Visible Light Photoredox Catalysts Research Articles

Carbon Dot as Visible-Light Photoredox Catalysts for a Myriad of Organic Transformations.

Metal-free catalysts for various organic transformations are of high demand now. In this study, we present a new carbon dot as an efficient metal-free nanophotocatalyst for carrying out a series of organic bond formation reactions. Using a single photocatalyst carbon dot, Csp2-Csp2, Csp2-B, Csp2-S, Csp2-Se, and C-P bond formation reactions were performed with a high yield of the corresponding products. Moreover, Csp2-H activation of the aromatic ring was achieved by merging the carbon dot photocatalyst with a transition metal. Interestingly, these carbon nanodot-based catalysts show good recyclability a few times without any significant loss of catalytic activity. The development of catalytic systems based on carbon dots has its merits vested in the advantageous properties of this nanomaterial, such as a robust chemical nature and cheap cost of preparation. This report demonstrates that a carbon dot indeed holds the potential to replace expensive metal-based catalysts as well as organic dyes in five different photoredox reactions.

Photochemical Sonogashira coupling reactions: beyond traditional palladium-copper catalysis.

Sonogashira coupling is one of the Nobel reactions discovered in 1975 to form a C-C bond using palladium and copper as co-catalysts. Incorporating alkyne functionalities either in macro or micro molecules by using this Sonogashira reaction has already proven its viability and relevance in the sphere of synthetic chemistry. While aiming for sustainable chemistry, in recent years, visible light photoredox catalysts have appeared as an advanced tool in this regard. In this review, we aim to portray a comprehensive summary of modern visible light photo redox catalyzed Sonogashira reaction, which will leave space for the readers to rethink alternative strategies to conduct the Sonogashira reaction. This review briefly describes the implementation of various metal-based nanomaterial photocatalysts, developing either copper or palladium-free photocatalytic methods, and organo-photolytic and bioinspired photocatalysts for the Sonogashira coupling reactions. Besides, this review also gives a concise overview of the mechanistic aspects and highlights selective examples for substrate scope.

Alternatives to Iridium: A Polyaza[7]helicene as a Strongly Reductive Visible Light Photoredox Catalyst.

The use of a readily accessible polyazahelicene as a strongly reducing metal-free alternative to the commonly used precious metal based photoredox catalysts is demonstrated. An improved two-step synthesis of the catalyst is described, and its photophysical properties with respect to its use as a photoredox catalyst are evaluated. Its activity under visible light irradiation is proven by application in two double radical light-driven multicomponent reactions. The azahelicene gave comparable results to an iridium-based catalyst originally used for the same transformations.

Enhanced synergistic catalysis by a light-harvesting binary organic dyes system based on FRET for cross-dehydrogenative-coupling reaction

Rose bengal (RB) showed photocatalytic performance for cross dehydrogenative-coupling (CDC) reaction. However, single organic dye only absorbs light of specific wavelength which results a lower light utilization efficiency. In this paper, we proposed a simple and effective approach based on Förster resonance energy transfer (FRET), to improve the performance of RB through the incorporation of rhodamine 6G (Rh6G) into RB solution and constructing a binary dyes system rhodamine 6G-rose bengal (Rh6G-RB). The energy transfer efficiency and antenna effect of Rh6G-RB were up to 58% and 10.8, respectively. The synergistic effect of RB and Rh6G exhibited excellent catalytic performance under low concentration (10−5 M). The catalytic rate constant (kd) for Rh6G-RB reached 0.026 mol/(L·min) at 35 °C for aza-Henry reaction of tertiary amines with nitromethane, which was 2.03 and 5.52 times higher than that of single RB and Rh6G respectively. Moreover, the synergy of Rh6G and RB was also observed in Mannich reaction of N-tertiary amine derivatives with acetone, which all afforded the coupling products with good to excellent conversions. Finally, the Rh6G-RB system was also demonstrated an efficient visible light photoredox catalyst in continuous flow fashion. The research opened a new avenue and convenient for developing of binary or ternary organic dyes systems based on FRET and advance the application of these dye systems in photocatalytic organic synthesis.

