Oxidative decarboxylation of 3,4-dihydroxymandelic acid to 3,4-dihydroxybenzaldehyde: electrochemical and HPLC analysis of the reaction mechanism.
Cyclic voltammetric and chronoamperometric data are consistent with a process in which 3,4-dihydroxymandelic acid (DOMA) is oxidized initially in a two-electron step to its corresponding o-benzoquinone. This species is unstable and undergoes the rate-determining loss of CO2 (k = 1.6 s-1 at pH 6 and 25 degrees C) to give an unobserved p-benzoquinone methide intermediate that rapidly isomerizes to 3,4-dihydroxybenzaldehyde (DOBAL), DOBAL is also electroactive at the applied potential and is oxidized in a two-electron step to 4-formyl-1,2-benzoquinone. Subsequent reactions of 4-formyl-1,2-benzoquinone include the oxidation of unreacted DOMA and the hydration of its aldehyde functional group. Oxidation of DOMA directly to its p-benzoquinone methide apparently does not occur. Derivatives of mandelic acid (e.g., 4-hydroxymandelic acid) that are expected to give only their corresponding p-benzoquinone methides upon oxidation afford redox behavior that differs distinctly from that for DOMA.
Czapla
T H
TH
Department of Entomology, Kansas State University, Manhattan 66506-3701.
Claeys
M R
MR
Morgan
T D
TD
Kramer
K J
KJ
Hopkins
T L
TL
Hawley
M D
MD
D001547:Benzaldehydes / Q000378:metabolism*; D002396:Catechols / Q000378:metabolism*; D002851:Chromatography, High Pressure Liquid; D003653:Decarboxylation; D004563:Electrochemistry; D006863:Hydrogen-Ion Concentration; D008333:Mandelic Acids / Q000378:metabolism*; D010084:Oxidation-Reduction
D001547:Benzaldehydes; D002396:Catechols; D008333:Mandelic Acids; C005581:protocatechualdehyde; C000989:3,4-dihydroxymandelic acid