Pushing Boundaries: What's Next in Metal-Free C-H Functionalization for Sulfenylation?

The synthesis of thioether derivatives has been explored widely due to the potential application of these derivatives in medicinal chemistry, pharmaceutical industry and material chemistry. Within this context, there has been an increasing demand for the environmentally benign construction of C-S bonds via C-H functionalization under metal-free conditions. In the present article, we highlight recent developments in metal-free sulfenylation that have occurred in the past three years. The synthesis of organosulfur compounds via a metal-free approach using a variety of sulfur sources, including thiophenols, disulfides, sulfonyl hydrazides, sulfonyl chlorides, elemental sulfur and sulfinates, is discussed. Non-conventional strategies, which refer to the development of thioether derivatives under visible light and electrochemically mediated conditions, are also discussed. The key advantages of the reviewed methodologies include broad substrate scope and high reaction yields under environmentally benign conditions. This comprehensive review will provide chemists with a synthetic tool that will facilitate further development in this field.

Identification of novel 3-aryl-1-aminoisoquinolines-based KRASG12C inhibitors: Rational drug design and expedient construction by CH functionalization/annulation

Developing a synthetic methodology to expediently construct a specific drug scaffold with the desired biological activity remains challenging. Herein, we describe a work on rational application of a synthetic methodology in the synthesis of KRASG12C inhibitors. Novel KRASG12C inhibitors were initially designed with 1-amino-3-aryl isoquinoline scaffold using structure-based drug design strategy. A ruthenium-catalyzed direct monoCH functionalization/annulation cascade reaction of amidines and sulfoxonium ylides was then developed with high versatility of substrates and good tolerance for polar functional groups. By using this reaction, the target compounds 1-amino-3-aryl isoquinolines were facilely prepared. Further in vitro tests led to identification of two novel lead compounds with KRASG12C inhibitory activity.

Palladium-Catalyzed Dearomatization of Benzothiophenes: Isolation and Functionalization of a Discrete Dearomatized Intermediate

A Pd-catalyzed decarboxylative dearomatization reaction of a heterocyclic substrate enables access to an uncommon reaction intermediate that rearomatizes in the presence of amine bases in a net C-H functionalization sequence. The dearomatized benzo[b]thiophene intermediate bears an exocyclic alkene that can be functionalized through cycloaddition and halogenation reactions to deliver complex heterocyclic products.

Directed evolution of P411 enzymes for amination of inert C-H bonds

Functionalizing inert C-H bonds selectively is a formidable task due to their strong bond energy and the difficulty of distinguishing chemically similar C-H bonds. While enzymatic oxygenation of C-H bonds is ubiquitous and well established, there is currently no known natural enzymatic process for direct nitrogen insertion. Instead, nature typically relies on pre-oxidized compounds for nitrogen incorporation. Direct biocatalytic C-H amination methods developed in the last few years are only selective for activated C-H bonds that contain specific groups such as benzylic, allylic, or propargylic groups. However, we recently used directed evolution to generate cytochrome P411 enzymes (engineered P450 enzymes with axial ligand mutation from cysteine to serine) that directly aminate inert C-H bonds with high site-, diastereo-, and enantioselectivity. Using these enzymes, we demonstrated the regiodivergent desymmetrization of methylcyclohexane, among other reactions. This chapter provides a comprehensive account of the experimental protocols used to evolve P411s for aminating unactivated C-H bonds. These methods are illustrative and can be adapted for other directed enzyme evolution campaigns.

Self-sufficient P450-reductase chimeras for biocatalysis

In recent years, cytochromes P450 have emerged as powerful, versatile biocatalysts for the site-selective functionalization of small molecules. Catalyzing an impressive range of chemical transformations, these enzymes have been widely used to effect C-H oxidation, biaryl coupling, and carbon-heteroatom bond formation, among many other reactions. However, the majority of P450s are multi-protein systems that employ secondary redox partners in key steps of the catalytic cycle, which limits their broader applicability. In response, the discovery of self-sufficient P450s, such as P450BM3 and P450RhF, has provided a template for the construction of artificial, self-sufficient P450-reductase fusions. In this chapter, we describe a procedure for the design, assembly, and application of two engineered, self-sufficient P450s of Streptomyces origin via fusion with an exogenous reductase domain. In particular, we generated artificial chimeras of P450s PtmO5 and TleB by linking them covalently with the reductase domain of P450RhF. Upon verification of their activities, both enzymes were employed in preparative-scale biocatalytic reactions. This approach can feasibly be applied to any P450 of interest, thereby laying the groundwork for the production of self-sufficient P450s for diverse chemical applications.

C-H Bond Functionalization of N-Heteroarenes Mediated by Selectfluor

Herein, recent developments for Selectfluor-mediated C-H functionalization of N-heteroarenes are described. This type of C-H bond activation is an attractive and competitive alternative to traditional methodologies, allowing the functionalization of a variety of chemical functions. In addition, Selectfluor is a more sustainable and economically accessible oxidant compared with expensive/toxic metals or hazardous peroxides. For a practical understanding, the current review classified systematically the reported strategies in four subsections as follows: (1) carbon-carbon formation, (2) carbon-nitrogen bond formation, (3) carbon-chalcogen bond, and (4) carbon-halogen bond formation. Mechanistic aspects and reaction conditions are fully discussed to provide an understanding of the aspects that govern C-H functionalization in N-heteroarenes mediated by Selectfluor.

