Intramolecular hydrogen bonding of the enol forms of β-ketoamides and β-ketothioamides. Deuterium isotope effects on 13C chemical shifts

Isotope effect on chemical shifts in hydrogen‐bonded systems

“…For thioamindes 14 involved in hydrogen bonding much the same pattern is seen ( Figure 12) with respect to distances around the hydrogen bond ring.…”

“…4 DC ¼ OðODÞ isotope effects can be predicted using the following equation: For o-hydroxythioketones, 4 DC ¼ SðODÞ is found to be rather large and negative 13 and also in enolic forms of b-ketothioamides. 14 The latter may in special cases show tautomerism 15 (see section on Equilibrium).…”

“…Electron-withdrawing groups will lead to more positive 4 DC ¼ OðODÞ. Very few compounds have been investigated 16,17 so far but the examples given in The values can again be explained by the distances around the six-membered hydrogen bond ring. An early negative example was that of 1,4-diaminophenyl-9,10-anthraquinone 16 (see Figure 9).…”

“…In that case one does not have to worry about the resonance form involving charges and we can therefore predict a positive 4 DC ¼ OðNDÞ. A few examples are known 14 as seen in Figure 11.…”

Tautomerism of enolic triacetylmethane, 2-acyl-1,3-cycloalkanediones, 5-acyl Meldrum’s acids and 5-acyl-1,3-dimethylbarbituric acids studied by means of deuterium isotope effects on13C chemical shifts

“…* sum For tautomeric enolic b-ketonamides the are * sum between 1.11 and 1.48 ppm. 16 The large observed * sum for 14È16 indicate that they are tautomeric. We have reported earlier13 the isotope e †ects on the enolic and carbonylic carbons for enolic b-diketones as a function of the molar fraction, x.…”