Chemoselective Synthesis of Polysubstituted Pyridines from Heteroaryl Fluorosulfates

Rapid Aminations of Functionalized Aryl Fluorosulfates in Water

Aryl fluorosulfates of varying complexities have been used in amination reactions in water using a new Pd oxidative addition complex (OAC-1) developed specifically to match the needs of the fine chemicals industry, not only in terms of functional group tolerance, but also reflecting time considerations associated with these important C-N couplings. Also especially noteworthy is that they replace both PFAS-related triflates and nonaflates, which are today out of favor due to recent government regulations. The new complex based on the BippyPhos ligand is used at low loadings and under aqueous micellar conditions. Moreover, it is easily prepared and stable to long term storage. DFT calculations on the OAC precatalyst compare well with the X-ray structure of the crystals with π-complexation to the aromatic system of the ligand and also confirm the NMR data showing a mixture of conformers in solution that differ from the X-ray structure in rotation of the phenyl and t-butyl ligand substituents. An extensive variety of coupling partners, including pharmaceutically relevant APIs, readily participate under mild and environmentally responsible reaction conditions.

Pd-, Cu-, and Ni-Catalyzed Reactions: A Comprehensive Review of the Efficient Approaches towards the Synthesis of Antibacterial Molecules

A strong synthetic tool for many naturally occurring chemicals, polymers, and pharmaceutical substances is transition metal-catalyzed synthesis. A serious concern to human health is the emergence of bacterial resistance to a broad spectrum of antibacterial medications. The synthesis of chemical molecules that are potential antibacterial candidates is underway. The main contributions to medicine are found to be effective in transition metal catalysis and heterocyclic chemistry. This review underlines the use of heterocycles and certain effective transition metals (Pd, Cu, and Ni) as catalysts in chemical methods for the synthesis of antibacterial compounds. Pharmaceutical chemists might opt for clinical exploration of these techniques due to their potential.

New Chemical Transformations Involving SO2 F2 -Mediated Alcohol Activation

Sulfuryl fluoride is a gas produced on a multi-ton scale for its use as a fumigant. In the last decades, it has gained interest in organic synthesis as a reagent with unique properties in terms of stability and reactivity when compared to other sulfur-based reagents. Sulfuryl fluoride has not only been used for sulfur-fluoride exchange (SuFEx) chemistry but also encountered applications in classic organic synthesis as an efficient activator of both alcohols and phenols, forming a triflate surrogate, namely a fluorosulfonate. A long-standing industrial collaboration in our research group drove our work on the sulfuryl fluoride-mediated transformations that will be highlighted below. We will first describe recent works on metal-catalyzed transformations from aryl fluorosulfonates while emphasizing the one-pot processes from phenol derivatives. In a second section, nucleophilic substitution reactions on polyfluoroalkyl alcohols will be discussed and the value of polyfluoroalkyl fluorosulfonates in comparison to alternative triflate and halide reagents will be brought to light.

Synthesis of aliphatic nitriles from cyclobutanone oxime mediated by sulfuryl fluoride (SO2F2)

A SO₂F₂-mediated ring-opening cross-coupling of cyclobutanone oxime derivatives with alkenes was developed for the construction of a range of δ-olefin-containing aliphatic nitriles with (E)-configuration selectivity. This new method features wide substrate scope, mild conditions, and direct N-O activation.

Aryl fluorosulfates: powerful and versatile partners in cross-coupling reactions

Aryl fluorosulfates are versatile building blocks in organic synthesis and have gained increasing attention in SuFEx (Sulfur Fluoride Exchange) click chemistry. They are easily and conveniently prepared from phenols using sulfuryl fluoride SO₂F₂ as a low-cost sulfonyl fluoride provider. Recently, they served as less toxic and more atom economical alternatives to triflates in an impressive number of carbon-carbon and carbon-heteroatom cross-coupling reactions. In this review, we summarize the current advances and developments in applying aryl fluorosulfates as electrophilic partners in cross-coupling reactions.

