Tautomerism - Delocalized Chemical Bonding - Introduction - March's Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 7th Edition (2013)

March's Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 7th Edition (2013)

Part I. Introduction

Chapter 2. Delocalized Chemical Bonding

2.N. Tautomerism431

There is another topic that is important for an understanding of chemical bonding in organic compounds. For most compounds, all the molecules are represented by a single structure. But for many compounds, there is a mixture of two or more structurally distinct compounds that are in rapid equilibrium. When this phenomenon, called tautomerism,432 exists, there is a rapid shift back and forth among the molecules. In most cases, it is a proton that shifts from one atom of a molecule to another. Mass spectrometry was used to study tautomerism,433 which takes several forms.

2.N.i Keto–Enol Tautomerism434444

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A very common form of tautomerism is that between a carbonyl compound containing an α hydrogen and its enol form:445 Such equilibria are pH dependent, as in the case of 2-acetylcyclohexanone.446 In simple cases (R2 = H, alkyl, OR, etc.), the equilibrium lies well to the left (Table 2.1). Examining the bond energies in Table 1.7 leads to an explanation for this fact. The keto form differs from the enol form by the presence of a C–H, a C–C, and a C=O bond, whereas the enol has a C=C, a C–O, and an O–H bond. The approximate sum of the first three is 359 kcal mol−1 (1500 kJ mol−1) and of the second three is 347 kcal mol−1 (1452 kJ mol−1). The keto form is thermodynamically more stable by ~12 kcal mol−1 (48 kJ mol−1), and in most cases the enol forms cannot normally be isolated.447 In certain cases, however, a larger amount of the enol form is present, and it can even be the predominant form.448 There are three main types of the more stable enols:449

Table 2.1 The Enol Content of Some Carbonyl Compounds.

Compound

Enol Content (%)

Reference

Acetone

6 × 10−7

435

PhCOCH3

1.1 × 10−6

436

Cyclopentanone

1 × 10−6

437

CH3CHO

6 × 10−5

438

Cyclohexanone

4 × 10−5

437

Butanal

5.5 × 10−4

439

(CH3)2CHCHO

1.4 × 10−2

439,440

Ph2CHCHO

9.1

441

CH3COOEt

No enol founda

437

CH3COCH2COOEt

8.4

442

CH3COCH2COCH3

80

353

PhCOCH2COCH3

89.2

437

EtOOCCH2COOEt

7.7 × 10−3

437

NimgCCH2COOEt

2.5 × 10−1

437

Indane-1-one

3.3 × 10−8

443

Malonamide

No enol found

444

a. Less than 1 part in 10 million.

1. Molecules in which the enolic double bond is in conjugation with another double bond. Some of these are shown in Table 2.1. Carboxylic esters have a much smaller enol content than ketones, for example. In molecules like acetoacetic ester (145), the enol is also stabilized by internal hydrogen bonding, which is unavailable to the keto form:

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Analysis of acetoacetamide by gas electron diffraction shows that it exists as a mixture of 63% enol tautomer and 37% diketo form at 74°C.450 There is a discussion of electron delocalization with respect to amides.451

2. Molecules that contain two or three bulky aryl groups.452 An example is 2,2-dimesitylethenol (146), where the keto content at equilibrium is only 5%.453 In cases such as this, steric hindrance (Sec. 4.Q.iv) destabilizes the keto form. In 146, the two aryl groups are ~ 120° apart, but in 147 they must move closer together (~ 109.5°). Such compounds are often called Fuson-type enols.454 There is one example of an amide with a bulky aryl group [N-methyl bis(2,4,6-triisopropylphenyl)acetamide] that has a measurable enol content, in sharp contrast to most amides.455

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3. Highly fluorinated enols (e.g., 148).456

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In this case, the enol form is not more stable than the keto form (149). The enol form is less stable, and converts to the keto form upon prolonged heating. It can, however, be kept at room temperature for long periods of time because the tautomerization Reaction (12-3) is very slow, owing to the electron-withdrawing power of the fluorines.

