Synthetic Utilization of α-Aminoalkyl Radicals and Related Species in Visible Light Photoredox Catalysis

Mechanistic Investigations into Amination of Unactivated Arenes via Cation Radical Accelerated Nucleophilic Aromatic Substitution

A mechanistic investigation into the amination of electron-neutral and electron-rich arenes using organic photoredox catalysis is presented. Kinetic and computational data support rate-limiting nucleophilic addition into an arene cation radical using both azole and primary amine nucleophiles. This finding is consistent with both fluoride and alkoxide nucleofuges, supporting a unified mechanistic picture using cation radical accelerated nucleophilic aromatic substitution (CRA-SNAr). Electrochemistry and time-resolved fluorescence spectroscopy confirm the key role solvents play in enabling selective arene oxidation in the presence of amines. The synthetic limitations of xanthylium salts are elucidated via photophysical studies. An alternative catalyst scaffold with improved turnover numbers is presented.

1,2-Radical Shifts in Photoinduced Synthetic Organic Transformations: A Guide to the Reactivity of Useful Radical Synthons

The exploration of 1,2-radical shift (RS) mechanisms in photoinduced organic reactions has provided efficient routes for the generation of important radical synthons in many chemical transformations. In this Review, the basic concepts involved in the traditional 1,2-spin-center shift (SCS) mechanisms in recently reported studies are discussed. In addition, other useful 1,2-RSs are addressed, such as those proceeding through 1,2-group migrations in carbohydrate chemistry, via 1,2-boron shifts, and by the generation of α-amino radicals. The discussion begins with a general overview of the basic aspects of 1,2-RS mechanisms, followed by a demonstration of their applicability in photoinduced transformations. The sections that follow are organized according to the mechanisms operating in combination with the 1,2-radical migration event. This contribution is not a comprehensive review but rather aims to provide an understanding of the topic, focused on the more recent advances in the field, and establishes a definition for the nomenclature that has been used to describe such mechanisms.

Selective desaturation of amides: a direct approach to enamides

C(sp3)-H bond desaturation has been an attractive strategy in organic synthesis. Enamides are important structural fragments in pharmaceuticals and versatile synthons in organic synthesis. However, the dehydrogenation of amides usually occurs on the acyl side benefitting from enolate chemistry like the desaturation of ketones and esters. Herein, we demonstrate an Fe-assisted regioselective oxidative desaturation of amides, which provides an efficient approach to enamides and β-halogenated enamides.

Access to chiral β-sulfonyl carbonyl compounds via photoinduced organocatalytic asymmetric radical sulfonylation with sulfur dioxide

An organocatalytic enantioselective radical reaction of potassium alkyltrifluoroborates, DABCO·(SO2)2 and α,β-unsaturated carbonyl compounds under photoinduced conditions is developed, which provides an efficient pathway for the synthesis of chiral β-sulfonyl carbonyl compounds in good yields with excellent enantioselectivity (up to 96% ee). Aside from α,β-unsaturated carbonyl compounds with auxiliary groups, common chalcone substrates are also well compatible with this organocatalytic system. This method proceeds through an organocatalytic enantioselective radical sulfonylation under photoinduced conditions, and represents a rare example of asymmetric transformation involving sulfur dioxide insertion.

Visible Light-Mediated Enantioselective Addition of α-Aminoalkyl Radicals to Ketones Catalyzed by Chiral Oxazaborolidinium Ion

The development of a visible light-mediated synthetic method for chiral 1,2-amino tertiary alcohols is described. In the presence of a chiral oxazaborolidinium ion catalyst and photosensitizer, the enantioselective addition of an α-aminoalkyl radical to aryl methyl ketones under visible light provides chiral 1,2-amino tertiary alcohol derivatives in high yields (up to 88%) with excellent enantioselectivities (up to 98% ee). With mechanistic studies such as radical trapping analysis, radical clock analysis, and the measurement of quantum yield, a plausible catalytic cycle is proposed.

