Benzylic C(sp3)H Perfluoroalkylation of Six-Membered Heteroaromatic Compounds

Mechanistic insights into the role of cyclodextrin in the regioselective radical CH trifluoromethylation of aromatic compounds

The regioselective radical CH trifluoromethylation of aromatic compounds have been shown to proceed in good yield and high regioselectivity when cyclodextrin (CD) is present. Yet, the reaction mechanism and the role of CD during the reaction have remained obscure. To this end, here we performed density functional theory (DFT) calculations to the conformations obtained by semiempirical quantum mechanical molecular dynamics calculations to reveal the reaction mechanism and the role of CD in controlling regioselectivity. The results show that metal salt increases the yield but do not affect the regioselectivity, which we further confirmed by an experiment. In contrast, multiple CD-substrate complex conformations and reaction pathways were obtained, and CD was shown to contribute to improving the regioselectivity by stabilizing the intermediate state via encapsulation. The present study indicates that CDs can increase the regioselectivity by stabilizing the intermediate and product states while only marginally affecting the transition state.

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.

Site-Selective Pyridylic C-H Functionalization by Photocatalytic Radical Cascades

An efficient pyridylic C(sp³ )-H functionalization has been developed through photocatalytic radical-mediated fluoroalkylation or cascade reactions. This method is enabled by the reversible formation of alkylidene dihydropyridine intermediates via the facile enolate formation of C4-alkyl N-amidopyridinium salts in the absence of an external base, thereby establishing the conditions necessary for subsequent intermolecular radical trapping. Rapid structural diversification of the pyridylic site can be achieved through photocatalytic multicomponent cascade reactions involving alkene trifluoromethylation, SO₂ -reincorporation, and sulfonyl radical addition. This operationally simple method features a broad substrate scope and high chemoselectivity and offers a unique approach for the rational modification of the heterobenzylic C-H bonds of pyridines and quinolines with uniform site-selective control. Furthermore, experimental and theoretical studies were performed to elucidate the reaction mechanism.

A photoinduced transient activating strategy for late-stage chemoselective C(sp3)–H trifluoromethylation of azines

The direct functionalization of C(sp³)–H bonds is an ultimately ideal synthetic strategy with high atom economy and step efficiency. However, the direct trifluoromethylation of electron-deficient heteroaryl adjacent C(sp³)–H bonds remains a formidable challenge. We have described a transient activating strategy involving a Tf-shift process and π–π stacking interaction for catalyst-free direct benzylic C(sp³)–H trifluoromethylation of azines, such as pyridine, pyrimidine, quinoline, dihydropyridinone, tetrahydroisoquinoline and tetrahydroquinazoline, with an air-stable crystalline imidazolium sulfonate reagent IMDN-Tf. This bench-stable cationic reagent offers a scalable and practical protocol for the late-stage modification of drug molecules with high site selectivity, which avoids the prefunctionalization and the use of stoichiometric metals and strong oxidants. Furthermore, comprehensive mechanistic studies revealed the determining effect of π–π stacking for the activation of azinylic C(sp³)–H bonds.

Late-stage C(sp³)–H functionalization of unactivated azines: the traceless Tf switching process offers ample opportunities for site-selective derivatization of heteroaryls, allowing for the rapid increase of molecular complexity.

A radical approach for the selective C-H borylation of azines

Boron functional groups are often introduced in place of aromatic carbon-hydrogen bonds to expedite small-molecule diversification through coupling of molecular fragments^1-3. Current approaches based on transition-metal-catalysed activation of carbon-hydrogen bonds are effective for the borylation of many (hetero)aromatic derivatives^4,5 but show narrow applicability to azines (nitrogen-containing aromatic heterocycles), which are key components of many pharmaceutical and agrochemical products⁶. Here we report an azine borylation strategy using stable and inexpensive amine-borane⁷ reagents. Photocatalysis converts these low-molecular-weight materials into highly reactive boryl radicals⁸ that undergo efficient addition to azine building blocks. This reactivity provides a mechanistically alternative tactic for sp² carbon-boron bond assembly, where the elementary steps of transition-metal-mediated carbon-hydrogen bond activation and reductive elimination from azine-organometallic intermediates are replaced by a direct, Minisci⁹-style, radical addition. The strongly nucleophilic character of the amine-boryl radicals enables predictable and site-selective carbon-boron bond formation by targeting the azine's most activated position, including the challenging sites adjacent to the basic nitrogen atom. This approach enables access to aromatic sites that elude current strategies based on carbon-hydrogen bond activation, and has led to borylated materials that would otherwise be difficult to prepare. We have applied this process to the introduction of amine-borane functionalities to complex and industrially relevant products. The diversification of the borylated azine products by mainstream cross-coupling technologies establishes aromatic amino-boranes as a powerful class of building blocks for chemical synthesis.

