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 Ti^III 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).