Copper-Catalyzed Trifluoromethylation of Terminal Alkenes through Allylic C–H Bond Activation

Copper-catalysed dehydrogenation or lactonization of C(sp3)-H bonds

Cytochrome P450 enzymes are known to catalyse bimodal oxidation of aliphatic acids via radical intermediates, which partition between pathways of hydroxylation and desaturation1,2. Developing analogous catalytic systems for remote C-H functionalization remains a significant challenge3-5. Here, we report the development of Cu(I)-catalysed bimodal dehydrogenation/lactonization reactions of synthetically common N-methoxyamides through radical abstractions of the γ-aliphatic C-H bonds. The feasibility of switching from dehydrogenation to lactonization is also demonstrated by altering reaction conditions. The use of a readily available amide as both radical precursor and internal oxidant allows for the development of redox-neutral C-H functionalization reactions with methanol as the sole side product. These C-H functionalization reactions using a Cu(I) catalyst with loading as low as 0.5 mol.% is applied to the diversification of a wide range of aliphatic acids including drug molecules and natural products. The exceptional compatibility of this catalytic system with a wide range of oxidatively sensitive functionality demonstrates the unique advantage of using a simple amide substrate as a mild internal oxidant.

Synthesis of meta-carbonyl phenols and anilines

Phenols and anilines are of extreme importance for medicinal chemistry and material science. The development of efficient approaches to prepare both compounds has thus long been a vital research topic. The utility of phenols and anilines directly reflects the identity and pattern of substituents on the benzenoid ring. Electrophilic substitutions remain among the most powerful synthetic methods to substituted phenols and anilines, yet in principle achieving ortho- and para-substituted products. Therefore, the selective preparation of meta-substituted phenols and anilines is the most significant challenge. We herein report an efficient copper-catalyzed dehydrogenation strategy to exclusively synthesize meta-carbonyl phenols and anilines from carbonyl substituted cyclohexanes. Mechanistic studies indicate that this transformation undergoes a copper-catalyzed dehydrogenation/allylic hydroxylation or amination/oxidative dehydrogenation/aromatization cascade process.

Visible-light-induced radical cascade cyclization: a catalyst-free synthetic approach to trifluoromethylated heterocycles

A visible-light-promoted research protocol for constructing dihydropyrido[1,2-a]indolone skeletons is herein described proceeding through a cascade cyclization mediated by trifluoromethyl radicals. This method allows the efficient synthesis of various indole derivatives without the need of photocatalysts or transition-metal catalysts. Mechanism experiments indicate that the process involves a radical chain process initiated by the homolysis of Umemoto's reagent. This straightforward method enables a rapid access to heterocycles containing a trifluoromethyl group.

Efficient Rh-Catalyzed Chemo- and Enantioselective Hydrogenation of 2-CF3-Chromen/Thiochromen-4-ones

A Rh-catalyzed highly chemo- and enantioselective hydrogenation of 2-CF3-chromen/thiochromen-4-ones was successfully established achieving excellent selectivity and high turnover numbers. Under mild conditions, a series of 2-CF3-chromen-4-ones were hydrogenated to provide the corresponding chiral 2-CF3-chroman-4-ones with excellent enantioselectivities (up to 99.9% ee) and achieve high turnover numbers (TON of up to 11,800). Moreover, the obtained hydrogenation products were also successfully transformed into other derivatives including the important intermediate of plasmepsin inhibitors with maintained enantiopurity.

Copper-Catalyzed Visible-Light-Induced Allylic Difluoromethylation of Unactivated Alkenes Using Difluoroacetic Acid

We herein describe a straightforward allylic difluoromethylation reaction of unactivated alkenes. Compared to cross-couplings of prefunctionalized allylic substrates for the construction of allylic CF₂H bonds, this reaction employs readily available alkenes as substrates under mild conditions. Difluoroacetic acid is used as an inexpensive and easy-to-handle source of CF₂H radical under visible light irradiation with PIDA. The copper catalyst plays an important role of diverting the reaction pathway toward allylic difluoromethylation as opposed to previously found hydrodifluoromethylation.

