Solvent Anions Enable Photoinduced Borylation and Phosphonation of Aryl Halides via EDA Complexes

Iodoarene Activation: Take a Leap Forward toward Green and Sustainable Transformations

Constructing chemical bonds under green sustainable conditions has drawn attention from environmental and economic perspectives. The dissociation of (hetero)aryl-halide bonds is a crucial step of most arylations affording (hetero)arene derivatives. Herein, we summarize the (hetero)aryl halides activation enabling the direct (hetero)arylation of trapping reagents and construction of highly functionalized (hetero)arenes under benign conditions. The strategies for the activation of aryl iodides are classified into (a) hypervalent iodoarene activation followed by functionalization under thermal/photochemical conditions, (b) aryl-I bond dissociation in the presence of bases with/without organic catalysts and promoters, (c) photoinduced aryl-I bond dissociation in the presence/absence of organophotocatalysts, (d) electrochemical activation of aryl iodides by direct/indirect electrolysis mediated by organocatalysts and mediators acting as electron shuttles, and (e) electrophotochemical activation of aryl iodides mediated by redox-active organocatalysts. These activation modes result in aryl iodides exhibiting diverse reactivity as formal aryl cations/radicals/anions and aryne precursors. The coupling of these reactive intermediates with trapping reagents leads to the facile and selective formation of C-C and C-heteroatom bonds. These ecofriendly, inexpensive, and functional group-tolerant activation strategies offer green alternatives to transition metal-based catalysis.

Metal-Free C-X Functionalization in Solid-State via Photochemistry in Ball Mills

We report the first metal- and catalyst-free protocol for the facile cross-coupling of aryl halides towards C-B, C-P and C-S bonds under solid-state ball milling conditions via UV light irradiation. The reactions can be performed in the absence of bulk solvents at room temperature in a mixer mill, yielding up to 99 % and being tolerant towards various functionalized aryl halides (X=I or Br). Furthermore, we developed a novel photoreactor design increasing the light intensity. With this we could demonstrate that our protocol surpasses classical solvent based as well as purely mechanochemical approaches in terms of green metrics and energy efficiency.

Recent progress in C-S bond formation via electron donor-acceptor photoactivation

Recent advancements in C-S bond formation via electron donor-acceptor (EDA) complex photoactivation have been remarkable. EDA complexes, which are composed of electron donors and acceptors, facilitate C-S bond construction under mild conditions through single-electron transfer events upon visible light irradiation. This review highlights the utilization of various sulfur-containing substrates, including diacetoxybenzenesulfonyl (DABSO), sulfonic acids, sodium sulfinates, sulfonyl chlorides, and thiophenols, in EDA-promoted sulfonylation and thiolation reactions, covering the works published since 2017 to date. These reactions offer novel, environmentally friendly pathways for the synthesis of sulfur-containing compounds.

Light-assisted functionalization of aryl radicals towards metal-free cross-coupling

The many synthetic possibilities that arise when using radical intermediates, in place of their polar counterparts, make contemporary radical chemistry research an exhilarating field. The introduction of photocatalysis has helped tame aryl radicals, leading to a resurgence of interest in their chemistry, and an expansion of viable coupling partners and attainable transformations. These methods are more selective and safer than classical approaches, and they utilize new radical precursors. Given the importance of sustainability in current organic synthesis and our interest in light-assisted metal-free transformations, this Review focuses on recent advances in the use of aryl radicals in photoinduced cross-couplings that do not rely on metals for the crucial bond-forming event, and it is structured according to the key step that the aryl radicals engage in.

