Acyl Fluorides from Carboxylic Acids, Aldehydes, or Alcohols under Oxidative Fluorination

In-Flow Generation of Thionyl Fluoride (SOF2) Enables the Rapid and Efficient Synthesis of Acyl Fluorides from Carboxylic Acids

Herein, we report an approach for generating thionyl fluoride (SOF2) from the commodity chemicals thionyl chloride (SOCl2) and potassium fluoride (KF). The methodology relies on a microfluidic device that can efficiently produce and dose this toxic gaseous reagent under extremely mild and safe conditions. Subsequently, the in situ-generated thionyl fluoride is reacted with an array of structurally and electronically differing carboxylic acids, leading to the direct and efficient synthesis of highly sought-after acyl fluorides. Importantly, our investigation also highlights the inherent modularity of this flow-based platform. We demonstrate the adaptability of this approach by not only synthesizing acyl fluorides but also directly converting carboxylic acids into a diverse array of valuable compounds such as esters, thioesters, amides, and ketones. This versatility showcases the potential of this approach for a wide range of synthetic applications, underscoring its significance in the realm of chemical synthesis.

Vibration-mediated long-wavelength photolysis of electronegative bonds beyond S0-S1 and S0-T1 transitions

Photolysis is an attractive method in organic synthesis to produce free radicals through direct bond cleavage. However, in this method, specific irradiation wavelengths of light have been considered indispensable for excitation through S0-Sn or S0-Tn transitions. Here we report the photoinduced homolysis of electronegative interelement bonds using light at wavelengths much longer than theoretically and spectroscopically predicted for the S0-Sn or S0-Tn transitions. This long-wavelength photolysis proceeds in N-Cl, N-F, and O-Cl bonds at room temperature under blue, green, and red LED irradiation, initiating diverse radical reactions. Through experimental, spectroscopic, and computational studies, we propose that this "hidden" absorption is accessible via electronic excitations from naturally occurring vibrationally excited ground states to unbonded excited states and is due to the electron-pair repulsion between electronegative atoms.

Deoxyfluorinated amidation and esterification of carboxylic acid by pyridinesulfonyl fluoride

Amide bond synthesis is one of the most used reactions in medicinal chemistry. We report an amide bond formation reaction through deoxyfluorinated carboxylic acids under mild conditions using 2-pyridinesulfonyl fluoride. The reaction procedure has been used in a one-pot synthesis of amides and esters via in situ generation of acyl fluoride. This one-pot synthetic method provides easy access to amides and esters. Using this method, we have sequentially synthesized a tetrapeptide and calceolarioside-B glycoside derivative with good yields.

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.

Synthesis of acyl fluorides through deoxyfluorination of carboxylic acids

A direct deoxyfluorination of carboxylic acids by utilizing inorganic potassium fluoride (KF) as a safe and inexpensive fluoride source has been developed. Both aryl carboxylic acids and cinnamyl carboxylic acids could be efficiently transformed into valuable acyl fluorides in moderate to high yields with good functional group tolerance. A scale-up reaction could be carried out smoothly under solvent-free conditions, which further demonstrated the practicality of this reaction in organic synthesis.

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.

Fluorocarbonylation via palladium/phosphine synergistic catalysis

Despite the growing importance of fluorinated organic compounds in pharmaceuticals, agrochemicals, and materials science, the introduction of fluorine into organic molecules is still a challenge, and no catalytic fluorocarbonylation of aryl/alkyl boron compounds has been reported to date. Herein, we present the development of palladium and phosphine synergistic redox catalysis of fluorocarbonylation of potassium aryl/alkyl trifluoroborate. Trifluoromethyl arylsulfonate (TFMS), which was used as a trifluoromethoxylation reagent, an easily handled and bench-scale reagent, has been employed as an efficient source of COF₂. The reaction operates under mild conditions with good to excellent yields and tolerates diverse complex scaffolds, which allows efficient late-stage fluorocarbonylation of marked small-molecule drugs. Mechanistically, the key intermediates of labile Brettphos-Pd(II)-OCF₃ complex and difluoro-Brettphos were synthesized and spectroscopically characterized, including X-ray crystallography. A detailed reaction mechanism involving the synergistic redox catalytic cycles Pd(II)/(0) and P(III)/(V) was proposed, and multifunction of phosphine ligand was identified based on ¹⁹F NMR, isotope tracing, synthetic, and computational studies.