Polynuclear heteroleptic ruthenium(II) photoredox catalysts: Evaluation in blue-light-mediated, regioselective thiol-ene reactions

Polynuclear heteroleptic ruthenium(II) photosensitisers combining either imine or amine-functionalised bipyridyl ligands were synthesised (via Schiff-base condensation/reductive amination reactions), characterised and investigated for their photoreactivity in the hydrothiolation reaction. Furthermore, electrochemical, electronic absorption and emission studies of the complexes were conducted. All the ligand-modified, heteroleptic complexes show red-shifted emission spectra (614–633 nm) relative to the canonical [Ru(bpy)3](PF6)2 complex (609 nm), attributed to the transition from the triplet MLCT excited state (3MLCT) to the ground state. The complexes were evaluated as visible-light photoredox catalysts in the radical hydrothiolation reaction of olefins (thiol-ene coupling) to afford thioethers. Control reactions performed in the absence of the photocatalyst resulted in either significantly lower yields (6%) or no product formation, demonstrating the role of the complexes as photoredox catalysts. The reactions carried out using trinuclear complexes (Ered (Ru2+*/+) = +0.300 V vs Ag/Ag+) resulted in increased yields in comparison with their respective mononuclear congeners and greater than those reported for [Ru(bpy)3](PF6)2, demonstrating the benefits of using polynuclear photocatalysts for photoinitiated redox catalysis reactions.

Oxime‐Derived Iminyl Radicals in Selective Processes of Hydrogen Atom Transfer and Addition to Carbon‐Carbon π‐Bonds

AbstractOximes represent one of the fundamental organic compound classes with a wide range of synthetic applications. In the last decade O‐substituted oximes were recognized as the synthetically available and versatile precursors of iminyl radicals via one‐electron oxidation or one‐electron reduction employing visible light photoredox catalysts, salts of abundant metals (such as Cu or Fe), or other convenient reagents. Iminyl radicals are powerful synthons for various processes of cyclization, ring‐opening, CH‐functionalization, and coupling. The present review is focused on the synthetic methods based on oxime‐derived iminyl radicals developed in the last few years excluding ring opening reactions of cyclic iminyl radicals that were summarized in recent publications. The review consists of two main parts: (1) reactions of iminyl radicals involving 1,n‐hydrogen atom transfer (n=5 in most cases) and (2) reactions involving the addition of iminyl radical to the carbon‐carbon π‐bond.magnified image

Polysulfide Anions as Visible Light Photoredox Catalysts for Aryl Cross-Couplings.

Polysulfide anions are endowed with unique redox properties, attracting considerable attentions for their applications in alkali metals-sulfur batteries. However, the employment of these anionic species in redox catalysis for small molecule synthesis remains underdeveloped due to their moderate-poor electrochemical potential in the ground state, whereas some of them are characterized by photoabsorptions in visible spectral regions. Herein, we disclose the use of polysulfide anions as visible light photoredox catalysts for aryl cross-coupling reactions. The reaction design enables single-electron reduction of aryl halides upon the photoexcitation of tetrasulfide dianions (S42-). The resulting aryl radicals are engaged in (hetero)biaryl cross-coupling, borylation, and hydrogenation in a redox catalytic regime involving S4• -/S42- and S3• -/S32- redox couples.

Light Runs Across Iron Catalysts in Organic Transformations.

Over the past decades, organometallic complexes with precious elements, such as ruthenium and iridium, are widely used as visible-light photoredox catalysts. Recently, more and more complexes based on earth-abundant and inexpensive elements have been used as sensitizers in photochemistry. Although the photoexcited state lifetimes of iron complexes are typically shorter than those of traditional photosensitizers, the utilization of iron catalysts in photochemistry has sprung up owing to their abundance, low price, nontoxicity, and novel properties, including exhibiting ligand to metal charge transfer states. This concept focuses on recent advances in light-driven iron catalysis in organic transformations, including iron/photoredox dual catalysis, light-induced iron photoredox catalysis and light-induced generation of active iron catalysts. The prospect for the future of this field is also discussed.

Lead-halides Perovskite Visible Light Photoredox Catalysts for Organic Synthesis.

Lead-halide perovskites has grabbed great attention in recent years due to its unique photophysical and electrical properties. It has open new window in organic solar cell application for commercial point. Research has extended its application further towards visible light photoredox catalysts for organic synthesis. Due to intrinsic redox properties, sharp absorption and emissions patterns makes lead-halide perovskites as promising photoredox catalyst. In the presence of light, perovskite absorbs light, generates electrons/holes, which can reduce or oxidize species at reactive centres of organic molecule, leads to organic transformation. Lead-halide perovskites are used as heterogeneous catalysts for small molecules activations like CO, CH4 and NO, also suitable photocatalysts for organic bond formations, oxidations, reductions, polymerizations and dimerization transformations. Recent literatures have set another milestone for perovskite materials in organic synthesis. This review provides information on lead-halides perovskite in visible-light photoredox catalysis such as C-C, C-X (X-halo, hetero atom) bond formations C-H arylation.

