Hydrogen bonding in enols

FIGURE 3. Hydrogen bonds in enol forms of a-(triphenylphosphonio)-substituted acetoacetic ester... 

Among numerous examples of the role of the chemical structure in tunneling rotation we select just one, connected with the effect of intramolecular hydrogen bond. In acetyl acetone in stable enol form... 

Any doubt about the existence of individual tautomers is now long past some tautomers can be crystallized separately (desmotropy), and others can be observed simultaneously in the same crystal (Section V,D,2) in summary, tautomers are not intrinsically different from isomers. Maybe it is worth mentioning that even two identical tautomers can differ. This is the case for the two intramolecular hydrogen-bonded (IMHB) enol tautomers of acetylacetone and for many NH-azoles they correspond to a doublewell profile for the proton transfer with both wells having the same energy (autotrope). 

The AS value for pentane-2,4-dione in Table 3 is compatible with a double-minimum potential for the intramolecular hydrogen bond in the enol. This was also the conclusion reached from the microwave and photoelectron spectra. For unsymmetrically substituted diketones, for example 1,1,1-trifluoropentane-2,4-dione, measurement of 13C chemical shifts has been used to estimate values for the equilibrium constant K = [R]/[L] in (38)... 

Dibenzoylmethane (8b) has been the subject of much interest as regards the possibility that its polymorphism is associated with keto-enol tautomerism. Chemical and spectroscopic studies showed that this is not so (33a). This compound had previously been reported to be trimorphic (33b), but one form appears, in fact, to be a eutectic mixture of the other two. The molecules in these two polymorphs are both in the same state of tautomerism they differ in the torsional angle about the (CH)-(CO) bond and in the type of hydrogen bonding in which they participate. It is noteworthy that solutions prepared from these forms at low temperature have differences in chemical and spectroscopic properties that are maintained for some time. For example, such solutions prepared and held at —35° react at different rates with FeCl3. 

The bifluoride ion, HFj 297 Strong hydrogen bonding in p-diketones 309 Keto/enol equilibrium 310 Structures 312 Hydrogen-bond energies 314 Vibrational modes 315 Nmr spectroscopy 317... 

Acetyl-2-hydroxy-2-methyl-2,3-dihydrobenzo-l,4-dioxan (42, R = H R = MeCO R = Me) exists as a mixture of two cyclic diastereomers 42B and B, both stabilized by an intramolecular hydrogen bond (42B-Z and 48- ). The proportion of the open-chain tautomer 42A, also containing an intramolecular hydrogen bond in the enolized pentane-l,3-dione moiety, is very small ( 1%) at equilibrium (89ZOR1273). 

In this report the authors describe a surprising solvent effect on enantioselectivi-ties. Alcoholic solvents afford the opposite enantiomer using the same enantiomeric series of catalyst Eq. 9. This profound effect is presumably due to hydrogen bonding in the transition state on the nucleophilic enol and/or the carbonyl acceptor Eq. 10. These electrostatic interactions can be visualized with Models E and F. Although the enantioselectivity is reversed the values remain lower than when toluene is used. 

Free ligands have been studied in order to obtain an insight into their structure, both in solution and in the solid state, and for comparison with their metal complexes. H NMR spectroscopy has been used to investigate the keto-enol equilibrium and the nature of the hydrogen bonds. In the case of optically active Schiff bases UV and CD spectra provided information about structure in solution. The Schiff bases that have been most widely examined are derivatives of acetylacetone, salicyl-aldehyde and hydroxymethylenecamphor, whose prototypes with en are shown in Figure 13. 

Using a transition state model for enolate formation and a database search, a thiourea with a pendant amine has been designed as a catalyst, and its ability to hydrogen bond the enolate of acetone explored.287 Both in-plane and out-of-plane hydrogen bonds, to a lone a pair and the carbonyl 7r-bond, respectively, were considered. 

Ordinarily we do not write the enol form of acetone or the keto form of phenol, although minuscule amounts do exist at equilibrium. But both forms of acetylacetone are seen in the NMR spectrum because equilibration is slow enough on the NMR scale and the enol form is stabilized by intramolecular hydrogen bonding. The enol form of acetone and the keto form of phenol are not thus stabilized furthermore, the aro-... 

Other 1,3-dicarbonyl compounds also exist largely in the enol form. In some examples there is an additional stabilizing factor, intramolecular hydrogen bonding. Diethyl malonate (diethyl propane-dioate) has a symmetrical enol stabilized by conjugation. The enol form is also stabilized by a very diethyl maionate favourable intramolecular hydrogen bond in a six-membered ring. 

However there are aspects of Leipert s work that can be taken more seriously especially his observation of the effect of deuteriation on the chemical shift Ad(1H, 2H). This isotopic shift has been instrumental in solving the problem of the hydrogen bond in AA cis enol tautomer - see Sect. III. 

Later, more sophisticated X-ray analysis gave a better idea of the position of the proton, and several structural determinations carried out using this diffraction method have been able to describe the ds enol hydrogen bond in terms not only of R(O--O) but... 

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