Site-Divergent Alkenyl C–H Fluoroallylation of Olefins Enabled by Tunable Rhodium Catalysis

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.

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.

Observation of unusual outer-sphere mechanism using simple alkenes as nucleophiles in allylation chemistry

Transition-metal catalyzed allylic substitution reactions of alkenes are among the most efficient methods for synthesizing diene compounds, driven by the inherent preference for an inner-sphere mechanism. Here, we present a demonstration of an outer-sphere mechanism in Rh-catalyzed allylic substitution reaction of simple alkenes using gem-difluorinated cyclopropanes as allyl surrogates. This unconventional mechanism offers an opportunity for the fluorine recycling of gem-difluorinated cyclopropanes via C - F bond cleavage/reformation, ultimately delivering allylic carbofluorination products. The developed method tolerates a wide range of simple alkenes, providing access to secondary, tertiary fluorides and gem-difluorides with 100% atom economy. DFT calculations reveal that the C - C bond formation goes through an unusual outer-sphere nucleophilic substitution of the alkenes to the allyl-Rh species instead of migration insertion, and the generated carbon cation then forms the C - F bond with tetrafluoroborate as a fluoride shuttle.

DFT Calculations Rationalize Unconventional Regioselectivity in PdII-Catalyzed Defluorinative Alkylation of gem-Difluorocyclopropanes with Hydrazones

Density functional theory (DFT) calculations have been conducted to gain insight into the unique formation of the branched alkylation product in the PdII-catalyzed defluorinative alkylation of gem-difluorocyclopropanes with hydrazones. The reaction is established to occur in sequence through oxidative addition, β-F elimination, η1-η3 isomerization, transmetalation, η3-η1 isomerization, 3,3'-reductive elimination, deprotonation/N2 extrusion, and proton abstraction. The rate-determining step of the reaction is identified as the β-F elimination, featuring an energy barrier of 28.6 kcal/mol. The 3,3'-reductive elimination transition states are the regioselectivity-determining transition states. The favorable noncovalent π-π interaction between the naphthyl group of gem-difluorocyclopropane and the phenyl group of hydrazone is found to be mainly responsible for the observed regioselectivity.

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.

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.

Sn(OTf)2-Catalyzed (3 + 2) Cycloaddition/Sulfur Rearrangement Reaction of Donor-Acceptor Cyclopropanes with Indoline-2-thiones

The (3 + 2) cycloaddition/sulfur rearrangement reaction of donor-acceptor cyclopropanes bearing a single keto acceptor with indoline-2-thiones has been realized. Under the catalysis of Sn(OTf)2, a series of functionalized 3-indolyl-4,5-dihydrothiophenes were synthesized with moderate to excellent yields.

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.

Aromatic nitrogen scanning by ipso-selective nitrene internalization

Nitrogen scanning in aryl fragments is a valuable aspect of the drug discovery process, but current strategies require time-intensive, parallel, bottom-up synthesis of each pyridyl isomer because of a lack of direct carbon-to-nitrogen (C-to-N) replacement reactions. We report a site-directable aryl C-to-N replacement reaction allowing unified access to various pyridine isomers through a nitrene-internalization process. In a two-step, one-pot procedure, aryl azides are first photochemically converted to 3H-azepines, which then undergo an oxidatively triggered C2-selective cheletropic carbon extrusion through a spirocyclic azanorcaradiene intermediate to afford the pyridine products. Because the ipso carbon of the aryl nitrene is excised from the molecule, the reaction proceeds regioselectively without perturbation of the remainder of the substrate. Applications are demonstrated in the abbreviated synthesis of a pyridyl derivative of estrone, as well as in a prototypical nitrogen scan.

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.

Synthesis of α-Difluoromethylene Ethers via Photoredox-Induced Hyperconjugative Ring Opening of gem-Difluorocyclopropanes

Fluorinated compounds have found widespread applications in pharmaceuticals, agrochemicals, and materials science. Precise construction of α-difluoromethylene ether (CF₂-O) moiety in organic molecules is of high demand. Herein, a visible light-promoted reaction protocol for the synthesis of α-difluoromethylene ether from gem-difluorocyclopropane is described. The key ring-opening step is induced by hyperconjugative interaction of cyclopropane with photo-oxidized aromatic rings. This reaction is easy scale-up, and the products bearing a synthetic handle enable their further manipulation.

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.

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.