Structure, synthesis, and chemical reactions of fluorinated cyclopropanes and cyclopropenes

Difluorocarbene-Promoted O-O Bond Activation of Peroxy Acids for Electrophilic Carboxylation of Boronic Acids

In this study, a difluorocarbene-promoted O-O bond activation of peroxy acids is developed through the insertion of difluorocarbene into O-H bond. This activation strategy in synergy with O-B coordination with boronic acids/ester greatly polarizes the O-O bond for in-situ generation of carboxylium species that reacts with the nucleophilic part of boronic acids in a concerted way to produce carboxylic esters. Good efficiency and functional group tolerance are demonstrated. Application of this method to the functionalization of a boronic acid drug used as HSL enzyme inhibitor produces smoothly the ester derivative. This difluorocarbene-mediated O-O bond activation strategy is conceptually different from traditional radical type methods, and is also complementary to conventional esterification methods with a distinct retro-synthetic disconnection.

Double strain-release enables formal C-O/C-F and C-N/C-F ring-opening metathesis

Metathesis reactions have been established as a powerful tool in organic synthesis. While great advances were achieved in double-bond metathesis, like olefin metathesis and carbonyl metathesis, single-bond metathesis has received less attention in the past decade. Herein, we describe the first C(sp3)-O/C(sp3)-F bond formal cross metathesis reaction between gem-difluorinated cyclopropanes (gem-DFCPs) and epoxides under rhodium catalysis. The reaction involves the formation of a highly electrophilic fluoroallyl rhodium intermediate, which is capable of reacting with the oxygen atom in epoxides as weak nucleophiles followed by C-F bond reconstruction. The use of two strained ring substrates is the key to the success of the formal cross metathesis, in which the double strain release accounts for the driving force of the transformation. Additionally, azetidine also proves to be a suitable substrate for this transformation. The reaction offers a novel approach for the metathesis of C(sp3)-O and C(sp3)-N bonds, presenting new opportunities for single-bond metathesis.

Palladium/Amine Dual-Catalyzed Tsuji-Trost Fluoroallylation of Aldehydes with gem-Difluorinated Cyclopropanes

We herein disclose the Pd/amine dual-catalyzed ring-opening cross-coupling reaction between gem-difluorinated cyclopropanes (gem-F2CPs) with aldehydes, which enables the diversity-oriented synthesis (DOS) of 2-fluoroallylic aldehydes bearing all-carbon quaternary centers with features of broad scope and excellent functional group tolerance. The synthetic value of this Tsuji-Trost system was further demonstrated by late-stage functionalization of natural product-derived gem-F2CPs and the diverse synthesis of various fluoroallylic aldehyde derivatives, including alcohol, alkyne, alkene, and amine.

B(C6F5)3/CPA-Catalyzed Aza-Diels-Alder Reaction of 3,3-Difluoro-2-Aryl-3H-indoles and Unactivated Dienes

Here we report B(C6F5)3/CPA-catalyzed enantioselective aza-Diels-Alder reaction of 3,3-difluoro-2-Aryl-3H-indoles with unactivated dienes to access chiral 10,10-difluoro-tetrahydropyrido[1,2-a]indoles. This protocol allows the formation of pyrazole-based C2-quaternary indolin-3-ones with high enantioselectivities and regioselectivities. Moreover, gram-scale synthesis of the 10,10-difluoro-tetrahydropyrido[1,2-a]indole skeleton was successfully achieved without any reduction in both yield and enantioselectivity.

Enantioselective Tsuji-Trost α-Fluoroallylation of Amino Acid Esters with Gem-Difluorinated Cyclopropanes

A novel enantioselective Tsuji-Trost-type cross coupling reaction between gem-difluorinated cyclopropanes and N-unprotected amino acid esters enabled by synergistic Pd/Ni/chiral aldehyde catalysis is presented herein. This transformation streamlined the diversity-oriented synthesis (DOS) of optically active α-quaternary α-amino acid esters bearing a linear 2-fluoroallylic motif, which served as an appealing platform for the construction of other valuable enantioenriched compounds. The key intermediates were confirmed by HRMS detection, while DFT calculations revealed that the excellent enantioselectivity was attributed to the stabilizing non-covalent interactions between the Pd(II)-π-fluoroallyl species and the Ni(II)-Schiff base complex.