Regioselective C-H Trifluoromethylation of Aromatic Compounds by Inclusion in Cyclodextrins

A regioselective radical C-H trifluoromethylation of aromatic compounds was developed using cyclodextrins (CDs) as additives. The C-H trifluoromethylation proceeded with high regioselectivity to afford the product in good yield, even on the gram scale. In the presence of CDs, some substrates underwent a single trifluoromethylation selectively, whereas mixtures of single- and double-trifluoromethylated products were formed in the absence of the CD. ¹H NMR experiments indicated that the regioselectivity was controlled by the inclusion of a substrate inside the CD cavity.

Iodine(III)-Mediated Fluorination/Semipinacol Rearrangement Cascade of 2-Alkylidenecyclobutanol Derivatives: Access to β-Monofluorinated Cyclopropanecarbaldehydes

A hypervalent iodine(III)-mediated ring-contractive fluorination reaction of 2-alkylidenecyclobutanol derivatives is presented. The protocol allows the facile synthesis of β-monofluorinated cyclopropanecarbaldehydes via a fluorination/semipinacol rearrangement cascade using nucleophilic Py·HF as the fluorine source. For challenging electron-rich arene substrates, the installation of a protecting group on the free alcohol is pivotal for maintaining the reaction efficiency. The synthetic utility was demonstrated by the scalability of this reaction and further transformations of the products.

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.

Copper-Catalyzed Annulation-Cyanotrifluoromethylation of 1,6-Enynes Toward 1-Indanones via a Radical Process

A new Cu(II)-catalyzed annulation-cyanotrifluoromethylation of 1,6-enynes with Togni's reagent and trimethylsilyl cyanide (TMSCN) has been established, enabling the direct construction of trifluoromethylated 1-indanones with an all-carbon quaternary center in good yields. This reaction was performed by using low-cost Cu(OTf)₂ as the catalyst and Togni's reagent as both the radical initiator and a CF₃ source, providing an efficient protocol for building up an 1-indanone framework with wide functional group compatibility. The reaction mechanism was proposed through a radical triggered addition/5-exo-dig cyclization/oxidation/nucleophilic cascade.

V. K, Hari K, Kaliyamoorthy A. Base-promoted 1,6-conjugate addition of alkylazaarenes to para-quinone methides

1,1,2-Triarylethanes embedded with an azaarene unit were prepared in a single step at ambient temperature via the sodium hexamethyldisilazide mediated 1,6-conjugate addition of unactivated alkylazaarenes on para-quinone methides (p-QMs).

Stereocontrolled Synthesis of Functionalized Azaheterocycles from Carbocycles through Oxidative Ring Opening/Reductive Ring Closing Protocols

Fluorine-containing organic scaffolds are of significant interest in medicinal chemistry. The incorporation of fluorine into biomolecules can lead to remarkable changes in their physical, chemical, and biological properties. There are already many drugs on the market, which contain at least one fluorine atom. Saturated functionalized azaheterocycles as bioactive substances have gained increasing attention in pharmaceutical chemistry. Due to the high biorelevance of organofluorine molecules and the importance of N-heterocyclic compounds, selective stereocontrolled procedures to the access of new fluorine-containing saturated N-heterocycles are considered to be a hot research topic. This account summarizes the synthesis of functionalized and fluorine-containing saturated azaheterocycles starting from functionalized cycloalkenes and based on oxidative ring cleavage of diol intermediates followed by ring expansion with reductive amination.

Copper-Catalyzed Remote C(sp3 )-H Trifluoromethylation of Carboxamides and Sulfonamides

Reported herein is an unprecedented protocol for trifluoromethylation of unactivated aliphatic C(sp³ )-H bonds. With Cu(OTf)₂ as the catalyst, the reaction of N-fluoro-substituted carboxamides (or sulfonamides) with Zn(CF₃ )₂ complexes provides the corresponding δ-trifluoromethylated carboxamides (or sulfonamides) in satisfactory yields under mild reaction conditions. A radical mechanism involving 1,5-hydrogen atom transfer of N-radicals followed by CF₃ -transfer from Cu^II -CF₃ complexes to the thus formed alkyl radicals is proposed.

A general and practical Ni-catalyzed C-H perfluoroalkylation of (hetero)arenes

A direct perfluoroalkylation of (hetero)arenes using the air- and moisture-stable complex (dppf)Ni(o-tol)Cl was developed (23 examples). The novel procedure allows for the synthesis of various fluorinated products and tolerates sensitive functional groups including aldehydes, free amino groups and several heterocycles.

Copper-catalyzed cross-dehydrogenative coupling between quinazoline-3-oxides and indoles

A novel and simple protocol for the synthesis of 4-(indole-3-yl)quinazolines via cross-dehydrogenative coupling of quinazoline-3-oxides and indoles under an air atmosphere has been developed.

Direct C-4 alkylation of quinazoline N-oxides with ethers via an oxidative cross-coupling reaction under metal-free conditions

A novel and efficient protocol for the direct C-4 alkylation of quinazoline-3-oxides via radical oxidative coupling between quinazoline 3-oxides and ethers in the presence of TBHP was developed. The reaction proceeded smoothly under metal-free conditions, which provided quinazoline-containing heterocyclic molecules in moderate to good yields.

