STRUCTURAL FEATURES CONTROLLING CATION-RADICAL LINE-BROADENING IN THE 1H-NMR SPECTRA OF PHENOL DERIVATIVES IN ACID MEDIA

Article abstract of DOI:10.1016/S0040-4020(01)86907-6

The hexachloroantimonates of cation radicals from 2,3,4,7,8,9-hexahydro-2,2,5,7,7,10-hexamethylbenzo<1,2-b;4,5-b'>dipyran (1) and the isomeric benzo<1,2-b;4,3-b'>dipyran (3) have been characterised.The production of cation radical from (1) in acid media is limited by acid concentration for weak acids and also by the presence of oxygen for strong acids.The cation radical species (Ar⁺.) is considered to originate from substrate (Ar) via protonation then electron-transfer from neutral substrate (Ar); only when this transfer is structurally favourable do phenol derivatives exhibit line broadening in their 1H-nmr spectra.Stereochemical effects greatly reduce sensitivity to acid, and in derivatives of benzene-1,2,4-triol the presence of two oxygen atoms in one ring is extremely disadvantageous.Since phenolic cation radicals are rather strong acids, their behaviour in weakly acid media is best studied (by oxidising agents or by cyclic voltammetry) through the ethers.