UVA- and Visible-Light-Mediated Generation of Carbon Radicals from Organochlorides Using Nonmetal Photocatalyst

Catalytic Reductive Homocoupling of Benzyl Chlorides Enabled by Zirconocene and Photoredox Catalysis

The bibenzyl skeleton is prevalent in numerous natural products and other biologically active compounds. Radical homocoupling provides a straightforward approach for synthesizing bibenzyls in a single step with the reductive homocoupling of benzyl halides undergoing extensive development. Unlike benzyl bromides and other tailored precursors used in visible-light-mediated homocoupling, benzyl chlorides offer greater abundance and chemical stability. Nevertheless, achieving chemoselective cleavage of the C-Cl bond poses significant challenges, with only a limited number of studies reported to date. Herein, we demonstrate a catalytic reductive homocoupling of benzyl chlorides facilitated by zirconocene and photoredox catalysis. This cooperative catalytic system promotes C-Cl bond cleavage in benzyl chlorides under mild conditions and supports the homocoupling of a wide range of benzyl chlorides, including those derived from pharmaceutical agents. Our preliminary mechanistic investigations highlight the pivotal role of hydrosilane in the catalytic cycle.

Activation and C-C Coupling of Aryl Iodides via Bismuth Photocatalysis

Within the emerging field of bismuth redox catalysis, the catalytic formation of C-C bonds using aryl halides would be highly desirable; yet such a process remains a synthetic challenge. Herein, we present a chemoselective bismuth-photocatalyzed activation and subsequent coupling of (hetero)aryl iodides with pyrrole derivatives to access C(sp2)-C(sp2) linkages through C-H functionalization. This unique reactivity is the result of the bismuth complex featuring two redox state-dependent interactions with light, which 1) activates the Bi(I) complex for oxidative addition via MLCT, and 2) promotes the homolytic cleavage of aryl Bi(III) intermediates through a LLCT process.

Radical Replacement Process for Ligated Boryl Radical-Mediated Activation of Unactivated Alkyl Chlorides for C(sp3)-C(sp3) Bond Formation

The ligated boryl radical (LBR) has emerged as a potent tool for activating alkyl halides in radical transformations through halogen-atom transfer (XAT). However, unactivated alkyl chlorides still present an open challenge for this strategy. We herein describe a new activation mode of the LBR for the activation of unactivated alkyl chlorides to construct a C(sp3)-C(sp3) bond. Mechanistic studies reveal that the success of the protocol relies on a radical replacement process between the LBR and unactivated alkyl chloride, forming an alkyl borane intermediate as the alkyl radical precursor. Aided with the additive K3PO4, the alkyl borane then undergoes one-electron oxidation, generating an alkyl radical. The incorporation of the radical replacement activation model to activate unactivated alkyl chlorides significantly enriches LBR chemistry, which has been applied to activate alkyl iodides, alkyl bromides, and activated alkyl chlorides via XAT.

A Porous Carbazolic Al-MOF for Efficient Aerobic Photo-Oxidation of Sulfides into Sulfoxides under Air

A robust, microporous, and photoactive aluminum-based metal-organic framework (Al-MOF, LCU-600) has been assembled by an in situ-formed [Al3O(CO2)6] trinuclear building unit and a tritopic carbazole ligand. LCU-600 shows a high water stability and permanent porosity for N2 and CO2 adsorption. Notably, the incorporation of photoresponsive carbazole moieties into LCU-600 makes it a highly efficient and recyclable photocatalyst for aerobic photo-oxidation of sulfides into sulfoxides under an air atmosphere at room temperature. Mechanism investigations unveil that photogenerated holes (h+), superoxide radical anion (O2•-), and singlet oxygen (1O2) are critical active spices for the photo-oxidation reaction performed in an air atmosphere.

Photo- and electro-chemical strategies for the activations of strong chemical bonds

The employment of light and/or electricity - alternatively to conventional thermal energy - unlocks new reactivity paradigms as tools for chemical substrate activations. This leads to the development of new synthetic reactions and a vast expansion of chemical spaces. This review summarizes recent developments in photo- and/or electrochemical activation strategies for the functionalization of strong bonds - particularly carbon-heteroatom (C-X) bonds - via: (1) direct photoexcitation by high energy UV light; (2) activation via photoredox catalysis under irradiation with relatively lower energy UVA or blue light; (3) electrochemical reduction; (4) combination of photocatalysis and electrochemistry. Based on the types of the targeted C-X bonds, various transformations ranging from hydrodefunctionalization to cross-coupling are covered with detailed discussions of their reaction mechanisms.