Organic dye-photocatalyzed fluoroalkylation of heteroarene-N-oxide derivatives

A direct C_Het–H perfluoroalkylation reaction of heteroaromatic-N-oxides has been achieved. Acid-catalyzed transformation of the perfluoroalkylated-N-oxides leads to 2-(perfluoroalkyl)benzo[f][1,3]oxazepines. De-oxygenation of the perfluoroalkylated heteroaromatic-N-oxides affords a regioselective radical perfluoroalkylation protocol.

Photoredox generation of the trifluoromethyl radical from borate complexes via single electron reduction

A trifluoromethyl radical is generated by single electron reduction of CF₃-substituted borate complexes.

Laser-Initiated Radical Trifluoromethylation of Peptides and Proteins: Application to Mass-Spectrometry-Based Protein Footprinting

Described is a novel, laser-initiated radical trifluoromethylation for protein footprinting and its broad residue coverage. ^. CF₃ reacts with 18 of the 20 common amino acids, including Gly, Ala, Ser, Thr, Asp, and Glu, which are relatively silent with regard to ^. OH. This new approach to footprinting is a bridge between trifluoromethylation in materials and medicinal chemistry and structural biology and biotechnology. Its application to a membrane protein and to myoglobin show that the approach is sensitive to protein conformational change and solvent accessibility.

Strategy for Stereoselective Metal-free α-Functionalization of 2-Azaaryl Acetates with N-Boc Imines

We report the first diastereo- and enantioselective formal Mannich reaction of 2-pyridyl acetates which gives rise to α- and β-functionalized 2-substituted pyridines. Key for success is the previous azaarene N-oxide formation enabling α-carbon deprotonation by a mild bifunctional Brønsted base and subsequent reaction with N-Boc imines under almost perfect stereocontrol.

Regio- and Stereoselective Radical Perfluoroalkyltriflation of Alkynes Using Phenyl(perfluoroalkyl)iodonium Triflates

A method for regio- and stereoselective anti-addition of the perfluoroalkyl and the triflate group of phenyl(perfluoroalkyl)iodonium triflates to alkynes is presented. The radical reaction uses cheap CuCl as a smart initiator and can be conducted in gram scale. The perfluoroalkyltriflated products are readily further functionalized, rendering this transformation valuable.

Advances in the Preparation of Fluorinated Isoquinolines: A Decade of Progress

Heterocyclic molecules incorporating fluorinated isoquinoline components are found in many medicinally and agriculturally important bioactive products as well as industrially impactful materials. Within the past decade, a variety of isoquinolinic ring assembly techniques has enabled the introduction of diverse fluorine-containing functionalities which can enhance potential bioactivity and industrial utility. This review examines recent noncatalyzed and transition metal catalyzed synthetic approaches to the assembly of isoquinoline derivatives that are ring-fluorinated and/or result in the incorporation of fluorine-containing functional groups. Specifically, efficient synthetic methods and regioselectivity in the incorporation of functional groups into isoquinoline ring systems are examined.

4-Position-Selective C-H Perfluoroalkylation and Perfluoroarylation of Six-Membered Heteroaromatic Compounds

The first 4-position-selective C-H perfluoroalkylation and perfluoroarylation of six-membered heteroaromatic compounds were achieved using nucleophilic perfluoroalkylation and perfluoroarylation reagents. The regioselectivity was controlled by electrophilically activating the heteroaromatic rings, while sterically hindering the 2-position, with a sterically bulky borane Lewis acid. The reaction proceeded in good yield, even in gram scale, and by a sequential reaction without isolating the intermediates. This reaction could be applied to late-stage trifluoromethylation of a bioactive compound.

5-Position-selective C–H trifluoromethylation of 8-aminoquinoline derivatives

A rare example of C–H trifluoromethylation at the remote position of 8-aminoquinoline derivatives was realized by using a CuCl catalyst/Togni's reagent system.

Direct Catalytic Asymmetric Mannich-Type Reaction of α- and β-Fluorinated Amides

The last two decades have witnessed the emergence of direct enolization protocols providing atom-economical and operationally simple methods to use enolates for stereoselective C-C bond-forming reactions, eliminating the inherent drawback of the preformation of enolates using stoichiometric amounts of reagents. In its infancy, direct enolization relied heavily on the intrinsic acidity of the latent enolates, and the reaction scope was limited to readily enolizable ketones and aldehydes. Recent advances in this field enabled the exploitation of carboxylic acid derivatives for direct enolization, offering expeditious access to synthetically versatile chiral building blocks. Despite the growing demand for enantioenriched fluorine-containing small molecules, α- and β-fluorinated carbonyl compounds have been neglected in direct enolization chemistry because of the competing and dominating defluorination pathway. Herein we present a comprehensive study on direct and highly stereoselective Mannich-type reactions of α- and β-fluorine-functionalized 7-azaindoline amides that rely on a soft Lewis acid/hard Brønsted base cooperative catalytic system to guarantee an efficient enolization while suppressing undesired defluorination. This protocol contributes to provide a series of fluorinated analogs of enantioenriched β-amino acids for medicinal chemistry.