Enantioselective, Catalytic Fluorolactonization Reactions with a Nucleophilic Fluoride Source

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.

Catalytic Enantioselective Aminative Difunctionalization of Alkenes

Enantioselective difunctionalization of alkenes offers a straightforward means for the rapid construction of enantioenriched complex molecules. Despite the tremendous efforts devoted to this field, enantioselective aminative difunctionalization remains a challenge, particularly through an electrophilic addition fashion. Herein, we report an unprecedented approach for the enantioselective aminative difunctionalization of alkenes via copper-catalyzed electrophilic addition with external azo compounds as nitrogen sources. A series of valuable cyclic hydrazine derivatives via either [3 + 2] cycloaddition or intramolecular cyclization have been achieved in high chemo-, regio-, enantio-, and diastereoselectivities. In this transformation, a wide range of functional groups, such as carboxylic acid, hydroxy, amide, sulfonamide, and aryl groups, could serve as nucleophiles. Importantly, a new cyano oxazoline chiral ligand was found to play a crucial role in the control of enantioselectivity.

Asymmetric Ruthenium-Catalyzed C-H Activation by a Versatile Chiral-Amide-Directing Strategy

A versatile and readily available chiral amide directing group has been developed for the ruthenium(II)-catalyzed asymmetric C-H activation. Asymmetric C-H activation of the related chiral benzamides with various olefins, aldehydes and propargylic alcohols has been accomplished with high stereoselectivities, affording a series of chiral products including 3,4-dihydroisocoumarins (up to 96 % ee), isocoumarins (up to 92 % ee), phthalides (up to 99 % ee), chiral bicyclo[2.2.1]heptanes (>20 : 1 dr), 4-alkylidene-3,4-dihydroisocoumarins (up to 97 % ee) and allenes (>20 : 1 dr). Importantly, our methodologies enabled concise syntheses of many biologically active compounds and natural products (e.g., Montroumarin, Cyclosporone E, Cyclosporone Q, Concentricolide, Chuangxinol, and Eleutherol).

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.

A Chemoselective Polarity-Mismatched Photocatalytic C(sp3 )-C(sp2 ) Cross-Coupling Enabled by Synergistic Boron Activation

We report the development of a C(sp3 )-C(sp2 ) coupling reaction using styrene boronic acids and redox-active esters under photoredox catalysis. The reaction proceeds through an unusual polarity-mismatched radical addition mechanism that is orthogonal to established processes. Synergistic activation of the radical precursor and organoboron are critical mechanistic events. Activation of an N-hydroxyphthalimide (NHPI) ester by coordination to boron enables electron transfer, with decomposition leading to a nucleofuge rebound, activating the organoboron to radical addition. The unique mechanism enables chemoselective coupling of styrene boronic acids in the presence of other alkene radical acceptors. The scope and limitations of the reaction, and a detailed mechanistic investigation are presented.

Forging Medium Rings via I(I)/I(III)-Catalyzed Diene Carbofunctionalization

A main-group catalysis-based strategy to access 8-membered carbocycles via the direct carbofunctionalization of 2-phenethyl-substituted 1,3-dienes is disclosed. Through the intervention of an I(I)/I(III) catalysis cycle, the synthesis of densely functionalized, fluorinated benzocyclooctenes can be achieved in an operationally simple manner. Modulating the oxidation/activation regime, and the external nucleophile, the process has been extended to unify the challenging cyclization with formation of allylic C-O, C-N, and C-C bonds (>30 examples). Derivatization of the product benzocyclooctenes is demonstrated together with X-ray conformational analysis, preliminary validation of enantioselective catalysis and a scalable resolution protocol.

Hypervalent Fluoro-iodane-Triggered Semipinacol Rearrangements: Synthesis of α-Fluoro Ketones

Hypervalent fluoro-λ3-iodanes have emerged as versatile reagents that provide unusual fluorination selectivities under mild reaction conditions. Here, we report on adding a semipinacol rearrangement, fluorination, and aryl migration cascade reaction of styrene derivatives. Thus, various cyclopentanones became accessible in up to 96% yield, all bearing tertiary C,F-carbon centers adjacent to the ketone group. Such fluorinated structural motifs are difficult to build with previously established methods. Preliminary experiments on enantioselective processes validated that asymmetric transformations are likewise feasible.

