Organic Photoredox-Catalyzed Synthesis of δ-Fluoromethylated Alcohols and Amines via 1,5-Hydrogen-Transfer Radical Relay

Hydrogen Atom Transfer Promoted by Carbon-Centered Biradicals via Energy Transfer Catalysis

ConspectusIn the past decade, visible-light-mediated photocatalysis has emerged as an applicable strategy for the generation of diverse radical species via single electron transfer (SET) and energy transfer (EnT) processes. Within this context, visible-light-mediated hydrogen atom transfer (HAT) has attracted major interest due to its mild and environmentally benign conditions applied in the selective activation of C-H bonds. Strategies employing C- and heteroatom-centered radical species to selectively activate C-H bonds have become versatile tools due to their mildness and good functional group compatibility for synthesizing value-added products. In this regard, a review on C-centered radical-promoted HAT processes was reported by Gevorgyan's group ( Chem. Sci. 2020, 11, 12974, DOI: 10.1039/d0sc04881j), and a review on visible-light-promoted remote C-H functionalization via 1,5-HAT was recently reported by Zhu's group ( Chem. Soc. Rev. 2021, 50, 7359, DOI: 10.1039/d0cs00774a). Compared to N- and O-centered radical-promoted HAT processes, C(sp3)-centered radical-promoted HAT is more challenging and less explored due to the small differences in C-H bond dissociation energies. Additionally, in the realm of C-centered radical-promoted HAT, the generation of C-centered radicals has mostly involved SET processes, while the EnT-mediated C-centered radical-promoted HAT process has been less discussed because of the considerable scarcity of related reports.As a result of the rapid advancement of EnT catalysis in synthetic chemistry, C-C-centered biradical species can be readily generated from C═C double bonds via the EnT process using a photocatalyst under visible-light irradiation. An array of transformations (such as cycloaddition and isomerization) involving these C-C-centered biradical species have been reported. Our group recognized these C-C-centered biradicals as practical initiators of the HAT process in C-H functionalization reactions and devoted considerable effort to this field. Initially, we used a triplet excited allene moiety to realize remote sp3 C-H bond activation successfully. Later, a visible-light-induced triplet biradical HAT reaction of diarylethylenes was disclosed by our group. To further enhance its synthetic applicability, we applied simple styrene derivatives to realize such a novel EnT-mediated HAT process. Detailed control experiments combined with comprehensive density functional theory calculations elucidated the reaction mechanisms of these biradical-mediated HAT reactions along with the subsequent transformations of the obtained products.In addition to the advancements achieved in our group, Bach, Petersen, and others also reported such EnT-mediated HAT processes utilizing aryl acrylamide or aryl acrylate derivatives. Given the rapid progress over the past 5 years and the lack of focused discussion in this area, it is necessary to highlight these reports as a complement to the area of visible-light-mediated HAT via carbon-to-carbon processes. We anticipate that this Account will offer worthwhile insights and serve as guidance for future related research.

Visible-Light-Induced Trifluoromethyl Radical Addition to Thiocarbonyl of Thioamide Derivatives

An effective trifluoromethyl radical addition to the thiocarbonyl of thioamide derivatives is described, which produces various trifluoromethylthiolated N-heterocycles such as 6-(trifluoromethylthio)phenanthridine, 2-(trifluoromethylthio)indole, and 2-(trifluoromethylthio)benzothiazole derivatives under visible-light irradiation. The process features advantages such as mild reaction conditions, a cheap and easily available trifluoromethyl source (CF3Br), and green energy, as well as broad substrate scope. The reaction mechanism is investigated in detail, and scale-up experiments are performed.

Photoredox-catalyzed C(sp2)-H trifluoromethylation of 3-methylene-isoindolin-1-ones under metal-free conditions

A facile synthetic method for direct C(sp2)-H bond trifluoromethylation of 3-methylene-isoindolin-1-ones under visible-light-induced metal-free conditions is presented. This protocol features mild reaction conditions, broad substrate scope and excellent functional group tolerance, resulting in a range of structurally diverse trifluoromethylated products in good to excellent yields.

Visible Light-Induced Metal-Free Fluoroalkylations

Fluoroalkylation is a crucial synthetic process that enables the modification of molecules with fluoroalkyl groups, which can enhance the properties of compounds and have potential applications in medicine and materials science. The utilization of visible light-induced, metal-free methods is of particular importance as it provides an environmentally friendly alternative to traditional methods and eliminates the potential risks associated with metal-catalyst toxicity. This Account describes our studies on visible light-induced, metal-free fluoroalkylation processes, which include the use of organic photocatalysts or EDA complexes. We have utilized organophotocatalysts such as Nile red, tri(9-anthryl)borane, and an indole-based tetracyclic complex, as well as catalyst-free EDA chemistry through photoactive halogen bond formation or an unconventional transient ternary complex formation with nucleophilic fluoroalkyl source. A variety of π-systems including arenes/heteroarenes, alkenes, and alkynes have been successfully fluoroalkylated under the developed reaction conditions.