De Novo Synthesis of Polysubstituted 3-Hydroxypyridines Via “Anti-Wacker”-Type Cyclization

We report an efficient method to prepare polysubstituted 3-hydroxypyridines from amino acids, propargyl alcohols, and arylboronic acids. The process involves Pd(0)-catalyzed anti-selective arylative cyclizations of N-propargyl-N-tosyl-aminoaldehydes with arylboronic acids (“anti-Wacker”-type cyclization), oxidation of the resulting 5-substituted-3-hydroxy-1,2,3,6-tetrahydropyridines to 3-oxo derivatives, and elimination of p-toluenesulfinic acid. This method provides diverse polysubstituted 3-hydroxypyridines, whose hydroxy group can be further substituted by a cross-coupling reaction via a triflate.

Thionyl Fluoride-Mediated One-Pot Substitutions and Reductions of Carboxylic Acids

Thionyl fluoride (SOF₂) is an underutilized reagent that is yet to be extensively studied for its synthetic applications. We previously reported that it is a powerful reagent for both the rapid syntheses of acyl fluorides and for one-pot peptide couplings, but the full scope of these nucleophilic acyl substitutions had not been explored. Herein, we report one-pot thionyl fluoride-mediated syntheses of peptides and amides (35 examples, 45-99% yields) that were not explored in our previous study. The scope of thionyl fluoride-mediated nucleophilic acyl substitutions was also expanded to encompass esters (24 examples, 64-99% yields) and thioesters (11 examples, 24-96% yields). In addition, we demonstrate that the scope of thionyl fluoride-mediated one-pot reactions can be extended beyond nucleophilic acyl substitutions to mild reductions of carboxylic acids using NaBH₄ (13 examples, 33-80% yields).

Nucleophilic Aromatic Substitution of 5-Bromo-1,2,3-triazines with Phenols

Nucleophilic aromatic substitution (S_NAr) reaction in classic textbook is a stepwise mechanism, and few examples of concerted reactions have been reported. Herein, we developed a concerted S_NAr reaction of 5-bromo-1,2,3-triazines with phenols in which the nonclassic mechanism of this reaction could be revealed by calculation. Furthermore, the resulting 5-aryloxy-1,2,3-triazines could be used as convenient precursors to access biologically important 3-aryloxy-pyridines in one-pot manner.

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.

SuFExable Isocyanides for Ugi Reaction: Synthesis of Sulfonyl Fluoro Peptides

Herein, the sulfonyl fluoro isocyanides were first developed as a new type of SuFExable synthon, and they are used as building blocks in the Ugi reaction (U-4CR). The Ugi reaction was established and the substrate scope was investigated, and various sulfonyl fluoro α-amino amides and peptides could be reached in a one-step synthesis. Therefore, this protocol opens a new vision for SuFExable building blocks and click chemistry, and it also provides a distinct approach to sulfonyl fluoro peptides.

Late-Stage 18F/19F Isotopic Exchange for the Synthesis of 18F-Labeled Sulfamoyl Fluorides

Synthesis of sulfamoyl [¹⁸F]fluorides has been a challenging topic owing to the inefficient nucleophilic radiofluorination of sulfamoyl derivatives. Herein, we report an ¹⁸F/¹⁹F isotopic exchange approach to synthesize various sulfamoyl [¹⁸F]fluorides, otherwise inaccessible via direct synthesis from amines, with high radiochemical yields up to 97% (30 examples). This late-stage labeling protocol offers an efficient route to yield functionalized molecules by diversifying the chemical library possessing sulfamoyl functionalities through nucleophilic ¹⁸F incorporation within nitrogen-containing sulfur(VI) frameworks.

