Ketones, protonation

Protonation of this anion can occur either at the a carbon or at oxygen Protonation of the a carbon simply returns the anion to the starting aldehyde or ketone Protonation of oxygen as shown m step 2 of Figure 18 3 produces the enol... 

In order to predict the structure of the product, you must identify the factors that will tend to favor selective ketal formation. Consider selective carbonyl protonation first. Obtain energies and atomic charges, and display electrostatic potential maps of the alternative protonated ketones (protonated ketone A, protonated ketone B). Identify the more stable isomer. Compare geometries and draw whatever Lewis structures are needed to account for your data. Why is one isomer more stable than the other Is the more stable isomer also that in which the positive charge is better delocalized Will the more stable isomer undergo nucleophilic attack more or less easily than the other Explain. 

Condensation of an aniline with a dione with loss of water provides enamine 16. Ketone protonation and cyclization forms 18 followed by loss of water provides quinoline 4. Some have suggested the formation of dication 19 as a requirement to cyclization. Cyclization of 19 to 20 and subsequent conversion to quinoline 4 requires loss of water and acid. Another rendering of the mechanism takes into account participation of an electron-donating group (EDG), which stabilizes intermediate 21. 

Acidic Carboxonium Ions (Hydroxylated Cations). Acidic carboxonium ions are generally obtained by protonation of carbonyl compounds.541,542 Aldehydes and ketones protonate on the carbonyl oxygen atom in superacid media at low temperatures, and the corresponding carboxonium ions can be directly observed543-551 [Eqs. (3.61) and (3.62)]. 

The concentration of the ketone enolate is higher than that of the aldehyde enolate. This is true under thermodynamic control as the stability of an enolate increases with its degree of substitution. It is also true under kinetic control since enolization is an acid-base equilibrium, the increased enolate concentration reflects the higher acidity of the ketone protons. 

The polarographic behavior of aromatic carbonyl compounds at the dropping mercury electrode in aqueous SSE s is rather complex 130 In acid medium two one-electron waves are observed, corresponding to reduction of the protonated ketone (protonation makes the reduction easier, shifting the first wave towards less cathodic potentials with decreasing pH) and the protonated anion radical, respectively ... 

In these last steps, two nucleophilic addition reactions take place to yield imine products. The mechanism has been worked out in greater detail in Section 19.8, but the essential steps are additions of phenylhydrazine, first to the imine, then to the ketone. Proton transfers are followed by eliminations, first of ammonia, then of H2O. 

Gil, M., Ji, X., Li, X., Na, H., Hampsay, J., Lu, Y. (2004). Direct synthesis of sulfonated aromatic poly(ether ether ketone) proton exchange membranes for fuel cell applications. /. Membrane Sci. 234, 75-81. 

Replacement of oxygen by nitrogen makes a molecule much more basic. For example the p.K j, of acetone is ca. 21 compared with a value of ca. 7 for the corresponding imine. At pH 7 the fraction of the ketone protonated is ca. 10 (Eqn. 15) whereas half of the imine exists in the protonated form (Eqn. 16). 

In 1997, Hashmi et al. observed the Pd-catalyzed homodimerization of 1,2-allenyl ketones affording2-substituted4-(4 -oxo-2 -alken-2 -yl)furans (Scheme 31) [23]. The reaction may proceed via intramolecular oxypalladation involving the carbonyl oxygen, leading to the formation of furanylpalladium intermediate 69, followed by intermolecular carbopalladation with a second molecule of 1,2-allenyl ketone. Protonation of the C-Pd bond in 70 afforded the product 66 and regenerated Pd(II) (Scheme 32). 

Electrocyclic reactions are not limited to neutral polyenes. The cyclization of a pentadienyl cation to a cyclopentenyl cation offers a useful entry to five-membered carbocycUc compounds. One such reaction is the Nazarov cyclization of divinyl ketones. Protonation or Lewis acid complexation of the oxygen atom of the carbonyl group of a divinyl ketone generates a pentadienyl cation. This cation undergoes electrocyclization to give an allyl cation within a cyclopentane ring. The allyl cation can lose a proton or be trapped, for example by a nucleophile. Proton loss occurs to give the thermodynamically more stable alkene and subsequent keto-enol tautomerism leads to the typical Nazarov product, a cyclopentenone (3.220). 

It was pointed out that with unsymmetrical ketones, proton-abstraction reactions normally lead to mixtures of the various possible enolates. The composition of the mixture is dependent on the extent of kinetic or thermodynamic control. Perhaps the simplest cases to consider are enolates generated by the methods discussed in Section 1.3. Here a single enolate is the dominant species, and subsequent alkylation leads mainly to its alkylation product. Scheme 1.6 shows some examples of alkylation of specific enolates. 

Zhong S, Cui X, Fu T, Na H (2008) Modification of sulfonated poly(ether ether ketone) proton exchange membrane for reducing methanol crossover. J Power Sources 180 23-28... 

Zhang Y, Fei X, Zhang G, Li H, Shao K, Zhu J, Zhao C, Liu Z, Han M, Na H (2010) Preparation and properties of epoxy-based cross-linked sulfonated poly(arylene ether ketone) proton exchange membrane for direct methanol fuel cell applications, bit J Hydrogen Energ 35 6409-6417... 

Zhong, S. Cui, X. Cai, H. Fu, T. Zhao, C. and Na, H. 2007. Crosshnked sulfonated polyfether ether ketone) proton exchange membranes for direct methanol fuel cell applications. 

Loss of either the a hydrogen from acetone or the hydroxyl hydrogen of the enol form of acetone leads to the same enolate. Protonation at carbon gives the ketone protonation at oxygen gives the enol. It is formation of the enolate that allows the equilibration to take place. 

The mechanism will have the same order of steps as any other mechanism involving a primary amine attacking a ketone (protonate, attack, deprotonate, protonate, lose water, and then deprotonate) ... 

X. Li, C. Liu, H. Lu, C. Zhao, Z. Wang, W. Xing, H. Na, Preparation and characterization of sulfonated poly(ether ether ketone ketone) proton exchange membranes for fuel cell application. /. Membr. Sci. 255,149-155 (2005). 

Cyclic Ketones.—Protonation of cyclohept-2-enone or cyclo-oct-2-enone at low temperature produces two stereoisomers of the form (322) and (323), where n = 1 or 2, respectively. ... 

The hydrate of the ester loses water in acid-catalyzed steps similar to those for loss of water from the hydrate of an aldehyde or ketone. Protonation of one of the two hydroxyl groups prepares it to leave as water. Loss of a proton from the remaining hydroxyl group in a solvent-mediated reaction gives the ester. 

Genies, C., Mercier, R., SiUion, B., Comet, N., Gebel, G., and Pineri, M. (2001b). Stability stody of sulfonated phthalic and naphthalenic polyimide stmctores in aqueous medium. Polymer 42, 5097. Gil, M., Ji, X. L., Li, X. F., Na, H. Hampsey, J. E., and Lu, Y. F. (2004). Direct synthesis of sulfonated aromatic poly (ether ether ketone) proton exchange membranes for fuel cell applications. J. Membr. Sci. 234, 75. 

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