Co(salophen)-Catalyzed Aerobic Oxidation of p-Hydroquinone: Mechanism and Implications for Aerobic Oxidation Catalysis

Abstract

Macrocyclic metal complexes and p-benzoquinones are commonly used as co-catalytic redox mediators in aerobic oxidation reactions. In an effort to gain insight into the mechanism and energetic efficiency of these reactions, we investigated Co(salophen)-catalyzed aerobic oxidation of p-hydroquinone. Kinetic and spectroscopic data suggest that the catalyst resting-state consists of an equilibrium between a Co(II)(salophen) complex, a Co(III)-superoxide adduct, and a hydrogen-bonded adduct between the hydroquinone and the Co(III)-O2 species. The kinetic data, together with density functional theory computational results, reveal that the turnover-limiting step involves proton-coupled electron transfer from a semi-hydroquinone species and a Co(III)-hydroperoxide intermediate. Additional experimental and computational data suggest that a coordinated H2O2 intermediate oxidizes a second equivalent of hydroquinone. Collectively, the results show how Co(salophen) and p-hydroquinone operate synergistically to mediate O2 reduction and generate the reactive p-benzoquinone co-catalyst.

Selective Aerobic Oxidation of Methylarenes to Benzaldehydes Catalyzed by N‐Hydroxyphthalimide and Cobalt(II) Acetate in Hexafluoropropan‐2‐ol

“…The active radical species are often generated from dioxygen molecules in the presence of redox metal catalysts [4b] such as Co 2+ /Co 3+ , [8] Mn 2+ /Mn 3+ , [7a, 9] Fe 2+ /Fe 3+ , [10] and Cu 1+ / Cu 2+ . [11] One of the most efficient species for abstracting weak hydrogen atoms is the phthalimido-N-oxyl (PINO) intermediate, formed from N-hydroxyphthalimide (NHPI) [2b, 12] under a dioxygen atmosphere in the presence of Co(OAc) 2 or Mn(OAc) 2 redox catalysts (Scheme 1 A).…”

Triazine based Mn (II) and Mn (II)/Ln (III) complexes: Synthesis, characterization and catecholase activities

“…Polymerization of the resulting quinones leads to dark colored polymers as a barrier against infections . Due to the presence of dinuclear Cu (II) in the active site of the natural enzyme, synthetic enzyme studies have focused on Cu (II) and Co (III) complexes . Although several Mn (III) and Mn (IV) complexes were reported as catecholase‐like mimetics, a few number of Mn (II) have been shown to possess catecholase activity …”

Oxidation von organischen Substraten mit einem redoxaktiven Guanidinkatalysator

“…[1,2] Die katalytische Oxidation von Catecholen und p-Hydrochinon mit O 2 wurde mit zweikernigen wie auch einkernigen Übergangsmetallkomplexen (vor allem Kupfer, Cobalt und Eisen) als Katalysatoren detailliert untersucht. [3][4][5][6][7] Stahl, Hammes-Schiffer et al klärten die Mechanismen solcher Reaktionen auf und gruppierten Chinone in zwei Familien:s olche mit hohen Redoxpotentialen wie 2,3-Dichlor-5,6-dicyan-1,4-benzochinon (DDQ) und bioinspirierte o-Chinone,d ie dem o-Chinon-Cofaktor ähneln, in Aminoxidasen sowie verwandten Enzymen vorkommen und niedrigere Redoxpotentiale aufweisen. [7,8] Spezielle o-Chinone ermçgli-chen die aerobe Oxidation von Aminen ohne die Notwendigkeit eines Metallkatalysators.…”

“…[1,2] Die katalytische Oxidation von Catecholen und p-Hydrochinon mit O 2 wurde mit zweikernigen wie auch einkernigen Übergangsmetallkomplexen (vor allem Kupfer, Cobalt und Eisen) als Katalysatoren detailliert untersucht. [7,8] Spezielle o-Chinone ermçgli-chen die aerobe Oxidation von Aminen ohne die Notwendigkeit eines Metallkatalysators. [7,8] Spezielle o-Chinone ermçgli-chen die aerobe Oxidation von Aminen ohne die Notwendigkeit eines Metallkatalysators.…”