Catalytic Geminal Difluorination of Styrenes for the Construction of Fluorine-rich Bioisosteres

Alkene 1,3-Difluorination via Transient Oxonium Intermediates

The 1,3-difunctionalization of unactivated alkenes is an under-explored transformation that leads to moieties that are otherwise challenging to prepare. Herein, we report a hypervalent iodine-mediated 1,3-difluorination of homoallylic (aryl) ethers to give unreported 1,3-difluoro-4-oxy groups with moderate to excellent diastereoselectivity. The transformation proceeds through a different mode of reactivity for 1,3-difunctionalization, in which a regioselective addition of fluoride opens a transiently formed oxonium intermediate to rearrange an alkyl chain. The optimized protocol is scalable and shown to proceed well with a variety of functional groups and substitution on the alkenyl chain, hence providing ready access to this fluorinated, conformationally controlled moiety.

Catalytic Ring Expanding Difluorination: An Enantioselective Platform to Access β,β-Difluorinated Carbocycles

Cyclic β,β-difluoro-carbonyl compounds have a venerable history as drug discovery leads, but limitations in the synthesis arsenal continue to impede chemical space exploration. This challenge is particularly acute in the arena of fluorinated medium rings where installing the difluoromethylene unit subtly alters the ring conformation by expanding the internal angle (∠C-CF2-C>∠C-CH2-C): this provides a handle to modulate physicochemistry (e.g. pKa). To reconcile this disparity, a highly modular ring expansion has been devised that leverages simple α,β-unsaturated esters and amides, and processes them to one-carbon homologated rings with concomitant geminal difluorination (6 to 10 membered rings, up to 95 % yield). This process is a rare example of the formal difluorination of an internal alkene and is enabled by sequential I(III)-enabled O-activation. Validation of enantioselective catalysis in the generation of unprecedented medium ring scaffolds is reported (up to 93 : 7 e.r.) together with X-ray structural analyses and product derivatization.

Determination of the Hammett Acidity of HF/Base Reagents

Harnessing the acidity of HF/base reagents is of paramount importance to improve the efficiency and selectivity of fluorination reactions. Yet, no general method has been reported to evaluate their acidic properties, and experimental designs are still relying on a trial-and-error approach. We report a new method based on 19F NMR spectroscopy which allows highly sensitive measures and short-time analyses. Advantageously, the basic properties of the indicators can be determined upstream by DFT calculations, affording a simple yet robust semiempirical approach. In particular, the indicators used in this study were rationally designed to fit on the conceptually appealing and commonly used Hammett scale. This method has been applied to commercially available and recently developed HF/base reagents.

Palladium-Based Dyotropic Rearrangement Enables A Triple Functionalization of Gem-Disubstituted Alkenes: An Unusual Fluorolactonization Reaction

We report in this paper a Pd(II)-catalyzed migratory gem-fluorolactonization of ene-carboxylic acids. Reaction of 4-methylenealkanoic acid derivatives with Selectfluor in the presence of Pd(OAc)2 (1.0 mol %) at room temperature affords fluorolactones in good to excellent yields. 2-(2-Methylenecycloalkanyl)acetic acids are transformed to bridged fluorolactones under identical conditions. One C-C, one C-O and one tertiary C-F bond were generated along the gem-disubstituted carbon-carbon double bond in this operationally simple transformation. Trapping experiments indicates that the reaction is initiated by a 5-exo-trig oxypalladation followed by Pd oxidation, regioselective ring-enlarging 1,2-alkyl/Pd(IV) dyotropic rearrangement and C-F bond forming reductive elimination cascade. Post-transformations of these fluorolactones taking advantage of the electrophilicity of the 1-fluoroalkylcarboxylate function are also documented.

