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 on 13C chemical shifts
✍ Scribed by Simon Bolvig; Fritz Duus; Poul Erik Hansen
- Publisher
- John Wiley and Sons
- Year
- 1998
- Tongue
- English
- Weight
- 290 KB
- Volume
- 36
- Category
- Article
- ISSN
- 0749-1581
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✦ Synopsis
Deuterium isotope e †ects on 13C nuclear shielding, n*C(OD), were investigated for a series of enolic triacetylmethane, 2-acyl-1,3-cycloalkanediones, 5-acyl MeldrumÏs acids and 5-acyl-1,3-dimethylbarbituric acids at di †erent temperatures. The enolic 2-acyl-1,3-cycloalkanediones, 5-acyl MeldrumÏs acids and 5-acyl-1,3-dimethylbarbituric acids all exhibit intramolecular enolÈenol tautomerism. For the Ðrst two the equilibrium constants were estimated from the deuterium isotope e †ects on the enolic and carbonylic carbons. The equilibrium constants were estimated to be 1.5 for the enolic 2-acyl-1,3-cyclohexanediones and 2-acetyl-1,3-cyclopentanedione, favouring the form having an endocyclic enolic double bond, and 0.8 for 5-acyl-1,3-dimethylbarbituric acids, favouring the form having an exocyclic enolic double bond. Apparently, the equilibrium position is una †ected by increasing the size of the acyl group, and therefore no distinct e †ects caused by steric hindrance were observed. The non-hydrogenbonded a-carbonyl group of enolic triacetylmethane, the 2-acyl-1,3-cycloalkanediones, 5-acyl MeldrumÏs acids and 5-acyl-1,3-dimethylbarbituric acids cause a high frequency shift of the OH 1H chemical shifts. A plot of the latter against the sum of 2*C(OD) ] 4*C(OD) shows a larger sum for the compounds apparently exhibiting intramolecular enolÈenol tautomerism than for compounds apparently not exhibiting such tautomerism.
1998 John Wiley & ( Sons, Ltd.