Mechanistic Insights into the Oxidation of Substituted Phenols via Hydrogen Atom Abstraction by a Cupric–Superoxo Complex

To
obtain mechanistic insights into the inherent reactivity patterns
for copper­(I)–O2 adducts, a new cupric–superoxo
complex [(DMM-tmpa)­CuII(O2•–)]+ (2) [DMM-tmpa = tris­((4-methoxy-3,5-di­methyl­pyridin-2-yl)­methyl)­amine]
has been synthesized and studied in phenol oxidation–oxygenation
reactions. Compound 2 is characterized by UV–vis,
resonance Raman, and EPR spectroscopies. Its reactions with a series
of para-substituted 2,6-di-tert-butyl­phenols
(p-X-DTBPs) afford 2,6-di-tert-butyl-1,4-benzo­quinone
(DTBQ) in up to 50% yields. Significant deuterium kinetic isotope
effects and a positive correlation of second-order rate constants
(k2) compared to rate constants for p-X-DTBPs plus cumylperoxyl radical reactions indicate a
mechanism that involves rate-limiting hydrogen atom transfer (HAT).
A weak correlation of (kBT/e) ln k2 versus Eox of p-X-DTBP indicates that
the HAT reactions proceed via a partial transfer of charge rather
than a complete transfer of charge in the electron transfer/proton
transfer pathway. Product analyses, 18O-labeling experiments,
and separate reactivity employing the 2,4,6-tri-tert-butyl­phenoxyl radical provide further mechanistic insights.
After initial HAT, a second molar equiv of 2 couples
to the phenoxyl radical initially formed, giving a CuII–OO–(ArO′) intermediate, which proceeds in the
case of p-OR-DTBP substrates via a two-electron oxidation
reaction involving hydrolysis steps which liberate H2O2 and the corresponding alcohol. By contrast, four-electron
oxygenation (O–O cleavage) mainly occurs for p-R-DTBP which gives 18O-labeled DTBQ and elimination of
the R group.