Enantioselective Cross-Exchange between C-I and C-C σ Bonds

Radical Addition-Enabled C-C σ-Bond Cleavage/Reconstruction to Access Functional Indanones: Total Synthesis of Carexane L

C-C σ-bond cleavage and reconstruction is a significant tool for structural modification in synthetic chemistry but it remains a formidable challenge to perform on unstrained skeletons. Herein, we describe a radical addition-enabled C-C σ-bond cleavage/reconstruction reaction of unstrained allyl ketones to access various functional indanones bearing a benzylic quaternary center. The synthetic utility of this method has been showcased by the first total synthesis of carexane L, an indanone-based natural product.

Recent advances in Pd-catalyzed asymmetric cyclization reactions

Over the past few decades, there have been major developments in transition metal-catalyzed asymmetric cyclization reactions, enabling the convenient access to a wide spectrum of structurally diverse chiral carbo- and hetero-cycles, common skeletons found in fine chemicals, natural products, pharmaceuticals, agrochemicals, and materials. In particular, a plethora of enantioselective cyclization reactions have been promoted by chiral palladium catalysts owing to their outstanding features. This review aims to collect the latest advancements in enantioselective palladium-catalyzed cyclization reactions over the past eleven years, and it is organized into thirteen sections depending on the different types of transformations involved.

Ligand-Controlled Nickel-Catalyzed Reactions of Benzocyclobutenones with Alkynyltrifluoroborates: Diverse Construction of Polysubstituted Naphthols

Ligand-controlled nickel-catalyzed selective cleavage of the C1-C2 or C1-C8 bond of benzocyclobutenones (BCBs) is reported. The delicate selection of dpppe or PMe₃ as the ligand led to predictably divergent synthesis of a wide range of 1-naphthols and 2-naphthols without C2 and C3 substituents, respectively, from BCBs and potassium alkynyltrifluoroborate, and the increase in the amount of PMe₃ resulted in tandem reaction of 2 equiv of BCB with the borate to afford 3,4,5-trisubstituted 2-naphthols. The fabulous ligand effect resulted in the facile and unique construction of multisubstituted naphthols with well-controlled regioselectivity and a high degree of structural diversity.

Palladium- and nickel-catalyzed cascade enantioselective ring-opening/coupling reactions of cyclobutanones

The chemistry of small ring compounds is an intriguing subject in organic chemistry. As the smallest stable cyclic aliphatic ketones, cyclobutanones have garnered tremendous attention owing to their intrinsic high reactivity such as transition-metal catalyzed C-C bond cleavage. In this context, transition-metal catalyzed formal cycloaddition of cyclobutanones via a "cut and sew" strategy has gained marvelous advances. In contrast, an alternative reaction paradigm, i.e., transition-metal catalyzed ring-opening reactions of cyclobutanones, is still underdeveloped. This feature article aims to summarize our efforts in developing enantioselective palladium-catalyzed ring-opening/coupling reactions and recently emerging nickel-catalyzed ring-opening/reductive coupling reactions of cyclobutanones with a tethered aryl halide. The possible mechanisms are briefly showcased and the advantages and limitations of each strategy as well as their synthetic applications in the synthesis of natural products or bioactive compounds are presented.