Reductive Activation of Aryl Chlorides by Tuning the Radical Cation Properties of N-Phenylphenothiazines as Organophotoredox Catalysts

Aryl chlorides as substrates for arylations present a particular challenge for photoredox catalytic activation due to their strong C(sp2 )-Cl bond and their strong reduction potential. Electron-rich N-phenylphenothiazines, as organophotoredox catalysts, are capable of cleaving aryl chlorides simply by photoinduced electron transfer without the need for an additional electrochemical activation setup or any other advanced photocatalysis technique. Due to the extremely strong reduction potential in the excited state of the N-phenylphenothiazines the substrate scope is high and includes aryl chlorides both with electron-withdrawing and electron-donating substituents. We evidence this reactivity for photocatalytic borylations and phosphonylations. Advanced time-resolved transient absorption spectroscopy in combination with electrochemistry was the key to elucidating and comparing the unusual photophysical properties not only of the N-phenylphenothiazines, but also of their cation radicals as the central intermediates in the photocatalytic cycle. The revealed photophysics allowed the excited-state and radical-cation properties to be fine-tuned by the molecular design of the N-phenylphenothiazines; this improved the photocatalytic activity.

Electroreductive Radical Borylation of Unactivated (Hetero)Aryl Chlorides Without Light by Using Cumulene-Based Redox Mediators

Single-electron transfer (SET) plays a critical role in many chemical processes, from organic synthesis to environmental remediation. However, the selective reduction of inert substrates (Ep/2 <-2 V vs Fc/Fc+ ), such as ubiquitous electron-neutral and electron-rich (hetero)aryl chlorides, remains a major challenge. Current approaches largely rely on catalyst photoexcitation to reach the necessary deeply reducing potentials or suffer from limited substrate scopes. Herein, we demonstrate that cumulenes-organic molecules with multiple consecutive double bonds-can function as catalytic redox mediators for the electroreductive radical borylation of (hetero)aryl chlorides at relatively mild cathodic potentials (approximately -1.9 V vs. Ag/AgCl) without the need for photoirradiation. Electrochemical, spectroscopic, and computational studies support that step-wise electron transfer from reduced cumulenes to electron-neutral chloroarenes is followed by thermodynamically favorable mesolytic cleavage of the aryl radical anion to generate the desired aryl radical intermediate. Our findings will guide the development of other sustainable, purely electroreductive radical transformations of inert molecules using organic redox mediators.

Sensitizer-Free Photochemical Regeneration of Benzimidazoline Organohydride

Organohydrides are an important class of organic compounds that can provide hydride anions for chemical and biochemical reactions, as demonstrated by reduced nicotinamide adenine dinucleotides serving as important natural redox cofactors. The coupling of hydride transfer from the organohydride to the substrate and subsequent regeneration of the organohydride from its oxidized form can realize organohydride-catalyzed reduction reactions. Depending on the structure of the organohydride, its hydridicity and ease of regeneration vary. Benzimidazoline (BIH) is one of the strongest synthetic C-H hydride donors; however, its reductive regeneration requires highly reducing conditions. In this study, we synthesized various oxidized and reduced forms of BIH derivatives with aryl groups at the 2-position and investigated their photophysical and electrochemical properties. 4-(Dimethylamino)phenyl-substituted BIH exhibited salient red-shifted absorption compared with other synthesized BIH derivatives, and visible-light-driven regeneration without using an external photosensitizer was achieved. This knowledge has significant implications for the future development of solar-energy-based catalytic photoreduction technologies that utilize organohydride regeneration strategies.

Photoinduced activation of alkyl chlorides

In recent years, the activation of unactivated alkyl chlorides through light-induced processes has emerged as a promising field in radical chemistry, and has led to new transformations in organic synthesis. Direct utilization of alkyl chlorides as C(sp3)-hybridized electrophiles enables the facile construction of carbon-carbon and carbon-heteroatom bonds. Furthermore, recent studies in medicinal chemistry indicate that their presence is associated with high levels of success in clinical trials. This review summarizes the recent advances in the photoinduced activation of unactivated alkyl chlorides and discusses the mechanistic aspects underlying these reactions. We anticipate that this review will serve as a valuable resource for researchers in the field of unactivated chemical bond functionalization, and inspire considerable developments in organic chemistry, drug synthesis, materials science and other related disciplines.

