Synthesis of gem-Difluorinated Oxa/Azaspiro[2.4]heptanes via Palladium-Catalyzed Spirocyclopropanation

A palladium-catalyzed spirocyclopropanation of gem-difluoroalkenes with π-allylpalladium 1,4-dipoles has been successfully developed, which gives a powerful and straightforward synthetic strategy for the construction of novel gem-difluorinated spirocyclic compounds, 6,6-difluoro-5-oxa/azaspiro[2.4]heptanes. The scope of gem-difluoroalkenes can be extended to styrenes, acrylic esters, and acrylamides to realize the installment of various functional groups and different heteroatoms on the spirocyclic skeletons, which could be converted to valuable compounds with potential biological activity. The mechanistic investigations revealed the competition between spirocyclopropanation and β-F elimination of π-allylpalladium zwitterionic intermediates.

Palladium-catalyzed alkynylation of allylic gem-difluorides

Herein, a palladium-catalyzed regioselective alkynylation, esterification, and amination of allylic gem-difluorides via C-F bond activation/transmetallation/β-C elimination or nucleophilic attack has been achieved. This innovative protocol showcases an extensive substrate range and operates efficiently under mild reaction conditions, resulting in high product yields and Z-selectivity. Particularly noteworthy is its exceptional tolerance towards a wide array of functional groups. This developed methodology provides effective and convenient routes to access a diverse array of essential fluorinated enynes, esters and amines.

A Journey of the Development of Privileged Difluorocarbene Reagents TMSCF2X (X = Br, F, Cl) for Organic Synthesis

ConspectusAs fluorine has played an increasingly important role in modulating the physical, chemical, and biological properties of organic molecules, the selective introduction of fluorine atom(s) or fluorinated moieties into target molecules has become a powerful tool in the development of new pharmaceuticals, agrochemicals, and functional materials. In this context, the difluoromethylene (CF2) and difluoromethyl (CF2H) groups are of special interest because of their ability to serve as bioisosteres of ethereal oxygen atoms and hydroxyl (OH) and thiol (SH) groups, respectively. Difluorocarbene is one of the most versatile reactive intermediates to incorporate CF2 and CF2H groups; however, before 2006, most of the previously known difluorocarbene reagents suffered from several drawbacks such as using ozone-depleting substances (ODSs), difficult-to-handle reagents, or harsh reaction conditions or having narrow substrate scope and/or low yields. Moreover, the reactivity of difluorocarbene generated from different precursors (reagents) was often unpredictable, since the difluorocarbene generation conditions (activation modes) of various difluorocarbene precursors are different, and these conditions may mismatch those required for subsequent difluorocarbene-involved transformations. Therefore, the development of new environmentally friendly and versatile difluorocarbene reagents, as well as the investigation of the mechanistic insights into difluorocarbene-involved reactions, has been highly desirable.In this Account, we summarize our contributions to the development of new difluorocarbene reagents and their applications in organic synthesis since 2006. We have developed seven new difluorocarbene reagents, including 2-chloro-2,2-difluoroacetophenone (1), chlorodifluoromethyl phenyl sulfone (2), S-difluoromethyl-S-phenyl-N-tosylsulfoximine (3), difluoromethyltri(n-butyl)ammonium chloride (4), (chlorodifluoromethyl)trimethylsilane (TMSCF2Cl, 5), (bromodifluoromethyl)trimethylsilane (TMSCF2Br, 6), and (trifluoromethyl)trimethylsilane (TMSCF3, 7). In this journey, we realized the key factor for an ideal difluorocarbene reagent that can be used for a broad range of reactions, that is, the reagent should allow various activation modes for the generation of difluorocarbene species, such as under basic/acidic/neutral conditions, at wide range of temperatures, and in different solvents, which are compatible with a wide range of difluorocarbene-involved transformations. Among all known difluorocarbene reagents, silanes TMSCF2X (X = Br, F, Cl) have stood out as privileged ones, which paves a new avenue for further developing difluorocarbene chemistry. In particular, TMSCF2Br was recognized as an "all-rounder": TMSCF2Br can be applied in almost all common difluorocarbene-involved reactions, and more importantly, TMSCF2Br also enables many other novel transformations that other difluorocarbene reagents cannot achieve, thanks to its unique structure and rich activation modes of releasing difluorocarbene under different reaction conditions. It can be expected that with the commercial availability of TMSCF2X reagents (X = Br, F, Cl) now, the development of difluorocarbene chemistry will be accelerated in the years to come.