Strategies for Nucleophilic C(sp3)-(Radio)Fluorination

This Perspective surveys the progress and current limitations of nucleophilic fluorination methodologies. Despite the long and rich history of C(sp³)-F bond construction in chemical research, the inherent challenges associated with this transformation have largely constrained nucleophilic fluorination to a privileged reaction platform. In recent years, the Doyle group─along with many others─has pursued the study and development of this transformation with the intent of generating deeper mechanistic understanding, developing user-friendly fluorination reagents, and contributing to the invention of synthetic methods capable of enabling radiofluorination. Studies from our laboratory are discussed along with recent developments from others in this field. Fluoride reagent development and the mechanistic implications of reagent identity are highlighted. We also outline the chemical space inaccessible by current synthetic technologies and a series of future directions in the field that can potentially fill the existing dark spaces.

A Catalytic Asymmetric Hydrolactonization

Despite recent advancements in the development of catalytic asymmetric electrophile induced lactonization reactions of olefinic carboxylic acids, the archetypical hydrolactonization has long remained an unsolved and well-recognized challenge. Here, we report the realization of a catalytic asymmetric hydrolactonization using a confined imidodiphosphorimidate (IDPi) Brønsted acid catalyst. The method is operationally simple, scalable, and compatible with a wide variety of substrates. Its potential is showcased with concise syntheses of the sesquiterpenes (-)-boivinianin A and (+)-gossonorol. Through in-depth physicochemical and DFT analyses, we derive a nuanced picture of the mechanism and enantioselectivity of this reaction.

Different Performances of BF3, BCl3, and BBr3 in Hypervalent Iodine-Catalyzed Halogenations

Herein, hypervalent iodine-catalyzed halogenation of aryl-activated alkenes using BX₃ (X = Cl, Br) as the halogen source and activating reagents was reported. Various halogenated 1,3-oxazine/2-oxazoline derivatives were obtained in good-to-high yields. Using BF₃ resulted in different substitute sites from BBr₃ and BCl₃ of the products, indicating different reactive intermediates and reaction pathways. The reaction underwent a "ligand coupling/oxidative addition/intermolecular nucleophilic attack/1,2-aryl migration/reductive elimination/intramolecular nucleophilic attack" cascade when BF₃ was applied as the halogen source, while 1,2-aryl migration has "disappeared" when the halogen source was BBr₃ or BCl₃. Possible catalytic cycles were proposed, and DFT calculations were conducted to demonstrate the differences among BX₃ (X = F, Cl, Br) in the hypervalent iodine-catalyzed halogenations.

Enantioselective anti-Dihalogenation of Electron-Deficient Olefin: A Triplet Halo-Radical Pylon Intermediate

The textbook alkene halogenation reaction establishes straightforward access to vicinal dihaloalkanes. However, a robust catalytic method for dihalogenizing electron-deficient olefins in an enantioselective manner is still under development, and its mechanism remains controversial. Herein, we disclose efficient regio-, anti-diastereo-, and enantioselective dibromination, bromochlorination, and dichlorination reactions of enones catalyzed by a chiral N,N'-dioxide/Yb(OTf)₃ complex. With the combination of electrophilic halogen and halide salts as halogenating agents, an array of homo- and heterodihalogenated derivatives is achieved in moderate to good enantioselectivities. Moreover, DFT calculations reveal that a novel triplet halo-radical pylon intermediate is probable in accounting for the exclusive regio- and anti-diastereoselectivity.

