Visible‐Light‐Induced Photoredox Catalysis of Dye‐Sensitized Titanium Dioxide: Selective Aerobic Oxidation of Organic Sulfides

TiO₂ photoredox catalysis has recently attracted much interest for use in performing challenging organic transformations under mild reaction conditions. However, the reaction scheme is hampered by the fact that TiO₂ can only be excited by UV light of wavelengths λ shorter than 385 nm. One promising strategy to overcome this issue is to anchor an organic, preferably metal‐free dye onto the surface of TiO₂. Importantly, we observed that the introduction of a catalytic amount of the redox mediator TEMPO [(2,2,6,6‐tetramethylpiperidin‐1‐yl)oxyl] ensured the stability of the anchored dye, alizarin red S, thereby resulting in the selective oxidation of organic sulfides with O₂. This result affirms the essential role of the redox mediator in enabling the organic transformations by visible‐light photoredox catalysis.     

Extending aromatic acids on TiO2 for cooperative photocatalysis with triethylamine: Violet light-induced selective aerobic oxidation of sulfides

Designing visible light photocatalysts with a metal oxide semiconductor as the starting material could expand a new horizon for the conversion and storage of solar energy. Here, the benchmark photocatalyst TiO₂ was used to pursue this goal by anchoring aromatic acids. Extending the aromatic acid was strategically deployed to design TiO₂ complexes with violet light-induced selective aerobic oxidation of sulfide as the probe reaction. With benzoic acid (BA) as the initial molecule, horizontally extending one or two benzene rings furnishes 2-naphthoic acid (2-NA) and 2-anthracene acid (2-AA). Moreover, triethylamine (TEA), an electron transfer mediator, was introduced to maintain the integrity of the anchored aromatic acids. Notably, there was a direct correlation between the π-conjugation of aromatic acid ligand and the selective aerobic oxidation of sulfides. Among the three aromatic acids, 2-AA delivered the best result over TiO₂ due to the most extensive π-conjugated system. Ultimately, violet light-induced selective aerobic oxidation of sulfides into corresponding sulfoxides was conveniently realized by cooperative photocatalysis of 2-AA-TiO₂ with 10 mol% of TEA. This work affords an extending strategy for designing the next-generation ligands for semiconductors to expand visible light-induced selective reactions.     

The Morita–Baylis–Hillman reaction for non-electron-deficient olefins enabled by photoredox catalysis

A strategy for overcoming the limitation of the Morita–Baylis–Hillman (MBH) reaction, which is only applicable to electron-deficient olefins, has been achieved via visible-light induced photoredox catalysis in this report. A series of non-electron-deficient olefins underwent the MBH reaction smoothly via a novel photoredox-quinuclidine dual catalysis. The in situ formed key β-quinuclidinium radical intermediates, derived from the addition of olefins with quinuclidinium radical cations, are used to enable the MBH reaction of non-electron-deficient olefins. On the basis of previous reports, a plausible mechanism is suggested. Mechanistic studies, such as radical probe experiments and density functional theory (DFT) calculations, were also conducted to support our proposed reaction pathways.

A strategy for overcoming the limitation of the Morita–Baylis–Hillman (MBH) reaction, which is only applicable to electron-deficient olefins, has been achieved via visible-light induced photoredox catalysis in this report.     

Thiourea dioxide (TUD): a robust organocatalyst for oxidation of sulfides to sulfoxides with TBHP under mild reaction conditions

Thiourea dioxide (TUD) has been reported for the first time as a highly efficient and selective organocatalyst for the oxidation of sulfides to sulfoxides in high to excellent yields by using tert-butylhydroperoxide as oxidant under mild reaction conditions.     

Electro- and acid-catalysis in tetrafluoroborate-based ionic liquid: new alternative routes for the oxidation of sulfides with UHP

By means of anodic activation, BF₄-based ionic liquid was found to play the triple role of electrolysis/reaction medium, supporting electrolyte and pre-catalyst versus the oxidation of sulfides with UHP. Galvanostatic electrolysis in the presence of substrate and UHP molecules allowed the fast, efficient and selective achievement of sulfoxides. Comparable results have been attained by acid catalysis based on R-CSA.     

Carbon–sulfur bond formation via photochemical strategies: An efficient method for the synthesis of sulfur-containing compounds

The development of green and convenient methods for C–S bond formation has received significant attention because C–S bond widely occurs in many important pharmaceutical and biological compounds.Recently, visible-light photoredox catalysis has been established as an efficient and general tool for the construction of C–C and C-heteroatom bonds. In this review, we have focused on the research on recent advances in C–S bond formation via visible-light photoredox catalysis, and the growing opportuni...     

