Visible-light photoredox-catalyzed C–O bond cleavage of diaryl ethers by acridinium photocatalysts at room temperature

Abstract

Cleavage of C–O bonds in lignin can afford the renewable aryl sources for fine chemicals. However, the high bond energies of these C–O bonds, especially the 4-O-5-type diaryl ether C–O bonds (~314 kJ/mol) make the cleavage very challenging. Here, we report visible-light photoredox-catalyzed C–O bond cleavage of diaryl ethers by an acidolysis with an aryl carboxylic acid and a following one-pot hydrolysis. Two molecules of phenols are obtained from one molecule of diaryl ether at room temperature. The aryl carboxylic acid used for the acidolysis can be recovered. The key to success of the acidolysis is merging visible-light photoredox catalysis using an acridinium photocatalyst and Lewis acid catalysis using Cu(TMHD)2. Preliminary mechanistic studies indicate that the catalytic cycle occurs via a rare selective electrophilic attack of the generated aryl carboxylic radical on the electron-rich aryl ring of the diphenyl ether. This transformation is applied to a gram-scale reaction and the model of 4-O-5 lignin linkages.

Photoinduced Arylation of Acridinium Salts: Tunable Photoredox Catalysts for C–O Bond Cleavage

“…Inspired by the pioneering work of Fukuzumi and his co-workers on the oxidation of benzyl alcohol using 9-phenyl-10-methylacridium as the OPC, and by the recent C–O bond cleavage of diaryl ethers by acridinium photocatalysts, the fragmentation of C β O–Ar bond induced by acridinium-based OPCs was investigated. The choice for 2-(2-methoxyphenoxy)-1-phenylethan-1-ol ( 31 ) as a model substrate was made because its phenolic unit is enriched by a methoxy substituent thus facilitating its single-electron oxidation.…”

Uranyl-Photocatalyzed Hydrolysis of Diaryl Ethers at Ambient Environment for the Directional Degradation of 4-O-5 Lignin

“… 19 Recently, the degradation of DPE was realized by Li et al through esterification under light with an acridinium photocatalyst with two molecules of phenols obtained via the following hydrolysis. 20 Meanwhile, the cleavage of C–C 21 and β-O-4 22 , 23 linkages of lignin were achieved with the development of photocatalysis. 24 − 27 …”

“…19 Recently, the degradation of DPE was realized by Li et al through esterification under light with an acridinium photocatalyst with two molecules of phenols obtained via the following hydrolysis. 20 Based on our previous study on uranyl species, 28 the ligandto-metal charge transfer (LMCT) mode and superior oxidative ability [E ox = +2.60 V vs SCE] 29−32 supply great potential for DPE activation [E ox = +1.88 V vs SCE]. 33 Herein, the photocatalyzed hydrolysis of diaryl ethers was established to afford two kinds of phenols via uranyl catalysis with visible light stimulation at room temperature and normal pressure (Scheme 1d).…”

“…A series of elegant strategies by Hartwig, Grubbs, Mauriello, and others have been developed for selective hydrogenolysis with or without the assistance of transition-metal catalysts. − Hydrolysis of DPE was reported by Katritzky et al and Lercher and co-workers − for cleaving the 4-O-5 linkage with great significance in lignin-first biorefinery, which is thermodynamically challenging with the reaction energy of −7.1 kcal/mol comparing to −14.2 kcal/mol in hydrogenolysis . Recently, the degradation of DPE was realized by Li et al through esterification under light with an acridinium photocatalyst with two molecules of phenols obtained via the following hydrolysis . Meanwhile, the cleavage of C–C and β-O-4 , linkages of lignin were achieved with the development of photocatalysis. − …”

Integrating Continuous-Flow Electrochemistry and Photochemistry for the Synthesis of Acridinium Photocatalysts Via Site-Selective C–H Alkylation

“…Photoredox catalysis holds great promise in sustainable organic synthesis and has been attracting broad interests in the past decade. − Key to the success of this chemistry and its further development resides in the development of catalysts with adjustable physical, photophysical, and electrochemical properties. Acridinium dye 1 (Scheme A), first introduced by Fukuzumi, , has become one of the most popular organic photocatalysts because of its favorable excited-state properties such as a high reduction potential ( E* red = 2.06 V vs standard calomel electrode (SCE)) and good lifetime (τ = 6 ns). ,, Recent work by the Nicewicz group has shown that a modification of the acridinium core with alkyl groups leads to more robust catalysts such as 2 that bear tert -butyl ( t Bu) groups at positions 3 and 6 (Scheme B). − While the acridinium dye 2 can be prepared by adding an aryl Grignard reagent to a t Bu-substituted acridone or the reaction of a doubly lithiated biarylether with an ester (Scheme B), the synthesis of acridinium dyes bearing other alkyl groups on the core remained rather challenging and underexplored.…”