Rh(I)-bisphosphine-catalyzed asymmetric, intermolecular hydroheteroarylation of α-substituted acrylate derivatives

Recent advances in Rh(I)-catalyzed enantioselective C-H functionalization

Chiral carbon-carbon (C-C) and carbon-heteroatom (C-X) bonds are pervasive and very essential in natural products, bioactive molecules, and functional materials, and their catalytic construction has emerged as one of the hottest research fields in synthetic organic chemistry. The last decade has witnessed vigorous progress in Rh(I)-catalyzed asymmetric C-H functionalization as a complement to Rh(II) and Rh(III) catalysis. This review aims to provide the most comprehensive and up-to-date summary covering the recent advances in Rh(I)-catalyzed C-H activation for asymmetric functionalization. In addition to the development of diverse reactions, chiral ligand design and mechanistic investigation (inner-sphere mechanism, outer-sphere mechanism, and 1,4-Rh migration) will also be highlighted.

Enantioselective Rh(I)-Catalyzed C-H Arylation of Ferroceneformaldehydes

As an important class of platform molecules, planar chiral ferrocene carbonyl compounds could be transformed into various functional groups offering facile synthesis of chiral ligands and catalysts. However, developing efficient and straightforward methods for accessing enantiopure planar chiral ferrocene carbonyl compounds, especially ferroceneformaldehydes, remains highly challenging. Herein, we report a rhodium(I)/phosphoramidite-catalyzed enantioselective C-H bond arylation of ferroceneformaldehydes. Readily available aryl halides such as aryl iodides, aryl bromides, and even aryl chlorides are suitable coupling partners in this transformation, leading to a series of planar chiral ferroceneformaldehydes in good yields and excellent enantioselectivity (up to 83% yield and >99% ee). The aldehyde group could be transformed into diverse functional groups smoothly, and enantiopure Ugi's amine and PPFA analogues could be synthesized efficiently. The latter was found to be a highly efficient ligand in Pd-catalyzed asymmetric allylic alkylation reactions. Mechanistic experiments supported the formation of imine intermediates as the key step during the reaction.

Acetate ion addition to and exchange in (1,5-cyclooctadiene)rhodium(I) acetate: relevance for the coagulation of carboxylic acid-functionalized shells of core-crosslinked micelle latexes

The solution behavior of complex [Rh(COD)(μ-OAc)]2 in the absence and presence of PPN+OAc- in dichloromethane has been investigated in detail by multinuclear NMR spectroscopy. Without additional acetate ions, the compound shows dynamic behavior at room temperature, consistent with an inversion of its C2v structure. Addition of PPN+OAc- reveals an equilibrated generation of [Rh(COD)(OAc)2]-. Rapid exchange is observed at room temperature between the neutral dimer and the anionic mononuclear complex, as well as between the anionic complex and free acetate. Lowering the temperature to 213 K freezes the exchange between the two Rh complexes, but fast exchange between the anionic Rh complex and free acetate maintains coalesced Me (1H and 13C) and COO (13C) NMR resonances. DFT calculations support the experimental data and lean in favour of a dissociative mechanism for the acetate exchange in [Rh(COD)(OAc)2]-. The acetate ligands in complex [Rh(COD)(μ-OAc)]2 are also exchanged in a biphasic (water/organic) system with the methacrylic acid (MAA) functions of hydrosoluble [MMA0.5-co-PEOMA0.5]30 copolymer chains (PEOMA = poly(ethylene oxide) methyl ether methacrylate), resulting in transfer of the Rh complex to the aqueous phase. Exchange with the MAA functions in the same polymer equally takes place for the chloride ligands of [Rh(COD)(μ-Cl)]2. The latter phenomenon rationalizes the coagulation of a core-crosslinked micelle (CCM) latex, where MMA functions are present on the hydrophilic CCM shell, when a dichloromethane solution of [Rh(COD)(μ-Cl)]2 is added.

Rhodium-Catalyzed Asymmetric C-H Functionalization Reactions

This review summarizes the advancements in rhodium-catalyzed asymmetric C-H functionalization reactions during the last two decades. Parallel to the rapidly developed palladium catalysis, rhodium catalysis has attracted extensive attention because of its unique reactivity and selectivity in asymmetric C-H functionalization reactions. In recent years, Rh-catalyzed asymmetric C-H functionalization reactions have been significantly developed in many respects, including catalyst design, reaction development, mechanistic investigation, and application in the synthesis of complex functional molecules. This review presents an explicit outline of catalysts and ligands, mechanism, the scope of coupling reagents, and applications.

