Resonance protonated ketone

The hydroxymethyl cation forms of protonated ketones (264) and aldehydes (265) contribute to the resonance hybrid. Based on 13C NMR studies,94 548 551 the degree of contribution of the hydroxymethyl cation forms can be quite accurately estimated. Similar studies have been carried out using 170 NMR spectroscopy.552 Recent theoretical studies (MP2/6-31G level)553 for protonated acetone have supported the... 

Oxygen-17 NMR provides a convenient means for monitoring perturbations of carbonyl groups which reduce carbon-oxygen TT bond order. Protonated ketones R2C0lT show the most dramatic effect. As is to be expected from the resonance forms and 5,... 

Here, the excited triplet ketone abstracts a hydrogen atom from the phenol to give a phenoxy cation-radical. This latter species is further oxidized by the ketyl radical to the resonance-stabilized ylide. The protonated ketone then collapses to regenerate ketone and give the acid, HX,. It is noteworthy that in contrast to the photosensitization of dialkylphenacylsulfonium salts which involves reduction of the onium salts, the above process results in the oxidation of the sulfonium salt. Although formally Scheme 12 is an oxidation, photosensitization closely resembles the non-photosensitized process with respect to the products which are formed, i.e., an ylide and a Bronsted acid. Hence, in this case, photosensitized photolysis is a reversible process and addition of monomers to pre-irradiated solutions does not result in polymerization. 

An a hydrogen of an aide hyde or a ketone is more acidic than most other protons bound to carbon Aldehydes and ketones are weak acids with pK s in the 16 to 20 range Their enhanced acidity IS due to the electron withdrawing effect of the carbon yl group and the resonance stabi lization of the enolate anion... 

Inductive and resonance stabilization of carbanions derived by proton abstraction from alkyl substituents a to the ring nitrogen in pyrazines and quinoxalines confers a degree of stability on these species comparable with that observed with enolate anions. The resultant carbanions undergo typical condensation reactions with a variety of electrophilic reagents such as aldehydes, ketones, nitriles, diazonium salts, etc., which makes them of considerable preparative importance. 

The NMR study of steroidal epoxides (discussed in section II-F) parallels that of the analogous thiiranes. It is possible to relate the location and configuration of the thiirane group with the angular methyl and thiirane proton resonances. The proton NMR relationships for the intermediate thiocyanatohydrins have been included inageneral NMR study of steroids. Electronic spectra may be used in the analysis of steroidal thiiranes. Spectroscopic measurements have shown the existence of a low intensity absorption in the 240-260 m region. The regular patterns of rotatory contributions of thiiranes which are comparable with those of ketones prompted an accumulation of ORD and CD data for steroidal thiiranes. 

Enamino ketones can protonate not only on nitrogen or carbon but also on oxygen to give 12,13, and 14, respectively. Enamino ketones form stable perchlorates, chlorides, bromides, and iodides, and examination of their infrared (21,22), ultraviolet (23), and nuclear magnetic resonance (24,25) spectra show these salts to be O protonated. The salts of 4-dialkylamino-... 

The salts of some enamines crystallize as hydrates. In such cases it is possible that they are derived from either the tautomeric carbinolamine or the amino ketone forms. Amino ketone salts (93) ( = 5, 11) can serve as examples. The proton resonance spectra of 93 show that these salts exist in the open-chain forms in trifluoroacetic acid solution, rather than in the ring-closed forms (94, n = 5, 11). The spectrum of the 6-methylamino-l-phenylhexanone cation shows a multiplet at about 2.15 ppm for phenyl, a triplet for the N-methyl centered at 7.0 ppm and overlapped by signals for the methylene protons at about 8.2 ppm. The spectrum of 93 ( = 11) was similar. These assignments were confirmed by determination of the spectrum in deuterium oxide. Here the N-methyl group of 93 showed a sharp singlet at about 7.4 ppm since the splitting in —NDjMe was much reduced from that of the undeuterated compound. 

The proton magnetic resonance spectrum of the product in trifluoroacetic acid shows that the isomeric purity is greater than 90%. The proton magnetic resonance spectral properties for the isomeric amino ketones in both trifluoroacetic acid and chloroform-d are as follows (solvent) 8 (multiplicity, number of protons, assignment, coupling constant J in Hz.) 1-... 

The ratio of the isomeric amino ketones in the crude product can be determined from the relative intensities of the signals for the (CH3)2C grouping in a proton magnetic resonance spectrum taken in trifluoroacetic acid (see Note 10). In chloroform-d these absorptions overlap. 

That the [19]annulenone 136 is diatropic is evident from the high field resonance of H13 compared to the adjacent external protons Ht2 and H14, and to all the olefinic protons of the homoannulene 137. Further evidence is the significantly lower field resonance of H3, H4, H17, and Hlg when compared with the similar protons H3 and H4 of the atropic ketones 143,144, and 145 (see below). 

Still another possibility in the base-catalyzed reactions of carbonyl compounds is alkylation or similar reaction at the oxygen atom. This is the predominant reaction of phenoxide ion, of course, but for enolates with less resonance stabilization it is exceptional and requires special conditions. Even phenolates react at carbon when the reagent is carbon dioxide, but this may be due merely to the instability of the alternative carbonic half ester. The association of enolate ions with a proton is evidently not very different from the association with metallic cations. Although the equilibrium mixture is about 92 % ketone, the sodium derivative of acetoacetic ester reacts with acetic acid in cold petroleum ether to give the enol. The Perkin ring closure reaction, which depends on C-alkylation, gives the alternative O-alkylation only when it is applied to the synthesis of a four membered ring ... 

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