Generation of Axially Chiral Fluoroallenes through a Copper-Catalyzed Enantioselective β-Fluoride Elimination

Electrochemical synthesis of allenyl silanes and allenyl boronic esters

Allenyl silanes and boronates are pivotal building blocks in organic synthesis. Nevertheless, their synthesis requires the manipulation of transition metal or highly reactive species. Hence, the development of more sustainable protocol is highly sought after. Here we show the electrochemical synthesis of allenyl silanes and allenyl boronic esters. This catalyst-free method proceeds under mild reaction conditions. The protocol for the synthesis of allenyl silanes shows an excellent efficiency and a good functional group tolerance. The allenyl silanes are isolated in good yields (28 examples, 45-95% yields) without the use of a transition metal catalyst and under mild reaction conditions. A similar protocol is developed for the synthesis of allenyl boronates, which are obtained in low to moderate yields (13 examples, 5-55% yields). Finally, a mechanism based on an oxidative generation of the silyl and boryl radicals is suggested to access these classes of allenes.

Capturing Unstable Carbanionic Intermediates via Halogen Transfer: Base-Promoted Oxidative Coupling Reactions of α,α-Difluoromethylarenes

We describe how the merger of deprotonation, halogenation, and substitution into compatible processes enables the productive functionalization of traditionally unstable carbanionic intermediates. This strategy enables the first oxidative coupling protocol of α,α-difluorobenzylic C─H bonds with heteronucleophiles. Here, transiently generated α,α-difluorobenzylic carbanionic intermediates undergo halogen transfer from 2-bromothiophenes to form electrophilic ArCF2Br compounds for in situ nucleophilic substitution, thereby avoiding α-fluoride elimination pathways that typically plague α-fluorocarbanions. This method streamlines the modular synthesis of α,α-difluorobenzyl(thio)ethers and led to the broader realization that halogen transfer to unstable carbanions is an enabling principle across diverse C(sp2)─H and C(sp3)─H systems.

Characterization of the Active Enantiomer and Mapping of the Stereospecific Intermolecular Pattern of a Reference P2X7 Allosteric Antagonist

The P2X purinergic receptor 7 (P2X7) has an essential role in inflammation, innate immunity, tumor progression, neurodegenerative diseases, and several other diseases, leading subsequently to the development of P2X7 modulators. AZ11645373 is a frequently studied P2X7 antagonist tool compound but always used as a racemic mixture. Racemic AZ11645373 can be separated into its respective enantiomers by chiral chromatography, albeit in small batches, and these were stereochemically intact over two years later, by chiral high-performance liquid chromatography (HPLC) analysis. On a higher scale, significant decomposition is observed during purification. One of the enantiomers was crystallized as a palladium complex, and its (R)-configuration was determined by single-crystal X-ray diffraction, further confirmed, in solution, by vibrational circular dichroism. Biological studies demonstrated that both (S)- and (R)-forms were able to fully inhibit human P2X7, but (R)-AZ11645373 was more potent, with an IC50 of 32.9 nM. Contrary to its effect on human P2X7, (S)-AZ11645373 was ineffective on mouse P2X7, while the (R)-AZ11645373 enantiomer was a full antagonist. These results demonstrated that the antagonistic effects of racemic AZ11645373 are mainly due to its (R)-enantiomer. Site-directed mutagenesis and molecular dynamics simulations indicated that the (R)-enantiomer may form specific interactions with Phe95 and the antagonists bound to other P2X7 monomers. Phe95 is situated in the allosteric binding site at the edge of the upper vestibule and appears to be the pivotal molecular gateway between AZ11645373 allosteric binding and locking of the closed state of the P2X7 channel. All together, these structure-function relationships should be helpful for drug design of P2X7 modulators.

Recent advances in the applications of gem-difluoromethylene alkynes

As a special class of alkynes, gem-difluoromethylene alkynes exhibit a variety of fascinating properties due to the presence of the gem-difluoro substitution. This substitution highlights the distinctive fluorine effects in influencing the chemoselectivity of reactions. As a result, chemical scientists have shown great interest and enthusiasm for investigating their reactions. In this review, we briefly summarize recent advances in transition metal-catalysed reactions of gem-difluoromethylene alkynes with multiple reaction pathways. Their mechanistic studies and challenges will be highlighted. The purpose of this review is to provide illustrations of elegant gem-difluoromethylene alkynes and thereby elicit further interest among synthetic chemists in developing innovative transformations of gem-difluoromethylene alkynes.