Regioselective, catalytic 1,1-difluorination of enynes

Fluorinated small molecules are prevalent across the functional small-molecule spectrum, but the scarcity of naturally occurring sources creates an opportunity for creative endeavour in developing routes to access these important materials. Iodine(I)/iodine(III) catalysis has proven to be particularly well-suited to this task, enabling abundant alkene substrates to be readily intercepted by in situ-generated λ3-iodanes and processed to high-value (di)fluorinated products. These organocatalysis paradigms often emulate metal-based processes by engaging the π bond and, in the case of styrenes, facilitating fluorinative phenonium-ion rearrangements to generate difluoromethylene units. Here we demonstrate that enynes are competent proxies for styrenes, thereby mitigating the recurrent need for aryl substituents, and enabling highly versatile homopropargylic difluorides to be generated in an operationally simple manner. The scope of the method is disclosed, together with application in target synthesis (>30 examples, up to >90% yield).

Iodine(III)-Mediated Migratory gem-Difluorinations: Synthesis of β Transformable Functionality Substituted gem-Difluoroalkanes

Geminal-difluoroalkanes featuring intriguing steric and electronic properties are of great significance in medicinal chemistry, and great progresses have been achieved for their synthesis. In recent years, iodine(III) reagent-mediated migratory gem-difluorination of alkenes has proved to be an efficient and powerful strategy to access to diverse gem-difluoroalkanes, especially those bearing a readily transformable functionality (TF), which are important for rapid assembly of complex gem-difluorinated molecules in a modular and diverse manner. In this review, we systematically summarize the recent development of iodine(III)-mediated migratory gem-difluorination reactions for the synthesis of gem-difluoroalkanes bearing a synthetically versatile TF at the β position. The reaction mechanism and the utilities of the products are also discussed. This review is presented and grouped basically according to the types of transformable functionalities within the products.

Cobalt-Catalyzed Wagner-Meerwein Rearrangements with Concomitant Nucleophilic Hydrofluorination

We report a cobalt-catalyzed Wagner-Meerwein rearrangement of gem-disubstituted allylarenes that generates fluoroalkane products with isolated yields up to 84 %. Modification of the counteranion of the N-fluoropyridinium oxidant suggests the substrates undergo nucleophilic fluorination during the reaction. Subjecting the substrates to other known metal-mediated hydrofluorination procedures did not lead to observable 1,2-aryl migration. Thus, indicating the unique ability of these cobalt-catalyzed conditions to generate a sufficiently reactive electrophilic intermediate capable of promoting this Wagner-Meerwein rearrangement.

Integrating I(I)/I(III) catalysis in reaction cascade design enables the synthesis of gem-difluorinated tetralins from cyclobutanols

Partially saturated, fluorine-containing rings are ubiquitous across the drug discovery spectrum. This capitalises upon the biological significance of the native structure and the physicochemical advantages conferred by fluorination. Motivated by the significance of aryl tetralins in bioactive small molecules, a reaction cascade has been validated to generate novel gem-difluorinated isosteres from 1,3-diaryl cyclobutanols in a single operation. Under the Brønsted acidity of the catalysis conditions, an acid-catalysed unmasking/fluorination sequence generates a homoallylic fluoride in situ. This species serves as the substrate for an I(I)/I(III) cycle and is processed, via a phenonium ion rearrangement, to an (isolable) 1,3,3-trifluoride. A final C(sp³)-F bond activation event, enabled by HFIP, forges the difluorinated tetralin scaffold. The cascade is highly modular, enabling the intermediates to be intercepted: this provides an expansive platform for the generation of structural diversity.

Catalytic, Regioselective 1,4-Fluorodifunctionalization of Dienes

A catalysis-based regioselective 1,4-fluorofunctionalization of trifluoromethyl substituted 1,3-dienes has been developed to access compact, highly functionalized products. The process allows E,Z-mixed dienes to be processed to a single E-alkene isomer, and leverages an inexpensive and operationally convenient I(I)/I(III) catalysis platform. The first example of catalytic 1,4-difluorination is disclosed and subsequently evolved to enable 1,4-hetero-difunctionalization, which allows δ-fluoro-alcohol and amine derivatives to be forged in a single operation. The protocol is compatible with a variety of nucleophiles including fluoride, nitriles, carboxylic acids, alcohols and even water thereby allowing highly functionalized products, with a stereocenter bearing both C(sp³ )-F and C(sp³ )-CF₃ groups, to be generated rapidly. Scalability (up to 3 mmol), and facile post-reaction modifications are demonstrated to underscore the utility of the method in expanding organofluorine chemical space.