Rhodium-Catalyzed Enantio- and Regioselective Allylation of Indoles with gem-Difluorinated Cyclopropanes

The use of gem-difluorinated cyclopropanes (gem-DFCPs) as fluoroallyl surrogates under transition-metal catalysis has drawn considerable attention recently but such reactions are restricted to producing achiral or racemic mono-fluoroalkenes. Herein, we report the first enantioselective allylation of indoles under rhodium catalysis with gem-DFCPs. This reaction shows exceptional branched regioselectivity towards rhodium catalysis with gem-DFCPs, which provides an efficient route to enantioenriched fluoroallylated indoles with wide substrate scope and good functional group tolerance.

High-Valent Copper Catalysis Enables Regioselective Fluoroarylation of Gem-Difluorinated Cyclopropanes

Transition-metal (TM) catalyzed reaction of gem-difluorinated cyclopropanes (gem-DFCPs) has drawn much attention recently. The reaction generally occurs via the activation of the distal C─C bond in gem-DFCPs by a low-valent TM through oxidative addition, eventually producing mono-fluoro olefins as the coupling products. However, achieving regioselective activation of the proximal C─C bond in gem-DFCPs that overcomes the intrinsic reactivity via TM catalysis remains elusive. Here, a new reaction mode of gem-DFCPs enabled by high-valent copper catalysis, which allows exclusive activation of the congested proximal C─C bond is presented. The reaction that achieves fluoroarylation of gem-DFCPs uses NFSI (N-fluorobenzenesulfonimide) as electrophilic fluoro reagent and arenes as the C─H nucleophiles, enabling the synthesis of diverse CF3-containing scaffolds. It is proposed that a high-valent copper species plays an important role in the regioselective activation of the proximal C─C bond possibly via a σ-bond metathesis.

Axially Modified Square-Pyramidal CoN4-F1 Sites Enabling High-Performance Zn-Air Batteries

Cobalt-nitrogen-carbon (Co-N-C) catalysts with a CoN4 structure exhibit great potential for oxygen reduction reaction (ORR), but the imperfect adsorption energy toward oxygen species greatly limits their reduction efficiency and practical application potential. Here, F-coordinated Co-N-C catalysts with square-pyramidal CoN4-F1 configuration are successfully synthesized using F atoms to regulate the axial coordination of Co centers via hydrothermal and chemical vapor deposition methods. During the synthesis process, the geometry structure of the Co atom converts from six-coordinated Co-F6 to square-pyramidal CoN4-F1 in the coordinatively unsaturated state, which provides an open binding site for the O2. The introduction of axial F atoms into the CoN4 plane alters the local atomic environment around Co, significantly improving the ORR activity and Zn-air batteries performance. In situ spectroscopy proves that CoN4-F1 sites strongly combine with the OOH* intermediate and facilitate the splitting of O-O bond, making OOH* readily decompose into O* and OH* via a dissociative pathway. Theoretical calculations confirm that the axial F atom effectively reduces the electronic density of the Co centers and facilitates the desorption of the OH* intermediate, efficiently accelerating the overall ORR kinetics. This work advances a feasible synthesis mechanism of axial ligands and provides a route to construct efficient high-coordination catalysts.

Selectivity in Rh-catalysis with gem-difluorinated cyclopropanes

Small-ring chemistry is a fascinating field in organic chemistry. gem-Difluorinated cyclopropanes, a unique class of cyclopropanes, have garnered significant interest due to their intrinsic high reactivity. In this context, gem-difluorinated cyclopropanes have been extensively investigated as fluoroallylic synthons in Pd-catalyzed ring-opening/cross-coupling reactions for the synthesis of monofluoroalkenes with linear or branched selectivity. In contrast, Rh-catalysis has revealed diverse selectivity in the reaction of gem-difluorinated cyclopropanes, such as regioselectivity, enantioselectivity, and chemoselectivity. This feature article aims to summarize our efforts towards developing Rh-catalyzed reactions of gem-difluorinated cyclopropanes, briefly discussing the design, selectivity, reaction mechanisms and future research prospects.

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.

