Multiple Approaches to the In Situ Generation of Anhydrous Tetraalkylammonium Fluoride Salts for SNAr Fluorination Reactions

Deoxyfluorination of Carboxylic Acids via an In Situ Generated Trifluoromethoxide Salt

An in situ generated pyridinium trifluoromethoxide salt (PyOCF3) is a highly effective deoxyfluorination reagent for the synthesis of acid fluorides. PyOCF3 is formed at room temperature from the reaction of 2,4-dinitro(trifluoromethoxy)benzene with 4-dimethylaminopyridine. PyOCF3 undergoes slow release of fluorophosgene and fluoride, which serve as the electrophile and nucleophile, respectively, for deoxyfluorination. A wide substrate scope and high functional group tolerance are demonstrated. Furthermore, the acid fluorides can be purified by filtration and telescoped to various known reactions.

Synthetic Advantages of Defluorinative C-F Bond Functionalization

Much progress has been made in the development of methods to both create compounds that contain C-F bonds and to functionalize C-F bonds. As such, C-F bonds are becoming common and versatile synthetic functional handles. This review summarizes the advantages of defluorinative functionalization reactions for small molecule synthesis. The coverage is organized by the type of carbon framework the fluorine is attached to for mono- and polyfluorinated motifs. The main challenges, opportunities and advances of defluorinative functionalization are discussed for each class of organofluorine. Most of the text focuses on case studies that illustrate how defluorofunctionalization can improve routes to synthetic targets or how the properties of C-F bonds enable unique mechanisms and reactions. The broader goal is to showcase the opportunities for incorporating and exploiting C-F bonds in the design of synthetic routes, improvement of specific reactions and advent of new methods.

A 2,2-Difluoroimidazolidine Derivative for Deoxyfluorination Reactions: Mechanistic Insights by Experimental and Computational Studies

A N-heterocyclic deoxyfluorinating agent SIMesF2 was synthesized by nucleophilic fluorination of N,N-1,3-dimesityl-2-chloroimidazolidinium chloride (3) at room temperature. SIMesF2 was applied to deoxyfluorinate carboxylic acids and alcohols and convert benzaldehyde into difluorotoluene. Mechanistic studies by NMR spectroscopy suggest reaction pathways of the carboxylic acid to acyl fluoride via outer-sphere fluorinations at an imidazolidinium ion by polyfluoride. DFT studies give further insight by exploring mechanistic details which distinguish the fluorination of aldehydes from that of carboxylic acids. Furthermore, a consecutive reaction sequence for the oxidation of an aldehyde followed by in situ fluorination of the generated carboxylic acid was developed.

LiBF4-Promoted Aromatic Fluorodetriazenation under Mild Conditions

An efficient and mild fluorination method through LiBF4-promoted aromatic fluorodetriazenation of 3,3-dimethyl-1-aryltriazenes is developed. The reaction proceeds smoothly and tends to complete within 2 h in the absence of a protic acid or strong Lewis acid. This method tolerates a wide range of functional groups and affords the aryl fluoride products in moderate to excellent yields.

Nucleophilic Fluorination Catalyzed by a Cyclometallated Rhodium Complex

Quantitative catalytic nucleophilic fluorination of a range of acyl chlorides to acyl fluorides was promoted by a cyclometallated rhodium complex [(η5,κ2C-C5Me4CH2C6F5CH2NC3H2NMe)- RhCl] (1). 1 can be prepared in high yields from commercially available starting materials using a one-pot method. The catalyst could be separated, regenerated, and reused. Rapid quantitative fluorination generated the fluoride analogue of the active pharmaceutical ingredient probenecid. Infrared in situ monitoring verified the clean conversion of the substrates to products. VTNA graphical kinetic analysis and DFT calculations lead to a postulated reaction mechanism involving a nucleophilic Rh-F bond.

Tetramethylammonium Fluoride: Fundamental Properties and Applications in C-F Bond-Forming Reactions and as a Base

Nucleophilic ionic sources of fluoride are essential reagents in the synthetic toolbox to access high added-value fluorinated building blocks unattainable by other means. In this review, we provide a concise description and rationale of the outstanding features of one of these reagents, tetramethylammonium fluoride (TMAF), as well as disclosing the different methods for its preparation, and how its physicochemical properties and solvation effects in different solvents are intimately associated with its reactivity. Furthermore, herein we also comprehensively describe its historic and recent utilization, up to December 2021, in C-F bond-forming reactions with special emphasis on nucleophilic aromatic substitution fluorinations with a potential sustainable application in industrial settings, as well as its use as a base capable of rendering unprecedented transformations.

Towards better syntheses of partially methylated carbohydrates?

We give an overview of the reported synthetic strategies towards partially methylated glycosides and discuss how better protocols could stem from catalytic site-selective transformations of carbohydrates and cleaner methylation reagents.

