Brønsted Base Assisted Photoredox Catalysis: Proton Coupled Electron Transfer for Remote C−C Bond Formation via Amidyl Radicals

SOMOphilic alkyne vs radical-polar crossover approaches: The full story of the azido-alkynylation of alkenes

We report the detailed background for the discovery and development of the synthesis of homopropargylic azides by the azido-alkynylation of alkenes. Initially, a strategy involving SOMOphilic alkynes was adopted, but only resulted in a 29% yield of the desired product. By switching to a radical-polar crossover approach and after optimization, a high yield (72%) of the homopropargylic azide was reached. Full insights are given about the factors that were essential for the success of the optimization process.

Photocatalytic Intramolecular Alkene Hydroamination of N-Alkoxy Ureas: An Approach to Imidazolinones

Imidazolinones were obtained in good yields by intramolecular hydroamination of N-alkoxy ureas in the presence of an organic photocatalyst and an inorganic base. In this reaction, the N-alkoxy urea anion generated by deprotonation undergoes photocatalyzed single-electron-transfer oxidation to generate the corresponding radical, which cyclizes to afford the imidazolinone ring. This new protocol grants access to an array of complex molecules containing a privileged imidazolinone core.

Photoinduced Copper-Catalyzed Aminoalkylation of Amino-Pendant Olefins

The combination of photo and copper catalysts has emerged as a novel paradigm in organic catalysis, which provides access to the acceleration of chemical synthesis. Herein, we describe an aminoalkylation of amino-dependent olefins with maleimides through a cooperative photo/copper catalytic system. In this report, the strategy allows the generation of a broad complex of functionalized nitrogenous molecules including oxazolidinones, 2-pyrrolidones, imidazolidinones, thiazolidinones, pyridines, and piperidines in the absence of an external photosensitizer and base. The approach is achieved through a photoinduced Cu(I)/Cu(II)/Cu(III) complex species of nitrogen nucleophiles, intermolecular radical addition, and hydrogen atom transfer (HAT) processes. The plausible mechanism is investigated by a series of control experiments and theoretical tests, including radical scavenging experiments, deuterium labeling experiments, ultraviolet-visible absorption, and cyclic voltammetry (CV) tests.

Copper-Catalyzed Sulfonylation/Cyclization of Pent-4-ynamides toward Sulfonyl-Functionalized Pyrrol-2-ones

A domino sulfonylation/intramolecular C-N coupling/dehydrogenation reaction was realized between pent-4-ynamides and sulfonyl chlorides by catalysis of Cu(acac)₂ and 2,2'-bis(diphenylphosphanyl)-1,1'-binaphthalene. The reaction provides a convenient approach to sulfonyl-functionalized pyrrol-2-ones. This method can also be applied to the synthesis of 3-alkylidene isoindolinones from 2-ethynyl-benzamides.

p-Methoxybenzyl-Radical-Promoted Chemoselective Protection of sec-Alkylamides

The hitherto difficult site-selective p-methoxybenzylation of secondary amides using p-methoxybenzylated alkylsilyl peroxides as a novel p-methoxybenzylation agent under copper catalysis is reported. The reaction proceeds under mild reaction conditions in a highly chemoselective manner. This approach was successfully applied to the site-selective p-methoxybenzylation of peptides.

Recent advances in visible light-induced C(sp3)–N bond formation

Synthetic chemists have long focused on selective C(sp ³)-N bond-forming approaches in response to the high value of this motif in natural products, pharmaceutical agents and functional materials. In recent years, visible light-induced protocols have become an important synthetic platform to promote this transformation under mild reaction conditions. These photo-driven methods rely on converting visible light into chemical energy to generate reactive but controllable radical species. This Review highlights recent advances in this area, mostly after 2014, with an emphasis placed on C(sp ³)-H bond activations, including amination of olefins and carbonyl compounds, and cross-coupling reactions.

