Enols, detection

Calcined barium hydroxide is an efficient catalyst for a number of base-mediated reactions. Among these, the Claisen-Schmidt condensation of acetophenones with benzaldehydes occurs in times as short as 10 min at room temperature (Eq. 15). The acetophenone enolate (detected by IR spectroscopy) is formed on the catalyst surface, where the reaction with the aldehyde takes place. The higher activity of the ketone enolate is interpreted by the authors as the result of "an increased vibrational state of the lattice", a formulation close to the mechano-chemical explanation. With the help of selective poisoning experiments, it is concluded that the enolate forms via a SET mechanism. 

The radicals that are formed from the enolate in this process are rapidly destroyed so that only the stable semidione species remains detectable for EPR study. Semidiones can also be generated oxidatively from ketones by reaction with oxygen in the presence of base. The diketone is presumably generated oxidatively and then reduced to the semidione via reduction by the enolate derived from the original ketone. 

The efficiency of reduction of benzophenone derivatives is greatly diminished when an ortho alkyl substituent is present because a new photoreaction, intramolecular hydrogen-atom abstraction, then becomes the dominant process. The abstraction takes place from the benzylic position on the adjacent alkyl chain, giving an unstable enol that can revert to the original benzophenone without photoreduction. This process is known as photoenolization Photoenolization can be detected, even though no net transformation of the reactant occurs, by photolysis in deuterated hydroxylic solvents. The proton of the enolic hydroxyl is rapidly exchanged with solvent, so deuterium is introduced at the benzylic position. Deuterium is also introduced if the enol is protonated at the benzylic carbon by solvent ... 

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 ... 

A solution whieh eontained ca. 95% of the O-deuterated 3-hydroxyfuran 22 was generated from the trimethylsilyl ether in [De]-DMS0/D20 with DCl at 32°C. After several hours it was eonverted into the keto tautomer 23 deulerated at position 2 (Seheme 9). In all solvents used (CCI4, [DgjDMSO, [Dg]aeetone, CD3OD) no detectable amount of the enol was present at equilibrium (89JA5346). 

NMR spectra of thiophene 77 indicate a small amount (8.4% in CS2) of the second keto tautomer 78. The enol tautomer 76 eould not be detected [78JCS(P1)292]. 

Equilibrium constants and activation parameters have been determined [76ACS(B)101] [for a review see 82AHC(30)127]. Ionization potentials for tautomeric 2-hydroxyselenophenes have been analyzed by comparison with IP data for compounds derived from either tautomeric form. The enol form could not be detected [75ACS(B)647]. 

Unlike most /3-diketones, the following /3-diketone has no detectable enol content and is about as acidic as acetone. Explain. 

Ghosh and co-workers have recently used the indanyl-derived auxiliary 69 (Table 1.9) in titanium enolate condensations with a range of aldehydes [34], Of the four possible diastereomers, only the anti 71 and syn TL were produced (the alternative anti and syn diastereomers were not detected by 1H or 13C NMR). The use of monodentate aliphatic aldehydes resulted in the formation of anti diastereomers... 

The chiral cyclic cobalt enolate 2 reacts with acetone to produce the aldol adduct 3 which arises from attack of the electrophile from the least hindered side of the enolate, i.e., away from the phosphane13. None of the other possible diastereomers was detected. 

The tautomeric equilibrium between enols and ketones or aldehydes is not normally a preparative reaction, though for some ketones both forms can be prepared (see p. 75 for a discussion of this and other aspects of tautomerism). For most ketones and aldehydes only the keto form is detectable under ordinary conditions, though the equilibrium must occur, since aldehydes and ketones often react through their enol forms. 

Mono- and polyl dric phenols and enols frequently form characteristically colored complexes with Fe + ions [4, 28, 29]. Here monohydric phenols usually produce reddish-violet colors, while pyrocatechol derivatives yield green chelates [4]. Detection of acetone using Legal s test is based on the formation of an iron complex [4]. The same applies to the thioglycolic acid reaction of the German Pharmacopoeia (DAB 9) [4, 30]. 

Hong, V. and Wrolstad, R.E., Use of HPLC separation/photodiode array detection for characterization of anthocyanins, J. Agric. Food Chem., 38, 708, 1990. Osmianski, J. and Lee, C.Y., Isolation and HPLC determination of phenolic compounds in red grapes. Am. J. Enol. Vitic., 41, 204, 1990. 

The reaction conditions were optimized to afford clean coupling of enol tosylate 32 using only a slight excess of amide 24 (1.05equiv) at 100 °C, 5mol% Pd2(dba)3/ dppb catalyst, and a toluene/tert-amyl alcohol solvent system. Even under the harsh reaction conditions required for complete conversion of the tosylate (100 °C, 20 h) no detectable E/Z isomerization was seen, providing further proof that the hindered nature of the enamide aids stability to isomerization. Treatment of the mixture with activated carbon (Darco KB-B) at the end of the reaction followed by isolation of the product by crystallization, afforded enamide 22 in 92% isolated yield. 