Regio-, Diastereo-, and Enantioselective Decarboxylative Hydroaminoalkylation of Dienol Ethers Enabled by Dual Palladium/Photoredox Catalysis

Intermolecular photocatalytic hydroaminoalkylation (HAA) of alkenes have emerged as a powerful method for the construction of alkyl amines. Although there are some studies aiming at stereoselective photocatalytic HAA reactions, the alkenes are limited to electrophilic alkenes. Herein, we report a highly regio-, diastereo-, and enantioselective HAA of electron-rich dienol ethers and α-amino radicals derived from α-amino acids using a unified photoredox and palladium catalytic system. This decarboxylative 1,2-Markovnikov addition enables the construction of vicinal amino tertiary ethers with high levels of regio- (up to >19 : 1 rr), diastereo- (up to >19 : 1 dr), and enantioselectivity control (up to >99 % ee). Mechanistic studies support a reversible hydropalladation as a key step.

A Germanium Catalyst Accelerates the Photoredox α-C(sp3)-H Alkylation of Primary Amines

Site-selective C(sp³)-H functionalizations using photoredox catalysis (PC) and hydrogen atom transfer (HAT) catalysis have received increasing attention. Here, we report a Ph₂GeCl₂ cocatalyst that greatly improves the yield of α-C(sp³)-H alkylation of primary amines catalyzed by a PC-HAT hybrid system. The α-position of the amino group selectively reacted even when weaker C-H bonds existed in the substrates. This finding may help the design of a novel site-selective hybrid catalysis.

Tropane and related alkaloid skeletons via a radical [3+3]-annulation process

Tropanes and related bicyclic alkaloids are highly attractive compounds possessing a broad biological activity. Here we report a mild and simple protocol for the synthesis of N-arylated 8-azabicyclo[3.2.1]octane and 9-azabicyclo[3.3.1]nonane derivatives. It provides these valuable bicyclic alkaloid skeletons in good yields and high levels of diastereoselectivity from simple and readily available starting materials using visible-light photoredox catalysis. These bicyclic aniline derivatives are hardly accessible via the classical Robinson tropane synthesis and represent a particularly attractive scaffold for medicinal chemistry. This unprecedented annulation process takes advantage of the unique reactivity of ethyl 2-(acetoxymethyl)acrylate as a 1,3-bis radical acceptor and of cyclic N,N-dialkylanilines as radical 1,3-bis radical donors. The success of this process relies on efficient electron transfer processes and highly selective deprotonation of aminium radical cations leading to the key α-amino radical intermediates.

Tethered photocatalyst-directed palladium-catalysed C-H allenylation of N-aryl tetrahydroisoquinolines

Harnessing radical intermediates in regioselective reactions presents a substantial challenge. Here, we report a novel control strategy through engineering covalently tethered transition metal-photocatalysts that conjoin Pd-phosphine and Ru/Ir photoredox units. This strategy allows us to override the innate regioselectivity of the Pd-catalysed C-H allenylation of N-aryl tetrahydroisoquinolines.

Recent Progress in Chromium-Mediated Carbonyl Addition Reactions

Organochromium(III) species are versatile nucleophiles in complex molecule synthesis due to their high functional group tolerance and chemoselectivity for aldehydes. Traditionally, carbonyl addition reactions of organochromium(III) species were performed through reduction of organohalides either using stoichiometric chromium(II) salts or catalytic chromium salts in the presence of stoichiometric reductants [such as Mn(0)]. Recently, alternative methods emerged involving organoradical formation from readily available starting materials (e.g., N-hydroxyphthalimide esters, alkenes, and alkanes), followed by trapping the radical with stoichiometric or catalytic chromium(II) salts. Such methods, especially using catalytic chromium(II) salts, will lead to the development of sustainable chemical processes minimizing salt wastes and number of synthetic steps. In this review, methods for generation of organochromium(III) species for addition reactions to carbonyl compounds, classified by nucleophiles are described.