Rhodium-catalyzed atroposelective access to trisubstituted olefins via C-H bond olefination of diverse arenes

The atroposelective synthesis of axially chiral acyclic olefins remains a daunting challenge due to their relatively lower racemization barriers, especially for trisubstituted ones. In this work, atroposelective C-H olefination has been realized for synthesis of open-chain trisubstituted olefins via C-H activation of two classes of (hetero)arenes in the coupling with sterically hindered alkynes. The employment of phenyl N-methoxycarbamates as arene reagents afforded phenol-tethered olefins, with the carbamate being a traceless directing group. The olefination of N-methoxy-2-indolylcarboxamides afforded the corresponding chiral olefin by circumventing the redox-neutral [4 + 2] annulation. The reactions proceeded with excellent Z/E selectivity, chemoselectivity, regioselectivity, and enantioselectivity in both hydroarylation systems.

Iridium-Catalyzed Enantioselective Intermolecular Hydroarylation of 1,1-Disubstituted Alkenes

The catalytic enantioselective hydroarylation of 1,1-disubstituted alkenes proceeded by using a cationic iridium/(R)-binap complex to give the corresponding adducts in high yields with high enantioselectivity. The reaction of arenes substituted with heteroaromatic directing groups proceeded to give the addition products linear-selectively. Methallylamine derivatives were good acceptors to obtain high enantioselectivities. The adduct bearing maleimide moiety was readily transformed into the β-chiral amine derivative without loss of the enantiomeric purity.

Iridium-Catalyzed Branch-Selective and Enantioselective Hydroalkenylation of α-Olefins through C-H Cleavage of Enamides

Catalytic branch-selective hydrofunctionalization of feedstock α-olefins to form enantioenriched chiral compounds is a particularly attractive yet challenging transformation in asymmetric catalysis. Herein we report an iridium-catalyzed asymmetric hydroalkenylation of α-olefins through directed C-H cleavage of enamides. This atom-economical addition process is highly branch-selective and enantioselective, delivering trisubstituted alkenes with an allylic stereocenter. DFT calculations reveal the origin of regio- and enantioselectivity.

Transition-metal-catalyzed C-H bond alkylation using olefins: recent advances and mechanistic aspects

Transition metal catalysis has contributed immensely to C-C bond formation reactions over the last few decades, and alkylation is no exception. The superiority of such methodologies over traditional alkylation is evident from minimal reaction steps, shorter reaction times, and atom economy while also allowing control over regio- and stereo-selectivity. In particular, hydrocarbonation of alkenes has grabbed increased attention due its fundamental ability to effectively and selectively synthesise a wide range of industrially and pharmaceutically relevant moieties. This review attempts to provide a scientific viewpoint and a systematic analysis of the recent developments in transition-metal-catalyzed alkylation of various C-H bonds using simple and activated olefins. The key features and mechanistic studies involved in these transformations are described briefly.

Enantio- and Regioselective Ni-Catalyzed para-C-H Alkylation of Pyridines with Styrenes via Intermolecular Hydroarylation

Direct asymmetric functionalization of the pyridyl C-H bond represents a longstanding challenge in organic chemistry. We herein describe the first enantioselective para-C-H activation of pyridines through the use of a Ni-Al bimetallic catalyst system and N-heterocyclic carbene (NHC) ligand for intermolecular hydroarylation of styrenes. The reaction procceds in high to excellent enantioselectivities (up to 98.5:1.5 er) and high site-selectivities for both styrene and pyridine components (up to >98:2). Consequently, a broad range of enantioenriched 1,1-diarylalkanes containing pyridine moieties could be prepared in a single step with 100% atom economy. Computational studies supported a mechanism involving a ligand-to-ligand H-transfer (LLHT) and reductive elimination sequence, with LLHT being the rate- and enantioselectivity-determining step. DFT studies indicate that the π-π stacking interaction between the NHC aryl fragment and trans-styrenes is critical for high reactivity and enantiocontrol.

