It is straightforward to show that a β-dicarbonyl, with at least one α-hydrogen, can transform bidirectionally into two enol forms by keto-enol tautomerism as illustrated in Fig. 1. Involved hydroxyl, carbonyl, and alkene functional groups of both enol forms contain six atoms that can contribute in a π-electronic resonance conjugation to induce a six-membered ring like structure referred to as a chelated ring. The noteworthy aim in ongoing research is to crosscheck the effects of different β-substitutions on the stabilities and IHB strengths of the two mentioned enol forms. Successive substitution of the CH3 groups of acetylacetone (AA, R1=R3=CH3, see Fig. 1) by −C(CH3)3 groups increase the enol content of β-dicarbonyl, that confirmed by 1H-NMR spectroscopy studies [1]. These studies indicate that 2,2,6,6- tetramethyl-3,5-heptanedione (TMHD, R1=R3=C(CH3)3) in the liquid phase is completely in the enol form, whereas the enol contents for 5,5-dimethyl-2,4-hexanedione (DMHD, R1=CH3, R3=C(CH3)3) and AA are 94% and 79% [2], respectively. The NMR chemical shifts of the enolated protons in these compounds have indicated that substitution of the CH3 groups of AA by t-But groups increase the IHB strength.

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