Room Temperature Defluorination of Poly(tetrafluoroethylene) by a Magnesium Reagent

Perfluoroalkyl substances (PFAS) are pervasive in the environment. The largest single use material within the PFAS compound class is poly(tetrafluoroethylene) (PTFE), a robust and chemically resistant polymer. Despite their widespread use and serious concerns about their role as pollutants, methods for repurposing PFAS are rare. Here we show that a nucleophilic magnesium reagent reacts with PTFE at room temperature, generating a molecular magnesium fluoride which is easily separated from the surface-modified polymer. The fluoride in turn can be used to transfer the fluorine atoms to a small array of compounds. This proof-of-concept study demonstrates that the atomic fluorine content of PTFE can be harvested and reused in chemical synthesis.

One-pot ester and thioester formation mediated by pentafluoropyridine (PFP)

Acyl fluorides are valuable synthetic intermediates, but in some cases they can be challenging to handle and difficult to isolate given their susceptibility to degradation. In addition, many reagents utilised to prepare acyl fluorides are incompatible with in situ generation strategies and require the acyl fluoride to be isolated before any further reaction can take place. The combination of these factors has meant that acyl fluorides are currently under investigated in nucleophilic substitution processes, and often only a limited substrate scope is tolerated where they have been used. Herein, we report that pentafluoropyridine can be utilised to generate acyl fluorides in situ under mild conditions, and that they can subsequently be used to generate a range of esters and thioesters. This methodology offers a simple one-pot synthesis of esters and thioesters directly from parent carboxylic acids.

Exogenous Ligand-Free NiH-Catalyzed Hydroacylation of Aryl Alkenes with Aroyl Fluorides

Acyl fluorides have emerged as efficient acyl group donors, but these attractive reagents have rarely been utilized in transition-metal-catalyzed hydroacylation. Herein we report a nickel hydride-catalyzed hydroacylation of aryl alkenes using aroyl fluorides. The reaction proceeds without recourse to an exogenous ligand under mild conditions. The synthetic utility of the present method is demonstrated by the glovebox-free, gram-scale reaction and the late-stage derivatization of complex molecules containing pharmaceutical frameworks.

Ni-catalysed deamidative fluorination of amides with electrophilic fluorinating reagents

We describe here a Ni-catalysed deamidative fluorination of diverse amides with electrophilic fluorinating reagents. Different types of amides including aromatic amides and olefinic amides were well compatible, affording the corresponding acyl fluorides in good to excellent yields.

Versatile Palladium-Catalyzed Approach to Acyl Fluorides and Carbonylations by Combining Visible Light- and Ligand-Driven Operations

We describe the development of a general palladium-catalyzed carbonylative method to synthesize acyl fluorides from aryl, heteroaryl, alkyl, and functionalized organic halides. Mechanistic analysis suggests that the reaction proceeds via the synergistic combination of visible light photoexcitation of Pd(0) to induce oxidative addition with a ligand-favored reductive elimination. These together create a unidirectional catalytic cycle that is uninhibited by the classical effect of carbon monoxide coordination. Coupling the catalytic formation of acyl fluorides with their subsequent nucleophilic reactions has opened a method to perform carbonylation reactions with unprecedented breadth, including the assembly of highly functionalized carbonyl-containing products.

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.

Gram-Scale Preparation of Acyl Fluorides and Their Reactions with Hindered Nucleophiles

A series of acyl fluorides was synthesized at 100 mmol scale using phase-transfer-catalyzed halogen exchange between acyl chlorides and aqueous bifluoride solution. The convenient procedure consists of vigorous stirring of the biphasic mixture at room temperature, followed by extraction and distillation. Isolated acyl fluorides (usually 7–20 g) display excellent purity and can be transformed into sterically hindered amides and esters when treated with lithium amide bases and alkoxides under mild conditions.

Electrophilic N-trifluoromethylthiophthalimide as a fluorinated reagent in the synthesis of acyl fluorides

Herein we report the deoxygenated fluorination of readily available carboxylic acids. A series of acyl fluorides have been synthesized using shelf-stable N-trifluoromethylthiophthalimide as a fluorinated reagent for the first time.

