Asymmetric C(sp2)H Activation

Pd-Catalyzed Atroposelective C-H Olefination: Diverse Synthesis of Axially Chiral Biaryl-2-carboxylic Acids

Axially chiral carboxylic acids are important motifs in chiral catalysts and ligands. We herein reported the synthesis of axially chiral carboxylic acids via Pd(II)-catalyzed atroposelective C-H olefination using carboxylic acid as the native directing group. A broad range of axial chiral biaryl-2-carboxylic acids were synthesized in good yields with high enantioselectivities (up to 84% yield with 99% ee). Gram-scale reaction and further transformation reactions also provide a platform for synthetic applications of this method.

Electrooxidative Rhodium(III)/Chiral Carboxylic Acid-Catalyzed Enantioselective C-H Annulation of Sulfoximines with Alkynes

The combination of achiral Cp*Rh(III) with chiral carboxylic acids (CCAs) represents an efficient catalytic system in transition metal-catalyzed enantioselective C-H activation. However, this hybrid catalysis is limited to redox-neutral C-H activation reactions and the adopt to oxidative enantioselective C-H activation remains elusive and pose a significant challenge. Herein, we describe the development of an electrochemical Cp*Rh(III)-catalyzed enantioselective C-H annulation of sulfoximines with alkynes enabled by chiral carboxylic acid (CCA) in an operationally friendly undivided cell at room temperature. A broad range of enantioenriched 1,2-benzothiazines are obtained in high yields with excellent enantioselectivities (up to 99 % yield and 98 : 2 er). The practicality of this method is demonstrated by scale-up reaction in a batch reactor with external circulation. A crucial chiral Cp*Rh(III) intermediate is isolated, characterized, and transformed, providing rational support for a Rh(III)/Rh(I) electrocatalytic cycle.

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.

Pd(II)-Catalyzed enantioselective C-H olefination toward the synthesis of P-stereogenic phosphinamides

P-Stereogenic phosphorus compounds are important structural elements in chiral ligands or organocatalysts. Herein, we report a Pd(II)-catalyzed enantioselective C-H olefination toward the synthesis of P-stereogenic phosphinamides using cheap commercially available L-pGlu-OH as a chiral ligand. A broad range of P-stereogenic phosphinamides were gained in good yields with high enantioselectivities (33 examples, up to 77% yield, 99% ee) via desymmetrization and kinetic resolution.

Probing Chiral Sulfoximine Auxiliaries in Ru(II)-Catalyzed One-Pot Asymmetric C-H Hydroarylation and Annulations with Alkynes

Developed herein is a chiral sulfoximine-enabled Ru(II)-catalyzed asymmetric C-H activation/functionalization involving intramolecular hydroarylation and functionalization/annulation of alkynes. This process constructs dihydrobenzofuran- or indoline-fused isoquinolinones having a tertiary or quaternary stereocenter with good yields and enantioselectivities (up to 97:3 enantiomeric ratio). The chiral sulfoxide precursor used in synthesizing the enantiopure sulfoximines is spontaneously eliminated during the reaction. It can be recovered without losing enantiopurity (∼99% enantiomeric excess) and reused.

Transition-metal-catalyzed atroposelective synthesis of axially chiral styrenes

Axially chiral styrenes, a type of atropisomer analogous to biaryls, have attracted great interest because of their unique presence in natural products and asymmetric catalysis. Since 2016, a number of methodologies have been developed for the atroposelective construction of these chiral skeletons, involving both transition metal catalysis and organocatalysis. In this feature article, we aim to provide a comprehensive understanding of recent advances in the asymmetric synthesis of axially chiral styrenes catalyzed by transition metals, integrating scattered work with different catalytic systems together. This feature article is cataloged into five sections according to the strategies, including asymmetric coupling, enantioselective C-H activation, central-to-axial chirality transfer, asymmetric alkyne functionalization, and atroposelective [2+2+2] cycloaddition.

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.

