Keto-enol exchange

These retro-Aldol and -Michael reactions can, obviously, follow an isomerization of the aldose to the corresponding ketose, leading thereby to different Aldol fragments or retro-Michael products. Keto-enol exchange as well as the retro-... 

Remember that any proton which is acidic enough is prone to undergo deuterium exchange. Methylene protons alpha to a carbonyl for example, may exchange if left standing with D2O for any length of time, as they can exchange via the keto-enol route (i.e., Structure 7.1). 

B. Keto-Enol Isomerization on Acidic Zeolite HZSM-5 Evidenced by H/D Exchange... 

X,Y=0,S,Se,Te], has been undertaken.628 The stabilities of different tautomeric forms of 4-hydroxycoumarins have been evaluated629 by MNDO calculations, and the four lowest-energy oxo-hydroxy tautomers of 5-fluorouracil have been studied630 using density functional methods. Semiempirical calculations have been carried out on the keto-enol tautomerism of triazolopyrimidines.631 A base-catalysed keto-enol tautomer-ism has been proposed632 to be responsible for the observed deuterium exchange of the hydrogens at the 3-position of diazepam when the molecule is treated with alkaline deuteriated methanol. 

Acid-catalysed hydrogen-deuterium exchange in norcamphor has also been investigated by Werstiuk and Banerjee (1977) (DOAc—D20—DC1 medium). It was observed that exo-deuteron addition to the enol is also preferred, but with a slightly smaller selectivity (x 190). This would mean that, if torsional factors cause preferential base-catalysed exo-exchange, they also occur for acid-catalysed keto-enol tautomerism. However, the absence of important torsional strain effects on the rate constants of acid-catalysed enolisation of cyclic and bicyclic ketones contradicts this assumption. 

The slow keto enol proton transfer means separate signals in the NMR spectrum for the tautomers. The second exchange (ii) is responsible for the line broadening and loss of multiplet structure of the NMR signal of the enol proton. The third type of proton motion, (iii), is not resolvable by NMR so that ways around this have been sought in order to obtain a time-averaged analysis of the proton s location in the cis enol. 

Enzymes thus far found to catalyze isomerization or epimerization by oxidation and reduction require the cofactor NAD, which is often very tightly bound, not being removed by dialysis, but only by treatment with charcoal. Enzymes for which a keto-enol mechanism has been suggested do not usually involve NAD. The two mechanisms should also be distinguishable by the nonoccurrence or occurrence, respectively, of hydrogen exchange with the solvent. 

Treating (l,2-dimethoxyethane-0,0 )lithiuni bis(trimethylsilyliminobenzoyl)phosphanide with tri-ftuoroacetic acid in 1,2-dimethoxyethane solution the cation can be exchanged for a proton to give bis(trimethylsilyliminobenzoyl)phosphane (Eq. 21). In contrast to diacylphosphanes which show a keto-enol equilibrium in solution [45], this compound has been found to exist only as an imino-enamine... 

Bidentate phosphate compounds. The organophosphorus bidentate extractants are neutral species extractants that undergo no keto-enolization and have no exchangeable hydrogens (as is the case in extraction by the bidentate diketones). They contain either two P=0 groups or one P=0 and one C=0 group. The carbamoylmethyl-phosphonates (CMPs) and carbamoylphosphonates (CPs) are examples of the latter, while the tetraalkyldiphosphonates and tetraalkyl-diphosphinedioxides [or bis-(disubstituted phosphinyl)-alkanes] are examples of the former. 

Also, all the -situated hydrogen atoms of ketones are readily replaceable by deuterium in an aqueous alkaline medium, since these atoms take part one after the other in the reversible keto-enol tautomerism. This applies, for instance, to the four -positions of cyclohexanone and to the one of 2,2,6-tri-methylcyclohexanone, whereas no exchange occurs in camphor or camphor-quinone which cannot enolize owing to the particular stereochemistry of the molecules.90 Open-chain ketones and also steroidal ketones91 that are not subject to this limitation are often used for exchange reactions. The following description of the preparation of [D6]acetone illustrates the point 29... 

In solution, isotopic incorporation of deuterium from deuterated solvents into metal-bound hydrogen is common, e.g., reaction of acetone-reaction occurs between the Os complex and acetone even at reflux temperature thus the isotopic exchange with acetone-isotopic exchange may therefore occur via a deprotonation/reprotonation pathway coupled with a keto-enol tautomerization. 

Neutral diketone complexes are not generally available and so it is of interest to find Mg(C104)2 2acacH 2H20. The keto form of the Ugand has been shown to predominate in the complex in solution, by use of H NMR, and althou exchange between free and complexed ligand is fast, the keto-enol tautomerization is slow. ... 

Structural studies on several hydroxycoumarins have ascertained that the significance of keto-enol tautomerism, H-bonding and H-D exchange is dependent on the position of the hydroxy group <97CJC365, 377>. 

Lastly, the assignment of the spectrum should indicate some protons which should not be used for quantitation - those labile NH and OH protons which are exchange broadened and therefore more difficult to integrate accurately, but which may also partially deuterate in solvents such as D2O or CD3OD. Keto-enol tautomerism may also cause exchange for aliphatic protons beta to a carbonyl group, for example, and some aromatic protons may also be surprisingly labile, as shown in Fig. 4.10. 

Treatment of mercuric acetate with l,l,l,2,2,3,3-heptafluoro-7,7-dimethyloctane-4,6-dione in 95% ethanol provides Hg(fod)2 quantitatively the mercury atom in this solid is bonded to only carbon, but in [ H ]acetone solution the equilibrium shown in Scheme 9 is set up, a 1 1 mixture of tautomers being obtained at — 60 °C. A dynamic n.m.r. study of this equilibrium, the first substantiated mercury keto-enol tautomerization, has revealed that the process is totally intramolecular it was not possible to decide whether or not intermolecular ligand exchange occurs this was attempted by mass spectrometric examination of an approximately equimolar mix-... 

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