Enol Forms of 1,3‐Indanedione, Their Stabilization by Strong Hydrogen Bonding, and Zwitterion‐Assisted Interconversion

Abstract

By analyzing NMR spectroscopic data, and supported by IR, UV/Vis, Raman, dielectrometry, and DFT techniques, a comprehensive study of the 1:2 adducts of picolinaldehyde and 1,3‐indanediones is presented. The parent indanedione derivative 5 exists in an equilibrium between all‐keto and enol forms, the latter being stabilized by an intramolecularO–H···N hydrogen bond. Only the all‐keto form was observed in the 5,6‐dimethoxy compound 6, whereas solely the enol tautomer was observed with its 5,6‐dichloro analogue 7. Polar solvents and low temperatures shift the equilibrium towards the enol tautomer in 5. The structure of adduct 8, formed with isonicotinaldehyde, prevents the formation of intramolecular O–H···N hydrogen bonds and thus it exists in the all‐keto form in low polar solvents. However, in DMSO solutions it adopts a zwitterionic form with a strong anionic O–···H···O hydrogen bond. Thus, the enol form in indanedione adducts was unequivocally characterized in solution and the factors that determine the keto–enol tautomerism, namely electronic effects, solvent, temperature, and intramolecular hydrogen bonds, have been methodically studied by spectroscopic and quantum mechanical methods.

Femtosecond Transient Absorption Spectroscopic Study of a Carbonyl-Containing Carotenoid Analogue, 2-(all-trans-Retinylidene)-indan-1,3-dione

“…shows resonance Raman spectra of C20Ind in the same set of solvents. Based on the assignment of the Raman bands in the previous papers, the Raman lines observed for C20Ind can be empirically assigned as below 44–49. The Raman lines in the 959 – 967 spectral region are assigned to the C-C-H out-of-plane bending vibration.…”

Syntheses of (−)-gabosine A, (+)-4-epi-gabosine A, (−)-gabosine E, and (+)-4-epi-gabosine E

“…15 It has been reported in the literature that intramolecular hydrogen bonding favors ketoeenol tautomeric equilibrium. 16 In our case, initially formed MBH adduct 20 may undergo Michael type addition with undefined nucleophile (may be solvent molecule or base) to give species 20a. Here in this species, oxygen of the enol can form a six membered hydrogen bond with proton of the hydroxymethyl group and at the same time the carbonyl oxygen of the acetoxy group can also form seven membered hydrogen bond with enolic hydrogen of the same enol to give species 20a 0 and thereby favoring the enol form over its keto form.…”

Conjugation of Different Aflatoxin Derivatives to Proteins and Their Use as Heterologous Antigens in Immunoassay Development

“…In organic solvents, the diketone structures like the one observed in AFB1 is stabilized at enol form with an H‐bond. However, in aqueous solutions, the availability of the solvent for H‐bonding destabilizes the enol form and the predominant form becomes the ketone form . This results in the availability of a small portion of AFB1 for reaction, and hence a low reaction yield.…”