Mn-Mediated Direct Regioselective C-H Trifluoromethylation of Imidazopyridines and Quinoxalines

A simple and highly efficient strategy has been developed for direct C-H trifluoromethylation at C-3 of imidazopyridines and C-8 of quinoxalines with readily available Langlois reagent through KMnO₄/AcOH system. This protocol showed broad substrate scope and afforded moderate-to-excellent yields of both products. It is the first report that the functionalization of quinoxalines occurred regioselectively at the C-8 position of quinoxalines. Mechanistic studies revealed that reaction proceeded via radical pathway.

Visible-light-induced cascade reaction: a sustainable approach towards molecular complexity

Photoredox catalysis has demonstrated rapid evolution in the field of synthetic organic chemistry. On the other hand, the splendour of cascade reactions in providing complex molecular architectures renders them a cutting-edge research area. Therefore, the merging of photocatalysis with cascade synthesis brings out a synthetic paradigm with immense potential. The development of photocascade catalysis for a target molecule with a particular molecular skeleton and stereochemical framework presents certain challenges but provides a robust and environmentally benign synthetic alternative. This comprehensive review assembles all the accomplishments and highlights of visible-light-induced cascade reactions with literature coverage up to October 2022.

Organic Photoredox Catalysts Exhibiting Long Excited-State Lifetimes

Organic photoredox catalysts with a long excited-state lifetime have emerged as promising alternatives to transition-metal-complex photocatalysts. This paper explains the effectiveness of using long-lifetime photoredox catalysts for organic transformations, focusing on the structures and photophysics that enable long excited-state lifetimes. The electrochemical potentials of the reported organic, long-lifetime photocatalysts are compiled and compared with those of the representative Ir(III)- and Ru(II)-based catalysts. This paper closes by providing recent demonstrations of the synthetic utility of the organic catalysts.

1 Introduction

2 Molecular Structure and Photophysics

3 Photoredox Catalysis Performance

4 Catalysis Mediated by Long-Lifetime Organic Photocatalysts

4.1 Photoredox Catalytic Generation of a Radical Species and its Addition to Alkenes

4.2 Photoredox Catalytic Generation of a Radical Species and its Addition to Arenes

4.3 Photoredox Catalytic Generation of a Radical Species and its Addition to Imines

4.4 Photoredox Catalytic Generation of a Radical Species and its Addition to Substrates Having C≡X Bonds (X=C, N)

4.5 Photoredox Catalytic Generation of a Radical Species and its Bond Formation with Transition Metals

4.6 Miscellaneous Reactions of Radical Species Generated by Photoredox Catalysis

5 Conclusions

Recent advances in transition-metal-free trifluoromethylation with Togni's reagents

Transition-metal-free trifluoromethylations have attracted significant research interest driven by the increasing importance of CF3-containing compounds.

Radical addition to the CC bond meets (1,n)-HAT: recent advances in the remote C(sp3)–H or C(sp2)–H functionalization of alkenes

This review summarizes the recent development of remote C(sp3)–H bond or aldehydic C(sp2)–H functionalizations enabled by intermolecular radical addition to CC bond/(1,n)-HAT tandem sequences.

A diastereoselective synthesis of cyclopentanones via photocatalytic reductive alkyltrifluoromethylation of ynones

A photocatalytic reductive alkyltrifluoromethylation of ynones with the Langlois reagent is developed, providing a regio- and diastereoselective access to trifluoromethylated cyclopentanones under mild conditions.

Photoredox-Catalyzed Synthesis of Remote Fluoroalkylated Azaarene Derivatives and the α-Deuterated Analogues via 1,n-Hydrogen-Atom-Transfer-Involving Radical Reactions

A modular strategy to access the remote fluoroalkylated azaarene derivatives and the α-deuterated analogues, which are the isosteres of many pharmaceutically important compounds, is reported. Transformations under the sustainable photoredox catalysis platform could efficiently experience cascade radical addition, 1,n-hydrogen atom transfer (HAT), and single-electron reduction to offer the crucial anions α to azaarenes. Through reacting with H₂O or the inexpensive D₂O, two series of valuable products were obtained in high yields with substantial deuterium incorporation. The work demonstrates that the HAT of the α-sp³ C-H of the electron-withdrawing azaarenes with alkyl radicals is viable.