Divergent regioselective Heck-type reaction of unactivated alkenes and N-fluoro-sulfonamides

The control of regioselectivity in Heck-type reaction of unactivated alkenes represents a longstanding challenge due to several detachable hydrogens in β–H elimination step, which generally afford either one specific regioisomer or a mixture. Herein, a copper-catalyzed intermolecular Heck-type reaction of unactivated alkenes and N-fluoro-sulfonamides with divergent regioselectivities is reported. The complete switch of regioselectivity mainly depends on the choice of different additives. Employment of alcohol solvent gives access to vinyl products, while the addition of carboxylate leads to the formation of allylic products. In addition, exclusion of these two promoting factors results in β-lactams via a C–N reductive elimination. This protocol shows a broad substrate scope for both alkenes and structurally diverse N-fluoro-sulfonamides, producing the corresponding products with excellent regio- and stereoselectivities. Further control experiments and DFT calculations provide in-depth insights into the reaction mechanism, highlighting the distinct effect of the additives on a bidentate auxiliary-stabilized Cu(III) intermediate.

The control of regioselectivity in Heck-type reactions of unactivated alkenes is challenging. Here, the authors realize regiodivergent Heck-type reactions of unactivated alkenes and N-fluoro-sulfonamides.

Radical Alkene-Trifluoromethylation-Triggered Nitrile Insertion/Remote Functionalization Relay Processes: Diverse Synthesis of Trifluoromethylated Azaheterocycles Enabled by Copper Catalysis

A copper-catalyzed alkene-trifluoromethylation-triggered nitrile insertion/remote functionalization relay process has been achieved, in which "interrupted" remote 1,n-difunctionalizations of alkenes with nitrile insertion can deliver iminyl radical intermediates instead of C-based radicals, followed by subsequent 1,n-HAT to furnish corresponding remote functionalization. This relay protocol enables a straightforward approach to streamline the assembly of structurally diverse trifluoromethylated azaheterocycles.

Metal-free allylic C-H nitrogenation, oxygenation, and carbonation of alkenes by thianthrenation

Selective functionalization of allylic C–H bonds into other chemical bonds is among the most straightforward and attractive, yet challenging transformations. Herein, a transition-metal-free protocol for direct allylic C–H nitrogenation, oxygenation, and carbonation of alkenes by thianthrenation was developed. This operationally simple protocol allows for the unified allylic C–H amination, esterification, etherification, and arylation of vinyl thianthrenium salts. Notably, the reaction furnishes multialkyl substituted allylic amines, ammonium salts, sulfonyl amides, esters, and ethers in good yields. The reaction proceeds under mild conditions with excellent functional group tolerance and could be applied to late-stage allylation of natural products, drug molecules and peptides with excellent chemoselectivity.

Diverse functionalizations of allylic C–H bonds of alkenes by thianthrenation have been demonstrated, featuring Z-selectivity to afford multi-alkyl substituted allylic esters, thioesters, ethers, amines, amides and arenes under metal-free conditions.

A straightforward access to trifluoromethylated natural products through late-stage functionalization

This review summarizes the applications of late-stage strategies in the direct trifluoromethylation of natural products in the past ten years, with particular emphasis on the reaction model of each method.

Non-noble Nickel-Modified Covalent Organic Framework for Partial Hydrogenation of Aromatic Terminal Alkynes

Developing non-noble metal-based catalysts with excellent performance for selective hydrogenation of alkynes under mild reaction conditions is highly desirable but still faces challenges. Herein, a non-noble nickel-modified covalent organic framework (Ni/COF) had been synthesized through a facile post-modified method and followed by reduction at a different temperature under a H₂/Ar atmosphere. The as-prepared catalysts were characterized by X-ray diffraction, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, Brunauer-Emmett-Teller, and Fourier transforms infrared, and the optimal H₃₅₀-Ni/COF presents excellent catalytic performance in the semihydrogenation of a series of aromatic terminal alkyne substrates, particularly in the partial hydrogenation of phenylacetylene with nearly full conversion and 85% selectivity toward styrene under mild reaction conditions (10 bar of H₂, 100 °C, and 1 h). Moreover, such a catalyst also exhibited satisfying stability after three consecutive cycles.

Electrochemical trifluoromethylation/cyclization for the synthesis of isoquinoline-1,3-diones and oxindoles

Herein, we describe a protocol for electrochemical cathode reduction to generate trifluoromethyl radicals. The trifluoromethylation reagent (IMDN-SO₂CF₃) used in this strategy is inexpensive and easy to obtain, and the reaction can be conducted efficiently without the addition of additional redox reagents. Using this strategy, we achieved electrochemical trifluoromethylation/cyclization for the synthesis of isoquinoline-1,3-diones and oxindoles. This protocol has good functional group tolerance and a broad substrate scope.

