Chain equilibrium

The ring-chain equilibrium position of substituted diazoalkylthiadiazoles is influenced by substituents and solvents and has been determined by NMR spectroscopy <1992JHC713> (cf. Section 11.07.8.1.3). 

Berkane, C. Mezoul, G. Lalot, T. Brigodiot, M. Lipase-catalyzed polyester synthesis in organic medium. Study of ring-chain equilibrium. Macromolecules 1997, 30, 7729-7734. 

The most powerful method currently available for the quantitative determination of ring-chain equilibrium constants is H-NMR spectroscopy. Often, two pairs of indicator signals of both tautomeric forms have been used, thereljy increasing the accuracy of equilibrium-constant determination. For structural analysis of the tautomers and quantitative measure-... 

Ring-chain equilibrium constants for 7A 7B have been measured by H-NMR as a function of pressure (1-2000 bar) to obtain the volumes of the intramolecular cyclization reaction and to compare them with those of the corresponding intermolecular analog [87JCS(P2)1477]. 

Ring-chain equilibrium constants Ky have been determined by using IR and H-NMR spectroscopy [93JCS(P2)635] for a series of 2-phenylacetylbenzoic (9) and benzyl-2-carboxylic (10) acids substituted in the phenyl group. A good linear correlation between Ky and the constants a or ct for the substituents X was obtained. As shown in Table I, for acids... 

Correlation Parameters of Ring-Chain Equilibrium Constants of 2-Phenylacetylbenzoic (9) AND Benzil-2 carboxylic (10) Acids"... 

Pyridylcarbonyl)- and 2-(2-quinolylcarbonyl)benzoic acids possess the open-chain structure in the solid state (84KGS1231). In dioxane solution, the ring-chain equilibrium is observed. Protonation of the pyridine or quinoline ring nitrogen atom leads to the formation of the protonated cyclic forms 11. Evidently, protonation stabilizes the tautomer that is the stronger base, i.e., the cyclic form. 

In the solid state, 3-benzoylpyridine-2-carboxylic acid exists as the open-chain form 12, stabilized by an intramolecular hydrogen bond. In dioxane solution, the ring-chain equilibrium was observed. The IR spectrum reveals that the hydrochloride of acid 12 exists as a mixture of the protonated cyclic and open-chain forms (86MII). 

Acetylpyridine-3-carboxamide 7V-oxides were isolated as cyclic isomers 26B (R = /-Pr, r-Bu) in the solid state (87MI4). In CD3OD solution, the isopropyl derivative retains the cyclic structure, but for the /-butyl derivative a ring-chain equilibrium was observed Ky = 1.10 H-NMR). 

Benzoyl-2,2,4,4-tetramethylbutyramide was obtained (84BAP335) only as the cyclic isomer, whereas both isomers were isolated for 4-benzoyl-3,3-dimethylbutyramide. The ring-chain equilibrium (A j 1) was rapidly attained in a solution of ether in the presence of traces of an acid catalyst, but this equilibration was accompanied by dehydration of the cyclic tautomer. The same equilibrium was reached, but more slowly, in the presence of basic catalysts, such as tertiary amines. 

The influence of solvent polarity on the ring-chain equilibrium is contradictory for hydroxy aldehydes or ketones of different structures (I-lOl). 

Introduction of a methoxy group (28, R = OMe) into the aryloxy side chain in the series of prostaglandin intermediates 28 has been found [90JCS(P1)751] to affect the ring-chain equilibrium significantly in favor of the cyclic tautomer. In CDCI3, when R = H, Kj = 1, and when R = OMe, Kt = 9. 

UKZ746) for 2-hydroxy-2-methyl-l,4-dioxane. The ring-chain equilibrium constants for the A -aroylmethyl-A/-(2-hydroxyethyl)-/V,/V-dimethyl-ammonium (aroylcholinium) salts 30A 30B (Y = NMc2, R = XQH4)... 

