Photochemical Organocatalytic Synthesis of Thioethers from Aryl Chlorides and Alcohols.

Thioethers, often found in pharmaceuticals and natural compounds, typically involve metal cross-coupling reactions, high temperatures, and the use of disagreeable thiols for their synthesis. Here we present a straightforward, thiol-free organocatalytic protocol that uses mild conditions to stitch together inexpensive alcohols and aryl chlorides, yielding a diverse array of aryl alkyl thioethers. Central to this approach was the discovery that tetramethylthiourea can serve as a simple sulfur source upon intercepting photochemically generated aryl radicals. To form radicals, we used a readily available indole thiolate organocatalyst that, when excited with 405 nm light, gained a strongly reducing power, enabling the activation of typically unreactive aryl chlorides via single-electron transfer. Radical trapping by the thiourea, followed by an alcohol attack via a polar path, resulted in the formation of thioether products.

Enhancing the potential of enantioselective organocatalysis with light.

Organocatalysis-catalysis mediated by small chiral organic molecules-is a powerful technology for enantioselective synthesis, and has extensive applications in traditional ionic, two-electron-pair reactivity domains. Recently, organocatalysis has been successfully combined with photochemical reactivity to unlock previously inaccessible reaction pathways, thereby creating new synthetic opportunities. Here we describe the historical context, scientific reasoning and landmark discoveries that were essential in expanding the functions of organocatalysis to include one-electron-mediated chemistry and excited-state reactivity.

Photochemical Synthesis of Thioesters from Aryl Halides and Carboxylic Acids.

Thioesters are important in synthesis, materials science, and biology, and their preparation traditionally relies on the use of disagreeable thiols. Here, we report a thiol-free protocol that stitches together widespread carboxylic acids and aryl halides, producing a diverse array of thioesters. Crucial to this strategy is the discovery that tetramethylthiourea can serve as both a sulfur source and, upon direct excitation by purple light, as a strong reductant, suitable for activating aryl halides via single-electron transfer. Coupling of the resulting aryl radicals provides an isothiouronium ion intermediate, which can be attacked by carboxylic acids via a polar pathway, affording the thioester products under mild conditions.

Reductive Cross-Coupling of Olefins via a Radical Pathway.

Olefins are widely available at low costs, which explains the usefulness of developing new methods for their functionalization. Here we report a simple protocol that uses a photoredox catalyst and an inexpensive thiol catalyst to stitch together two olefins, forming a new C-C bond. Specifically, an electron-poor olefin is reduced by the photoredox catalyst to generate, upon protonation, a carbon radical, which is then captured by a neutral olefin. This intermolecular cross-coupling process provides a tool for rapidly synthesizing sp3-dense molecules from olefins using an unconventional disconnection.

Photochemical Organocatalytic Functionalization of Pyridines via Pyridinyl Radicals.

We report a photochemical method for the functionalization of pyridines with radicals derived from allylic C-H bonds. Overall, two substrates undergo C-H functionalization to form a new C(sp2)-C(sp3) bond. The chemistry harnesses the unique reactivity of pyridinyl radicals, generated upon single-electron reduction of pyridinium ions, which undergo effective coupling with allylic radicals. This novel mechanism enables distinct positional selectivity for pyridine functionalization that diverges from classical Minisci chemistry. Crucial was the identification of a dithiophosphoric acid that masters three catalytic tasks, sequentially acting as a Brønsted acid for pyridine protonation, a single electron transfer (SET) reductant for pyridinium ion reduction, and a hydrogen atom abstractor for the activation of allylic C(sp3)-H bonds. The resulting pyridinyl and allylic radicals then couple with high regioselectivity.

A General Light-Driven Organocatalytic Platform for the Activation of Inert Substrates.

Due to their strong covalent bonds and low reduction potentials, activating inert substrates is challenging. Recent advances in photoredox catalysis offered a number of solutions, each of which useful for activating specific inert bonds. Developing a general catalytic platform that can consistently target a broad range of inert substrates would be synthetically useful. Herein, we report a readily available indole thiolate organocatalyst that, upon excitation with 405 nm light, acquires a strongly reducing power. This excited-state reactivity served to activate, by single-electron reduction, strong C-F, C-Cl, and C-O bonds in both aromatic and aliphatic substrates. This catalytic platform was versatile enough to promote the reduction of generally recalcitrant electron-rich substrates (Ered <-3.0 V vs SCE), including arenes that afforded 1,4-cyclohexadienes. The protocol was also useful for the borylation and phosphorylation of inert substrates with a high functional group tolerance. Mechanistic studies identified an excited-state thiolate anion as responsible of the highly reducing reactivity.

Tetrachlorophthalimides as Organocatalytic Acceptors for Electron Donor-Acceptor Complex Photoactivation.

Excitation of photoactive electron donor-acceptor (EDA) complexes is an effective way to generate radicals. Applications in a catalytic regime typically use catalytic donors. Herein, we report that readily available electron-poor tetrachlorophthalimides can act as effective organocatalytic acceptors to trigger the formation of EDA complexes with a variety of radical precursors not amenable to previous catalytic methods. Excitation with visible light generates carbon radicals under mild conditions. The versatility of this EDA complex catalytic platform allowed us to develop mechanistically distinct radical reactions, including in combination with a cobalt-based catalytic system. Quantum yield measurements established that a closed catalytic cycle is operational, which hints at the ability of tetrachlorophthalimides to readily turn over and govern each catalytic cycle.