Palladium-Catalyzed Cross-Couplings by C-O Bond Activation

Although palladium-catalyzed cross-coupling of aryl halides and reactive pseudohalides has revolutionized the way organic molecules are constructed today across various fields of chemistry, comparatively less progress has been made in the palladium-catalyzed cross-coupling of less reactive C-O electrophiles. This is despite the fact that the use of phenols and phenol derivatives as bench-stable cross-coupling partners has been well-recognized to bring about major advantages over aryl halides, such as (1) natural abundance of phenols, (2) avoidance of toxic halides, (3) orthogonal cross-coupling conditions, (4) prefunctionalization of phenolic substrates by electrophilic substitution or C-H functionalization, (5) ready availability of phenols from a different pool of precursors than aryl halides. In this review, we present an overview of recent advances made in the field of palladium-catalyzed cross-coupling of C-O electrophiles with a focus on (1) catalytic systems, (2) reaction type, and (3) class of C-O coupling partners. Although the field has been historically dominated by nickel catalysis, it is now evident that the use of more versatile, more functional group tolerant and highly active palladium catalysts supported by appropriately designed ancillary ligands enables the cross-coupling with improved substrate scope and generality, and likely represents a practical solution to the broadly applicable cross-coupling of various C-O bonds across diverse chemical disciplines. The review covers the period through June 2020.

Continuous flow synthesis of aryl aldehydes by Pd-catalyzed formylation of phenol-derived aryl fluorosulfonates using syngas

This communication describes the palladium-catalyzed reductive carbonylation of aryl fluorosulfonates (ArOSO2F) using syngas as an inexpensive and sustainable source of carbon monoxide and hydrogen. The conversion of phenols to aryl fluorosulfonates can be conveniently achieved by employing the inexpensive commodity chemical sulfuryl fluoride (SO2F2) and base. The developed continuous flow formylation protocol requires relatively low loadings for palladium acetate (1.25 mol%) and ligand (2.5 mol%). Good to excellent yields of aryl aldehydes were obtained within 45 min for substrates containing electron withdrawing substituents, and 2 h for substrates containing electron donating substituents. The optimal reaction conditions were identified as 120 °C temperature and 20 bar pressure in dimethyl sulfoxide (DMSO) as solvent. DMSO was crucial in suppressing Pd black formation and enhancing reaction rate and selectivity.

Highly Selective Room-Temperature Suzuki-Miyaura Coupling of Bromo-2-sulfonyloxypyridines for Unsymmetrical Diarylpyridines

A new and mild synthetic approach has been developed for the synthesis of pharmaceutically important unsymmetrical diarylpyridines via chemoselective Suzuki-Miyaura coupling reactions of bromo-2-sulfonyloxypyridines. Most reactions allow for facile access to aryl-2-sulfonyloxypyridines at room temperature in yields of 5-99% with excellent chemoselectivity in the presence of Pd(OAc)₂ (2.0 mol %) and Ad₂BnP (2.4 mol %). The second arylation of the remaining tosyl or triflyl group in the monoarylpyridine derivatives obtained was successfully accomplished for the synthesis of unsymmetrical 2,3-, 2,4-, 2,5-, and 2,6-diarylpyridine derivatives. Furthermore, a one-pot synthesis of unsymmetrical diarylpyridines starting from bromo-2-sulfonyloxypyridine was accomplished to demonstrate the practical convenience. Finally, with this method, an antibacterial agent, a topoisomerase inhibitor, and etoricoxib, a nonsteroidal anti-inflammatory drug, were successfully synthesized from the corresponding bromo-2-hydroxypyridines in overall yields of 80, 86, and 49%, respectively.

Site-Selective, Modular Diversification of Polyhalogenated Aryl Fluorosulfates (ArOSO2 F) Enabled by an Air-Stable PdI Dimer

Since 2014, the interest in aryl fluorosulfates (ArOSO2 F) as well as their implementation in powerful applications has continuously grown. In this context, the enabling capability of ArOSO2 F will strongly depend on the substitution pattern of the arene, which ultimately dictates its overall function as drug candidate, material, or bio-linker. This report showcases the modular, substrate-independent, and fully predictable, selective functionalization of polysubstituted arenes bearing C-OSO2 F, C-Br, and C-Cl sites, which makes it possible to diversify the arene in the presence of OSO2 F or utilize OSO2 F as a triflate surrogate. Sequential and triply selective arylations and alkylations were realized within minutes at room temperature, using a single and air-stable PdI dimer.

