With the recent soaring production of natural gas, the use of methane and other light hydrocarbon feedstocks as starting materials in synthetic transformations is becoming increasingly economically attractive, although it remains chemically challenging. We report the development of photocatalytic C-H amination, alkylation, and arylation of methane, ethane, and higher alkanes under visible light irradiation at ambient temperature. High catalytic efficiency (turnover numbers up to 2900 for methane and 9700 for ethane) and selectivity were achieved using abundant, inexpensive cerium salts as photocatalysts. Ligand-to-metal charge transfer excitation generated alkoxy radicals from simple alcohols that in turn acted as hydrogen atom transfer catalysts. The mixed-phase gas/liquid reaction was adapted to continuous flow, enabling the efficient use of gaseous feedstocks in scalable photocatalytic transformations.

https://science.sciencemag.org/content/sci/early/2018/07/25/science.aat9750.full.pdf

Selective functionalization of methane, ethane, and higher alkanes by cerium photocatalysis

Visible-light photoredox catalysis offers a distinct activation mode complementary to thermal transition metal catalyzed reactions. The vast majority of photoredox processes capitalizes on precious metal ruthenium(II) or iridium(III) complexes that serve as single-electron reductants or oxidants in their photoexcited states. As a low-cost alternative, organic dyes are also frequently used but in general suffer from lower photostability. Copper-based photocatalysts are rapidly emerging, offering not only economic and ecological advantages but also otherwise inaccessible inner-sphere mechanisms, which have been successfully applied to challenging transformations. Moreover, the combination of conventional photocatalysts with copper(I) or copper(II) salts has emerged as an efficient dual catalytic system for cross-coupling reactions.

Copper's rapid ascent in visible-light photoredox catalysis.

The targeted choice of specific photocatalysts has been shown to play a critical role for the successful realization of challenging photoredox catalytic transformations. Herein, we demonstrate the successful implementation of a rational design strategy for a series of deliberate structural manipulations of cyanoarene-based, purely organic donor–acceptor photocatalysts, using 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) as a starting point. Systematic modifications of both the donor substituents as well as the acceptors’ molecular core allowed us to identify strongly oxidizing as well as strongly reducing catalysts (e.g., for an unprecedented detriflation of unactivated naphthol triflate), which additionally offer remarkably balanced redox potentials with predictable trends. Especially halogen arene core substitutions are instrumental for our targeted alterations of the catalysts’ redox properties. Based on their preeminent electrochemical and photophysical characteristics, all novel, purely...

A Toolbox Approach To Construct Broadly Applicable Metal-Free Catalysts for Photoredox Chemistry: Deliberate Tuning of Redox Potentials and Importance of Halogens in Donor–Acceptor Cyanoarenes

In recent years, visible light-driven organic photochemical synthesis has attracted wide research interest from academic and industrial communities due to its features of green and sustainable chemistry. In this flourishing area, Chinese chemists have devoted great efforts to different aspects of synthetic chemistry. This review will summarize their representative work according to the following categories: C–H functionalization, synthesis of aromatic aza-heterocycles, asymmetric organic photochemical synthesis, transformations of small molecules and biomolecule-compatible reactions.

Visible light-driven organic photochemical synthesis in China

In this invited Perspective, recent developments and possible future directions of research on photoactive coordination compounds made from nonprecious transition metal elements will be discussed. The focus is on conceptually new, structurally well-characterized complexes with excited-state lifetimes between 10 ps and 1 ms in fluid solution for possible applications in photosensitizing, light-harvesting, luminescence and catalysis. The key metal elements considered herein are Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Mo, W and Ce in various oxidation states equipped with diverse ligands, giving access to long-lived excited states via a range of fundamentally different types of electronic transitions. Research performed in this area over the past five years demonstrated that a much broader spectrum of metal complexes than what was long considered relevant exhibits useful photophysics and photochemistry.

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Photoactive Complexes with Earth-Abundant Metals

We demonstrate the application of ligand-to-metal charge transfer (LMCT) excitation to the direct catalytic generation of energetically challenging alkoxy radicals from alcohols through a coordination–LMCT–homolysis process with an abundant and inexpensive cerium salt as the catalyst. This catalytic manifold provides a simple and efficient way to utilize the characteristic reactivity and selectivity of transient alkoxy radicals for δ-selective C–H bond functionalization. Under mild redox-neutral conditions without the need for prefunctionalization, this method provides a versatile platform to access molecular complexity from simple and abundant alcohols.

δ-Selective Functionalization of Alkanols Enabled by Visible-Light-Induced Ligand-to-Metal Charge Transfer.

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.

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

We describe a cerium-catalyzed aerobic oxidative ring expansion for the expedient construction of synthetically challenging macrolactones under visible-light conditions. Cyanoanthracene has been employed as co-catalyst to accelerate the turnover of the cerium cycle leading to a fast conversion within 20 min of irradiation. Taking advantage of the high efficiency and operationally simple conditions, a collection of over 100 macrolactones equipped with ring systems ranging from 9- to 19-membered macrocycles have been prepared from simple building blocks. Moreover, the enabling potential of this strategy to simplify the generation of molecular complexity has been demonstrated through the concise synthesis of sonnerlactone.

Photocatalytic Aerobic Oxidative Ring Expansion of Cyclic Ketones to Macrolactones by Cerium and Cyanoanthracene Catalysis

Employing methane as a C1 feedstock for commodity chemicals represents a sustainable and empowering opportunity for chemical catalysis. However, practical demands of industrial development have imposed serve challenges to utilize coeffective catalysts and reagents as well as ecologically benign conditions for highly efficient and selective C(sp)–H bond functionalizations, constantly driving the development of catalytic strategies. The aerobic oxidative carbonylation of methane represents an ideal approach for sustainable acetic acid synthesis, yet poses a longstanding challenge in selectivity for C2:C1 products. In recent years, metallaphotocatalysis in which photoredox catalysts operate in synergy with transition metal catalysts to facilitate both 1and 2-electron reaction steps, has provided intriguing opportunities to achieve selective cross-couplings otherwise unattainable. Here, we demonstrate the utilization of an iron terpyridine catalyst to facilitate multiple 1and 2-electron processes, including photoexcitation, radical generation, oxidative radical capture and carbonylation, in the form of methane aerobic carbonylation to acetic acid. High levels of catalytic efficiency (turnover numbers up to 31480) and selectivity (C2/C1 ratio up to 142:1) have been achieved with a robust iron terpyridine photocatalyst at ambient temperature under LED irradiation. Importantly, this practical catalytic manifold has been adapted to the selective oxidative carbonylation of various feedstock alkanes.

Hui Pan

Papers

Selective functionalization of methane, ethane, and higher alkanes by cerium photocatalysis

δ-Selective Functionalization of Alkanols Enabled by Visible-Light-Induced Ligand-to-Metal Charge Transfer.

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

Photocatalytic Aerobic Oxidative Ring Expansion of Cyclic Ketones to Macrolactones by Cerium and Cyanoanthracene Catalysis

Photocatalytic aerobic oxidative carbonylation of methane by iron terpyridine catalysts