Pd/IPrBIDEA-Catalyzed Hydrodefluorination of gem-Difluorocyclopropanes: Regioselective Synthesis of Terminal Fluoroalkenes

Developing new strategies to enable chemo- and regioselective reductions is an important topic in chemical research. Herein, an efficient and regioselective Pd/IPrBIDEA-catalyzed ring-opening hydrodefluorination of gem-difluorocyclopropanes to access terminal fluoroalkenes is developed. The success of this transformation was attributed to the use of 3,3-dimethylallyl Bpin as a novel hydride donor. DFT calculations suggest that a direct 3,4'-hydride transfer via a 9-membered cyclic transition state is more favorable, which combined with the irreversibility of the reaction enables the unusual selectivity for the less thermodynamically stable terminal alkene isomer. This reaction mode is also applicable to a variety of regioselective allylic and propargyl reductions.

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.

Palladium-Catalyzed Fluorinative Bifunctionalization of Aziridines and Azetidines with gem-Difluorocyclopropanes

An unprecedented Pd-catalyzed fluorinative bifunctionalization of aziridines and azetidines was successfully developed via regioselective C-C and C-F bond cleavage of gem-difluorocyclopropanes, leading to various β,β'-bisfluorinated amines and β,γ-bisfluorinated amines. This reaction was achieved by incorporating a 2-fluorinated allyl group and a fluorine atom scissored from gem-difluorocyclopropane in 100 % atom economy for the first time. The mechanistic investigations indicated that the reaction underwent amine attacking 2-fluorinated allyl palladium complex to generate η2 -coordinated N-allyl aziridine followed by fluoride ligand transfer affording the final β- and γ-fluorinated amines.

Reductive Catalytic Difluorocarbene Transfer via Palladium Catalysis

A palladium-catalyzed reductive difluorocarbene transfer reaction that tames difluorocarbene to couple with two electrophiles has been developed, representing a new mode of difluorocarbene transfer reaction. The approach uses low-cost and bulk industrial chemical chlorodifluoromethane (ClCF2 H) as the difluorocarbene precursor. It produces a variety of difluoromethylated (hetero)arenes from widely available aryl halides/triflates and proton sources, featuring high functional group tolerance and synthetic convenience without preparing organometallic reagents. Experimental mechanistic studies reveal that an unexpected Pd0/II catalytic cycle is involved in this reductive reaction, wherein the oxidative addition of palladium(0) difluorocarbene ([Pd0 (Ln )]=CF2 ) with aryl electrophile to generate the key intermediate aryldifluoromethylpalladium [ArCF2 Pd(Ln )X], followed by reaction with hydroquinone, is responsible for the reductive difluorocarbene transfer.

Synthesis of Selectively gem-Difluorinated Molecules; Chiral gem-Difluorocyclopropanes via Chemo-Enzymatic Reaction and gem-Difluorinated Compounds via Radical Reaction

The incorporation of fluorine atoms into an organic compound can alter the chemical reactivity or biological activity of the resulting compound due to the strong electron withdrawing nature of the fluorine atom. We have synthesized many original gem-difluorinated compounds and described the results in four sections. The first section describes the synthesis of optically active-gem-difluorocyclopropanes via the chemo-enzymatic reaction; we applied these compounds to liquid crystalline molecules, then further discovered a potent DNA cleavage activity for the gem-difluorocyclopropane derivatives. The second section describes the synthesis of selectively gem-difluorinated compounds via a radical reaction; we synthesized fluorinated analogues of a sex pheromone of the male African sugarcane borer, Eldana saccharina, and used the compounds as proof for investigating the origin of pheromone molecule recognition on the receptor protein. The third involves the synthesis of 2,2-difluorinated-esters by visible light-driven radical addition of 2,2-difluoroacetate with alkenes or alkynes in the presence of an organic pigment. The last section describes the synthesis of gem-difluorinated compounds via the ring-opening of gem-difluorocyclopropanes. We further developed a novel method of synthesizing gem-difluorohomoallylic alcohols via the ring-opening of gem-difluorocyclopropane and aerobic oxidation by photo-irradiation in the presence of an organic pigment. Since gem-difluorinated compounds that were prepared by the present method have two olefinic moieties with a different reactivity at the terminal position, we accomplished the synthesis of four types of gem-difluorinated cyclic alkenols via the ring-closing-metathesis (RCM) reaction.