Frequently, when the enol content is high, both forms can be isolated. The pure keto form of acetoacetic ester melts at −39 °C, while the enol is a liquid even at −78 °C. Each can be kept at room temperature for days if catalysts, (e.g., acids or bases) are rigorously excluded.457 Even the simplest enol, vinyl alcohol (CH2=CHOH), has been prepared in the gas phase at room temperature, where it has a half-life of ~ 30 min.458 The enol Me2C=CCHOH is indefinitely stable in the solid state at −78 °C and has a half-life of ~ 24 h in the liquid state at 25 °C.459 When both forms cannot be isolated, the extent of enolization is often measured by NMR.460

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The extent of enolization461 is greatly affected by solvent,462 concentration, and temperature. Lactone enols, for example, have been shown to be stable in the gas phase, but unstable in solution.463 Another example is acetoacetic ester, which has an enol content of 0.4% in water and 19.8% in toluene.464 In this case, water reduces the enol concentration by hydrogen bonding with the carbonyl, making this group less available for internal hydrogen bonding. The effect of temperature is clear from the enol content of pentan-2,4-dione (CH3COCH2COCH3), which was found to be 95, 68, and 44%, respectively, at 22, 180, and 275 °C.465 When a strong base is present, both the enol and the keto form can lose a proton. The resulting anion (the enolate ion) is the same in both cases. Since 150 and 151 differ only in placement of electrons, they are not tautomers, but canonical forms. The true structure of the enolate ion is a hybrid of 150 and 151 although 151 contributes more, since in this form the negative charge is on the more electronegative atom.

2.N.ii Other Proton-Shift Tautomerism

The valence tautomerism is discussed of a proton from either tautomer is the same because of resonance. Some examples follows:466

1. Phenol–Keto Tautomerism.467

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For most simple phenols, this equilibrium lies well to the side of the phenol, which is aromatic. For phenol itself, there is no evidence for the existence of the keto form.468 However, the keto form becomes important and may predominate: (1) where certain groups, (e.g., a second OH group or an N=O group), are present;469 (2) in systems of fused aromatic rings470; (3) in heterocyclic systems. In many heterocyclic compounds in the liquid phase or in solution, the keto form is more stable,471 although in the vapor phase the positions of many of these equilibria are reversed.472 For example, 152 is the only form detectable in ethanolic solution in the equilibrium between 4-pyridone (152) and 4-hydroxypyridine (153), while 153 predominates in the vapor phase.472 In other heterocycles, the hydroxy-form predominates. 2-Hydroxypyridone (154) and pyridone-2-thiol (156)473 are in equilibrium with their tautomers, 2-pyridone (155) and pyridine-2-thione (157), respectively. In both cases, the most stable form is the hydroxy or thiol tautomer, 154 and 156.474

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2. Nitroso–Oxime Tautomerism.

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The equilibrium shown for formaldehyde oxime and nitrosomethane illustrates this process.475 In molecules where the products are stable, the equilibrium lies far to the right, and as a rule nitroso compounds are stable only when there is not a hydrogen.

3. Aliphatic Nitro Compounds Are in Equilibrium with Aci Forms.

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The nitro form is much more stable than the aci form, in sharp contrast to the parallel case of nitroso-oxime tautomerism, undoubtedly because the nitro form has resonance not found in the nitroso case. Aci forms of nitro compounds are also called nitronic acids and azinic acids.

4. Imine–Enamine Tautomerism.476

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Enamines are normally stable only when no hydrogen is attached to the nitrogen (R2C=CR–NR2). Otherwise, the imine form predominates.477 The energy of various imine–enamine tautomers has been calculated.478 In the case of 6-aminofulvene-1-aldimines, tautomerism was observed in the solid state, as well as in solution.479 Porphyrins and porphycenes also undergo this type of tautomerism, and the two tautomers may be imaged using single-molecule spectroscopy.480

5. Ring–Chain Tautomerism. Ring–chain tautomerism481 occurs in sugars (aldehyde vs the pyranose or furanose structures), and in γ-oxocarboxylic acids.482 In benzamide carbaldehyde, (159), whose ring-chain tautomer is 158, the equilibrium favors the cyclic form (159).483 Similarly, benzoic acid 2-carbaldehyde (160) exists largely as the cyclic form (161).484 In these latter cases, and in many others, this tautomerism influences chemical reactivity. Conversion of 160 to an ester, for example, is difficult since most standard methods lead to the OR derivative of 161 rather than the ester of 160. Ring–chain tautomerism also occurs in spriooxathianes,485 in decahydroquinazolines (e.g. 162 and 163),486 in other 1,3-heterocycles,487 and in 2-ferrocenyl-2,4-dihydro-1H-3,1-benzoxazine derivatives.488

