Nickel(IV)-Catalyzed C-H Trifluoromethylation of (Hetero)arenes

Photocatalytic Radical Bis(trifluoromethyl)carbinolation of Alkenes and Heteroarenes

Structural modification of drug candidates with fluorine atoms has revolutionized drug discovery, frequently enhancing pharmacological properties. However, the strategic integration of privileged bis(trifluoromethyl)carbinol moiety remains challenging, primarily due to the overreliance on closed-shell strategies typically involving toxic gaseous hexafluoroacetone. Alternative radical-based strategies exist but are constrained by the inherently weak electrophilicity of the key bis(trifluoromethyl)carbinol radical intermediate, significantly limiting the scope of accessible transformations. Here we report the development of a novel masked bis(trifluoromethyl)carbinolation reagent, enabling the efficient generation of bis(trifluoromethyl)carbinol radical species with significantly enhanced electrophilicity under mild photocatalytic conditions. This approach facilitates chloro- and oxy-bis(trifluoromethyl)carbinolation of versatile alkenes, alongside highly selective C2-H bis(trifluoromethyl)carbinolation of diverse heteroarenes, providing streamlined access to a structurally diverse array of alkyl and aryl bis(trifluoromethyl)carbinols previously considered synthetically challenging.

Unlocking the Potential of CF3-Alkynes in Gold-Catalyzed Oxygen Transfer: A Direct Route to Trifluoromethylated Compounds

We report on gold-catalyzed oxygen transfer utilizing, for the first time, electron-deficient trifluoromethylated alkynes as substrates. This double O-transfer process provides a direct and convenient one-step route to difficult-to-obtain trifluoromethylated 1,2-diketones. Due to the relatively mild oxidative reaction conditions (rt to 60 °C, 2,6-dichloropyridine N-oxide as an oxidizer, 3 mol % JohnPhosAuNTf2), various functional substituents were tolerated (19 examples; yields up to 95%). The synthetic utility of the obtained trifluoromethylated dicarbonyl products was further explored, and these studies included one-pot multicomponent syntheses of valuable CF3-substituted indoles and benzofurans.

C-C and C-O bond formation reactivity of nickel complexes supported by the pyridinophane MeN3C ligand

The pyridinophane ligands RN3CX (X = H, Br) are well-established scaffolds that facilitate and stabilize nickel oxidative addition complexes to the proximal C(aryl)-X bond. In this study, we report the synthesis, detailed characterization, and reactivity of a series of NiII and NiIII complexes supported by the MeN3CX ligand. Our findings demonstrate that NiII complexes can be oxidized to readily yield well-defined NiIII species. Excitingly, the Ni-disolvento complexes exhibit catalytic trifluoroethoxylation to generate the C-O coupled product. In addition, the NiIII-halide complex undergoes transmetallation with a Grignard reagent and subsequent C-C reductive elimination, while the β-hydride elimination side reaction is suppressed, outperforming its NiII analogue.

Synthesis of Iron(IV) Alkynylide Complexes and Their Reactivity to Form 1,3-Diynes

The isolation of thermally unstable and highly reactive organoiron(IV) complexes is a challenge for synthetic chemists. In particular, the number of examples where the C-based ligand is not part of the chelating ligand remains scarce. These compounds are of interest because they could pave the way to designing catalytic cycles of bond forming reactions proceeding via organoiron(IV) intermediates. Herein, we report the synthesis and characterization, including single-crystal X-ray diffraction, of a family of alkynylferrates(III) and Fe(IV) alkynylide complexes. The alkynylferrates(III) are formed by transmetalation of the Fe(III) precursor [(N3N')FeIII] (N3N'3- is tris(N-tert-butyldimethylsilyl-2-amidoethyl)amine) with lithium alkynylides, and their further one-electron oxidation enables the synthesis of the corresponding Fe(IV) alkynylides. The electronic structure of this family of organometallic Fe(III) and Fe(IV) complexes has been thoroughly investigated by spectroscopic methods (EPR, NMR, 57Fe Mössbauer, X-Ray absorption (XAS) and emission (XES) spectroscopies) and theoretical calculations. While alkynylferrates(III) are sluggish to engage into C-C bond forming processes, the Fe(IV) alkynylides react to afford 1,3-diynes at room temperature. A bimolecular reductive elimination from a bimetallic Fe(IV) intermediate to form the 1,3-diynes is proposed based on the mechanistic investigations performed.