Synthesis and Fluorescence Properties of Imidazopyridine-Linked Coumarins via Tandem C(sp2)-H Functionalization/Decarboxylation Reaction

A catalyst-, oxidant-free and green synthetic route for direct access to a series of novel imidazopyridine-linked coumarins has been devised through tandem C(sp²)-H functionalization/decarboxylation reaction in ethyl acetate as a sustainable medium. Moreover, the utilities of ensured products in further organic synthesis were conducted by Suzuki-Miyaura and Sonogashira cross-coupling reactions. The fluorescence characteristics of the produced molecules are appropriate, and the synthesized scaffolds could promisingly garner future attention in clinical diagnostics and bioimaging research.

Recent Developments in Direct C-H Functionalization of Quinoxalin-2(1H)-Ones via Heterogeneous Catalysis Reactions

In recent years, Web of Science has published nearly one hundred reports per year on quinoxalin-2(1H)-ones, which have attracted great interest due to their wide applications in pharmaceutical and materials fields, especially in recyclable heterogeneous catalytic reactions for direct C-H functionalisation. This review summarises for the first time the methods and reaction mechanisms of heterogeneous catalytic reactions of quinoxalin-2(1H)-ones, including six major types of heterogeneous catalysts involved. The heterogeneous reactions of quinoxalin-2(1H)-ones are summarised by classifying different types of catalytic materials (graphitic phase carbon nitride, MOF, COF, ion exchange resin, piezoelectric materials, and microsphere catalysis). In addition, this review discusses the future development of heterogeneous catalytic reactions of quinoxalin-2(1H)-ones, including the construction of C-B/Si/P/R_F/X/Se bonds by heterogeneous catalytic reactions, the enrichment of heterogeneous catalysts such as metal oxides, graphene-based composites, doped metal nanoparticles, and molecular sieve-based porous materials, asymmetric synthesis, and other areas. The aim of this review is to contribute to the development of green and sustainable heterogeneous reaction methods for quinoxalin-2(1H)-ones with applications in materials chemistry and pharmacology.

Pyridine C(sp2)-H bond functionalization under transition-metal and rare earth metal catalysis

Pyridine is a crucial heterocyclic scaffold that is widely found in organic chemistry, medicines, natural products, and functional materials. In spite of the discovery of several methods for the synthesis of functionalized pyridines or their integration into an organic molecule, new methodologies for the direct functionalization of pyridine scaffolds have been developed during the past two decades. In addition, transition-metal-catalyzed C-H functionalization and rare earth metal-catalyzed reactions have flourished over the past two decades in the development of functionalized organic molecules of concern. In this review, we discuss recent achievements in the transition-metal and rare earth metal-catalyzed C-H bond functionalization of pyridine and look into the mechanisms involved.

Strategies Toward the Catalyst-Free α-C-H Functionalizations of Tertiary Amines

α-C-H functionalization of tertiary amines has been a highly studied field for the past two decades because several important nitrogen containing heterocycles or compounds can be synthesized through this strategy. Though transition metal catalysts and some metal-free catalysts are mainly used for these reactions, a few catalyst-free reactions have recently been efficiently performed. Catalyst-free reactions are cost-effective, less sensitive to air/moisture, easier to operate, have a simple purification process, and are relatively environment-friendly. In this article, we have summarized all the α-C-H functionalization reactions of tertiary amines performed without using any external catalysts. The content of this article will undoubtedly encourage readers to do more work in this area.

An update on late-stage functionalization in today's drug discovery

INTRODUCTION

Late-stage functionalization (LSF) allows for the introduction of new chemical groups toward the end of a synthetic sequence, which means new molecules can be rapidly accessed without laborious de novo chemical synthesis. Over the last decade, medicinal chemists have begun to implement LSF strategies into their drug discovery programs, affording benefits such as efficient access to diverse libraries to explore structure-activity relationships and the improvement of physicochemical and pharmacokinetic properties.

AREAS COVERED

An overview of the key advancements in LSF methodology development from 2019 to 2022 and their applicability to drug discovery is provided. In addition, several examples from both academia and industry where LSF methodologies have been applied by medicinal chemists to their drug discovery programs are presented.

EXPERT OPINION

Utilization of LSF by medicinal chemists is on the rise, both in academia and in industry. The maturation of the LSF field to produce methodologies bearing increased regioselectivity, scope, and functional group tolerance is envisaged to narrow the gap between methodology development and medicinal chemistry research. The authors predict that the sheer versatility of these techniques in facilitating challenging chemical transformations of bioactive molecules will continue to increase the efficiency of the drug discovery process.

