxide which showed no evidence of racemization in solution at ambient temperature via the achiral oxepin (26) <79JCS(Pl)2437>. 

As in the oxepin-arene oxide system, the resonance effect will also influence the position of equilibrium in the analogous organosulfur series. Compounds (46) and (53) thus appear to exist exclusively in the thiepin form. Since the resonance factor would favor the 1,2-episulfide of naphthalene over the thiepin tautomer (54) it is highly improbable that this thiepin will be detectable at ambient temperature. Both thiepins (46) and (55) have been isolated as thermolabile compounds (78JOC3379, 81MI51700). 

In the solid state, the equilibrium is in favor of the hydrido complex (III), and its crystal structure and that of the osmium(II) analog have been determined (38). Chatt also observed that, on heating the equilibrium mixture of (II) and (III), naphthalene was eliminated and the product Ru(dmpe)2 was also a tautomeric mixture. Here the tautomer-ism involves breaking and re-formation of carbon-hydrogen bonds in the methyl groups of the phosphine ligands (IV and V) ... 

The pifas for C-protonation of the phenols and phenoxide ions are compared with values for the unsubstituted aromatic molecules in Table 3. The focus on pKn rather than pAR is because the equilibrium constants for hydration of the keto tautomers of the phenols have not been measured or estimated. The values of Ap and ApKf show the magnitude of the oxygen substituent effects relative to the parent aromatic molecules. Again the substituent effects are large, and much larger for O (more then 20 log units) than OH ( 10 log units). At first, it is surprising that the effects are so similar for the benzene, naphthalene, and anthracene. Once more this arises because the pWa reflects the stability of... 

Bromination of (47) led to 3-azido-4-bromoisoquinoline (261). On the other hand, trifluoroacetic acid caused only the tetrazolo tautomer (262) to exist in the equilibrium of (47) with its azido partner. This is due to elimination of the unfavored o- quinoid structure (47) from the tautomeric equilibrium and formation of the more stable naphthalenic structure (262) which, consequently, is preferred over the azido form (8UOC843). To extend this equilibrium study further, NMR investigations on substituted derivatives of (47) indicated that electron-releasing groups on the isoquinoline favor the tetrazolo tautomer whereas electron-attracting substituents favor the azido tautomer. Furthermore, the tetrazole was found to be preferred in DMSO solution relative to chloroform solution (81JOC843). 

The simplest member of the polycyclic aromatic hydrocarbon (PAH) series, naphthalene, may in principle form four possible arene oxide-oxepin tautomeric pairs (A-D). In practice, the valence tautomers that have an intact aromatic-ring structure 11, 12, 100, 101 predominate. This discussion of arene oxide synthesis... 

The tautomeric properties of hydroxynaphthalenes show in the most unambiguous manner that the naphthalene system is less aromatic than that of benzene. The benzoannelation appreciably destabilizes the aromatic tautomers not only among phenols but also in the arene series . Therefore, even the monohydroxy naphthalenes display in their chemical reactions properties typical for the tautomeric keto form. 

The 3(5)-(l -hydroxy-2 -naphthyl)pyrazole and the 3(5)-(2 -hydroxy-1 naphthyl)p3trazoles have been synthesized and fully characterized (95JOC3427). NMR ( H and and UV (absorption and emission) spectroscopies in different solvents were used to determine the major tautomers, the coplanarity of both rings (naphthyl and pyrazolyl) and the existence of hydrogen bonds. The photostability of the compounds was addressed. In 3(5)-(2 -hydroxy-l -naphthyl)pyrazole, the peri interaction between the naphthalene proton and the pyrazole yielded nonplanar structures. 

An alternative method for the preparation of benz[c]azepines and their pyrido analogues is the treatment of nitronaphthalenes and nitroquinolines with dimethyl phosphate under basic conditions <85CC1792, 91J0C1283>. Thus, treatment of 2-nitronapthalene or 6-nitroquinoline with secondary amines and sodium methoxide affords the phosphonates (250 X = CH or N) whereas 1-nitro-naphthalene and 5-nitro- or 8-nitroquinoline form a mixture of the tautomers (251 X and Y = CH or N) and (252 X and Y = CH or N). 

Tautomeric trifluoromethyl-substituted spirocyclic quinazolines 693 and 694 were formed when p-benzoquinone imines 692 (synthesized by electrochemical oxidation of the corresponding p-anisidine derivatives 691) were heated in DMSO at 120 °C (Scheme 145) [409], The reaction was affected by solvent (DMSO giving the highest yields) and nature of the substituents in 692, In the case of naphthalene derivatives (e.g. 695), thecyclization gave single tautomers (e.g. 696). A synchronous mechanism was proposed for this transformation. 

Photolysis of (227) and its norcaradiene valence tautomer proceeds via two [l,7]-shifts to give (228), which undergoes a [1,3]-hydrogen shift followed by rearrangement to (223 R = H) and then loss of methylene to give, finally, naphthalene. ... 

The relative stabifization of the E or K tautomers in 2-4 depends mostly on the position of benzoannelation (Figure 10.3). In contrast to 2 and 3, imines of 3-hydroxy-2-naphthaldehyde 4 exist exclusively as the enolimine tautomers [50 - 54]. Loss of aromaticity in 2 and 3 is restricted to one ring of the naphthalene moiety, whereas in 4 both rings are involved, which leads to destabilization of the... 

Aza-15-crown-5 containing 4-(phenyldiazenyl)naphthalen-l-ol 38 exists in the enolimine form E. Complexation with alkaline-earth metal cations reverses the equilibrium to tautomer K (Figure 10.26). This process is accompanied by strong bathochromic (60-100nm) and hyperchromic effects in the absorption spectra [123]. 

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