DNA-Encoded Libraries: Aryl Fluorosulfonates as Versatile Electrophiles Enabling Facile On-DNA Suzuki, Sonogashira, and Buchwald Reactions

Using (hetero)aryl fluorosulfonates as versatile electrophiles, facile on-DNA cross-coupling reactions of Suzuki, Sonogashira, and Buchwald are reported here. Notably, all of these reactions show excellent functional group tolerance, mild reaction conditions (relative low temperature and open to air), rich heterocyclic coupling partners, and more importantly, DNA-compatibility. Thus, these new reactions based on efficient formation of C(sp2)-C(sp2), C(sp2)-C(sp), and C(sp2)-N bonds are highly amenable to synthesis of DNA-encoded libraries with great molecular diversity.

Installation of -SO2F groups onto primary amides

A protocol of SO₂F₂-mediated installation of sulfonyl fluoride onto primary amides has been developed providing a new portal to sulfur(VI) fluoride exchange (SuFEx) click chemistry. The generated molecules contain pharmaceutically important amide and -SO₂F moieties for application in the discovery of new therapeutics.

Biocompatible SuFEx Click Chemistry: Thionyl Tetrafluoride (SOF4 )-Derived Connective Hubs for Bioconjugation to DNA and Proteins

We report here the development of a suite of biocompatible SuFEx transformations from the SOF4 -derived iminosulfur oxydifluoride hub in aqueous buffer conditions. These biocompatible SuFEx reactions of iminosulfur oxydifluorides (R-N=SOF2 ) with primary amines give sulfamides (8 examples, up to 98 %), while the reaction with secondary amines furnish sulfuramidimidoyl fluoride products (8 examples, up to 97 %). Likewise, under mild buffered conditions, phenols react with the iminosulfur oxydifluorides (Ar-N=SOF2 ) to produce sulfurofluoridoimidates (13 examples, up to 99 %), which can themselves be further modified by nucleophiles. These transformations open the potential for asymmetric and trisubstituted linkages projecting from the sulfur(VI) center, including versatile S-N and S-O connectivity (9 examples, up to 94 %). Finally, the SuFEx bioconjugation of iminosulfur oxydifluorides to amine-tagged single-stranded DNA and to BSA protein demonstrate the potential of SOF4 -derived SuFEx click chemistry in biological applications.

Cascade Process for Direct Transformation of Aldehydes (RCHO) to Nitriles (RCN) Using Inorganic Reagents NH2OH/Na2CO3/SO2F2 in DMSO

A simple, mild, and practical process for direct conversion of aldehydes to nitriles was developed feathering a wide substrate scope and great functional group tolerability (52 examples, over 90% yield in most cases) using inorganic reagents (NH₂OH/Na₂CO₃/SO₂F₂) in DMSO. This method allows for transformations of readily available, inexpensive, and abundant aldehydes to highly valuable nitriles in a pot, atom, and step-economical manner without transition metals. This protocol will serve as a robust tool for the installation of cyano-moieties to complicated molecules.

Nickel-Catalyzed Carboxylation of Aryl and Heteroaryl Fluorosulfates Using Carbon Dioxide

The development of efficient and practical methods to construct carboxylic acids using CO₂ as a C1 synthon is of great importance. Nickel-catalyzed carboxylation of aryl fluorosulfates and heteroaryl fluorosulfates with CO₂ is described, affording arene carboxylic acids with good to excellent yields under mild conditions. In addition, a one-pot phenol fluorosulfation/carboxylation is developed.

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.

The growing applications of SuFEx click chemistry

SuFEx (Sulfur Fluoride Exchange) is a modular, next generation family of click reactions, geared towards the rapid and reliable assembly of functional molecules.

Regioselective installation of fluorosulfate (–OSO2F) functionality into aromatic C(sp2)–H bonds for the construction of para-amino-arylfluorosulfates

The synthesis of novel para-amino-arylfluorosulfates was achieved through installing fluorosulfate functionality into aromatic C(sp²)–H bonds mediated by sulfuryl fluoride.