Recent Advances in Regioselective C-H Bond Functionalization of Free Phenols

Phenols are important readily available synthetic building blocks and starting materials for organic synthetic transformations, which are widely found in agrochemicals, pharmaceuticals, and functional materials. The C-H functionalization of free phenols has proven to be an extremely useful tool in organic synthesis, which provides efficient increases in phenol molecular complexity. Therefore, approaches to functionalizing existing C-H bonds of free phenols have continuously attracted the attention of organic chemists. In this review, we summarize the current knowledge and recent advances in ortho-, meta-, and para-selective C-H functionalization of free phenols in the last five years.

Noble-Metal-Free C-H Allylation of Tetrahydroisoquinolines Using a Cobalt-Organophotoredox Dual Catalyst System

Metallaphotoredox-catalyzed allylation represents an emerging synthetic methodology that enables allylic substitution using nucleophilic radical species. The C-H allylation of N-aryl tetrahydroisoquinolines is an innovative example in this area and allows access to synthetically useful precursors for the further derivatization of tetrahydroisoquinolines. However, previous methods have required the use of noble metals, which has hampered their application due to concerns over their sustainability. Here we report the C-H allylation of N-aryl tetrahydroisoquinolines using a cobalt/organophotoredox dual catalyst system. Based on precedent, control experiments and controlled irradiation experiments, a mechanism for the cobalt/photoredox-catalyzed allylation that involves a π-allyl cobalt complex is proposed.

Photocatalytic sequential C-H functionalization expediting acetoxymalonylation of imidazo heterocycles

The importance of functionalized imidazo heterocycles has often been featured in several impactful research both from academia and industry. Herein, we report a direct C-3 acetoxymalonylation of imidazo heterocycles using relay C-H functionalization enabled by organophotocatalysis starring zinc acetate in the triple role of an activator, ion scavenger as well as an acetylating reagent. The mechanistic investigation revealed a sequential sp² and sp³ C-H activation, followed by functionalization driven by zinc acetate coupled with the photocatalyst PTH. A variety of imidazo[1,2-a]pyridines and related heterocycles were explored as substrates along with several active methylene reagents, all generating the products with excellent yields and regioselectivity, thus confirming excellent functional group tolerability.

Aliphatic C-H Functionalization Using Pyridine N-Oxides as H-Atom Abstraction Agents

The alkylation and heteroarylation of unactivated tertiary, secondary, and primary C(sp3)-H bonds was achieved by employing an acridinium photoredox catalyst along with readily available pyridine Noxides as hydrogen atom transfer (HAT) precursors under visible light. Oxygen-centered radicals, generated by single-electron oxidation of the Noxides, are the proposed key intermediates whose reactivity can be easily modified by structural adjustments. A broad range of aliphatic C-H substrates with electron-donating or -withdrawing groups as well as various olefinic radical acceptors and heteroarenes were well tolerated.

Demetallation of organometallic and metal-mediated reactions

The use of stoichiometric organometallic reagents and stoichiometric metals formed the basis of vast majority of classical reactions for constructing carbon-carbon bonds. The indispensable requirement of stoichiometric metals for such reactions constitutes significant challenges in terms of resource sustainability, operational safety, and chemical-waste management. The recent developments in C-H functionalizations, hydrogenative alkene/alkyne addition to electrophiles, the hydrazone umpolung chemistry, and other emerging fields such as the electrosynthesis and photoredox chemistry provide potential solutions to overcome these inherent challenges.

Controlling Non-Native Cobalamin Reactivity and Catalysis in the Transcription Factor CarH

Vitamin B₁₂ derivatives catalyze a wide range of organic transformations, but B₁₂-dependent enzymes are underutilized in biocatalysis relative to other metalloenzymes. In this study, we engineered a variant of the transcription factor CarH, called CarH*, that catalyzes styrene C-H alkylation with improved yields (2-6.5-fold) and selectivity relative to cobalamin. While the native function of CarH involves transcription regulation via adenosylcobalamin (AdoCbl) Co(III)-carbon bond cleavage and β-hydride elimination to generate 4',5'-didehydroadenosine, CarH*-catalyzed styrene alkylation proceeds via non-native oxidative addition and olefin addition coupled with a native-like β-hydride elimination. Mechanistic studies on this reaction echo findings from earlier studies on AdoCbl homolysis to suggest that CarH* selectivity results from its ability to impart a cage effect on radical intermediates. These findings lay the groundwork for the development of B₁₂-dependent enzymes as catalysts for non-native transformations.

DABCO-promoted photocatalytic C-H functionalization of aldehydes

Herein we present a direct application of DABCO, an inexpensive and broadly accessible organic base, as a hydrogen atom transfer (HAT) abstractor in a photocatalytic strategy for aldehyde C–H activation. The acyl radicals generated in this step were arylated with aryl bromides through a well stablished nickel cross-coupling methodology, leading to a variety of interesting aryl ketones in good yields. We also performed computational calculations to shine light in the HAT step energetics and determined an optimized geometry for the transition state, showing that the hydrogen atom transfer between aldehydes and DABCO is a mildly endergonic, yet sufficiently fast step. The same calculations were performed with quinuclidine, for comparison of both catalysts and the differences are discussed.

