Predominance of the triketo tautomer in acyldipivaloylmethanes in solution and the solid state

• Five 1,3,3′-triketones derived from dipivaloylmethane have been prepared.
• Both in the solid and in solution no enol tautomers were detected for any of the compounds.
• Slow isotopic exchange in CD3OD solution indicates presence of a minute amount of the enol.
• The predominance of the triketo tautomer is attributed to steric hindrance in enol molecules.

A series of five acyldipivaloylmethanes was prepared and studied with respect to keto–enol tautomerism. In the solid state all the compounds exist as triketo tautomers with the triketo group of approximate C3 symmetry. MNR and IR spectroscopy were employed to study the compounds in a variety of solvents. No diketoenol tautomers were detected in any of the solutions. However, a slow deuteration was noticed in the CD3OD solution of acetyldipivaloylmethane which indicates presence of a minute amount of the enol form of this compound. The predominance of the triketo tautomer in all the compounds was explained by the destabilisation of the enol due to steric repulsions of the bulky tert-butyl substituents.

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