Rapid and column-free syntheses of acyl fluorides and peptides using ex situ generated thionyl fluoride

Thionyl fluoride (SOF₂) was first isolated in 1896, but there have been less than 10 subsequent reports of its use as a reagent for organic synthesis. This is partly due to a lack of facile, lab-scale methods for its generation. Herein we report a novel protocol for the ex situ generation of SOF₂ and subsequent demonstration of its ability to access both aliphatic and aromatic acyl fluorides in 55-98% isolated yields under mild conditions and short reaction times. We further demonstrate its aptitude in amino acid couplings, with a one-pot, column-free strategy that affords the corresponding dipeptides in 65-97% isolated yields with minimal to no epimerization. The broad scope allows for a wide range of protecting groups and both natural and unnatural amino acids. Finally, we demonstrated that this new method can be used in sequential liquid phase peptide synthesis (LPPS) to afford tri-, tetra-, penta-, and decapeptides in 14-88% yields without the need for column chromatography. We also demonstrated that this new method is amenable to solid phase peptide synthesis (SPPS), affording di- and pentapeptides in 80-98% yields.

Direct Synthesis of Tri-/Difluoromethyl Ketones from Carboxylic Acids by Cross-Coupling with Acyloxyphosphonium Ions

A simple and straightforward approach to the synthesis of trifluoromethyl and difluoromethyl ketones from widely available carboxylic acids is disclosed. The transformation utilizes an acyloxyphosphonium ion as the active electrophile, conveniently generated in situ from the carboxylic acid substrate by using commodity chemicals. The utility of the reaction system is exemplified by its chemoselectivity, with tolerance to a variety of important functional groups. The late-stage functionalization of carboxylic acid active pharmaceutical ingredients and pharmaceutically relevant compounds is also discussed.

Green chemistry meets medicinal chemistry: a perspective on modern metal-free late-stage functionalization reactions

The progress of drug discovery and development is paced by milestones reached in organic synthesis. In the last decade, the advent of late-stage functionalization (LSF) reactions has represented a valuable breakthrough. Recent literature has defined these reactions as the chemoselective modification of complex molecules by means of C-H functionalization or the manipulation of endogenous functional groups. Traditionally, these diversifications have been accomplished by organometallic means. However, the presence of metals carries disadvantages related to their cost, environmental hazard and health risks. Fundamentally, green chemistry directives can help minimize such hazards through the development of metal-free LSF methodologies. In this review, we expand the current discussion on metal-free LSF reactions by providing an overview of C(sp2)-H, and C(sp3)-H functionalizations, as well as the utilization of heteroatom-containing functional groups as chemical handles. Selected topics such as metal-free cross-dehydrogenative coupling (CDC) reactions, organocatalysis, electrochemistry and photochemistry are also discussed. By writing the first review on metal-free LSF methodologies, we aim to highlight current advances in the field with examples that reveal specific challenges and solutions, as well as future research opportunities.

Complete deconstruction of SF6 by an aluminium(I) compound

The room-temperature activation of SF₆, a potent greenhouse gas, is reported using a monovalent aluminium(i) reagent to form well-defined aluminium(iii) fluoride and aluminium(iii) sulfide products. New reactions have been developed to utilise the aluminium(iii) fluoride and aluminium(iii) sulfide as a nucleophilic source of F⁻ and S²⁻ for a range of electrophiles. The overall reaction sequence results in the net transfer of fluorine or sulfur atoms from an environmentally detrimental gas to useful organic products.

The room-temperature activation of SF₆, a potent greenhouse gas, is reported using a monovalent aluminium(i) reagent to form well-defined aluminium(iii) fluoride and aluminium(iii) sulfide products.

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%.

Deoxyfluorination of Carboxylic Acids with CpFluor: Access to Acyl Fluorides and Amides

3,3-Difluoro-1,2-diphenylcyclopropene (CpFluor), a bench-stable fluorination reagent, has been developed in the deoxyfluorination of carboxylic acids to afford various acyl fluorides. This all-carbon-based fluorination reagent enabled the efficient transformation of (hetero)aryl, alkyl, alkenyl, and alkynyl carboxylic acids to the corresponding acyl fluorides under the neutral conditions. This deoxyfluorination method was featured by the synthesis of acyl fluorides with in-situ formed CpFluor, as well as the one-pot amidation reaction of carboxylic acids via in-situ formed acyl fluorides.

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.