Farbselektive Photokatalyse: kontrollierte Bindungsaktivierung durch Redoxpotentialregulation über die Anregungslichtfarbe

Catalytic Asymmetric Redox-Neutral [3+2] Photocycloadditions of Cyclopropyl Ketones with Vinylazaarenes Enabled by Consecutive Photoinduced Electron Transfer

Consecutive photoinduced electron transfer (ConPET) is a powerful and atom-economical protocol to overcome the limitations of the intrinsic redox potential of visible light-absorbing photosensitizers, thereby considerably improving the substrate and reaction types. Likely because such an exothermic single-electron transfer (SET) process usually does not require the aid of chiral catalysts, resulting in an inevitable racemic background reaction, notably, no enantioselective manifolds have been reported. Herein, we report on the viability of cooperative ConPET and chiral hydrogen-bonding catalysis for the [3+2] photocycloaddition of cyclopropyl ketones with vinylazaarenes. In addition to enabling the first use of olefins that preferentially interact with chiral catalysts, this catalysis platform paves the way for the efficient synthesis of pharmaceutically and synthetically important cyclopentyl ketones functionalized by azaarenes with high yields, ees and dr. The robust capacity of the method can be further highlighted by the low loading of the chiral catalyst (1.0 mol %), the good compatibility of both 2-azaarene and 3-pyridine-based olefins, and the successful concurrent construction of three stereocenters on cyclopentane rings involving an elusive but important all-carbon quaternary.

Photocatalytic Carboxylation of C-N Bonds in Cyclic Amines with CO2 by Consecutive Visible-Light-Induced Electron Transfer

Visible-light photocatalytic carboxylation with CO₂ is highly important. However, it still remains challenging for reluctant substrates with low reduction potentials. Herein, we report a novel photocatalytic carboxylation of C-N bonds in cyclic amines with CO₂ via consecutive photo-induced electron transfer (ConPET). It is also the first photocatalytic reductive ring-opening reaction of azetidines, pyrrolidines and piperidines. This strategy is practical to transform a variety of easily available cyclic amines to valuable β-, γ-, δ- and ϵ-amino acids in moderate-to-excellent yields. Moreover, the method also features mild and transition-metal-free conditions, high selectivity, good functional-group tolerance, facile scalability and product derivations. Mechanistic studies indicate that the ConPET might be the key to generating highly reactive photocatalysts, which enable the reductive activation of cyclic amines to generate carbon radicals and carbanions as the key intermediates.

Visible-Light-Promoted Switchable Selective Oxidations of Styrene Over Covalent Triazine Frameworks in Water

Using photocatalytic oxidation to convert basic chemicals into high value compounds in environmentally benign reaction media is a current focus in catalytic research. The challenge lies in gaining controllability over product formation selectivity. We design covalent triazine frameworks as heterogeneous, metal-free, and recyclable photocatalysts for visible-light-driven switchable selective oxidation of styrene in pure water. Selectivity in product formation was achieved by activation or deactivation of the specific photogenerated oxygen species. Using the same photocatalyst, by deactivation of photogenerated H₂ O₂ , benzaldehyde was obtained with over 99 % conversion and over 99 % selectivity as a single product. The highly challenging and sensitive epoxidation of styrene was carried out by creating peroxymonocarbonate as an initial epoxidation agent in the presence of bicarbonate, which led to formation of styrene oxide with a selectivity up to 76 % with near quantitative conversion. This study demonstrates a preliminary yet interesting example for simple control over switchable product formation selectivity for challenging oxidation reactions of organic compounds in pure water.

Visible Light-Mediated Cyclisation Reaction for the Synthesis of Highly-Substituted Tetrahydroquinolines and Quinolines

Condensation of 2-vinylanilines and conjugated aldehydes followed by an efficient light-mediated cyclisation selectively yields either substituted tetrahydroquinolines with typically high dr, or in the presence of an iridium photocatalyst the synthesis of quinoline derivatives is demonstrated. These atom economical processes require mild conditions, with the substrate scope demonstrating excellent site selectivity and functional group tolerance, including azaarene-bearing substrates. A thorough experimental mechanistic investigation explores multiple pathways and the key role that imine and iminium intermediates play in the absorption of visible light to generate reactive excited states. The synthetic utility of the reactions is demonstrated on gram scale quantities in both batch and flow, alongside further manipulation of the medicinally relevant products.