Photoinduced selective perfluoroalkylation of terminal alkynes via electron donor-acceptor complexes

Herein, we report a photoinduced selective perfluoroalkylation of terminal alkynes driven by the noncovalent interaction between a thymol anion and fluoroalkyl iodides. By precisely tuning the reaction solvent, a wide range of 37 structurally diverse perfluoroalkylated alkynes and alkenes, including ibuprofen, empagliflozin, galactose, isoxepac and indomethacin, were obtained in up to 92% yields. Mechanistic studies reveal the formation of EDA complexes between the thymol anion and fluoroalkyl iodides. This strategy may provide an important complement to traditional approaches to prepare useful perfluoroalkylated alkynes and alkenes.

Difluoromethylated Difunctionalization of Alkenes under Visible Light

Difluoromethylated compounds usually act as bioisosteres for alcohol functional groups and show unique physicochemical and biological properties. The cyano-difluoromethylation of alkenes using 5-((difluoromethyl)sulfonyl)-1-phenyl-1H-tetrazole as a CF2H radical difluoromethyl precursor was developed to afford nitriles including a CF2H group. A low-cost, stable, easily handled 5-((difluoromethyl)sulfonyl)-1-methyl-1H-tetrazole (DFSMT) was synthesized and applied as the radical CF2H reagent. Using DFSMT as the radical CF2H precursor, the oxyl-difluoromethylation of alkenes was developed to obtain difluoromethylated ether products. All of the reactions showed good functional group tolerability. Initial mechanistic experiments indicated that the CF2H radical was involved as the key active intermediate.

Divergent Synthesis of F- and CF3-Containing N-Fused Heterocycles Enabled by Fragmentation Cycloaddition of β-CF3-1,3-Enynes with N-Aminopyridiniums Ylides

The two novel cyclization modes of β-CF3-1,3-enynes are presented herein for the divergent construction of F- and CF3-containing N-fused heterocycles. Fluorinated pyrazolo[1,5-a]pyridines were afforded from β-CF3-1,3-enynes with N-aminopyridiniums ylides via detrifluoromethylative [2 + 3] cyclizations, followed by fluorine transfer from a CF3 unit. Whereas reaction with N-aminoisoquinoliniums ylides gave CF3-substituted pyrrolo[2,1-a]isoquinoline by unprecedented fragmentation [3 + 2]-cycloadditions. Additionally, gram-scale experiments and synthetic utility are demonstrated by further derivatization of fluorinated heterocycles.

Palladium-Catalyzed Cross-Coupling of gem-Difluorocyclopropanes with gem-Diborylalkanes for the Synthesis of Boryl-Substituted Fluorinated Alkenes

This study presents a novel method for the regioselective coupling of gem-difluorinated cyclopropanes with gem-diborylmethane, utilizing a Pd-catalyst system. This innovative approach enables the synthesis of 2-fluoroalkenyl monoboronate scaffolds with high Z-selectivity. The resulting products undergo further transformations, including oxidation, Suzuki cross-coupling, and trifluoroborylation, all of which are achieved with good yields. This work introduces a valuable synthetic pathway to access important fluorinated compounds for various applications in organic chemistry.