Copper-catalysed difluorocarbene transfer enables modular synthesis

The use of metal catalysts to produce and control the reactivity of carbenes has long offered a powerful approach to organic synthesis; however, difluorocarbene transfer catalysed by metal is an outlier and remains a substantial challenge. In that context, copper difluorocarbene chemistry has been elusive so far. Here we report the design, synthesis, characterization and reactivity of isolable copper(I) difluorocarbene complexes, which enable the development of a copper-catalysed difluorocarbene transfer reaction. The method offers a strategy for the modular synthesis of organofluorine compounds from simple and readily available components. This strategy facilitates a modular difluoroalkylation by coupling difluorocarbene with two inexpensive feedstocks, silyl enol ethers and allyl/propargyl bromides, in a one-pot reaction via copper catalysis, providing a diversity of difluoromethylene-containing products without laborious multistep synthesis. The approach enables access to various fluorinated skeletons of medicinal interest. Mechanistic and computational studies consistently reveal a mechanism involving nucleophilic addition to an electrophilic copper(I) difluorocarbene.

Generation of difluorocarbenes and introduction of fluorinated one carbon units into carbonyl and related compounds

Difluorocarbene is a simple and versatile one-carbon unit for synthesizing acyclic and cyclic organofluorine compounds. However, the use of difluorocarbene in organic synthesis has been relatively limited because of the harsh conditions required for its generation, the toxicity of the precursors, and undesired dimerization. This feature article provides an account of (i) the generation of free and metal difluorocarbenes from trimethylsilyl 2,2-difluoro-2-(fluorosulfonyl)acetate (TFDA) or BrCF₂CO₂Li/Na and (ii) their application to the facile synthesis of valuable organofluorine compounds. The difluorocarbenes thus generated react with (thio)carbonyl compounds and silyl dienol ethers to provide a wide variety of products such as (a) difluoromethyl (thio)ethers, (b) fluorinated thiophenes, (c) fluorinated thia/oxazoles, (d) fluorinated cyclopentanones and (e) difluoroalkenes.

Transition-Metal-Free Controllable Single and Double Difluoromethylene Formal Insertions into C-H Bonds of Aldehydes with TMSCF2 Br

We have developed a new strategy for controllable single and double difluoromethylene (CF₂ ) formal insertions into C-H bonds of aldehydes with nearly full selectivity under transition-metal-free conditions. The key to the success of controllable CF₂ insertions lies in the well-defined formation of 2,2-difluoroenolsilyl ether and 2,2,3,3-tetrafluorocyclopropanolsilyl ether intermediates using difluorocarbene reagent TMSCF₂ Br (TMS=trimethylsilyl). These two intermediates can react with various electrophiles including proton sources and various halogenation reagents, allowing for the access to diverse arrays of ketones containing difluoromethylene (CF₂ ) and tetrafluoroethylene (CF₂ CF₂ ) units. The first synthesis of relatively stable 2,2,3,3-tetrafluorocyclopropanolsilyl ethers has been achieved, which offers a new platform to explore other unknown chemical space.

Difluorocarbene-Triggered Acyl Rearrangement Reaction: A Strategy for the Direct Introduction of the gem-Difluoromethylene Group

A novel metal- and catalyst-free dearomative reaction of 2-oxypyridines to construct gem-difluoromethylenated N-substituted 2-pyridones has been developed. The reaction involves an attractive acyl rearrangement from O to CF₂ of difluorocarbene-derived pyridinium ylides, which provides a new strategy for the direct introduction of the gem-difluoromethylene group with high efficiency and selectivity as well as broad substrate scope. Gram-scale synthesis and synthetic transformations have also been demonstrated.

