Cerium-Catalyzed C–H Functionalizations of Alkanes Utilizing Alcohols as Hydrogen Atom Transfer Agents

Abstract

Modern photoredox catalysis has traditionally relied upon metal-to-ligand charge-transfer (MLCT) excitation of metal polypyridyl complexes for the utilization of light energy for the activation of organic substrates. Here, we demonstrate the catalytic application of ligand-to-metal charge-transfer (LMCT) excitation of cerium alkoxide complexes for the facile activation of alkanes utilizing abundant and inexpensive cerium trichloride as the catalyst. As demonstrated by cerium-catalyzed C–H amination and the alkylation of hydrocarbons, this reaction manifold has enabled the facile use of abundant alcohols as practical and selective hydrogen atom transfer (HAT) agents via the direct access of energetically challenging alkoxy radicals. Furthermore, the LMCT excitation event has been investigated through a series of spectroscopic experiments, revealing a rapid bond homolysis process and an effective production of alkoxy radicals, collectively ruling out the LMCT/homolysis event as the rate-determining step of this C–H functionalization.

Photocatalyst-Independent Photoredox Ring-Opening Polymerization of O-Carboxyanhydrides: Stereocontrol and Mechanism

“…The alkoxy radical-mediated β-scission-like reaction has been recently reported in Ce and Mn mediated photoredox reactions. 45,[75][76][77] It is also known that free primary alcohols are good O- 44,45 We noticed that the absence of Zn complex seemed not to affect this side reaction (Fig. 4B).…”

“…25,26 The utilization of earthabundant metals as visible-light photocatalysts has recently gained great interest in order to develop sustainable photocatalysts. 27 Recent reported light-induced metal catalysts include Cu, [28][29][30][31] Co, [32][33][34][35] Ni, 26,[36][37][38] Mn, [39][40][41] Ce [42][43][44][45] and Fe 46,47 complexes. Such metal complexes usually involve photoinduced single-electron transfer (SET) between substrate and catalyst, and allow for reactions that occur directly from their photoexcited states, without the assistance of exogenous photocatalysts that harvest light to transfer electron or energy.…”

Catalytic generation of alkoxy radicals from unfunctionalized alcohols

“…Expanding upon this protocol for intermolecular HAT, Liu, Zuo, and coworkers in 2020 again utilized simple alcohols as tunable HAT catalysts for the C-H amination and alkylation of liquid hydrocarbons (Scheme 26). 76 Extensive selectivity studies reveal that methanol-catalyzed conditions generally favor abstraction from weaker C-H bonds, while sterically bulky alcohols, such as tert-butanol or 1-adamantanol, promote abstraction from less sterically encumbered bonds. Electron-decient alcohols, such as TCE or 2,2,2-triuoroethanol, possess stronger O-H bonds and, thus, decrease selectivity for weaker C-H bonds when used as HAT catalysts.…”

Photocatalyst-Independent Photoredox Ring-Opening Polymerization of O-Carboxyanhydrides: Stereocontrol and Mechanism

“…33,34 The utilization of earth-abundant metals as visible-light photocatalysts has recently gained great interest in order to develop sustainable photocatalysts. 35 Recently reported light-induced metal catalysts include Cu, [36][37][38][39] Co, [40][41][42][43] Ni, 34,[44][45][46] Mn, [47][48][49] Ce [50][51][52][53] and Fe 54,55 complexes. Such metal complexes usually involve photoinduced single-electron transfer between the substrate and the catalyst, and allow for reactions that occur directly from their photoexcited states, without the assistance of exogenous photocatalysts that harvest light to transfer electron or energy.…”

“…The alkoxy radical-mediated β-scission-like reaction has been recently reported in Ce and Mn mediated photoredox reactions. 53,[83][84][85] It is also known that free primary alcohols are good Onucleophiles and prone to 2eoxidations that generate carboxylic acids or aldehydes under oxidative conditions. 52,53 We noticed that the absence of Zn complex seemed to not affect this side reaction (Fig.…”