Acetone tautomers

The mean chemical shifts of A- unsubstituted pyrazoles have been used to determine the tautomeric equilibrium constant, but the method often leads to erroneous conclusions (76AHC(Sl)l) unless the equilibrium has been slowed down sufficiently to observe the signals of individual tautomers (Section 4.04.1.5.1). When acetone is used as solvent it is necessary to bear in mind the possibility (depending on the acidity of the pyrazole and the temperature) of observing the signals of the 1 1 adduct (55) whose formation is thermodynamically favoured by lowering the solution temperature (79MI40407). A similar phenomenon is observed when SO2 is used as solvent. 

Analogously, pyrazolyl-aluminate and -indate ligands have been prepared <75JCS(D)749) and their chelating properties evaluated with cobalt, nickel, copper and zinc. Gallyl derivatives of pyrazoles and indazoles have been extensively studied by Storr and Trotter e.g. 75CJC2944) who determined several X-ray structures of these compounds. These derivatives exist in the solid state as dimers, such as (212) and (288). A NMR study in acetone solution showed the existence of a slow equilibrium between the dimer (212) and two identical tautomers (289) and (290) (Section 4.04.1.5.1) (81JOM(215)157). 

The coupling of enamines with aromatic diazonium salts has been used for the syntheses of monoarylhydrazones of a-diketones (370,488-492) and a-ketoaldehydes (488,493). Cleavage of the initial enamine double bond and formation of the phenylhydrazone of acetone and acetophenone has been reported with the enamines of isobutyraldehyde and 2-phenylpropionalde-hyde. Rearrangement of the initial coupling product to the hydrazone tautomer is not possible in these examples. 

Which tautomer is lower in energy, acetone or propen-2-oll Use equation (1) to calculate the equilibrium distribution of the two at room temperature. If an experiment is capable of detecting concentrations as low as 1 % of the total, would you expect to observe both keto and enol forms of acetone at room temperature ... 

Which tautomer is more stable Would you expect to be able to observe both tautomers at room temperature Rationalize any differences between this keto-enol equihbrium and that above involving acetone and propen-2-ol. 

Fig. 2. The proton magnetic resonance spectrum of 5-nitrobenzofuroxan, in acetone at — Sl C. The bands marked by arrows arise from the 5-nitro tautomer.

Most carbonyl compounds exist almost exclusively in the keto form at equilibrium, and it s usually difficult to isolate the pure enol. For example, cyclohexanone contains only about 0.0001% of its enol tautomer at room temperature, and acetone contains only about 0.000 000 1% enol. The percentage of enol tautomer is even less for carboxylic acids, esters, and amides. Even though enols are difficult to isolate and are present only to a small extent at equilibrium., they are nevertheless responsible for much of the chemistry of carbonyl compounds because they are so reactive. 

Another transformation observed with a naturally occurring bacteriochlorin is the tautomer-ization of the exocyclic C — C double bond in bacleriochlorophyll b (3) in acetone which gives 7,8-dehydrobacteriochlorophyll a (4), a chlorin-type product (configurations at C7 and of the 8-ethylidene group have not been established).112,12... 

In the interest of clarity only one tautomer is shown for acetone and for 12.78-12.80. 

The high selectivity of the catalyst in forming ( )-alkenes can be used in interesting ways (eq. 1). For example, in acetone-iie solution, within 15 min at room temperature allyl alcohol is converted to nearly pure enol (E)-26. Under these mild conditions, the product slowly isomerizes to the more stable aldehyde tautomer. We know of one other report of rapid enol formation from allyl alcohol, using a Rh... 

Kostic el al. discovered that Pd11 complexes, when attached to tryptophan residues, can rapidly cleave peptides in acetone solutions to which a stoichiometric amount of water is added, for hydrolysis.436 The indole tautomer in which a hydrogen has moved from the nitrogen to C(3) is named indolenine. Its palladium(II) complexes that are coordinated via the nitrogen atom have been characterized by X-ray crystallography and spectroscopic methods.451 Binuclear dimeric complexes between palladium(II) and indole-3-acetate involve cyclopalladation.452 Bidentate coordination to palladium(II) through the N(l) and the C(2) atoms occurs in binuclear complexes.453 Reactions of palladium(II) complexes with indole-3-acetamide and its derivatives produced new complexes of unusual structure. Various NMR, UV, IR, and mass spectral analyses have revealed bidentate coordination via the indole carbon C(3) and the amide oxygen.437... 

