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Suppression Mechanism for Enol–Enol Isomerization of 2‐Substituted Dimedones
Author(s) -
China Hideyasu,
Okada Yutaka,
Dohi Toshifumi
Publication year - 2015
Publication title -
asian journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.846
H-Index - 44
eISSN - 2193-5815
pISSN - 2193-5807
DOI - 10.1002/ajoc.201500069
Subject(s) - chemistry , isomerization , enol , intramolecular force , photochemistry , intermolecular force , nuclear magnetic resonance spectroscopy , solvent , proton nmr , keto–enol tautomerism , computational chemistry , molecule , tautomer , stereochemistry , organic chemistry , catalysis
2‐Substituted dimedones, described as having a simple chemical structure, are a type of cyclic β‐diketone with enol–enol interconversion. The influence of the enol‐enol isomerism interferes with structure analysis of many dimedones in solution by NMR spectroscopy. It was demonstrated that the isomerism of the 2‐substituted dimedones depends on solvent, time, solute concentration, and temperature. Negative activation energy of the isomerization was determined by the NMR line shape and spin‐spin relaxation analyses, which is an extraordinary phenomenon found for the first time in the molecular dynamics of NMR. Suppression of the isomerization by heating, dilution of the solute, and time control in several low‐polarity solvents enabled analysis of the enol form in detail. It was clarified that deceleration of the intramolecular chemical exchange rate in the isomerization was responsible for the decrease in the intermolecular hydrogen‐bonding effect in the suppression mechanism.