Homogeneous Light‐Driven Catalytic Direct Carboxylation with CO2

Carbon dioxide is a ubiquitous and inexpensive one‐carbon source for chemical synthesis, and the efficient incorporation of CO₂ into organic molecules is of widespread research interest both for economic and ecological reasons. The methodologies to employ carbon dioxide as a single‐carbon unit to construct molecules relevant for agrochemical and pharmaceutical research include many elegant approaches, including asymmetric transformations. Even though remarkable achievements have been made in the field of light‐driven catalysis, especially photoredox catalysis, homogeneous light‐driven catalytic carboxylation by employing CO₂ as the key reagent has only become a subject of increasing attention in recent years. Therefore, this concise review will discuss the latest advances in this research area.     

Oxygen‐Controlled Catalysis by Vitamin B12‐TiO2: Formation of Esters and Amides from Trichlorinated Organic Compounds by Photoirradiation

An oxygen switch in catalysis of the cobalamin derivative (B₁₂)‐TiO₂ hybrid catalyst for the dechlorination of trichlorinated organic compounds has been developed. The covalently bound B₁₂ on the TiO₂ surface transformed trichlorinated organic compounds into an ester and amide by UV light irradiation under mild conditions (in air at room temperature), while dichlorostilbenes (E and Z forms) were formed in nitrogen from benzotrichloride. A benzoyl chloride was formed as an intermediate of the ester and amide, which was detected by GC‐MS. The substrate scope of the synthetic strategy is demonstrated with a range of various trichlorinated organic compounds. A photo‐duet reaction utilizing the hole and conduction band electron of TiO₂ in B₁₂‐TiO₂ for the amide formation was also developed.     

One‐Pot Tandem Photoredox and Cross‐Coupling Catalysis with a Single Palladium Carbodicarbene Complex

The combination of conventional transition‐metal‐catalyzed coupling (2 e^? process) and photoredox catalysis (1 e^? process) has emerged as a powerful approach to catalyze difficult cross‐coupling reactions under mild reaction conditions. Reported is a palladium carbodicarbene (CDC) complex that mediates both a Suzuki–Miyaura coupling and photoredox catalysis for C?N bond formation upon visible‐light irradiation. These two catalytic pathways can be combined to promote both conventional transition‐metal‐catalyzed coupling and photoredox catalysis to mediate C?H arylation under ambient conditions with a single catalyst in an efficient one‐pot process.     

The Redox Coupling Effect in a Photocatalytic RuII‐PdII Cage with TTF Guest as Electron Relay Mediator for Visible‐Light Hydrogen‐Evolving Promotion

A nanocage coupling effect from a redox Ru^II‐Pd^II metal–organic cage (MOC‐16) is demonstrated for efficient photochemical H₂ production by virtue of redox–guest modulation of the photo‐induced electron transfer (PET) process. Through coupling with photoredox cycle of MOC‐16, tetrathiafulvalene (TTF) guests act as electron relay mediator to improve the overall electron transfer efficiency in the host–guest system in a long‐time scale, leading to significant promotion of visible‐light driven H₂ evolution. By contrast, the presence of larger TTF‐derivatives in bulk solution without host–guest interactions results in interference with PET process of MOC‐16, leading to inefficient H₂ evolution. Such interaction provides an example to understand the interplay between the redox‐active nanocage and guest for optimization of redox events and photocatalytic activities in a confined chemical nanoenvironment.     

Visible‐Light Photoredox‐Catalyzed and Copper‐Promoted Trifluoromethoxylation of Arenediazonium Tetrafluoroborates

We report the development of photoredox‐catalyzed and copper‐promoted trifluoromethoxylation of arenediazonium tetrafluoroborates, with trifluoromethyl arylsulfonate (TFMS) as the trifluoromethoxylation reagent. This new method takes advantage of visible‐light photoredox catalysis to generate the aryl radical under mild conditions, combined with copper‐promoted selective trifluoromethoxylation. The reaction is scalable, tolerates a wide range of functional groups, and proceeds regioselectively under mild reaction conditions. Furthermore, mechanistic studies suggested that a Cs[Cu(OCF₃)₂] intermediate might be generated during the reaction.     