Skeletal Ring Contractions via I(I)/I(III) Catalysis: Stereoselective Synthesis of cis-α,α-Difluorocyclopropanes

The clinical success of α,α-difluorocyclopropanes, combined with limitations in the existing synthesis portfolio, inspired the development of an operationally simple, organocatalysis-based strategy to access cis-configured derivatives with high levels of stereoselectivity (up to >20:1 cis:trans). Leveraging an I(I)/I(III)-catalysis platform in the presence of an inexpensive HF source, it has been possible to exploit disubstituted bicyclobutanes (BCBs) as masked cyclobutene equivalents for this purpose. In situ generation of this strained alkene, enabled by Brønsted acid activation, facilitates an unprecedented 4 → 3 fluorinative ring contraction, to furnish cis-α,α-difluorinated cyclopropanes in a highly stereoselective manner (up to 88% yield). Mechanistic studies are disclosed together with conformational analysis (X-ray crystallography and NMR) to validate cis-α,α-difluorocyclopropanes as isosteres of the 1,4-dicarbonyl moiety. Given the importance of this unit in biology and the foundational n_o → π* interactions that manifest themselves in this conformation (e.g., collagen), it is envisaged that the title motif will find application in focused molecular design.

Divergent Synthesis of gem-Difluorinated Oxa-Spirocyclohexadienones by One-Pot Sequential Reactions of p-Hydroxybenzyl Alcohols with Difluoroenoxysilanes

A new efficient formal [2 + 3] cyclization of p-hydroxybenzyl alcohols with difluoroenoxysilanes has been established. This convenient one-pot sequential procedure enables the divergent construction of highly functionalized gem-difluorinated oxa-spirocyclohexadienones under mild conditions. As opposed to the common C1 synthons in previous studies, difluoroenoxysilanes acted as new 3-atom (CCO) synthons for the first time here. The AcOH and H₂O generated in the reaction are critical for the reactions to proceed smoothly.

Hypervalent iodine-mediated β-difluoroalkylboron synthesis via an unusual 1,2-hydrogen shift enabled by boron substitution

A hypervalent iodine-mediated gem-difluorination allows the facile synthesis of β-difluoroalkylborons. An unusual 1,2-hydrogen migration, triggered by boron substitution, is involved.

Electrophilic Fluorination of Alkenes via Bora-Wagner-Meerwein Rearrangement. Access to β-Difluoroalkyl Boronates

The electrophilic fluorination of geminal alkyl substituted vinyl‐Bmida derivatives proceeds via bora‐Wagner–Meerwein rearrangement. According to DFT modelling studies this rearrangement occurs with a low activation barrier via a bora‐cyclopropane shaped TS. The Bmida group has a larger migration aptitude than the alkyl moiety in the Wagner–Meerwein rearrangement of the presented electrophilic fluorination reactions.

Synthesis of Dihydroxazines and Fluorinated Oxazepanes Using a Hypervalent Fluoroiodine Reagent

Application of a hypervalent fluoroiodane for the regiodivergent synthesis of dihydroxazines and fluorinated oxazepanes from allylaminoethanol was investigated. The reaction was carried out under mild conditions and gave the products in moderate to good yields. The selectivity of this transformation is controlled by the substituents of the allylaminoethanol.

Expanding organofluorine chemical space: the design of chiral fluorinated isosteres enabled by I(i)/I(iii) catalysis

Short aliphatic groups are prevalent in bioactive small molecules and play an essential role in regulating physicochemistry and molecular recognition phenomena. Delineating their biological origins and significance have resulted in landmark developments in synthetic organic chemistry: Arigoni's venerable synthesis of the chiral methyl group is a personal favourite. Whilst radioisotopes allow the steric footprint of the native group to be preserved, this strategy was never intended for therapeutic chemotype development. In contrast, leveraging H → F bioisosterism provides scope to complement the chiral, radioactive bioisostere portfolio and to reach unexplored areas of chiral chemical space for small molecule drug discovery. Accelerated by advances in I(i)/I(iii) catalysis, the current arsenal of achiral 2D and 3D drug discovery modules is rapidly expanding to include chiral units with unprecedented topologies and van der Waals volumes. This Perspective surveys key developments in the design and synthesis of short multivicinal fluoroalkanes under the auspices of main group catalysis paradigms.