Metal-Free Direct C3-H Alkylation and Arylation of Quinoxalin-2(1H)-Ones with Inert Alkyl and Aryl Chlorides

In the present manuscript, we reported the first visible-light-enabled direct C3-H alkylation/arylation of quinoxalin-2(1H)-ones with unactivated alkyl/aryl chlorides under metal-free conditions. A wide range of unactivated alkyl and aryl chlorides containing different functionalities are coupled with a variety of quinoxalin-2(1H)-one derivatives under mild reaction conditions to afford the C3-alkyl/aryl substituted quinoxalin-2(1H)-ones in moderate to good yields.

Manipulation of an electron by photoirradiation in the electron-catalyzed cross-coupling reaction

An electron has recently been shown to catalyze the cross-coupling reaction of organometallic compounds with aryl halides. In terms of green and sustainable chemistry, the electron catalysis is much more desirable than the inevitably used transition metal catalysis but a high temperature of more than 100°C is required to achieve it. Here, we disclose that visible light photoirradiation accelerates the electron-catalyzed reaction of arylzinc reagents with aryl halides with the aid of a photoredox catalysis. Photoexcitation of a photoredox catalyst and an anion radical intermediate respectively affects the supply and transfer of the electron catalyst, promoting the cross-coupling reaction to proceed at room temperature. The supply of the electron catalyst by the photoredox catalysis makes the scope of aryl halides wider.

Metal-free reduction of CO2 to formate using a photochemical organohydride-catalyst recycling strategy

Increasing levels of CO₂ in the atmosphere is a problem that must be urgently resolved if the rise in current global temperatures is to be slowed. Chemically reducing CO₂ into compounds that are useful as energy sources and carbon-based materials could be helpful in this regard. However, for the CO₂ reduction reaction (CO₂RR) to be operational on a global scale, the catalyst system must: use only renewable energy, be built from abundantly available elements and not require high-energy reactants. Although light is an attractive renewable energy source, most existing CO₂RR methods use electricity and many of the catalysts used are based on rare heavy metals. Here we present a transition-metal-free catalyst system that uses an organohydride catalyst based on benzimidazoline for the CO₂RR that can be regenerated using a carbazole photosensitizer and visible light. The system is capable of producing formate with a turnover number exceeding 8,000 and generates no other reduced products (such as H₂ and CO).

Assessing Carbazole Derivatives as Single-Electron Photoreductants

The electron-donating capabilities of carbazoles have stimulated interest in their use as photoinduced single-electron reductants. Due to the modularity of the carbazole, a further broadening and understanding of their reactivity could be achieved by manipulating the structure. Herein, eight carbazole derivatives were synthesized, characterized, and assessed as single-electron photoreductants in the hydrodehalogenation of aryl halides and the arylation of N-methylpyrrole.

Mechanism of a Luminescent Dicopper System That Facilitates Electrophotochemical Coupling of Benzyl Chlorides via a Strongly Reducing Excited State

Photochemical radical generation has become a modern staple in chemical synthesis and methodology. Herein, we detail the photochemistry of a highly reducing, highly luminescent dicopper system [Cu2] (Eox* ≈ -2.7 V vs SCE; τ0 ≈ 10 μs) within the context of a model reaction: single-electron reduction of benzyl chlorides. The dicopper system is mechanistically well defined. As we show, it is the [Cu2]* excited state that serves as the outer-sphere photoreductant of benzyl chloride substrates; the ground-state oxidized byproduct, [Cu2]+, is electrochemically recycled, demonstrating a catalytic electrophotochemical C-C coupling process.

