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

Abstract

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.

Aerobic oxidation of 2,4,6-tri-tert-butylphenol to quinones catalyzed by copper(II) complexes of an N-octylated bis-benzimidazolyl ligand

“…species. Such a reactive species has been reported earlier [11]. The existence of a copper(II)-hydroperoxo species in the present case is strengthened by the isolation of a peroxydimer in the present study.…”

“…A mechanism involving tert-butyl migration and a 6-hydroperoxy-2,4-cyclohexadienone intermediate is also postulated, involving a copper(I) mediated oxidation [10]. A recent study at low temperature employs a cupric-superoxo complex for the study of oxidation of substituted di-tertbutylphenols and proposes a cupric superoxo-phenoxyl radical as intermediate forming 2,6-di-tert-butyl-1,4-benzoquinone [11]. Another study proposed a saturation kinetics in phenolic O-H oxidation using mononuclear Mn(III)-OH complex [12].…”

Cooperative Effects in Copper Polyamido­amine Dendrimer Complexes Catalyzing the Reduction of Molecular Oxygen

“…For similar copper-oxygen species with bipyridyl ligands re-ported by Karlin, [34] a possible mechanism was proposed ( Figure 7) to describe the catalytic behavior reported. The Cu 2+ bipyridyl complexes are expected to be reduced to Cu + species in the presence of DDT.…”

Switchover of the Mechanism between Electron Transfer and Hydrogen‐Atom Transfer for a Protonated Manganese(IV)–Oxo Complex by Changing Only the Reaction Temperature

“…[1][2][3][4][5][6][7] There are three reaction mechanisms proposed in hydrogen transfer,such as electron transfer (ET) followed by proton transfer (PT), one-step hydrogen-atom transfer (HAT), and PT followed by ET (Scheme 1). [1a, 8-12] Theo nestep HATm echanism implies that the hydrogen-atom transfer reaction occurs without atransient intermediate when an electron and ap roton are transferred at the same time.I n contrast, the sequential pathway involves mechanistically distinct ET and PT steps.Inparticular,there has been alongstanding ambiguity as to the mechanistic borderline where aHAT pathway is changed to arate-determining ET pathway followed by aPTpathway or vice versa.…”