Synthesis of P-Stereogenic Phosphinamides via Pd(II)-Catalyzed Enantioselective C-H Alkynylation

P-Stereogenic phosphinamides represent important structural elements in chiral organocatalysts and bioactive compounds. Herein, we report Pd(II)-catalyzed enantioselective C-H alkynylation using cheap commercially available l-pyroglutamic acid as a chiral ligand. A range of structurally diverse P-stereogenic phosphinamides was prepared in good yields with high enantioselectivities via desymmetrization and kinetic resolution. A tailor-made congested directing group, N-ethyl-N-(3-methylpyridin-2-yl)amino, was crucial for the reactivity.

Copper-catalyzed asymmetric C(sp2)-H arylation for the synthesis of P- and axially chiral phosphorus compounds

Transition metal-catalyzed C-H bond functionalization is an important method in organic synthesis, but the development of methods that are lower cost and have a less environmental impact is desirable. Here, a Cu-catalyzed asymmetric C(sp²)-H arylation is reported. With diaryliodonium salts as arylating reagents, a range of ortho-arylated P-chiral phosphonic diamides were obtained in moderate to excellent yields with high enantioselectivities (up to 92% ee). Meanwhile, enantioselective C-3 arylation of diarylphosphine oxide indoles was also realized under similar conditions to construct axial chirality.

(SCp)Rhodium-Catalyzed Asymmetric Satoh-Miura Reaction for Building-up Axial Chirality: Counteranion-Directed Switching of Reaction Pathways

Satoh-Miura reaction is an important method for extending π-systems by forging multi-substituted benzene rings via double aryl C-H activation and annulation with alkynes. However, the development of highly enantioselective Satoh-Miura reaction remains rather challenging. Herein, we report an asymmetric Satoh-Miura reaction between 1-aryl benzo[h]isoquinolines and internal alkynes enabled by a SCpRh-catalyst. Judiciously choosing the counteranion of the Rh-catalyst is crucial for the desired reactivity over the competitive formation of azoniahelicenes. Detailed mechanistic studies support the proposal of counteranion-directed switching of reaction pathways in Rh-catalyzed asymmetric C-H activation.

Electrochemical Ruthenium-Catalysed C–H Activation in Water Through Heterogenization of a Molecular Catalyst

Efficient catalytic oxidative C–H activation of organic substrates remains an important challenge in synthetic chemistry. Here, we show that the combination of a transition metal catalyst, surface immobilisation and an...

Thioether-enabled palladium-catalyzed atroposelective C–H olefination for N−C and C−C axial chirality

Thioethers allowed for highly atroposelective C–H olefinations by a palladium/chiral phosphoric acid catalytic system under ambient air. Both N–C and C–C axial chiral (hetero)biaryls were successfully constructed, leading to a broad range of axially chiral N-aryl indoles and biaryls with excellent enantioselectivities up to 99% ee. Experimental and computational studies were conducted to unravel the walking mode for the atroposelective C–H olefination. A plausible chiral induction model for the enantioselectivity-determining step was established by detailed DFT calculations.

Thioethers allowed for highly atroposelective C–H olefinations by a palladium/chiral phosphoric acid catalytic system under ambient air.

Coordination-assisted, transition-metal-catalyzed enantioselective desymmetric C–H functionalization

Recent advances in transition-metal-catalyzed enantioselective desymmetric C–H functionalization are summarized.

Kinetic resolution of sulfur-stereogenic sulfoximines by Pd(ii)-MPAA catalyzed C-H arylation and olefination

A direct Pd(ii)-catalyzed kinetic resolution of heteroaryl-enabled sulfoximines through an ortho-C-H alkenylation/arylation of arenes has been developed. The coordination of the sulfoximine pyridyl-motif and the chiral amino acid MPAA ligand to the Pd(ii)-catalyst controls the enantio-discriminating C(aryl)-H activation. This method provides access to a wide range of enantiomerically enriched unreacted aryl-pyridyl-sulfoximine precursors and C(aryl)-H alkenylation/arylation products in good yields with high enantioselectivity (up to >99% ee), and selectivity factor up to >200. The coordination preference of the directing group, ligand effect, geometry constraints, and the transient six-membered concerted-metalation-deprotonation species dictate the stereoselectivity; DFT studies validate this hypothesis.

