Room Temperature Deoxyfluorination of Benzaldehydes and α-Ketoesters with Sulfuryl Fluoride and Tetramethylammonium Fluoride

Selective Defluorination of Trifluoromethyl Substituents by Conformationally Induced Remote Substitution

The selective reduction of an aromatic trifluoromethyl substituent to a difluoromethyl substituent may be achieved by base-promoted elimination to form a difluoro-p-quinomethide which is trapped by an intramolecular nucleophile. High yields are obtained when the nucleophilic trap entails the conformationally favoured cyclisation of an aminoisobutyric acid (Aib) derivative. The resulting cyclised difluoromethyl-substituted arylimidazolidinone products are readily converted to versatile difluoromethyl-substituted aldehydes by reduction and hydrolysis. Defluorination is successful on a range of benzenoid (both para and ortho CF3-substituted) and heterocyclic substrates. Double defluorination may likewise be achieved sequentially, or in a single step, from an Aib dipeptide derivative.

Recent Advancements in the Synthesis of α-fluoroalkylated Azine-derived Heterocycles through Direct Fluorination

The review highlights recent advancements in the synthesis of α-fluoro and α,α- difluoroalkylated azines, focusing on two main approaches. The first approach involves nucleophilic deoxofluorination, wherein α-hydroxy- or α-oxoalkylated azines are treated with diethylaminosulfur trifluoride or other S-F reagents to introduce fluorine atoms. The second approach employs direct electrophilic benzylic fluorination, whereby alkylazines undergo fluorination using N-F reagents. Both methods provide flexibility in designing and synthesizing fluoroalkylated heterocycles.

Cobalt-catalyzed decarboxylative difluoroalkylation of nitrophenylacetic acid salts

The selective installation of fluorine-containing groups into biologically relevant molecules has been used as a common strategy for the development of pharmaceutically active molecules. However, the selective incorporation of gem-difluoromethylene groups next to sterically demanding secondary and tertiary alkyl groups remains a challenge. Herein, we report the first cobalt-catalyzed regioselective difluoroalkylation of carboxylic acid salts. The reaction allows for the facile construction of various difluoroalkylated products in good yields tolerating a wide range of functionalities on either reaction partner. The potential of the method is illustrated by the late-stage functionalization of molecules of biological relevance. Mechanistic studies support the in situ formation of a cobalt(i) species and the intermediacy of difluoroalkyl radicals, thus suggesting a Co(i)/Co(ii)/Co(iii) catalytic cycle.

Deoxyfluorination of 1°, 2°, and 3° Alcohols by Nonbasic O-H Activation and Lewis Acid-Catalyzed Fluoride Shuttling

A method for deoxyfluorination of aliphatic primary, secondary, and tertiary alcohols is reported, employing a nontrigonal phosphorus triamide for base-free alcohol activation in conjunction with an organic soluble fluoride donor and a triarylborane fluoride shuttling catalyst. Mechanistic experiments are consistent with a reaction that proceeds by the collapse of an oxyphosphonium fluoroborate ion pair with fluoride transfer. The substrate scope complements existing deoxyfluorination methods and enables the preparation of homochiral secondary and tertiary alkylfluorides by stereoinversion of the substrate alcohol.

SO2F2-Mediated Fluorination of P(O)-H and P(O)-OH Compounds under Mild Conditions

With the increasing relevance of organophosphorus fluorine compounds in the pharmaceutical industry, their synthesis has attracted great attention. Herein, we report an efficient fluorination strategy for P(O)-H and P(O)-OH compounds using sulfuryl fluoride as the fluorination reagent. Avoiding the use of expensive or complex prepreparation reagents for fluoridation, this strategy could conveniently construct a variety of fluorophosphonates and phosphonofluoridates under mild conditions and without additional oxidants.

Electrochemically Driven Deoxygenative Borylation of Alcohols and Carbonyl Compounds

We present a new, unified approach for the transformation of benzylic and allylic alcohols, aldehydes, and ketones into boronic esters under electroreductive conditions. Key to our strategy is the use of readily available pinacolborane, which serves both as an activator and an electrophile by first generating a redox-active trialkylborate species and then delivering the desired deoxygenatively borylated product. This strategy is applicable to a variety of substrates and can be employed for the late-stage functionalization of complex molecules.

Room temperature deoxofluorination of aromatic aldehydes with XtalFluor-E under highly concentrated conditions

The synthesis of difluoromethyl-containing compounds exploiting the deoxofluorination reaction of aromatic aldehydes using XtalFluor-E is described. This transformation occurs at room temperature under highly concentrated conditions, i.e., with no added solvent. A wide range of difluoromethyl-containing compounds was obtained in 21 to 87% isolated yields.

Photoredox Catalyzed Single C-F Bond Activation of Trifluoromethyl Ketones: A Solvent Controlled Divergent Access of gem-Difluoromethylene Containing Scaffolds

Selective defluorinative functionalization of trifluoromethyl ketones is a long-standing challenge owing to the exhaustive mode of the process. To meet the demands for the installation of the gem-difluoromethylene unit for the construction of the molecular architectures of well-known pharmaceuticals and agrochemicals, a distinct pathway is thereby highly desirable. Here, a protocol is introduced that allows the divergent synthesis of gem-difluoromethylene group containing tetrahydrofuran derivatives and linear ketones via single C-F bond activation of trifluoromethyl ketones using visible-light photoredox catalysis in the presence of suitable olefins as trapping partner. The choice of appropriate solvent and catalyst plays a significant role in controlling the divergent behavior of this protocol. Highly reducing photo-excited catalysts are found to be responsible for the generation of α,α-difluoromethyl ketone (DFMK) radicals as the key intermediate via a SET process. This protocol also results in a high diastereoselectivity towards the formation of partially fluorinated cyclic ketal derivatives with simultaneous construction of one C-C and two C-O bonds. State-of-the-art DFT calculations are performed to address the origin of diastereoselectivity as well as the divergence of this protocol.