Axial Ligand Enables Synthesis of Allenylsilane through Dirhodium(II) Catalysis

Described herein is a dirhodium(II)-catalyzed silylation of propargyl esters with hydrosilanes, using tertiary amines as axial ligands. By adopting this strategy, a range of versatile and useful allenylsilanes can be achieved with good yields. This reaction not only represents a SN2'-type silylation of the propargyl derivatives bearing a terminal alkyne moiety to synthesize allenylsilanes from simple hydrosilanes, but also represents a new application of dirhodium(II) complexes in catalytic transformation of carbon-carbon triple bond. The highly functionalized allenylsilanes that are produced can be transformed into a series of synthetically useful organic molecules. In this reaction, an intriguing ON-OFF effect of the amine ligand was observed. The reaction almost did not occur (OFF) without addition of Lewis base amine ligand. However, the reaction took place smoothly (ON) after addition of only catalytic amount of amine ligand. Detailed mechanistic studies and density functional theory (DFT) calculations indicate that the reactivity can be delicately improved by the use of tertiary amine. The fine-tuning effect of the tertiary amine is crucial in the formation of the Rh-Si species via a concerted metalation deprotonation (CMD) mechanism and facilitating β-oxygen elimination.

Copper-catalyzed highly switchable defluoroborylation and hydrodefluorination of 1-(trifluoromethyl)alkynes

CF2-containing compounds hold significant potential in drug discovery, organic synthesis, and materials science. However, synthesizing various CF2-containing building blocks from a single compound remains challenging. Here, we present a Cu-catalyzed, switchable defluoroborylation and hydrodefluorination of trifluoromethylated alkynes, yielding four types of CF2-containing compounds. The chemo- and regio-selective sp2/sp3 1,2-diborylation and sp2 monoborylation of 1-(trifluoromethyl)alkynes are controlled by adjusting the solvent and ligand quantity. Additionally, altering the base allows selective generation of gem-difluoroalkenes or difluoromethylalkenes. Notably, our method prevents over-defluorination of the CF3 group on unsaturated C-C bonds during nucleophilic additions, preserving the pharmaceutically valuable CF2 group. Experimental data and density functional theory (DFT) calculations elucidate the regioselectivities of Cu-Bpin addition and the regulatory role of the ligand in selective deborylation processes.

Chemo-, regio-, and stereoselective tetrafunctionalization of fluoroalkynes enables divergent synthesis of 5-7-membered azacycles

Alkyne annulation has been widely used in organic synthesis for the construction of azacycles with unique structural and physicochemical properties. However, the analogous transformation of fluoroalkynes remains a challenge and has seen limited progress. Herein we report a 1,2,3,4-tetrafunctionalization of polyfluoroalkynes for the divergent construction of 5-7-membered (E)-1,2-difluorovinyl azacycles. The use of the fluorine atom as a detachable "activator" not only obviates the use of any transition metal catalysts and oxidizing reagents, but also ensures the [3-5 + 2]-annulation and defluorinative functionalization of fluoroalkynes with high chemo-, regio-, and stereoselectivities. This method exhibits a broad substrate scope, good functional group tolerance, and excellent scalability, providing a modular platform for accessing fluorinated skeletons of medicinal and biological interest. The late-stage modification of complex molecules, the multi-component 1,2-diamination of fluoroalkyne, and the synthesis of valuable organofluorides from the obtained products further highlight the real-world utility of this fluoroalkyne annulation technology.

Rhodium-Catalyzed Highly Enantioselective Hydroboration of Acyclic Tetrasubstituted Alkenes Directed by an Amide

Although progress has been made in enantioselective hydroboration of di- and trisubstituted alkenes over the past decades, enantioselective hydroboration of tetrasubstituted alkenes with high diastereo- and enantioselectivities continues as an unmet challenge since the 1950s due to its extremely low reactivity and the difficulties to simultaneously control the regio- and stereoselectivity of a tetrasubstituted alkene. Here, we report highly regio-, diastereo-, and enantioselective catalytic hydroboration of diverse acyclic tetrasubstituted alkenes. The delicate interplay of an electron-rich rhodium complex and coordination-assistance forms a highly adaptive catalyst that effectively overcomes the low reactivity and controls the stereoselectivity. The generality of the catalyst system is exemplified by its efficacy across various tetrasubstituted alkenes with diverse steric and electronic properties.