Highly Diastereoselective Synthesis of γ-Lactams Enabled by Photoinduced Deaminative [3 + 2] Annulation Reaction

The photoinitiated deaminative [3 + 2] annulation reaction of N-aminopyridinium salts with alkenes for the synthesis of functionalized γ-lactams is described. This transformation shows good functional group tolerance as well as excellent diastereoselectivity. Preliminary studies suggest that the employed N-aminopyridinium salts generate the key amidyl radical intermediates through N-N bond cleavage via a photoinduced single-electron transfer (SET) process. The amidyl radical species would add to the double bond of alkenes, followed by a radical-mediated annulation process, to afford the desired γ-lactams.

Electricity mediated [3+2]-cycloaddition of N-sulfonylcyclopropanes with olefins via N-centered radical intermediates: access to cyclopentane analogs

An external oxidant free electrochemical strategy is designed towards the β-scission of strained C-C bonds in cyclopropylamine. Moreover, the mechanistic studies ascertained that the methodology encompasses the N-center radical (NCRs) route and provides access to di- or tri-substituted cyclopentane analogs.

Photoinduced remote regioselective radical alkynylation of unactivated C-H bonds

A method for the remote regioselective alkynylation of unactivated C(sp³)-H bonds in diverse aliphatic amides by photogenerated amidyl radicals has been developed. The site-selectivity is dominated via a 1,5-hydrogen atom transfer (HAT) process of the amide. Mild reaction conditions and high regioselectivity are demonstrated in this methodology.

Strategies to Generate Nitrogen-centered Radicals That May Rely on Photoredox Catalysis: Development in Reaction Methodology and Applications in Organic Synthesis

For more than 70 years, nitrogen-centered radicals have been recognized as potent synthetic intermediates. This review is a survey designed for use by chemists engaged in target-oriented synthesis. This review summarizes the recent paradigm shift in access to and application of N-centered radicals enabled by visible-light photocatalysis. This shift broadens and streamlines approaches to many small molecules because visible-light photocatalysis conditions are mild. Explicit attention is paid to innovative advances in N-X bonds as radical precursors, where X = Cl, N, S, O, and H. For clarity, key mechanistic data is noted, where available. Synthetic applications and limitations are summarized to illuminate the tremendous utility of photocatalytically generated nitrogen-centered radicals.

Photochemical and Electrochemical Applications of Proton-Coupled Electron Transfer in Organic Synthesis

We present here a review of the photochemical and electrochemical applications of multi-site proton-coupled electron transfer (MS-PCET) in organic synthesis. MS-PCETs are redox mechanisms in which both an electron and a proton are exchanged together, often in a concerted elementary step. As such, MS-PCET can function as a non-classical mechanism for homolytic bond activation, providing opportunities to generate synthetically useful free radical intermediates directly from a wide variety of common organic functional groups. We present an introduction to MS-PCET and a practitioner's guide to reaction design, with an emphasis on the unique energetic and selectivity features that are characteristic of this reaction class. We then present chapters on oxidative N-H, O-H, S-H, and C-H bond homolysis methods, for the generation of the corresponding neutral radical species. Then, chapters for reductive PCET activations involving carbonyl, imine, other X═Y π-systems, and heteroarenes, where neutral ketyl, α-amino, and heteroarene-derived radicals can be generated. Finally, we present chapters on the applications of MS-PCET in asymmetric catalysis and in materials and device applications. Within each chapter, we subdivide by the functional group undergoing homolysis, and thereafter by the type of transformation being promoted. Methods published prior to the end of December 2020 are presented.

Silver-catalyzed cascade cyclization and functionalization of N-aryl-4-pentenamides: an efficient route to γ-lactam-substituted quinone derivatives

The synthesis of γ-lactam-substituted quinone derivatives through a Ag₂O-catalyzed cascade cyclization and functionalization of N-aryl-4-pentenamides has been developed. Related 2-oxazolidinone substituted quinone products can be also obtained with N-aryl allyl carbamates. The reactions proceed through an amidyl radical-initiated 5-exo-trig cyclization and followed radical addition to quinones. They provide an efficient route to various γ-lactam-substituted quinone derivatives with a wide range of substrate scope.

The synthesis of γ-lactam and related 2-oxazolidinone substituted quinone derivatives through a Ag₂O-catalyzed cascade cyclization and functionalization of N-ary-4-pentenamides and N-aryl allyl carbamates has been developed.