Carboxylic Acid Deoxyfluorination and One-Pot Amide Bond Formation Using Pentafluoropyridine (PFP)

This work describes the application of pentafluoropyridine (PFP), a cheap commercially available reagent, in the deoxyfluorination of carboxylic acids to acyl fluorides. The acyl fluorides can be formed from a range of acids under mild conditions. We also demonstrate that PFP can be utilized in a one-pot amide bond formation via in situ generation of acyl fluorides. This one-pot deoxyfluorination amide bond-forming reaction gives ready access to amides in yields of ≤94%.

Tetramethylammonium Fluoride Alcohol Adducts for SNAr Fluorination

Nucleophilic aromatic fluorination (S_NAr) is among the most common methods for the formation of C(sp²)-F bonds. Despite many recent advances, a long-standing limitation of these transformations is the requirement for rigorously dry, aprotic conditions to maintain the nucleophilicity of fluoride and suppress the generation of side products. This report addresses this challenge by leveraging tetramethylammonium fluoride alcohol adducts (Me₄NF·ROH) as fluoride sources for S_NAr fluorination. Through systematic tuning of the alcohol substituent (R), tetramethylammonium fluoride tert-amyl alcohol (Me₄NF·t-AmylOH) was identified as an inexpensive, practical, and bench-stable reagent for S_NAr fluorination under mild and convenient conditions (80 °C in DMSO, without the requirement for drying of reagents or solvent). A substrate scope of more than 50 (hetero) aryl halides and nitroarene electrophiles is demonstrated.

Deoxyfluorination of Carboxylic Acids with KF and Highly Electron-Deficient Fluoroarenes

A deoxyfluorination reaction of carboxylic acids using potassium fluoride (KF) and highly electron-deficient fluoroarenes is reported here, giving acyl fluorides in moderate to excellent yield (57-92% based on NMR integration and 34-95% for isolated examples).

Organocatalyzed Fluoride Metathesis

A new organocatalyzed fluoride metathesis reaction between fluoroarenes and carbonyl derivatives is reported. The reaction exchanges fluoride (F^-) and alternate nucleophiles (OAc^-, OCO₂R^-, SR^-, Cl^-, CN^-, NCS^-). The approach provides a conceptually novel route to manipulate the fluorine content of organic molecules. When the fluorination and defluorination steps are combined into a single catalytic cycle, a byproduct free and 100% atom-efficient reaction can be achieved.

Development of SNAr Nucleophilic Fluorination: A Fruitful Academia-Industry Collaboration

The identification of reliable, general, and high yielding methods for the formation of C(sp²)-fluorine bonds remains a major challenge for synthetic organic chemists. A very common approach involves nucleophilic aromatic fluorination (S_NAr fluorination) reactions of aryl chlorides or nitroarenes. Despite being known for more than a century, traditional S_NAr fluorination reactions suffer from significant limitations, particularly on a process scale. These include the high cost of common reagents [e.g., cesium fluoride (CsF)], a requirement for elevated temperatures and long reaction times, poor functional group tolerance, and the need for rigorous exclusion of water. This Account summarizes our collaboration with Corteva Agriscience (previously Dow Agrosciences) to address many of these challenges. This collaboration has provided a platform for fundamental scientific advances involving the development of new methods, reagents, and substrates for mild and high yielding nucleophilic fluorination reactions.Our early studies established that the combination of potassium fluoride (KF) and superstoichiometric tetrabutylammonium chloride (Bu₄NCl) serves as a cost-effective alternative to CsF for the S_NAr fluorination of chloropicolinate substrates. However, these reactions still require elevated temperatures (>130 °C) and afford moderate yields due to competing decomposition of the substrate and product. The need for high temperature is largely due to slow reaction rates resulting from the low concentration of the active fluorinating reagent [anhydrous tetrabutylammonium fluoride (Bu₄NF)] under these conditions. To address this issue, we developed several strategies for generating high concentration solutions of anhydrous tetraalkylammonium fluoride in situ by combining fluorine-containing electrophiles (e.g., hexafluorobenzene, acyl fluorides, sulfonyl fluorides) with tetraalkylammonium nucleophiles (R₄NCN or R₄NOR). These systems enable S_NAr fluorination under unusually mild conditions, affording nearly quantitative yield with chloropicolinate substrates at room temperature. However, the high cost of the electrophiles and the generation of large quantities of byproducts in the R₄NF-forming step render this approach unsuitable for process scale applications. As an alternative, we next explored anhydrous tetramethylammonium fluoride (Me₄NF) for these transformations. This highly reactive fluoride source can be synthesized directly from inexpensive KF and Me₄NCl and then dried by heating under vacuum. Unlike Bu₄NF, it is not susceptible to Hofmann elimination. As such, anhydrous Me₄NF is stable and isolable, as well as highly effective for the room temperature S_NAr fluorination of chloropicolinates and other electron deficient substrates.The studies with anhydrous R₄NF drew our attention to another challenge associated with traditional S_NAr fluorination reactions: their limitation to substrates bearing resonance electron-withdrawing groups. We hypothesized that this challenge could be addressed by circumventing the Meisenheimer intermediate, a canonical mechanistic feature of S_NAr fluorination. By designing reactions that involve an alternative concerted delivery of the fluoride to the ipso C(sp²) center, we developed a deoxyfluorination of arylfluorosulfonates using anhydrous Me₄NF. This reaction exhibits a broad scope with respect to the aryl electrophile, with substrates bearing both electron-withdrawing (CN, ester, CF₃, Cl) and moderately electron donating (phenyl, alkyl) substituents participating in deoxyfluorination. These deoxyfluorination conditions were also expanded to nonaromatic substrates, including aldehydes and benzylic/aliphatic alcohols.This Account concludes by delineating several ongoing challenges and opportunities in this fast-moving field. For instance, one important future direction will be to address the high moisture sensitivity of these transformations. In addition, the application of these new reagents and methods in the synthesis of pharmaceuticals, agrochemicals, and PET imaging agents will continue to test the versatility and functional group compatibility of these methods.