Cu-Catalyzed Regioselective Silylation of Chloro-Substituted Allenyl-Bdan

A copper-catalyzed efficient regioselective silylation reaction of chloro-substituted allenyl-Bdan was developed. Under mild reaction conditions, allenyl and propargyl silane compounds can be selectively obtained in moderate to high yields by adjusting the bases and solvents used in the reactions. This study offers direct and efficient methods for synthesizing multifunctionalized allenyl and propargyl silane compounds from the same initial material of chloro-substituted allenyl-Bdan.

Enantioselective and Regiodivergent Synthesis of Propargyl- and Allenylsilanes through Catalytic Propargylic C-H Deprotonation

We report a highly enantioselective intermolecular C-H bond silylation catalyzed by a phosphoramidite-ligated iridium catalyst. Under reagent-controlled protocols, propargylsilanes resulting from C(sp3)-H functionalization, as well the regioisomeric and synthetically versatile allenylsilanes, could be obtained with excellent levels of enantioselectivity and good to excellent control of propargyl/allenyl selectivity. In the case of unsymmetrical dialkyl acetylenes, good to excellent selectivity for functionalization at the less-hindered site was also observed. A variety of electrophilic silyl sources (R3SiOTf and R3SiNTf2), either commercial or in situ-generated, were used as the silylation reagents, and a broad range of simple and functionalized alkynes, including aryl alkyl acetylenes, dialkyl acetylenes, 1,3-enynes, and drug derivatives were successfully employed as substrates. Detailed mechanistic experiments and DFT calculations suggest that an η3-propargyl/allenyl Ir intermediate is generated upon π-complexation-assisted deprotonation and undergoes outer-sphere attack by the electrophilic silylating reagent to give propargylic silanes, with the latter step identified as the enantiodetermining step.

Synergistic Pd/Cu-Catalyzed 1,5-Double Chiral Inductions

Much attention has been focused on the catalytic asymmetric creation of single chiral centers or two adjacent stereocenters. However, the asymmetric construction of two nonadjacent stereocenters is of significant importance but is challenging because of the lack of remote chiral induction models. Herein, based on a C═C bond relay strategy, we report a synergistic Pd/Cu-catalyzed 1,5-double chiral induction model. All four stereoisomers of the target products bearing 1,5-nonadjacent stereocenters involving both allenyl axial and central chirality could be obtained divergently by simply changing the combination of two chiral catalysts with different configurations. Control experiments and DFT calculations reveal a novel mechanism involving 1,5-oxidative addition, contra-thermodynamic η3-allyl palladium shift, and conjugate nucleophilic substitution, which play crucial roles in the control of reactivity, regio-, enantio-, and diastereoselectivity. It is expected that this C═C bond relay strategy may provide a general protocol for the asymmetric synthesis of structural motifs bearing two distant stereocenters.

A review of frustrated Lewis pair enabled monoselective C-F bond activation

Frustrated Lewis pair (FLP) bond activation chemistry has greatly developed over the last two decades since the seminal report of metal-free reversible hydrogen activation. Recently, FLP systems have been utilized to allow monoselective C-F bond activation (at equivalent sites) in polyfluoroalkanes. The problem of 'over-defluorination' in the functionalization of polyfluoroalkanes (where multiple fluoro-positions are uncontrollably functionalized) has been a long-standing chemical problem in fluorocarbon chemistry for over 80 years. FLP mediated monoselective C-F bond activation is complementary to other solutions developed to address 'over-defluorination' and offers several advantages and unique opportunities. This perspective highlights some of these advantages and opportunities and places the development of FLP mediated C-F bond activation into the context of the wider effort to overcome 'over-defluorination'.

