Fluorierung in späten Synthesestadien: extravagante Neuheit oder nützliches Hilfsmittel?

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.

Syntheses and Synthetic Applications of Functionalized Propargylic and Allylic Fluorides

This account presents the synthesis and application of propargylic and allylic fluorides containing hydroxy or carbonyl functional groups. In particular, the Barbier-type reaction of difluoropropargyl bromides with aldehydes or chloroformates provides versatile propargylic fluorides, and the organocatalytic fluorination of dienamine intermediates has been demonstrated as an effective method to obtain allylic fluorides stereoselectively. Additionally, mechanistic insights into such reactions are discussed with the aid of density functional theory calculations. The report also describes the preparation of fluorinated 1,7-diyne or 1,7-enyne derivatives of these compounds. These propargylic and allylic fluorides can be used as building blocks for fluorinated heterocycles, such as fluorinated furans, tetrahydrofurans, and lactams. Additionally, fluorinated bi- or tri-heterocyclic compounds can be synthesized via transition-metal-catalyzed reactions with fluorinated 1,7-diyne or 1,7-enyne derivatives.

Studying Regioisomer Formation in the Pd-Catalyzed Fluorination of Cyclic Vinyl Triflates: Evidence for in situ Ligand Modification

Pd-catalyzed nucleophilic fluorination reactions are important methods for the synthesis of fluoroarenes and fluoroalkenes. However, these reactions can generate a mixture of regioisomeric products that are often difficult to separate. While investigating the Pd-catalyzed fluorination of cyclic vinyl triflates, we observed that the addition of a substoichiometric quantity of TESCF3 significantly improved the regioselectivity of the reaction. Herein, we report a combined experimental and computational study on the mechanism of this transformation focusing on the role of TESCF3 . The poor regioselectivity of the reaction in the absence of additives results from the formation of LPd-cyclohexyne complexes (L=biaryl monophosphine ligand). When TESCF3 is added to the reaction mixture, the generation of the Pd-cyclohexyne complexes is diminished by an unexpected pathway involving the dearomatization of the ligand by nucleophilic attack from a trifluoromethyl anion (CF3 - ).

5-(Trifluorovinyl)dibenzothiophenium Triflate: Introducing the 1,1,2-Trifluoroethylene Tether in Drug-Like Structures

This manuscript reports the synthesis and structure of an unprecedented sulfonium salt, 5-(trifluorovinyl)dibenzothiophenium triflate, and its use as a versatile reagent for the introduction of the bioisosteric 1,1,2-trifluoroethylene linker in drug-like structures. The protocol developed consists of the reaction of this compound with alcohols and phenols to deliver a complete set of 1,2,2-trifluoro-2-(alkoxy-/aryloxy)ethyl sulfonium salts, which have been purified by column chromatography and fully characterized. Subsequent single electron reduction under mild photochemical conditions efficiently affords the corresponding fluoroalkyl radicals that are trapped either intra- or intermolecularly through their reaction with (hetero)arenes. Theoretical calculations are used to evaluate the conformational consequences derived from the presence of the CF₂ -CHF tether.

Diastereoselective Transfer of Tri(di)fluoroacetylsilanes-Derived Carbenes to Alkenes

Stereoselective cyclopropanation reaction of alkenes is usually achieved by metal complexes via singlet-metal-carbene intermediates. However, previous transition-metal-catalyzed cyclopropanation of alkenes with acylsilanes afforded low diastereoselectivity. Herein, we report the first visible-light-induced transition-metal-free cyclopropanation reaction of terminal alkenes with trifluoroacetylsilanes and difluoroacetylsilanes. Both aromatic and aliphatic alkenes as well as electron-deficient alkenes are suitable substrates for the highly cis-selective [2+1] cyclization reaction. A combination of experimental and computational studies identified triplet carbenes as being key intermediates in this transformation. The gram scale reaction and late-stage functionalization demonstrated the synthetic potential of this strategy.

Construction of Carbon-Fluorine Bonds via Copper-Catalyzed/-Mediated Fluorination Reactions

The construction of carbon-fluorine bonds is an important yet challenging task in organic synthesis. Transition metal-catalyzed/-mediated C-F bond forming processes have recently emerged as a viable strategy and provided access to value-added monofluorinated compounds. A dramatic increase in fluorination methods using inexpensive and earth-abundant copper can be seen in the past decade surpassing those using palladium and silver. This review discusses the recent development of Cu-catalyzed/-mediated formation of C(sp² )-F and C(sp³ )-F bonds.