Photochemical Organocatalytic Benzylation of Allylic C-H Bonds.

We report a radical-based organocatalytic method for the direct benzylation of allylic C-H bonds. The process uses nonfunctionalized allylic substrates and readily available benzyl radical precursors and is driven by visible light. Crucial was the identification of a dithiophosphoric acid that performs two distinct catalytic roles, sequentially acting as a catalytic donor for the formation of photoactive electron donor-acceptor (EDA) complexes and then as a hydrogen atom abstractor. By mastering these orthogonal radical generation paths, the organic catalyst enables the formation of benzylic and allylic radicals, respectively, to then govern their selective coupling. The protocol was also used to design a three-component radical process, which increased the synthetic potential of the chemistry.

Asymmetric catalytic formation of quaternary carbons by iminium ion trapping of radicals.

An important goal of modern organic chemistry is to develop new catalytic strategies for enantioselective carbon-carbon bond formation that can be used to generate quaternary stereogenic centres. Whereas considerable advances have been achieved by exploiting polar reactivity, radical transformations have been far less successful. This is despite the fact that open-shell intermediates are intrinsically primed for connecting structurally congested carbons, as their reactivity is only marginally affected by steric factors. Here we show how the combination of photoredox and asymmetric organic catalysis enables enantioselective radical conjugate additions to ß,ß-disubstituted cyclic enones to obtain quaternary carbon stereocentres with high fidelity. Critical to our success was the design of a chiral organic catalyst, containing a redox-active carbazole moiety, that drives the formation of iminium ions and the stereoselective trapping of photochemically generated carbon-centred radicals by means of an electron-relay mechanism. We demonstrate the generality of this organocatalytic radical-trapping strategy with two sets of open-shell intermediates, formed through unrelated light-triggered pathways from readily available substrates and photoredox catalysts--this method represents the application of iminium ion activation (a successful catalytic strategy for enantioselective polar chemistry) within the realm of radical reactivity.

Lewis Base-Catalysed Enantioselective Radical Conjugate Addition for the Synthesis of Enantioenriched Pyrrolidinones.

We report a catalytic asymmetric protocol for the preparation of chiral pyrrolidinones proceeding via a radical pathway. The chemistry exploits the combination of photoredox catalysis and Lewis base catalysis to realise the first example of asymmetric radical conjugate addition to α,ß-unsaturated anhydrides and esters. The reaction is initiated by photoredox activation of N-arylglycines to generate, upon decarboxylation, α-amino radicals. These radicals are then intercepted stereoselectively by α,ß-unsaturated acyl ammonium intermediates, whose formation is mastered by a chiral isothiourea organocatalyst. Cyclisation leads to catalyst turnover and formation of enantioenriched pyrrolidinones. The utility of the protocol was demonstrated with application to the synthesis of biologically-active γ-amino butyric acids.

Enantioselective Biocascade Catalysis with a Single Multifunctional Enzyme.

Asymmetric catalytic cascade processes offer direct access to complex chiral molecules from simple substrates and in a single step. In biocatalysis, cascades are generally designed by combining multiple enzymes, each catalyzing individual steps of a sequence. Herein, we report a different strategy for biocascades based on a single multifunctional enzyme that can promote multiple stereoselective steps of a domino process by mastering distinct catalytic mechanisms of substrate activation in a sequential way. Specifically, we have used an engineered 4-oxalocrotonate tautomerase (4-OT) enzyme with the ability to form both enamines and iminium ions and combine their mechanisms of catalysis in a complex sequence. This approach allowed us to activate aldehydes and enals toward the synthesis of enantiopure cyclohexene carbaldehydes. The multifunctional 4-OT enzymes could promote both a two-component reaction and a triple cascade characterized by different mechanisms and activation sequences.

A General Organocatalytic System for Electron Donor-Acceptor Complex Photoactivation and Its Use in Radical Processes.

We report herein a modular class of organic catalysts that, acting as donors, can readily form photoactive electron donor-acceptor (EDA) complexes with a variety of radical precursors. Excitation with visible light generates open-shell intermediates under mild conditions, including nonstabilized carbon radicals and nitrogen-centered radicals. The modular nature of the commercially available xanthogenate and dithiocarbamate anion organocatalysts offers a versatile EDA complex catalytic platform for developing mechanistically distinct radical reactions, encompassing redox-neutral and net-reductive processes. Mechanistic investigations, by means of quantum yield determination, established that a closed catalytic cycle is operational for all of the developed radical processes, highlighting the ability of the organic catalysts to turn over and iteratively drive every catalytic cycle. We also demonstrate how the catalysts' stability and the method's high functional group tolerance could be advantageous for the direct radical functionalization of abundant functional groups, including aliphatic carboxylic acids and amines, and for applications in the late-stage elaboration of biorelevant compounds and enantioselective radical catalysis.