Selective Methylation of Amides, N-Heterocycles, Thiols, and Alcohols with Tetramethylammonium Fluoride

We herein disclose the use of tetramethylammonium fluoride (TMAF) as a direct and selective methylating agent of a variety of amides, indoles, pyrroles, imidazoles, alcohols, and thiols. The method is characterized by operational simplicity, wide scope, and ease of purification. Our computational studies suggest a concerted methylation-deprotonation as the preferred reaction pathway.

Direct Access to Acyl Fluorides from Carboxylic Acids Using a Phosphine/Fluoride Deoxyfluorination Reagent System

A fast and simple method for deoxyfluorination of carboxylic acids is presented. The protocol employs commodity chemicals (PPh₃, NBS, fluoride), affording products in excellent yields under mild conditions. Acyloxyphosphonium ion, the key reaction intermediate, was identified by NMR spectroscopic methods. Brønsted acidic conditions are essential for efficient C-F bond formation. The protocol displays scalability, high functional group tolerance, chemoselectivity, and easy purification of products. Deoxyfluorination of active pharmaceutical ingredients was established.

Cobalt-Catalyzed C-F Bond Borylation of Aryl Fluorides

A mild and practical cobalt-catalyzed defluoroborylation of fluoroarenes is presented for the first time. The method permits straightforward functionalization of fluoroarenes, with high selectivity for borylation of C-F over C-H bonds, and a tolerance for aerobic conditions. Furthermore, two-step ¹⁸F-fluorination was achieved for expanding the scope of ¹⁸F-positron emission tomography probes.

Visible-Light Photocatalysis: Does It Make a Difference in Organic Synthesis?

Visible-light photocatalysis has evolved over the last decade into a widely used method in organic synthesis. Photocatalytic variants have been reported for many important transformations, such as cross-coupling reactions, α-amino functionalizations, cycloadditions, ATRA reactions, or fluorinations. To help chemists select photocatalytic methods for their synthesis, we compare in this Review classical and photocatalytic procedures for selected classes of reactions and highlight their advantages and limitations. In many cases, the photocatalytic reactions proceed under milder reaction conditions, typically at room temperature, and stoichiometric reagents are replaced by simple oxidants or reductants, such as air, oxygen, or amines. Does visible-light photocatalysis make a difference in organic synthesis? The prospect of shuttling electrons back and forth to substrates and intermediates or to selectively transfer energy through a visible-light-absorbing photocatalyst holds the promise to improve current procedures in radical chemistry and to open up new avenues by accessing reactive species hitherto unknown, especially by merging photocatalysis with organo- or metal catalysis.

Nucleophilic fluorination facilitated by a CsF–CaF2 packed bed reactor in continuous flow

A simple to prepare, dry and handle packed bed reactor carrying CsF on CaF₂, towards nucleophilic fluorinations in continuous flow, is reported.

Potassium fluoride activation for the nucleophilic fluorination reaction using 18-crown-6, [2.2.2]-cryptand, pentaethylene glycol and comparison with the new hydro-crown scaffold: a theoretical analysis

Ion-pair binding in the hydro-crown ether leads to more effective catalytic cycle for nucleophilic fluorination.

Ex Situ Generation of Sulfuryl Fluoride for the Synthesis of Aryl Fluorosulfates

A convenient transformation of phenols into the corresponding aryl fluorosulfates is presented: the first protocol to completely circumvent direct handling of gaseous sulfuryl fluoride (SO₂F₂). The proposed method employs 1,1'-sulfonyldiimidazole as a precursor to generate near-stoichiometric amounts of SO₂F₂ gas using a two-chamber reactor. With NMR studies, it was shown that this ex situ gas evolution is extremely rapid, and a variety of phenols and hydroxylated heteroarenes were fluorosulfated in good to excellent yields.