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There are many other highly specialized cases of proton-shift tautomerism, including an internal Michael reaction (see 15-24) in which 2-(2,2-dicyano-1-methylethenyl)benzoic acid (164) exists largely in the open-chain form rather than its tautomer (162) in the solid state, but in solution there is an increasing amount of 165 as the solvent becomes more polar.489

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Notes

1. See Wheland, G.W. Resonance in Organic Chemistry, Wiley, NY, 1955.

2. There are other methods. See Streitwieser, Jr., A. Molecular Orbital Theory for Organic Chemists, Wiley, NY, 1961, pp. 27–29; Hirst, D.M.; Linnett, J.W. J. Chem. Soc. 1962, 1035; Firestone, R.A. J. Org. Chem. 1969, 34, 2621.

3. Pullman, A. Prog. Org. Chem. 1958, 4, 31, p. 33.

4. See Clarkson, D.; Coulson, C.A.; Goodwin, T.H. Tetrahedron 1963, 19, 2153. See also, Herndon, W.C.; Párkányi, C. J. Chem. Educ. 1976, 53, 689.

5. See Dewar, M.J.S. Mol. Struct. Energ. 1988, 5, 1.

6. Shaik, S.S.; Hiberty, P.C.; Lefour, J.; Ohanessian, G. J. Am. Chem. Soc. 1987, 109, 363; Stanger, A.; Vollhardt, K.P.C. J. Org. Chem. 1988, 53, 4889. See also, Jug, K.; Köster, A.M. J. Am. Chem. Soc. 1990, 112, 6772; Aihara, J. Bull. Chem. Soc. Jpn. 1990, 63, 1956.

7. See Pullman, A. Prog. Org. Chem. 1958, 4, 31, p. 36; Clarkson, D.; Coulson, C.A.; Goodwin, T.H. Tetrahedron 1963, 19, 2153. For a MO picture of aromaticity, see Pierrefixe, S.C.A.H.; Bickelhaupt, F.M. Chem. Eur. J. 2007, 13, 6321.

8. See Yates, K. Hückel Molecular Orbital Theory, Academic Press, NY, 1978; Coulson, C.A.; O'Leary, B.; Mallion, R.B. Hückel Theory for Organic Chemists, Academic Press, NY, 1978; Lowry, T.H.; Richardson, K.S. Mechanism and Theory in Organic Chemistry, 3rd ed., Harper and Row, NY, 1987, pp. 100–121.

9. Pople, J.A. Trans. Faraday Soc. 1953, 49, 1375, J. Phys. Chem. 1975, 61, 6; Dewar, M.J.S. The Molecular Orbital Theory of Organic Chemistry, McGraw-Hill, NY, 1969; Dewar, M.J.S., in Aromaticity, Pub. no. 21, 1967, pp. 177–215. See Merino, G.; Vela, A.; Heine, T. Chem. Rev. 2005, 105, 3812; Poater, J.; Duran, M.; Solà, M.; Silvi, B. Chem. Rev. 2005, 105, 3911.

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11. See Herndon, W.C. Prog. Phys. Org. Chem. 1972, 9, 99.

12. Hoffmann, R. J. Chem. Phys. 1963, 39, 1397. See Yates, K. Hückel Molecular Orbital Theory, Academic Press, NY, 1978, pp. 190–201.

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14. Hehre, W.J.; Radom, L.; Schleyer, P.v.R.; Pople, J.A. Ab Initio Molecular Orbital Theory, Wiley, NY, 1986; Clark, T. A Handbook of Computational Chemistry, Wiley, NY, 1985, pp. 233–317; Richards, W.G.; Cooper, D.L. Ab Initio Molecular Orbital Calculations for Chemists, 2nd ed., Oxford University Press, Oxford, 1983.

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21. Clark, T. A Handbook of Computational Chemistry, Wiley, NY, 1985, p. 141.

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