Premium
Hydrogen‐Atom Transfer Reactions from ortho ‐Alkoxy‐Substituted Phenols: An Experimental Approach
Author(s) -
Amorati Riccardo,
Menichetti Stefano,
Mileo Elisabetta,
Pedulli Gian Franco,
Viglianisi Caterina
Publication year - 2009
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200802454
Subject(s) - chemistry , intramolecular force , substituent , alkoxy group , hammett equation , phenols , hydrogen atom , medicinal chemistry , radical , hydrogen atom abstraction , bond dissociation energy , hydrogen bond , photochemistry , stereochemistry , reaction rate constant , molecule , dissociation (chemistry) , organic chemistry , kinetics , alkyl , physics , quantum mechanics
Deceptively simple : Joint results obtained by EPR and IR spectroscopies and inhibited autoxidation studies allow an explanation, on quantitative grounds, of the factors determining the H‐atom donating abilities of 2‐alkoxyphenols (see graphic; blue: 2‐methoxyphenol, red: cyclic phenol).The role of intramolecular hydrogen bonding (HB) on the bond‐dissociation enthalpy (BDE) of the phenolic OH and on the kinetics of H‐atom transfer to peroxyl radicals ( k inh ) of several 2‐alkoxyphenols was experimentally quantified by the EPR equilibration technique and by inhibited autoxidation studies. These compounds can be regarded as useful models for studying the H‐atom abstraction from 2‐OR phenols, such as many lignans, reduced coenzyme Q and curcumin. The effects of the various substituents on the BDE(OH) of 2‐methoxy, 2‐methoxy‐4‐methyl, 2,4‐dimethoxyphenols versus phenol were measured in benzene solution as −1.8; −3.7; −5.4 kcal mol −1 , respectively. In the case of polymethoxyphenols, significant deviations from the BDE(OH) values predicted by the additive effects of the substituents were found. The logarithms of the k inh constants in cumene were inversely related to the BDE(OH) values, obeying a linear Evans–Polanyi plot with the same slope of other substituted phenols and a y ‐axis intercept slightly smaller than that of 2,6‐dimethyl phenols. In the cases of phenols having the 2‐OR substituent included in a five‐membered condensed ring (i.e, compounds 9 – 11 ), both conformational isomers in which the OH group points toward or away from the oxygen in position 2 were detected by FTIR spectroscopy and the intramolecular HB strength was thus estimated. The contribution to the BDE(OH) of the ortho‐ OR substituent in 9 , corrected for intramolecular HB formation, was calculated as −5.6 kcal mol −1 . The similar behaviour of cyclic and non‐cyclic ortho‐ alkoxy derivatives clearly showed that the preferred conformation of the OMe group in ortho‐ methoxyphenoxyl radicals is that in which the methyl group points away from the phenoxyl oxygen, in contrast to the geometries predicted by DFT calculations.