Visible-Light-Induced Trifluoromethylation of Allylic Alcohols

An organic photoredox-catalyzed dehydroxylative trifluoromethylation of allylic alcohols was developed in an environmentally benign manner. In this reaction, the readily available CF₃SO₂Na was selected as the trifluoromethylation reagent. The in situ generated byproduct SO₂ was reutilized to activate C-OH bond, which enabled this dehydroxylative trifluoromethylation to be performed conveniently. A variety of multifunctionalized CF₃-allylic compounds were obtained in high yields and excellent stereoselectivity.

Cu-Catalyzed Oxidative Allylic C-H Arylation of Inexpensive Alkenes with (Hetero)Aryl Boronic Acids

Herein, we present a regioselective Cu-catalyzed oxidative allylic C(sp³)-H arylation by radical relay using a broad range of heteroaryl boronic acids with inexpensive and readily available unactivated terminal and internal olefins. This C(sp²)-C(sp³) allyl coupling has the advantage of using cheap, abundant, and nontoxic Cu₂O without the need to use prefunctionalized alkenes, thus offering an alternative method to allylic arylation reactions that employ more traditional coupling partners with preinstalled leaving groups (LGs) at the allylic position.

Zinc–Brønsted acid mediated practical hydrotrifluoromethylation of alkenes with CF3Br

Hydrotrifluoromethylation of alkenes was developed through a novel generation of CF3˙ via the combination of Zn and PhCO2H with CF3Br.

β-Selective Cu(II)-Catalyzed Dehydrogenative Enamination of Alkylbenzenes

A Cu(II)-catalyzed dehydrogenative enamination that couples alkyl-substituted electron-rich arenes and N-fluorobenzenesulfonimide to forge enamine products is reported. This C-N bond-forming procedure occurs selectively at the β-position of the alkyl group. Both aniline and anisole derivatives are tolerated under these conditions, to afford styryl amines. A reaction mechanism involving quinone methide and styrene intermediates is proposed.

Intermolecular oxidative amination of unactivated alkenes by dual photoredox and copper catalysis

Oxidative amination of alkenes via amidyl radical addition is potentially an efficient method to generate allylic amines, which are versatile synthetic intermediates to bioactive compounds and organic materials. Here by combining photochemical generation of amidyl radicals with Cu-mediated β-H elimination of alkyl radicals, we have developed an intermolecular oxidative amination of unactivated alkenes. The reaction relies on tandem photoredox and copper catalysis, and works for both terminal and internal alkenes. The radical nature of the reaction and the mild conditions lead to high functional group tolerance.

Oxidative amination via amidyl radical addition of unactivated alkenes was realized by dual photoredox and copper catalysis.

Electrochemical Tandem Fluoroalkylation-Cyclization of Vinyl Azides: Access to Trifluoroethylated and Difluoroethylated N-Heterocycles

A transition-metal- and oxidant-free electrochemical strategy for radical fluoroalkylation of vinyl azides was developed. The reaction was carried out under mild conditions by using inexpensive and bench-stable R_fSO₂Na (R_f = CF₃, CF₂H) as fluorination reagents. Depending on the starting material, both the electrochemical radical cyclization and dearomatization products could be obtained. This method provides a green and safe approach to synthesize fluorinated nitrogen heterocycles.

Recent Developments in Heck-Type Reaction of Unactivated Alkenes and Alkyl Electrophiles

The Mizoroki–Heck reaction is considered as one of the most ingenious and widely used methods for constructing C–C bonds. This reaction mainly focuses on activated olefins (styrenes, acrylates, or vinyl ethers) and aryl/vinyl (pseudo) halides. In comparison, the studies on unactivated alkenes and alkyl electrophiles are far less due to the low reactivity, poor selectivity, as well as competitive β-H elimination. In the past years, a growing interest has thus been devoted and significant breakthroughs have been achieved in the employment of unactivated alkenes and alkyl electrophiles as the reaction components, and this type of coupling is called as Heck-type or Heck-like reaction, which distinguishes from the traditional Heck reaction. Herein, we give a brief summary on Heck-type reaction between unactivated alkenes and alkyl electrophlies, covering its initial work, recent advancements, and mechanistic discussions.