Photocatalytic/Cu‐Promoted C−H Activations: Visible‐light‐Induced ortho‐Selective Perfluoroalkylation of Benzamides

A visible‐light‐induced and copper‐promoted perfluoroalkylation of benzamides was successfully developed under the assistance of an 8‐aminoquinoline directing group. It provides a straightforward method for the synthesis of ortho‐perfluoroalkyl‐substituted benzoic acid derivatives. The reaction employs a cheap organic dye eosin Y as the photoredox catalyst and is run under the irradiation of a 26 W fluorescent LED light bulb.     

Photoredox Catalytic α‐Alkoxypentafluorosulfanylation of α‐Methyl‐ and α‐Phenylstyrene Using SF6

SF₆ was applied as pentafluorosulfanylation reagent to prepare ethers with a vicinal SF₅ substituent through a one‐step method involving photoredox catalysis. This method shows a broad substrate scope with respect to applicable alcohols for the conversion of α‐methyl and α‐phenyl styrenes. The products bear a new structural motif with two functional groups installed in one step. The alkoxy group allows elimination and azidation as further transformations into valuable pentafluorosulfanylated compounds. These results confirm that non‐toxic SF₆ is a useful SF₅ transfer reagent if properly activated by photoredox catalysis, and toxic reagents are completely avoided. In combination with light as an energy source, a high level of sustainability is achieved. Through this method, the proposed potential of the SF₅ substituent in medicinal chemistry, agrochemistry, and materials chemistry may be exploited in the future.     

A Robust,Precious‐Metal‐Free Dye‐Sensitized Photoanode for Water Oxidation: A Nanosecond‐Long Excited‐State Lifetime through a Prussian Blue Analogue

Herein, we establish a simple synthetic strategy affording a heterogeneous, precious metal‐free, dye‐sensitized photoelectrode for water oxidation, which incorporates a Prussian blue (PB) structure for the sensitization of TiO₂ and water oxidation catalysis. Our approach involves the use of a Fe(CN)₅ bridging group not only as a cyanide precursor for the formation of a PB‐type structure but also as an electron shuttle between an organic chromophore and the catalytic center. The resulting hetero‐functional PB‐modified TiO₂ electrode demonstrates a low‐cost and easy‐to‐construct photoanode, which exhibits favorable electron transfers with a remarkable excited state lifetime on the order of nanoseconds and an extended light absorption capacity of up to 500 nm. Our approach paves the way for a new family of precious metal‐free robust dye‐sensitized photoelectrodes for water oxidation, in which a variety of common organic chromophores can be employed in conjunction with CoFe PB structures.     

Functionalization of C‐H Bonds by Photoredox Catalysis

Visible‐light photoredox catalysis has been successfully used in the functionalization of inert C?H bonds including C(sp²)‐H bonds of arenes and C(sp³)‐H bonds of aliphatic compounds over the past decade. These transformations are typically promoted by the process of single‐electron‐transfer (SET) between substrates and photo‐excited photocatalyst upon visible light irradiation (household bulbs or LEDs). Compared with other synthetic strategies, such as the transition‐metal catalysis and traditional radical reactions, visible‐light photoredox approach has distinct advantages in terms of operational simplicity and practicability. Versatile direct functionalization of inert C(sp²)‐H and C(sp³)‐H bonds including alkylation, trifluoromethylation, arylation and amidation, has been achieved using this practical strategy.     

Acceptorless Dehydrogenation of N‐Heterocycles by Merging Visible‐Light Photoredox Catalysis and Cobalt Catalysis

Herein, the first acceptorless dehydrogenation of tetrahydroquinolines (THQs), indolines, and other related N‐heterocycles, by merging visible‐light photoredox catalysis and cobalt catalysis at ambient temperature, is described. The potential applications to organic transformations and hydrogen‐storage materials are demonstrated. Primary mechanistic investigations indicate that the catalytic cycle occurs predominantly by an oxidative quenching pathway.     

