Competitive Desulfonylative Reduction and Oxidation of α-Sulfonylketones Promoted by Photoinduced Electron Transfer with 2-Hydroxyaryl-1,3-dimethylbenzimidazolines under Air

Triarylamine-Substituted Benzimidazoliums as Electron Donor-Acceptor Dyad-Type Photocatalysts for Reductive Organic Transformations

Triarylamine-substituted benzimidazoliums (BI⁺-PhNAr₂), new electron donor-acceptor dyad molecules, were synthesized. Their photocatalytic properties for reductive organic transformations were explored using absorption and fluorescence spectroscopy, redox potential determinations, density functional theory calculations, transient absorption spectroscopy, and reduction reactions of selected substrates. The results show that irradiation of BI⁺-PhNAr₂ promotes photoinduced intramolecular electron transfer to form a long-lived (∼300 μs) charge shifted state (BI^•-PhN^•+Ar₂). In the pathway for photocatalysis of reduction reactions of substrates, BI^•-PhN^•+Ar₂ is subsequently transformed to the neutral benzimidazolyl radical (BI^•-PhNAr₂) by single-electron transfer from the donor 1,3-dimethyl-2-phenylbenzimidazoline (BIH-Ph) serving as a cooperative agent. Among the benzimidazoliums explored, the bromo-substituted analogue BI⁺-PhN(C₆H₄Br-p)₂ in conjunction with BIH-Ph demonstrates the most consistent catalytic performance.

Red Light-Based Dual Photoredox Strategy Resembling the Z-Scheme of Natural Photosynthesis

Photoredox catalysis typically relies on the use of single chromophores, whereas strategies, in which two different light absorbers are combined, are rare. In photosystems I and II of green plants, the two separate chromophores P₆₈₀ and P₇₀₀ both absorb light independently of one another, and then their excitation energy is combined in the so-called Z-scheme, to drive an overall reaction that is thermodynamically very demanding. Here, we adapt this concept to perform photoredox reactions on organic substrates with the combined energy input of two red photons instead of blue or UV light. Specifically, a Cu^I bis(α-diimine) complex in combination with in situ formed 9,10-dicyanoanthracenyl radical anion in the presence of excess diisopropylethylamine catalyzes ca. 50 dehalogenation and detosylation reactions. This dual photoredox approach seems useful because red light is less damaging and has a greater penetration depth than blue or UV radiation. UV-vis transient absorption spectroscopy reveals that the subtle change in solvent from acetonitrile to acetone induces a changeover in the reaction mechanism, involving either a dominant photoinduced electron transfer or a dominant triplet-triplet energy transfer pathway. Our study illustrates the mechanistic complexity in systems operating under multiphotonic excitation conditions, and it provides insights into how the competition between desirable and unwanted reaction steps can become more controllable.

Photocatalytic turnover of CO2 under visible light by [Re(CO)3(1-(1,10) phenanthroline-5-(4-nitro-naphthalimide))Cl] in tandem with the sacrificial donor BIH

Optimized photocatalytic conversion of CO2 requires new potent catalysts that can absorb visible light. The photocatalytic reduction of CO2 using rhenium(i) has been demonstrated but suffers from low turnover. Herein, we describe a [Re(CO)3(1-(1,10)phenanthroline-5-(4-nitro-naphthalimide))Cl] photocatalyst, which when combined with the sacrificial donor 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole, results in selective production of formic acid and a high turnover number of 533 and turnover frequency of 356 h-1. Single-crystal X-ray diffraction and DFT studies are also discussed.

A Photocatalytic System Composed of Benzimidazolium Aryloxide and Tetramethylpiperidine 1-Oxyl to Promote Desulfonylative α-Oxyamination Reactions of α-Sulfonylketones

A new photocatalytic system was developed for carrying out desulfonylative α-oxyamination reactions of α-sulfonylketones in which α-ketoalkyl radicals are generated. The catalytic system is composed of benzimidazolium aryloxide betaines (BI⁺-ArO^-), serving as visible light-absorbing electron donor photocatalysts, and 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO), playing dual roles as an electron donor for catalyst recycling and a reagent to capture the generated radical intermediates. Information about the detailed nature of BI⁺-ArO^- and the photocatalytic processes with TEMPO was gained using absorption spectroscopy, electrochemical measurements, and density functional theory calculations.

Desulfonylation via Radical Process: Recent Developments in Organic Synthesis

As the "chemical chameleon", sulfonyl-containing compounds and their variants have been merged with various types of reactions for the efficient construction of diverse molecular architectures by taking advantage of their incredible reactive flexibility. Currently, their involvement in radical transformations, in which the sulfonyl group typically acts as a leaving group via selective C-S, N-S, O-S, S-S, and Se-S bond cleavage/functionalization, has facilitated new bond formation strategies which are complementary to classical two-electron cross-couplings via organometallic or ionic intermediates. Considering the great influence and synthetic potential of these novel avenues, we summarize recent advances in this rapidly expanding area by discussing the reaction designs, substrate scopes, mechanistic studies, and their limitations, outlining the state-of-the-art processes involved in radical-mediated desulfonylation and related transformations. With a specific emphasis on their synthetic applications, we believe this review will be useful for medicinal and synthetic organic chemists who are interested in radical chemistry and radical-mediated desulfonylation in particular.

Selective Synthesis of Acylated Cross-Benzoins from Acylals and Aldehydes via N-Heterocyclic Carbene Catalysis

The utility of acylals as building blocks for selective cross-benzoin synthesis was explored in this study. The synthesis of α-acetoxyketones (O-acyl cross-benzoins) was achieved via selective N-heterocyclic carbene-catalyzed cross-benzoin reactions using acylals as aldehyde equivalents. Thus, the combination of ortho-substituted phenyl acylals and aromatic/aliphatic aldehydes as coupling substrates using bicyclic triazolium salts as precatalysts and potassium carbonate as a base in THF at reflux temperature selectively yielded O-acyl cross-benzoins.