Synthesis of Highly Reactive Sulfone Iminium Fluorides and Their Use in Deoxyfluorination and Sulfur Fluoride Exchange Chemistry

We report the synthesis of sulfone iminium fluorides (SIFs), a reactive class of sulfur(VI) molecules. The synthesis is tolerant of a variety of substituents on the sulfur and nitrogen components. The SIF reagents were applied to the deoxyfluorination of alcohols and carboxylic acids, providing high yields of fluorinated products in 60 s at room temperature. The SIF reagents were then utilized in sulfur fluoride exchange (SuFEx), creating the first ionic SuFEx products to date.

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.

Deoxygenative Functionalizations of Aldehydes, Ketones and Carboxylic Acids

Conversion of carbonyl compounds, including aldehydes, ketones and carboxylic acids, into functionalized alkanes via deoxygenation would be highly desirable from a sustainability perspective and very enabling in chemical synthesis. This review covers the recent methodology development in carbonyl and carboxyl deoxygenative functionalizations, highlighting some typical and significant contributions in this field. These advances will be categorized based on types of bond formation, and in each part, selected examples will be discussed from their generalized mechanistic perspectives. Four summarized reactivity modes of aldehydes and ketones during the deoxygenation, namely, bis-electrophile, carbenoid, bis-nucleophile and alkyl radical, are presented, while the carboxylic acids are deoxygenated mainly via activated carbonyl or acetal intermediates.

Achievements in fluorination using variable reagents through a deoxyfluorination reaction

The application of fluorination reagent for the direct conversion of alcohols and phenols to fluorinated analogues.

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.

Slicing and Splicing of Bromodifluoro-N-arylacetamides: Dearomatization and Difunctionalization of Pyridines

Copper-catalyzed dearomatization and difunctionalization of pyridines have been disclosed, in which bromodifluoro-N-arylacetamide was sliced into five fragments and three or four of them were transferred to pyridine partners. Through this reaction, novel N-difluoromethyl-2-imine dihydropyridine derivatives can be conveniently accessed from commercially available 4-amino substituted pyridines. This strategy demonstrates a novel fluorination method featuring high atom economy, environmental friendliness, an easily available catalyst, and simple operation.

Sulfuryl Fluoride Mediated Synthesis of Amides and Amidines from Ketoximes via Beckmann Rearrangement

A metal-free and redox-neutral method for Beckmann rearrangement employing inexpensive and readily available SO₂ F₂ gas is described. The reported transformation proceeds at ambient temperature and is compatible with a wide range of sterically and electronically diverse aromatic, heteroaromatic, aliphatic and lignin-like oximes providing amides in good to excellent yields. The reaction proceeds through the formation of an imidoyl fluoride intermediate that can also be used for the synthesis of amidines.

Controlled Release of Reactive Gases: A Tale of Taming Carbon Monoxide

This Personal Account describes the development of air-stable and solid precursors for on-demand release of carbon monoxide. In combination with the development of a two-chamber reactor, COware®, CO liberation can be achieved under safe working conditions, as well as allowing transition metal-mediated carbonylations with stoichiometric carbon monoxide. Particularly appealing is the adaptability of this chemical technology for the preparation of carbon isotope labeled bioactive compounds.

Late-Stage Conversion of a Metabolically Labile Aryl Methyl Ether-Containing Natural Product to Fluoroalkyl Analogues

We report the conversion of aryl methyl ethers and phenols into six fluoroalkyl analogues through late-stage functionalization of a natural product-derived FDA-approved therapeutic. This series of short synthetic sequences exploits a combination of both modern and traditional methods and demonstrates that some recently reported methods do not always work as well as desired on a natural product-like scaffold. Nonetheless, reaction optimization can deliver sufficient quantities of each target analogue for medicinal chemistry purposes. In some cases, classical reactions and synthetic sequences still outcompete modern organofluorine transformations, which should encourage the continued search for improved reactions. Overall, the project provides a valuable synthetic roadmap for medicinal chemists to access a range of fluorinated therapeutic candidates with distinct physicochemical properties relative to the original O-based analogue.

δ-C-H Mono- and Dihalogenation of Alcohols

Alkoxy radicals have long been known to enable remote C-H functionalization via 1,5-hydrogen atom abstraction. However, methods for their generation traditionally have relied upon highly oxidizing metals, ultraviolet radiation, or preformed peroxide intermediates, which has prevented the development of many desirable transformations. Herein we report a new bench-stable precursor that decomposes to free alkoxy radicals via a previously unreported single-electron oxidation pathway. This new precursor enables the fluorination and chlorination of remote C-H bonds under exceptionally mild conditions with exceedingly high monoselectivity. Iterative use of this precursor enables the introduction of a second halogen atom, granting access to remote dihalide motifs, including CF₂ and CFCl.

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.

Ex situ gas generation for lab scale organic synthesis

This review discusses recent examples of ex situ generated gaseous reagents, and their use in organic synthesis.

Applications of sulfuryl fluoride (SO2F2) in chemical transformations

A number of novel methodologies concerning the chemical, biological and medicinal applications of sulfuryl fluoride (SO₂F₂) gas have dramatically improved year by year.