Electrochemical Radiofluorination of Small Molecules: New Advances

The development of new ¹⁸ F-based radiopharmaceuticals constantly demands innovations in the search for new radiofluorination methods. [¹⁸ F]fluoride is the simplest and most convenient chemical form of the isotope for the synthesis of ¹⁸ F-based radiopharmaceuticals. The ease of production and handling, as well as the possibility of obtaining high molar activities, makes it the preferred choice for radiofluorination. However, the use of [¹⁸ F]fluoride in late-stage radiofluorination comes with challenges, especially for the radiolabeling of electron-rich molecules where S_N 2 and S_N Ar reactions are not suitable. New developments in fluorination chemistry have been extensively studied to overcome these difficulties. Selective electrochemical oxidation of precursors, using a controlled potential, is one method to create reactive intermediates and overcome the activation energy required for nucleophilic fluorination of electron-rich moieties. This method has been used for years in cold fluorination of organic molecules and more recently has been adapted as an alternative to traditional radiofluorination methods. Although relatively young, this field stands out as a promising route for the synthesis of new PET probes as well as fluorinated pharmaceuticals. This review focuses on recent advances in electrochemical radiofluorination as an alternative for the late-stage radiolabeling of organic molecules.

Silver-Promoted Fluorination Reactions of α-Bromoamides

Silver-promoted C-F bond formation in α-bromoamides by using AgF under mild conditions is reported. This simple method enables access to tertiary, secondary, and primary alkyl fluorides involving biomolecular scaffolds. This transformation is applicable to primary and secondary amides and shows broad functional-group tolerance. Kinetics experiments revealed that the reaction rate increased in the order of 3°>2°>1° α-carbon atom. In addition, it was found that the acidic amide proton plays an important role in accelerating the reaction. Mechanistic studies suggested generation of an aziridinone intermediate that undergoes subsequent nucleophilic addition to form the C-F bond with stereospecificity (i.e., retention of configuration). The synthesis of sterically hindered alcohols and ethers by using Ag^I is also demonstrated. Examples of reactions of α-bromoamides with O nucleophiles are presented.

Hydrofluorination of Alkenes: A Review

In this minireview, we explore the different approaches used to perform the hydrofluorination reaction of alkenes. Contrary to other hydrohalogenation reactions, the hydrofluorination requires specific conditions due to the lower reactivity of HF. Over the years, many different approaches have been explored among which the use of HF complexes has particularly proved to be useful as these reagents are easier to handle. The enantioselective hydrofluorination has been demonstrated using electrophilic sources of fluorine, while radical fluorination proved compatible with a vast range of functional groups that are generally problematic with strong acids and some fluoride sources. This review will cover the different conditions developed through the years, starting with the first reported addition using gaseous HF, up to the most recent method described in October 2020.

Ruthenium-Catalyzed Site-Selective Trifluoromethylations and (Per)Fluoroalkylations of Anilines and Indoles

Introducing (per)fluoroalkyl groups into arenes continues to be an interesting, but challenging area in organofluorine chemistry. We herein report an ortho-selective C-H perfluoroalkylation including trifluoromethylations of anilines and indoles without the need of protecting groups using Rf I and Rf Br as commercially available reagents. The availability and price of the starting materials and the inherent selectivity make this novel methodology attractive for the synthesis of diverse (per)fluoroalkylated building blocks, for example, for bioactive compounds and materials.

Light-Mediated Formal Radical Deoxyfluorination of Tertiary Alcohols through Selective Single-Electron Oxidation with TEDA2+

The synthesis of tertiary alkyl fluorides through a formal radical deoxyfluorination process is described herein. This light-mediated, catalyst-free methodology is fast and broadly applicable allowing for the preparation of C-F bonds from (hetero)benzylic, propargylic, and non-activated tertiary alcohol derivatives. Preliminary mechanistic studies support that the key step of the reaction is the single-electron oxidation of cesium oxalates-which are readily available from the corresponding tertiary alcohols-with in situ generated TEDA2+. (TEDA: N-(chloromethyl)triethylenediamine), a radical cation derived from Selectfluor®.

Synthesis of Aryl Triflones through the Trifluoromethanesulfonylation of Benzynes

The direct synthesis of aryl triflones, that is, trifluoromethanesulfonyl arenes, was achieved through the trifluoromethanesulfonylation of benzynes. The trifluoromethanesulfonyl group, one of the fluorinated functional groups, is a highly electron-negative and mild lipophilic substituent. Aryl triflones have high potential in the synthesis of bioactive compounds and specialty materials. The treatment of 2-(trimethylsilyl)aryl trifluoromethanesulfonates with cesium fluoride in the presence of 15-crown-5 generated benzynes, which reacted with sodium trifluoromethanesulfinate followed by protonation with tBuOH under heating conditions, provided aryl triflones in moderated to good yields. Both symmetrical and unsymmetrical triflones were nicely accessed under the same reaction conditions. Interestingly, the trifluoromethanesulfonylation of unsymmetrical benzyne precursors proceeded smoothly to furnish corresponding aryl triflones in good yields with good to high regioselectivities. The balance of polarization of electric charge as well as steric hindrance of the benzyne intermediates are central factors to control the outcome of regioselectivity.