Operando X-ray absorption and EPR evidence for a single electron redox process in copper catalysis

An unprecedented single electron redox process in copper catalysis is confirmed using operando X-ray absorption and EPR spectroscopies. The oxidation state of the copper species in the interaction between Cu(ii) and a sulfinic acid at room temperature, and the accurate characterization of the formed Cu(i) are clearly shown using operando X-ray absorption and EPR evidence. Further investigation of anion effects on Cu(ii) discloses that bromine ions can dramatically increase the rate of the redox process. Moreover, it is proven that the sulfinic acids are converted into sulfonyl radicals, which can be trapped by 2-arylacrylic acids and various valuable β-keto sulfones are synthesized with good to excellent yields under mild conditions.     

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.

Strong bis-cyclometalated iridium photoreductants, in combination with a single sacrificial reductant, enable visible-light-promoted reductive activation of a variety of challenging substrates under simple and general reaction conditions.     

Sequential C–O decarboxylative vinylation/C–H arylation of cyclic oxalates via a nickel-catalyzed multicomponent radical cascade

A selective, sequential C–O decarboxylative vinylation/C–H arylation of cyclic alcohol derivatives enabled by visible-light photoredox/nickel dual catalysis is described. This protocol utilizes a multicomponent radical cascade process, i.e. decarboxylative vinylation/1,5-HAT/aryl cross-coupling, to achieve efficient, site-selective dual-functionalization of saturated cyclic hydrocarbons in one single operation. This synergistic protocol provides straightforward access to sp³-enriched scaffolds and an alternative retrosynthetic disconnection to diversely functionalized saturated ring systems from the simple starting materials.

A selective, sequential C–O decarboxylative vinylation/C–H arylation of cyclic alcohol derivatives enabled by visible-light photoredox/nickel dual catalysis has been described.     

Selective and efficient oxidation of sulfides to sulfoxides with N-bromosuccinimide in the presence of β-cyclodextrin in water

A simple and highly selective oxidation of sulfides to sulfoxides with N-bromosuccinimide (NBS) catalyzed by β-cyclodextrin in water has been developed. A series of sulfides were oxidized selectively at room temperature in excellent yields. This reaction proceeds without over-oxidation to sulfones under mild conditions using water as a solvent and has many advantages over the existing methodologies.     

Evidence of Two Key Intermediates Contributing to the Selectivity of P450‐Biomimetic Oxidation of Sulfides to Sulfoxides and Sulfones

A mild process for the selective oxidation of sulfides is in great demand. Therefore, probing the mechanism underlying the biological oxidation of sulfides under ambient conditions may provide valuable insights for the development of such a reaction. Based on porphyrin models of P450 enzymes, evidence of two key intermediates, Int0 and Int1 , in this reaction is provided. Spectroscopic studies indicated the formation of a hydroperoxide‐iron(III) species ( Int0 ) upon addition of H₂O₂. This intermediate proved to be highly selective for sulfoxide production. By contrast, a defined porphyrin oxoiron(IV) cation radical ( Int1 ) directly reacted with sulfoxides, leading selectively to the corresponding sulfones. Interestingly, the available sulfoxides reversibly act as a new axial ligand for Int0 forming a more active species Int0 _SO. The amount of Int0 increased in the presence of alkyl, aryl, or aromatic sulfides, while Int1 formed in the absence of these sulfides. Thus, sulfoxides and sulfones would selectively form under conditions that favor the corresponding intermediates, which elucidate the biological oxidation pathway.     

Synergistic cascade catalysis by metal nanoparticles and Lewis acids in hydrogen autotransfer

Of the many types of catalysis involving two or more catalysts, synergistic catalysis is of great interest because novel reactions or reaction pathways may be discovered when there is synergy between the catalysts. Herein, we describe a synergistic cascade catalysis, in which immobilized Au/Pd bimetallic nanoparticles and Lewis acids work in tandem to achieve the N-alkylation of primary amides to secondary amides with alcohols via hydrogen autotransfer. When Au/Pd nanoparticles were used with metal triflates, a significant rate acceleration was observed, and the desired secondary amides were obtained in excellent yields. The metal triflate is thought to not only facilitate the addition of primary amides to aldehydes generated in situ, but also enhance the returning of hydrogen from nanoparticles to hydrogen-accepting intermediates. This resulted in a more rapid turnover of the nanoparticle catalyst, and ultimately translated into an increase in the overall rate of the reaction. The two catalysts in this co-catalytic system work in a synergistic and cascade fashion, resulting in an efficient hydrogen autotransfer process.     

Hydrogenation of Sulfoxides to Sulfides under Mild Conditions Using Ruthenium Nanoparticle Catalysts

The first demonstration of the hydrogenation of sulfoxides under atmospheric H₂ pressure is reported. The highly efficient reaction is facilitated by a heterogeneous Ru nanoparticle catalyst. The mild reaction conditions enable the selective hydrogenation of a wide range of functionalized sulfoxides to the corresponding sulfides. The high redox ability of RuO_x nanoparticles plays a key role in the hydrogenation.     

12-Tungstocobaltate(II) catalyzed selective oxidation of sulfides to sulfoxides using aqueous hydrogen peroxide under solvent free conditions

12-Tungstocobaltate(II) is found to be a selective, recyclable catalyst for the oxidation of sulfides into sulfoxides. The reaction was carried out under solvent free conditions using 30% H₂O₂.     