Synthesis, Structure, and Redox Reactivity of Ni Complexes Bearing a Redox and Acid-Base Non-innocent Ligand with NiII, NiIII, and NiIV Formal Oxidation States

A series of Ni complexes bearing a redox and acid-base noninnocent tetraamido macrocyclic ligand, H4-(TAML-4) {H4-(TAML-4) = 15,15-dimethyl-5,8,13,17-tetrahydro-5,8,13,17-tetraaza-dibenzo[a,g]cyclotridecene-6,7,14,16-tetraone}, with formal oxidation states of NiII, NiIII, and NiIV were synthesized and characterized structurally and spectroscopically. The X-ray crystallographic analysis of the Ni complexes revealed a square planar geometry, and the [Ni(TAML-4)] complex with the formal oxidation state of NiIV was characterized to be [NiIII(TAML-4•+)] with the oxidation state of the NiIII ion and the one-electron oxidized TAML-4 ligand, TAML-4•+. The NiIII oxidation state and the TAML-4 radical cation ligand, TAML-4•+, were supported by X-ray absorption spectroscopy and density functional theory calculations. The reversible interconversions between [NiII(TAML-4)]2- and [NiIII(TAML-4)]- and between [NiIII(TAML-4)]- and [NiIII(TAML-4•+)] were demonstrated in spectroelectrochemical measurements as well as in chemical oxidation and reduction reactions. The reactivities of [NiIII(TAML-4)]- and [NiIII(TAML-4•+)] were then investigated in hydride transfer reactions using NADH analogs. Hydride transfer from 9,10-dihydro-10-methylacridine (AcrH2) to [NiIII(TAML-4•+)] was found to proceed via electron transfer (ET) from AcrH2 to [NiIII(TAML-4•+)] with no deuterium kinetic isotope effect (kH/kD = 1.0(2)). In contrast, hydride transfer from AcrH2 to [NiIII(TAML-4)]- proceeded much more slowly via a concerted proton-coupled electron transfer (PCET) process with kH/kD = 7.0(5). In the latter reaction, an electron and a proton were transferred to the NiIII center and the TAML-4 ligand, respectively. The mechanisms of the ET by [NiIII(TAML-4•+)] and the concerted PCET by [NiIII(TAML-4)]- were ascribed to the different redox potentials of the Ni complexes.

Continuous activation of phenoxide and CF3I for multiple trifluoromethylations

The introduction of multiple trifluoromethyl (CF3) groups into aromatic compounds remains a significant challenge in synthetic chemistry. Here, we report an unprecedented visible light-promoted multiple trifluoromethylation of phenols using commercially available CF3I. The key to success lies in our discovery of a "continuous activation strategy" that enables sequential trifluoromethylations through single-electron transfer from photoexcited phenoxide to CF3I until all ortho or para positions are occupied. This practical method provides access to previously inaccessible multi-CF3-substituted phenols under mild conditions, opening revolutionary possibilities for the design of fluorine-containing functional materials.

Skeletal Editing of Aromatic N-Heterocycles via Hydroborative Cleavage of C-N Bonds-Scope, Mechanism, and Property

Skeletal editing of the core structure of heterocycles offers new opportunities for chemical construction and is a promising yet challenging research topic that has recently gained increasing interest. However, several limitations of the reported systems remain to be addressed. For example, the reagents employed are generally in high-energy, such as chlorocarbene precursors, nitrene species, and metal carbenes, which are also associated with low atomic efficiencies. Thus, the development of simple systems for the skeletal editing of heterocycles is still desired. Herein, a straightforward and facile BH3-mediated skeletal editing of readily available indoles, benzimidazoles, and several other aromatic heterocycles is reported. Structurally diverse products were readily obtained, including tetrahydrobenzo azaborinines, diazaboroles, O-anilinophenylethyl alcohols, benzene-1,2-diamines, and more. Density functional theory (DFT) calculations and natural bond orbital (NBO) analysis revealed a BH3-induced C-N bond cleavage reaction pathway. An exciting and counterintuitive indole hydroboration phenomenon of -BH2 shift from C3-position to C2-position was disclosed. Moreover, the photophysical properties of the synthesized diazaboroles were studied, and an interestingly and pronounced aggregation-induced emission (AIE) behavior was disclosed.