1 Introduction

2 Alkylation

2.1 Branch-Selective Reductive Alkylation of Aldehydes Using Unactivated Alkenes

2.2 Linear-Selective Alkylation of Aldehydes

2.2.1 Catalytic Decarboxylative Alkylation of Aldehydes Using NHPI ­Esters

2.2.2 Catalytic Reductive Alkylation of Aldehydes Using Unactivated Alkenes

2.2.3 Alkylation of Aldehydes via C(sp3)–H Bond Functionalization of Unactivated Alkanes

2.3 Catalytic α-Aminoalkylation of Carbonyl Compounds

3 Allylation

3.1 Catalytic Allylation of Aldehydes via Three-Component Coupling

3.2 Catalytic Allylation of Aldehydes via C(sp3)–H Bond Functionalization of Alkenes

4 Propargylation: Catalytic Propargylation of Aldehydes via Three-Component Coupling

5 Conclusion

Azetidine synthesis enabled by photo-induced copper-catalysis via [3+1] radical cascade cyclization

Azetidines are an important type of saturated, highly strained, four-membered, nitrogen-containing heterocyclic compound. These compounds serve as important raw materials, intermediates, and catalysts in organic synthesis, as well as important active units in amino acids, alkaloids, and pharmaceutically active compounds. Thus, the development of an efficient and concise method to construct azetidines is of great significance in multiple disciplines. In this work, we reported on the photo-induced copper-catalyzed radical annulation of aliphatic amines with alkynes to produce azetidines. This reaction occurred in a two- or three-component manner. The alkynes efficiently captured photogenerated α-aminoalkyl radicals, forming vinyl radicals, which initiated tandem 1,5-hydrogen atom transfer and 4-exo-trig cyclization. Density functional theory calculations indicated that the tertiary radical intermediate was critical for the success of cyclization. In addition, the resulting saturated azetidine scaffolds possessed vicinal tertiary-quaternary and even quaternary-quaternary centers.

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Azetidines, four-membered N-heterocyclic compounds, are valuable targets for synthesis

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The first [3 + 1] cyclization approach is enabled by visible-light-induced copper catalysis

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This atom economic synthesis is characterized by double C-H activation

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This technology features operational simplicity, cheap catalyst, and broad substrate scope

Photoredox catalysis powered by triplet fusion upconversion: arylation of heteroarenes

In this work, the feasibility of triplet fusion upconversion (TFU, also named triplet-triplet annihilation upconversion) technology for the functionalization (arylation) of furans and thiophenes has been successfully proven. Activation of aryl halides by TFU leads to generation of aryl radical intermediates; trapping of the latter by the corresponding heteroarenes, which act as nucleophiles, affords the final coupling products. Advantages of this photoredox catalytic method include the use of very mild conditions (visible light, standard conditions), employment of commercially available reactants and low-loading metal-free photocatalysts, absence of any sacrificial agent (additive) in the medium and short irradiation times. The involvement of the high energetic delayed fluorescence in the reaction mechanism has been evidenced by quenching studies, whereas the two-photon nature of this photoredox arylation of furans and thiophenes has been manifested by the dependence on the energy source power. Finally, the scaling-up conditions have been gratifyingly afforded by a continuous-flow device.

Visible-light-mediated intramolecular radical cyclization of α-brominated amide-tethered alkylidenecyclopropanes

A ring-opening/cyclization cascade reaction of α-brominated amide-tethered alkylidenecyclopropanes in the presence of photocatalyst 4CzIPN under visible-light irradiation was developed to afford polycyclic benzazepine derivatives in good yields with broad substrate scope and good functional tolerance. A plausible mechanism involving a halogen atom transfer (XAT) process and a radical chain process is proposed for this reaction. This study provides a concise and practical strategy for the synthesis of benzazepine derivatives.

Photoredox-Catalyzed C-H Functionalization Reactions

The fields of C-H functionalization and photoredox catalysis have garnered enormous interest and utility in the past several decades. Many different scientific disciplines have relied on C-H functionalization and photoredox strategies including natural product synthesis, drug discovery, radiolabeling, bioconjugation, materials, and fine chemical synthesis. In this Review, we highlight the use of photoredox catalysis in C-H functionalization reactions. We separate the review into inorganic/organometallic photoredox catalysts and organic-based photoredox catalytic systems. Further subdivision by reaction class─either sp2 or sp3 C-H functionalization─lends perspective and tactical strategies for use of these methods in synthetic applications.