Metal-catalyzed asymmetric heteroarylation of alkenes: diverse activation mechanisms

This review summarizes the state-of-the-art in transition metal-catalyzed asymmetric alkylation of heteroarenes using alkenes (covering literature from 2000 to late 2021). Based on elementary reactions on metals for substrate activation, these reactions are broadly classified in several categories: (A) concerted oxidative addition of heteroaryl C-H bonds on rhodium(I) and iridium(I), (B) ligand-to-ligand hydrogen transfer (LLHT) on low-valent 3d metal complexes of nickel and cobalt, (C) different ways for deprotonation of heteroaryl C-H bonds by late transition metal complexes, especially palladium, including electrophilic aromatic substitution and a related mechanism, base-assisted intramolecular electrophilic substitution, concerted and nonconcerted metalation deprotonation, (D) σ-bond metathesis by d⁰ early transition metal complexes, (E) electrophilic activation of olefins by Pd(II), Pt(II) and Au(I), and (F) metal hydride insertion of aryl olefins and dienes. The demand to achieve enantiocontrol in the heteroarylation reactions has also driven innovation in chiral ancillary ligands, exemplified by extremely bulky, chiral N-heterocyclic carbenes for nickel catalysts, bulky monodentate oxazolines for Wacker-type reactions and chiral cyclopentadienyl ligands for half-sandwich complexes of scandium.

Recent advances in catalytic enantioselective direct C–H bond functionalization of electron-deficient N-containing heteroarenes

Catalytic enantioselective direct C–H bond functionalization of electron-deficient N-containing heteroarenes represents one of the most straightforward and powerful protocols to construct diverse enantioenriched highly functionalized N-heteroarenes.

Atom and step economical synthesis of acyclic quaternary centers via iridium-catalyzed hydroarylative cross-coupling of 1,1-disubstituted alkenes

Quaternary benzylic centers are accessed with high atom and step economy by intermolecular Ir-catalyzed hydroarylation of non-polarized 1,1-disubstituted alkenes.

Cp*Co(III)-Catalyzed Enantioselective Hydroarylation of Unactivated Terminal Alkenes via C-H Activation

Enantioselective hydroarylation of unactivated terminal akenes constitutes a prominent challenge in organic chemistry. Herein, we reported a Cp*Co(III)-catalyzed asymmetric hydroarylation of unactivated aliphatic terminal alkenes assisted by a new type of tailor-made amino acid ligands. Critical to the chiral induction was the engaging of a novel noncovalent interaction (NCI), which has seldomly been disclosed in the C-H activation area, arising from the molecular recognition among the organocobalt(III) intermediate, the coordinated alkene, and the well-designed chiral ligand. A broad range of C2-alkylated indoles were obtained in high yields and excellent enantioselectivities. DFT calculations revealed the reaction mechanism and elucidated the origins of chiral induction in the stereodetermining alkene insertion step.

Enantioselective Intermolecular Murai-Type Alkene Hydroarylation Reactions

Strategies that enable the efficient assembly of complex building blocks from feedstock chemicals are of paramount importance to synthetic chemistry. Building upon the pioneering work of Murai and co-workers in 1993, C–H-activation-based enantioselective hydroarylations of alkenes offer a particularly promising framework for the step- and atom-economical installation of benzylic stereocenters. This short review presents recent intermolecular enantioselective Murai-type alkene hydroarylation methodologies and the mechanisms by which they proceed.

1 Introduction

2 Enantioselective Hydroarylation Reactions of Strained Bicyclic Alkenes

3 Enantioselective Hydroarylation Reactions of Electron-Rich Acyclic Alkenes

4 Enantioselective Hydroarylation Reactions of Electron-Poor Acyclic Alkenes

5 Enantioselective Hydroarylation Reactions of Minimally Polarized Acyclic Alkenes

6 Conclusion and Outlook

Synthetic Scope of Brønsted Acid-Catalyzed Reactions of Carbonyl Compounds and Ethyl Diazoacetate

The comprehensive study of the reactions of carbonyl compounds and ethyl diazoacetate in the presence of a Brønsted acid catalyst is described. In result, a broad range of 3-oxo-esters were synthesized from a variety of ketones and aliphatic aldehydes by 1,2-aryl/alkyl/hydride shift. Aryl-methyl ketones produced only aryl-migrated products, whereas other ketones yielded a mixture of products. For diaryl ketones, the identity of two inseparable migrated products was confirmed by two-dimensional NMR spectroscopy.

Copper-catalyzed oxidative cyclization of glycine derivatives toward 2-substituted benzoxazoles

A novel and straightforward intramolecular cyclization of glycine derivatives to 2-substituted benzoxazoles through copper-catalyzed oxidative C-H/O-H cross-coupling was described. A variety of glycine derivatives involving short peptides underwent cross-dehydrogenative-coupling readily to afford diverse 2-substituted benzoxazoles. The synthetic method has the advantages of simple operation, broad substrate scope and mild reaction conditions, thus providing an alternative effective approach for benzoxazole construction.