Trifluorinated Tetralins via I(I)/I(III)-Catalysed Ring Expansion: Programming Conformation by [CH2 CH2 ] → [CF2 CHF] Isosterism

Saturated, fluorinated carbocycles are emerging as important modules for contemporary drug discovery. To expand the current portfolio, the synthesis of novel trifluorinated tetralins has been achieved. Fluorinated methyleneindanes serve as convenient precursors and undergo efficient difluorinative ring expansion with in situ generated p‐TolIF₂ (>20 examples, up to >95 %). A range of diverse substituents are tolerated under standard catalysis conditions and this is interrogated by Hammett analysis. X‐ray analysis indicates a preference for the CH−F bond to occupy a pseudo‐axial orientation, consistent with stabilising σ_C−H→σ_C−F* interactions. The replacement of the symmetric [CH₂−CH₂] motif by [CF₂−CHF] removes the conformational degeneracy intrinsic to the parent tetralin scaffold leading to a predictable half‐chair. The conformational behavior of this novel structural balance has been investigated by computational analysis and is consistent with stereoelectronic theory.

Iodine(III)-Mediated Fluorination/Semipinacol Rearrangement Cascade of 2-Alkylidenecyclobutanol Derivatives: Access to β-Monofluorinated Cyclopropanecarbaldehydes

A hypervalent iodine(III)-mediated ring-contractive fluorination reaction of 2-alkylidenecyclobutanol derivatives is presented. The protocol allows the facile synthesis of β-monofluorinated cyclopropanecarbaldehydes via a fluorination/semipinacol rearrangement cascade using nucleophilic Py·HF as the fluorine source. For challenging electron-rich arene substrates, the installation of a protecting group on the free alcohol is pivotal for maintaining the reaction efficiency. The synthetic utility was demonstrated by the scalability of this reaction and further transformations of the products.

Difluorination of α-(bromomethyl)styrenes via I(I)/I(III) catalysis: facile access to electrophilic linchpins for drug discovery

Simple α-(bromomethyl)styrenes can be processed to a variety of 1,1-difluorinated electrophilic building blocks via I(I)/I(III) catalysis. This inexpensive main group catalysis strategy employs p-TolI as an effective organocatalyst when combined with Selectfluor® and simple amine·HF complexes. Modulating Brønsted acidity enables simultaneous geminal and vicinal difluorination to occur, thereby providing a platform to generate multiply fluorinated scaffolds for further downstream derivatization. The method facilitates access to a tetrafluorinated API candidate for the treatment of amyotrophic lateral sclerosis. Preliminary validation of an enantioselective process is disclosed to access α-phenyl-β-difluoro-γ-bromo/chloro esters.

A Chiral Pentafluorinated Isopropyl Group via Iodine(I)/(III) Catalysis

An I(I)/(III) catalysis strategy to construct an enantioenriched fluorinated isostere of the i Pr group is reported. The difluorination of readily accessible α-CF3 -styrenes is enabled by the in situ generation of a chiral ArIF2 species to forge a stereocentre with the substituents F, CH2 F and CF3 (up to 95 %, >20:1 vicinal:geminal difluorination). The replacement of the metabolically labile benzylic proton results in a highly preorganised scaffold as was determined by X-ray crystallography (π→σ* and stereoelectronic gauche σ→σ* interactions). A process of catalyst editing is disclosed in which preliminary validation of enantioselectivity is placed on a structural foundation.

Modern approaches towards the synthesis of geminal difluoroalkyl groups

This review will cover the importance of and most recent approaches toward geminal difluoroalkyl groups. Transition metal-mediated, photochemical, organocatalytic, and other methods as well as their mechanistic implications will be discussed, with special emphasis on applications to biologically-relevant compounds.