Oxy-Borylenes as Photoreductants: Synthesis and Application in Dehalogenation and Detosylation Reactions

While the range of accessible borylenes has significantly broadened over the last decade, applications remain limited. Herein, we present tricoordinate oxy-borylenes as potent photoreductants that can be readily activated by visible light. Facile oxidation of CAAC stabilized oxy-borylenes (CAAC)(IPr₂ Me₂ )BOR (R=TMS, CH₂ CH₂ C₆ H₅ , CH₂ CH₂ (4-F)C₆ H₄ ) to their corresponding radical cations is achieved with mildly oxidizing ferrocenium ion. Cyclovoltammetric studies reveal ground-state redox potentials of up to -1.90 V vs. Fc^+/0 for such oxy-borylenes placing them among the strongest organic super electron donors. Their ability as photoreductants is further supported by theoretical studies and showcased by the application as stoichiometric reagents for the photochemical hydrodehalogenation of aryl chlorides, aryl bromides and unactivated alkyl bromides as well as the detosylation of anilines.

Tuning Deazaflavins Towards Highly Potent Reducing Photocatalysts Guided by Mechanistic Understanding – Enhancement of the Key Step by the Internal Heavy Atom Effect

Deazaflavins are well suited for reductive chemistry acting via a consecutive photo‐induced electron transfer, in which their triplet state and semiquinone – the latter is formed from the former after electron transfer from a sacrificial electron donor – are key intermediates. Guided by mechanistic investigations aiming to increase intersystem crossing by the internal heavy atom effect and optimising the concentration conditions to avoid unproductive excited singlet reactions, we synthesised 5‐aryldeazaflavins with Br or Cl substituents on different structural positions via a three‐component reaction. Bromination of the deazaisoalloxazine core leads to almost 100 % triplet yield but causes photo‐instability and enhances unproductive side reactions. Bromine on the 5‐phenyl group in ortho position does not affect the photostability, increases the triplet yield, and allows its efficient usage in the photocatalytic dehalogenation of bromo‐ and chloroarenes with electron‐donating methoxy and alkyl groups even under aerobic conditions. Reductive powers comparable to lithium are achieved.

Controlling Chemoselectivity of Catalytic Hydroboration with Light

The ability to selectively react one functional group in the presence of another underpins efficient reaction sequences. Despite many designer catalytic systems being reported for hydroboration reactions, which allow introduction of a functional handle for cross‐coupling or act as mild method for reducing polar functionality, these platforms rarely deal with more complex systems where multiple potentially reactive sites exist. Here we demonstrate, for the first time, the ability to use light to distinguish between ketones and carboxylic acids in more complex molecules. By taking advantage of different activation modes, a single catalytic system can be used for hydroboration, with the chemoselectivity dictated only by the presence or absence of visible light.

Photophysics of Perylene Diimide Dianions and Their Application in Photoredox Catalysis

The two‐electron reduced forms of perylene diimides (PDIs) are luminescent closed‐shell species whose photochemical properties seem underexplored. Our proof‐of‐concept study demonstrates that straightforward (single) excitation of PDI dianions with green photons provides an excited state that is similarly or more reducing than the much shorter‐lived excited states of PDI radical monoanions, which are typically accessible after biphotonic excitation with blue photons. Thermodynamically demanding photocatalytic reductive dehalogenations and reductive C−O bond cleavage reactions of lignin model compounds have been performed using sodium dithionite acts as a reductant, either in aqueous solution or in biphasic water–acetonitrile mixtures in the presence of a phase transfer reagent. Our work illustrates the concept of multi‐electron reduction of a photocatalyst by a sacrificial reagent prior to irradiation with low‐energy photons as a means of generating very reactive excited states.

Photocatalysis in Supramolecular Fluorescent Metallacycles and Metallacages

The utilization of photocatalytic techniques for achieving light-to-fuel conversion is a promising way to ease the shortage of energy and degradation of the ecological environment. Fluorescent metallacycles and metallacages have drawn considerable attention and have been used in widespread fields due to easy preparation and their abundant functionality including photocatalysis. This review covers recent advances in photocatalysis in discrete supramolecular fluorescent metallacycles and metallacages. The developments in the utilization of the metallacycles skeletons and the effect of fluorescence-resonance energy transfer for photocatalysis are discussed. Furthermore, the use of the ligands decorated by organic chromophores or redox metal sites in metallacages as photocatalysts and their ability to encapsulate appropriate catalytic cofactors for photocatalysis are summarized. For the sake of brevity, macrocycles and cages with inorganic coordination complexes such as ruthenium complexes and iridium complexes are not included in this minireview.

