An ESR spectroscopic investigation of pH-dependent rearrangements of the vinyl radical formed from the reaction between the hydroxyl radical and butynedioic acid. Routes to ˙CH2CO2H and ˙CH(CO2–)C(O)CO2–, and CIDEP effects

Abstract

ESR experiments reveal the occurrence of two unusual rearrangements of the vinyl radical HO2CC(OH)C(CO2H)˙(1) in aqueous solution. The formation of ˙CH2CO2H from (1) at low pH is believed to result from a rapid 1,4-hydrogen shift (k ca. 104 s–1) followed by ketonization, decarboxylation, and decarbonylation. Reaction to produce ˙CH(CO2–)C(O)CO2–(2) at high pH is believed to occur via intramolecular deprotonation of the hydroxy group by a carboxylate function, followed by reprotonation (at carbon) of the intermediate anion radical. Rapid reduction of (2) and its protonated form by TiIII leads to the detection of hydroxyl adducts of the corresponding enol and, at high pH, to the observation of novel CIDEP effects in the ESR spectrum of (2).