CF3-substituted carbocations: underexploited intermediates with great potential in modern synthetic chemistry

"The extraordinary instability of such an "ion" accounts for many of the peculiarities of organic reactions" - Franck C. Whitmore (1932). This statement from Whitmore came in a period where carbocations began to be considered as intermediates in reactions. Ninety years later, pointing at the strong knowledge acquired from the contributions of famous organic chemists, carbocations are very well known reaction intermediates. Among them, destabilized carbocations - carbocations substituted with electron-withdrawing groups - are, however, still predestined to be transient species and sometimes considered as exotic ones. Among them, the CF3-substituted carbocations, frequently suggested to be involved in synthetic transformations but rarely considered as affordable intermediates for synthetic purposes, have long been investigated. This review highlights recent and past reports focusing on their study and potential in modern synthetic transformations.

Revisiting the ‘Phenonium Phenomenon’: Regiodivergent Opening of Nonsymmetrical Phenonium Ions with Halide Nucleophiles

Formation of phenonium ions through anchimeric assistance (neighboring-group participation) of aryl rings has been known since 1949. Although these reactive intermediates have been studied extensively by physical organic chemists, their potential as control elements in synthesis is underutilized. Presented here are our laboratory’s recently reported first examples of regiodivergent openings of nonsymmetrical phenonium ions with chloride nucleophiles. The selectivity of these processes is under reagent control. The reactions are operationally simple and permit the stereospecific synthesis of complex chiral building blocks from readily accessible epoxide starting materials.

1 Introduction

2 Select Examples of Phenonium Ion Methodology

3 Regiodivergent Opening of Nonsymmetrical Phenonium Ions

4 Summary and Outlook

Formation of synthetically relevant CF3-substituted phenonium ions in superacid media

Under superacidic conditions, CF₃-substituted phenonium ions can be generated to furnish original CF₃-substituted dihydrostilbenes of interest.

Iodoarene-Catalyzed Oxyamination of Unactivated Alkenes to Synthesize 5-Imino-2-Tetrahydrofuranyl Methanamine Derivatives

Reported here is the room-temperature metal-free iodoarene-catalyzed oxyamination of unactivated alkenes. In this process, the alkenes are difunctionalized by the oxygen atom of the amide group and the nitrogen in an exogenous HNTs₂ molecule. This mild and open-air reaction provided an efficient synthesis to N-bistosyl-substituted 5-imino-2-tetrahydrofuranyl methanamine derivatives, which are important motifs in drug development and biological studies. Mechanistic study based on experiments and density functional theory calculations showed that this transformation proceeds via activation of the substrate alkene by an in situ generated cationic iodonium(III) intermediate, which is subsequently attacked by an oxygen atom (instead of nitrogen) of amides to form a five-membered ring intermediate. Finally, this intermediate undergoes an S_N2 reaction by NTs₂ as the nucleophile to give the oxygen and nitrogen difunctionalized 5-imino-2-tetrahydrofuranyl methanamine product. An asymmetric variant of the present alkene oxyamination using chiral iodoarenes as catalysts also gave promising results for some of the substrates.

Enantioselective Construction of Tertiary Fluoride Stereocenters by Organocatalytic Fluorocyclization

1,1-Disubstituted styrenes with internal oxygen and nitrogen nucleophiles undergo oxidative fluorocyclization reactions with in situ generated chiral iodine(III)-catalysts. The resulting fluorinated tetrahydrofurans and pyrrolidines contain a tertiary carbon–fluorine stereocenter. Application of a new 1-naphthyllactic acid-based iodine(III)-catalyst allows the control of tertiary carbon–fluorine stereocenters with up to 96% ee. Density functional theory calculations are performed to investigate the details of the mechanism and the factors governing the stereoselectivity of the reaction.

