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Tautomers are constitutional isomers of organic compounds that readily interconvert by a chemical reaction called tautomerization.^[1]^[2]^[3] This reaction commonly results in the formal migration of a hydrogen atom or proton, accompanied by a switch of a single bond and adjacent double bond. The concept of tautomerizations is called tautomerism. Because of the rapid interconversion, tautomers are generally considered to be the same chemical compound. Tautomerism is a special case of structural isomerism and can play an important role in non-canonical base pairing in DNA and especially RNA molecules.

Chemistry

Aromaticity provides some stability to the triple lactim form of these triple lactam-lactim tautomers. ^{[clarification needed]}

In solutions in which tautomerization is possible, a chemical equilibrium of the tautomers will be reached. The exact ratio of the tautomers depends on several factors, including temperature, solvent, and pH.^[4]^{[non-primary source needed]}^{[better source needed]}

Common tautomeric pairs include:^{[citation needed]}

ketone - enol, e.g., for acetone (see: keto–enol tautomerism);
enamine - imine;
ketene - ynol, e.g., for ethenone;
nitroso - oxime;
amide - imidic acid, e.g., during nitrile hydrolysis reactions;
lactam - lactim, an amide-imidic acid tautomerism in heterocyclic rings, e.g., in the nucleobases guanine, thymine, and cytosine;
enamine - enamine, e.g., during pyridoxalphosphate-catalyzed enzymatic reactions; and
anomers of reducing sugars in solution, which interconvert through an intermediate open chain form.

Prototropy

Prototropy is the most common form of tautomerism and refers to the relocation of a proton.^[5] Prototropic tautomerism may be considered as a subset of acid-base behavior. Prototropic tautomers are sets of isomeric protonation states with the same empirical formula and total charge. Tautomerizations are catalyzed by:^{[citation needed]}

bases, involving a series of steps: deprotonation, formation of a delocalized anion (e.g., an enolate), and protonation at a different position of the anion; and
acids, involving a series of steps: protonation, formation of a delocalized cation, and deprotonation at a different position adjacent to the cation).

There are two specific further subcategories of tautomerizations. Annular tautomerism is a type of prototropic tautomerism wherein a proton can occupy two or more positions of a heterocyclic system, for example, 1H- and 3H-imidazole; 1H-, 2H- and 4H- 1,2,4-triazole; 1H- and 2H- isoindole.^[4]^{[non-primary source needed]}^{[better source needed]} Ring–chain tautomers occur when the movement of the proton is accompanied by a change from an open structure to a ring, such as the open chain and pyran forms of glucose and furan form of fructose.^{[citation needed]}

Valence tautomerism

Valence tautomerism is a type of tautomerism in which single and/or double bonds are rapidly formed and ruptured, without migration of atoms or groups.^[6] It is distinct from prototropic tautomerism, and involves processes with rapid reorganisation of bonding electrons. An example of this type of tautomerism can be found in bullvalene. Another example is open and closed forms of certain heterocycles, such as azide - tetrazole or mesoionic münchnone-acylamino ketene. Valence tautomerism requires a change in molecular geometry and should not be confused with canonical resonance structures or mesomers.

References

^ Antonov L (2013). Tautomerism: Methods and Theories (1st ed.). Weinheim: Wiley-VCH. ISBN 978-3-527-33294-6.
^ Smith MB, March J (2001). Advanced Organic Chemistry (5th ed.). New York: Wiley Interscience. pp. 1218–1223. ISBN 0-471-58589-0.
^ Katritzky AR, Elguero J et al. (1976). The Tautomerism of heterocycles. New York: Academic Press. ISBN 0-12-020651-X.
^ ^a ^b Roman M. Balabin (2009). "Tautomeric equilibrium and hydrogen shifts in tetrazole and triazoles: Focal-point analysis and ab initio limit". J. Chem. Phys. 131 (15): 154307. Bibcode:2009JChPh.131o4307B. doi:10.1063/1.3249968. ^{[non-primary source needed]}^{[better source needed]}
^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "Tautomerism".
^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "Valence tautomerization".

English Journal

A "keto-enol tautomerization"-based response mechanism: a novel approach to stimuli-responsive supramolecular gel.

Lin Q1, Lu TT, Lou JC, Wu GY, Wei TB, Zhang YM.
Chemical communications (Cambridge, England).Chem Commun (Camb).2015 Jul 2. [Epub ahead of print]
A novel approach to stimuli-responsive gel termed the "keto-enol tautomerization"-based response mechanism was proposed. By tautomerization, vinyl ketone-based gelator can be self-assembled into an organogel () accompanied by strong AIE. shows reversible dual-channel response for S2-. The response p
PMID 26135904

Origin of the Individual Basicity of Corrole NH-Tautomers: A Quantum Chemical Study on Molecular Structure and Dynamics, Kinetics, and Thermodynamics.

Beenken W1, Maes W, Kruk M2, Martínez T3, Presselt M1,3,4.
The journal of physical chemistry. A.J Phys Chem A.2015 Jul 2;119(26):6875-83. doi: 10.1021/acs.jpca.5b02869. Epub 2015 Jun 19.
Free-base corroles exist as individual NH-tautomers that may differ in their spectral and chemical properties. The present paper focuses on the origin of the basicity difference between two AB2-pyrimidinylcorrole NH-tautomers, which has been tentatively attributed to differences in the weak out-of-p
PMID 26052732

A Reversible Proton Relay Process Mediated by Hydrogen-Bonding Interactions in [FeFe]Hydrogenase Modeling.

Chu KT1,2,3, Liu YC1, Huang YL1, Hsu CH1, Lee GH4, Chiang MH5,6.
Chemistry (Weinheim an der Bergstrasse, Germany).Chemistry.2015 Jun 26. doi: 10.1002/chem.201501114. [Epub ahead of print]
A reversible and temperature-dependent proton-relay process is demonstrated for a Fe2 complex possessing a terminal thiolate in the presence of nitrogen-based acids. The terminal sulfur site (St ) of the complex forms a hydrogen-bond interaction with N,N-dimethylanilinium acid at 183 K. The Fe2 co
PMID 26118674

Japanese Journal

Synthesis of Metallo-supramolecular Polymers with a Bis-ONO-tridentate Ligand by Coordination Programming

Kanao Miki,Higuchi Masayoshi
Chemistry Letters 43(7), 1075-1077, 2014
… Azo-hydrazo tautomerization in L1 was indicated by the NMR and IR spectra. …
NAID 130004868258

Ab Initio QM/MM-MC Study on Hydrogen Transfer of Glycine Tautomerization in Aqueous Solution : Helmholtz Energy Changes along Water-mediated and Direct Processes

MIYAMOTO Hidenori,AIDA Misako
Chemistry letters 42(6), 598-600, 2013-06-05
NAID 10031176020

Ab Initio QM/MM-MC Study on Hydrogen Transfer of Glycine Tautomerization in Aqueous Solution: Helmholtz Energy Changes along Water-mediated and Direct Processes

Miyamoto Hidenori,Aida Misako
Chemistry Letters 42(6), 598-600, 2013
… Helmholtz free-energy changes in aqueous solution along two paths of glycine tautomerization were investigated by using an ab initio QM/MM-MC method with the thermodynamic perturbation theory. …
NAID 130004426966