Olefin Hydroarylation Catalyzed by a Single-Component Cobalt(-I) Complex

A single-component Co(-I) catalyst, [(PPh₃)₃Co(N₂)]Li(THF)₃, has been developed for olefin hydroarylations with (N-aryl)aryl imine substrates. More than 40 examples were examined under mild reaction conditions to afford the desired alkyl-arene product in good to excellent yields. Catalysis occurs in a regioselective manner to afford exclusively branched products with styrene-derived substrates or linear products for aliphatic olefins. Electron-withdrawing functional groups (e.g., -F, -CF₃, and -CO₂Me) were tolerated under the reaction conditions.

Copper-Catalyzed Enantioconvergent Cross-Coupling of Racemic Alkyl Bromides with Azole C(sp2 )-H Bonds

The development of enantioconvergent cross-coupling of racemic alkyl halides directly with heteroarene C(sp² )-H bonds has been impeded by the use of a base at elevated temperature that leads to racemization. We herein report a copper(I)/cinchona-alkaloid-derived N,N,P-ligand catalytic system that enables oxidative addition with racemic alkyl bromides under mild conditions. Thus, coupling with azole C(sp² )-H bonds has been achieved in high enantioselectivity, affording a number of potentially useful α-chiral alkylated azoles, such as 1,3,4-oxadiazoles, oxazoles, and benzo[d]oxazoles as well as 1,3,4-triazoles, for drug discovery. Mechanistic experiments indicated facile deprotonation of an azole C(sp² )-H bond and the involvement of alkyl radical species under the reaction conditions.

Recent Developments in Transition-Metal Catalyzed Direct C-H Alkenylation, Alkylation, and Alkynylation of Azoles

The transition metal-catalyzed C–H bond functionalization of azoles has emerged as one of the most important strategies to decorate these biologically important scaffolds. Despite significant progress in the C–H functionalization of various heteroarenes, the regioselective alkylation and alkenylation of azoles are still arduous transformations in many cases. This review covers recent advances in the direct C–H alkenylation, alkylation and alkynylation of azoles utilizing transition metal-catalysis. Moreover, the limitations of different strategies, chemoselectivity and regioselectivity issues will be discussed in this review.

Nickel/NHC-Catalyzed Asymmetric C-H Alkylation of Fluoroarenes with Alkenes: Synthesis of Enantioenriched Fluorotetralins

Chiral polyfluoroarene derivatives are an important scaffold in chemistry. An unprecedented enantioselective C-H alkylation of polyfluoroarenes with alkenes is described. The reaction employs bulky chiral N-heterocyclic carbene (NHC) ligands for nickel catalysts to enable exclusive activation of C-H bonds over C-F bonds and complete endo-selective C-H annulation and excellent enantioselectivity. A wide variety of chiral fluorotetralins, compounds otherwise difficultly accessed but serve as important bioisosteric analogs of both tetralin and heterocycle units for drug design, are expediently synthesized from easily available substrates. To our knowledge, this is the first example of catalytic enantioselective C-H functionalization of polyfluoroarenes.

Mild polyaddition and polyalkylation based on the carbon–carbon bond formation reaction of active methylene

The Michael addition and alkylation reaction of active methylene compounds (AMCs) with two active hydrogens had been investigated extensively in organic chemistry, while the polymerization of AMCs had few studies. Herein, we reported active methylene-based polyaddition and polyalkylation catalyzed via an organic superbase under ambient conditions. A model polymerization was first conducted between ethylene glycol diacrylate (EGDA) and methyl cyanoacetate (MCA). The molecular weight (M_w) of the model polymer was up to 50 500 g mol⁻¹ with a high yield (99%). Eight AMCs were selected and a high-throughput parallel synthesizing instrument (HTPSI) was used to synthesize semi-library polymers of AMCs and EGDA via a Michael type polyaddition. The obtained AMC-based polymers had low cell cytotoxicity. Elastomers with cyanogen groups could be prepared using trimethylolpropane triacrylate (TMPTA) as a crosslinker. Furthermore, three dihalogen compounds were explored to polymerize with MCA and malononitrile via alkylation reactions. The pendent cyanogen or ester groups of the polymers could be reduced by lithium aluminum hydride. Novel polymer families were constructed based on the polyaddition and polyalkylation of AMCs.

Active methylene-based polyaddition and polyalkylation catalyzed via an organic superbase were studied in a high-throughput parallel synthesizing instrument.

Mild complexation protocol for chiral CpxRh and Ir complexes suitable for in situ catalysis

Mild complexations of chiral cyclopentadienes with rhodium(i) and iridium(i) precursors enable user-friendly in situ complex formation for catalytic applications.