Enantioselective Synthesis of 3-Fluorochromanes via Iodine(I)/Iodine(III) Catalysis

The chromane nucleus is common to a plenum of bioactive small molecules where it is frequently oxidized at position 3. Motivated by the importance of this position in conferring efficacy, and the prominence of bioisosterism in drug discovery, an iodine(I)/iodine(III) catalysis strategy to access enantioenriched 3-fluorochromanes is disclosed (up to 7:93 e.r.). In situ generation of ArIF2 enables the direct fluorocyclization of allyl phenyl ethers to generate novel scaffolds that manifest the stereoelectronic gauche effect. Mechanistic interrogation using deuterated probes confirms a stereospecific process consistent with a type IIinv pathway.

Fluorohydration of alkynes via I(I)/I(III) catalysis

Substrate specificity is ubiquitous in biological catalysis, but less pervasive in the realm of small-molecule catalysis. Herein, we disclose an intriguing example of substrate specificity that was observed whilst exploring catalysis-based routes to generate α-fluoroketones from terminal and internal alkynes under the auspices of I(I)/I(III) catalysis. Utilising p-TolI as an inexpensive organocatalyst with Selectfluor^® and amine/HF mixtures, the formation of protected α-fluoroketones from simple alkynes was realised. Whilst the transient p-TolIF₂ species generated in situ productively engaged with pentynyl benzoate scaffolds to generate the desired α-fluoroketone motif, augmentation or contraction of the linker suppressed catalysis. The prerequisite for this substructure was established by molecular editing and was complemented with a physical organic investigation of possible determinants.

Visible-light promoted three-component tandem reaction to synthesize difluoromethylated oxazolidin-2-imine

An effective synthetic method to achieve difluoromethylated oxazolidin-2-imine has been developed via visible-light promoted three-component tandem reaction of aryl allylamines, 2-BTSO2CF2H (BT = Benzothiazole) and isocyanates. This method features mild reaction conditions and good functional group tolerance, and the reaction mechanism was confirmed by experiments and interpreted by quantum chemical calculations.

Photocatalytic Fluoro Sulfoximidations of Styrenes

Reactions of difluoroiodotoluene with NH‐sulfoximines provide new hypervalent iodine(III) reagents, which photocatalytically transfer a fluoro and a sulfoximidoyl group onto styrenes with high regioselectivity. The substrate scope is broad with respect to both sulfoximines and olefins. Following an operationally simple protocol, a large library of fluorine‐containing N‐functionalized sulfoximines can be accessed. Results from mechanistic investigations revealed the importance of radical intermediates.

Silver-Mediated Intermolecular Iodotrifluoromethoxylation of Alkenes

For the first time, intermolecular iodotrifluoromethoxylation between alkenes and NIS with AgF as the catalyst and TFMS as the trifluoromethoxylation reagent has been explored. The practical processes, good functional group tolerance, and easy scalability make this reaction an attractive protocol for the synthesis of trifluoromethoxylated iodides, which can be readily used for further synthetic manipulation.

Synthesis of difluoromethylated allenes through trifunctionalization of 1,3-enynes

Organofluorine compounds have shown their great value in many aspects. Moreover, allenes are also a class of important compounds. Fluorinated or fluoroalkylated allenes might provide an option as candidates for drug and material developments, as allenes allow a great number of valuable transformations. Herein, we report a metal-free synthesis of difluoromethylated allenes via regioselective trifunctionalization of 1,3-enynes. This method proceeds through double C–F bond formation with concomitant introduction of an amino group to the allene. Synthetic applications are conducted and preliminary mechanistic studies suggest that a two-step pathway is involved. DFT calculations revealed an unusual dibenzenesulfonimide-assisted fluorination/fluoroamination with NFSI. In addition, kinetic reaction study revealed the induction period of both major and side products to support the proposed reaction mechanism. This work offers a convenient approach for the synthesis of a range of difluoromethylated allenes and is also a rare example of trifunctionalization of 1,3-enynes.

Fluorinated or fluoroalkylated allenes are versatile building blocks for medicinal and material chemistry. Here, the authors show a regioselective trifunctionalization of 1,3-enynes proceeding through double C-F bond formation and concomitant installation of a -NSO₂Ph group to the allene moiety.