Titanium–salan‐catalyzed asymmetric sulfoxidations with H2O2: design of more versatile catalysts

Titanium–salan complexes with 3,3’‐diphenyl substituents in the salicylidene rings of the salan ligand are efficient sulfoxidation catalysts, capable of catalyzing the asymmetric oxidation of bulky aryl benzyl sulfides with H₂O₂ with good to high enantioselectivities. In this paper, substituent effects on titanium‐salan‐catalyzed enantioselective oxidation of sulfides to sulfoxides have been systematically investigated. Titanium–salan catalysts with halogen substituents at the 5,5’‐positions (3,3’‐H₂dihydrogen substituted) have been found to catalyze the oxidation of both bulky aryl benzyl sulfides and small alkyl phenyl sulfides with good to high enantioselectivities. Kinetic data witness a direct attack of the sulfide to the electrophilic active oxygen species; a consistent reaction mechanism is proposed. Copyright © 2013 John Wiley & Sons, Ltd.     

Allylic C(sp3)–H alkylation via synergistic organo- and photoredox catalyzed radical addition to imines

A new catalytic method for the direct alkylation of allylic C(sp³)–H bonds from unactivated alkenes via synergistic organo- and photoredox catalysis is described. The transformation achieves an efficient, redox-neutral synthesis of homoallylamines with broad functional group tolerance, under very mild reaction conditions. Mechanistic investigations indicate that the reaction proceeds through the N-centered radical intermediate which is generated by the allylic radical addition to the imine.

A new catalytic method for the direct alkylation of allylic C(sp³)–H bonds from unactivated alkenes via synergistic organo- and photoredox catalysis is described.     

Regiodivergent sulfonylarylation of 1,3-enynes via nickel/photoredox dual catalysis

Catalytic difunctionalization of 1,3-enynes represents an efficient and versatile approach to rapidly assemble multifunctional propargylic compounds, allenes and 1,3-dienes. Controlling selectivity in such addition reactions has been a long-standing challenging task due to multiple reactive centers resulting from the conjugated structure of 1,3-enynes. Herein, we present a straightforward method for regiodivergent sulfonylarylation of 1,3-enynes via dual nickel and photoredox catalysis. Hinging on the nature of 1,3-enynes, diverse reaction pathways are feasible: synthesis of α-allenyl sulfones via 1,4-sulfonylarylation, or preparation of (E)-1,3-dienyl sulfones with high chemo-, regio- and stereoselectivity through 3,4-sulfonylarylation. Notably, this is the first example that nickel and photoredox catalysis are merged to achieve efficient and versatile difunctionalization of 1,3-enynes.

A mild reaction protocol for regiodivergent sulfonylarylation of 1,3-enynes via dual nickel and photoredox catalysis has been developed, which led to efficient synthesis of α-allenyl sulfones or 1,3-dienyl sulfones.     

Selective oxidation of sulfides to sulfoxides with cyanuric chloride and urea–hydrogen peroxide adduct

Although a number of methods have been developed for the selective oxidation of sulfides to sulfoxides, the need remains for alternative efficient, reliable strategies that can be generally applied to various sulfides and that use readily available reagents under mild reaction conditions. Herein, we report the use of urea–hydrogen peroxide adduct (UHP) and cyanuric chloride in CH₃CN at room temperature to convert sulfides to sulfoxides in excellent yields. In particular, this protocol produced sulfoxides with aromatic rings bearing electron-withdrawing groups in excellent yields.     

Characterizing chain processes in visible light photoredox catalysis

The recognition that Ru(bpy)₃²⁺ and similar visible light absorbing transition metal complexes can be photocatalysts for a variety of synthetically useful organic reactions has resulted in a recent resurgence of interest in photoredox catalysis. However, many of the critical mechanistic aspects of this class of reactions remain poorly understood. In particular, the degree to which visible light photoredox reactions involve radical chain processes has been a point of some disagreement that has not been subjected to systematic analysis. We have now performed quantum yield measurements to demonstrate that three representative, mechanistically distinct photoredox processes involve product-forming chain reactions. Moreover, we show that the combination of quantum yield and luminescence quenching experiments provides a rapid method to estimate the length of these chains. Together, these measurements constitute a robust, operationally facile strategy for characterizing chain processes in a wide range of visible light photoredox reactions.     

A highly efficient reusable homogeneous copper catalyst for the selective aerobic oxygenation sulfides to sulfoxides

Readily available copper showed efficient activity and great selectivity for the homogeneous catalysis of oxidation of sulfides to sulfoxides using molecular oxygen as the oxidant. The reaction proceeds under mild conditions in the presence of a catalytic amount of TEMPO. Importantly, the catalysts could be conveniently recovered and reused. And this methodology was proved to be applicable for the transformation of various aromatic and aliphatic sulfides into the corresponding sulfoxides with high conversion and high selectivity.