Synthesis of Spiroindenyl-2-Oxindoles through Palladium-Catalyzed Spirocyclization of 2-Bromoarylamides and Vinyl Bromides

An expeditious approach to the construction of spiroindenyl-2-oxindoles was developed via a palladium-catalyzed spirocyclization reaction of 2-bromoarylamides with vinyl bromides. The reaction formed spiropalladacycles as the intermediates via carbopalladation and the C–H functionalization of 2-bromoarylamides. The spiropalladacycles reacted with vinyl bromides to form spiroindenyl-2-oxindoles. A Heck process rather than vinylic C–H functionalization was involved in the reaction.

Ten Years of Glory in the α-Functionalizations of Acetophenones: Progress Through Kornblum Oxidation and C-H Functionalization

This review article focuses on the α-functionalization of acetophenones involving Kornblum oxidation and C-H functionalizations. Although various other strategies, such as classical approaches, enamine approaches and umpolung strategy are also known for this functionalization, here we discuss mainly the Kornblum oxidation approach and C-H functionalization strategy as they have advantages over the others. In Kornblum oxidation, the reaction uses iodine and dimethylsulfoxide and proceeds through the formation of arylglyoxal as the key intermediate. In C-H functionalization, the reaction requires metal, or metal-free catalyst, and generates radical intermediate in most cases. α-Functionalization of acetophenones is very important because of their huge applications in the synthesis of various natural products and pharmaceuticals and, therefore, a number of research articles have been published in this area. However, no review articles are available so far. In this article, we present a succinct discussion of various important and novel reactions, along with their mechanisms, published since 2012 to date. We believe that this first review article in this field will give readers one-stop information on this topic and encourage further intriguing work in this area.

Site-selective and metal-free C-H nitration of biologically relevant N-heterocycles

The site-selective and metal-free C-H nitration reaction of quinoxalinones and pyrazinones as biologically important N-heterocycles with t-butyl nitrite is described. A wide range of quinoxalinones were efficiently applied in this transformation, providing C7-nitrated quinoxalinones without undergoing C3-nitration. From the view of mechanistic point, the radical addition reaction exclusively occurred at the electron-rich aromatic region beyond electron-deficient N-heterocycle ring. This is a first report on the C7-H functionalization of quinoxalinones under metal-free conditions. In contrast, the nitration reaction readily takes place at the C3-position of pyrazinones. This transformation is characterized by the scale-up compatibility, mild reaction conditions, and excellent functional group tolerance. The applicability of the developed method is showcased by the selective reduction of NO₂ functionality on the C7-nitrated quinoxalinone product, providing aniline derivatives. Combined mechanistic investigations aided the elucidation of a plausible reaction mechanism.

Additive and Counterion Effects in Iron-Catalyzed Reactions Relevant to C-C Bond Formation

The use of iron catalysts in carbon-carbon bond forming reactions is of interest as an alternative to precious metal catalysts, offering reduced cost, lower toxicity, and different reactivity. While well-defined ligands such as N-heterocyclic carbenes (NHCs) and phosphines can be highly effective in these reactions, additional additives such as N-methylpyrrolidone (NMP), N,N,N',N'-tetramethylethylenediamine (TMEDA), and iron salts that alter speciation can also be employed to achieve high product yields. However, in contrast to well-defined iron ligands, the roles of these additives are often ambiguous, and molecular-level insights into how they achieve effective catalysis are not well-defined. Using a unique physical-inorganic in situ spectroscopic approach, detailed insights into the effect of additives on iron speciation, mechanism, and catalysis can inform further reaction development. In this Perspective, recent advances will be discussed as well as ongoing challenges and potential opportunities in iron-catalyzed reactions.

Engineering P450 TamI as an Iterative Biocatalyst for Selective Late-Stage C-H Functionalization and Epoxidation of Tirandamycin Antibiotics

Iterative P450 enzymes are powerful biocatalysts for selective late-stage C-H oxidation of complex natural product scaffolds. These enzymes represent useful tools for selectivity and cascade reactions, facilitating direct access to core structure diversification. Recently, we reported the structure of the multifunctional bacterial P450 TamI and elucidated the molecular basis of its substrate binding and strict reaction sequence at distinct carbon atoms of the substrate. Here, we report the design and characterization of a toolbox of TamI biocatalysts, generated by mutations at Leu101, Leu244, and/or Leu295, that alter the native selectivity, step sequence, and number of reactions catalyzed, including the engineering of a variant capable of catalyzing a four-step oxidative cascade without the assistance of the flavoprotein and oxidative partner TamL. The tuned enzymes override inherent substrate reactivity, enabling catalyst-controlled C-H functionalization and alkene epoxidation of the tetramic acid-containing natural product tirandamycin. Five bioactive tirandamycin derivatives (6-10) were generated through TamI-mediated enzymatic synthesis. Quantum mechanics calculations and MD simulations provide important insights into the basis of altered selectivity and underlying biocatalytic mechanisms for enhanced continuous oxidation of the iterative P450 TamI.

