Substituent effects on keto–enol tautomerization of β-diketones from X-ray structural data and DFT calculations

Single crystal X-ray structure determinations of six crystals 1–6 of β-diketones, the related DFT calculations as well as a systematic investigation, on the CSD (Cambridge Structural Database) files, of all acyclic β-diketones having at least one α-hydrogen, in both β-diketo and β-keto–enol tautomeric forms, are reported. In spite of the stabilization energy gained by the formation of strong intramolecular O–H⋯O resonace assisted hydrogen bonds (RAHB) a certain number of non-enolized structures were retrieved. The structural data show that the steric and electronic properties of the substituents play a definite role in tuning the hydrogen bond strength and determining the enolic site but the driving force able to shift from the more common β-keto–enol tautomer to the β-diketo one can be only the steric hindrance of bulky groups. In this context the substituents in position 2 play a crucial role in establishing the tautomeric form. In fact, while the 2-unsubstituted β-diketones (or 2-substitued by a group linked by a sp2carbon) assume almost exclusively the β-keto–enol form with some exceptions for very bulky substituents, β-diketones carrying 2-alkyl substituents, in general, display the β-diketo tautomeric form. The only exceptions are the 2-alkyl curcumin derivatives where the planar β-keto–enol group is stabilized by extended π-conjugation within the whole molecule and by the absence of short contacts between the alkyl R2groups and R1 or R3 substituents. DFT calculations on the six compounds, 1–6, show that in the four more overcrowded structures, 3–6, the trans-β-diketo tautomer is more stable than the Z-β-keto–enol isomer unlike what happens for 1 and 2 where the Z-β-keto–enol isomer is the most stable by a few kcal mol−1. Thereby, the occurrence of the trans-β-diketo tautomer for all compounds, in the crystal, can be interpreted in terms of the existence of a large activation energy in the mechanism to attain the Z-β-keto–enol isomer containing an intramolecular O–H⋯O hydrogen bond.