A Metal-Free, Photocatalytic Method for Aerobic Alkane Iodination

Abstract

Halogenation is an important alkane functionalization strategy, but O 2 is widely considered the most desirable terminal oxidant. Here, the aerobic iodination of alkanes, including methane, was performed using catalytic [ n Bu 4 N]Cl and light irradiation (390 nm). Up to 10 turnovers of CH 3 I were obtained from CH 4 and air, using a stop-flow microtubing system. Mechanistic studies using cyclohexane as the substrate revealed important details about the iodination reaction. Iodine (I 2 ) serves multiple roles in the catalysis: (1) as the alkyl radical trap, (2) as a precursor for the light absorber, and (3) as a mediator of aerobic oxidation. The alkane activation is attributed to Cl • derived from photofragmentation of the electron donor−acceptor complex of I 2 and Cl − . The kinetic profile of cyclohexane iodination showed that aerobic oxidation of I 3 − to produce I 2 in CH 3 CN is turnoverlimiting.

Chloride Oxidation by One- or Two-Photon Excitation of N-Phenylphenothiazine

“…Chlorine atoms have long been known as a powerful tool for C–H activation yet historically have seen little use in practical applications due to the harsh reaction conditions necessary to generate Cl • . Over the last decade, a number of groundbreaking studies have demonstrated facile chloride oxidation using photoredox catalysts, which have enabled new approaches to functionalize inert C–H bonds. − Additionally, environmentally motivated applications rely on chloride oxidation as an important fundamental process for HCl splitting to store solar energy − or for electric gradient generation for sea water desalination. − However, the vast majority of these examples rely on noble metal photocatalysts to generate chlorine atoms because the one-electron reduction potential, (Cl • /– ), requires an incredibly potent photooxidant. − Indeed, chloride ions are so redox inert that they are frequently used as counterions for common photocatalysts such as ruthenium polypyridyl compounds. , Synthesizing ruthenium or iridium coordination compounds that are strong photooxidants requires skillful ligand engineering ,− and such catalysts often suffer from stability issues. − Moreover, photocatalysts based on noble metals are impractical at scale given the low natural abundance of these rare elements. Herein, we report a rare example of light-induced chloride oxidation catalyzed by an inexpensive, commercially available organic photocatalyst through unconventional one- and two-photon pathways.…”

Aerobic Partial Oxidation of Alkanes Using Photodriven Iron Catalysis

“…The degradation rate of MeTFA under reaction conditions was assessed and shown to be much slower than the rate of its formation, with >90% MeTFA remaining after 24 h (Figure ). Regardless of the mechanism for the formation of MeTFA, the monooxidized product is stable under reaction conditions, indicating a protective effect of the ester moiety . Under the conditions in Figure , the different solubilities of methane and MeTFA play a role.…”

Titanium(IV) Chloride-Catalyzed Photoalkylation via C(sp3)–H Bond Activation of Alkanes

“…Thus, chlorine radicals are capable of cleaving ubiquitously prevalent C­(sp 3 )–H bonds of alkanes (BDE ∼ 100 kcal/mol) through HAT . Recently, single-electron oxidation of chloride ions ( E = 1.36 V vs NHE) facilitated by photoredox catalysis, electrocatalysis, protonated N -heteroaromatics, singlet oxygen, and iodine under photothermal conditions has been established as a versatile approach for generating chlorine radicals. Further significant contributions were made by Aggarwal, in which chlorine radical generation via bimolecular homolytic substitution between chloroborate and an oxy radical allowed for alkane borylation .…”