Overcoming a Radical Polarity Mismatch in Strain-Release Pentafluorosulfanylation of [1.1.0]Bicyclobutanes: An Entryway to Sulfone- and Carbonyl-Containing SF5-Cyclobutanes

The first assortment of achiral pentafluorosulfanylated cyclobutanes (SF5-CBs) are now synthetically accessible through strain-release functionalization of [1.1.0]bicyclobutanes (BCBs) using SF5Cl. Methods for both chloropentafluorosulfanylation and hydropentafluorosulfanylation of sulfone-based BCBs are detailed herein, as well as proof-of-concept that the logic extends to tetrafluoro(aryl)sulfanylation, tetrafluoro(trifluoromethyl)sulfanylation, and three-component pentafluorosulfanylation reactions. The methods presented enable isolation of both syn and anti isomers of SF5-CBs, but we also demonstrate that this innate selectivity can be overridden in chloropentafluorosulfanylation; that is, an anti-stereoselective variant of SF5Cl addition across sulfone-based BCBs can be achieved by using inexpensive copper salt additives. Considering the SF5 group and CBs have been employed individually as nonclassical bioisosteres, structural aspects of these unique SF5-CB "hybrid isosteres" were then contextualized using SC-XRD. From a mechanistic standpoint, chloropentafluorosulfanylation ostensibly proceeds through a curious polarity mismatch addition of electrophilic SF5 radicals to the electrophilic sites of the BCBs. Upon examining carbonyl-containing BCBs, we also observed rare instances whereby radical addition to the 1-position of a BCB occurs. The nature of the key C(sp3)-SF5 bond formation step - among other mechanistic features of the methods we disclose - was investigated experimentally and with DFT calculations. Lastly, we demonstrate compatibility of SF5-CBs with various downstream functionalizations.

Hydrothiolation of Triisopropylsilyl Acetylene Sulfur Pentafluoride - Charting the Chemical Space of β-SF5 Vinyl Sulfides

Recently, we suggested liquid and high-boiling TIPS-CC-SF5 (TASP) as a versatile reagent to access so far elusive SF5-containing building blocks by less specialized laboratories under bench-top conditions. The synthesis of non-aromatic SF5 building blocks generally requires on-site fluorination or pentafluorosulfanylation steps employing toxic and/or gaseous reagents. Herein, we underline the versatility of this reagent by reporting a benign bench-top protocol for the synthesis of Z-configured β-pentafluorosulfanylated vinyl sulfides in good to excellent yields (up to 99 %) with exclusive (Z)-diasteroselectivity and broad functional group tolerance. This method exploits an in-situ protodesilylation-hydrothiolation sequence. This so far uncharted class of compounds was characterized by means of NMR-spectroscopy as well as SC-XRD. Furthermore, we suggest the reaction to proceed via a kinetically controlled closed-shell reaction pathway, corroborated by in-silico experiments.

Strain-Release Pentafluorosulfanylation and Tetrafluoro(aryl)sulfanylation of [1.1.1]Propellane: Reactivity and Structural Insight

We leveraged the recent increase in synthetic accessibility of SF5 Cl and Ar-SF4 Cl compounds to combine chemistry of the SF5 and SF4 Ar groups with strain-release functionalization. By effectively adding SF5 and SF4 Ar radicals across [1.1.1]propellane, we accessed structurally unique bicyclopentanes, bearing two distinct elements of bioisosterism. Upon evaluating these "hybrid isostere" motifs in the solid state, we measured exceptionally short transannular distances; in one case, the distance rivals the shortest nonbonding C⋅⋅⋅C contact reported to date. This prompted SC-XRD and DFT analyses that support the notion that a donor-acceptor interaction involving the "wing" C-C bonds is playing an important role in stabilization. Thus, these heretofore unknown structures expand the palette for highly coveted three-dimensional fluorinated building blocks and provide insight to a more general effect observed in bicyclopentanes.

Oxidative Fluorination of Heteroatoms Enabled by Trichloroisocyanuric Acid and Potassium Fluoride

In synthetic method development, the most rewarding path is seldom a straight line. While our initial entry into pentafluorosulfanyl (SF₅) chemistry did not go according to plan (due to inaccessibility of reagents such as SF₅Cl at the time), a “detour” led us to establish mild and inexpensive oxidative fluorination conditions that made aryl‐SF₅ compound synthesis more accessible. The method involved the use of potassium fluoride and trichloroisocyanuric acid (TCICA)—a common swimming pool disinfectant—as opposed to previously employed reagents such as F₂, XeF₂, HF, and Cl₂. Thereafter, curiosity led us to explore applications of TCICA/KF as a more general approach to the synthesis of fluorinated Group 15, 16, and 17 heteroatoms in organic scaffolds; this, in turn, prompted SC‐XRD, VT‐NMR, computational, and physical organic studies. Ultimately, it was discovered that TCICA/KF can be used to synthesize SF₅Cl, enabling SF₅ chemistry in an unexpected way.