Rhodium-Catalyzed Atroposelective Access to Axially Chiral Olefins via C-H Bond Activation and Directing Group Migration

Axially chiral open-chain olefins represent an underexplored class of chiral platform. In this report, two classes of tetrasubstituted axially chiral acyclic olefins have been accessed in excellent enantioselectivity and regioselectivity via C-H activation of (hetero)arenes assisted by a migratable directing group en route to coupling with sterically hindered alkynes. The coupling of indoles bearing an N-aminocarbonyl directing group afforded C-N axially chiral acrylamides with the assistance of a racemic zinc carboxylate additive. DFT studies suggest a β-nitrogen elimination-reinsertion pathway for the directing group migration. Meanwhile, the employment of N-phenoxycarboxamide delivered C-C axially chiral enamides via migration of the oxidizing directing group. Experiments suggest that in both cases the (hetero)arene substrate adopts a well-defined orientation during the C-H activation, which in turn determines the disposition of the alkyne in migratory insertion. Synthetic applications of representative chiral olefins are demonstrated.

Synthesis of Planar Chiral 2-Aryl Aroylferrocenes via Palladium-Catalyzed C-C Bond-Cleavage/Ring-Opening Reaction

A palladium-catalyzed ring-opening reaction of optically active ferrocenyl tertiary alcohols for the construction of planar chiral ketones is reported. The stereochemistry of the hydroxyl group in ferrocenyl alcohols markedly affects reaction: ferrocenyl alcohols with a β-hydroxyl group show better reactivity and chemoselectivity than the corresponding α-hydroxyl analogues. The treatment of α-hydroxyl substrates with trifluoroacetic acid successfully realizes the inversion of the orientation of hydroxyl group to the corresponding β-analogues.

Twofold C-H Activation-Based Enantio- and Diastereoselective C-H Arylation Using Diarylacetylenes as Rare Arylating Reagents

C-H bond activation has been established as an attractive strategy to access axially chiral biaryls, and the most straightforward method is direct C-H arylation of arenes. However, the arylating source has been limited to several classes of reactive and bulky reagents. Reported herein is rhodium-catalyzed 1:2 coupling of diarylphosphinic amides and diarylacetylenes for enantio- and diastereoselective construction of biaryls with both central and axial chirality. This twofold C-H activation reaction stays contrast to the previously explored Miura-Satoh type 1:2 coupling of arenes and alkynes in terms of chemoselectivity and proceeded under mild conditions with the alkyne acting as a rare arylating reagent. Both C-H activation events are stereo-determining and are under catalyst control, with the 2^nd C-H activation being diastereo-determining in a remote fashion. Analysis of the stereochemistry of the major and side products suggests moderate enantioselectivity of the initial C-H activation-desymmetrization process. However, the minor (R) rhodium vinyl intermediate is consumed more readily in undesired protonolysis, eventually resulting in high enantio- and diastereoselectivity of the major product.

Rhodium-Catalyzed C-H Activation-Based Construction of Axially and Centrally Chiral Indenes through Two Discrete Insertions

Reported herein is asymmetric [3+2] annulation of arylnitrones with different classes of alkynes catalyzed by chiral rhodium(III) complexes, with the nitrone acting as an electrophilic directing group. Three classes of chiral indenes/indenones have been effectively constructed, depending on the nature of the substrates. The coupling system features mild reaction conditions, excellent enantioselectivity, and high atom-economy. In particular, the coupling of N-benzylnitrones and different classes of sterically hindered alkynes afforded C-C or C-N atropochiral pentatomic biaryls with a C-centered point-chirality in excellent enantio- and diastereoselectivity (45 examples, average 95.6 % ee). These chiral center and axis are disposed in a distal fashion and they are constructed via two distinct migratory insertions that are stereo-determining and are under catalyst control.

Enantioselective Ruthenium-Catalyzed C-H Alkylations by a Chiral Carboxylic Acid with Attractive Dispersive Interactions

Asymmetric ruthenium-catalyzed C-H alkylations were enabled by a chiral C2-symmetric carboxylic acid. The mild cooperative ruthenium(II) catalysis set the stage for the assembly of chiral tetrahydrocarbazoles and cyclohepta[b]indoles with high levels of enantioselectivity at room temperature. Mechanistic studies by experiment and computation identified a fast C-H ruthenation, along with a rate- and enantio-determining proto-demetalation. The asymmetric induction was governed by weak attractive secondary dispersion interactions as found in NCI analysis of the key transition states.