A practical complexation method for chiral cyclopentadienyl (Cp^x) iridium and rhodium complexes is described. The procedure uses the free Cp^xH with stable and commercially available rhodium(i) and iridium(i) salts without base or additive. The conditions are mild and do not require the exclusion of air and moisture. A salient feature is the suitability for in situ complexations enhancing the user-friendliness of Cp^x ligands in asymmetric catalysis. DFT-calculations confirm an intramolecular proton abstraction pathway by either the bound acetate or methoxide. Furthermore, the superior facial selectivity of the proton abstraction step enabled the development of TMS-containing trisubstituted Cp^x ligands which display improved enantioselectivities for the benchmarking dihydroisoquinolone synthesis.

Remote Nucleophilic Allylation by Allylrhodium Chain Walking

Metal migration through a carbon chain is a versatile method for achieving remote functionalization. However, almost all known examples involve the overall net migration of alkylmetal species. Here, we report that allylrhodium species obtained from hydrorhodation of 1,3-dienes undergo chain walking toward esters, amides, or (hetero)arenes over distances of up to eight methylene units. The final, more highly conjugated allylrhodium species undergo nucleophilic allylation with aldehydes and with an imine to give Z-homoallylic alcohols and amines, respectively.

Conjugate Addition of Perfluoroarenes to α,β-Unsaturated Carbonyls Enabled by an Alkoxide-Hydrosilane System: Implication of a Radical Pathway

Conjugate addition of organometallic reagents to α,β-unsaturated carbonyls is a key strategy for the construction of carbon-carbon bond in organic synthesis. Although direct C-H addition to unsaturated bonds via transition metal catalysis is explored in recent years, electron-deficient arenes that do not bear directing groups continue to be challenging. Herein we disclose the first example of a conjugate addition of perfluoroarenes to α,β-unsaturated carbonyls enabled by an alkoxide-hydrosilane system. The reaction is convenient to carry out at room temperature over a broad range of substrates and reactants to furnish synthetically versatile products in high to excellent yields. Mechanistic experiments in combination with computational studies suggest that a radical pathway is most likely operative in this transformation. The hypervalent silicate and silanide species, which are relevant to the proposed mechanism, were observed experimentally by NMR and single crystal X-ray diffraction analyses.

Nickel(0)-catalyzed linear-selective hydroarylation of unactivated alkenes and styrenes with aryl boronic acids

Herein, we describe the first linear-selective hydroarylation reaction of unactivated alkenes and styrenes with aryl boronic acids, which was achieved by introducing a directing group on the alkenes. This efficient, scalable reaction serves as a method for modular assembly of structurally diverse alkyl arenes, including γ-aryl butyric acid derivatives, which are widely utilized as chemical building blocks for the synthesis of various drugs and other biologically active compounds.

Enantioselective Ni-Al Bimetallic Catalyzed exo-Selective C-H Cyclization of Imidazoles with Alkenes

A Ni-Al bimetallic catalyzed enantioselective C-H exo-selective cyclization of imidazoles with alkenes has been developed. A series of bi- or polycyclic imidazoles with β-stereocenter were obtained in up to 98% yield and >99% ee. The bifunctional SPO ligand-promoted bimetallic catalysis proved to be critical to this challenging stereocontrol.

NiH-Catalyzed Reductive Relay Hydroalkylation: A Strategy for the Remote C(sp3 )-H Alkylation of Alkenes

The terminal-selective, remote C(sp³ )-H alkylation of alkenes was achieved by a relay process combining NiH-catalyzed hydrometalation, chain walking, and alkylation. This method enables the construction of unfunctionalized C(sp³ )-C(sp³ ) bonds under mild conditions from two simple feedstock chemicals, namely olefins and alkyl halides. The practical value of this transformation is further demonstrated by the large-scale and regioconvergent alkylation of isomeric mixtures of olefins at low catalyst loadings.

Heterogeneous Co-catalyzed direct 2-alkylation of azoles with ethers

The direct 2-alkylation of oxazoles and thiazoles with ethers through cross-dehydrogenative coupling reaction using Co-containing mesoporous zeolite ETS-10 as the heterogeneous catalyst is described. The basic Co-containing mesoporous zeolite ETS-10 catalyst facilitates this cross-dehydrogenative coupling reaction through metal-base synergy catalytic principle.

The direct 2-alkylation of oxazoles and thiazoles with ethers through cross-dehydrogenative coupling reaction using Co-containing mesoporous zeolite ETS-10 as the heterogeneous catalyst is described.

Palladium-catalyzed chemoselective anaerobic oxidation of N-heterocycle-containing alcohols

A palladium-catalyzed chemoselective anaerobic oxidation for N-heterocycle-containing alcohols has been achieved with chloroarenes as oxidants.