A photoelectrocatalytic system as a reaction platform for selective radical-radical coupling

The selection of electrode material is a critical factor that determines the selectivity of electrochemical organic reactions. However, the fundamental principles governing this relationship are still largely unexplored. Herein, we demonstrate a photoelectrocatalytic (PEC) system as a promising reaction platform for the selective radical-radical coupling reaction owing to the inherent charge-transfer properties of photoelectrocatalysis. As a model reaction, the radical trifluoromethylation of arenes is shown on hematite photoanodes without employing molecular catalysts. The PEC platform exhibited superior mono- to bis-trifluoromethylated product selectivity compared to conventional electrochemical methods utilizing conducting anodes. Electrochemical and density functional theory (DFT) computational studies revealed that controlling the kinetics of anodic oxidation of aromatic substrates is essential for increasing reaction selectivity. Only the PEC configuration could generate sufficiently high-energy charge carriers with controlled kinetics due to the generation of photovoltage and charge-carrier recombination, which are characteristic features of semiconductor photoelectrodes. This study opens a novel approach towards selective electrochemical organic reactions through understanding the intrinsic physicochemical properties of semiconducting materials.

Nickel-Mediated Divergent Trifluoromethylation of Chlorinated and Brominated Phenol Derivatives through Chemoselective Cleavage of Ar-O and Ar-Cl(Br) Bonds

We report herein that nickel-mediated trifluoromethylation of chlorinated and brominated phenol derivatives ClArOTs and BrArOTf gave chloro(bromo)trifluoromethylarenes through the chemoselective cleavage of Ar-O bonds. Furthermore, under similar reaction conditions, the chemoselective trifluoromethylation of Ar-Cl and Ar-Br bonds of ClArOPiv and BrArOTs was achieved to give trifluoromethylated phenol derivatives.

Efficient Trifluoromethylation of Halogenated Hydrocarbons Using Novel [(bpy)Cu(O2CCF2SO2F)2] Reagent

This study introduces a novel trifluoromethylating reagent, [(bpy)Cu(O2CCF2SO2F)2], notable for not only its practical synthesis from cost-effective starting materials and scalability but also its nonhygroscopic nature. The reagent demonstrates high efficiency in facilitating trifluoromethylation reactions with various halogenated hydrocarbons, yielding products in good yields and exhibiting broad functional group compatibility. The development of [(bpy)Cu(O2CCF2SO2F)2] represents an advancement in the field of organic synthesis, potentially serving as a valuable addition to the arsenal of existing trifluoromethylating agents.

Electron Donor-Acceptor Complex Driven Photocatalyst-Free Trifluoromethylation of Heterocycles

Heterocyclic trifluoromethylation is efficiently initiated through a photochemical reaction utilizing an electron donor-acceptor (EDA) complex, proceeding smoothly without the use of photocatalysts, transition-metal catalysts, or additional oxidants. This method has been optimized through extensive experimentation, demonstrating its versatility and efficacy across various substrates, including quinoxalinones, coumarins, and indolones. Notably, this approach enables the practical synthesis of trifluoromethylated quinoxalinones on a gram scale. Mechanistic investigations that incorporate radical trapping and ultraviolet/visible spectroscopy, confirmed the formation of the an EDA complex and elucidated the reaction pathways. This study highlights the crucial role of EDA photoactivation in trifluoromethylation, significantly expanding the application scope of EDA complexes in chemical synthesis.