A lower molecular weight methyl ketone and an olefin are isolated as products of this reaction. That the enol is formed as a primary product which rearranges to the ketone follows from its detection in the IR spectrum of gaseous 2-pentanone upon photolysis. 3 In addition to the ketone and olefinic products, one usually obtains varying amounts of cyclobutanols. 

It was unfortunate that we did not detect any product derived from a diketone in the reaction of w-dimethoxybenzene with tetrafluoroben-zyne. We therefore carried out a reaction of tetrafluorobenzyne with 1,3,5-trimethoxybenzene. The di-enol ether (80) could not be isolated, and after the removal of unreacted 1,3,5-trimethoxybenzene we isolated the phenolic acid (81) in good yield. This compound is undoubtedly formed by the hydrolysis of (80) followed by a retro-Claisen condensation, and aromatisation as shown below. 

The measurement of Tg is straightforward when Trp<triplet lifetime can be shortened by addition of triplet quenchers, and the values of Tg for PPVK and CoPT(l) have been obtained using this approach (6). Figure 2 shows a typical trace corresponding to the decay of the biradical from PTVK, as monitored at l 15nm. The triplet state is in this case too short lived to be detectable the residual absorbance observed after decay of the biradical is due to the enol. 

Nanosecond flash kinetic spectroscopy was also carried out on 2-hydroxy benzophenone and the copolymer (11). No transients could be detected in the nanosecond time scale, suggesting that the ground state enol [S (lb) in scheme 1] has a lifetime less than 1 x 10 9 sec. These results strongly imply that processes (3) and (4) are responsible for the deactivation of singlet energy in these systems. A small, non zero triplet yield is postulated in the copolymer both to account for the photodegradation data and the transient spectral data. Triplet... 

The optically active Schiff bases containing intramolecular hydrogen bonds are of major interest because of their use as ligands for complexes employed as catalysts in enantioselective reactions or model compounds in studies of enzymatic reactions. In the studies of intramolecularly hydrogen bonded Schiff bases, the NMR spectroscopy is widely used and allows detection of the presence of proton transfer equilibrium and determination of the mole fraction of tautomers [21]. Literature gives a few names of tautomers in equilibrium. The OH-tautomer has been also known as OH-, enol- or imine-form, while NH tautomer as NH-, keto-, enamine-, or proton-transferred form. More detail information concerning the application of NMR spectroscopy for investigation of proton transfer equilibrium in Schiff bases is presented in reviews.42-44... 

The ionization potentials, using mass spectrometry, for both 2-hydroxy-and 3-hydroxythiophenes have been compared with data for compounds derived from either tautomeric form in order to analyze the tautomeric composition.124 125 In the 2-hydroxy-substituted system the enol isomer could not be detected. Of the two possible unsaturated lactones the oc,/l-unsaturated form was the major isomer. In the 3-hydroxy-substituted case both the oxo form and the enol form are important. The position of the equilibrium was compared with those of the corresponding furan and sele-nophene systems for both isomers. 

If one compares the solvolyses of 2-bromo-l,l-diphenyl-4-(p-methoxyphenyl)-but-l-en-3-yne (57) and 4.4-diphenyl-1 -bromo-1 -(/ -mcthoxyphcny l)-buta-1,2,3-tricncs (58, X = Br) in aqueous ethanol (equation 21), the destabilization of the intermediate cation 59 by the large inductive effect of the triple bond as compared to its conjugative effect is evident42. Only in the case of 58 could the substitution product butatrienyl enol ether 60 be isolated in 40% yield, while it was only detected by UV and IR spectroscopy in the solvolysis product of 57. The faster observed reaction rate of 58 as compared to 57 was ascribed to a difference in their ground-state energies42. 

Although the tautomeric ratios of the 4 species have not been measured directly, it is known that in aqueous solution the keto-N2H form dominates, while the keto-NlH form is only detectable in non-polar solvents. An analysis of experimental data concluded that in aqueous solution the stability (lowest free energy) is in the order keto-N2H > imino-N2H > enol-NlH > keto-NIH. In the gas phase, calculations predict that the keto-N2H form is the least stable. While solvation is found to favour this species, which is the most polar, this stabilisation is not enough to reverse the order of stability. It is thus clearly predicted that the keto-NIH tautomer is the most stable in... 

The enol form 10 of pyrrolam A 10 could not be detected despite its aromatic character <2004JOC6105>. Also, its pyrrolinone analogues 34 exist mainly as /3-enaminones <1985TL833> the tautomeric hydroxypyrrole 34 was only observed when R = C02Et (Scheme 1) <1988JOC5680>. 

---