Asymmetric Fluorofunctionalizations with Carboxylate-Based Phase-Transfer Catalysts

Fluorine is an attractive element in the field of pharmaceutical and agrochemical chemistry due to its unique properties. Considering the chiral environment in nature, where enantiomers often show different biological activities, the introduction of fluorine atom(s) into organic molecules to make chiral fluorinated compounds is an important subject. Herein, we describe the story of the development of our chiral carboxylate-based phase-transfer catalysts and their applications for asymmetric fluorocyclizations of alkenes bearing a carboxylic acid, an amide, and an oxime as an internal nucleophile with a dicationic fluorinating reagent, Selectfluor. We also describe dearomative fluorinations of indole derivatives, 2-naphthols, and resorcinols.

Progress in organocatalysis with hypervalent iodine catalysts

Hypervalent iodine compounds as environmentally friendly and relatively inexpensive reagents have properties similar to transition metals. They are employed as alternatives to transition metal catalysts in organic synthesis as mild, nontoxic, selective and recyclable catalytic reagents. Formation of C-N, C-O, C-S, C-F and C-C bonds can be seamlessly accomplished by hypervalent iodine catalysed oxidative functionalisations. The aim of this review is to highlight recent developments in the utilisation of iodine(III) and iodine(V) catalysts in the synthesis of a wide range of organic compounds including chiral catalysts for stereoselective synthesis. Polymer-, magnetic nanoparticle- and metal organic framework-supported hypervalent iodine catalysts are also described.

Regio- and Enantioselective Intermolecular Aminofluorination of Alkenes via Iodine(I)/Iodine(III) Catalysis

The regio‐ and enantio‐selective, intermolecular vicinal fluoroamination of α‐trifluoromethyl styrenes has been achieved by enantioselective I^I/I^III catalysis. Leveraging C₂‐symmetric resorcinol‐based aryl iodide catalysts, it has been possible to intercept the transient iodonium intermediate using simple nitriles, which function as both the solvent and nucleophile. In situ Ritter reaction provides direct access to the corresponding amides (up to 89 % yield, e.r. 93 : 7). This main group catalysis paradigm inverts the intrinsic regioselectivity of the uncatalyzed process, thereby providing facile access to tertiary, benzylic stereocenters bearing both CF₃ and F groups. Privileged phenethylamine pharmacophores can be generated in which there is complete local partial charge inversion (CF₃^δ−/F^δ− versus CH₃^δ+/H^δ+). Crystallographic analyses of representative β‐fluoroamide products reveal highly pre‐organized conformations that manifest the stereoelectronic gauche effect.

Stereocontrolled Synthesis of Fluorinated Isochromans via Iodine(I)/Iodine(III) Catalysis

The success of saturated, fluorinated heterocycles in contemporary drug discovery provides a stimulus for creative endeavor in main group catalysis. Motivated by the ubiquity of isochromans across the bioactive small molecule spectrum, the prominence of the anomeric effect in regulating conformation, and the metabolic lability of the benzylic position, iodine(I)/iodine(III) catalysis has been leveraged for the stereocontrolled generation of selectively fluorinated analogs. To augment the current arsenal of fluorocyclization reactions involving carboxylic acid derivatives, the reaction of readily accessible 2‐vinyl benzaldehydes is disclosed (up to >95 : 05 d.r. and 97 : 03 e.r.). Key stereoelectronic interactions manifest themselves in the X‐ray crystal structures of the products, thereby validating the [CH₂‐CHF] fragment as a stereoelectronic mimic of the [O‐CH(OR)] acetal motif.

Catalytic Synthesis of 5-Fluoro-2-oxazolines: Using BF3·Et2O as the Fluorine Source and Activating Reagent

Hypervalent iodine catalyst-catalyzed nucleophilic fluorination of unsaturated amides using BF₃·Et₂O as the fluorine source and activating reagent was reported. Various 5-fluoro-2-oxazoline derivatives were synthesized in good to excellent yields (up to 95% isolated yield) within 10 min. The process was efficient and metal-free under mild conditions. A mechanism involving a fluorination/1,2-aryl migration/cyclization cascade was proposed on the basis of previous work and experimental results.