Density Functional Theory Analysis of the Copolymerization of Cyclopropenone with Ethylene Using a Palladium Catalyst

Density functional theory has been used to elucidate the mechanism of Pd copolymerization of cyclopropenone with ethylene. The results reveal that introducing ethylene and cyclopropenone to Pd catalyst is thermodynamically feasible and generates the α,β-unsaturated ketone unit (UnitA). Cis-mode insertion and Path A_1a are the most favorable reaction routes for ethylene and cyclopropenone, respectively. Moreover, cyclopropenone decomposition can generate CO in situ without a catalyst or with a Pd catalyst. The Pd-catalyzed decomposition of cyclopropenone exhibits a lower reaction barrier (22.7 kcal/mol) than its direct decomposition. Our study demonstrates that incorporating CO into the Pd catalyst can generate the isolated ketone unit (UnitB). CO is formed first; thereafter, UnitB is generated. Therefore, the total energy barrier of UnitB generation, accounting for the CO barrier, is 22.7 kcal/mol, which is slightly lower than that of UnitA generation (24.0 kcal/mol). Additionally, the possibility of copolymerizing ethylene, cyclopropenone, and allyl acetate (AAc) has been investigated. The free energy and global reactivity index analyses indicate that the cyclopropenone introduction reaction is more favorable than the AAc insertion, which is consistent with the experimental results. Investigating the copolymerization mechanism will help to develop of a functionalization strategy for polyethylene polymers.

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.

Rh-catalyzed regio-switchable cross-coupling of gem-difluorinated cyclopropanes with allylboronates to structurally diverse fluorinated dienes

The control of linear/branched selectivity is one of the major focuses in transition-metal catalyzed allyl-allyl cross-coupling reactions, in which bond connection occurs at the terminal site of both the allyl fragments forming different types of 1,5-dienes. Herein, terminal/internal regioselectivity is investigated and found to be switchable in allyl-allyl cross-coupling reactions between gem-difluorinated cyclopropanes and allylboronates. The controlled terminal/internal regioselectivity arises from the fine-tuning of the rhodium catalytic system. Fluorinated 1,3-dienes, 1,4-dienes and 1,5-dienes are therefore produced in good yields with respectively isomerized terminal, internal, and terminal regioselectivity.

Three-Component Coupling of Anilines, Amines, and Difluorocarbene to Access Formamidines

A one-pot three-component reaction of two anilines (or one aniline and one alkylamine) and in situ-generated difluorocarbene is developed herein to enable efficient construction of formamidines. Crucial formimidoyl fluoride intermediate RN═CHF is proposed from the reaction of a primary aniline and difluorocarbene. Ensuing nucleophilic iminyl substitution of this intermediate with a second amine allows cross-condensation of the two amines to produce formamidines. When only one type of primary aniline is used as the substrate, the difluoromethylated homo-condensation products can also be produced under a 1:1 molar ratio of aniline/difluorocarbene. Intramolecular variant of this method allows concise synthesis of benzimidazoquinazolines and nitrogen-fused/spirocyclic compounds, showing the potential of this method in organic synthesis. More interesting reactions are anticipated by exploiting the reactivity of difluorocarbene and primary amines to isocyanides or the formimidoyl fluoride intermediates.

The reactivity of the trapping reaction of the benzene-bridged boron/phosphorus-based frustrated Lewis pair with difluorocarbene and its group 14 analogs: A theoretical investigation

The trapping reactions of carbene analogs G14F₂ (G14 = group 14 element) by the benzene-bridged B/P-Rea frustrated Lewis pair (FLPs) molecule are studied using density functional theory (B3LYP-D3(BJ)/def2-TZVP). Our theoretical investigations predict that only the CF₂ intermediate rather than other heavy carbene analogs can be trapped by the B/P-Rea FLP-type molecule. Energy decomposition analysis-natural orbitals for chemical valence (EDA-NOCV) analyses indicate that the bonding nature of the G14F₂ catching reactions by the B/P-Rea FLP-type molecule is a donor-acceptor (singlet-singlet) interaction rather than an electron-sharing (triplet-triplet) interaction. Moreover, EDA-NOCV and frontier molecular orbital (FMO) theory findings strongly suggest that the lone pair (LP) (P) → vacant p-π-orbital (G14F₂ ) interaction rather than the empty σ-orbital (B) ← sp² -σ-orbital (G14F₂ ) interaction plays a predominant role in establishing its bonding condition during the G14F₂ trapping reaction with the B/P-Rea FLP-associated molecule. Our activation strain model findings reveal that the atomic radius of the G14 element of G14F₂ plays a key role in determining the activation barrier of the G14F₂ trapping reactions by the benzene-bridged B/P-Rea FLP. The valence bond state correlation diagram (VBSCD) model developed by Shaik is used to rationalize the calculated results. The VBSCD findings demonstrate that in the present trapping reactions, the singlet triplet splitting of G14F₂ plays a significant role in influencing its reaction barrier and reaction enthalpy. Our theoretical results demonstrate that the relationship between the geometrical parameters of the transition states and the corresponding reaction free energy barriers agrees well with the findings based on the Hammond postulate.