Synthetic Advantages of Defluorinative C-F Bond Functionalization

Much progress has been made in the development of methods to both create compounds that contain C-F bonds and to functionalize C-F bonds. As such, C-F bonds are becoming common and versatile synthetic functional handles. This review summarizes the advantages of defluorinative functionalization reactions for small molecule synthesis. The coverage is organized by the type of carbon framework the fluorine is attached to for mono- and polyfluorinated motifs. The main challenges, opportunities and advances of defluorinative functionalization are discussed for each class of organofluorine. Most of the text focuses on case studies that illustrate how defluorofunctionalization can improve routes to synthetic targets or how the properties of C-F bonds enable unique mechanisms and reactions. The broader goal is to showcase the opportunities for incorporating and exploiting C-F bonds in the design of synthetic routes, improvement of specific reactions and advent of new methods.

Nickel-catalysed asymmetric hydromonofluoromethylation of 1,3-enynes for enantioselective construction of monofluoromethyl-tethered chiral allenes

An unprecedented nickel-catalysed enantioselective hydromonofluoromethylation of 1,3-enynes is developed, allowing the diverse access to monofluoromethyl-tethered axially chiral allenes, including the challenging deuterated monofluoromethyl (CD2F)-tethered ones that are otherwise inaccessible. It represents the first asymmetric 1,4-hydrofunctionalization of 1,3-enynes using low-cost asymmetric nickel catalysis, thus opening a new avenue for the activation of 1,3-enynes in reaction development. The utility is further verified by its broad substrate scope, good functionality tolerance, mild conditions, and diversified product elaborations toward other valuable fluorinated structures. Mechanistic experiments and DFT calculations provide insights into the reaction mechanism and the origin of the enantioselectivity.

Vibrational circular dichroism spectroscopy in the C-D, XY, and XYZ stretching region

Vibrational circular dichroism (VCD) spectroscopy is a powerful technique for structural analysis of chiral molecules, but information available from VCD spectra of large molecular systems can be limited by severe overlap of vibrational bands. While common chiral molecules do not absorb in the 1900-2400 cm-1 region, observation of VCD signals in this spectrally-isolated region is possible for molecules containing C-D, XY, and XYZ chromophores. Thus, a strategic introduction of these chromophores to a target molecule may produce VCD signals informative for molecular structures. VCD spectroscopy in the 1900-2400 cm-1 region is a rather unexplored research field and its basic properties remain to be investigated. This perspective article discusses insight obtained so far on the usefulness and physicochemical aspects of VCD spectroscopy in this region with briefly summarizing previous experimental VCD studies including classic examples as well as our recent results. We show that anharmonic effects such as overtones and combination bands often complicate VCD patterns. On the other hand, some molecules exhibit characteristic VCD signals that can be well interpreted by harmonic DFT spectral calculations for structural analysis. This article also discusses several examples of the use of this region for studying solute-solvent interactions and for VCD signal augmentation.

A Pd-catalyzed highly selective three-component protocol for trisubstituted allenes

Herein we report the first example of a Pd-catalyzed highly selective three-component reaction of alkynyl-1,4-diol dicarbonates, organoboronic acids, and malonate anions for the efficient synthesis of trisubstituted 2,3-allenyl malonates not readily available by the known protocols. The reaction demonstrates an excellent regio- and chemo-selectivity for both the oxidative addition referring to the two C-O bonds and the subsequent coupling with the nucleophile with a remarkable functional group compatibility. A series of control experiments confirm a unique mechanism involving β-O elimination forming alka-1,2,3-triene and the subsequent insertion of its terminal C[double bond, length as m-dash]C bond into the Ar-Pd bond.

Nickel-Catalyzed Three-Component Tandem Radical Cyclization 1,5-Difunctionalization of 1,3-Enynes and Alkyl Bromide

A nickel-catalyzed three-component tandem radical cyclization reaction of aryl bromides with 1,3-enynes and aryl boric acids to construct γ-lactam-substituted allene derivatives has been described. This protocol provides lactam alkyl radicals through the free radical cyclization process, which can be effectively used to participate in the subsequent multicomponent coupling reaction so that 1,3-enynes could directly convert into corresponding poly-substituted allene compounds. In addition, this efficient method enjoys a broad substrate scope and provides a series of 1,5-difunctionalized allenes in a one-pot reaction.