The H-NMR spectra were recorded at different temperatures to determine the ring-chain equilibrium constants in solutions of 2-N- 2-hydroxyethyl)amino-5-methyl-l,4-benzoquinones 33 (93M1053). Greater... 

Ring-Chain Equilibrium CoNSTANrs" por o-Hydroxymethyl-SuBSTiTUTED Arene Carbal.dehydes AND Arylketones (34-36)... 

Ring-Chain Equilibrium Constants" for 2-(2-Hydroxyethoxy)benzaldehydes (37A 37B) in Various Solvents... 

Another case of an or//io-substituent steric-assistance effect has been described (90JOC3891) for the rather rare situation of a ring-chain equilibrium involving a seven-membered ring closure. The equilibrium was ob-... 

The ring-chain equilibrium of o-hydroxyphenoxymethyl ketones 42A 42B (see Tables VI and VII) and the diastereomeric equilibrium 42B 42B (Z/E) were investigated thoroughly by means of H-NMR... 

Ring-Chain Equilibrium 42A 42B (R = R = H R = Ph) Constants for Various Solvents and the Correlation Parameters for the... 

H-NMR and C-NMR spectroscopy demonstrated a three-component equilibrium (83G91) in ( 03)280 solutions of the diastereomeric monooximes of 5-methyl-2-phenacylcyclohexanone 54 and 55. The same methods revealed the ring-chain equilibrium 56A 56B (X = H2, R = Ph) in solutions of 5-hydroxy-5-methyl-2-phenylisoxazolidines (89KGS823). An increase in the solvent proton-accepting ability shifted the equilibrium in favor of the open-chain tautomer 56A in CCI4, Kj = 1.0 and in ( 03)280,... 

Four diastereomers of 2-ethynyl-7a-hydroxy-3a,7-dimethyl-4,6-diphe-nylperhydropyrrolo[3,2-c]pyridine (58B) were recently isolated (92KGS-903). It was established by means of H-NOESY and C-NMR spectroscopy that the stereochemical differences in the structures of the diastereomers involve cis and transfusion of the rings and different configurations at C(2). In solution and in the gas phase, the corresponding ring-chain equilibrium 58A 58B was detected, with two open-chain epimers (different configurations at C—NH2). 

Ring-Chain Equilibrium Constants for 2-Hydroxy Indolines 58 AND 59 ... 

A ring-chain equilibrium displaced in favor of the cyclic tautomer was observed [78JBC(253)5407 83LA1623] in neutral aqueous solutions of 5-carbamido- 66 n = 3 Xt = 5.67) and 5-guanidino-2-oxovaleric 67 n = 3 Xt = 3.17) acids. In aqueous solutions of acid 67 (w = 3), H-NMR spectroscopy detected the presence of 4% of the open-chain tautomer hydrate 67A (n = 3). In acidic medium, the amount of this hydrate is higher. The dipolar cyclic structure of 67B (n = 3) in the solid state was established on the basis of X-ray diffraction data [83AX(C)1240]. Both isomers 67A and... 

Mass-spectrometric investigation (83KGS1273) showed that the more substituted A-(3-oxoalkyl)thioureas 69 (R = R = R = Me R = H R = alkyl, Ar) exhibit a ring-chain equilibrium in the gas phase, but the less substituted derivatives 69 (R = R = H R = R = Me R = Me, Et, Bu) exist only as open-chain isomers. Clear evidence of the stabilizing effect of gem-dimethyl substitution on the cyclic form (Thorpe-Ingold effect) was presented for a gas-phase equilibrium. The mass-spectrometric data are rather similar to those obtained for this system in solution, which indicates that this equilibrium depends mainly on structural, rather than solvation, factors. 

The small value of the coefficient a and the similar values of px and py in the one- and two-parameter equations indicate that the substituents X and Y exert independent effects on the ring-chain equilibrium. 

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