Dearomative Iodocyclization of N-(o-Alkynyl)aryl Isoindole

We present a dearomative iodocyclization of N-(o-alkynyl)aryl isoindole here, which affords various biologically active benzoindoleazine skeletons containing alkenyl iodine. The products can further undergo cycloaddition or coupling reactions to afford a series of highly functionalized N-fused polycyclic scaffolds.

Construction of Benzimidazolone Derivatives via Aryl Iodide Catalyzed Intramolecular Oxidative C-H Amination

The first aryl iodide catalyzed intramolecular C-H amination of phenylurea has been disclosed for high-efficiency synthesis of benzimidazolone derivatives in excellent yields (up to 97%) by an operationally simple one-step organocatalytic oxidative process. Fluorinated protic alcohols can efficiently accelerate the conversion of this transformation. The straightforward method has good functional group tolerance and can be performed with an inexpensive and readily accessible catalyst with high proficiency.

The applications of catalytic asymmetric halocyclization in natural product synthesis

Catalytic asymmetric halocyclization of olefinic substrate has evolved rapidly and been well utilized as a practical strategy for constructing enantioenriched cyclic skeletons in natural product synthesis.

The aryl iodine-catalyzed organic transformation via hypervalent iodine species generated in situ

The application of hypervalent iodine species generated in situ in organic transformations has emerged as a useful and powerful tool in organic synthesis, allowing for the construction of a series of bond formats via oxidative coupling. Among these transformations, the catalytic aryl iodide can be oxidized to hypervalent iodine species, which then undergoes oxidative reaction with the substrates and the aryl iodine regenerated again once the first cyclic cycle of the reaction is completed. This review aims to systematically summarize and discuss the main progress in the application of in situ-generated hypervalent iodine species, providing references and highlights for synthetic chemists who might be interested in this field of hypervalent iodine chemistry.

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.

Structure Dependence in Asymmetric Deprotonative Fluorination and Fluorocyclization Reactions of Allylamine Derivatives with Linked Binaphthyl Dicarboxylate Phase-Transfer Catalyst

The asymmetric fluorofunctionalization of γ,γ-disubstituted allylamine derivatives (e.g., 3, 7, and 8) was investigated using our dianionic phase-transfer catalyst. Depending on the substituents on the alkene moiety, the reaction afforded chiral allylic fluorides and fluorinated dihydrooxazines in a highly enantioselective manner (up to 99% ee). The absolute stereochemistry of these products was found to be opposite to that in our previously reported fluorocyclization of γ-monosubstituted allylic amides (e.g., 13 and 14). To probe this interesting phenomenon, we investigated the influence of the substitution pattern of the alkene moiety on the reaction by means of NMR experiments and kinetic studies. The rate laws of the deprotonative fluorination and the fluorocyclization of γ,γ-disubstituted substrates were v = k[cat]^0.6, while that of the fluorocyclization of γ-monosubstituted substrates was v = k[substrate][cat]^0.4. An exponent of less than 1 suggests the involvement of an aggregated state of the catalyst ion pair in the catalytic cycle. Interestingly, a positive nonlinear effect was observed in the reactions of the γ,γ-disubstituted substrates, while a negative nonlinear effect was observed in the case of the γ-monosubstituted substrates. Thus, the reaction pathway depends on the presence or absence of an alkyl substituent at the γ position of the substrates, and on the basis of our mechanistic studies we propose that the active catalytic species for γ,γ-disubstituted substrates is a catalyst ion pair aggregate, whereas that for γ-monosubstituted substrates is the more active monomeric catalyst ion pair species, even though its concentration would be low.

Organoiodine-Catalyzed Enantioselective Intramolecular Oxy­aminations of Alkenes with N-(Fluorosulfonyl)carbamate

Organoiodine-catalyzed enantioselective intramolecular oxyaminations were realized by the use of benzyl N-(fluorosulfonyl)carbamate as the exogenous nitrogen source. The method allows access to enantioenriched lactones and oxazolines, starting from γ,δ- and δ,ε-unsaturated esters and N-allyl amides, respectively.