Access to 2-thio/selenoquinolines via domino reaction of isocyanides with sulfur and selenium in water

A domino reaction of o-alkenylaryl isocyanides with elemental sulfur and selenium in pure water was developed for the efficient and green synthesis of quinoline-2-thione and diquinolyl diselenide derivatives. Mechanistical investigation reveals that intramolecular nucleophilic addition of an alkenyl group to the in situ generated isothio/isoselenocyanate accounts for the formation of a quinoline-ring. Moreover, this transformation is also amendable for the convenient preparation of 2-fluoromethylthio-/seleno-quinolines by a one-pot three-component reaction.

Carbon-Carbon Bond Cleavage for Late-Stage Functionalization

Late-stage functionalization (LSF) introduces functional group or structural modification at the final stage of the synthesis of natural products, drugs, and complex compounds. It is anticipated that late-stage functionalization would improve drug discovery's effectiveness and efficiency and hasten the creation of various chemical libraries. Consequently, late-stage functionalization of natural products is a productive technique to produce natural product derivatives, which significantly impacts chemical biology and drug development. Carbon-carbon bonds make up the fundamental framework of organic molecules. Compared with the carbon-carbon bond construction, the carbon-carbon bond activation can directly enable molecular editing (deletion, insertion, or modification of atoms or groups of atoms) and provide a more efficient and accurate synthetic strategy. However, the efficient and selective activation of unstrained carbon-carbon bonds is still one of the most challenging projects in organic synthesis. This review encompasses the strategies employed in recent years for carbon-carbon bond cleavage by explicitly focusing on their applicability in late-stage functionalization. This review expands the current discourse on carbon-carbon bond cleavage in late-stage functionalization reactions by providing a comprehensive overview of the selective cleavage of various types of carbon-carbon bonds. This includes C-C(sp), C-C(sp2), and C-C(sp3) single bonds; carbon-carbon double bonds; and carbon-carbon triple bonds, with a focus on catalysis by transition metals or organocatalysts. Additionally, specific topics, such as ring-opening processes involving carbon-carbon bond cleavage in three-, four-, five-, and six-membered rings, are discussed, and exemplar applications of these techniques are showcased in the context of complex bioactive molecules or drug discovery. This review aims to shed light on recent advancements in the field and propose potential avenues for future research in the realm of late-stage carbon-carbon bond functionalization.

Tunable Synthesis of Monofluoroalkenes and Gem-Difluoroalkenes via Solvent-Controlled Rhodium-Catalyzed Arylation of 1-Bromo-2,2-difluoroethylene

Divergent synthesis of fluorine-containing scaffolds starting from a suite of raw materials is an intriguing topic. Herein, we report the solvent-controlled rhodium-catalyzed tunable arylation of 1-bromo-2,2-difluoroethylene. The selection of the reaction solvents provides switchable defluorinated or debrominated arylation from readily available feedstock resources (both arylboronic acids/esters and 1-bromo-2,2-difluoroethylene are commercially available). This switch is feasible because of the difference in coordination ability between the solvent (CH2 Cl2 or CH3 CN) and the rhodium center, resulting in different olefin insertion. This protocol allows the convenient synthesis of monofluoroalkenes and gem-difluoroalkenes, both of which are important scaffolds in the fields of medicine and materials. Moreover, this newly developed solvent-regulated reaction system can be applied to the site-selective dechlorinated arylation of trichloroethylene. Overall, this study provides a useful strategy for the divergent synthesis of fluorine-containing scaffolds and provides insight into the importance of solvent selection in catalytic reactions.

Pd-IHept-Catalyzed Ring-Opening of gem-Difluorocyclopropanes with Malonates Via Selective C-C Bond Cleavage: Synthesis of Monofluoroalkenes

Monofluoroalkene scaffolds are frequently found in various functional molecules. Herein, we report a Pd-IHept-catalyzed (NHC = N-heterocyclic carbene) defluorinative functionalization approach for the synthesis of monofluoroalkenes from gem-difluorocyclopropanes and malonates. The flexible yet sterically hindered N,N'-bis(2,6-di(4-heptyl)phenyl)imidazol-2-ylidene ligand plays a key role in ensuring the high reaction efficiency. In addition, sterically hindered 1,1- and 1,2-disubstituted gem-difluorocyclopropanes could also be used in this transformation.