Review and Theoretical Analysis of Fluorinated Radicals in Direct CAr-H Functionalization of (Hetero)arenes

We highlight key contributions in the field of direct radical CAr- H (hetero)aromatic functionalization involving fluorinated radicals. A compilation of Functional Group Transfer Reagents and their diverse activation mechanisms leading to the release of radicals are discussed. The substrate scope for each radical is analyzed and classified into three categories according to the electronic properties of the substrates. Density functional theory computational analysis provides insights into the chemical reactivity of several fluorinated radicals through their electrophilicity and nucleophilicity parameters. Theoretical analysis of their reduction potentials also highlights the remarkable correlation between electrophilicity and oxidizing ability. It is also established that highly fluorinated radicals (e.g. ⋅OCF3) are capable of engaging in single-electron transfer (SET) processes rather than radical addition, which is in good agreement with experimental literature data. A reactivity scale, based on activation barrier of addition of these radicals to benzene is also elaborated using the high accuracy DLPNO-(U)CCSD(T) method.

nBu4N+[AgI(CF3)2]-: Trifluoromethylated Argentate Derived from Fluoroform and Its Reaction with (Hetero)Aryl Diazonium Salts

The preparation of a well-defined trifluoromethylated argentate nBu4N+[Ag(CF3)2]- 1 from fluoroform was described. The complex was stable in the solid state and in solution under an inert atmosphere. Treatment of a variety of (hetero)aryl diazonium tetrafluoroborates with nBu4N+[Ag(CF3)2]- 1 generated trifluoromethylated (hetero)arenes in good to excellent yields. Preliminary experiments were conducted, and a reasonable mechanism of the reaction was proposed.

Uncovering a CF3 Effect on X-ray Absorption Energies of [Cu(CF3 )4 ]- and Related Copper Compounds by Using Resonant Diffraction Anomalous Fine Structure (DAFS) Measurements

Understanding the electronic structures of high-valent metal complexes aids the advancement of metal-catalyzed cross coupling methodologies. A prototypical complex with formally high valency is [Cu(CF3 )4 ]- (1), which has a formal Cu(III) oxidation state but whose physical analysis has led some to a Cu(I) assignment in an inverted ligand field model. Recent examinations of 1 by X-ray spectroscopies have led previous authors to contradictory conclusions, motivating the re-examination of its X-ray absorption profile here by a complementary method, resonant diffraction anomalous fine structure (DAFS). From analysis of DAFS measurements for a series of seven mononuclear Cu complexes including 1, here it is shown that there is a systematic trifluoromethyl effect on X-ray absorption that blue shifts the resonant Cu K-edge energy by 2-3 eV per CF3 , completely accounting for observed changes in DAFS profiles between formally Cu(III) complexes like 1 and formally Cu(I) complexes like (Ph3 P)3 CuCF3 (3). Thus, in agreement with the inverted ligand field model, the data presented herein imply that 1 is best described as containing a Cu(I) ion with dn count approaching 10.

Recent advances on non-precious metal-catalysed fluorination, difluoromethylation, trifluoromethylation, and perfluoroalkylation of N-heteroarenes

This review highlights the recent advances, from 2015 to 2023, on the introduction of organo-fluorine derivatives at the N-heteroarene core. Notable features considering new technologies based on organofluorine compounds such as: (i) approaches based on non-precious metal catalysis (Fe, Co, Mn, Ni, etc.), (ii) the development of new strategies using non-precious metal-catalysts for the introduction of organo-fluorinine derivatives using N-heterocycles with one or more heteroatoms, (iii) newer reagents for fluorination, difluoromethylation, trifluoromethylation, or perfluoroalkylation of N-heteroarenes using different approaches, (iv) mechanistic studies on various catalytic transformations, as and when required, and (v) the synthetic applications of various bio-active organo-fluorine compounds, including post-synthetic drug derivatization, are discussed.

Bi-Catalyzed Trifluoromethylation of C(sp2)-H Bonds under Light

We disclose a Bi-catalyzed C-H trifluoromethylation of (hetero)arenes using CF3SO2Cl under light irradiation. The catalytic method permits the direct functionalization of various heterocycles bearing distinct functional groups. The structural and computational studies suggest that the process occurs through an open-shell redox manifold at bismuth, comprising three unusual elementary steps for a main group element. The catalytic cycle starts with rapid oxidative addition of CF3SO2Cl to a low-valent Bi(I) catalyst, followed by a light-induced homolysis of Bi(III)-O bond to generate a trifluoromethyl radical upon extrusion of SO2, and is closed with a hydrogen-atom transfer to a Bi(II) radical intermediate.