Photoredox Activation of Inert Alkyl Chlorides for the Reductive Cross-Coupling with Aromatic Alkenes

The inertness of chloroalkanes has precluded them as coupling partners for cross-coupling reactions. Herein we disclose a general strategy for the activation of inert alkyl chlorides through photoredox catalysis and their use as coupling partners with alkenes. The catalytic system is formed by [Ni(OTf)(Py₂ ^Ts tacn)](OTf) (1_Ni ), which is responsible for the Csp³ -Cl bond activation, and [Ir(^NMe2 bpy)(ppy)₂ ]PF_6, (PC_Ir ^NMe2 ), which is the photoredox catalyst. Combined experimental and theoretical studies show an in situ photogenerated Ni^I intermediate ([Ni(Py₂ ^Ts tacn)]⁺ ) which is catalytically competent for the Csp³ -Cl bond cleavage via a S_N 2 mechanism for primary alkyl chlorides, forming carbon-centered free radicals, which react with the olefin leading to the formation of the Csp³ -Csp³ bond. These results suggest inert alkyl chlorides can be electrophiles for developing new intermolecular strategies in which low-valent aminopyridine nickel complexes act as key catalytic species.

Visible-Light Enabled C(sp3)-C(sp2) Cross-Electrophile Coupling via Synergistic Halogen-Atom Transfer (XAT) and Nickel Catalysis

We herein report the first visible-light-mediated cross-coupling of unactivated alkyl iodides with aryl bromides through synergistic halogen atom transfer (XAT) and nickel catalysis. This simple protocol operates under mild reaction conditions and tolerates a variety of functional groups affording C(sp³)-C(sp²) cross-coupling products in good to moderate yields.

Photoredox-Catalyzed C-H Functionalization Reactions

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.

Direct photo-induced reductive Heck cyclization of indoles for the efficient preparation of polycyclic indolinyl compounds

The photo-induced cleavage of C(sp2)-Cl bonds is an appealing synthetic tool in organic synthesis, but usually requires the use of high UV light, photocatalysts and/or photosensitizers. Herein is described a direct photo-induced chloroarene activation with UVA/blue LEDs that can be used in the reductive Heck cyclization of indoles and without the use of a photocatalyst or photosensitizer. The indole compounds examined display room-temperature phosphorescence. The photochemical reaction tolerates a panel of functional groups including esters, alcohols, amides, cyano and alkenes (27 examples, 50-88% yields), and can be used to prepare polycyclic compounds and perform the functionalization of natural product analogues in moderate to good yields. Mechanistic experiments, including time-resolved absorption spectroscopy, are supportive of photo-induced electron transfer between the indole substrate and DIPEA, with the formation of radical intermediates in the photo-induced dearomatization reaction.

Visible-Light-Mediated C-I Difluoroallylation with an α-Aminoalkyl Radical as a Mediator

Herein, we report a protocol for direct visible-light-mediated C-I difluoroallylation reactions of α-trifluoromethyl arylalkenes with alkyl iodides at room temperature with an α-aminoalkyl radical as a mediator. The protocol permits efficient functionalization of various α-trifluoromethyl arylalkenes with cyclic and acyclic primary, secondary, and tertiary alkyl iodides and is scalable to the gram level. This mild protocol uses an inexpensive mediator and is suitable for late-stage functionalization of complex natural products and drugs.

Sensitization-initiated electron transfer via upconversion: mechanism and photocatalytic applications

Sensitization-initiated electron transfer (SenI-ET) describes a recently discovered photoredox strategy that relies on two consecutive light absorption events, triggering a sequence of energy and electron transfer steps. The cumulative energy input from two visible photons gives access to thermodynamically demanding reactions, which would be unattainable by single excitation with visible light. For this reason, SenI-ET has become a very useful strategy in synthetic photochemistry, but the mechanism has been difficult to clarify due to its complexity. We demonstrate that SenI-ET can operate via sensitized triplet-triplet annihilation upconversion, and we provide the first direct spectroscopic evidence for the catalytically active species. In our system comprised of fac-[Ir(ppy)₃] as a light absorber, 2,7-di-tert-butylpyrene as an annihilator, and N,N-dimethylaniline as a sacrificial reductant, all photochemical reaction steps proceed with remarkable rates and efficiencies, and this system is furthermore suitable for photocatalytic aryl dehalogenations, pinacol couplings and detosylation reactions. The insights presented here are relevant for the further rational development of photoredox processes based on multi-photon excitation, and they could have important implications in the greater contexts of synthetic photochemistry and solar energy conversion.

