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.

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.

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.

Applications of Transition Metal-Catalyzed ortho-Fluorine-Directed C-H Functionalization of (Poly)fluoroarenes in Organic Synthesis

The synthesis of organic compounds efficiently via fewer steps but in higher yields is desirable as this reduces energy and reagent use, waste production, and thus environmental impact as well as cost. The reactivity of C-H bonds ortho to fluorine substituents in (poly)fluoroarenes with metal centers is enhanced relative to meta and para positions. Thus, direct C-H functionalization of (poly)fluoroarenes without prefunctionalization is becoming a significant area of research in organic chemistry. Novel and selective methodologies to functionalize (poly)fluorinated arenes by taking advantage of the reactivity of C-H bonds ortho to C-F bonds are continuously being developed. This review summarizes the reasons for the enhanced reactivity and the consequent developments in the synthesis of valuable (poly)fluoroarene-containing organic compounds.

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.

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.

Pd-catalyzed access to mono- and di-fluoroallylic amines from primary anilines

The Pd-catalyzed highly selective synthesis of mono- and di-2-fluoroallylic amines from gem-difluorocyclopropanes and ubiquitous unprotected primary anilines is herein described. Initial kinetic investigations suggest a first order in the gem-difluorocyclopropane substrate, as well as a circa zeroth order in the aniline coupling partner. The newly produced fluoroallylic motifs should find important applications in synthetic as well as medicinal chemistry and stimulate the further development of coupling methods based on strained cyclic building blocks.

Cobalt-Catalyzed Fluoroallyllation of Carbonyls via C-C Activation of gem-Difluorocyclopropanes

A new Co-catalyzed sequential C-C and C-F activation of gem-difluorinated cyclopropanes (gem-FCPs) to form nucleophilic fluoroallylcobalt, followed by addition to aldehydes, is reported. The protocol features the regioselective cleavage of dual chemical bonds of readily available gem-FCPs to prepare easily separable linear (Z)- and (E)-fluorinated homoallylic alcohols with a broad scope. This discovery established a new strategy for the efficient transformation of gem-FCPs as well as the synthesis of challenging fluorinated homoallylic alcohols.

3d Metal Complexes in T-shaped Geometry as a Gateway to Metalloradical Reactivity

ConspectusLow-valent, low-coordinate 3d metal complexes represent a class of extraordinarily reactive compounds that can act as reagents and catalysts for challenging bond-activation reactions. The pursuit of these electron-deficient metal complexes in low oxidation states demands ancillary ligands capable of providing not only energetic stabilization but also sufficiently high steric bulk at the metal center. From this perspective, pincer ligands are particularly advantageous, as their prearranged, meridional coordination mode scaffolds the active center while the substituents of the peripheral donor atoms provide effective steric shielding for the coordination sphere. In a T-shaped geometry, the transition metal complexes possess a precisely defined vacant coordination site, which, combined with the often observed high-spin electron configuration, exhibits unusually high selectivity of these compounds with respect to one-electron redox chemistry. In light of the intractable reaction pathways typically observed with related electronically unsaturated 3d transition metal complexes, the pincer coordination mode enables the isolation of low-valent compounds with more controlled and unique reactivity. We have thus investigated a series of T-shaped metal(I) complexes using three different types of pincer ligands, which may be regarded as "metalloradicals" due to their selectively exposed unpaired electrons.These compounds display remarkably high thermal stability and represent rarely observed "naked" monovalent metal species featuring both monomeric and dimeric structures. Extensive reactivity studies using various organic substrates highlight a strong tendency of these paramagnetic compounds to undergo one-electron oxidation, leading to the isolation of a plethora of metal(II) species with reduced organic ligands as unusual structural elements. The exploration of C₂ symmetric T-shaped Ni(I) complexes as asymmetric catalysts also shows success in enantioselective hydrodehalogenation of geminal dihalogenides. In addition, this specific class of low-valent, low-coordinate complexes can be further diversified by introducing redox-active pincer ligands or building homobimetallic systems with two T-shaped units.This Account focuses on the discussion of selected examples of iron, cobalt, and nickel pincer complexes bearing a [P,N,P] or [N,N,N] donor set; however, their electronic structure and radical-type reactivity can be broadly extended to other pincer systems. The availability of various types of pincer ligands should allow fine-tuning of the reactivity of the T-shaped complexes. Given the unprecedented reactivity observed with these compounds, we expect the studies of T-shaped 3d metal complexes to be a fertile field for advancing base metal catalysis.

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.

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.