Electro-mediated PhotoRedox Catalysis for Selective C(sp3 )-O Cleavages of Phosphinated Alcohols to Carbanions

We report a novel example of electro-mediated photoredox catalysis (e-PRC) in the reductive cleavage of C(sp3 )-O bonds of phosphinated alcohols to alkyl carbanions. As well as deoxygenations, olefinations are reported which are E-selective and can be made Z-selective in a tandem reduction/photosensitization process where both steps are photoelectrochemically promoted. Spectroscopy, computation, and catalyst structural variations reveal that our new naphthalene monoimide-type catalyst allows for an intimate dispersive precomplexation of its radical anion form with the phosphinate substrate, facilitating a reactivity-determining C(sp3 )-O cleavage. Surprisingly and in contrast to previously reported photoexcited radical anion chemistries, our conditions tolerate aryl chlorides/bromides and do not give rise to Birch-type reductions.

Using Light to Modify the Selectivity of Transition Metal Catalysed Transformations

Light has a remarkable and often unique ability to promote chemical reactions. In combination with transition metal catalysis, it offers exciting opportunities to modify catalyst function in a non‐invasive manner, most frequently being reported to switch on or accelerate reactions that do not occur in the dark. However, the ability to completely change reactivity or selectivity between two different reaction outcomes is considerably less common. In this Minireview we bring together examples of this concept and highlight their mechanistically distinct approaches. Our overview demonstrates how these non‐natural, photo‐switchable systems provide key fundamental mechanistic insights, enhancing our understanding and stimulating development of new catalytic activity, and how this might lead to tangible applications, impacting fields such as polymer chemistry.

Chromoselective Synthesis of Sulfonyl Chlorides and Sulfonamides with Potassium Poly(heptazine imide) Photocatalyst

Among external stimuli used to promote a chemical reaction, photocatalysis possesses a unique one-light. Photons are traceless reagents that provide an exclusive opportunity to alter chemoselectivity of the photocatalytic reaction varying the color of incident light. This strategy may be implemented by using a sensitizer capable to activate a specific reaction pathway depending on the excitation light. Herein, we use potassium poly(heptazine imide) (K-PHI), a type of carbon nitride, to generate selectively three different products from S-arylthioacetates simply varying the excitation light and otherwise identical conditions. Namely, arylchlorides are produced under UV/purple, sulfonyl chlorides with blue/white, and diaryldisulfides at green to red light. A combination of the negatively charged polyanion, highly positive potential of the valence band, presence of intraband states, ability to sensitize singlet oxygen, and multi-electron transfer is shown to enable this chromoselective conversion of thioacetates.

Photo Click Reaction of Acylsilanes with Indoles

Light-mediated coupling of acylsilanes with indoles is reported. This photo click reaction occurs under mild conditions (415 nm) mostly in quantitative yield and provides stable silylated N,O-acetals via light mediated siloxycarbene generation with subsequent indole-N-H insertion. We show that this very efficient and fully atom economic coupling process can be applied to conjugate complex systems, as documented by the clicking of carbohydrates with indole alkaloids. The method is also applicable to the conjugation of polymer chains. The linking acetal moiety can be readily cleaved and it is also shown that wavelength-selective coupling and cleavage with acyl silanes bearing a second photoactive moiety is possible. This is documented by a successful polymerization/depolymerization sequence and by a polymer folding/unfolding process.

Phenyl Silicates with Substituted Catecholate Ligands: Synthesis, Structural Studies and Reactivity

While the generation of aryl radicals by photoredox catalysis under reductive conditions is well documented, it has remained challenging under an oxidative pathway. Because of the easy photo-oxidation of alkyl bis-catecholato silicates, a general study of phenyl silicates bearing substituted catecholate ligands has been achieved. The newly synthesized phenyl silicates have been fully characterized, and their reactivity has been explored. It was found that, thanks to the substitution of the catecholate moiety, notably with the 4-cyanocatecholato ligand, the phenyl radical could be generated and trapped. Computational studies provided a rationale for these findings.

Chromoselective Photocatalysis Enables Stereocomplementary Biocatalytic Pathways*

Controlling the selectivity of a chemical reaction with external stimuli is common in thermal processes, but rare in visible-light photocatalysis. Here we show that the redox potential of a carbon nitride photocatalyst (CN-OA-m) can be tuned by changing the irradiation wavelength to generate electron holes with different oxidation potentials. This tuning was the key to realizing photo-chemo-enzymatic cascades that give either the (S)- or the (R)-enantiomer of phenylethanol. In combination with an unspecific peroxygenase from Agrocybe aegerita, green light irradiation of CN-OA-m led to the enantioselective hydroxylation of ethylbenzene to (R)-1-phenylethanol (99 % ee). In contrast, blue light irradiation triggered the photocatalytic oxidation of ethylbenzene to acetophenone, which in turn was enantioselectively reduced with an alcohol dehydrogenase from Rhodococcus ruber to form (S)-1-phenylethanol (93 % ee).