Iridium-Based Visible-Light Photoredox Catalyst for Radical Alkylation

Iridium-Based Visible-Light Photoredox Catalyst for Radical Alkylation

Hantzsch Ester as a Visible-Light Photoredox Catalyst for Transition-Metal-Free Coupling of Arylhalides and Arylsulfinates.

Diethyl 2,6-dimethyl-1,4-dihydropyridine-3,5-dicarboxylate (HEH) has been utilized as a visible-light photoredox catalyst for the cross coupling of arylhalides and arylsulfinates without transition metal, sacrificial agent, and mediator. This method is compatible with various functional groups and provides diaryl sulfones in good to high yields. Mechanistic studies indicate that this reaction undergoes the stepwise light irradiation of HE- , single electron transfer (SET) in donor-acceptor complex (DAC) from *HE- to arylhalide, trapping of aryl radical with sulfinate, and SET oxidation of sulfone radical anion by HE. to sulfone by the DAC method.

Visible-Light-Induced Oxidation/[3 + 2] Cycloaddition/Oxidative Aromatization to Construct Benzo[a]carbazoles from 1,2,3,4-Tetrahydronaphthalene and Arylhydrazine Hydrochlorides.

An efficient synthesis of benzo[a]carbazoles via visible-light-induced tandem oxidation/[3 + 2] cycloaddition/oxidative aromatization reactions was reported. The benzylic C(sp3)-H of tetrahydronaphthalene was activated through visible-light photoredox catalyst with oxygen as the clean oxidant under mild reaction conditions. This protocol proceeds efficiently with broad substrate scope, and the mechanism study was performed.

Naphthalene diimides with improved solubility for visible light photoredox catalysis.

Five core-substituted naphthalene diimides bearing two dialkylamino groups were synthesized as potential visible light photoredox catalysts and characterized by methods of optical spectroscopy and electrochemistry in comparison with one unsubstituted naphthalene diimide as reference. The core-substituted naphthalene diimides differ by the alkyl groups at the imide nitrogens and at the nitrogens of the two substituents at the core in order to enhance their solubility in DMF and thereby enhance their photoredox catalytic potential. The 1-ethylpropyl group as rather short and branched alkyl substituent at the imide nitrogen and the n-propyl group as short and unbranched one at the core amines yielded the best solubilities. The electron-donating diaminoalkyl substituents together with the electron-deficient aromatic core of the naphthalene diimides increase the charge-transfer character of their photoexcited states and thus shift their absorption into the visible light (500–650 nm). The excited state reduction potential was estimated to be approximately +1.0 V (vs SCE) which is sufficient to photocatalyze typical organic reactions. The photoredox catalytic activity in the visible light range was tested by the α-alkylation of 1-octanal as benchmark reaction. Irradiations were performed with LEDs in the visible light range between 520 nm and 640 nm. The irradiation by visible light together with the use of an organic dye instead of a transition metal complex as photoredox catalyst improve the sustainability and make photoredox catalysis “greener”.

Photoredox‐Catalyzed Tandem Demethylation of N,N‐Dimethyl Anilines Followed by Amidation with α‐Keto or Alkynyl Carboxylic Acids

AbstractWe report herein, a biomimetic approach for highly selective monodemethylation of N,N‐dimethyl anilines to generate secondary amines and subsequent coupling with α‐ketocarboxylic acids or alkynyl carboxylic acids to form α‐ketoamides or alkynamides respectively under visible light photoredox catalyst in a single operation. From the deuterium‐labeling experiment, it was probed that demethylation is the slowest step in this tandem process. Whereas, control experiments and spectroscopic studies revealed that photoredox catalyst is also involved in the subsequent amidation step. The reaction proceeds smoothly at room temperature providing moderate to excellent yield of the coupling products. The amides have also been converted to a series of biologically active spiro compounds.magnified image

Synthesis of Isoxazolidines by Intramolecular Hydroamination of N-Alkoxyamides in the Presence of a Visible-Light Photoredox Catalyst

Abstract N-Acyl isoxazolidines were obtained in moderate to good yields by intramolecular hydroamination of N-alkoxyamides in the presence of a ruthenium photocatalyst. In this reaction, the N-alkoxyamide anion generated by deprotonation undergoes photocatalyzed single-electron-transfer (SET) oxidation to generate the corresponding radical, which cyclizes to afford the isoxazolidine ring. Notably, this method was applicable to a macrocyclic substrate, affording the corresponding 12-membered macrocycle-containing product.

