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Two tautomers of an amino acid: (1) neutral and (2) zwitterionic forms.

Tautomers are constitutional isomers of organic compounds that readily interconvert.^[1]^[2]^[3] This reaction commonly results in the relocation of a proton. Although it is a complicated concept, tautomerism is relevant to the behavior of amino acids and nucleic acids, two of the fundamental building blocks of life.

The concept of tautomerizations is called tautomerism. The chemical reaction interconverting the two is called tautomerization.

Examples

Aromaticity provides some stability to the triple lactim form of these triple lactam-lactim tautomers.

Tautomerization is pervasive in organic chemistry. It is typically associated with polar molecules and ions containing functional groups that are at least weakly acidic. Most commonly tautomers exist in pairs, which means that the proton is located at one of two positions. Common tautomeric pairs include:^[4]

amino acid-ammonium carboxylate, which applies to the building blocks of the proteins.
ketone - enol, e.g., for acetone (see: keto–enol tautomerism);
enamine - imine;
lactam - lactim, an amide-imidic acid tautomerism in the nucleobases guanine, thymine, and cytosine;
enamine - enamine, e.g., during pyridoxalphosphate-catalyzed enzymatic reactions; and

Keto-enol tautomerization typically strongly favors the keto tautomer, but an important exception is the case of 1,3-diketones such as acetylacetone.

Some examples of tautomers

Some more specialized examples are known including nitroso - oxime, ketene - ynol, amide - imidic acid (e.g., the latter is encountered during nitrile hydrolysis reactions);

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).

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.^[6]^{[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.^[4]

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.^[7] 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 Smith, Michael B.; March, Jerry (2007), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (6th ed.), New York: Wiley-Interscience, ISBN 0-471-72091-7
^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "Tautomerism".
^ 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–) "Valence tautomerization".

English Journal

Synthesis, structural characterization, antimicrobial activities and theoretical investigations of some 4-(4-aminophenylsulfonyl) phenylimino) methyl)-4-(aryldiazenyl) phenol.

Ghasemian M1, Kakanejadifard A2, Karami T3.
Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy.Spectrochim Acta A Mol Biomol Spectrosc.2016 Nov 5;168:190-8. doi: 10.1016/j.saa.2016.06.007. Epub 2016 Jun 6.
The azo-azomethine dyes with a different substitution have been designed from the reaction of 4,4'-diaminodiphenyl sulfone with 2-hydroxy-5-(aryldiazenyl)benzaldehyde. The compounds have been characterized by elemental analysis, Mass, IR, UV-Vis, TGA-DTA and NMR spectroscopy. The solvatochromism beh
PMID 27294547

Encapsulation of AGE-breaker Alagebrium by Cucurbit[7]uril Improved the Stability of both its Carbonyl -Hydrogen and Thiazolium C(2)-Hydrogen.

Li S1, Ding YF2, Yin H3, Wang C4, Bardelang D5, Wang LH6, Wang R7.
Chemistry, an Asian journal.Chem Asian J.2016 Sep 8. doi: 10.1002/asia.201601153. [Epub ahead of print]
As determined by both 1H NMR and UV-Vis spectroscopic titration, electrospray ionization mass spectrometry (ESI-MS), isothermal titration calorimetry (ITC), and DFT molecular modeling, AGE-breaker alagebrium (ALA) was found to form 1:1 guest-host inclusion complexes with cucurbit[7]uril (CB[7]), wit
PMID 27605465

Stepwise Regioselective Hydrogenation of cis-2-C60 (CF2 )2 Homofullerene with [6,6]-Open/Closed Valence Tautomerism.

Bogdanov VP1, Semivrazhskaya OO1, Belov NM1, Troyanov SI1, Markov VY1, Ioffe IN1, Kemnitz E2, Goryunkov AA3.
Chemistry (Weinheim an der Bergstrasse, Germany).Chemistry.2016 Sep 8. doi: 10.1002/chem.201602351. [Epub ahead of print]
The homofullerene compound cis-2-C60 (CF2 )2 , which has an unusual kind of open/closed valence tautomerism undergoes consecutive regioselective hydrogenation at bridgehead carbon atoms upon reduction with Zn/Cu couple in H2 O-toluene mixture. The tautomerism barrier in cis-2-C60 (CF2 )2 is negligib
PMID 27607901

Japanese Journal

クロミックイオン液体

吉田幸大,齋藤軍治
名城大学農学部学術報告 = Scientific reports of the Faculty of Agriculture, Meijo University (51), 43-53, 2015-03
NAID 40020411627

SYNTHESIS, BIOLOGICAL ACTIVITIES, AND TAUTOMERISM OF 4-QUINOLONES AND RELATED COMPOUNDS

Kurasawa Yoshihisa,Sasaki Kenji
Heterocycles : an international journal for reviews and communications in heterocyclic chemistry 91(1), 1-39, 2015-01-01
NAID 40020363620

Unusual Tautomeric Energetics in 4n Azaheterocycles

Hatanaka Masashi
Bulletin of the Chemical Society of Japan advpub(0), 2015
… The same analysis is also applicable for hydroxyl/oxo tautomerism, and results in very stable lactams. …
NAID 130005069523