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.

Photocatalytic Alkyl Radical Addition Tandem Oxidation of Alkenyl Borates

Photocatalytic oxidation is a popular transformation way for organic synthesis and is widely applied in academia and industry. Herein, we report a blue light-induced alkylation-oxidation tandem reaction for the synthesis of diverse ketones by combining alkyl radical addition and oxidation of alkenyl borates. This reaction shows excellent functional group compatibility in acceptable yields, and diversity of radical precursors is applicable.

Merging Copper(I) Photoredox Catalysis and Iodine(III) Chemistry for the Oxy-monofluoromethylation of Alkenes

A simple process for the oxy-monofluoromethylation of alkenes is described. In combination with visible-light copper(I) photoredox catalysis, an easily accessible iodine(III) reagent containing monofluoroacetoxy ligands serves as a powerful source of a monofluoromethyl (CH₂ F) radical, enabling the step economical synthesis of γ-fluoro-acetates from a broad range of olefinic substrates under mild conditions. Applications to late-stage diversification of alkenes derived from complex molecules, amino acids and the synthesis of fluoromethylated heterocycles are also demonstrated.

Visible-Light-Induced Difluoroalkylation of Alkenes and Alkynes with Fluoro-Containing Hypervalent Iodane (III) Reagents Under Photo-Catalyst-Free Conditions

A visible-light-induced difluoroalkylation of unactivated alkenes by fluoro-containing hypervalent iodine-based difluoroalkylation reagent was achieved for the first time under photo-catalyst-free conditions. Moreover, the same reaction conditions were applicable to the difluoroalkylation of alkynes to give the hydrodifluoroalkylation products in moderate to excellent yields. The readily available reagent, broad substrate scope, and photo-catalyst-free conditions make this protocol an efficient and environmental friendly method for the hydrodifluoroalkylation of alkenes and alkynes.

Versatile Fluorine-Containing Building Blocks: β-CF3-1,3-enynes

The development of diversity-oriented synthesis based on fluorine-containing building blocks has been one of the hot research fields in fluorine chemistry. β-CF₃-1,3-enynes, as one type of fluorine-containing building blocks, have attracted more attention in the last few years due to their distinct reactivity. Numerous value-added trifluoromethylated or non-fluorinated compounds which have biologically relevant structural motifs, such as O-, N-, and S-heterocycles, carboncycles, fused polycycles, and multifunctionalized allenes were synthesized from these fluorine-containing building blocks. This review summarizes the most significant developments in the area of synthesis of organofluorine compounds based on β-CF₃-1,3-enynes, providing a detailed overview of the current state of the art.

Multifunctional Waterborne Polyurethane Microreactor-Based Approach to Fluorocarbon Composite Latex Coatings with Double Self-Healing and Excellent Synergistic Performances

In this article, chlorotrifluoroethylene (CTFE)-based fluorocarbon composite latexes and their coatings are successfully fabricated by an environmentally friendly preparation method based on a new multifunctional waterborne polyurethane (MFWPU) dispersion. It is worth noting that the MFWPU acts as the sole system stabilizer as well as microreactor and simultaneously endows the composite coating with excellent double self-healing performance and adhesion. Moreover, the introduction of a dynamic disulfide bond in the polyurethane dispersion entrusts the coating with excellent scratch self-healing performance. Simultaneously, carbon-carbon double bonds in the polyurethane dispersion increase the compatibility between the core polymer and shell polymer. The fluorine-containing chain segments can be distributed in the coating evenly during the self-assembly film-forming process of composite particles so that the original element composition of the worn coating surface can restore the original element composition after heating, and the coating presents a regeneration ability, which further and verifies the usefulness of the double self-healing model of the coating. Afterward, efficient recovery and durability, which are two contradictory properties of scratch self-healing polymers, are optimized to obtain a composite coating with excellent comprehensive performance. The research results regarding the composite system may provide a valuable reference for the structural design and application of waterborne fluorocarbon functional coatings in the future.