Photons or Electrons? A Critical Comparison of Electrochemistry and Photoredox Catalysis for Organic Synthesis

Redox processes are at the heart of synthetic methods that rely on either electrochemistry or photoredox catalysis, but how do electrochemistry and photoredox catalysis compare? Both approaches provide access to high energy intermediates (e.g., radicals) that enable bond formations not constrained by the rules of ionic or 2 electron (e) mechanisms. Instead, they enable 1e mechanisms capable of bypassing electronic or steric limitations and protecting group requirements, thus enabling synthetic chemists to disconnect molecules in new and different ways. However, while providing access to similar intermediates, electrochemistry and photoredox catalysis differ in several physical chemistry principles. Understanding those differences can be key to designing new transformations and forging new bond disconnections. This review aims to highlight these differences and similarities between electrochemistry and photoredox catalysis by comparing their underlying physical chemistry principles and describing their impact on electrochemical and photochemical methods.

Electrochemical synthesis for α-arylation of ketones using enol acetates and aryl diazonium salts

In this article, an electrochemical method has been developed to achieve the α-arylation of ketones by reacting aryl diazonium salts with enol acetates. The broad scope of the reaction toward...

Recent advances in visible-light mediated functionalization of olefins and alkynes using copper catalysts

Over the past decade, visible-light photoredox catalysis has blossomed as a powerful strategy and offers a discrete activation mode complementary to thermal controlled reactions. Visible-light-mediated photoredox catalysis also offers exciting...

Isothiourea-Catalysed Enantioselective Radical Conjugate Addition under Batch and Flow Conditions

The photocatalytic generation of α-amino radicals is combined with chiral isothiourea derived α,β-unsaturated acyl ammonium intermediates. The reaction proceeds via a [3+2] radical-polar crossover mechanism to generate γ-lactams in good...

Alkyl-GeMe3: Neutral Metalloid Radical Precursors upon Visible-Light Photocatalysis

Single-electron transfer (SET) oxidation of ionic hypervalent complexes, representatively alkyltrifluoroborates (Alkyl-BF3-) and alkylbis(catecholato)silicates (Alkyl-Si(cat)2-), have contributed substantially to alkyl radical generation compared to alkali or alkaline earth organometallics because of their excellent activity-stability balance. Herein, we report another proposal using neutral metalloid compounds, Alkyl-GeMe3, as radical precursors. Compared to Alkyl-BF3- and Alkyl-Si(cat)2-, Alkyl-GeMe3 show comparable activity in radical addition reactions. Moreover, Alkyl-GeMe3 gives the first success of group 14 tetraalkyl nucleophiles in nickel catalyzed cross-coupling. Meanwhile, the neutral nature of these organogermanes supplemented the limination of ionic precursors in purification and derivatization. A preliminary mechanism study corresponds to the procedure that alkyl radical generates from tetraalkylgermane radical cation with the assistance of a nucleophile, which may also enlighten the development of more non-ionic alkyl radical precursors with metalloid center.

Stereoselective Synthesis of Tertiary Allylic Amines by Titanium-Catalyzed Hydroaminoalkylation of Alkynes with Tertiary Amines

Intermolecular hydroaminoalkylation reactions of symmetrical and unsymmetrical alkynes with tertiary amines take place in the presence of catalytic amounts of TiBn₄, Ph₃C[B(C₆F₅)₄], and a sterically demanding aminopyridinato ligand precursor. The resulting products, synthetically and pharmaceutically useful tertiary β,γ‐disubstituted allylic amines, are formed in convincing yields and with excellent stereoselectivity. Particularly promising for future applications is the fact that even the industrial side product trimethylamine can be used as a substrate.

Photoredox Chemistry with Organic Catalysts: Role of Computational Methods

Organic catalysts have the potential to carry out a wide range of otherwise thermally inaccessible reactions via photoredox routes. Early demonstrated successes of organic photoredox catalysts include one-electron CO₂ reduction and H₂ generation via water splitting. Photoredox systems are challenging to study and design owing to the sheer number and diversity of phenomena involved, including light absorption, emission, intersystem crossing, partial or complete charge transfer, and bond breaking or formation. Designing a viable photoredox route therefore requires consideration of a host of factors such as absorption wavelength, solvent, choice of electron donor or acceptor, and so on. Quantum chemistry methods can play a critical role in demystifying photoredox phenomena. Using one-electron CO₂ reduction with phenylene-based chromophores as an illustrative example, this perspective highlights recent developments in quantum chemistry that can advance our understanding of photoredox processes and proposes a way forward for driving the design and discovery of organic catalysts.