Scope and limitations of absolute configuration determination of allenic natural products using the CCC stretching VCD signal

The allene functional group in natural products isolated so far exists in a non-racemic form, but its axial chirality is difficult to elucidate. Allenes exhibit a characteristic antisymmetric CCC stretching mode at around 1950 cm^-1, and their VCD properties have not been studied in detail. This work, for the first time, applied VCD spectroscopy to allenic natural products and allenic molecules with other asymmetric centers focusing on the antisymmetric CCC stretching mode. This vibrational mode yielded a negligibly weak VCD signal for several molecules, but in the presence of electron-withdrawing and/or conjugating substituents, it generated a stronger one. Its sign was found to be influenced by the nature of substituents. These findings should deepen the understanding of the VCD properties of the allene functional group and should be useful for future studies of chiral allenes.

Ferrocene derivatives with planar chirality and their enantioseparation by liquid-phase techniques

In the last decade, planar chiral ferrocenes have attracted a growing interest in several fields, particularly in asymmetric catalysis, medicinal chemistry, chiroptical spectroscopy and electrochemistry. In this frame, the access to pure or enriched enantiomers of planar chiral ferrocenes has become essential, relying on the availability of efficient asymmetric synthesis procedures and enantioseparation methods. Despite this, in enantioseparation science, these metallocenes were not comprehensively explored, and very few systematic analytical studies were reported in this field so far. On the other hand, enantioselective high-performance liquid chromatography has been frequently used by organic and organometallic chemists in order to measure the enantiomeric purity of planar chiral ferrocenes prepared by asymmetric synthesis. On these bases, this review aims to provide the reader with a comprehensive overview on the enantioseparation of planar chiral ferrocenes by discussing liquid-phase enantioseparation methods developed over time, integrating this main topic with the most relevant aspects of ferrocene chemistry. Thus, the main structural features of ferrocenes and the methods to model this class of metallocenes will be briefly summarized. In addition, planar chiral ferrocenes of applicative interest as well as the limits of asymmetric synthesis for the preparation of some classes of planar chiral ferrocenes will also be discussed with the aim to orient analytical scientists towards 'hot topics' and issues which are still open for accessing enantiomers of ferrocenes featured by planar chirality.

Regioselective Aromatic Perfluoro-tert-butylation Using Perfluoro-tert-butyl Phenyl Sulfone and Arynes

The selective introduction of perfluoro-tert-butyl group (PFtB, the bulkier analogue of CF₃ group) into arenes has long been sought after but remains a formidable task. We herein report the first general synthetic protocol to realize aromatic perfluoro-tert-butylation. The key to the success is the identification of PFtB phenyl sulfone as a new source of PFtB anion, which reacts with arynes in a highly regioselective manner to afford perfluoro-tert-butylated arenes in high yields. The application of the method is demonstrated by the preparation of sensitive ¹⁹F-labeled NMR probes with an extraordinary resolving ability.

Rhodium(I)-Catalyzed Defluorinative Coupling of Boronic Acids with Monofluoroalkenes

We herein describe a highly selective Rh(I)-catalyzed defluorinative coupling of boronic acids with (E)-β-monofluoroacrylates. In contrast to previous methods, the trisubstituted (Z)-alkene products were obtained in excellent dr with an inversion of double bond geometry. Experimental and computational studies established that Rh(I)-facilitated β-F elimination is favored over competing β-H elimination and protodemetalation.

Photo and copper dual catalysis for allene syntheses from propargylic derivatives via one-electron process

Different from the traditional two-electron oxidative addition-transmetalation-reductive elimination coupling strategy, visible light has been successfully integrated into transition metal-catalyzed coupling reaction of propargylic alcohol derivatives highly selectively forming allenenitriles: specifically speaking, visible light-mediated Cu-catalyzed cyanation of propargylic oxalates has been realized for the general, efficient, and exclusive syntheses of di-, tri, and tetra-substituted allenenitriles bearing various synthetically versatile functional groups. A set of mechanistic studies, including fluorescence quenching experiments, cyclic voltammetric measurements, radical trapping experiments, control experiments with different photocatalyst, and DFT calculation studies have proven that the current reaction proceeds via visible light-induced redox-neutral reductive quenching radical mechanism, which is a completely different approach as compared to the traditional transition metal-catalyzed two-electron oxidative addition processes.