(Hetero)aryl-SVI Fluorides: Synthetic Development and Opportunities

(Hetero)arylsulfur compounds where the S atom is in the oxidation state VI represent a large percentage of the molecular functionalities present in organic chemistry. More specifically, (hetero)aryl‐S^VI fluorides have recently received enormous attention because of their potential as chemical biology probes, as a result of their reactivity in a simple, modular, and efficient manner. Whereas the synthesis and application of the level 1 fluorination at S^VI atoms (sulfonyl and sulfonimidoyl fluorides) have been widely studied and reviewed, the synthetic strategies towards higher levels of fluorination (levels 2 to 5) are somewhat more limited. This Minireview evaluates and summarizes the progress in the synthesis of highly fluorinated aryl‐S^VI compounds at all levels, discussing synthetic strategies, reactivity, the advantages and disadvantages of the synthetic procedures, the proposed mechanisms, and the potential upcoming opportunities.

In situ ortho-lithiation/functionalization of pentafluorosulfanyl arenes

A general method for ortho-functionalization of pentafluorosulfanyl arenes has been developed. ortho-Lithiation with lithium tetramethylpiperidide at -60 °C in the presence of silicon, germanium, and tin electrophiles affords trapped products in moderate to high yields. Precise temperature regimes and the presence of electrophiles during lithiation are important for successful reactions, since the pentafluorosulfanyl group acts as a competent leaving group at temperatures above -40 °C. Fluoro, bromo, iodo, enolizable keto, cyano, ester, amide, and unsubstituted amino functionalities are compatible with the reaction conditions. Conversion of 2-dimethylsilylpentafluorosulfanyl benzene to 2-halosubstituted derivatives, useful as starting materials in cross-coupling chemistry, was also demonstrated.

Photoredox Catalytic Pentafluorosulfanylative Domino Cyclization of α-Substituted Alkenes to Oxaheterocycles by Using SF6

Virtually inert sulfur hexafluoride becomes a precious pentafluorosulfanylation agent, if properly activated by photoredox catalysis, to access α-fluoro and α-alkoxy SF5 -compounds. This advanced protocol converts SF6 in the presence of alkynols as bifunctional C-C- and C-O-bond forming reagents directly into pentafluorosulfanylated oxygen-containing heterocycles in a single step from α-substituted alkenes. The proposed mechanism is supported by theoretical calculations and gives insights not only in the pentafluorosulfanylation step but also into formation of the carbon-carbon bond and is in full agreement with Baldwin's cyclization rules. The key step is a radical type 5-, 6- respectively 7-exo-dig-cyclization. The synthesized oxaheterocycles cannot be simply prepared by other synthetic methods, show a high level of structural complexity and significantly expand the scope of pentafluorosulfanylated building blocks valuable for medicinal and material chemistry.

Structural characterization and Hirshfeld surface analysis of 2-iodo-4-(penta-fluoro-λ6-sulfan-yl)benzo-nitrile

The title compound, C7H3F5INS, a penta-fluoro-sulfanyl (SF5) containing arene, was synthesized from 4-(penta-fluoro-sulfan-yl)benzo-nitrile and lithium tetra-methyl-piperidide following a variation to the standard approach, which features simple and mild conditions that allow direct access to tri-substituted SF5 inter-mediates that have not been demonstrated using previous methods. The mol-ecule displays a planar geometry with the benzene ring in the same plane as its three substituents. It lies on a mirror plane perpendicular to [010] with the iodo, cyano, and the sulfur and axial fluorine atoms of the penta-fluoro-sulfanyl substituent in the plane of the mol-ecule. The equatorial F atoms have symmetry-related counterparts generated by the mirror plane. The penta-fluoro-sulfanyl group exhibits a staggered fashion relative to the ring and the two hydrogen atoms ortho to the substituent. S-F bond lengths of the penta-fluoro-sulfanyl group are unequal: the equatorial bond facing the iodo moiety has a longer distance [1.572 (3) Å] and wider angle compared to that facing the side of the mol-ecules with two hydrogen atoms [1.561 (4) Å]. As expected, the axial S-F bond is the longest [1.582 (5) Å]. In the crystal, in-plane C-H⋯F and N⋯I inter-actions as well as out-of-plane F⋯C inter-actions are observed. According to the Hirshfeld analysis, the principal inter-molecular contacts for the title compound are F⋯H (29.4%), F⋯I (15.8%), F⋯N (11.4%), F⋯F (6.0%), N⋯I (5.6%) and F⋯C (4.5%).