Investigations on the Photochemical Reaction Mechanisms of Selected Dibenzoylmethane Compounds

In this work, combined time-resolved spectroscopies of femtosecond transient absorption, nanosecond transient absorption, and DFT calculations were performed to unravel the photocyclization reaction mechanisms of selected dibenzoylmethane (DBM) derivatives, including 2-chloro-1,3-diphenylpropan-1,3-dione (1a), 2-chloro-1-(3,5-dimethoxyphenyl)-3-phenylpropan-1,3-dione (1b), 2-chloro-2-fluoro-1,3-diphenylpropan-1,3-dione (1c), and 2-chloro-2-fluoro-1,3-di(4-methoxyphenyl)propan-1,3-dione (1d). Photocyclization reaction mechanisms for 1a and 1b are similar, where a C-Cl heterolysis occurs yielding an α-ketocation intermediate, followed by cyclization to generate the cation species. On the other hand, 1c and 1d undergo dechlorination primarily producing a radical species, which further experiences cyclization yielding cyclized radical species. The dominant factor leading to the different reaction mechanisms is the involvement of a fluorine atom bonded at α-C. Due to the meta-effect, the p-methoxy substitution on the benzene ring inhibits the photocyclization reaction and reduces the yield of photocyclization.

Selective photoredox direct arylations of aryl bromides in water in a microfluidic reactor

Carrying out photoredox direct arylation couplings between aryl halides and aryls in aqueous solutions of surfactants enables unprecedented selectivity with respect to the competing dehalogenation process, thanks to the partition coefficient of the selected sacrificial base. The use of a microfluidic reactor dramatically improves the reaction time, without eroding the yields and selectivity. The design of a metal free sensitizer, which also acts as the surfactant, sizeably improves the overall sustainability of arylation reactions and obviates the need for troublesome purification from traces of metal catalysts. The generality of the method is investigated over a range of halides carrying a selection of electron withdrawing and electron donating substituents.

Unveiling Potent Photooxidation Behavior of Catalytic Photoreductants

We describe a photocatalytic system that reveals latent photooxidant behavior from one of the most reducing conventional photoredox catalysts, N-phenylphenothiazine (PTH). This aerobic photochemical reaction engages difficult to oxidize feedstocks, such as benzene, in C(sp²)-N coupling reactions through direct oxidation. Mechanistic studies are consistent with activation of PTH via photooxidation and with Lewis acid cocatalysts scavenging inhibitors inextricably formed in this process.

Photophysical Properties and Redox Potentials of Photosensitizers for Organic Photoredox Transformations

Photoredox catalysis has proven to be a powerful tool in synthetic organic chemistry. The rational design of photosensitizers with improved photocatalytic performance constitutes a major advancement in photoredox organic transformations. This review summarizes the fundamental ground-state and excited-state photophysical and electrochemical attributes of molecular photosensitizers, which are important determinants of their photocatalytic reactivity.

Photocatalytic Reductive C-O Bond Cleavage of Alkyl Aryl Ethers by Using Carbazole Catalysts with Cesium Carbonate

Methods to activate the relatively stable ether C-O bonds and convert them to other functional groups are desirable. One-electron reduction of ethers is a potentially promising route to cleave the C-O bond. However, owing to the highly negative redox potential of alkyl aryl ethers (E^red < -2.6 V vs SCE), this mode of ether C-O bond activation is challenging. Herein, we report the visible-light-induced photocatalytic cleavage of the alkyl aryl ether C-O bond using a carbazole-based organic photocatalyst (PC). Both benzylic and non-benzylic aryl ethers underwent C-O bond cleavage to form the corresponding phenol products. Addition of Cs₂CO₃ was beneficial, especially in reactions using a N-H carbazole PC. The reaction was proposed to occur via single-electron transfer (SET) from the excited-state carbazole to the substrate ether. Interaction of the N-H carbazole PC with Cs₂CO₃ via hydrogen bonding exists, which enables a deprotonation-assisted electron-transfer mechanism to operate. In addition, the Lewis acidic Cs cation interacts with the substrate alkyl aryl ether to activate it as an electron acceptor. The high reducing ability of the carbazole combined with the beneficial effects of Cs₂CO₃ made this otherwise formidable SET event possible.