Catalytic Asymmetric β-Oxygen Elimination

A catalytic enantioselective β-O-elimination reaction is reported in the form of a zirconium-catalyzed asymmetric opening of meso-ketene acetals. Furthermore, a regiodivergent β-O-elimination is demonstrated. The reaction proceeds under mild conditions, at low catalyst loadings, and produces chiral monoprotected cis-1,2-diols in good yield and enantiomeric excess. The combination with a Mitsunobu reaction or a one-pot hydroboration/Suzuki reaction sequence then gives access to additional diol and aminoalcohol building blocks. A stereochemical analysis supported by DFT calculations reveals that a high selectivity in the hydrozirconation step is also important for achieving high enantioselectivity, although it does not constitute the asymmetric step. This insight is crucial for the future development of related asymmetric β-elimination reactions.

Synthesis of Allenyl-Bdan via Cu(I)-Catalyzed Borylation of Propargyl gem-Dichlorides

A copper-catalyzed borylation of propargyl dichlorides was developed under mild reaction conditions. The corresponding chloro-substituted allenyl-Bdan products were obtained in good yields. The utilities of allenyl-Bdan products were examined by their diverse derivatizations.

Nickel-catalyzed reductive defluorination of iodo allylic gem-difluorides: allenyl monofluoride synthesis

As a potential fluorinated synthon, there have been only limited reports on fluorinated allene synthesis and applications due to concerns about their stability. Here, we developed a nickel-catalyzed reductive defluorination of iodo allyl gem-difluorides to afford allenyl monofluorides under mild conditions with good functional group tolerance, which were easily converted to other C-F bond compounds, such as alkyl and alkenyl fluorides. Preliminary mechanistic studies suggested that monofluoroallenes were yielded by β-F elimination of the alkenyl C-Ni intermediates from the oxidative addition of C-I bonds to a nickel(0) catalyst, while zinc regenerates the catalyst and closes the catalytic cycle.

Recent advances in the metal-catalyzed asymmetric synthesis of chiral allenes

Recent advances in the metal-catalyzed asymmetric synthesis of chiral allenes are summarized. This review is categorized based on the starting material, including alkynes, racemic allenes, and conjugated dienes.

Nickel-Catalyzed Stereo- and Enantioselective Cross-Coupling of gem-Difluoroalkenes with Carbon Electrophiles by C-F Bond Activation

Stereo- and enantioselective cross-electrophile coupling involving C-F bond activation is reported. Treatment of gem-difluoroalkenes with racemic benzyl electrophiles in the presence of a chiral nickel complex using B₂ pin₂ as a stoichiometric reductant allows the construction of a C(sp² )-C(sp³ ) bond under mild conditions, affording a broad range of monofluoroalkenes bearing stereogenic allylic centers. Initial mechanistic studies indicate that a radical chain pathway may be operating, wherein the ester group in the gem-difluoroalkene promotes C-F bond activation through oxidative addition to a Ni species.

Synthesis of ArCF2X and [18F]Ar-CF3 via Cleavage of the Trifluoromethylsulfonyl Group

A versatile synthesis of ArCF₂X and [¹⁸F]Ar-CF₃ type compounds from readily available ArCF₂SO₂CF₃ has been developed. Diverse nucleophiles, including weak nucleophiles such as halides (¹⁸F^-, Cl^-, Br^-, and I^-), RSH, and ROH, could react with ArCF₂SO₂CF₃ efficiently to give the corresponding difluoromethylene products. The control experiments and the Hammett plot indicated that the reaction might proceed through a difluorocarbocation intermediate generated from the steric hindrance-assisted cleavage of the trifluoromethylsulfonyl group.

Iridium(III) Catalyzed Z-Selective Allylic Arylation of α-Fluoro But-1-enoic Acid Amides via β-F-Elimination in Water

Allylic arylation of α-fluoro but-1-enoic acid amides with arylboronic acids was carried out in water by comparing the catalytic activity of iridium(III) and rhodium(III). Ir(III) has shown a strong superiority over Rh(III) to give allyl-aryl coupling products with excellent stereoselectivity in favor of the Z-isomer. The origin of high stereoselectivity is perhaps because of the a coordination of iridium Ir-N or Ir-O.