Highly Chemoselective Synthesis of Azaarene-Equipped CF3-Tertiary Alcohols under Metal-Free Conditions and Their Fungicidal Activities

A highly chemoselective reaction between α,β-unsaturated trifluoromethyl ketones with azaarenes under metal-free conditions was carried out, affording a range of valuable azaarene-equipped CF₃-tertiary alcohols in moderate to excellent yields (up to 95% yield) with good tolerance of functional groups, and their structures were confirmed by NMR, HRMS, and X-ray diffraction for validation. This method features simple reaction conditions (only solvent), high atom- and step-economy, and broad substrate scope. Moreover, most of the target products exhibited promising fungicidal activities, and compound 3al exhibited 91.65% fungicidal activity against R. solani, with an EC₅₀ value of 0.18 mg/mL.

Reagent-Controlled Highly Stereoselective Difluoromethylation: Efficient Access to Chiral α-Difluoromethylamines from Ketimines

A reagent-controlled highly stereoselective reaction between (S)-difluoromethyl phenyl sulfoximine 1 and imines is reported, and this synthetic method provides a variety of enantiomerically enriched α-difluoromethyl amines. The main pros of this approach include high efficiency, high stereoselectivity, and a broad substrate scope, which is probably achieved through a non-chelating transition state.

C3-Selective Trifluoromethylthiolation and Difluoromethylthiolation of Pyridines and Pyridine Drugs via Dihydropyridine Intermediates

Herein, we report a method for unprecedented C3-selective C-H tri- and difluoromethylthiolation of pyridines. The method relies on borane-catalyzed pyridine hydroboration for generation of nucleophilic dihydropyridines; these intermediates react with trifluoromethylthio and difluoromethylthio electrophiles to form functionalized dihydropyridines, which then undergo oxidative aromatization. The method can be used for late-stage functionalization of pyridine drugs for the generation of new drug candidates.

Photocatalytic difluoromethylarylation of unactivated alkenes via a (hetero)aryl neophyl-like radical migration

Photoredox-catalyzed addition of the difluoromethylradical to unactivated alkenes has been found to trigger neophyl-like aryl and heteroaryl migrations which allowed the construction of a diverse series of difluoromethyl ketones. The reaction featured mild reaction conditions and broad substrate scope.

Photoinduced Three-Component Difluoroalkylation of Quinoxalinones with Alkenes via Difluoroiodane(III) Reagents

An environmentally friendly strategy for the photocatalyzed three-component reaction between quinoxalinones, alkenes, and hypervalent iodine(III) reagents is disclosed. The new designed difluoroiodane(III) reagent shows excellent reactivity, providing a wide range of difluoroalkyl-substituted quinoxaline-2(1H)-ones in moderate to excellent yields under mild conditions. Experimental studies demonstrated that a difluoroalkyl radical intermediate was involved in this reaction.

Synthesis of Aryl Perfluorocyclopropyl Ethers via [2 + 1] Cyclopropanation Using TMSCF2Br Reagent

Aryl perfluorocyclopropyl ethers have been synthesized for the first time by [2 + 1] cyclopropanation between aryl trifluorovinyl ethers and a commercially available TMSCF₂Br reagent. This cycloaddition reaction between two fluorine-containing reactants proceeds smoothly in toluene at 120 °C in the presence of a catalytic amount of n-Bu₄NBr, and the reaction tolerates a variety of functional groups. A wide range of aryl trifluorovinyl ethers, easily accessible from phenols, were successfully transformed to aryl perfluorocyclopropyl ethers.