Stereoselective Oxidative Cyclization of N-Allyl Benzamides to Oxaz(ol)ines

This study presents an enantioselective oxidative cyclization of N-allyl carboxamides via a chiral triazole-substituted iodoarene catalyst. The method allows the synthesis of highly enantioenriched oxazolines and oxazines, with yields of up to 94% and enantioselectivities of up to 98% ee. Quaternary stereocenters can be constructed and, besides N-allyl amides, the corresponding thioamides and imideamides are well tolerated as substrates, giving rise to a plethora of chiral 5-membered N-heterocycles. By applying a multitude of further functionalizations, we finally demonstrate the high value of the observed chiral heterocycles as strategic intermediates for the synthesis of other enantioenriched target structures.

Catalytic asymmetric nucleophilic fluorination using BF3·Et2O as fluorine source and activating reagent

Fluorination using chiral catalytic methods could result in a direct access to asymmetric fluorine chemistry. However, challenges in catalytic asymmetric fluorinations, especially the longstanding stereochemical challenges existed in BF₃·Et₂O-based fluorinations, have not yet been addressed. Here we report the catalytic asymmetric nucleophilic fluorination using BF₃·Et₂O as the fluorine reagent in the presence of chiral iodine catalyst. Various chiral fluorinated oxazine products were obtained with good to excellent enantioselectivities (up to >99% ee) and diastereoselectivities (up to >20:1 dr). Control experiments (the desired fluoro-oxazines could not be obtained when Py·HF or Et₃N·3HF were employed as the fluorine source) indicated that BF₃·Et₂O acted not only as a fluorine reagent but also as the activating reagent for activation of iodosylbenzene.

Catalytic asymmetric fluorination remains elusive, especially the longstanding stereochemical challenges which exist in BF₃Et₂O-based fluorinations. Here the authors show a catalytic asymmetric nucleophilic fluorination using BF₃·Et₂O as the fluorine reagent in the presence of chiral iodine catalyst.

Fluorocyclization of Allyl Alcohols and Amines to Access 3-Functionalized Oxetanes and Azetidines

An efficient method to prepare 3-functionalized oxetanes and azetidines has been realized by fluorocyclization of readily available 2-azidoallyl/2-alkoxyallyl alcohols and amines. Notably, this is the first example applying the fluorocyclization strategy to construct four-membered heterocycles. The pendant electron-donating group (-N₃ or -OR) plays a crucial role in polarizing the C═C double bond and facilitating the cyclization process, as verified by DFT and experimental studies.

Aminolactonization of Unactivated Alkenes Catalyzed by Aryl Iodine

A one-step protocol of the aryl iodine-catalyzed aminolactonization of unactivated alkenes under oxidation conditions was first reported to efficiently construct diverse amino lactones in a short time using HNTs₂ as the compatible nitrogen source. In addition, we investigated the influence of the reaction rate based on the structure of the iodoarene precatalyst, which revealed the selective adjustment effect on aminolactonization and oxylactonization. Finally, preliminary experiments verified the feasibility of asymmetric aminolactonization catalyzed by a chiral iodoarene precatalyst.

Organoiodine-Catalyzed Enantioselective Intermolecular Oxyamination of Alkenes

Metal-free, catalytic enantioselective intermolecular oxyamination of alkenes is realized by use of organoiodine(I/III) chemistry. The protocol is applicable toward aryl- and alkyl-substituted alkenes with high enantioselectivity and electronically controlled regioselectivity. The oxyaminated products can be easily deprotected in one step to reveal free amino alcohols in high yields without loss of enantioselectivity. A key to our success is the discovery of a virtually unexplored chemical entity, N-(fluorosulfonyl)carbamate, as a bifunctional N,O-nucleophile.

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.

Recent Application of Chiral Aryliodine Based on the 2-Iodo­resorcinol Core in Asymmetric Catalysis

Chiral iodoarenes have been steadily increasing in importance in recent years, especially in enantioselective synthesis and catalysis. Since the development of the concept of chiral iodine(I/III) catalysis, the use of various chiral aryliodine frameworks has been explored in this area. This short review gives an overview of the use of chiral hypervalent iodine(I/III) reagents based on the 2-iodoresorcinol core with two attached two lactic side chains bearing ester or amide groups for the catalytic enantioselective dearomatization of phenol compounds, asymmetric oxidation of alkenes, and enantioselective α-functionalization of carbonyl compounds highlighting the excellent reactivities in terms of yield and enantioselectivity.