Photoredox catalysis on unactivated substrates with strongly reducing iridium photosensitizers

Photoredox catalysis has emerged as a powerful strategy in synthetic organic chemistry, but substrates that are difficult to reduce either require complex reaction conditions or are not amenable at all to photoredox transformations. In this work, we show that strong bis-cyclometalated iridium photoreductants with electron-rich β-diketiminate (NacNac) ancillary ligands enable high-yielding photoredox transformations of challenging substrates with very simple reaction conditions that require only a single sacrificial reagent. Using blue or green visible-light activation we demonstrate a variety of reactions, which include hydrodehalogenation, cyclization, intramolecular radical addition, and prenylation via radical-mediated pathways, with optimized conditions that only require the photocatalyst and a sacrificial reductant/hydrogen atom donor. Many of these reactions involve organobromide and organochloride substrates which in the past have had limited utility in photoredox catalysis. This work paves the way for the continued expansion of the substrate scope in photoredox catalysis.

Studies of hydrogen isotope scrambling during the dehalogenation of aromatic chloro-compounds with deuterium gas over palladium catalysts

Catalytic dehalogenation of aromatic halides using isotopic hydrogen gas is an important strategy for labelling pharmaceuticals, biochemicals, environmental agents and so forth. To extend, improve and further understand this process, studies have been carried out on the scrambling of deuterium isotope with protium during the catalytic deuterodehalogenation of model aryl chlorides using deuterium gas and a palladium on carbon catalyst in tetrahydrofuran solution. The degree of scrambling was greatest with electron-rich chloroarene rings. The tetrahydrofuran solvent and the triethylamine base were not the source of the undesired protium; instead, it arose, substantially, from the water content of the catalyst, though other sources of protium may also be present on the catalyst. Replacement of the Pd/C catalyst with one prepared in situ by reduction of palladium trifluoroacetate with deuterium gas and dispersed upon micronised polytetrafluoroethylene led to much reduced scrambling (typically 0-6% compared with up to 40% for palladium on carbon) and to high atom% abundance, regiospecific labelling. The improved catalytic system now enables efficient polydeuteration via the dehalogenation of polyhalogenated precursors, making the procedure viable for the preparation of MS internal standards and, potentially, for high specific activity tritium labelling.

A case of chain propagation: α-aminoalkyl radicals as initiators for aryl radical chemistry

The generation of aryl radicals from the corresponding halides by redox chemistry is generally considered a difficult task due to their highly negative reduction potentials. Here we demonstrate that α-aminoalkyl radicals can be used as both initiators and chain-carriers for the radical coupling of aryl halides with pyrrole derivatives, a transformation often employed to evaluate new highly reducing photocatalysts. This mode of reactivity obviates for the use of strong reducing species and was also competent in the formation of sp² C-P bonds. Mechanistic studies have delineated some of the key features operating that trigger aryl radical generation and also propagate the chain process.

Photo-triggered hydrogen atom transfer from an iridium hydride complex to unactivated olefins

Many photoactive metal complexes can act as electron donors or acceptors upon photoexcitation, but hydrogen atom transfer (HAT) reactivity is rare. We discovered that a typical representative of a widely used class of iridium hydride complexes acts as an H-atom donor to unactivated olefins upon irradiation at 470 nm in the presence of tertiary alkyl amines as sacrificial electron and proton sources. The catalytic hydrogenation of simple olefins served as a test ground to establish this new photo-reactivity of iridium hydrides. Substrates that are very difficult to activate by photoinduced electron transfer were readily hydrogenated, and structure-reactivity relationships established with 12 different olefins are in line with typical HAT reactivity, reflecting the relative stabilities of radical intermediates formed by HAT. Radical clock, H/D isotope labeling, and transient absorption experiments provide further mechanistic insight and corroborate the interpretation of the overall reactivity in terms of photo-triggered hydrogen atom transfer (photo-HAT). The catalytically active species is identified as an Ir(ii) hydride with an IrII-H bond dissociation free energy around 44 kcal mol-1, which is formed after reductive 3MLCT excited-state quenching of the corresponding Ir(iii) hydride, i.e. the actual HAT step occurs on the ground-state potential energy surface. The photo-HAT reactivity presented here represents a conceptually novel approach to photocatalysis with metal complexes, which is fundamentally different from the many prior studies relying on photoinduced electron transfer.