Visible-light-promoted olefinic trifluoromethylation of enamides with CF3SO2Na

A visible-light-promoted olefinic C-H trifluoromethylation of enamides was developed by employing cheap and stable Langlois' reagent as the CF₃ source. A series of β-CF₃ enamides were obtained in moderate to good yields with high E-isomer selectivity under mild conditions. Preliminary mechanistic studies suggest that molecular oxygen acts as the terminal oxidant for this net oxidative process, and the E isomer selectivity could be well explained by a base-assisted deprotonation of the cation intermediate.

Accessing HIV-1 Protease Inhibitors through Visible-Light-Mediated Sequential Photocatalytic Decarboxylative Radical Conjugate Addition-Elimination-Oxa-Michael Reactions

A photocatalytic decarboxylative radical conjugate addition-elimination-oxa-Michael reaction of hydroxyalkylated carboxylic acids with cyclopentenones is developed to construct diverse cyclopentanonyl-fused functionalized 5-7 membered cyclic ethers. The stereoselective synthetic strategy is amenable to substructural variation, establishing a direct total synthetic route to two diastereomers of C3-amino cyclopentyltetrahydrofuranyl-derived potent HIV-1 protease inhibitors with low nanomolar IC₅₀ values.

Visible light photoredox-catalysed remote C–H functionalisation enabled by 1,5-hydrogen atom transfer (1,5-HAT)

The generation of heteroatom-centred radicals followed by intramolecular 1,5-HAT and functionalisation of the translocated carbon-centred radical is an efficient way to functionalize chemo- and regio-selectively the remote unactivated C(sp³)–H bond.

Organic thermally activated delayed fluorescence (TADF) compounds used in photocatalysis

Organic compounds that show Thermally Activated Delayed Fluorescence (TADF) have become wildly popular as next-generation emitters in organic light emitting diodes (OLEDs). Since 2016, a subset of these have found increasing use as photocatalysts. This review comprehensively highlights their potential by documenting the diversity of the reactions where an organic TADF photocatalyst can be used in lieu of a noble metal complex photocatalyst. Beyond the small number of TADF photocatalysts that have been used to date, the analysis conducted within this review reveals the wider potential of organic donor-acceptor TADF compounds as photocatalysts. A discussion of the benefits of compounds showing TADF for photocatalysis is presented, which paints a picture of a very promising future for organic photocatalyst development.

Catalytic Direct α-Amination of Arylacetic Acid Synthons with Anilines

A unique α-amination approach using various anilines has been developed for arylacetic acids via adaptation as benzazoles. The reaction proceeds through a single electron transfer mechanism utilizing an iron-based catalyst system to access α-(N-arylamino)acetic acid equivalents. Modification of approved drugs, facile cleavage of the benzazole auxiliary, and tolerance of amide linkage forming conditions constitute the potential applicability of this strategy.

Recent Synthetic Applications of the Hypervalent Iodine(III) Reagents in Visible-Light-Induced Photoredox Catalysis

The synergistic combination of visible-light-induced photoredox catalysis with hypervalent iodine(III) reagents (HIRs) represents a particularly important achievement in the field of hypervalent iodine chemistry, and numerous notable organic transformations were achieved in a mild and environmentally benign fashion. This account intends to summarize recent synthetic applications of HIRs in visible-light-induced photoredox catalysis, and they are organized in terms of the photochemical roles of HIRs played in reactions.

Total Synthesis of (-)-Maximiscin

A short, enantioselective synthesis of (-)-maximiscin, a structurally intriguing metabolite of mixed biosynthetic origin, is reported. A retrosynthetic analysis predicated on maximizing ideality and efficiency led to several unusual disconnections and tactics. Formation of the central highly oxidized pyridone ring through a convergent coupling at the end of the synthesis simplified the route considerably. The requisite building blocks could be prepared from feedstock materials (derived from shikimate and mesitylene). Strategies rooted in hidden symmetry recognition, C-H functionalization, and radical retrosynthesis played key roles in developing this concise route.

Transition-metal-free radical relay cyclization of vinyl azides with 1,4-dihydropyridines involving a 1,5-hydrogen-atom transfer: access to α-tetralone scaffolds

The remote C(sp³)–H functionalization enabled by a radical-mediated 1,5-hydrogen-atom transfer (HAT) process using vinyl azides and 1,4-dihydropyridines as precursors has been described.

Exploring the ring-opening reactions of imidazo[1,5-a]quinolines for the synthesis of imides under photochemical conditions

The ring-opening reaction of imidazo[1,5-a]quinolines under photoredox conditions has been described. With Eosin Y as the organophotoredox catalyst, synthetically useful and medicinally important imides were obtained in moderate to excellent yields under mild reaction conditions.