1 Introduction

2 Enantioselective Dearomatization of Phenol Derivatives

3 Asymmetric Oxidation of Alkenes

4 Enantioselective α-Functionalization of Carbonyl Compounds

5 Conclusion and Outlook

Enantioselective Aryl-Iodide-Catalyzed Wagner-Meerwein Rearrangements

We report a strategy for effecting catalytic, enantioselective carbocationic rearrangements through the intermediacy of alkyl iodanes as stereodefined carbocation equivalents. Asymmetric Wagner-Meerwein rearrangements of β-substituted styrenes are catalyzed by the C₂-symmetric aryl iodide 1 to provide access to enantioenriched 1,3-difluorinated molecules possessing interesting and well-defined conformational properties. Hammett and kinetic isotope effect studies, in combination with computational investigations, reveal that two different mechanisms are operative in these rearrangement reactions, with the pathway depending on the identity of the migrating group. In reactions involving alkyl-group migration, intermolecular fluoride attack is product- and enantio-determining. In contrast, reactions in which aryl rearrangement occurs proceed through an enantiodetermining intramolecular 1,2-migration prior to fluorination. The fact that both pathways are promoted by the same chiral aryl iodide catalyst with high enantioselectivity provides a compelling illustration of generality across reaction mechanisms in asymmetric catalysis.

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.

Catalytic Enantioselective Synthesis of Difluorinated Alkyl Bromides

We report an iodoarene-catalyzed enantioselective synthesis of β,β-difluoroalkyl bromide building blocks. The transformation involves an oxidative rearrangement of α-bromostyrenes, utilizing HF-pyridine as the fluoride source and m-CPBA as the stoichiometric oxidant. A catalyst decomposition pathway was identified, which, in tandem with catalyst structure-activity relationship studies, facilitated the development of an improved catalyst providing higher enantioselectivity with lower catalyst loadings. The versatility of the difluoroalkyl bromide products was demonstrated via highly enantiospecific substitution reactions with suitably reactive nucleophiles. The origins of enantioselectivity were investigated using computed interaction energies of simplified catalyst and substrate structures, providing evidence for both CH-π and π-π transition state interactions as critical features.

Fluorofunctionalizations of C-C Multiple Bonds and C-H Bonds

In spite of only a few naturally occurring products having one or more fluorine atoms, organofluorine compounds have been widely utilized in pharmaceutical, agrochemical, and functional material science fields due to the characteristic properties of the fluorine atom. Therefore, the development of new methods for the introduction of fluorine-containing functional groups has been a long-standing research topic. This article discusses our contributions to this area. The first topic is on the trifluoromethylations of C-C multiple bonds using Togni reagent based on our working hypothesis that hypervalent iodine could be activated by coordination of the carbonyl moiety to the Lewis acid catalyst. The second topic relates to asymmetric fluorofunctionalization of alkenes. A newly designed phase-transfer catalyst consisting of a carboxylate anion functioning as a phase-transfer agent and a primary hydroxyl group as a site that captures the anionic substrate was revealed to be an effective catalyst for asymmetric fluorolactonization. Inspired by the mechanistic studies of fluorolactonization, we produced a linked binaphthyl dicarboxylate catalyst, which catalyzes the 6-endo-fluorocyclization and the deprotonative fluorination of allylic amides in a highly enantioselective manner. The third topic is on C-H fluorofunctionalizations using either catalysis or photoactivation. Benzylic trifluoromethylation, which is still a rare reaction, using Togni reagent and aromatic C-H trifluoromethylation using Umemoto reagent under simple photoirradiation conditions were achieved. In addition, the Csp³-H fluorination of alkyl phthalimide derivatives is demonstrated.