Synthesis of Tri‐ and Difluoromethoxylated Compounds by Visible‐Light Photoredox Catalysis

Trifluoromethoxy (OCF₃) and difluoromethoxy (OCF₂H) groups are fluorinated structural motifs that exhibit unique physicochemical characteristics. Incorporation of these substituents into organic molecules is a highly desirable approach used in medicinal chemistry and drug discovery processes to alter the properties of a parent compound. Recently, tri‐ and difluoromethyl ethers have received increasing attention and several innovative strategies to access these valuable functional groups have been developed. The focus of this Minireview is the use of visible‐light photoredox catalysis in the synthesis of tri‐ and difluoromethyl ethers. Recent photocatalytic strategies for the formation of O?CF₃, C?OCF_3, O?CF₂H, and C?OCF₂H bonds as well as other transformations leading to the construction of OR_F groups are discussed herein.     

Visible‐Light‐Mediated Decarboxylation/Oxidative Amidation of α‐Keto Acids with Amines under Mild Reaction Conditions Using O2

Photochemistry has ushered in a new era in the development of chemistry, and photoredox catalysis has become a hot topic, especially over the last five years, with the combination of visible‐light photoredox catalysis and radical reactions. A novel, simple, and efficient radical oxidative decarboxylative coupling with the assistant of the photocatalyst [Ru(phen)₃]Cl₂ is described. Various functional groups are well‐tolerated in this reaction and thus provides a new approach to developing advanced methods for aerobic oxidative decarboxylation. The preliminary mechanistic studies revealed that: 1) an SET process between [Ru(phen)₃]²⁺* and aniline play an important role; 2) O₂ activation might be the rate‐determining step; and 3) the decarboxylation step is an irreversible and fast process.     

Redox-Mediated Alcohol Oxidation Coupled to Hydrogen Gas Formation in a Dye-Sensitized Photosynthesis Cell

This work reports a dye-sensitized photoelectrochemical cell (DSPEC) that couples redox-mediated light-driven oxidative organic transformations to reductive hydrogen (H₂) formation. The DSPEC photoanode consists of a mesoporous anatase TiO₂ film on FTO (fluorine-doped tin oxide), sensitized with the thienopyrroledione-based dye AP11 , while H₂ was formed at a FTO-Pt cathode. Irradiation of the dye-sensitized photoanode transforms 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) to the oxidized TEMPO (TEMPO⁺), which acts as a chemical oxidant for the conversion of benzyl alcohol. The TEMPO^0/+ couple, previously used as redox mediator in DSSC, mediates efficient electron transfer from the organic substrate to the photo-oxidized dye. A DSPEC photoreactor was designed that allows in situ monitoring the reaction progress by infrared spectroscopy and gas chromatography. Sustained light-driven oxidation of benzyl alcohol to benzaldehyde within the DSPEC photoreactor, using of TEMPO as mediator, demonstrated the efficiency of the device, with a photocurrent of 0.4 mA cm⁻², approaching quantitative Faradaic efficiency and exhibiting excellent device stability.     

Visible‐Light‐Irradiated Graphitic Carbon Nitride Photocatalyzed Diels–Alder Reactions with Dioxygen as Sustainable Mediator for Photoinduced Electrons

Photocatalytic Diels–Alder (D–A) reactions with electron rich olefins are realized by graphitic carbon nitride (g‐C₃N₄) under visible‐light irradiation and aerobic conditions. This heterogeneous photoredox reaction system is highly efficient, and the apparent quantum yield reaches a remarkable value of 47 % for the model reaction. Dioxygen plays a critical role as electron mediator, which is distinct from the previous reports in the homogeneous Ru^II complex photoredox system. Moreover, the reaction intermediate vinylcyclobutane is captured and monitored during the reaction, serving as a direct evidence for the proposed reaction mechanism. The cycloaddition process is thereby determined to be the combination of direct [4+2] cycloaddition and [2+2] cycloaddition followed by photocatalytic rearrangement of the vinylcyclobutane intermediate.     

Visible‐Light‐Induced Formal [3+2] Cycloaddition for Pyrrole Synthesis under Metal‐Free Conditions

A photocatalytic formal [3+2] cycloaddition of 2H‐azirines with alkynes has been achieved under irradiation by visible light in the presence of organic dye photocatalysts. This transformation provides efficient access to highly functionalized pyrroles in good yields and has been applied to the synthesis of drug analogues. A primary trial of photocascade catalysis merging energy transfer and redox neutral reactions was shown to be successful.     