Enantioselective Formation of All-Carbon Quaternary Stereocenters in gem-Difluorinated Cyclopropanes via Rhodium-Catalyzed Stereoablative Kinetic Resolution

Herein, we report an effective method to offer chiral gem-difluorinated cyclopropanes containing an all-carbon quaternary stereocenter by rhodium-catalyzed stereoablative kinetic resolution. The activation of a sterically hindered all-carbon quaternary C-C bond through oxidative addition with a chiral rhodium complex is proposed as the enantiodetermining step. A wide range of gem-difluorinated cyclopropanes can be obtained with excellent ee values (ee = 87% to >99.9%), which are demonstrated to be useful chiral fluorine-containing building blocks by a series of postfunctionalizations.

M-CPOnes: transition metal complexes with cyclopropenone-based ligands for light-triggered carbon monoxide release

A new class of CO-releasing molecules, M-CPOnes, was prepared combining cyclopropenone-based ligands for CO release with the modular scaffold of transition metal complexes. In proof-of-concept studies, M-CPOnes based on Zn^II, Fe^II and Co^II are stable in the dark but undergo light-triggered CO release with the cyclopropenone substituents and metal ions enabling tuning of the photophysical properties. Furthermore, the choice of metal allows the use of different spectroscopic methods to monitor photodecarbonylation from fluorescence spectroscopy to UV/vis spectroscopy and paramagnetic NMR spectroscopy. The modularity of M-CPOnes from the metal ion to the cyclopropenone substitution and potential for further functionalisation of the ligand make M-CPOnes appealing for tailored functionality in applications.

Copper-Catalyzed Ring-Opening Defluoroborylation of gem-Difluorinated Cyclobutenes: A General Route to Bifunctional 1,3-Dienes and Their Applications

The exploration of the reactivity of gem-difluorinated small-size rings has continuously drawn attention in recent years but is limited to three-membered carbocycles. Herein we report a copper-catalyzed reaction of gem-fluorinated cyclobutenes with bis(pinacolato)diboron (B₂pin₂). A sequence of defluoroborylation and a ring-opening process produces B,F-bifunctional 1,3-dienes in a stereoselective manner. The transformation together with the efficient downstream coupling of the boronate and the fluoride moieties collectively constitutes a modular route to highly functionalized and stereocontrolled 1,3-dienes.

Difluoromethylation of Heterocycles via a Radical Process

Difluoromethylation is of prime importance for its applicability in functionalizing diverse fluorine-containing heterocycles, which are core moieties of various biologically and pharmacologically active ingredients. Due to their significant biological and...

TMSCF2 Br-Enabled Fluorination-Aminocarbonylation of Aldehydes: Modular Access to α-Fluoroamides

A protocol for the modular assembly of the α-fluoroamide motif has been developed, which provides a practical method for the efficient synthesis of structurally diverse α-fluoroamides from easily available aldehydes and tertiary amines through a three-component fluorination-aminocarbonylation process. The key to the success of this process is taking advantage of the multiple roles of the unique difluorocarbene reagent TMSCF₂ Br (TMS=trimethylsilyl). The mechanism of the process involves the 1,2-fluorine and oxygen migrations of the in situ formed TMS-protected α-aminodifluoromethyl carbinol intermediates, which represents a new type of deoxyfluorination reaction.