Synthesis and Characterization of a Stable Nickelocenium Dication Salt

We report the synthesis and characterization of the nickelocenium cations [NiCp2 ]⋅+ and [NiCp2 ]2+ as their [F-{Al(ORF )3 }2 ]- (Cp = C5 H5 ; RF =C(CF3 )3 ) salts. Diamagnetic [NiCp2 ]2+ represents the first example for the isolation of an unsubstituted parent metallocene dication. Both salts were generated by reacting neutral NiCp2 with [NO]+ [F-{Al(ORF )3 }2 ]- in 1,2,3,4-tetrafluorobenzene (4FB). The salts were characterized by single crystal X-ray diffraction (XRD), indicating shorter metal-ligand bond lengths for the higher charged salt. Powder XRD shows the salts to be phase pure, cyclic voltammetry in 4FB gave quasi reversible redox waves at -0.44 (0→1) and +1.17 V (1→2) vs Fc/Fc+ . The 1 H NMR of [NiCp2 ]2+ is a singlet at 8.6 ppm, whereas paramagnetic [NiCp2 ]⋅+ is significantly shifted upfield to -103.1 ppm.

Catalytic C-H Trifluoromethylation of Arenes and Heteroarenes via Visible Light Photoexcitation of a Co(III)-CF3 Complex

A cobalt photocatalyst for direct trifluoromethylation of (hetero)arene C(sp2)-H bonds is described and shown to operate via visible light activation of a Co-CF3 intermediate, which functions as a combined chromophore and organometallic reaction center. Chemical oxidations of previously reported (OCO)Co complexes containing a redox-active [OCO] pincer ligand afford a Co-CF3 complex two oxidation states above Co(II). Computational and spectroscopic studies are consistent with formulation of the product as [(OCO•)CoIII(CF3)(THF)(OTf)] (II) containing an open-shell [OCO•]1- radical ligand bound to a S = 0 Co(III) center. II is thermodynamically stable, but exposure to blue (440 nm) light induces Co-CF3 bond homolysis and release of •CF3, which is trapped by radical acceptors including TEMPO•, (hetero)arenes, or the radical [OCO•] ligand in II. The latter comprises a competitive degradation pathway, which is overcome under catalytic conditions by using excess substrate. Accordingly, generation of II from the reaction of [(OCO)CoIIL] (III) (L = THF, MeCN) with Umemoto's dibenzothiophenium trifluoromethylating reagent (1) followed by photolytic Co-CF3 bond activation completes a photoredox catalytic cycle for C-H (hetero)arene trifluoromethylation utilizing visible light. Electronic structure and photophysical studies, including time-dependent density functional theory (TDDFT) calculations, suggest that Co-CF3 bond homolysis at II occurs via an ligand-to-metal charge-transfer (LMCT) (OCO0)CoII(CF3) state, revealing ligand redox activity as a critical design feature and establishing design principles for the use of base metal chromophores for selectivity in photoredox bond activations occurring via free radical intermediates.

Handling fluorinated gases as solid reagents using metal-organic frameworks

Fluorine is an increasingly common substituent in pharmaceuticals and agrochemicals because it improves the bioavailability and metabolic stability of organic molecules. Fluorinated gases represent intuitive building blocks for the late-stage installation of fluorinated groups, but they are generally overlooked because they require the use of specialized equipment. We report a general strategy for handling fluorinated gases as benchtop-stable solid reagents using metal-organic frameworks (MOFs). Gas-MOF reagents are prepared on gram-scale and used to facilitate fluorovinylation and fluoroalkylation reactions. Encapsulation of gas-MOF reagents within wax enables stable storage on the benchtop and controlled release into solution upon sonication, which represents a safer alternative to handling the gas directly. Furthermore, our approach enables high-throughput reaction development with these gases.

Regioselective C-H Trifluoromethylation and Its Related Reactions of (Hetero)aromatic Compounds

Fluorinated functional groups, including trifluoromethyl group, play important roles in the development of drugs, agrochemicals, and organic functional materials. Therefore, the development of highly effective and practical reactions to introduce fluorinated functional groups into (hetero)aromatic compounds is highly desirable. We have achieved several regioselective C-H trifluoromethylation and related reactions by electrophilic and nucleophilic activation of six-membered heteroaromatic compounds and steric protection of aromatic compounds. These reactions proceed in good to excellent yields, even on a gram scale, with high functional group tolerance, and are applicable to the regioselective trifluoromethylation of drug molecules. In this personal account, the background of the introduction reactions of fluorinated functional groups, our reaction designs to achieve regioselective C-H trifluoromethylation and the related reactions of (hetero)aromatic compounds are explained.

