A Rational Approach to Organo‐Photocatalysis: Novel Designs and Structure‐Property Relationships

Fluoroalkyl Sulfoximines for Versatile Photocatalytic Radical Fluoroalkylations

Fluoroalkyl sulfoximines, which serve as electron-accepting fluoroalkyl radical sources, are easy-to-handle, solid, and bench-stable chemicals. Fluoroalkyl radicals can be generated from sulfoximine reagents using strong one-electron injectors, such as a highly reducing photoredox catalyst in the excited state. Our group has developed photocatalytic radical di- and mono-fluoromethylation and α-monofluoroalkylation of olefins with the corresponding fluoroalkyl sulfoximines. In this personal account, appropriate combinations of fluoroalkyl sulfoximines and photoredox catalysts, leading to successful radical fluoroalkylation, have been discussed.

N,N-Diphenyl Dihydrophenazines: Using π-Extension to Access Dicationic Multifunctional Materials

Dihydrophenazines are receiving increasing attention due to applications in numerous fields of chemistry, from light emission to organo-photocatalysis. Despite this growing interest and numerous works involving the preparation of radical cations based on this scaffold, the isolation and study of the aromatic dications obtained by 2 electron oxidation of dihydrophenazines is still mostly unexplored. From this point of view, along with the substitution at the N atoms generally used to tune dihydrophenazine properties, the π-extension of the phenazine core could play a crucial role in making dicationic states accessible. This could result in an extension of the knowledge on these elusive dications and in potentially highly interesting applications ranging from material science to molecular actuators.

Diquat Based Dyes: A New Class of Photoredox Catalysts and Their Use in Aerobic Thiocyanation

A series of new organic donor-π-acceptor dyes incorporating a diquat moiety as a novel electron-acceptor unit have been synthesized and characterized. The analytical data were supported by DFT calculations. These dyes were explored in the aerobic thiocyanation of indoles and pyrroles. Here they showed a high photocatalytic activity under visible light, giving isolated yields of up to 97 %. In addition, the photocatalytic activity of standalone diquat and methyl viologen through formation of an electron donor acceptor complex is presented.

Visible-Light-Active Coumarin- and Quinolinone-Based Photocatalysts and Their Applications in Chemical Transformations

Organic dyes have been actively studied as useful photocatalysts because they allow access to versatile structural flexibility and green synthetic applications. The identification of a new class of robust organic chromophores is, therefore, in high demand to increase structural diversity and variability. Although coumarins and quinolinones have long been acknowledged as organic chromophores, their ability to participate in photoinduced transformations is somewhat less familiar. Fascinated by their chromophoric features and adaptable platform, our group is interested in the identification of fluorescent bioactive molecules and in the development of new photoinduced synthetic methods using coumarins and quinolinones as photocatalysts. This account provides an overview of our recent progress in the discovery and application of light-absorbing coumarin and quinolinone derivatives in photochemistry and medicinal chemistry.

The Rational Design of Reducing Organophotoredox Catalysts Unlocks Proton-Coupled Electron-Transfer and Atom Transfer Radical Polymerization Mechanisms

Photocatalysis has become a prominent tool in the arsenal of organic chemists to develop and (re)imagine transformations. However, only a handful of versatile organic photocatalysts (PCs) are available, hampering the discovery of new reactivities. Here, we report the design and complete physicochemical characterization of 9-aryl dihydroacridines (9ADA) and 12-aryl dihydrobenzoacridines (12ADBA) as strong reducing organic PCs. Punctual structural variations modulate their molecular orbital distributions and unlock locally or charge-transfer (CT) excited states. The PCs presenting a locally excited state showed better performances in photoredox defunctionalization processes (yields up to 92%), whereas the PCs featuring a CT excited state produced promising results in atom transfer radical polymerization under visible light (up to 1.21 Đ, and 98% I*). Unlike all the PC classes reported so far, 9ADA and 12ADBA feature a free NH group that enables a catalytic multisite proton-coupled electron transfer (MS-PCET) mechanism. This manifold allows the reduction of redox-inert substrates including aryl, alkyl halides, azides, phosphate and ammonium salts (E_red up to -2.83 vs SCE) under single-photon excitation. We anticipate that these new PCs will open new mechanistic manifolds in the field of photocatalysis by allowing access to previously inaccessible radical intermediates under one-photon excitation.