Stereoconvergent Synthesis of Monofluoroalkenes via Photoinduced Dual Decarboxylative Cross-Coupling of α-Fluoroacrylic Acids with Redox-Active Esters

Herein, a new strategy for the synthesis of monofluoroalkenes via employing α-fluoroacrylic acids and N-hydroxyphthalimide (NHPI) redox-active esters as coupling partners has been developed. This decarboxylative reaction enabled the formation of C(sp²)-C(sp³) bonds to provide a practical and efficient approach for the construction of a variety of monofluoroalkenes, which are key structural motifs in organic chemistry, under mild reaction conditions. The protocol exhibited excellent functional group compatibility and delivered monofluoroalkene products with excellent Z-stereoselectivity. This work also provides a platform for the modification of complex biologically active molecules containing carboxylic acids.

Visible-light-promoted trifluoromethylselenolation of ortho-hydroxyarylenaminones

Development of an efficient process that employs easy to handle and shelf-stable reagents for the synthesis of trifluoromethylselenylated heterocyclics remains a daunting challenge in organic synthesis. Herein, we report a...

Iron-catalyzed decarboxylative and oxidative decarbonylative cross-coupling: a new strategy for the synthesis of monofluoroalkenes

Herein, an iron(ii)-catalyzed decarboxylative and oxidative decarbonylative cross-coupling of α-fluoro cinnamic acids with aliphatic aldehydes is presented.

State of knowledge in photoredox-catalysed direct difluoromethylation

The combination of visible light photoredox catalysis with direct difluoromethylation has allowed the synthesis of a large choice of CF2H-containing value-added molecules under very mild reaction conditions.

Tunable photocatalytic oxysulfonylation and chlorosulfonylation of α-CF3 alkenes with sulfonyl chlorides

Tunable photoredox-catalyzed chlorosulfonylation and oxysulfonylation of α-trifluoromethylstyrenes with sulfonyl chloride were facilely achieved by simply manipulating the photocatalyst and solvent.

Copper-catalyzed direct monofluoroalkenylation of C(sp3)–H bonds via decarboxylation of α-fluoroacrylic acids

Herein, a protocol for the copper-catalyzed direct monofluoroalkenylation of C(sp3)–H bonds is reported.

Synthesis of oxindoles bearing a stereogenic 3-fluorinated carbon center from 3-fluorooxindoles

3,3-Disubstituted oxindoles bearing a stereogenic 3-fluorinated carbon center are privileged structural motifs present in many bioactive molecules. The straightforward functionalization of 3-fluorooxindoles constitutes a powerful method for the synthesis of 3-fully substituted 3-fluorooxindoles, taking advantage of the ease of preparation of 3-fluorooxindoles with different substitution patterns and the atom efficiency of chemical reactions. In the past decade, many papers have appeared on the synthesis of 3-fully substituted 3-fluorooxindoles from 3-fluorooxindoles. Importantly, many asymmetric catalytic methods have been developed for the enantioselective synthesis of these valuable compounds. This review summarizes the achievements in this area, and overviews synthetic opportunities that still exist.

Three-component 1,2-dicarbofunctionalization of alkenes involving alkyl radicals

1,2-Dicarbofunctionalization of alkenes represents an appealing strategy for chemical bond formation in organic synthesis, which could enable the rapid construction of molecular complexity from simple and readily available starting materials by incorporating two functional groups onto a carbon-carbon double bond in one step. In this field, the dicarbofunctionalization of alkenes with different alkyl radicals in a controlled manner represents an elegant and versatile strategy to access structurally diverse functionalized alkanes, which have witnessed significant progress over the last five years. Due to the importance of alkyl radicals in organic synthesis and medicinal chemistry, this review provides a comprehensive perspective on the development of alkyl radical precursors including electrophilic precursors such as alkyl halides, alkyl peroxides, alkyl NHP esters, cycloketone oxime esters, and Katritzky pyridinium salts, and nucleophilic precursors such as alkyl acids, alkyl oxalates, alkylborates, alkylsilicates, and unactivated hydrocarbons, which generate alkyl radicals by photocatalysis or transition metal catalysis to engage in dicarbofunctionalization under oxidative reaction conditions, redox-neutral conditions, or reductive conditions. The mechanisms of these dicarbofunctionalization reactions have also been discussed in detail.

