Recent trends in phenol synthesis by photocatalytic oxidation of benzene

Abstract

Phenol is an important intermediate for manufacturing chemical products in industrial. In recent decades, phenol synthesis by one-pot oxidation of benzene has aroused tremendous interest in phenol synthesis due to...

Direct C–H Hydroxylation of N-Heteroarenes and Benzenes via Base-Catalyzed Halogen Transfer

“…These concepts are illustrated in Figure b for access to biaryl phenols ( 68 – 71 ) built from the inexpensive 3-(trifluoromethyl)­benzonitrile scaffold. We note that hydroxylation occurs on the more electron-deficient aryl ring in a complementary fashion to other benzene C–H hydroxylation and halogenation protocols. − Here, our site-selectivity studies explain the regioisomeric outcomes, as substitution is selective for the most sterically accessible position adjacent to the nitrile.…”

“…Benzenes with other acidifying substituents, such as amides ( 42 , 44 ), sulfonamides ( 43 ), and trifluoromethyl groups ( 47 , 48 ) also undergo hydroxylation. Hydroxylation of compounds with multiple aryl rings occurs selectively on the more electron-deficient arene, a trend that contrasts electrophilic aromatic substitution and hydroxylation methods. − High selectivity is also observed for substrates with multiple acidic C–H sites, including biaryl systems ( 46 – 48 ) and those with potentially coordinating pyridyl substituents ( 48 ) . For substrates in Table , the remaining mass balance is primarily unfunctionalized benzene substrate with the moderate yields likely caused by a lower acidity or a relatively challenging S N Ar step…”

“…For example, several well-established approaches for benzene hydroxylation rely on electrophilic oxygen intermediates or single electron arene oxidation pathways that are best suited for electron-neutral to -rich arenes. This includes a variety of impressive protocols, such as enzymatic, Fe-catalyzed, electro­(photo)­catalytic, and phthaloyl peroxide-promoted benzene C–H hydroxylation, among others. − Metal-catalyzed C–H activation methods are not as sensitive to arene electronic properties and provide phenols and O -acylated derivatives in the presence of O -oxidants. , However, these methods typically require directing groups for high selectivity and are often inhibited on N -heteroarenes due to catalyst coordination, although Yu recently addressed these issues in the context of a Pd-catalyzed method for ortho -hydroxylation of carboxylic groups on pyridines and benzenes . Several alternative oxidation approaches such as vicarious S N Ar, peracid-promoted, and microbial metabolic methods can overcome these reactivity challenges but are generally limited to specific substrates or by synthetic practicality. − Therefore, despite much effort dedicated to advancing aromatic C–H hydroxylation, there remains a lack of mechanistic strategies suited for N -heteroarenes and electron-deficient benzenes, especially those devoid of directing groups or that provide alternative selectivity to existing (multistep) protocols.…”

Boosting the Efficiency of Photoactive Rod-Shaped Nanomotors via Magnetic Field-Induced Charge Separation

“…Then, the photogenerated charge pairs react with H 2 O 2 or hydroxyl groups adsorbed on the surface of the nanomotors to produce hydroxyl radicals (step 2). These photogenerated hydroxyl radicals directly attack the benzene compound, leading to the formation of a hydroxyl cyclohexadienyl radical, which generates phenol after losing a proton (step 3). − …”