Hydrogen Bonding: Regulator for Nucleophilic Fluorination

The recent advances in nucleophilic fluorination, regulated through hydrogen bonding interactions are summarized. Two main categories of fluorine nucleophiles are discussed. Alkali-metal fluorides are widely used in various fluorination transformations because they are inexpensive and safe nucleophilic fluorine sources. But the non-controllable nucleophilicity and strong basicity of some of them cause undesired side reactions, which led to the introduction of hydrogen bonding to fine tune their nucleophilicity and basicity. In contrast, an HF-based fluorine nucleophile, HF/DMPU, is in some aspects superior to the conventional HF/pyridine (Olah's reagent) or HF/Et3 N because of the higher hydrogen bond basicity of DMPU. It has been used in several nucleophilic fluorinations such as fluorination of alkynes, fluoro-Prins reaction and fluorination of aziridines.

The Versatile Behavior of Platinum Alkyne Complexes towards XeF2 : Formation of Fluorovinyl and Fluorido Complexes

Reactions of platinum(0) tolane complexes, bearing a chelating ligand with P and N donor atoms, with the electrophilic fluorinating agent XeF₂ give facile access to platinum(II) β-fluorovinyl fluorido complexes. A series of new platinum(II) β-fluorovinyl complexes have been synthesized and were structurally characterized. Further oxidation with XeF₂ led to ortho-metalated platinum(IV) fluorido compounds. Additional reactions of platinum(0) tolane complexes, bearing a chelating P,P donor ligand, with XeF₂ led to a variety of fluorido and fluorovinyl complexes.

Targeted Fluoro Positioning for the Discovery of a Potent and Highly Selective Matrix Metalloproteinase Inhibitor

The incorporation of fluorine atoms into functional molecules is of wide interest in synthetic organic chemistry as well as cognate disciplines. In particular, in medicinal chemistry, there is a strong desire to positively influence the physicochemical molecular properties of drug compounds by introducing fluorine into biologically active molecules. Here, we present targeted fluoro positioning as the key design principle of converting a weak matrix metalloproteinase‐13 (MMP‐13) inhibitor into a very potent (IC₅₀=6 nm) and highly selective (selectivity factors of >1000 over MMP‐1, 2, 3, 7, 8, 9, 10, 12, 14) inhibitor with excellent plasma and microsomal stability, and no binding to the hERG channel (hERG: human ether‐a‐go‐go related gene).

Palladium-Catalyzed Fluorination of Cyclic Vinyl Triflates: Effect of TESCF3 as an Additive

A method for the palladium-catalyzed fluorination of cyclic vinyl triflates has been developed. As with several previous palladium-catalyzed fluorination reactions using fluoride salts, controlling the regioselectivity presented a challenge in developing a practical synthetic procedure. The addition of triethyl(trifluoromethyl)silane (TESCF3 ) was found to effectively address this problem and resulted in drastically improved regioselectivities in this palladium-catalyzed fluorination reaction. This discovery, along with the use of a new biarylphosphine ligand, allowed for the development of an efficient and highly regioselective protocol for the fluorination of vinyl triflates. This method is compatible with a range of sensitive functional groups and provides access to five-, six-, and seven-membered cyclic vinyl fluorides.

The Uranyl Cation as a Visible-Light Photocatalyst for C(sp(3) )-H Fluorination

The fluorination of unactivated C(sp(3) )-H bonds remains a desirable and challenging transformation for pharmaceutical, agricultural, and materials scientists. Previous methods for this transformation have used bench-stable fluorine atom sources; however, many still rely on the use of UV-active photocatalysts for the requisite high-energy hydrogen atom abstraction event. Uranyl nitrate hexahydrate is described as a convenient, hydrogen atom abstraction catalyst that can mediate fluorinations of certain alkanes upon activation with visible light.

A Versatile Organocatalytic Approach for the Synthesis of Enantioenriched gem-Difluorinated Compounds

The combination of a practical and highly enantioselective organocatalytic reaction, which allows the stereoselective introduction of a benzodithiol group, with a fluorination step, gives a new and effective strategy for the stereoselective synthesis of difluorinated building blocks. The benzodithiol group is a versatile and chameleonic group that can be further functionalized before fluorination, giving customized and tailored useful synthetic strategies. As an example of the application of this facile strategy, the effective enantioselective synthesis of difluoroarundic acid is described.

Consecutive C-F bond activation and C-F bond formation of heteroaromatics at rhodium: the peculiar role of FSi(OEt)3

C-F activation of 2,3,5,6-tetrafluoropyridine at [Rh{Si(OEt)3}(PEt3)3] (1) yields [Rh{2-(3,5,6-C5F3HN)}(PEt3)3] (2) and FSi(OEt)3, but in an unprecedented consecutive reaction FSi(OEt)3 acts as a fluoride source to give [Rh(4-C5F4N)(PEt3)3] (4) by regeneration of the C-F bond and C-H activation. Analogous refluorination steps were observed for other 2-pyridyl rhodium complexes. NMR spectroscopic studies revealed a delicate balance between the feasibility for C-F bond formation accompanied by a C-H activation and the occurrence of competing reactions such as hydrodefluorinations induced by the intermediary presence of H2.

Organonickel(IV) chemistry: a new catalyst?

With scorpionate ligands finding their way into organonickel chemistry, the state of the art of present-day nickel(IV) chemistry is highlighted. Will rapid CX coupling reactions emerge as a domain of higher-oxidation-state nickel chemistry?