Integrating TEMPO and Its Analogues with Visible‐Light Photocatalysis

Visible light has risen to become a very important facilitator for selective radical reactions enabled by well‐cognized photocatalysts. The renaissance of visible‐light photocatalysis on this matter partly relies on integrating it with other fields of catalysis. In parallel, 2,2,6,6‐tetramethylpiperidin N‐oxide (TEMPO), a quintessential persistent radical, has a wide range of uses owing to its exceptional redox behavior, which gives rise to its latest prominence in catalysis. Therefore, integrating the catalysis of TEMPO with photocatalysis to perform visible‐light‐induced selective reactions becomes a very convenient marriage of merits. In this context, the integration of different types of photocatalysts, including metal complexes, metal‐free organic dyes, and semiconductors, with TEMPO for outstanding organic transformations will be summarized. To expand further the catalytic repertoire, the integration of TEMPOH analogues such as NHPI (N‐hydroxyphthalimide) and NHS (N‐hydroxysuccinimide) with photocatalysis will also be discussed. Hopefully, these advances will pave the way for more breakthroughs by integrating TEMPO and its analogues with photocatalysis to lead to a valuable blueprint for visible‐light‐induced selective organic transformations.     

Metal‐Free,Room‐Temperature,Radical Alkoxycarbonylation of Aryldiazonium Salts through Visible‐Light Photoredox Catalysis

The first radical alkoxycarboxylation of aryldiazonium salts using CO gas through visible‐light‐induced photoredox catalysis (16 W blue LEDs) has been developed. This reaction is entirely metal‐free, is carried out at room temperature with a low loading of an organic dye as a photocatalyst (0.5 mol %), and provides a wide range of arylcarboxylic acid esters in high yields. Importantly, this photocatalytic system can be successfully extended to other carboxylation reactions.     

Visible‐light‐sensitive photoredox catalysis by iron complexes: Applications in cationic and radical polymerization reactions

The development of iron complexes for the photoredox catalysis is a huge challenge. Indeed, Iron complexes can be ideal candidates due to their potential visible light absorption and redox properties but also because they are less toxic, inexpensive and environmentally friendly compared to other catalysts. In the present paper, a series of novel iron complexes have been synthesized and utilized to initiate the free radical promoted cationic polymerization of epoxides or the free radical polymerization of acrylates through photoredox catalysis processes upon exposure to near UV (385 nm) or visible violet (405 nm) light emitting diodes (LEDs). When combined with an iodonium salt and N‐vinylcarbazole, the iron complex‐based photoinitiating systems are able to generate radicals, cations, and radical cations. The initiation efficiency is investigated through real‐time Fourier transform infrared spectroscopy and a satisfactory initiating ability is found. The mechanisms for the generation of the reactive initiating species through photoredox catalysis are studied by different methods (steady state photolysis, cyclic voltammetry and electron spin resonance spin trapping techniques) and discussed in detail. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 2247–2253     

Metal‐Free,Room‐Temperature,Radical Alkoxycarbonylation of Aryldiazonium Salts through Visible‐Light Photoredox Catalysis

The first radical alkoxycarboxylation of aryldiazonium salts using CO gas through visible‐light‐induced photoredox catalysis (16 W blue LEDs) has been developed. This reaction is entirely metal‐free, is carried out at room temperature with a low loading of an organic dye as a photocatalyst (0.5 mol %), and provides a wide range of arylcarboxylic acid esters in high yields. Importantly, this photocatalytic system can be successfully extended to other carboxylation reactions.     

Visible‐Light‐Promoted Trifluoromethylthiolation of Styrenes by Dual Photoredox/Halide Catalysis

Herein, we report a new visible‐light‐promoted strategy to access radical trifluoromethylthiolation reactions by combining halide and photoredox catalysis. This approach allows for the synthesis of vinyl–SCF₃ compounds of relevance in pharmaceutical chemistry directly from alkenes under mild conditions with irradiation from household light sources. Furthermore, alkyl–SCF₃‐containing cyclic ketone and oxindole derivatives can be accessed by radical‐polar crossover semi‐pinacol and cyclization processes. Inexpensive halide salts play a crucial role in activating the trifluoromethylthiolating reagent towards photoredox catalysis and aid the formation of the SCF₃ radical.