Solvent Dictated Organic Transformations

Solvent plays an important role in many chemical reactions. The C-H activation has been one of the most powerful tools in organic synthesis. These reactions are often assisted by solvents which not only provide a medium for the chemical reactions but also facilitate reaching to the product stage. The solvent helps the reaction profile both chemically and energetically to reach the targeted product. Organic transformations via C-H activation from the solvent assistance perspective has been discussed in this review. Various solvents such as tetrahydrofuran (THF), MeCN, dichloromethane (DCM), dimethoxyethane (DME), 1,2-dichloroethane (1,2-DCE), dimethylformamide (DMF), dimethylsulfoxide (DMSO), isopropyl nitrile (iPrCN), 1,4-dioxane, AcOH, trifluoroacetic acid (TFA), Ac2O, PhCF3, chloroform (CHCl3), H2O, N-methylpyrrolidone (NMP), acetone, methyl tert-butyl ether (MTBE), toluene, p-xylene, alcohols, MeOH, 1,1,1-trifluoroethanol (TFE), 1,1,1,3,3,3-hexafluoroisopropanol (HFIP), tert-amyl alcohol and their roles are discussed. The exclusive role of the solvent in various transformations has been deliberated by highlighting the substrate scope, along with the proposed mechanisms. For easy classification, the review has been divided into three parts: (i) solvent-switched divergent C-H activation; (ii) C-H bond activation with solvent as the coupling reagent, and (iii) C-H activation with solvent caging and solvent-assisted electron donor acceptor (EDA) complex formation and autocatalysis.

Visible Light Photoredox Catalysis in the Synthesis of Phosphonate-Substituted 1,10-Phenanthrolines

Photoredox-catalyzed phosphonylation of bromo-substituted 1,10-phenanthrolines under visible light irradiation was studied. The reaction was shown to proceed under mild conditions with Eosin Y as a photocatalyst in DMSO under blue light irradiation. It is transition-metal-free and affords the target phosphonate-substituted 1,10-phenanthrolines in moderate yields (26-51%) in 22 to 40 h. The rate and selectivity of the reaction depend largely on the position of the bromine atom, as well as on the nature and position of other substituents in the 1,10-phenanthroline core.

Photoinduced Phosphoniumation of Aryl Halides and Arylthianthrenium Salts via an Electron Donor-Acceptor Complex

Owing to their remarkable practicality and utility, phosphonium salts have attracted substantial interest and are widely applied in critical areas, such as medicine, materials science, and catalysis. Herein, we developed a facile and photocatalyst/metal-free synthetic strategy for the preparation of phosphonium salts utilizing aryl halides/arylthianthrenium salts as aryl radical precursors. This approach is disclosed to undergo an efficient light-induced electron donor-acceptor pathway, facilitating the synthesis of a structurally diverse range of phosphonium salts.

Visible Light-Induced Electron-Donor-Acceptor-Mediated C-3 Coupling of Quinoxalin-2(1H)-ones with Unactivated Aryl Iodides

Visible light-induced C-3 arylation of quinoxalin-2(1H)-ones with abundantly available aryl iodides with good yields via an electron-donor-acceptor (EDA)-complex formation have been accomplished. The radical scavenging, Electron paramagnetic resonance (EPR), UV-visible experiments, density functional theory (DFT), and quantum yield studies revealed that the reaction went through a radical pathway via a single electron transfer (SET) process. Furthermore, the protocol could also be applied to the synthesis of biologically active molecules, illustrating the practicality of the present protocol.

Photocatalytic Proficiency of Cinnoline Moiety for Cross-Coupling Reactions: A Two in One Photocatalyst

Herein, we have developed a new class of organic photocatalysts that can mimic transition metals for several oxidative and reductive organic cross-coupling transformations. Due to its wide potential window in both the oxidation and reduction ranges, cinnoline exhibits dual catalytic activity under visible light illumination, acting as both a photoreductant and photooxidant.