Recent Opportunities and Challenges in Selective C-H Functionalization of Methyl Azaarenes: a Highlight from 2010 to 2020 Literatures

Azaarenes are unique scaffolds frequently found in pharmaceuticals. Herein, we have summarized the recent synthetic available methods in C-H functionalization of methylazaarenes from 2010 to 2020. Multiple approaches involving halogenation and alkylation via different methods, such as alkenylation, oxidative functionalization, and cyclization of the methylazaarenes will be discussed in this review.

Metal-Free Direct C-H Functionalization of Quinoxalin-2(1H)-Ones to Produce 3-Vinylated Quinoxalin-2(1H)-Ones in the Presence of Alkenes

A novel and efficient C 3-H vinylation reaction with quinoxalin-2(1H)-one as the substrate, in the presence of alkenes, under metal-free conditions, is reported herein. The reaction leads to the formation of new carbon-carbon bonds that exhibit moderate to good reactivities. The vinylation of quinoxalin-2(1H)-ones, in the presence of alkenes, is an attractive process that can be potentially utilized to produce biologically active 3-vinylated quinoxalin-2(1H)-ones.

Recent Advances on Synthetic Methodology Merging C-H Functionalization and C-C Cleavage

The functionalization of C-H bonds has become a major thread of research in organic synthesis that can be assessed from different angles, for instance depending on the type of catalyst employed or the overall transformation that is carried out. This review compiles recent progress in synthetic methodology that merges the functionalization of C-H bonds along with the cleavage of C-C bonds, either in intra- or intermolecular fashion. The manuscript is organized in two main sections according to the type of substrate in which the cleavage of the C-C bond takes place, basically attending to the scission of strained or unstrained C-C bonds. Furthermore, the related research works have been grouped on the basis of the mechanistic aspects of the different transformations that are carried out, i.e.,: (a) classic transition metal catalysis where organometallic intermediates are involved; (b) processes occurring via radical intermediates generated through the use of radical initiators or photochemically; and (c) reactions that are catalyzed or mediated by suitable Lewis or Brønsted acid or bases, where molecular rearrangements take place. Thus, throughout the review a wide range of synthetic approaches show that the combination of C-H and C-C cleavage in single synthetic operations can serve as a platform to achieve complex molecular skeletons in a straightforward manner, among them interesting carbo- and heterocyclic scaffolds.

Recent Advances in Rapid Synthesis of Non-proteinogenic Amino Acids from Proteinogenic Amino Acids Derivatives via Direct Photo-Mediated C-H Functionalization

Non-proteinogenic amino acids have attracted tremendous interest for their essential applications in the realm of biology and chemistry. Recently, rising C-H functionalization has been considered an alternative powerful method for the direct synthesis of non-proteinogenic amino acids. Meanwhile, photochemistry has become popular for its predominant advantages of mild conditions and conservation of energy. Therefore, C-H functionalization and photochemistry have been merged to synthesize diverse non-proteinogenic amino acids in a mild and environmentally friendly way. In this review, the recent developments in the photo-mediated C-H functionalization of proteinogenic amino acids derivatives for the rapid synthesis of versatile non-proteinogenic amino acids are presented. Moreover, postulated mechanisms are also described wherever needed.

Recent Developments in Transition-Metal Catalyzed Direct C-H Alkenylation, Alkylation, and Alkynylation of Azoles

The transition metal-catalyzed C–H bond functionalization of azoles has emerged as one of the most important strategies to decorate these biologically important scaffolds. Despite significant progress in the C–H functionalization of various heteroarenes, the regioselective alkylation and alkenylation of azoles are still arduous transformations in many cases. This review covers recent advances in the direct C–H alkenylation, alkylation and alkynylation of azoles utilizing transition metal-catalysis. Moreover, the limitations of different strategies, chemoselectivity and regioselectivity issues will be discussed in this review.

The Cyclic Nitronate Route to Pharmaceutical Molecules: Synthesis of GSK's Potent PDE4 Inhibitor as a Case Study

An efficient asymmetric synthesis of GlaxoSmithKline’s potent PDE4 inhibitor was accomplished in eight steps from a catechol-derived nitroalkene. The key intermediate (3-acyloxymethyl-substituted 1,2-oxazine) was prepared in a straightforward manner by tandem acylation/(3,3)-sigmatropic rearrangement of the corresponding 1,2-oxazine-N-oxide. The latter was assembled by a (4 + 2)-cycloaddition between the suitably substituted nitroalkene and vinyl ether. Facile acetal epimerization at the C-6 position in 1,2-oxazine ring was observed in the course of reduction with NaBH₃CN in AcOH. Density functional theory (DFT) calculations suggest that the epimerization may proceed through an unusual tricyclic oxazolo(1,2)oxazinium cation formed via double anchimeric assistance from a distant acyloxy group and the nitrogen atom of the 1,2-oxazine ring.