Aryl dechlorination and defluorination with an organic super-photoreductant

Direct excitation of the commercially available super-electron donor tetrakis(dimethylamino)ethylene (TDAE) with light-emitting diodes at 440 or 390 nm provides a stoichiometric reductant that is able to reduce aryl chlorides and fluorides. The method is very simple and requires only TDAE, substrate, and solvent at room temperature. The photoactive excited state of TDAE has a lifetime of 17.3 ns in cyclohexane at room temperature and an oxidation potential of ca.-3.4 V vs. SCE. This makes TDAE one of the strongest photoreductants able to operate on the basis of single excitation with visible photons. Direct substrate activation occurs in benzene, but acetone is reduced by photoexcited TDAE and substrate reduction takes place by a previously unexplored solvent radical anion mechanism. Our work shows that solvent can have a leveling effect on the photochemically available redox power, reminiscent of the pH-leveling effect that solvent has in acid-base chemistry.

Carbazole based Electron Donor Acceptor (EDA) catalysis for the synthesis of biaryl and aryl-heteroaryl compounds

A highly regioselective, carbazole based Electron Donor Acceptor (EDA) catalyzed synthesis of biaryl and aryl-heteroaryl compounds is described. Various indole and carbazole derivatives were screened for the Homolytic Aromatic Substitution (HAS) reaction. Tetrahydrocarbazole (THC) was very efficient for the HAS transformation and proceeded via a complex formation between diazonium salt and electron rich tetrahydrocarbazole. The UV-Vis spectroscopy technique has been used to confirm the complex formation. The in situ generated EDA complex even in a catalytic amount is found to be efficient for the Single Electron Transfer (SET) process without any photoactivation. Biaryl compounds, 2-phenylfuran, 2-phenylthiophene, and 2-phenylpyrrole and bioactive compounds such as dantrolene and canagliflozin have been synthesized in moderate to excellent yields.

Potent Reductants via Electron-Primed Photoredox Catalysis: Unlocking Aryl Chlorides for Radical Coupling

We describe a new catalytic strategy to transcend the energetic limitations of visible light by electrochemically priming a photocatalyst prior to excitation. This new catalytic system is able to productively engage aryl chlorides with reduction potentials hundreds of millivolts beyond the potential of Na0 in productive radical coupling reactions. The aryl radicals produced via this strategy can be leveraged for both carbon-carbon and carbon-heteroatom bond-forming reactions. Through direct comparison, we illustrate the reactivity and selectivity advantages of this approach relative to electrolysis and photoredox catalysis.

Laser-Induced Wurtz-Type Syntheses with a Metal-Free Photoredox Catalytic Source of Hydrated Electrons

Upon irradiation with ns laser pulses at 355 nm, 2-aminoanthracene in SDS micelles readily produces hydrated electrons. These "super-reductants" rapidly attack substrates such as chloro-organics and convert them into carbon-centred radicals through dissociative electron transfer. For a catalytic cycle, the aminoanthracene needs to be restored from its photoionization by-product, the radical cation, by a sacrificial donor. The ascorbate monoanion can only achieve this across the micelle-water interface, but the monoanion of ascorbyl palmitate results in a fully micelle-contained regenerative electron source. The shielding by the micelle in the latter case not only increases the life of the catalyst but also strongly suppresses the interception of the carbon-centred radicals by the hydrogen-donating ascorbate moiety; and in conjunction with the high local concentrations effected by the pulsed laser, termination by radical dimerization thus dominates. We have obtained a complete and consistent picture through monitoring the individual steps and the assembled system by flash photolysis on fast and slow timescales, from microseconds to minutes; and in preparative studies on a variety of substrates, we have achieved up to quantitative dimerization with a turnover on the order of 1 mmol per hour.

Catalytic Strategy for Regioselective Arylethylamine Synthesis

A mild, modular, and practical catalytic system for the synthesis of the highly privileged phenethylamine pharmacophore is reported. Using a unique combination of organic catalysts to promote the transfer of electrons and hydrogen atoms, this system performs direct hydroarylation of vinyl amine derivatives with a wide range of aryl halides (including aryl chlorides). This general and highly chemoselective protocol delivers a broad range of arylethylamine products with complete regiocontrol. The utility of this process is highlighted by its scalability and the modular synthesis of an array of bioactive small molecules.

Utilising excited state organic anions for photoredox catalysis: activation of (hetero)aryl chlorides by visible light-absorbing 9-anthrolate anions

A novel photocatalytic concept based on photoexcitation of an organic anionic ground state catalyst for direct C–H (het)arylations using (het)aryl chlorides.