SO2F2 mediated transformation of pyrazolones into pyrazolyl fluorosulfates

The construction of a class of novel N-heterocyclic molecules containing both pyrazole and fluorosulfate functionalities was achieved through the reactions of pyrazolones with SO₂F₂ in good to excellent yields.

Protein Glycosylation through Sulfur Fluoride Exchange (SuFEx) Chemistry: The Key Role of a Fluorosulfate Thiolactoside

Protein glycosylation is the most complex post-translational modification process. More than 50 % of human cells proteins are glycosylated, whereas bacteria such as E. coli do not have this modification machinery. Indeed, the carbohydrate residues in natural proteins affect their folding, immunogenicity, and stability toward proteases, besides controlling biological properties and activities. It is therefore important to introduce such structural modification in bioengineered proteins lacking the presence of carbohydrate residues. This is not trivial as it requires reagents and conditions compatible with the protein's stability and reactivity. This work reports on the introduction of lactose moieties in two natural proteins, namely ubiquitin (Ub) and l-asparaginase II (ANSII). The synthetic route employed is based on the sulfur(VI) fluoride exchange (SuFEx) coupling of a lactose tethered arylfluorosulfate (Lact-Ar-OSO₂ F) with the ϵ-NH₂ group of lysine residues of the proteins. This metal-free click SuFEx reaction relies on the properties of the fluorosulfate employed, which is easily prepared in multigram scale from available precursors and reacts chemoselectively with the ϵ-NH₂ group of lysine residues under mild conditions. Thus, iterative couplings of Lact-Ar-OSO₂ F to Ub and ANSII, afforded multiple glycosylations of these proteins so that up to three and four Lact-Ar-OSO₂ groups were introduced in Ub and ANSII, respectively, via the formation of a sulfamoyl (OSO₂ -NH) linkage.

N-Silylenamines as Reactive Intermediates: Hydroamination for the Modular Synthesis of Selectively Substituted Pyridines

A modular and selective synthesis of mono-, di-, tri-, tetra-, and pentasubstituted pyridines is reported. Hydroamination of alkynes with N-silylamine using a bis(amidate)bis(amido)titanium(IV) precatalyst furnishes the regioselective formation of N-silylenamines. Addition of α,β-unsaturated carbonyls to the crude mixtures followed by oxidation affords 47 examples of pyridines in yields of up to 96%. This synthetic route allows for the synthesis of diverse pyridines containing variable substitution patterns, including pharmaceutically relevant 2,4,5-trisubstituted pyridines, using this one-pot protocol.

SuFEx Chemistry of Thionyl Tetrafluoride (SOF4 ) with Organolithium Nucleophiles: Synthesis of Sulfonimidoyl Fluorides, Sulfoximines, Sulfonimidamides, and Sulfonimidates

Thionyl tetrafluoride (SOF4 ) is a valuable connective gas for sulfur fluoride exchange (SuFEx) click chemistry that enables multidimensional linkages to be created via sulfur-oxygen and sulfur-nitrogen bonds. Herein, we expand the available SuFEx chemistry of SOF4 to include organolithium nucleophiles, and demonstrate, for the first time, the controlled projection of sulfur-carbon links at the sulfur center of SOF4 -derived iminosulfur oxydifluorides (R1 -N=SOF2 ). This method provides rapid and modular access to sulfonimidoyl fluorides (R1 -N=SOFR2 ), another array of versatile SuFEx connectors with readily tunable reactivity of the S-F handle. Divergent connections derived from these valuable sulfonimidoyl fluoride units are also demonstrated, including the synthesis of sulfoximines, sulfonimidamides, and sulfonimidates.

Pd-Catalyzed one-pot dehydroxylative coupling of phenols with K4[Fe(CN)6] mediated by SO2F2: a practical method for the direct conversion of phenols to aryl nitriles

A SO₂F₂ mediated Pd-catalyzed one-pot dehydroxylative cyanation of phenols to aryl nitriles was accomplished by employing (K₄[Fe(CN)₆]) as the cyanating reagent.