When metal-catalyzed C-H functionalization meets visible-light photocatalysis

While aiming at sustainable organic synthesis, over the last decade particular attention has been focused on two modern fields, C-H bond activation, and visible-light-induced photocatalysis. Couplings through C-H bond activation involve the use of non-prefunctionalized substrates that are directly converted into more complex molecules, without the need of a previous functionalization, thus considerably reduce waste generation and a number of synthetic steps. In parallel, transformations involving photoredox catalysis promote radical reactions in the absence of radical initiators. They are conducted under particularly mild conditions while using the visible light as a cheap and economic energy source. In this way, these strategies follow the requirements of environment-friendly chemistry. Regarding intrinsic advantages as well as the complementary mode of action of the two catalytic transformations previously introduced, their merging in a synergistic dual catalytic system is extremely appealing. In that perspective, the scope of this review aims to present innovative reactions combining C-H activation and visible-light induced photocatalysis.

"On Water" Palladium Catalyzed Direct Arylation of 1H-Indazole and 1H-7-Azaindazole

The C3 direct arylation of 1H-indazole and 1H-7-azaindazole has been a significant challenge due to the lack of the reactivity at this position. In this paper, we describe a mild and an efficient synthesis of new series of C3-aryled 1H-indazoles and C3-aryled 1H-7-azaindazoles via a C3 direct arylation using water as solvent. On water, PPh₃ was effective as a ligand along with a lower charge of the catalyst Pd(OAc)₂ (5 mol%) at 100 °C, leading to C3-aryled 1H-indazoles or C3-aryled 1H-7-azaindazoles in moderate to good yields.

Photocatalytic trifluoromethoxylation of arenes and heteroarenes in continuous-flow

The first example of photocatalytic trifluoromethoxylation of arenes and heteroarenes under continuous-flow conditions is described. Application of continuous-flow microreactor technology allowed to reduce the residence time up to 16 times in comparison to the batch procedure, while achieving similar or higher yields. In addition, the use of inorganic bases was demonstrated to increase the reaction yield under batch conditions.

Fluorofunctionalizations of C-C Multiple Bonds and C-H Bonds

In spite of only a few naturally occurring products having one or more fluorine atoms, organofluorine compounds have been widely utilized in pharmaceutical, agrochemical, and functional material science fields due to the characteristic properties of the fluorine atom. Therefore, the development of new methods for the introduction of fluorine-containing functional groups has been a long-standing research topic. This article discusses our contributions to this area. The first topic is on the trifluoromethylations of C-C multiple bonds using Togni reagent based on our working hypothesis that hypervalent iodine could be activated by coordination of the carbonyl moiety to the Lewis acid catalyst. The second topic relates to asymmetric fluorofunctionalization of alkenes. A newly designed phase-transfer catalyst consisting of a carboxylate anion functioning as a phase-transfer agent and a primary hydroxyl group as a site that captures the anionic substrate was revealed to be an effective catalyst for asymmetric fluorolactonization. Inspired by the mechanistic studies of fluorolactonization, we produced a linked binaphthyl dicarboxylate catalyst, which catalyzes the 6-endo-fluorocyclization and the deprotonative fluorination of allylic amides in a highly enantioselective manner. The third topic is on C-H fluorofunctionalizations using either catalysis or photoactivation. Benzylic trifluoromethylation, which is still a rare reaction, using Togni reagent and aromatic C-H trifluoromethylation using Umemoto reagent under simple photoirradiation conditions were achieved. In addition, the Csp³-H fluorination of alkyl phthalimide derivatives is demonstrated.

Photocatalytic deaminative benzylation and alkylation of tetrahydroisoquinolines with N-alkylpyrydinium salts

A ruthenium-catalyzed photoredox coupling of substituted N-aryltetrahydroisoquinolines (THIQs) and different bench-stable pyridinium salts was successfully developed to give fast access to 1-benzyl-THIQs. Furthermore, secondary alkyl and allyl groups were also successfully introduced via the same method. Additionally, the typically applied N-phenyl group in the THIQ substrate could be replaced by the cleavable p-methoxyphenyl (PMP) group and successful N-deprotection was demonstrated.

Synthesis of triphenylene-fused phosphole oxides via C-H functionalizations

The synthesis of triphenylene-fused phosphole oxides has been achieved through two distinct C–H functionalization reactions as key steps. The phosphole ring was constructed by a three-component coupling of 3-(methoxymethoxy)phenylzinc chloride, an alkyne, and dichlorophenylphosphine, involving the regioselective C–H activation of the C2 position of the arylzinc intermediate via 1,4-cobalt migration. The resulting 7-hydroxybenzo[b]phosphole derivative was used for further π-extension through Suzuki–Miyaura couplings and a Scholl reaction, the latter closing the triphenylene ring. The absorption and emission spectra of the thus-synthesized compounds illustrated their nature as hybrids of triphenylene and benzo[b]phosphole.

Synthesis of Elaborate Benzofuran-2-carboxamide Derivatives through a Combination of 8-Aminoquinoline Directed C-H Arylation and Transamidation Chemistry

Herein, we present a short and highly modular synthetic route that involves 8-aminoquinoline directed C-H arylation and transamidation chemistry, and which enables access to a wide range of elaborate benzofuran-2-carboxamides. For the directed C-H arylation reactions, Pd catalysis was used to install a wide range of aryl and heteroaryl substituents at the C3 position of the benzofuran scaffold in high efficiency. Directing group cleavage and further diversification of the C3-arylated benzofuran products were then achieved in a single synthetic operation through the utilization of a one-pot, two-step transamidation procedure, which proceeded via the intermediate N-acyl-Boc-carbamates. Given the high efficiency and modularity of this synthetic strategy, it constitutes a very attractive method for generating structurally diverse collections of benzofuran derivatives for small molecule screening campaigns.