1 Introduction

2 Intramolecular Heck-Type Reaction of Unactivated Alkenes and Alkyl Electrophiles

2.1 Cobalt-Catalyzed Intramolecular Heck-Type Reaction

2.2 Palladium-Catalyzed Intramolecular Heck-Type Reaction

2.3 Nickel-Catalyzed Intramolecular Heck-Type Reaction

2.4 Photocatalysis and Multimetallic Protocol for Intramolecular Heck-Type Reaction

3 Intermolecular Heck-Type Reaction of Unactivated Alkenes and Alkyl Electrophiles

3.1 Electrophilic Trifluoromethylating Reagent as Reaction Partners

3.2 Alkyl Electrophiles as Reaction Partners

4 Oxidative Heck-Type Reaction of Unactivated Alkenes and Alkyl Radicals

5 Conclusions and Outlook

Photocatalytic Palladium-Catalyzed Fluoroalkylation of Styrene Derivatives

A visible light induced palladium-catalyzed fluoroalkylation method was developed. The Heck-type alkyl coupling reaction enables the introduction of trifluoroethyl, difluoroethyl and other fluoroalkyl fragment into styrenes under mild reaction conditions without the use of additional photosensitizers and ensures access to fluoroalkylated olefins on a broad scale.

Overcoming the Deallylation Problem: Palladium(II)-Catalyzed Chemo-, Regio-, and Stereoselective Allylic Oxidation of Aryl Allyl Ether, Amine, and Amino Acids

We report herein a Pd(II)/bis-sulfoxide-catalyzed intramolecular allylic C-H acetoxylation of aryl allyl ether, amine, and amino acids with the retention of a labile allyl moiety. Mechanistically, the reaction proceeds through a distinct double-bond isomerization from the allylic to the vinylic position followed by intramolecular carboxypalladation and the β-hydride elimination pathway. For the first time, C-H oxidation of N-allyl-protected amino acids to furnish five-membered heterocycles through 1,3-syn-addition is established with excellent diastereoselectivity.

Photoredox-Catalyzed Selective Synthesis of Allylic Perfluoroalkanes from Alkenes

We report herein a novel photoredox-catalyzed synthesis of allylic trifluoromethanes. The use of sulfilimino iminium as a source of trifluoromethyl radicals proves crucial to achieving high selectivity. Importantly, both styrene derivatives and unactivated alkenes are for the first time suitable partners for this process. The mild reaction conditions are compatible with a variety of functional groups. Remarkably, this method is readily broadened to other perfluoroalkyl groups (R_F =CFCl₂ , CF₂ Br, C₄ F₉ ). An extensive mechanistic study is also provided.

Silver-Mediated Perfluoroalkylation of Terminal Alkynes with Perfluoroalkyl Iodides

The incorporation of a perfluoroalkyl group (R_F) into drug candidates has become an increasingly important strategy in drug molecule design. In this study, the silver-mediated perfluoroalkylation reaction based on the addition-elimination process of terminal alkynes which was initiated by a perfluoroalkyl radical to form a C(sp)-R_F bond has been developed. The reaction proceeds under mild conditions using readily available, low-cost perfluoroalkyl iodides as the sources of the R_F group. This method allows access to a variety of perfluoroalkylated alkynes.

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.

Radical Pentafluoroethylation of Unactivated Alkenes Using CuCF2CF3

We have found that the CuCF₂CF₃ complex generated from low-cost pentafluoroethane is a convenient and practical source for the CF₂CF₃ radical under aerobic conditions at room temperature. Using this system, readily available unactivated alkenes can be pentafluoroethylated to provide novel allylic CF₂CF₃ compounds with excellent E-selectivity and functional group tolerability. Mechanistic studies including TEMPO-CF₂CF₃ trapping and radical clock experiments provided strong evidence for radical pathways, offering a new opportunity for copper-mediated radical perfluoroalkylation.

Copper-Catalyzed Oxidative Trifluoromethylation of Terminal Alkenes Using AgCF3

A mild and convenient reaction for oxidative trifluoromethylation of terminal alkenes was developed using in situ generated AgCF₃ in the presence of a copper catalyst. The reaction proceeded under an air atmosphere to afford trifluoromethylated allylic compounds in moderate to good yield. This reaction, with no need for highly hygroscopic or corrosive reagents, features not only a simple operation but also various functional group tolerances.

