The fields of C-H functionalization and photoredox catalysis have garnered enormous interest and utility in the past several decades. Many different scientific disciplines have relied on C-H functionalization and photoredox strategies including natural product synthesis, drug discovery, radiolabeling, bioconjugation, materials, and fine chemical synthesis. In this Review, we highlight the use of photoredox catalysis in C-H functionalization reactions. We separate the review into inorganic/organometallic photoredox catalysts and organic-based photoredox catalytic systems. Further subdivision by reaction class-either sp2 or sp3 C-H functionalization-lends perspective and tactical strategies for use of these methods in synthetic applications.

Photoredox-Catalyzed C-H Functionalization Reactions.

Transition-metal-catalysed direct C(sp3)−H functionalization of primary aliphatic amines is an attractive – but elusive – process that could provide efficient access to biologically and pharmaceutically important compounds. Now, a palladium-catalysed γ-arylation of primary alkylamines is achieved using an inexpensive, catalytic and transient directing group.

Site-Selective C−H Arylation of Primary Aliphatic Amines Enabled by a Catalytic Transient Directing Group

Organic compounds that show Thermally Activated Delayed Fluorescence (TADF) have become wildly popular as next-generation emitters in organic light emitting diodes (OLEDs). Since 2016, a subset of these have found increasing use as photocatalysts. This review comprehensively highlights their potential by documenting the diversity of the reactions where an organic TADF photocatalyst can be used in lieu of a noble metal complex photocatalyst. Beyond the small number of TADF photocatalysts that have been used to date, the analysis conducted within this review reveals the wider potential of organic donor–acceptor TADF compounds as photocatalysts. A discussion of the benefits of compounds showing TADF for photocatalysis is presented, which paints a picture of a very promising future for organic photocatalyst development.

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Organic thermally activated delayed fluorescence (TADF) compounds used in photocatalysis

Site-selective C-H functionalization in chemical feedstocks is a challenging and useful reaction in the broad field of chemical research. Here, we report a modular photochemical platform for the site-selective C-H pyridylation of unactivated hydrocarbons via the unique synergistic effects of triplet excited anthraquinone and an amidyl radical-based reverse hydrogen atom transfer (RHAT) agent. The selective pyridylation of tertiary and secondary C(sp3)-H bonds in abundant chemical feedstocks was achieved by employing various N-aminopyridinium salts in a highly selective fashion, thus providing a new catalytic system for the direct construction of high-value-added compounds under ambient reaction conditions. Moreover, this operationally simple protocol is applicable to a variety of linear-, branched-, and cyclo-alkanes and more complex molecules with high degrees of site selectivity under visible-light conditions, which provides rapid and straightforward access to versatile synthons for upgrading feedstocks under mild, metal-free reaction conditions.

Site-Selective Direct C-H Pyridylation of Unactivated Alkanes by Triplet Excited Anthraquinone.

Herein, we conceptualized a transient mediator approach that has the capability of para-selective C-H functionalization of monosubstituted aromatics. This approach is enabled by in situ generation of a versatile sulfonium salt via highly electrophilic phenoxathiine or thianthrene dication intermediate which can be readily generated from its sulfoxide with trifluoromethanesulfonic anhydride. Preliminary mechanistic study implied that the remarkable para selectivity might be related to the incredible electrophilicity of thianthrene dication intermediate. The versatility of this approach was demonstrated via para-borylation of various monosubstituted simple aromatics combining the sulfonium salt formation with further photocatalyzed transformation.

Para -selective borylation of monosubstituted benzenes using a transient mediator

Cross-Dehydrogenative Coupling of Strong C(sp3)-H with N-Heteroarenes through Visible-Light-Induced Energy Transfer.

The monofluoroalkene substructure plays a crucial role in various fields of organic synthesis, pharmacology and materials science. However, a redox-neutral direct monofluoroalkenylation of prevalent alkyl sources still remains very challenging. Herein, the first monofluoroalkenylation of naturally abundant unactivated carboxylic acids via organic photoredox catalytic decarboxylation has been developed. This redox-neutral approach avoids the utilization of presynthesized redox-active esters and high energy irradiation, providing a mild and efficient strategy for direct decarboxylative cross-coupling reactions of unactivated carboxylic acids. Furthermore, it allows the remarkable and challenging late-stage monofluoroalkenylation of complex molecular architectures such as bioactive ramipril, gemfibrozil, dehydroabietic acid, enoxolone and saccharide-derived cyclic α-oxy acid.

Organic photoredox catalytic decarboxylative cross-coupling of gem-difluoroalkenes with unactivated carboxylic acids

A para‐C–H Functionalization of Aniline Derivatives via In situ Generated Bulky Hypervalent Iodinium Reagents

C5-selective trifluoromethylation of 8-amino quinolines via photoredox catalysis

A general visible-light mediated ortho-C-H trifluoromethylation of aniline derivatives by using a low-cost and stable Langlois reagent (CF3SO2Na) as the "CF3" source has been developed. In contrast to previous reports, this strategy allowed access to elusive trifluoromethyl lactams. Furthermore, mechanism experiments revealed that a copper/photoredox dual catalytic mechanism enabled general trifluoromethylation of aniline derivatives.

Chao Tian

Papers

Cross-Dehydrogenative Coupling of Strong C(sp3)-H with N-Heteroarenes through Visible-Light-Induced Energy Transfer.

Organic photoredox catalytic decarboxylative cross-coupling of gem-difluoroalkenes with unactivated carboxylic acids

A para‐C–H Functionalization of Aniline Derivatives via In situ Generated Bulky Hypervalent Iodinium Reagents

C5-selective trifluoromethylation of 8-amino quinolines via photoredox catalysis

Visible-Light Mediated ortho-Trifluoromethylation of Aniline Derivatives.