Oxidation of 3-hydroxythiophenes [thiophen-3(2H)-ones]: An EPR study of monomeric and dimeric intermediates

Hydrogen abstraction from thiophen-3(2H)-ones gives 3-oxothiophen-2-yl radicals which have been observed by EPR spectroscopy at temperatures below ambient. The structures, enthalpies of formation, and spin densities of these species have been investigated by semiempirical SCF MO methods. The radicals readily couple to give dimers which (except for 2-substituted 3-hydroxythiophenes) are susceptible to a second hydrogen abstraction step to give extensively delocalized dimeric radicals; EPR spectroscopy showed the 5,5'-disubstituted dimeric radicals to be significantly persistent even at higher temperatures. A mechanism for the oxidation of thiophen-3(2H)-ones by air and one-electron oxidants is proposed and justified.