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

Palladium-Catalyzed C-H Allylation of Electron-Deficient Polyfluoroarenes with gem-Difluorinated Cyclopropanes

A palladium-catalyzed C-H allylation of electron-deficient polyfluoroarenes with gem-difluorinated cyclopropanes is reported. It provides a useful and facile approach to 2-fluoroallylic polyfluoroarenes in moderate to excellent yields with high Z-selectivity. In addition, this new approach has good functional group compatibility and broad substrate scope.

Highly Stereoselective Intramolecular Carbofluorination of Internal α,β-Ynones Promoted by Selectfluor

Herein, we report a metal-free intramolecular carbofluorination protocol for the synthesis of tetrasubstituted monofluoroalkenes from internal α,β-ynones and Selectfluor with both high stereoselectivity and broad functional group tolerance. The chelation between tetrafluoroborate anion and the oxygen present in the aldehyde group rendered the reaction highly stereoselective, with the tetrafluoroborate serving as the direct fluorine source. Therefore, with addition of sodium tetrafluoroborate, Selectfluor could be reused several times without sacrificing reactivity.

(2-Fluoroallyl)pyridinium tetrafluoroborates: novel fluorinated electrophiles for Pd-catalyzed allylic substitution

An efficient two-step approach to 2-fluoroallyl amines was developed that involves the synthesis of (2-fluoroallyl)pyridinium tetrafluoroborates from readily available gem-bromofluorocyclopropanes and the application of the former as novel and stable 2-fluoroallyl electrophiles for Pd-catalyzed allylic substitution.

Rhodium Catalyzed Regioselective C-H Allylation of Simple Arenes via C-C Bond Activation of Gem-difluorinated Cyclopropanes

Herein, we report a rhodium catalyzed directing-group free regioselective C-H allylation of simple arenes. Readily available gem-difluorinated cyclopropanes can be employed as highly reactive allyl surrogates via a sequence of C-C and C-F bond activation, providing allyl arene derivatives in good yields with high regioselectivity under mild conditions. The robust methodology enables facile late-stage functionalization of complex bioactive molecules. The high efficiency of this reaction is also demonstrated by the high turnover number (TON, up to 1700) of the rhodium catalyst on gram-scale experiments. Preliminary success on kinetic resolution of this transformation is achieved, providing a promising access to enantio-enriched gem-difluorinated cyclopropanes.

The preparation and properties of 1,1-difluorocyclopropane derivatives

Recently, the functionalization of organic molecules with fluorine substituents has grown rapidly due to its applications in such fields as medicine, agriculture or materials sciences. The aim of this article is to review the importance of 1,1-difluorocyclopropane derivatives in synthesis. It will examine the role of the fluorine substituents in both ring-forming and ring-opening reactions, as well as methods for obtaining difluorocyclopropanes as single enantiomers. Several examples are provided to highlight the biological importance of this class of compounds.

Palladium-catalyzed allylic alkylation dearomatization of β-naphthols and indoles with gem-difluorinated cyclopropanes

A palladium-catalyzed allylic alkylation dearomatization of β-naphthols and indoles with gem-difluorinated cyclopropanes has been developed for the first time.

Recent advances in pincer-nickel catalyzed reactions

Organometallic catalysts have played a key role in accomplishing numerous synthetically valuable organic transformations that are either otherwise not possible or inefficient. The use of precious, sparse and toxic 4d and 5d metals are an apparent downside of several such catalytic systems despite their immense success over the last several decades. The use of complexes containing Earth-abundant, inexpensive and less hazardous 3d metals, such as nickel, as catalysts for organic transformations has been an emerging field in recent times. In particular, the versatile nature of the corresponding pincer-metal complexes, which offers great control of their reactivity via countless variations, has garnered great interest among organometallic chemists who are looking for greener and cheaper alternatives. In this context, the current review attempts to provide a glimpse of recent developments in the chemistry of pincer-nickel catalyzed reactions. Notably, there have been examples of pincer-nickel catalyzed reactions involving two electron changes via purely organometallic mechanisms that are strikingly similar to those observed with heavier Pd and Pt analogues. On the other hand, there have been distinct differences where the pincer-nickel complexes catalyze single-electron radical reactions. The applicability of pincer-nickel complexes in catalyzing cross-coupling reactions, oxidation reactions, (de)hydrogenation reactions, dehydrogenative coupling, hydrosilylation, hydroboration, C-H activation and carbon dioxide functionalization has been reviewed here from synthesis and mechanistic points of view. The flurry of global pincer-nickel related activities offer promising avenues in catalyzing synthetically valuable organic transformations.