Nickel-Catalyzed Reductive Coupling for Transforming Unactivated Aryl Electrophiles into β-Fluoroethylarenes

We report herein a facile synthetic method for converting unactivated (hetero)aryl electrophiles into β-fluoroethylated (hetero)arenes via nickel-catalyzed reductive cross-couplings. This coupling reaction features the involvement of FCH₂ CH₂ radical intermediate rather than β-fluoroethyl manganese species which provides effective solutions to the problematic β-fluoride side eliminations. The practical value of this protocol is further demonstrated by the late-stage modification of several complex ArCl or ArOH-derived bioactive molecules.

Structural Mechanism of Regioselectivity in an Unusual Bacterial Acyl-CoA Dehydrogenase

Terminal alkenes are easily derivatized, making them desirable functional group targets for polyketide synthase (PKS) engineering. However, they are rarely encountered in natural PKS systems. One mechanism for terminal alkene formation in PKSs is through the activity of an acyl-CoA dehydrogenase (ACAD). Herein, we use biochemical and structural analysis to understand the mechanism of terminal alkene formation catalyzed by an γ,δ-ACAD from the biosynthesis of the polyketide natural product FK506, TcsD. While TcsD is homologous to canonical α,β-ACADs, it acts regioselectively at the γ,δ-position and only on α,β-unsaturated substrates. Furthermore, this regioselectivity is controlled by a combination of bulky residues in the active site and a lateral shift in the positioning of the FAD cofactor within the enzyme. Substrate modeling suggests that TcsD utilizes a novel set of hydrogen bond donors for substrate activation and positioning, preventing dehydrogenation at the α,β position of substrates. From the structural and biochemical characterization of TcsD, key residues that contribute to regioselectivity and are unique to the protein family were determined and used to identify other putative γ,δ-ACADs that belong to diverse natural product biosynthetic gene clusters. These predictions are supported by the demonstration that a phylogenetically distant homologue of TcsD also regioselectively oxidizes α,β-unsaturated substrates. This work exemplifies a powerful approach to understand unique enzymatic reactions and will facilitate future enzyme discovery, inform enzyme engineering, and aid natural product characterization efforts.

Synthesis of aziridines with multiple chiral substitutions by copper-catalyzed diastereoselective radical aminotrifluoromethylation of alkenes

A one-step catalytic and diastereoselective method for the synthesis of aziridines possessing multiple chiral substitutions by radical aminotrifluoromethylation of alkenes has been developed for the first time.

A Direct Approach to Decoration of Bioactive Compounds via C-H Amination Reaction

The development of new methods to achieve the direct synthesis of bioactive organic molecules is always an important topic in organic synthesis. We hereby demonstrate that N-methoxyamide is an excellent amino source in the iridium-catalyzed intermolecular C-H amination reaction. The linkage of two bioactive organic molecules can be well achieved with this new protocol. More than 20 examples of decorated bioactive compounds were reported, which can facilitate the discovery of new bioactive molecules.

Halogen-bond mediated efficient storage of extremely volatile perfluoroiodides in ionic liquids

Highly volatile and valuable reagents, perfluoroiodides were shown to complex with ionic liquids with the aid of a halogen-bond between a diverse set of anions and the electrophilic iodine atoms in perfluorinated iodides. The affinity of anions in various ionic liquids towards perfluoroiodides was examined by 19F NMR. Making use of the non-volatility of ionic liquids, we demonstrate here that perfluoroiodides can be stored and released controllably, on demand, from an ionic liquid matrix.

Metal-Free Construction of the C(sp3)-CF3 Bond: Trifluoromethylation of Hydrazones with Togni's Reagent under Mild Conditions

A metal-free trifluoromethylation of hydrazones with Togni's reagent under mild conditions was developed. Various functional groups including ester, methoxy, dimethoxy, nitro, halogen, and heterocyclic compounds were tolerated. This simple and green strategy provides a practical tool to construct C(sp³)-CF₃ bonds.

Metal-Free Visible-Light-Promoted Trifluoromethylation of Vinylcyclopropanes Using Pyrylium Salt as a Photoredox Catalyst

Visible-light-induced metal-free trifluoromethylation of activated, carbocyclic, and unactivated vinylcyclopropanes via a ring-opening reaction using the Langlois reagent (CF₃SO₂Na) is reported to synthesize allylic trifluoromethylated derivatives. Allylic trifluoromethylation was achieved by a photo-oxidative single electron transfer (SET) process at an ambient temperature and under metal-free conditions and visible-light irradiation using pyrylium salt as a photoredox catalyst. The reported methodology has an operational simplicity, broad substrate scope, high functional group tolerance, and scalability.