Single crystal X-ray structure determinations of six crystals 1-6 of beta-diketones, the related DFT calculations as well as a systematic investigation, on the CSD (Cambridge Structural Database) files, of all acyclic beta-diketones having at least one alpha-hydrogen, in both beta-diketo and beta-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 beta-keto-enol tautomer to the beta-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 beta-diketones (or 2-substitued by a group linked by a sp2 carbon) assume almost exclusively the beta-keto-enol form with some exceptions for very bulky substituents, beta-diketones carrying 2-alkyl substituents, in general, display the beta-diketo tautomeric form. The only exceptions are the 2-alkyl curcumin derivatives where the planar beta-keto-enol group is stabilized by extended pi-conjugation within the whole molecule and by the absence of short contacts between the alkyl R2 groups 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-beta-diketo tautomer is more stable than the Z-beta-keto-enol isomer unlike what happens for 1 and 2 where the Z-beta-keto-enol isomer is the most stable by a few kcal mol_1. Thereby, the occurrence of the trans-beta-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-beta-keto-enol isomer containing an intramolecular O-H_ _ _O hydrogen bond.
Substituent effects on keto-enol tautomerization of beta-diketones from X-ray structural data and DFT calculations
BERTOLASI, Valerio;FERRETTI, Valeria;GILLI, Paola;
2008
Abstract
Single crystal X-ray structure determinations of six crystals 1-6 of beta-diketones, the related DFT calculations as well as a systematic investigation, on the CSD (Cambridge Structural Database) files, of all acyclic beta-diketones having at least one alpha-hydrogen, in both beta-diketo and beta-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 beta-keto-enol tautomer to the beta-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 beta-diketones (or 2-substitued by a group linked by a sp2 carbon) assume almost exclusively the beta-keto-enol form with some exceptions for very bulky substituents, beta-diketones carrying 2-alkyl substituents, in general, display the beta-diketo tautomeric form. The only exceptions are the 2-alkyl curcumin derivatives where the planar beta-keto-enol group is stabilized by extended pi-conjugation within the whole molecule and by the absence of short contacts between the alkyl R2 groups 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-beta-diketo tautomer is more stable than the Z-beta-keto-enol isomer unlike what happens for 1 and 2 where the Z-beta-keto-enol isomer is the most stable by a few kcal mol_1. Thereby, the occurrence of the trans-beta-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-beta-keto-enol isomer containing an intramolecular O-H_ _ _O hydrogen bond.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