Asymmetric Transfer Hydrogenation of gem-Difluorocyclopropenyl Esters: Access to Enantioenriched gem-Difluorocyclopropanes

Catalytic enantioselective access to disubstituted functionalized gem-difluorocyclopropanes, which are emerging fluorinated motifs of interest in medicinal chemistry, was achieved through asymmetric transfer hydrogenation of gem-difluorocyclopropenyl esters, catalyzed by a Noyori-Ikariya (p-cymene)-ruthenium(II) complex, with (N-tosyl-1,2-diphenylethylenediamine) as the chiral ligand and isopropanol as the hydrogen donor. The resulting cis-gem-difluorocyclopropyl esters were obtained with moderate to high enantioselectivity (ee=66-99 %), and post-functionalization reactions enable access to valuable building blocks incorporating a cis- or trans-gem-difluorocyclopropyl motif.

Access to Divergent Fluorinated Enynes and Arenes via Palladium-Catalyzed Ring-Opening Alkynylation of gem-Difluorinated Cyclopropanes

Herein, we describe a palladium-catalyzed alkynylation of gem-difluorinated cyclopropanes via C-C bond activation/C-F bond cleavage, followed by C-C(sp) coupling. The new approach proceeds with broad substrate scope under mild reaction conditions, whereas both 1,1-disubstituted and complex-molecule-modified gem-difluorinated cyclopropanes react smoothly with high stereoselectivity. The developed method provides efficient and convenient ways access to diversity of important fluorinated enynes and arenes by slightly modification of the reaction conditions.

Catalyzing the Hydrodefluorination of CF3-Substituted Alkenes by PhSiH3. H• Transfer from a Nickel Hydride

The hydrodefluorination of CF₃-substituted alkenes can be catalyzed by a nickel(II) hydride bearing a pincer ligand. The catalyst loading can be as low as 1 mol%. gem-Difluoroalkenes containing a number of functional groups can be formed in good to excellent yields by a radical mechanism initiated by H• transfer from the nickel hydride. The relative reactivity of various substrates supports the proposed mechanism, as does a TEMPO trapping experiment.

Hydrodehalogenation of alkyl halides catalyzed by a trichloroniobium complex with a redox active α-diimine ligand

A high-valent d⁰ niobium(v) complex, (α-diimine)NbCl₃ (1), bearing a dianionic redox-active α-diimine ligand served as a catalyst for a hydrodehalogenation reaction of alkyl halides in the presence of PhSiH₃. During the catalytic reaction, the redox-active α-diimine ligand allowed the complex to reversibly release and accept one-electron through switching its coordination mode between a dianionic folded form and a monoanionic planar one.

Intramolecular C-H Functionalization Followed by a [2σ + 2π] Addition via an Intermediate Nickel-Nitridyl Complex

Irradiation of a disphenoidal Ni(II) azido complex, [Cz ^tBu(Pyr ^iPr)₂NiN₃] (1), revealed an unprecedented nickel complex, [Cz ^tBu(Pyr ^iPr)(NH₂-Pyr ^iPr)] (2), in >90% isolated yield. As evidenced by single-crystal X-ray diffraction, 2 is produced by double intramolecular C-H activation of a putative nickel-nitridyl intermediate, [Cz ^tBu(Pyr ^iPr)₂Ni-⃛N^•]. Calculations support the generation of an intermediate with significant nitridyl radical character after the loss of N₂, which, in turn, undergoes tandem C-H activations, leading to functionalized intermediates and products. This is an unprecedented example of transient Ni-⃛N^•-promoted intramolecular C-H functionalization, followed by a [2_σ + 2_π] addition, yielding bis-metallacyclic product 2. Complex 2 is also observed from the reaction of Ni(I) precursor Cz ^tBu(Pyr ^iPr)₂Ni (3) and Me₃SiN₃, suggesting a unique thermal route toward a masked nickel-nitridyl intermediate.

Low-coordinate first-row transition metal complexes in catalysis and small molecule activation

In this Perspective, we will highlight selected examples of transition metal complexes with low coordination numbers whose high reactivity has been exploited in catalysis and the activation of small molecules featuring strong bonds (N₂, CO₂, and CO).

The open d-shell enforces the active space in 3d metal catalysis: highly enantioselective chromium(ii) pincer catalysed hydrosilylation of ketones

A square-planar chromium precatalyst with its active space shaped by the metal centre was employed in an unprecedented enantioselective hydrosilylation of ketones.

Redox-state dependent activation of silanes and ammonia with reverse polarity (PCcarbeneP)Ni complexes: electrophilic vs. nucleophilic carbenes

A rigidified PC_alkylP ligand allowed for the synthesis and characterization of cationic and radical PC_CarbeneP nickel complexes in which the carbene anchor of the pincer framework is electrophilic rather than nucleophilic.