Oxygen transfer reactivity mediated by nickel perfluoroalkyl complexes using molecular oxygen as a terminal oxidant

Nickel perfluoroethyl and perfluoropropyl complexes supported by naphthyridine-type ligands show drastically different aerobic reactivity from their trifluoromethyl analogs resulting in facile oxygen transfer to perfluoroalkyl groups or oxygenation of external organic substrates (phosphines, sulfides, alkenes and alcohols) using O₂ or air as a terminal oxidant. Such mild aerobic oxygenation occurs through the formation of spectroscopically detected transient high-valent Ni^III and structurally characterized mixed-valent Ni^II-Ni^IV intermediates and radical intermediates, resembling O₂ activation reported for some Pd dialkyl complexes. This reactivity is in contrast with the aerobic oxidation of naphthyridine-based Ni(CF₃)₂ complexes resulting in the formation of a stable Ni^III product, which is attributed to the effect of greater steric congestion imposed by longer perfluoroalkyl chains.

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

Covering: 2011 to 2021Trifluoromethyl (CF3)-modified natural products have attracted increasing interest due to their magical effect in binding affinity and/or drug metabolism and pharmacokinetic properties. However, the chemo and regioselective construction of natural products (NPs) bearing a CF3 group still remains a long-standing challenge due to the complex chemical scaffolds and diverse reactive sites of NPs. In recent years, the development of late-stage functionalization strategies, including metal catalysis, organocatalysis, light-driven reactions, and electrochemical synthesis, has paved the way for direct trifluoromethylation process. In this review, we summarize the applications of these strategies in the late-stage trifluoromethylation of natural products in the past ten years with particular emphasis on the reaction model of each method. We also discuss the challenges, limitations, and future prospects of this approach.

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.

Copper-Catalyzed Hydroamination of gem-Difluoroalkenes Access to Diversified α-Difluoromethyl Amines

The difluoromethyl group (CF₂H) is of great importance in medicinal chemistry. We report herein an efficient method for the synthesis of diversified α-difluoromethyl amines through copper-catalyzed hydroamination of gem-difluoroalkenes, where the C-N bond formed via a α-CF₂H transition-metal intermediate. This new reaction proceeds through Cu-H insertion to gem-difluoroalkenes and gives valuable alkyl-CF₂H-containing compounds, which overcome the much more challenged β-F elimination from α-fluoroalkyl organocopper species. The reaction exhibits broad substrate scope with readily available starting materials and commercial catalysis.

Electrogenerated Nickel Catalyst for C-N Cross-Coupling

Arylamines represent a class of compounds widely found in natural products and pharmaceuticals. Among methodologies devoted to their synthesis, nickel-catalyzed amination of aryl halides constitutes one of the most employed conventional strategies. However, C-N cross-couplings often involve elaborated nickel complexes, which are expensive and/or air and moisture sensitive. To circumvent this issue, we herein report an electrochemical method based on a sacrificial anode process to in situ generate a catalytic amount of nickel salts allowing amination of aryl halides. The approach, simple to set up, proceeds under mild reaction conditions and enables access to a large panel of arylamines.

Transition-metal-free electrochemical oxidative C(sp2)-H trifluoromethylation of aryl aldehyde hydrazones

A simple protocol of metal-free C-H trifluoromethylation of hydrazones via electrolysis was developed. This environment-friendly transformation showed high efficiency, good tolerance, and scaled-up functionalization, providing the desired products in moderate to good yields. At the same time, a high yield can be obtained for the substrates either bearing an electron-donating group or an electron-withdrawing group by using different trifluoromethyl reagents. In addition, the radical mechanism was confirmed by the control experiment.