Generation and Application of Aryl Radicals Under Photoinduced Conditions

Photoinduced aryl radical generation is a powerful strategy in organic synthesis that facilitates the formation of diverse carbon-carbon and carbon-heteroatom bonds. The synthetic applications of photoinduced aryl radical formation in the synthesis of complex organic compounds, including natural products, physiologically significant molecules, and functional materials, have received immense attention. An overview of current developments in photoinduced aryl radical production methods and their uses in organic synthesis is given in this article. A generalized idea of how to choose the reagents and approach for the generation of aryl radicals is described, along with photoinduced techniques and associated mechanistic insights. Overall, this article offers a critical assessment of the mechanistic results as well as the selection of reaction parameters for specific reagents in the context of radical cascades, cross-coupling reactions, aryl radical functionalization, and selective C-H functionalization of aryl substrates.

Photoinduced Trifluoromethylselenolation of Aryl Halides with [Me4N][SeCF3]

A visible-light-induced metal-free trifluoromethylselenolation of aryl iodides and bromides with [Me4N][SeCF3] is described. The reaction was conducted at ambient temperature by successfully harnessing the light-sensitive SeCF3 reagent. Mechanistically, the EDA complexes between aryl halide and the -SeCF3 anion or the base might be formed and excited by light, which subsequently undergo intracomplex SET processes to generate aryl and •SeCF3 radicals as key intermediates, allowing a convenient and green access to various aryl trifluoromethyl selenoethers.

Site-selective and metal-free C-H phosphonation of arenes via photoactivation of thianthrenium salts

Aryl phosphonates are prevalent moieties in medicinal chemistry and agrochemicals. Their chemical synthesis normally relies on the use of precious metals, harsh conditions or aryl halides as substrates. Herein, we describe a sustainable light-promoted and site-selective C-H phosphonation of arenes via thianthrenation and the formation of an electron donor-acceptor complex (EDA) as key steps. The method tolerates a wide range of functional groups including biomolecules. The use of sunlight also promotes this transformation and our mechanistic investigations support a radical chain mechanism.

Access to Benzocyclic Boronates via Light-Promoted Intramolecular Arylborylation of Alkenes

Benzocyclic boronates have attracted increasing research interest in drug chemistry and organic synthesis in recent years. Herein, we report a facile access to benzocyclic boronates through photopromoted intramolecular arylborylation of allyl aryldiazonium salts. This simple protocol features a broad scope, allowing the formation of variously functionalized borates bearing dihydrobenzofuran, dihydroindene, benzothiophene, and indoline skeletons under mild and sustainable conditions.

Visible-Light-Induced N-Heterocyclic Carbene-Catalyzed Single Electron Reduction of Mono-Fluoroarenes

Fluoroarenes are abundant and readily available feedstocks. However, due to the high reduction potentials of mono-fluoroarenes, their photoreduction remains a continuing challenge, motivating the development of efficient activation modes to address this issue. This report presents the blue light-induced N-heterocyclic carbene (NHC)-catalyzed single electron reduction of mono-fluoroarenes for biaryl cross-couplings. We discovered that under blue light irradiation, NHC/tBuOK combination could construct powerful photoactive architectures to promote single electron transfer for C_aryl -F bond reduction via forming highly reducing NHC radical anion. Notably, the strategy was also successful to reduce C_aryl -O, C_aryl -N, and C_aryl -S bonds for biaryl cross-couplings.

Light-Induced Activation of C-X Bond via Carbonate-Assisted Anion-π Interactions: Applications to C-P and C-B Bond Formation

We have presented a carbonate anion assisted photochemical protocol for the C-X bond activation. Anion-π interactions have been leveraged to generate aryl radicals from easily accessible aryl halides that are further utilized in C-P and C-B bond formation reactions with excellent reactivity and broad functional group tolerance. Spectroscopic investigations and DFT studies were conducted for mechanistic insights. This inexpensive method alleviates the use of a photocatalyst and the need of preactivation of the substrate for the light-induced activation of C-X bonds.

Recent advances in photochemical construction of aromatic C–P bonds via C–hetero bond cleavage

Photochemical C–P bond cross-coupling in aromatics via C–X (X = F, Cl, Br, I), C–N bond and C–O bond cleavages with/without photosensitizer were summarized in this review.