Chiral Catalysts for Pd0 -Catalyzed Enantioselective C-H Activation

In the past few decades, processes that involve transition-metal catalysis have represented a major part of the synthetic chemist's toolbox. Recently, the interest has shifted from the well-established cross-coupling reactions to C-H bond functionalization, thus making it a current frontier of transition-metal-catalyzed reactions. Constant progress in this field has led to the discovery of enantioselective methods to generate and control various types of stereogenic elements, thereby demonstrating its high value to generate scalemic chiral molecules. The present review is dedicated to enantioselective Pd⁰ -catalyzed C-H activation, which may be considered as an evolution of Pd⁰ -catalyzed cross-couplings, with a focus on the different chiral ligands and catalysts that enable these transformations.

Pd(ii)-Catalyzed enantioconvergent twofold C–H annulation to access atropisomeric aldehydes: a platform for diversity-oriented-synthesis

The first Pd(ii)-catalyzed atroposelective dual C–H annulative strategy for diverse synthesis of functionalized axially chiral biaryls was developed.

Chiral Transient Directing Group Strategies in Asymmetric Synthesis

The development of novel methodologies for catalytic enantioselective functionalization reactions enabled by chiral transient directing groups is accompanying in a paradigm shift in the field of asymmetric synthesis. In particular, these highly atom- and step-economic enantioinduction processes commonly proceed either via enantioselective C-H functionalization, or via enantioselective hydroarylation of the pro-chiral substrates generating point, axial or planar chirality. The use of the transient directing group strategy in C-H functionalizations precludes the stoichiometric installations and removal of directing groups and enables efficient, more compatible and economical chemical routes. This minireview highlights asymmetric transition-metal-catalyzed methodologies involving chiral transient directing groups together with the scope, utility and future perspective of the field.

Rhodium-Catalyzed Atroposelective Oxidative C-H/C-H Cross-Coupling Reaction of 1-Aryl Isoquinoline Derivatives with Electron-Rich Heteroarenes

Rhodium(III)-catalyzed enantioselective oxidative C-H/C-H cross-coupling reaction between two arenes is disclosed. With the combination of a chiral CpRh(III) complex and a chiral carboxylic acid additive, the direct coupling reactions between 1-aryl isoquinoline derivatives and electron-rich heteroarenes such as thiophenes, furans, benzothiophenes, and benzofurans are realized via a double C-H functionalization process. A series of axially chiral compounds are obtained in excellent yields and enantioselectivities (up to 99% yield and 99% ee). Mechanistic studies suggest that both C-H bond cleavages may not be the turnover-limiting step.

Transition Metal-Catalyzed Enantioselective C-H Functionalization via Chiral Transient Directing Group Strategies

Transition metal-catalyzed enantioselective functionalization of C-H bond, the most abundant functionality in organic molecules, has emerged as an expedient synthetic approach to streamline the synthesis of complex chiral molecules. Despite significant progress, traditional directing group-enabled strategies require additional steps for the installation and removal of directing groups from the target molecule. The recently developed asymmetric C-H functionalization using chiral transient directing groups (cTDGs) offers a promising alternative that can circumvent this obstacle and therefore simplify the process. In this Minireview, we briefly discuss the advent and recent advances of this emerging concept, with an emphasis on discussing the creation of various stereogenic centers and the developments of cTDGs. Applications in natural product synthesis and ligand derivatizations are also discussed. We hope this Minireview will highlight the great potential of this strategy and help to inspire further endeavors.

Enantioselective Pallada-Electrocatalyzed C-H Activation by Transient Directing Groups: Expedient Access to Helicenes

Asymmetric pallada-electrocatalyzed C-H olefinations were achieved through the synergistic cooperation with transient directing groups. The electrochemical, atroposelective C-H activations were realized with high position-, diastereo-, and enantio-control under mild reaction conditions to obtain highly enantiomerically-enriched biaryls and fluorinated N-C axially chiral scaffolds. Our strategy provided expedient access to, among others, novel chiral BINOLs, dicarboxylic acids and helicenes of value to asymmetric catalysis. Mechanistic studies by experiments and computation provided key insights into the catalyst's mode of action.