Ligand-controlled regioselective and chemodivergent defluorinative functionalization of gem-difluorocyclopropanes with simple ketones

Modulating the reaction selectivity is highly attractive and pivotal to the rational design of synthetic regimes. The defluorinative functionalization of gem-difluorocyclopropanes constitutes a promising route to construct β-vinyl fluorine scaffolds, whereas chemo- and regioselective access to α-substitution patterns remains a formidable challenge. Presented herein is a robust Pd/NHC ligand synergistic strategy that could enable the C-F bond functionalization with exclusive α-regioselectivity with simple ketones. The key design adopted enolates as π-conjugated ambident nucleophiles that undergo inner-sphere 3,3'-reductive elimination warranted by the sterically hindered-yet-flexible Pd-PEPPSI complex. The excellent branched mono-defluorinative alkylation was achieved with a sterically highly demanding IHept ligand, while subtly less bulky SIPr acted as a bifunctional ligand that not only facilitated α-selective C(sp3)-F cleavage, but also rendered the newly-formed C(sp2)-F bond as the linchpin for subsequent C-O bond formation. These examples represented an unprecedented ligand-controlled regioselective and chemodivergent approach to various mono-fluorinated terminal alkenes and/or furans from the same readily available starting materials.

Double Capture of Difluorocarbene by 2-Aminostyrenes Enables the Construction of 3-(2,2-Difluoroethyl)-2-fluoroindoles

We report herein an efficient strategy to construct 3-(2,2-difluoroethyl)-2-fluoroindoles from activated o-aminostyrenes with ethyl bromodi-fluoroacetate as a difluorocarbene source. Through double capture of a difluorocarbene, two different types of fluorine motifs are incorporated into the products with simultaneous construction of one C-N and two C-C bonds, without the need for transition metals. This reaction features high efficiency and excellent functional group compatibility and has great potential in the late-stage modifications of pharmaceutical molecules and natural products.

Difluorocarbene-Induced Ring-Opening Difluoromethylation-Halogenation of Cyclic (Thio)Ethers with TMSCF2 X (X=Br, Cl)*

The ring-opening difluoromethylation-halogenation of cyclic (thio)ethers is reported through a simple strategy relying on carbon-chalcogen bond activation with difluorocarbene. The reaction proceeds through in situ protonation of the previously little-known difluoromethylene oxonium or sulfonium ylide intermediate followed by ring-opening with halide ion to afford halogenated acyclic difluoromethyl (thio)ethers that can then be employed for further elaboration. TMSCF₂ X (X=Br, Cl) are unique reagents to achieve this synthetic purpose, which serve as both the difluorocarbene source and the halide ion source.

Three-Component Couplings among Heteroarenes, Difluorocyclopropenes, and Water via C-H Activation

Three-component couplings have been realized for efficiently constructing various nitrogen-containing skeletons via C-H activation, where difluorocyclopropenes have been first identified as coupling partners. Many substrates including sp² and sp³ C-H substrates were well tolerated, furnishing the corresponding products in good yields. Furthermore, a catalyst-dependent reaction was also developed, enabling divergent construction of two different frameworks. The application value of these reactions was demonstrated in gram-scale experiments with as little as 1 mol % catalyst.

Synthesis of Quinolines via the Metal-free Visible-Light-Mediated Radical Azidation of Cyclopropenes

We report the synthesis of quinolines using cyclopropenes and an azidobenziodazolone (ABZ) hypervalent iodine reagent as an azide radical source under visible-light irradiation. Multisubstituted quinoline products were obtained in 34-81% yield. The reaction was most efficient for 3-trifluoromethylcyclopropenes, affording valuable 4-trifluoromethylquinolines. The transformation probably proceeds through the cyclization of an iminyl radical formed by the addition of the azide radical on the cyclopropene double bond, followed by ring-opening and fragmentation.

Rhodium-Catalyzed Direct Allylation of Simple Arenes by Using Gem-Difluorinated Cyclopropanes as Allyl Surrogates

Gem-difluorinated cyclopropanes have become an important type of allyl surrogate in transition-metal-catalyzed ring-opening processes, as demonstrated recently through various important advances, especially with palladium catalysis. The versatile fluorinated allyl species generated in this way from gem-difluorinated cyclopropanes exhibit unique advantages compared with conventional allyl sources. By using gem-difluorinated cyclopropanes as allyl surrogates, we achieved a direct allylation of simple arenes through rhodium catalysis under mild conditions. This transformation permits directing-group-free allylation of simple arenes, including electron-neutral, electron-rich, and electron-deficient ones. Here, we give a brief introduction to this area and we discuss our thoughts regarding our recent work and its design.

1 Introduction

2 Our Design

3 Condition Optimization and Substrate Scope

4 Applications in Synthesis

5 Mechanistic Discussions

6 Conclusion and Outlook