The control of stereochemistry by the pentafluorosulfanyl group

The influence of pentafluorosulfanylation on biological activity has been revealed in numerous comparative studies of biologically active compounds, but considerably less is known about the influence of pentafluorosulfanylation on reactivity. Among the distinctive properties of the pentafluorosulfanyl group is the profound dipole moment that results from introduction of this substituent. It has been shown that dipolar effects coupled with the steric demand of the SF5 group may be employed to influence the stereochemistry of reactions, especially those processes with significant charge separation in the transition state. The Staudinger ketene-imine cycloaddition reaction is an ideal platform for investigation of dipolar control of diastereoselectivity by the pentafluorosulfanyl group.

SF5-Pyridylaryl-λ3-iodonium salts and their utility as electrophilic reagents to access SF5-pyridine derivatives in the late-stage of synthesis

Electrophilic pentafluorosulfanyl (SF₅) heteroarylation of target molecules using novel reagents is described. Unsymmetrical diaryliodonium reagents 1 having 2-SF₅-pyridine have been synthesized in good yields. They are efficient electrophilic reagents for carbon and heterocentered nucleophiles, producing the corresponding SF₅-pyridine derivatives in good to excellent yields.

Fluorine and Fluorinated Motifs in the Design and Application of Bioisosteres for Drug Design

The electronic properties and relatively small size of fluorine endow it with considerable versatility as a bioisostere and it has found application as a substitute for lone pairs of electrons, the hydrogen atom, and the methyl group while also acting as a functional mimetic of the carbonyl, carbinol, and nitrile moieties. In this context, fluorine substitution can influence the potency, conformation, metabolism, membrane permeability, and P-gp recognition of a molecule and temper inhibition of the hERG channel by basic amines. However, as a consequence of the unique properties of fluorine, it features prominently in the design of higher order structural metaphors that are more esoteric in their conception and which reflect a more sophisticated molecular construction that broadens biological mimesis. In this Perspective, applications of fluorine in the construction of bioisosteric elements designed to enhance the in vitro and in vivo properties of a molecule are summarized.

Racemic and enantioselective metal-catalyzed synthesis of SF5-containing diarylmethanols

The racemic and enantioselective metal-catalyzed addition of arylboronic acids to 4- and 3-(pentafluorosulfur)benzaldehydes is reported. The racemic synthesis was accomplished using a Pd-based system and a wide range of arylboronic acids could be used, resulting in yields of 42 to 98% of the corresponding SF₅-containing diarylmethanols. A ruthenium-based system, along with (R,R)-Me-BIPAM as the chiral ligand, was investigated and optimized for the enantioselective version. In this case, while the chiral SF₅-containing diarylmethanols were generally obtained in good yields (up to 94%) and enantioselectivities (up to 98% ee), limitations were also observed. For instance, 4-(pentafluorosulfur)benzaldehyde generally provided slightly better yields than 3-(pentafluorosulfur)benzaldehyde. In addition, lower yields and enantioselectivities were observed when using either 4- and 3-substituted arylboronic acids bearing electron-withdrawing (e.g., CO₂Et, NO₂, CF₃) or 2-substituted arylboronic acids (regardless of the nature of the substituent). Overall, the SF₅-containing diarylmethanols described herein represent novel and potentially useful fluorinated building blocks for the synthesis of biologically active compounds.

Application of the Pentafluorosulfanyl Group as a Bioisosteric Replacement

The success of fluorinated molecules in drug design has led medicinal chemists to search for new fluorine-containing substituents. A major recently developed group is the pentafluorosulfanyl group. This group is stable under physiological conditions and displays unique physical and chemical properties. There are currently few synthetic methods to install the SF₅ group, yet efforts to integrate this group into lead optimization continue unabated. Typically, the SF₅ group has been used as a replacement for trifluoromethyl, tert-butyl, halogen, or nitro groups. In this review, the use of the SF₅ group as a bioisosteric replacement for each of these three functionalities is compared and contrasted across various groups of biologically active molecules. The organization and presentation of these data should be instructive to medicinal chemists considering to design synthetic strategies to access SF₅ -substituted molecules.