Visible Light-Driven Reductive Azaarylation of Coumarin-3-carboxylic Acids

In the manuscript, reductive and decarboxylative azaarylation of coumarin-3-carboxylic acids is described. It utilizes the photocatalytic activation of (cyano)azaarenes in the presence of fac-Ir(ppy)₃ as a photocatalyst. The methodology is versatile and provides access to biologically relevant 4-substituted-chroman-2-ones. Visible light, photoredox catalyst, base, anhydrous solvent, and inert atmosphere constitute key parameters for the success of the described strategy. The developed methodology involves a wide range of coumarin-3-carboxylic acids as well as (cyano)azaarenes.

Stereoselective Synthesis of Atropisomeric Acridinium Salts by the Catalyst‐Controlled Cyclization of ortho ‐Quinone Methide Iminiums

Quinone methides are fundamental intermediates for a wide range of reactions in which catalyst stereocontrol is often achieved by hydrogen bonding. Herein, we describe the feasibility of an intramolecular Friedel–Crafts 6π electrocyclization through ortho‐quinone methide iminiums stereocontrolled by a contact ion pair. A disulfonimide catalyst activates racemic trichloroacetimidate substrates and imparts stereocontrol in the cyclization step, providing a new avenue for selective ortho‐quinone methide iminium functionalization. A highly stereospecific oxidation readily transforms the enantioenriched acridanes into rotationally restricted acridiniums. Upon ion exchange, the method selectively affords atropisomeric acridinium tetrafluoroborate salts in high yields and an enantioenrichment of up to 93 : 7 e.r. We envision that ion‐pairing catalysis over ortho‐quinone methide iminiums enables the selective synthesis of a diversity of heterocycles and aniline derivatives with distinct stereogenic units.

Photoredox-Catalyzed Defluorinative Functionalizations of Polyfluorinated Aliphatic Amides and Esters

Selective C-F bond functionalization of perfluoalkyl units has huge potential towards accessing functionalized organofluorinated compounds, but remains chanllenging due to the high C-F bond strength and inherent selectivity challenges. We report a new catalytic approach for the selective functionalization of the strong C-F bonds in polyfluorinated aliphatic esters and amides. This simple reaction proceeds in mild and operationally fashion with divergent conversions, including hydrodefluorination, defluoroalkylation, and defluoroalkenlylation, affording a diverse array of important partially fluorinated motifs. Straightforward downstream chemistry towards fluorinated alcohols, amines and drug derivatives highlights the potential of the protocol.

Radical α-Trifluoromethoxylation of Ketones under Batch and Flow Conditions by Means of Organic Photoredox Catalysis

The first light-driven method for the α-trifluoromethoxylation of ketones is reported. Enol carbonates react with N-trifluoromethoxy-4-cyano-pyridinium, using the photoredox catalyst 4-CzIPN under 456 nm irradiation, affording the α-trifluoromethoxy ketones in ≤50% isolated yield and complete chemoselectivity. As shown by 29 examples, the reaction is general and proceeds very rapidly under batch (1 h) and flow conditions (2 min). Diverse product manipulations demonstrate the synthetic potential of the disclosed method in accessing elusive trifluoromethoxylated bioactive ingredients.

Acridinium Salts and Cyanoarenes as Powerful Photocatalysts: Opportunities in Organic Synthesis

The use of organic photocatalysts has revolutionized the field of photoredox catalysis, as it allows access to reactivities that were traditionally restricted to transition-metal photocatalysts. This Minireview reports recent developments in the use of acridinium ions and cyanoarene derivatives in organic synthesis. The activation of inert chemical bonds as well as the late-stage functionalization of biorelevant molecules are discussed, with a special focus on their mechanistic aspects.