Chromoselective Photocatalysis Enables Stereocomplementary Biocatalytic Pathways

Controlling the selectivity of a chemical reaction with external stimuli is common in thermal processes, but rare in visible-light photocatalysis. Here we show that the redox potential of a carbon nitride photocatalyst (CN-OA-m) can be tuned by changing the irradiation wavelength to generate electron holes with different oxidation potentials. This tuning was the key to realizing photo-chemo-enzymatic cascades that give either the (S)- or the (R)-enantiomer of phenylethanol. In combination with an unspecific peroxygenase from Agrocybe aegerita, green light irradiation of CN-OA-m led to the enantioselective hydroxylation of ethylbenzene to (R)-1-phenylethanol (99 % ee). In contrast, blue light irradiation triggered the photocatalytic oxidation of ethylbenzene to acetophenone, which in turn was enantioselectively reduced with an alcohol dehydrogenase from Rhodococcus ruber to form (S)-1-phenylethanol (93 % ee).

Excited State Anions in Organic Transformations

Utilizing light is a smart way to fuel chemical transformations as it allows the energy to be selectively focused on certain molecules. Many reactions involving electronically excited species proceed via open-shell intermediates, which offer novel and unique routes to expand the hitherto used synthetic toolbox in organic chemistry. The direct conversion of non-prefunctionalized, less activated compounds is a highly desirable goal to pave the way towards more sustainable and atom-economic chemical processes. Photoexcited closed-shell anions have been shown to reach extreme potentials in single electron transfer reactions and reveal unusual excited-state reactivity. It is, therefore, surprising that their use as a reagent or photocatalyst is limited to a few examples. In this Review, we briefly discuss the characteristics of anionic photochemistry, highlight pioneering work, and show recent progress which has been made by utilizing photoexcited anionic species in organic synthesis.

Flow Photochemistry as a Tool in Organic Synthesis

Photochemical transformations of molecular building blocks have become an important and widely recognized research field in the past decade. Detailed and deep understanding of novel photochemical catalysts and reaction concepts with visible light as the energy source has enabled a broad application portfolio for synthetic organic chemistry. In parallel, continuous-flow chemistry and microreaction technology have become the basis for thinking and doing chemistry in a novel fashion with clear focus on improved process control for higher conversion and selectivity. As can be seen by the large number of scientific publications on flow photochemistry in the recent past, both research topics have found each other as exceptionally well-suited counterparts with high synergy by combining chemistry and technology. This review will give an overview on selected reaction classes, which represent important photochemical transformations in synthetic organic chemistry, and which benefit from mild and defined process conditions by the transfer from batch to continuous-flow mode.

Tandem Photoredox Catalysis: Enabling Carbonylative Amidation of Aryl and Alkylhalides

We report a new visible-light-mediated carbonylative amidation of aryl, heteroaryl, and alkyl halides. A tandem catalytic cycle of [Ir(ppy)₂ (dtb-bpy)]⁺ generates a potent iridium photoreductant through a second catalytic cycle in the presence of DIPEA, which productively engages aryl bromides, iodides, and even chlorides as well as primary, secondary, and tertiary alkyl iodides. The versatile in situ generated catalyst is compatible with aliphatic and aromatic amines, shows high functional-group tolerance, and enables the late-stage amidation of complex natural products.

Synthetic Photoelectrochemistry

Photoredox catalysis (PRC) and synthetic organic electrochemistry (SOE) are often considered competing technologies in organic synthesis. Their fusion has been largely overlooked. We review state-of-the-art synthetic organic photoelectrochemistry, grouping examples into three categories: 1) electrochemically mediated photoredox catalysis (e-PRC), 2) decoupled photoelectrochemistry (dPEC), and 3) interfacial photoelectrochemistry (iPEC). Such synergies prove beneficial not only for synthetic "greenness" and chemical selectivity, but also in the accumulation of energy for accessing super-oxidizing or -reducing single electron transfer (SET) agents. Opportunities and challenges in this emerging and exciting field are discussed.