Silylboronate-Mediated Defluorosilylation of Aryl Fluorides with or without Ni-Catalyst

The defluorosilylation of aryl fluorides to access aryl silanes was achieved under transition-metal-free conditions via an inert C-F bond activation. The defluorosilylation, mediated by silylboronates and KOtBu, proceeded smoothly at room temperature to afford various aryl silanes in good yields. Although a comparative experiment indicated that Ni catalyst facilitated this transformation more efficiently, the transition-metal-free protocol is advantageous from a green chemistry perspective.

Application of Oxidative Ring Opening/Ring Closing by Reductive Amination Protocol for the Stereocontrolled Synthesis of Functionalized Azaheterocycles

The current Account gives an insight into the synthesis of some N-heterocyclic β-amino acid derivatives and various functionalized saturated azaheterocycles accessed from substituted cycloalkenes via ring C=C bond oxidative cleavage followed by ring closing across double reductive amination. The ring-cleavage protocol has been accomplished according to two common approaches: a) Os-catalyzed dihydroxylation/NaIO4 vicinal diol oxidation and b) ozonolysis. A comparative study on these methodologies has been investigated. Due to the everincreasing relevance of organofluorine chemistry in drug research as well as of the high biological potential of β-amino acid derivatives several illustrative examples to the access of various fluorine-containing piperidine or azepane β-amino acid derivatives are also presented in the current Account.

1 Introduction

2 Olefin-Bond Transformation by Oxidative Ring Cleavage

3 Synthesis of Saturated Azaheterocycles via Oxidative Ring-Opening/Ring-Closing Double Reductive Amination

3.1 Importance of Fluorine-Containing Azaheterocycles in Pharmaceutical Research

3.2 Synthesis of Azaheterocyclic Amino Acid Derivatives with a Piperidine or Azepane Framework through Oxidative Ring Opening/Reductive Amination

3.2.1 Synthesis of Piperidine β-Amino Esters

3.2.2 Synthesis of Azepane β-Amino Esters

3.2.3 Synthesis of Fluorine-Containing Piperidine γ-Amino Esters

3.3 Synthesis of Tetrahydroisoquinoline Derivatives through Oxidative Ring Opening/Reductive Amination Protocol

3.4 Synthesis of Functionalized Benzazepines through Reductive Amination

3.4.1 Synthesis of Benzo[c]azepines

3.4.2 Synthesis of Benzo[d]azepines

3.5 Synthesis of Various N-Heterocycles via Ozonolysis/Reductive Amination

3.5.1 Synthesis of Compounds with an Azepane Ring

3.5.2 Synthesis of Piperidine β-Amino Acids and Piperidine-Fused β-Lactams

3.5.3 Synthesis of γ-Lactams with a Piperidine Ring

3.5.4 Synthesis of other N-Heterocycles

4 Summary and Outlook

5 List of Abbreviations

Asymmetric Synthesis of 2,2-Difluorotetrahydrofurans through Palladium-Catalyzed Formal [3+2] Cycloaddition

The asymmetric synthesis of 2,2-difluorinated tetrahydrofurans was accomplished via enantioselective formal [3+2] cycloaddition catalyzed by palladium. The asymmetric reaction between gem-difluoroalkenes and racemic vinyl epoxides or vinylethylene carbonates resulted in the formation of enantioenriched 2,2-difluorotetrahydrofurans with an enantioselectivity up to 98 %. Notably, the reaction used the readily available (R)-BINAP as the ligand at a low loading and yielded a wide variety of difluorinated products in moderate to high yields. Both chiral diastereomers could be obtained in a single sequence.