Regio- and Stereoselective Rhodium(II)-Catalyzed C-H Functionalization of Cyclobutanes

Recent developments in controlled C-H functionalization transformations continue to inspire new retrosynthetic disconnections. One tactic in C-H functionalization is the intermolecular C-H insertion reaction of rhodium bound carbenes. These intermediates can undergo highly selective transformations through the modulation of the ligand framework of the rhodium catalyst. This work describes our continued efforts towards differentiating C-H bonds in the same molecule by judicious catalyst choice. Substituted cyclobutanes which exist as a mixture of interconverting conformers and possess neighboring C-H bonds within a highly strained framework are the targets herein for challenging the current suite of catalysts. While most C-H functionalization tactics focus on generating 1,2-disubstituted cyclobutanes via substrate-controlled directing group methods, the regiodivergent methods in this paper provide access to chiral 1,1-disubstituted and cis-1,3-disubstituted cyclobutanes simply by changing the catalyst identity, thus permitting entry to novel chemical space.

A Continuing Career in Biocatalysis: Frances H. Arnold

On the occasion of Professor Frances H. Arnold's recent acceptance of the 2018 Nobel Prize in Chemistry, we honor her numerous contributions to the fields of directed evolution and biocatalysis. Arnold pioneered the development of directed evolution methods for engineering enzymes as biocatalysts. Her highly interdisciplinary research has provided a ground not only for understanding the mechanisms of enzyme evolution but also for developing commercially viable enzyme biocatalysts and biocatalytic processes. In this Account, we highlight some of her notable contributions in the past three decades in the development of foundational directed evolution methods and their applications in the design and engineering of enzymes with desired functions for biocatalysis. Her work has created a paradigm shift in the broad catalysis field.

Evaluation of Transition Metal Catalysts in Electrochemically Induced Aromatic Phosphonation

Voltammetry provides important information on the redox properties of catalysts (transition metal complexes of Ni, Co, Mn, etc.) and their activity in electrocatalytic reactions of aromatic C–H phosphonation in the presence of a phosphorus precursor, for example, dialkyl-H-phosphonate. Based on catalytic current growth of oxidation or reduction of the metal catalysts (Co^II, Mn^II, Ni^II, Mn^II/Ni^II, Mn^II/Co^II, and Co^II/Ni^II), quantitative characteristics of the regeneration of catalysts were determined, for example, for Mn^II, Ni^II and Mn^II/Ni^II, Co^II/Ni^II pairs. Calculations confirmed the previously made synthetic observations on the synergistic effect of certain metal ions in binary catalytic systems (Mn^IIbpy/Ni^IIbpy and Ni^IIbpy/Co^IIbpy); for mixtures, the observed rate constants, or TOF, were 690 s⁻¹ and 721 s⁻¹, respectively, and product yields were higher for monometallic catalytic systems (up to 71% for bimetallic catalytic systems and ~30% for monometallic catalytic systems). In some cases, the appearance of pre-waves after adding H-phosphonates confirmed the preceding chemical reaction. It also confirmed the formation of metal phosphonates in the time scale of voltammetry, oxidizing or reducing at lower potentials than the original (RO)₂P(O)H and metal complex, which could be used for fast diagnostics of metal ion and dialkyl-H-phosphonate interactions. Electrochemical transfer of an electron to (from) metal phosphonate generates a phosphonyl radical, which can then react with different arenes to give the products of aromatic C–H phosphonation.

Electrochemical C(sp3)-H Fluorination

A simple and robust method for electrochemical alkyl C-H fluorination is presented. Using a simple nitrate additive, a widely available fluorine source (Selectfluor), and carbon-based electrodes, a wide variety of activated and unactivated C-H bonds were converted to their C-F congeners. The scalability of the reaction was also demonstrated with a 100 gram preparation of fluorovaline.

Electrochemical Umpolung of Bromide: Transition-Metal-Free Bromination of Indole C⁻H Bond

A facile and sustainable electrochemical umpolung of bromide ion protocol was developed under mild reaction conditions. Transition metal catalysts and exogenous chemical oxidants were obviated for the bromination of C⁻H bond. Notably, graphite rod, which is commercially available at supermarkets and is inexpensive, was employed as the electrode material. This operationally easy and environmentally friendly approach accomplished the synthesis of 3-bromoindole in excellent yield and regioselectivity.

Metal-free C-H mercaptalization of benzothiazoles and benzoxazoles using 1,3-propanedithiol as thiol source

A facile and effective C-H functionalization strategy for the synthesis of 2-mercaptobenzothiazoles and 2-mercaptobenzoxazoles is described. 1,3-Propanedithiol was employed to convert benzothiazoles and benzoxazoles to the corresponding heteroarylthiols in the presence of potassium hydroxide and DMSO. This novel protocol is featured by direct C-H mercaptalization of heteroarenes and a simple reaction system.

