Reaction of singlet-excited 2,3-diazabicyclo[2.2.2]oct-2-ene and tert-butoxyl radicals with aryl-substituted benzofuranones.
5,7-Di-tert-butyl-3-aryl-3H-benzofuran-2-ones are lactones with potential antioxidant activity, owing to their abstractable benzylic C-H hydrogens. The fluorescence quenching of the azoalkane 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO), an established probe for the hydrogen-donor propensity of chain-breaking antioxidants, was investigated for 16 aryl-substituted benzofuranone derivatives [m,m-(CF3)2, p-CN, m-CN, p-CF3, p-COOCH3, m-CF3, p-Cl, p-F, H, m-CH3, p-CH3, m,p-(CH3)2, p-OCH3, o-CH3, o-CF3, o,m-(CH3)2]. Analysis of the rate data in terms of a linear free energy relationship yielded a reaction constant of rho = +0.35. This implies that n,pi*-excited DBO acts as nucleophilic species. In contrast, hydrogen abstraction of tert-butoxyl radicals from the benzofuranones was accelerated by electron-donating substituents (rho = -0.23), in conformity with the electrophilic character of oxygen-centered alkoxyl radicals. Possible implications for the optimization of the hydrogen-donor propensity of antioxidants through structural variation are discussed.
School of Engineering and Science, International University Bremen, Campus Ring 1, D-28759 Bremen, Germany.
Cecilia Vannesjö
CV
Lundgren
Apurba L
AL
Koner
Michael
M
Tinkl
Uwe
U
Pischel
Werner M
WM
Nau
D000975:Antioxidants / Q000138:chemical synthesis* / Q000737:chemistry; D001572:Benzofurans / Q000737:chemistry*; D019086:Bridged Bicyclo Compounds, Heterocyclic / Q000737:chemistry*; D005453:Fluorescence; D010087:Oxides / Q000737:chemistry*; D026082:Singlet Oxygen / Q000737:chemistry
C484077:2,3-diazabicyclo(2.2.2)oct-2-ene; D000975:Antioxidants; D001572:Benzofurans; D019086:Bridged Bicyclo Compounds, Heterocyclic; D010087:Oxides; D026082:Singlet Oxygen; C052893:tert-butoxy