Chemo‐ and Regioselective Direct Hydroxylation of Arenes with Hydrogen Peroxide Catalyzed by a Divanadium‐Substituted Phosphotungstate

Abstract

Peroxid hinein, Phenol heraus: [γ‐PW10O38V2(μ‐OH)2]3− katalysiert die Hydroxylierung einer Vielzahl aromatischer Verbindungen mit wässrigem H2O2 mit hoher Aktivität. Das System ist regioselektiv: Monosubstituierte Benzolderivate ergeben para‐Phenole, während Alkylarene mit reaktiven C sp 3‐H‐Bindungen in der Seitenkette chemoselektiv, ohne nennenswerte Bildung von seitenkettenoxygenierten Produkten hydroxyliert werden.

Immediate hydroxylation of arenes to phenols via V-containing all-silica ZSM-22 zeolite triggered non-radical mechanism

“…Anisole with strong electron-donating side-chain and bulky naphthalene were also chemo- and region-selectively oxidized into o -hydroxyanisole and 2-hydroxynaphthalene, respectively. All the reactions were fulfilled within 30 s, affording much higher TOF values (>1000 h −1 ) than previous systems (Supplementary Tables 6 , 7 ) 20 , 23 , 24 , 28 , 29 . These results suggest wide substrate compatibility for the selective activation of aromatic C sp2 –H bond.…”

“…Direct one-step hydroxylation of the benzene ring with economically and environmentally benign oxidants such as hydrogen peroxide (H 2 O 2 ) is one of the most promising alternatives for the production of phenol and its derivatives 8 , 10 , 13 – 20 . However, this process faces two major challenges: (1) low catalysis efficiency due to the inherent inertness of the aromatic ring C sp2 –H bonds and the undesirable but inevitable over-oxidation of the product phenols; 20 – 22 (2) preferential side-chain oxidation of the C sp3 –H bonds in substituted arenes like toluene rather than aromatic ring oxidation of the C sp2 –H bonds with higher bond dissociation energies 23 , 24 .…”

“…For decades, direct hydroxylation of arenes with H 2 O 2 has been broadly explored in both homogeneous and heterogeneous catalysis over numerous catalysts such as zeolites 17 , 25 , polyoxometallates 23 , 26 , carbon 19 , and metal complexes 27 , 28 . Recently, very high turnover numbers (TON) and turnover frequencies (TOF), as well as high selectivity for aromatic ring oxidation of substituted arenes, were achieved by homogeneous catalysts 20 , 23 , 24 , 29 . Heterogeneous catalysis is preferred due to the facile catalyst separation and reuse 8 , 10 , 13 – 17 , 25 , 26 , but the aforementioned challenges are more difficult to overcome.…”

“…Vanadium (V) is among the most widespread transition metals to catalyze the hydroxylation of arenes with H 2 O 2 8 , 16 , 23 , 26 , 30 . Its abundant coordination mode enables versatile activities but significantly complicates the catalytic active species, especially when employed as a heterogeneous catalyst 31 – 34 .…”

Base Catalysis by Mono- and Polyoxometalates

“…These protonated sites play an important role in the activation of oxidants and the catalytic oxidation abilities are strongly dependent on the protonation states [95][96][97][98][99][100]102,103].…”