Consecutive Photoinduced Electron Transfer (conPET): The Mechanism of the Photocatalyst Rhodamine 6G

The dye rhodamine 6G can act as a photocatalyst through photoinduced electron transfer. After electronic excitation with green light, rhodamine 6G takes an electron from an electron donor, such as N,N-diisopropylethylamine, and forms the rhodamine 6G radical. This radical has a reduction potential of around -0.90 V and can split phenyl iodide into iodine anions and phenyl radicals. Recently, it has been reported that photoexcitation of the radical at 420 nm splits aryl bromides into bromide anions and aryl radicals. This requires an increase in reduction potential, hence the electronically excited rhodamine 6G radical was proposed as the reducing agent. Here, we present a study of the mechanism of the formation and photoreactions of the rhodamine 6G radical by transient absorption spectroscopy in the time range from femtoseconds to minutes in combination with quantum chemical calculations. We conclude that one photon of 540 nm light produces two rhodamine 6G radicals. The lifetime of the photoexcited radicals of around 350 fs is too short to allow diffusion-controlled interaction with a substrate. A fraction of the excited radicals ionize spontaneously, presumably producing solvated electrons. This decay produces hot rhodamine 6G and hot rhodamine 6G radicals, which cool with a time constant of around 10 ps. In the absence of a substrate, the ejected electrons recombine with rhodamine 6G and recover the radical on a timescale of nanoseconds. Photocatalytic reactions occur only upon excitation of the rhodamine 6G radical, and due to its short excited-state lifetime, the electron transfer to the substrate probably takes place through the generation of solvated electrons as an additional step in the proposed photochemical mechanism.

Consecutive Charging of a Perylene Bisimide Dye by Multistep Low-Energy Solar-Light-Induced Electron Transfer Towards H2 Evolution

A photocatalytic system containing a perylene bisimide (PBI) dye as a photosensitizer anchored to titanium dioxide (TiO₂) nanoparticles through carboxyl groups was constructed. Under solar‐light irradiation in the presence of sacrificial triethanolamine (TEOA) in neutral and basic conditions (pH 8.5), a reaction cascade is initiated in which the PBI molecule first absorbs green light, giving the formation of a stable radical anion (PBI^.−), which in a second step absorbs near‐infrared light, forming a stable PBI dianion (PBI²⁻). Finally, the dianion absorbs red light and injects an electron into the TiO₂ nanoparticle that is coated with platinum co‐catalyst for hydrogen evolution. The hydrogen evolution rates (HERs) are as high as 1216 and 1022 μmol h⁻¹ g⁻¹ with simulated sunlight irradiation in neutral and basic conditions, respectively.

Multi-Photon Excitation in Photoredox Catalysis: Concepts, Applications, Methods

The energy of visible photons and the accessible redox potentials of common photocatalysts set thermodynamic limits to photochemical reactions that can be driven by traditional visible-light irradiation. UV excitation can be damaging and induce side reactions, hence visible or even near-IR light is usually preferable. Thus, photochemistry currently faces two divergent challenges, namely the desire to perform ever more thermodynamically demanding reactions with increasingly lower photon energies. The pooling of two low-energy photons can address both challenges simultaneously, and whilst multi-photon spectroscopy is well established, synthetic photoredox chemistry has only recently started to exploit multi-photon processes on the preparative scale. Herein, we have a critical look at currently developed reactions and mechanistic concepts, discuss pertinent experimental methods, and provide an outlook into possible future developments of this rapidly emerging area.

Photochemical Functionalization of Helicenes

Herein, a visible-light photochemical approach for practical helicene functionalization at very mild reaction conditions is described. The photochemical reactions allow for the regiospecific and innate late-stage functionalization of helicenes and are easily executed either through the activation of C(sp2 )-Br bonds in helicenes using K2 CO3 as inorganic base or direct C(sp2 )-H helicene bond functionalization under oxidative photoredox reaction conditions. Overall, using these transformations six different functional groups are introduced to the helicene scaffold through C-C and four different C-heteroatom bond-forming reactions.

Merging Photocatalysis with Electrochemistry: The Dawn of a new Alliance in Organic Synthesis

The merging of a homogeneous photocatalytic system with an electrochemical cell, having exchanged electrons as the only common point, has been recently demonstrated. This combination opens unexplored pathways in synthesis and allowed net-oxidative photocatalytic processes to be realized in the absence of a chemical oxidant, including: 1) the C-H alkylation of heteroarenes and 2) the coupling of azoles with arenes in the presence of an electrogenerated photocatalyst.

Neutral Organic Super Electron Donors Made Catalytic

Neutral organic super electron donors (SEDs) display impressive reducing power but, until now, it has not been possible to use them catalytically in radical chain reactions. This is because, following electron transfer, these donors form persistent radical cations that trap substrate-derived radicals. This paper unlocks a conceptually new approach to super electron donors that overcomes this issue, leading to the first catalytic neutral organic super electron donor.