The Mannich reaction is best discussed via an example. A mixture of dimethylamine, formaldehyde and acetone under mild acidic conditions gives N,N-dimethyl-4-aminobutan-2-one. This is a two-stage process, beginning with the formation of an iminium cation from the amine and the more reactive of the two carbonyl compounds, in this case the aldehyde. This iminium cation then acts as the electrophile for addition of the nucleophile acetone. Now it would be nice if we could use the enolate anion as the nucleophile, as in the other reactions we have looked at, but under the mild acidic conditions we cannot have an anion, and the nucleophile must be portrayed as the enol tautomer of acetone. The addition is then unspectacular, and, after loss of a proton from the carbonyl, we are left with the product. 

The reaction of D-lyxose (7) with 2-methoxypropene gave17 a high yield of 2,3-O-isopropylidene-D-lyxofuranose (9), previously obtained18 in low yield by the conventional procedure [with acetone in the presence of copper(II) sulfate and sulfuric acid]. This reaction probably proceeds through the pyranoid tautomer 8, a kinetic product of the reaction that tautomerizes rapidly to the thermodynamic prod-... 

Nitropropane exists in equilibrium with its tautomer propane-2-nitronic acid, present in physiological media as the anion propane-2-nitronate. Dayal et al. (1991) have investigated the metabolism of both 2-nitropropane and propane-2-nitronate in mouse microsomes and found that the oxidative denitrification of propane-2-nitronate was 5-10 times more extensive than that of 2-nitropropane. Acetone formed by this reaction affords the source of 002 detected when 2-nitro -C prop iiic was administered in vivo. However, neither 2-nitropropane nor propane-2-nitronate was cytotoxic to mouse hepatocytes. 

Yamamoto has recently described a novel catalytic, asymmetric aldol addition reaction of enol stannanes 19 and 21 with aldehydes (Eqs. 8B2.6 and 8B2.7) [14]. The stannyl ketones are prepared solvent-free by treatment of the corresponding enol acetates with tributyltin methoxide. Although, in general, these enolates are known to exist as mixtures of C- and 0-bound tautomers, it is reported that the mixture may be utilized in the catalytic process. The complexes Yamamoto utilized in this unprecedented process are noteworthy in their novelty as catalysts for catalytic C-C bond-forming reactions. The active complex is generated upon treatment of Ag(OTf) with (R)-BINAP in THF. Under optimal conditions, 10 mol % catalyst 20 effects the addition of enol stannanes with benzaldehyde, hydrocinnamaldehyde, or cinnamaldehyde to give the adducts of acetone, rerf-butyl methyl ketone (pinacolone), and acetophenone in good yields and 41-95% ee (Table 8B2.3). 

Erlenmeyer was first to consider ends as hypothetical primary intermediates in a paper published in 1880 on the dehydration of glycols.1 Ketones are inert towards electrophilic reagents, in contrast to their highly reactive end tautomers. However, the equilibrium concentrations of simple ends are generally quite low. That of 2-propenol, for example, amounts to only a few parts per billion in aqueous solutions of acetone. Nevertheless, many important reactions of ketones proceed via the more reactive ends, and enolization is then generally rate-determining. Such a mechanism was put forth in 1905 by Lapworth,2 who showed that the bromination rate of acetone in aqueous acid was independent of bromine concentration and concluded that the reaction is initiated by acid-catalyzed enolization, followed by fast trapping of the end by bromine (Scheme 1). This was the first time that a mechanistic hypothesis was put forth on the basis of an observed rate law. More recent work... 

Compound 156 adopted the 3H tautomer exclusively, even in hydrogen bond acceptor solvents, such as acetone-46. It was proposed by analogy with the 1,5-benzodiazepine system which adopts an analogous tautomer, the seven-membered ring of 156 is likely to be nonplanar and, regarding NMR spectra, interconversion between two equivalent forms may be present. 

Pochan, Baldwin and Flygare have analyzed the microwave spectra of cyclopropanone and the isotopic isomers 13Ci, 13C2, and 2,2-dideutero-cyclopropanone.63) The rotational transitions were determined by studying the Stark effect (the shifts and splittings of lines produced by an electric field). The type of transition observed for cyclopropanone was consistent with C v symmetry and the sum of the moments of inertia (/a + /b — Ic) suggested that all four protons are out-of-plane. These data eliminate such structural alternatives as the dipolar oxyallyl tautomer 82 and allene oxide 83. The electric dipole moment (fi ) was calculated to be 2.67 0.10 D, which corresponds to an average of those of acetone (2.93 D) 65> and formaldehyde (2.34D).6 )... 

Claramunt et al. [22] used a H and 13C NMR to study the tautomer-isim of omeprazole in solution. The tautomeric equilibrium constant, KT = 0.59 in tetrahydrofuran at 195 K, is in favor of the 6-methoxy tautomer. The assignment of the signals was made by comparison with its two N-methyl derivatives in acetone-d6 and through theoretical calculations of the absolute shieldings (GIAO/DFT/6-3111++G ). 

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