Photochemical Organocatalytic Regio- and Enantioselective Conjugate Addition of Allyl Groups to Enals.

We report the first catalytic enantioselective conjugate addition of allyl groups to α,ß-unsaturated aldehydes. The chemistry exploits the visible-light-excitation of chiral iminium ions to activate allyl silanes towards the formation of allylic radicals, which are then intercepted stereoselectively. The underlying radical mechanism of this process overcomes the poor regio- and chemoselectivity that traditionally affects the conjugate allylation of enals proceeding via polar pathways. We also demonstrate how this organocatalytic strategy could selectively install a valuable prenyl fragment at the ß-carbon of enals.

A General Organocatalytic System for Enantioselective Radical Conjugate Additions to Enals.

Herein, we report a general iminium ion-based catalytic method for the enantioselective conjugate addition of carbon-centered radicals to aliphatic and aromatic enals. The process uses an organic photoredox catalyst, which absorbs blue light to generate radicals from stable precursors, in combination with a chiral amine catalyst, which secures a consistently high level of stereoselectivity. The generality of this catalytic platform is demonstrated by the stereoselective interception of a wide variety of radicals, including non-stabilized primary ones which are generally difficult to engage in asymmetric processes. The system also served to develop organocatalytic cascade reactions that combine an iminium-ion-based radical trap with an enamine-mediated step, affording stereochemically dense chiral products in one-step.

Synthetic Methods Driven by the Photoactivity of Electron Donor-Acceptor Complexes.

The association of an electron-rich substrate with an electron-accepting molecule can generate a new molecular aggregate in the ground state, called an electron donor-acceptor (EDA) complex. Even when the two precursors do not absorb visible light, the resulting EDA complex often does. In 1952, Mulliken proposed a quantum-mechanical theory to rationalize the formation of such colored EDA complexes. However, and besides a few pioneering studies in the 20th century, it is only in the past few years that the EDA complex photochemistry has been recognized as a powerful strategy for expanding the potential of visible-light-driven radical synthetic chemistry. Here, we explain why this photochemical synthetic approach was overlooked for so long. We critically discuss the historical context, scientific reasons, serendipitous observations, and landmark discoveries that were essential for progress in the field. We also outline future directions and identify the key advances that are needed to fully exploit the potential of the EDA complex photochemistry.

A Photochemical Organocatalytic Strategy for the α-Alkylation of Ketones by using Radicals.

Reported herein is a visible-light-mediated radical approach to the α-alkylation of ketones. This method exploits the ability of a nucleophilic organocatalyst to generate radicals upon SN 2-based activation of alkyl halides and blue light irradiation. The resulting open-shell intermediates are then intercepted by weakly nucleophilic silyl enol ethers, which would be unable to directly attack the alkyl halides through a traditional two-electron path. The mild reaction conditions allowed functionalization of the α position of ketones with functional groups that are not compatible with classical anionic strategies. In addition, the redox-neutral nature of this process makes it compatible with a cinchona-based primary amine catalyst, which was used to develop a rare example of enantioselective organocatalytic radical α-alkylation of ketones.

Amide Synthesis by Nickel/Photoredox-Catalyzed Direct Carbamoylation of (Hetero)Aryl Bromides.

Herein, we report a one-electron strategy for catalytic amide synthesis that enables the direct carbamoylation of (hetero)aryl bromides. This radical cross-coupling approach, which is based on the combination of nickel and photoredox catalysis, proceeds at ambient temperature and uses readily available dihydropyridines as precursors of carbamoyl radicals. The method's mild reaction conditions make it tolerant of sensitive-functional-group-containing substrates and allow the installation of an amide scaffold within biologically relevant heterocycles. In addition, we installed amide functionalities bearing electron-poor and sterically hindered amine moieties, which would be difficult to prepare with classical dehydrative condensation methods.

Photochemical C-H Hydroxyalkylation of Quinolines and Isoquinolines.

We report herein a visible light-mediated C-H hydroxyalkylation of quinolines and isoquinolines that proceeds via a radical path. The process exploits the excited-state reactivity of 4-acyl-1,4-dihydropyridines, which can readily generate acyl radicals upon blue light absorption. By avoiding the need for external oxidants, this radical-generating strategy enables a departure from the classical, oxidative Minisci-type pattern and unlocks a unique reactivity, leading to hydroxyalkylated heteroarenes. Mechanistic investigations provide evidence that a radical-mediated spin-center shift is the key step of the process. The method's mild reaction conditions and high functional group tolerance accounted for the late-stage functionalization of active pharmaceutical ingredients and natural products.

Photochemical Asymmetric Nickel-Catalyzed Acyl Cross-Coupling.

Photochemical enantioselective nickel-catalyzed cross-coupling reactions are difficult to implement. We report a visible-light-mediated strategy that successfully couples symmetrical anhydrides and 4-alkyl dihydropyridines (DHPs) to afford enantioenriched α-substituted ketones under mild conditions. The chemistry does not require exogenous photocatalysts. It is triggered by the direct excitation of DHPs, which act as a radical source and as a reductant, facilitating the turnover of the chiral catalytic nickel complex.