Applications of Halogen-Atom Transfer (XAT) for the Generation of Carbon Radicals in Synthetic Photochemistry and Photocatalysis

The halogen-atom transfer (XAT) is one of the most important and applied processes for the generation of carbon radicals in synthetic chemistry. In this review, we summarize and highlight the most important aspects associated with XAT and the impact it has had on photochemistry and photocatalysis. The organization of the material starts with the analysis of the most important mechanistic aspects and then follows a subdivision based on the nature of the reagents used in the halogen abstraction. This review aims to provide a general overview of the fundamental concepts and main agents involved in XAT processes with the objective of offering a tool to understand and facilitate the development of new synthetic radical strategies.

Thiol-Mediated α-Amino Radical Formation via Visible-Light-Activated Ion-Pair Charge-Transfer Complexes

Visible-light-activated electron donor-acceptor complexes offer distinct reaction pathways for the synthesis of complex molecules under mild conditions. Herein, we report a method for the reductive generation of α-amino radicals via the reaction of a visible-light-activated ion-pair charge-transfer complex formed between an in situ-generated alkyl-iminium ion and a thiophenolate. This distinct activation mode is demonstrated through the development of a multicomponent coupling reaction to form substituted aminomethyl-cyclopentanes from secondary amines, cyclopropyl aldehydes, and alkenes. The operationally straightforward transformation displays broad scope and provides a means to generate cyclic amine-containing scaffolds from readily available feedstocks.

Site-Selective α-C-H Functionalization of Trialkylamines via Reversible Hydrogen Atom Transfer Catalysis

Trialkylamines are widely found in naturally occurring alkaloids, synthetic agrochemicals, biological probes, and especially pharmaceuticals agents and preclinical candidates. Despite the recent breakthrough of catalytic alkylation of dialkylamines, the selective α-C(sp³)-H bond functionalization of widely available trialkylamine scaffolds holds promise to streamline complex trialkylamine synthesis, accelerate drug discovery, and execute late-stage pharmaceutical modification with complementary reactivity. However, the canonical methods always result in functionalization at the less-crowded site. Herein, we describe a solution to switch the reaction site through fundamentally overcoming the steric control that dominates such processes. By rapidly establishing an equilibrium between α-amino C(sp³)-H bonds and a highly electrophilic thiol radical via reversible hydrogen atom transfer, we leverage a slower radical-trapping step with electron-deficient olefins to selectively forge a C(sp³)-C(sp³) bond with the more-crowded α-amino radical, with the overall selectivity guided by the Curtin-Hammett principle. This subtle reaction profile has unlocked a new strategic concept in direct C-H functionalization arena for forging C-C bonds from a diverse set of trialkylamines with high levels of site selectivity and preparative utility. Simple correlation of site selectivity and ¹³C NMR shift serves as a qualitative predictive guide. The broad consequences of this dynamic system, together with the ability to forge N-substituted quaternary carbon centers and implement late-stage functionalization techniques, hold potential to streamline complex trialkylamine synthesis and accelerate small-molecule drug discovery.

Syntheses of new chiral chimeric photo-organocatalysts

A new family of chiral chimeric photo-organocatalysts derived from phosphoric acid were synthesized and their spectroscopic and electrochemical properties were investigated. Then, the ability of these photo-activable molecules to catalyse an asymmetric tandem electrophilic β-amination of enecarbamates was evaluated.

Switchable, Reagent-Controlled Diastereodivergent Photocatalytic Carbocyclisation of Imine-Derived α-Amino Radicals

A reagent-controlled stereodivergent carbocyclisation of aryl aldimine-derived, photocatalytically generated, α-amino radicals possessing adjacent conjugated alkenes, affording either bicyclic or tetracyclic products, is described. Under net reductive conditions using commercial Hantzsch ester, the α-amino radical species underwent a single stereoselective cyclisation to give trans-configured amino-indane structures in good yield, whereas using a substituted Hantzsch ester as a milder reductant afforded cis-fused tetracyclic tetrahydroquinoline frameworks, resulting from two consecutive radical cyclisations. Judicious choice of the reaction conditions allowed libraries of both single and dual cyclisation products to be synthesised with high selectivity, notable predictability, and good-to-excellent yields. Computational analysis employing DFT revealed the reaction pathway and mechanistic rationale behind this finely balanced yet readily controlled photocatalytic system.

