Photocatalytic Reductive C–O Bond Cleavage of Alkyl Aryl Ethers by Using Carbazole Catalysts with Cesium Carbonate

Abstract

Methods to activate the relatively stable ether C–O bonds and convert them to other functional groups are desirable. One-electron reduction of ethers is a potentially promising route to cleave the C–O bond. However, owing to the highly negative redox potential of alkyl aryl ethers (E red < −2.6 V vs SCE), this mode of ether C–O bond activation is challenging. Herein, we report the visible-light-induced photocatalytic cleavage of the alkyl aryl ether C–O bond using a carbazole-based organic photocatalyst (PC). Both benzylic and non-benzylic aryl ethers underwent C–O bond cleavage to form the corresponding phenol products. Addition of Cs2CO3 was beneficial, especially in reactions using a N–H carbazole PC. The reaction was proposed to occur via single-electron transfer (SET) from the excited-state carbazole to the substrate ether. Interaction of the N–H carbazole PC with Cs2CO3 via hydrogen bonding exists, which enables a deprotonation-assisted electron-transfer mechanism to operate. In addition, the Lewis acidic Cs cation interacts with the substrate alkyl aryl ether to activate it as an electron acceptor. The high reducing ability of the carbazole combined with the beneficial effects of Cs2CO3 made this otherwise formidable SET event possible.

High Triplet Energy Iridium(III) Isocyanoborato Complex for Photochemical Upconversion, Photoredox and Energy Transfer Catalysis

“…Lastly, the photochemical degradation of a lignin model compound was performed (Figure c). The degradation of lignin is of interest for the production of low molecular-weight aromatic compounds from biomass; however, conventional methods for this reaction are energy-intense, and therefore, significant efforts toward the photochemical lignin degradation were made in recent years. − After a reaction time of 2 h with [Ir­(dFN­(Me) 2 ppy) 2 ­(BCF) 2 ] − as photocatalyst and TEA as reductant, the reaction was complete and gave an NMR-yield of 93% and 66%, respectively, for the two degradation products (Figure c) and a substrate conversion of 100%. Control experiments in the absence of [Ir­(dFN­(Me) 2 ppy) 2 ­(BCF) 2 ] − were performed for all photoreductions and resulted in only minor (<5%) product formation after the indicated reaction times (Figures S49–S58).…”

Arylamines as More Strongly Reducing Organic Photoredox Catalysts than fac-[Ir(ppy)3]

“…In addition, the same group developed visible-light-induced reductive C−O bond cleavage of alkyl aryl ethers with carbazole-based OPCs (Scheme 9b). 126 In this reaction, the N−H-type carbazole (32) showed better catalytic activity. Interestingly, the photoand electrochemical properties of 7 and 32 were not greatly different, but spectroscopic studies implied that 32 interacts with Cs 2 CO 3 via hydrogen bonding to give a weakly bound complex, which would facilitate the reaction more efficiently.…”

“…Aryl radicals generated by this method could be used for coupling reactions with various (hetero)­arenes, and it is possible for 7 to reduce fluorobenzene. In addition, the same group developed visible-light-induced reductive C–O bond cleavage of alkyl aryl ethers with carbazole-based OPCs (Scheme b) . In this reaction, the N–H-type carbazole ( 32 ) showed better catalytic activity.…”

Metal-free reduction of CO2 to formate using a photochemical organohydride-catalyst recycling strategy

“…This class of carbazoles is characterized by a red-shifted absorption of approximately 80 nm as compared to the parent carbazole, and absorbs visible light. 52,53 The photocatalytic reduction of BI + (I -) to its reduced-state BIH employing 1 as a PC remarkably proceeded in 44% yield under visible light irradiation (400 nm) in a water-containing solvent system in the presence of ascorbic acid (H2A) as a sacrificial reductant (Fig. 2).…”