Oxidative Deprotection of p-Methoxybenzyl Ethers via Metal-Free Photoredox Catalysis.

An efficient and greener deprotection method for p-methoxybenzyl (PMB) ethers using a metal-free visible light photoredox catalyst and air and ammonium persulfate as the terminal oxidants is presented. Various functional groups and protecting groups were tolerated in the developed method to achieve good to excellent yields in short reaction times. Significantly, the developed method was compatible with PMB ethers derived from primary, secondary, and tertiary alcohols and a gram-scale reaction. Mechanistic studies support a proposed reaction mechanism that involves single electron oxidation of the PMB ether.

Semiconductors as heterogeneous visible light photoredox catalysts in combined dual metal catalyzed C–H functionalizations

A protocol for C–H olefination with heterogeneous photocatalysts and visible light together with transition metals has been developed.

9,10-Phenanthrenedione as Visible-Light Photoredox Catalyst: A Green Methodology for the Functionalization of 3,4-Dihydro-1,4-Benzoxazin-2-Ones through a Friedel-Crafts Reaction

A visible-light photoredox functionalization of 3,4-dihydro-1,4-benzoxazin-2-ones through a Friedel-Crafts reaction with indoles using an inexpensive organophotoredox catalyst is described. The reaction uses a dual catalytic system that is formed by a photocatalyst simple and cheap, 9,10-phenanthrenedione, and a Lewis acid, Zn(OTf)2. 5W white LEDs are used as visible-light source and oxygen from air as a terminal oxidant, obtaining the corresponding products with good yields. The reaction can be extended to other electron-rich arenes. Our methodology represents one of the most valuable and sustainable approach for the functionalization of 3,4-dihydro-1,4-benzoxazin-2-ones, as compared to the reported procedures. Furthermore, several transformations were carried out, such as the synthesis of the natural product cephalandole A and a tryptophol derivative.

Recyclable alkylated fac-Ir(ppy)3 complex as a visible-light photoredox catalyst for the synthesis of 3-trifluoromethylated and 3-difluoroacetylated coumarins

A recyclable fac-tris(heptadecanyl-2-phenylpyridine) iridium [fac-Ir(hdppy)3] photocatalyst was synthesized. The hexane-phase-selective solubility of fac-Ir(hdppy)3 in a thermomorphic multicomponent solvent (TMS) allowed its easy recycling by automatic liquid/liquid separation at room temperature. The excellent catalytic and recoverable activities of fac-Ir(hdppy)3 were demonstrated via trifuoromethylation and difluoroacetylation reactions of phenyl 3-phenylpropiolate under visible-light irradiation in a TMS system.

Highly efficient organic photocatalysts discovered via a computer-aided-design strategy for visible-light-driven atom transfer radical polymerization

Organocatalysed photoredox-mediated atom transfer radical polymerization (O-ATRP) is a very promising polymerization method as it eliminates concerns associated with transition-metal contamination of polymer products. However, reducing the amount of catalyst and expanding the monomer scope remain major challenges in O-ATRP. Herein, we report a systematic computer-aided-design strategy to identify powerful visible-light photoredox catalysts for O-ATRP. One of our discovered organic photoredox catalysts controls the polymerization of methyl methacrylate at sub-ppm catalyst loadings (0.5 ppm—a very meaningful amount enabling the direct use of polymers without a catalyst removal process); that is, 100–1,000 times lower loadings than other organic photoredox catalysts reported so far. Another organic photoredox catalyst with supra-reducing power in an excited state and high redox stability facilitates the challenging polymerization of the non-acrylic monomer styrene, which is not successful using existing photoredox catalysts. This work provides access to diverse challenging organic/polymer syntheses and makes O-ATRP viable for many industrial and biomedical applications. Organocatalysed photoredox-mediated atom transfer radical polymerization is a very promising method, although many challenges still lie ahead. Now, Kwon, Gierschner, Kim and co-workers present a computer-aided-design strategy to identify organic photoredox catalysts for this process. The success of the design strategy is demonstrated by polymerizations of methyl methacrylate and styrene.