Metal-Free Aminomethylation of Aromatic Sulfones Promoted by Eosin Y

A metal-free α-aminomethylation of heteroaryls promoted by eosin Y under green light irradiation is reported. A large variety of α-trimethylsilylamines as precursor of α-aminomethyl radical species were engaged to functionalize sulfonyl-heteroaryls following a Homolytic Aromatic Substitution (HAS) pathway. This method has provided a range of α-aminoheteroaryl compounds including a functionalized natural product. The mechanism of this late-stage functionalization of aryls was investigated and suggests the formation of a sulfonyl radical intermediate over a reductive quenching cycle.

Anti-Markovnikov hydro(amino)alkylation of vinylarenes via photoredox catalysis

Photoredox catalysis is a powerful means to generate odd-electron species under mild reaction conditions from a wide array of radical precursors. Herein, we present the application of this powerful catalytic manifold to address the hydroalkylation and hydroaminoalkylation of electronically diverse vinylarenes. This reaction allows for generalized alkene hydroalkylation leveraging common alkyl radical precursors, such as organotrifluoroborate salts and carboxylic acids. Furthermore, utilizing easily accessible α-silyl amine reagents or tertiary amines directly, secondary and tertiary amine moieties can be installed onto monoaryl and diaryl alkenes to access valuable products, including γ,γ-diarylamines pharmacophores. Thus, under a unified system, both hydroalkylation and hydroaminoalkylation of alkenes are achieved. The substrate scope is evaluated through 57 examples, the synthetic utility of the method is demonstrated, and preliminary mechanistic insights are presented.

Many useful chemical scaffolds include carbon or nitrogen substitutions at two or three atoms away from benzene. Here, the authors show a unified hydroalkylation and hydroaminoalkylation protocol to access these structures via a regioselective photocatalytic addition to simple styrenes.

Cooperative Photoredox- and Nickel-Catalyzed Alkylative Cyclization Reactions of Alkynes with 4-Alkyl-1,4-dihydropyridines

Cooperative photoredox- and nickel-catalyzed alkylative cyclization reactions of iodoalkynes with 4-alkyl-1,4-dihydropyridines as alkylation reagents under visible light irradiation have been achieved to afford the corresponding alkylated cyclopentylidenes in good to high yields. Introduction of substituents at the propargylic position of iodoalkynes has led to the stereoselective formation of E-isomers. The present reaction system provides a novel synthetic method for alkylative cyclization reactions of both terminal and internal alkynes with cooperative photoredox and nickel catalysis.

Direct α-Alkenylation of Cyclic Amines with Maleimides through Fe(III)-Catalyzed C(sp3)-H/C(sp2)-H Cross Dehydrogenative Coupling

Presented herein is a novel and efficient α-C(sp³)-H alkenylation of cyclic amines with maleimides. Mechanistically, this C(sp³)-H/C(sp²)-H cross dehydrogenative coupling (CDC) reaction involves a cascade procedure including oxidative α-amino radical formation from the cyclic amine substrate and nucleophilic addition of the in situ formed α-amino radical onto the electron-deficient carbon-carbon double bond of maleimide followed by oxidation and β-elimination. Notably, this direct α-functionalization provides an effective alternative to the conventional ionic reaction mode, in which an imine or iminium intermediate is formed to react with electron-rich coupling partners other than electron-deficient ones. In general, this method features readily available and structurally diverse substrates, a green and economical catalyst, a unique reaction pathway, mild reaction conditions, high efficiency, and excellent atom economy. This new reaction enriches the application of Fe(III)-catalyzed C(sp³)-H activation and functionalization.

Visible-Light-Induced C-H Bond Aminoalkylation of Heterocycles by the Decarboxylation Coupling of Amino Acids

An efficient visible-light-induced decarboxylative coupling reaction of N-protecting α-amino acids with heterocycles for the generation of aminoalkylated heterocycles is presented. A series of aminoalkylated heterocycles were obtained in moderate to good yields. Attractive features of this process include the generation of aminomethyl radical by an inexpensive organic photocatalyst under transition-metal-free conditions.