One-step selective hydroxylation of benzene to phenol with hydrogen peroxide catalysed by copper complexes incorporated into mesoporous silica–alumina

Abstract

Benzene was hydroxylated with hydrogen peroxide (H2O2) in the presence of catalytic amounts of copper complexes in acetone to yield phenol at 298 K. At higher temperature, phenol was further hydroxylated with H2O2 by catalysis of copper complexes to yield p-benzoquinone. The kinetic study revealed that the rate was proportional to concentrations of benzene and H2O2, but to the square root of concentration of a copper(II) complex ([Cu(tmpa)]2+: tmpa = tris(2-pyridylmethyl)amine). The addition of a spin trapping reagent resulted in formation of a spin adduct of hydroperoxyl radical (HO2•), as observed by EPR spectroscopy, inhibiting phenol formation. HO2• produced by the reaction of [Cu(tmpa)]2+ with H2O2 acts as a chain carrier for the radical chain reactions for formation of phenol. When [Cu(tmpa)]2+ was incorporated into mesoporous silica-alumina (Al-MCM-41) by a cation exchange reaction, the selectivity to production of phenol was much enhanced by prevention of hydroxylation of phenol, which was not adsorbed to Al-MCM-41. The high durability with turnover number of 4320 for the hydroxylation of benzene to phenol with H2O2 was achieved using [Cu(tmpa)]2+ incorporated into Al-MCM-41 as an efficient and selective catalyst.

Decoding the Mechanism of Intramolecular Cu-Directed Hydroxylation of sp3 C–H Bonds

“…This supports our mechanistic proposal, as O-centered radicals (i.e., superoxide) could be quenched by aromatic solvents to form oxidation products and thus would not give the active oxidant H 2 O 2 (see the SI for further details). 15,47,48 …”

Immobilization of Molecular Catalysts for Enhanced Redox Catalysis

“…Immobilization of [Cu II (tmpa)] 2+ on Al‐MCM‐41 resulted in significant improvement of the selectivity to produce only phenol without further oxidation to p ‐benzoquinone (Figure b), because of the strong acid sites of Al‐MCM‐41, which can capture phenol to prevent further oxidation (vide supra) . The TON reached 4320 after 118 h, demonstrating a high durability of [Cu II (tmpa)] 2+ @Al‐MCM‐41 …”

“… (a) Time profiles of production of phenol (•) and p ‐benzoquinone (▴) in the catalytic benzene hydroxylation (2.1 m ), and generation of p ‐benzoquinone (×) in the catalytic oxidation of phenol (2.1 m ) with H 2 O 2 (2.1 m ) by use of [Cu II (tmpa)] 2+ (67 μ m ) in acetone (4.75 mL) at 298 K. (b) Time profiles of production of phenol (•) and p ‐benzoquinone (▴) in the catalytic benzene hydroxylation (2.1 m ) with H 2 O 2 (2.1 m ) by use of [Cu II (tmpa)] 2+ @Al‐MCM‐41 (1.0 μ m ) in acetone (4.75 mL) at 298 K. Reprinted with permission from Ref. . Copyright 2016, Royal Society of Chemistry.…”

Catalytic Hydroxylation of Benzene to Phenol by Dioxygen with an NADH Analogue

“…The hyperfine splitting pattern of the observed radical clearly indicates that HO 2 . is trapped by DMPO to produce DMPO‐OOH, which has a g value of 2.0061 and a (H)=10.7 G and a (N)=13.0 G. The observed spectrum (part a) agrees with the computer simulation spectrum (part b) in Figure b . The EPR signal due to the HO .…”

“…reacts with benzene to produce the radical adduct, which abstracts hydrogen from AcrH 2 to give the corresponding hydroperoxide, accompanied by generation of acridinyl radical (AcrH . ) to constitute the radical chain reaction (Scheme ) …”