Photoredox-Catalyzed Difunctionalization of Unactivated Olefins for Synthesizing Lactam-Substituted gem-Difluoroalkenes

Herein, the synthesis of lactam-substituted gem-difluoroalkenes has been developed through a photoredox-catalyzed radical cascade reaction. This developed photoredox-catalyzed, Brønsted base-assisted intramolecular 5-exo-trig cyclization/intermolecular radical addition/β-fluoride elimination reaction offers a simple method for producing lactam, carbamate, or urea-substituted gem-difluoroalkenes with good functional group tolerance and high yields.

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.

Photoredox-catalyzed aminofluorosulfonylation of unactivated olefins

The development of efficient approaches to access sulfonyl fluorides is of great significance because of the widespread applications of these structural motifs in many areas, among which the emerging sulfur(vi) fluoride exchange (SuFEx) click chemistry is the most prominent. Here, we report the first three-component aminofluorosulfonylation of unactivated olefins by merging photoredox-catalyzed proton-coupled electron transfer (PCET) activation with radical relay processes. Various aliphatic sulfonyl fluorides featuring a privileged 5-membered heterocyclic core have been efficiently afforded under mild conditions with good functional group tolerance. The synthetic potential of the sulfonyl fluoride products has been examined by diverse transformations including SuFEx reactions and transition metal-catalyzed cross-coupling reactions. Mechanistic studies demonstrate that amidyl radicals, alkyl radicals and sulfonyl radicals are involved in this difunctionalization transformation.

Brønsted Base-Catalyzed Formal Reductive [3+2] Annulation of 4,4,4-Trifluorocrotonate and α-Iminoketones

A formal reductive [3+2] annulation of 4,4,4-trifluorocrotonate and α-iminoketones was developed under Brønsted base catalysis. A single phosphazene base efficiently catalyzes the one-pot tandem reaction involving two mechanistically different elementary processes, namely the chemoselective reduction of an imine moiety of α-iminoketones with thiols as the reductant and the subsequent intermolecular Michael addition of an enolate of α-aminoketones concomitant with lactam formation. This operationally simple method provides β-trifluoromethyl-substituted γ-lactams with a tetrasubstituted carbon as a single diastereomer.

Amidyl radical-mediated aminodifluoroallylation of alkenes via photoredox catalysis

A practical and redox-neutral method is developed for the synthesis of gem-difluoroalkene-containing N-heterocycles via photoredox catalysis.

Photoredox-neutral alkene aminoarylation for the synthesis of 1,4,5,6-tetrahydropyridazines

A mild and redox-neutral protocol is developed for the synthesis of 1,4,5,6-tetrahydropyridazines via photoredox catalysis.

SOMOphilic alkynylation using acetylenic sulfones as functional reagents

Recent advancements in SOMOphilic alkynylation reactions by using acetylenic sulfones as functional reagents are summarized.

Manganese-Catalyzed N-F Bond Activation for Hydroamination and Carboamination of Alkenes

A visible-light-promoted method for generating amidyl radicals from N-fluorosulfonamides via a manganese-catalyzed N-F bond activation strategy is reported. This protocol employs a simple manganese complex, Mn₂(CO)₁₀, as the precatalyst and a cheap silane, (MeO)₃SiH, as both the hydrogen-atom donor and the F-atom acceptor, enabling intramolecular/intermolecular hydroaminations of alkenes, two-component carboamination of alkenes, and even three-component carboamination of alkenes. A wide range of valuable aliphatic sulfonamides can be readily prepared using these practical reactions.

Intermolecular radical carboamination of alkenes

Vicinal alkene carboamination is a highly efficient and practical synthetic strategy for the straightforward preparation of diverse and valuable amine derivatives starting from simple compounds. During the last decade that approach has found continuous research interests and various practical methods have been developed using transition-metal catalysis. Driven by the renaissance of synthetic radical chemistry, intermolecular radical alkene carboamination comprising a C-C bond and a C-N bond forming step has been intensively investigated recently culminating in novel strategies and improved protocols which complement existing methodologies. Radical alkene carboamination can be achieved via three different reaction modes. Such cascades can proceed through N-radical addition to an alkene with subsequent C-C bond formation leading to 2,1-carboamination products. Alternatively, the C-C bond can be installed prior to the C-N bond via initial C-radical addition to the alkene with subsequent β-amination resulting in 1,2-carboamination. The third mode comprises initial single electron oxidation of the alkene to the corresponding alkene radical cation that gets trapped by an N-nucleophile and the cascade is terminated by radical C-C bond formation. In this review, the three different conceptual approaches will be discussed and examples from the recent literature will be presented. Further, the reader will get insights into the mechanism of the different transformations.

