Halogen-Bonding-Promoted Photoinduced C-X Borylation of Aryl Halide Using Phenol Derivatives

Visible-Light-Mediated Borylation of Arenes via an Electron Donor-Acceptor Complex of Thianthrenium Salts

Aryl borates are one of the most important synthetic materials in organic synthesis. Herein, we report a visible-light-induced electron donor-acceptor (EDA)-based borylation protocol between thianthrenium salts and 1,4-diazabicyclo[2.2.2]octane (DABCO) for the synthesis of various aryl boronates without a photocatalyst or stepwise processing. Remarkably, this photochemical strategy is compatible with a wide range of sensitive functional groups and affords excellent yields. Moreover, the quantitative recovery of thianthrene for feedstock synthesis lowers the reaction cost.

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.

Visible-Light-Induced Synthesis of Esters via a Self-Propagating Radical Reaction

We herein disclose a visible-light-induced synthesis of O-aryl esters through the cross-dehydrogenative coupling of aldehydes with phenols using BrCCl3, in which phenolate functions as both a substrate and a photosensitizer. This transition-metal- and photocatalyst-free visible-light-induced esterification is suitable for a wide range of substrates and gives moderate to excellent yields (up to 95%). Mechanistic studies provided evidence of a self-propagating radical reaction involving homolytic cleavage of the aldehydic C-H bond and the formation of acyl bromides. BrCCl3 serves as an oxidant and a hydrogen atom transfer (HAT) agent.

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.

Utilizing weakly donor-acceptor ternary π-conjugated architecture to achieve single-component white luminescence and stimulus-responsive room-temperature phosphorescence

Purely organic room-temperature phosphorescence (RTP) has garnered substantial attention for its delayed emission, environmental sensitivity, and potential diverse applications. However, the quest for high-performance RTP materials has always been a challenge. In this study, we introduce novel weakly donor-acceptor (D-A) ternary π-conjugated architecture to construct an efficient RTP system. The strategy utilizes synergistic effects of the analogous El-Sayed rule, halogen-free heavy-atom effect, reduction of the singlet-triplet energy gap, and manipulation of flexible molecular conformation. A remarkable enhancement in the phosphorescence-to-fluorescence ratio was achieved, elevating from 0.4 in carbazole to 35.2 in DBTDBTCZ. Furthermore, the RTP system demonstrates single-component white luminescence, yielding warm and cool white colors. Intriguingly, we unveil the novel position-dependent heavy-atom effects, discerningly promoting intersystem crossing or phosphorescence decay. Benefiting from efficient RTP, multifunctional applications of real-time humidity monitoring, oxygen sensing, anti-counterfeiting labeling, and white lighting are demonstrated.

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.

Halogen-Bonding-Enabled Photoinduced Atom Transfer Radical Addition/Cyclization Reaction Leading to Tricyclic Heterocycles

Atom transfer radical addition (ATRA) reactions are crucial for the dual functionalization of unsaturated hydrocarbons. Radical generation, pivotal in ATRA, has seen advancements from thermal to photochemical methods. Recent focus on halogen-bonding-based radical generation, including our group's innovative photochemical approach, offers cost-effective alternatives to transition-metal-dependent photocatalysts. This eliminates the need for high-energy UV light, enhancing the efficiency with noncovalent interactions.

Photochemical Synthesis of Lactones, Cyclopropanes and ATRA Products: Revealing the Role of Sodium Ascorbate

Light-mediated processes have received significant attention, since they have re-surfaced unconventional reactivity platforms, complementary to conventional polar chemistry. γ-Lactones and cyclopropanes are prevalent moieties, found in numerous natural products and pharmaceuticals. Among various methods for their synthesis, light-mediated protocols are coming to the spotlight, although these are contingent upon the use of photoorgano- or metal-based catalysts. Herein, we introduce a novel photochemical activation of iodo-reagents via the use of cheap sodium ascorbate or ascorbic acid to enable their homolytic scission and addition onto double bonds. The developed protocol was applied successfully to the formal [3+2] cycloaddition for the synthesis of γ-lactones, traditional atom transfer radical addition (ATRA) reactions and the one-pot two-step conversion of alkenes to cyclopropanes. In all cases, the desired products were obtained in good to high yields, while the reaction mechanism was thoroughly investigated. Depending on the nature of the iodo-reagent, a halogen or a hydrogen-bonded complex is formed, which initiates the process.

Aminative Suzuki-Miyaura coupling

The Suzuki-Miyaura and Buchwald-Hartwig coupling reactions are widely used to form carbon-carbon (C-C) and carbon-nitrogen (C-N) bonds, respectively. We report the incorporation of a formal nitrene insertion process into the Suzuki-Miyaura reaction, altering the products from C-C-linked biaryls to C-N-C-linked diaryl amines and thereby joining the Suzuki-Miyaura and Buchwald-Hartwig coupling pathways to the same starting-material classes. A combination of a bulky ancillary phosphine ligand on palladium and a commercially available amination reagent enables efficient reactivity across aryl halides and pseudohalides, boronic acids and esters, and many functional groups and heterocycles. Mechanistic insights reveal flexibility on the order of bond-forming events, suggesting potential for expansion of the aminative cross-coupling concept to encompass diverse nucleophiles and electrophiles as well as four-component variants.

Photochemical Halogen-Bonding Promoted Synthesis of Vinyl Sulfones via Vinyl and Sulfonyl Radicals

A photochemical halogen-bonding-assisted synthesis of vinyl sulfones via radical-radical cross-coupling of vinyl bromines and sodium sulfinates is developed. This methodology offers a facile and efficient approach to various vinyl sulfones with excellent functional group tolerance under metal-, photocatalyst-, base-, and oxidant-free conditions. The reaction is also applicable for the late-stage functionalization of drug molecules and the hectogram scale. Moreover, instead of sodium sulfites being prepared, these reactions could also be conducted using sulfonyl chlorides in a one-pot method.

Photocatalyst- and Transition Metal-Free Light-Induced Borylation Reactions

The increasing global warming concerns have propelled a surge in the demand for sustainable energy sources within the domain of synthetic organic chemistry. A particularly prominent area of research has been the development of mild synthetic strategies for generating heterocyclic compounds. Heterocyclic compounds containing boron have notably risen to prominence as pivotal reagents in a myriad of organic transformations, showcasing their wide-ranging applicability. This comprehensive review is aimed at collecting the literature pertaining to borylation reactions induced by light, specifically focusing on photocatalyst-free and transition metal-free methodologies. The central emphasis is on delving into selective mechanistic investigations. The amalgamation and analysis of these research insights elucidate the substantial potential inherent in eco-friendly approaches for synthesizing heterocyclic compounds, thus propelling the landscape of sustainable organic chemistry.