Transition metal-free oxidative and deoxygenative C-H/C-Li cross-couplings of 2H-imidazole 1-oxides with carboranyl lithium as an efficient synthetic approach to azaheterocyclic carboranes

The direct C-H functionalization methodology has first been applied to perform transition metal-free C-H/C-Li cross-couplings of 2H-imidazole 1-oxides with carboranyllithium. This atom- and step-economical approach, based on one-pot reactions of nucleophilic substitution of hydrogen (SN H) in non-aromatic azaheterocycles, affords novel imidazolyl-modified carboranes of two types (N-oxides and their deoxygenative analogues), which are particularly of interest in the design of advanced materials.

Development of Highly Chemoselective Oxidative Transformations by Designing Organoradicals

To conduct organic synthesis in the field of pharmaceutical science, methodologies that can easily and quickly supply compounds with high drug-likeness are highly desirable. Based on the original catalyst design concept "Radical-Conjugated Redox Catalysis (RCRC)" established during my research, various C(sp³)-H functionalizations and protein modifications have been developed, taking advantage of the high reactivity and chemoselectivity of the single-electron transfer process. This review focuses on the eight-year research efforts by my collaborators and me, from conception to results.

Rhodium(III)-catalyzed C-H functionalization of C-alkenyl azoles with sulfoxonium ylides for the synthesis of bridgehead N-fused [5,6]-bicyclic heterocycles

The synthesis of bridgehead N-fused [5,6]-bicyclic heterocycles via rhodium(III)-catalyzed C-H functionalization of C-alkenyl azoles with sulfoxonium ylides is disclosed. Reactions proceeded in good to high yields for a range of aryl, heteroaryl and alkyl sulfoxonium ylides. In addition, 2-alkenyl imidazoles with different substitution patterns as well as C-alkenyl triazoles were effective inputs. The reaction could also be performed under straightforward bench top conditions.

Understanding and Improving the Activity of Flavin-Dependent Halogenases via Random and Targeted Mutagenesis

Flavin-dependent halogenases (FDHs) catalyze the halogenation of organic substrates by coordinating reactions of reduced flavin, molecular oxygen, and chloride. Targeted and random mutagenesis of these enzymes have been used to both understand and alter their reactivity. These studies have led to insights into residues essential for catalysis and FDH variants with improved stability, expanded substrate scope, and altered site selectivity. Mutations throughout FDH structures have contributed to all of these advances. More recent studies have sought to rationalize the impact of these mutations on FDH function and to identify new FDHs to deepen our understanding of this enzyme class and to expand their utility for biocatalytic applications.

Rapid access to the core skeleton of the [3 + 2]-type dimeric pyrrole-imidazole alkaloids by triplet ketone-mediated C-H functionalization

The ability of triplet ketones to abstract a hydrogen atom from hydrocarbons is reminiscent of that of the high-spin metal-oxo complexes in C-H oxidation enzymes. In practice, the reactivity of triplet ketones is easier to control and applicable to promoting a wider range of reactions. We demonstrate herein the synthetic utility of triplet ketone-mediated C-addition of methanol to cyclopentenone derivatives with an expedient synthesis of the core skeleton of the [3+2]-type dimeric pyrrole-imidazole alkaloids. Remarkably, this photochemical C-H functionalization reaction is highly regioselective and can tolerate a good range of functional groups.

Chemical synthesis of 7α-hydroxypregnenolone, a neuroactive steroid that stimulates locomotor activity

7α-Hydroxypregnenolone is an endogenous neuroactive steroid that stimulates locomotor activity. A synthesis of 7α-hydroxypregnenolone from pregnenolone, which takes advantage of an orthogonal protecting group strategy, is described. In detail, the C7-position was oxidized with CrO₃ and 3,5-dimethylpyrazole to yield a 7-keto steroid intermediate. The resulting 7-ketone was stereoselectively reduced to the 7α-hydroxy group with lithium tri-sec-butylborohydride. In contrast, reduction of the same 7-ketone intermediate with NaBH₄ resulted in primarily the 7β-hydroxy epimer. Furthermore, in an alternative route to the target compound, the 7α-hydroxy group was successfully incorporated by direct C-H allylic benzoyloxylation of pregnenolone-3-acetate with CuBr and tert-butyl peroxybenzoate followed by saponification. The disclosed syntheses to 7-oxygenated steroids are amenable to potentially obtain other biologically active sterols and steroids.

Ketone-catalyzed photochemical C(sp3)-H chlorination

Photoexcited arylketones catalyze the direct chlorination of C(sp³)-H groups by N-chlorosuccinimide. Acetophenone is the most effective catalyst for functionalization of unactivated C-H groups while benzophenone provides better yields for benzylic C-H functionalization. Activation of both acetophenone and benzophenone can be achieved by irradiation with a household compact fluorescent lamp. This light-dependent reaction provides a better control of the reaction as compared to the traditional chlorination methods that proceed through a free radical chain propagation mechanism.