Direct Synthesis of α-Aryl-α-Trifluoromethyl Alcohols via Nickel Catalyzed Cross-Electrophile Coupling

A nickel-catalyzed reductive cross-electrophile coupling between the redox-active N-trifluoroethoxyphthalimide and iodoarenes is documented. The protocol reproduces a formal arylation of trifluoroacetaldehyde under mild conditions in high yields (up to 88 %) and with large functional group tolerance (30 examples). A combined computational and experimental investigation revealed a pivotal solvent assisted 1,2-Hydrogen Atom Transfer (HAT) process to generate a nucleophilic α-hydroxy-α-trifluoromethyl C-centered radical for the Csp2 -Csp3 bond forming process.

Ligand-free nickel catalyzed perfluoroalkylation of arenes and heteroarenes

We describe a "ligand-free" Ni-catalyzed perfluoroalkylation of heteroarenes to produce a diverse array of trfiluoromethyl, pentafluoroethyl and heptafluoropropyl adducts. Catalysis proceeds at room temperature via a radical pathway. The catalytic protocol is distinguished by its simplicity, and its wide scope demonstrates the potential in the late-stage functionalization of drug analogues and peptides.

Synthesis of FeIII and FeIV Cyanide Complexes Using Hypervalent Iodine Reagents as Cyano-Transfer One-Electron Oxidants

We disclose a new reactivity mode for electrophilic cyano λ3 -iodanes as group transfer one-electron oxidants to synthesize FeIII and FeIV cyanide complexes. The inherent thermal instability of high-valent FeIV compounds without π-donor ligands (such as oxido (O2- ), imido (RN2- ) or nitrido (N3- )) makes their isolation and structural characterization a very challenging task. We report the synthesis of an FeIV cyanide complex [(N3 N')FeCN] (4) by two consecutive single electron transfer (SET) processes from FeII precursor [(N3 N')FeLi(THF)] (1) with cyanobenziodoxolone (CBX). The FeIV complex can also be prepared by reaction of [(N3 N')FeIII ] (3) with CBX. In contrast, the oxidation of FeII with 1-cyano-3,3-dimethyl-3-(1H)-1,2-benziodoxole (CDBX) enables the preparation of FeIII cyanide complex [(N3 N')FeIII (CN)(Li)(THF)3 ] (2-LiTHF ). Complexes 4 and 2-LiTHF have been structurally characterized by single crystal X-ray diffraction and their electronic structure has been examined by Mössbauer, EPR spectroscopy, and computational analyses.

Synthesis, structure, and electrochemical properties of [LNi(Rf)(C4F8)]- and [LNi(Rf)3]- complexes

The new anionic nickelate complexes [(MeCN)Ni(C₄F₈)(CF₃)]^-, [(MeCN)Ni(C₄F₈)(C₂F₅)]^-, [(IMes)Ni(C₄F₈)(CF₃)]^-, [(IMes)Ni(CF₃)₃]^- (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), and [(F-NHC)Ni(R_f)₃]^- (F-NHC = 1,3-bis(2,4-F₂Ph), 2,4,6-F₃Ph- or 3,4,5-F₃Ph)imidazol-2-ylidene; (R_f = CF₃ or C₂F₅) were synthesized and structurally characterized. The electrochemical properties of all new compounds were revealed by cyclic voltammetry studies and compared to the known CF₃ analogue [(MeCN)Ni(CF₃)₃]^-. The IMes-coordinated complexes exhibited initial oxidation events that were well-separated from a second oxidation process in the cyclic voltammograms. The complexes containing F-substituted NHC ligands [(F-NHC)Ni(CF₃)₃]^- are structurally quite similar to the IMes derivative and reveal also two separated oxidation waves in their cyclic voltammograms. The absolute potentials as well as the separation between the two waves vary with the substitution pattern, suggesting that the NHC ligand environment (NHC = N-heterocyclic carbene) is an interesting platform for the development of new redox-triggered reactions that release trifluoromethyl and perfluoroalkyl radicals upon oxidation.

Recent Advances in Transition-Metal Mediated Trifluoromethylation Reactions

Fluorine compounds are known for their abundance in more than 20% of pharmaceutical and agrochemical products mainly due to the enhanced lipophilicity, metabolic stability and pharmacokinetic properties of organofluorides. Consequently,...