Silver-induced self-immolative Cl–F exchange fluorination of arylsulfur chlorotetrafluorides: synthesis of arylsulfur pentafluorides

Cl–F exchange fluorination of (Het)ArSF₄Cl without fluorinating reagents is disclosed.

Generation of ortho-SF5-Benzyne and Its Diels-Alder Reactions with Furans: Synthesis of 1-SF5-Naphthalene, Its Derivatives, and 1,6(1,7)-Bis-SF5-naphthalenes

Generation of ortho-SF₅-benzyne was achieved by a lithiation/elimination sequence starting from 2-fluoro-SF₅-benzene. The highly reactive ortho-SF₅-benzyne intermediate was trapped by furan or 2-methylfuran in situ, and the obtained stable Diels-Alder adducts were subjected to the series of further chemical transformation, which led to the formation of previously unknown 1-SF₅-naphthalene and its derivatives with bromo, amino, hydroxy, and methyl substituents, including bis-SF₅-substituted naphthalenes. NMR spectroscopy experiments revealed characteristic through-space coupling between the SF₅-group's equatorial fluorines and proton/carbon nuclei of -H, -CH₃, and -OH substituents in the peri-position to the SF₅-group of 1-SF₅-naphthalenes.

Structure-Activity Study of Bioisosteric Trifluoromethyl and Pentafluorosulfanyl Indole Inhibitors of the AAA ATPase p97

Exploratory SAR studies of a new phenyl indole chemotype for p97 inhibition revealed C-5 indole substituent effects in the ADPGlo assay that did not fully correlate with either electronic or steric factors. A focused series of methoxy-, trifluoromethoxy-, methyl-, trifluoromethyl-, pentafluorosulfanyl-, and nitro-analogues was found to exhibit IC50s from low nanomolar to double-digit micromolar. Surprisingly, we found that the trifluoromethoxy-analogue was biochemically a better match of the trifluoromethyl-substituted lead structure than a pentafluorosulfanyl-analogue. Moreover, in spite of their almost equivalent strongly electron-depleting effect on the indole core, pentafluorosulfanyl- and nitro-derivatives were found to exhibit a 430-fold difference in p97 inhibitory activities. Conversely, the electronically divergent C-5 methyl- and nitro-analogues both showed low nanomolar activities.

Pyridine-promoted dediazoniation of aryldiazonium tetrafluoroborates: Application to the synthesis of SF5-substituted phenylboronic esters and iodobenzenes

Pyridine promotes dediazoniation of aryldiazonium tetrafluoroborates. The formed aryl radicals were trapped with B2pin2, iodine, or tetrahydrofuran to afford boronic esters, iodobenzenes and benzenes, respectively. The application to the synthesis of (pentafluorosulfanyl)phenylboronic esters, iodo(pentafluorosulfanyl)benzenes and (pentafluorosulfanyl)benzene is shown.

Applications of Fluorine in Medicinal Chemistry

The role of fluorine in drug design and development is expanding rapidly as we learn more about the unique properties associated with this unusual element and how to deploy it with greater sophistication. The judicious introduction of fluorine into a molecule can productively influence conformation, pKa, intrinsic potency, membrane permeability, metabolic pathways, and pharmacokinetic properties. In addition, (18)F has been established as a useful positron emitting isotope for use with in vivo imaging technology that potentially has extensive application in drug discovery and development, often limited only by convenient synthetic accessibility to labeled compounds. The wide ranging applications of fluorine in drug design are providing a strong stimulus for the development of new synthetic methodologies that allow more facile access to a wide range of fluorinated compounds. In this review, we provide an update on the effects of the strategic incorporation of fluorine in drug molecules and applications in positron emission tomography.

Evaluation of tert-butyl isosteres: case studies of physicochemical and pharmacokinetic properties, efficacies, and activities

The tert-butyl group is a common motif in medicinal chemistry. Its incorporation into bioactive compounds is often accompanied by unwanted property modulation, such as increased lipophilicity and decreased metabolic stability. Several alternative substituents are available for the drug discovery process. Herein, physicochemical data of two series of drug analogues of bosentan and vercirnon are documented as part of a comparative study of tert-butyl, pentafluorosulfanyl, trifluoromethyl, bicyclo[1.1.1]pentanyl, and cyclopropyl-trifluoromethyl substituents.