Reductive coupling of imines with redox-active esters by visible light photoredox organocatalysis

The direct alkylation of imines with redox-active esters by visible light photoorganocatalysis provides a direct way for accessing α-branched secondary amines which are found in numerous bioactive molecules.

Alkynylation of radicals: spotlight on the "Third Way" to transfer triple bonds

The alkynylation of radical intermediates has been known since a long time, but had not been broadly applied in synthetic chemistry, in contrast to the alkynylation of either electrophiles or nucleophiles. In the last decade however, it has been intensively investigated leading to new disconnections to introduce versatile triple bonds into organic compounds. Nowadays, such processes are important alternatives to classical nucleophilic and electrophilic alkynylations. Efficient alkyne transfer reagents, in particular arylsulfones and hypervalent iodine reagents were introduced. Direct alkynylation, as well as cascade reactions, were subsequently developed. If relatively harsh conditions were required in the past, a new era began with progress in photoredox and transition metal catalysis. Starting from various radical precursors, alkynylations under very mild reaction conditions were rapidly discovered. This review covers the evolution of radical alkynylation, from its emergence to its current intensive stage of development. It will focus in particular on improvements for the generation of radicals and on the extension of the scope of radical precursors and alkyne sources.

Transition State Asymmetry in C-H Bond Cleavage by Proton-Coupled Electron Transfer

The selective transformation of C-H bonds is a longstanding challenge in modern chemistry. A recent report details C-H oxidation via multiple-site concerted proton-electron transfer (MS-CPET), where the proton and electron in the C-H bond are transferred to separate sites. Reactivity at a specific C-H bond was achieved by appropriate positioning of an internal benzoate base. Here, we extend that report to reactions of a series of molecules with differently substituted fluorenyl-benzoates and varying outer-sphere oxidants. These results probe the fundamental rate versus driving force relationships in this MS-CPET reaction at carbon by separately modulating the driving force for the proton and electron transfer components. The rate constants depend strongly on the pK_a of the internal base, but depend much less on the nature of the outer-sphere oxidant. These observations suggest that the transition states for these reactions are imbalanced. Density functional theory (DFT) was used to generate an internal reaction coordinate, which qualitatively reproduced the experimental observation of a transition state imbalance. Thus, in this system, homolytic C-H bond cleavage involves concerted but asynchronous transfer of the H⁺ and e^-. The nature of this transfer has implications for synthetic methodology and biological systems.

Chemistry With N-Centered Radicals Generated by Single-Electron Transfer-Oxidation Using Photoredox Catalysis

This review covers the recent literature on oxidative generation of N-centered radicals using photoredox catalysis. The concept of proton-coupled electron transfer is briefly discussed. Applications of such reactive N-centered radicals in cascade processes comprising arene amidation, alkene amidation, C—C bond cleavage reactions, and remote C—H functionalization are addressed. In addition, novel reagents allowing for clean oxidative N-radical generation are discussed.

Synthesis of N-H Bearing Imidazolidinones and Dihydroimidazolones Using Aza-Heck Cyclizations

The synthesis of unsaturated, unprotected imidazolidinones via an aza-Heck reaction is described. This palladium-catalyzed process allows for the cyclization of N-phenoxy ureas onto pendant alkenes. The reaction has broad functional group tolerance, can be applied to complex ring topologies, and can be used to directly prepare mono- and bis-unprotected imidazolidinones. By addition of Bu4 NI, dihydroimidazolones can be accessed from the same starting materials. Improved conditions for preparing unsaturated, unprotected lactams are also reported.

Visible light catalyzed synthesis of quinolines from (aza)-Morita–Baylis–Hillman adducts

A mild and efficient protocol for the synthesis of quinoline scaffolds from (aza)-MBH adducts under visible light catalysis has been established.