Oxidative C–C/C–X coupling in organometallic nickel complexes: insights from DFT

NiIII and NiIV-center complexes prefer direct reductive elimination than reacting through five-coordinate intermediates. 32+ complex in the presence of Cl− undergoes Cl–Csp2 elimination preferably over Cl–Csp3 and Csp3–Csp2 elimination.

Highly Regio- and Enantioselective Hydrosilylation of gem-Difluoroalkenes via Nickel Catalysis

The synthesis of small organic molecules with a difluoromethylated stereocenter is particularly attractive in drug discovery. Herein, we developed an efficient method for the direct generation of difluoromethylated stereocenters through Ni(0)-catalyzed regio - and enantioselective hydrosilylation of gem -difluoroalkenes. The reaction also represents the enantioselective construction of carbon(sp 3 )-silicon bonds with nickel catalysis, which provides an atom- and step-economical synthesis route of high-value optically active α-difluoromethylsilanes. This protocol features with readily available starting materials and commercial chiral catalysis, broad substrates spanning a range of functional groups with high yield (up to 99% yield) and excellent enantioselectivity (up to 96% ee). The enantioenriched products undergo a variety of stereospecific transformations. Preliminary mechanistic studies were performed.

An Efficient Approach to 2-CF3-Indoles Based on ortho-Nitrobenzaldehydes

The catalytic olefination reaction of 2-nitrobenzaldehydes with CF3CCl3 afforded stereoselectively trifluoromethylated ortho-nitrostyrenes in up to 88% yield. The reaction of these alkenes with pyrrolidine permits preparation of α-CF3-β-(2-nitroaryl) enamines. Subsequent one pot reduction of nitro-group by Fe-AcOH-H2O system initiated intramolecular cyclization to afford 2-CF3-indoles. Target products can be prepared in up to 85% yields. Broad synthetic scope of the reaction was shown as well as some followed up transformations of 2- CF3-indole.

Photoinduced Trifluoromethylation of Arenes and Heteroarenes Catalyzed by High-Valent Nickel Complexes

We describe a series of air-stable Ni^III complexes supported by a simple, robust naphthyridine-based ligand. Access to the high-valent oxidation state is enabled by the CF₃ ligands on the nickel, while the naphthyridine exhibits either a monodentate or bidentate coordination mode that depends on the oxidation state and sterics, and enables facile aerobic oxidation of Ni^II to Ni^III . These Ni^III complexes act as efficient catalysts for photoinduced C(sp² )-H bond trifluoromethylation reactions of (hetero)arenes using versatile synthetic protocols. This blue LED light-mediated catalytic protocol proceeds via a radical pathway and demonstrates potential in the late-stage functionalization of drug analogs.

Cross-Coupling through Ag(I)/Ag(III) Redox Manifold

In ample variety of transformations, the presence of silver as an additive or co-catalyst is believed to be innocuous for the efficiency of the operating metal catalyst. Even though Ag additives are required often as coupling partners, oxidants or halide scavengers, its role as a catalytically competent species is widely neglected in cross-coupling reactions. Most likely, this is due to the erroneously assumed incapacity of Ag to undergo 2e^- redox steps. Definite proof is herein provided for the required elementary steps to accomplish the oxidative trifluoromethylation of arenes through Ag^I /Ag^III redox catalysis (i. e. CEL coupling), namely: i) easy Ag^I /Ag^III 2e^- oxidation mediated by air; ii) bpy/phen ligation to Ag^III ; iii) boron-to-Ag^III aryl transfer; and iv) ulterior reductive elimination of benzotrifluorides from an [aryl-Ag^III -CF₃ ] fragment. More precisely, an ultimate entry and full characterization of organosilver(III) compounds [K]⁺ [Ag^III (CF₃ )₄ ]^- (K-1), [(bpy)Ag^III (CF₃ )₃ ] (2) and [(phen)Ag^III (CF₃ )₃ ] (3), is described. The utility of 3 in cross-coupling has been showcased unambiguously, and a large variety of arylboron compounds was trifluoromethylated via [Ag^III (aryl)(CF₃ )₃ ]^- intermediates. This work breaks with old stereotypes and misconceptions regarding the inability of Ag to undergo cross-coupling by itself.