Janus All-Cis 2,3,4,5,6-Pentafluorocyclohexyl Building Blocks Applied to Medicinal Chemistry and Bioactives Discovery Chemistry

Monoalkylated derivatives of the unusually polar all‐cis 2,3,4,5,6‐ pentafluorocyclohexyl (Janus face) motif are prepared starting from an aryl hydrogenation of 2,3,4,5,6‐ pentafluorophenylacetate methyl ester 15. The method used Zeng's Rh(CAAC) carbene catalyst 4 in the hydrogenation following the protocol developed by Glorius. The resultant Janus pentafluorocyclohexylacetate methyl ester 16 was converted to the corresponding alcohol 18, aldehyde 13, bromide 29 and azide 14 through functional group manipulations, and some of these building blocks were used in Ugi‐multicomponent and Cu‐catalysed click reactions. NBoc protected pentafluoroarylphenylalanine methyl ester 35 was also subject to an aryl hydrogenation, and then deprotection to generate the Janus face β‐pentafluorocyclohexyl‐alanine amino acid 15, which was incorporated into representative members of an emerging class of candidate antiviral compounds. Log P measurements demonstrate that the all‐cis 2,3,4,5,6‐pentafluorocyclohexyl ring system is more polar than a phenyl ring. In overview the paper introduces new building blocks containing this Janus ring and demonstrates their progression to molecules typically used in bioactives discovery programmes.

Three-component reaction of gem-difluorinated cyclopropanes with alkenes and B2pin2 for the synthesis of monofluoroalkenes

Borylative difunctionalization of alkenes has emerged as a powerful approach for synthesizing highly functionalized molecules. Herein, dual Cu/Pd-catalysed borylfluoroallylation of alkenes was smoothly achieved by using gem-difluorinated cyclopropanes and B2pin2, providing the corresponding monofluoroalkene scaffolds in moderate to high yields with excellent stereoselectivity. Moreover, an array of synthetic building blocks can be obtained by downstream transformations.

DFT Mechanistic Study of IrIII/NiII-Metallaphotoredox-Catalyzed Difluoromethylation of Aryl Bromides

The work by MacMillan et al. ( Angew. Chem., Int. Ed. 2018, 57, 12543-12548) developed an Ir^III/Ni^II-metallaphotoredox-catalyzed difluoromethylation strategy of aryl bromides using CHF₂Br as the CHF₂ reagent in the presence of tris(trimethylsilyl)silane. Here, we present a density functional theory (DFT)-based computational study to understand special dual catalysis promoting the C(sp²)-C(sp³) coupling. The calculated results show that the energetically more favorable pathway involves the reductive quenching of a photocatalyst (Ir^III/*Ir^III/Ir^II/Ir^III) and a Ni⁰-initiated catalytic cycle (Ni⁰/Ni^I/Ni^III/Ni^I/Ni⁰ or Ni⁰/Ni^II/Ni^III/Ni^I/Ni⁰). The calculations reveal not only the mechanistic details delivering the difluoromethylarene product but also the molecular-level picture of the generation of Ni⁰ species from the Ni^II precatalyst. Moreover, the calculations also rationalize the observed stoichiometric effect of CHF₂Br in the reactions of aryl bromides with different substituted groups.

Asymmetric α-electrophilic difluoromethylation of β-keto esters by phase transfer catalysis

An efficient and enantioselective α-electrophilic difluoromethylation of β-keto esters has been achieved by phase-transfer catalysis. This procedure is applicable to different kinds of β-keto esters with a series of cinchona-derived C-2' aryl-substituted phase-transfer catalysts. The reaction gives the corresponding products in good enantioselectivities (up to 83% ee) and yields (up to 92%) with high C/O regioselectivities (up to 98 : 2). Moreover, the C/O selectivity of β-keto esters could be easily reversed and controlled. This asymmetric difluoromethylation provided a novel and efficient way for introducing chiral C-CF2H groups.