Atropisomers with Axial and Point Chirality: Synthesis and Applications

Enantiopure atropisomers have become increasingly important in asymmetric synthesis and catalysis, pharmaceutical science, and material science since the discovery of inherent features of axial chirality originating from rotational restriction. Despite the advances made in this field to date, it remains highly desirable to construct structurally diverse atropisomers with potentially useful functions. We propose superposition to match axial and point chirality as a potentially useful strategy to access structurally complex and diverse building blocks for organic synthesis and pharmaceutical science because merging atropisomeric backbones with one or more extra chiral elements can topologically broaden three-dimensional environments to create complex scaffolds with multiple tunable parameters. Over the past decade, we have successfully implemented a strategic design for the superposition of axial and point chirality to develop a series of enantiopure atropisomers and have utilized the synergistic functions of these molecules to enhance chirality transfer in various catalytic asymmetric transformations.In this Account, we present several novel atropisomers with superposed axial and point chirality developed in our laboratory. In our studies, this superposition strategy was used to design and synthesize both biaryl and non-biaryl atropisomers from commercially available chiral sources. Consequently, these atropisomers were used to demonstrate the importance of the synergetic functions of axial and point chirality in specific enantioselective reactions. For example, aromatic amide-derived atropisomers, simplified as Xing-Phos arrays, were broadly employed in Ag-catalyzed [3 + 2] cycloaddition by a series of reactions of aldiminoesters with activated alkenes and imines, as well as being used as chiral solvating agents for the discrimination of optically active mandelic acid derivatives. Considering the powerful potential of non-biaryl atropisomers for asymmetric catalysis, we also explored the transition-metal-catalyzed enantioselective construction of a novel backbone of non-biaryl atropisomers (Ar-alkene, Ar-N axis) bearing both axial and point chirality for the design and synthesis of chiral ligands and functional molecules.The studies presented herein are expected to stimulate further research efforts on the development of functional atropisomers by superposition of matching axial and point chirality. In addition to tunable electron and stereohindrance effects, the synergy between matching chiral elements of axial/point chirality and functional groups is proven to be a special function that cannot be ignored for promoting reactivity and chirality-transfer efficiency in enantioselective synthesis. Consequently, our novel types of scaffolds with superposed axial and point chirality that are capable of versatile coordination with various metal catalysts in asymmetric catalysis highlight the power of the superposition of matching axial and point chirality for the construction of synthetically useful atropisomers.

Formal C-H/C-I Metathesis: Site-Selective C-H Iodination of 2-Aryl Benzoic Acid Derivatives Using Aryl Iodide

C-H functionalization via functional group metathesis is extremely rare. A protocol of remote site-selective C-H iodination of 2-aryl benzoic acid derivatives via formal C(sp²)-H/C(sp²)-I metathesis using readily available 1-iodo-4-methoxy-2-nitrobenzene as the mild iodinating reagent was reported herein. A range of 2-aryl benzoic acid derivatives including 2-(naphthalen-1-yl)benzoic acids and [1,1'-binaphthalene]-2-carboxylic acids were iodinated under mild conditions to give valuable iodinated products in a site- and chemo-selective fashion.

Catalytic Enantioselective Synthesis of γ-Lactams with β-Quaternary Centers via Merging of C-C Activation and Sulfonyl Radical Migration

Transition-metal-catalyzed C-C activation has become synthetically valuable; however, it rarely involves single-electron downstream processes. To expand the repertoire of C-C activation, here we describe the discovery of a Rh-catalyzed enantioselective C-C activation involving migration of a sulfonyl radical. This reaction directly transforms cyclobutanones containing a sulfonamide-tethered 1,3-diene moiety into γ-lactams containing a β-quaternary center with excellent enantioselectivity. This unusual process involves cleavage of C-C and N-S bonds and subsequent formation of C-N and C-S bonds. The reaction also exhibits broad functional group tolerance and a good substrate scope. A combined experimental and computational mechanistic study suggested that the reaction goes through a Rh(I)-mediated oxidative addition into the cyclobutanone C-C bond followed by a Rh(III)-triggered N-S bond homolysis and sulfonyl radical migration.

Merging C-C σ-bond activation of cyclobutanones with CO2 fixation via Ni-catalysis

A carboxylative Ni-catalyzed tandem C-C σ-bond activation of cyclobutanones followed by CO₂-electrophilic trapping is documented as a direct route to synthetically valuable 3-indanone-1-acetic acids. The protocol shows an adequate functional group tolerance and useful chemical outcomes (yield up to 76%) when AlCl₃ is adopted as an additive. Manipulations of the targeted cyclic scaffolds and a mechanistic proposal based on experimental evidence complete the investigation.

Reversible C-C bond formation using palladium catalysis

A widely appreciated principle is that all reactions are fundamentally reversible. Observing reversible transition metal-catalysed reactions, particularly those that include the cleavage of C-C bonds, is more challenging. The development of palladium- and nickel-catalysed carboiodination reactions afforded access to the cis and trans diastereomers of the iodo-dihydroisoquinolone products. Using these substrates, an extensive study investigating the reversibility of C-C bond formation using a simple palladium catalyst was undertaken. Herein we report a comprehensive investigation of reversible C-C bond formation using palladium catalysis employing diastereomeric neopentyl iodides as the starting point. It was shown that both diastereomers could be converted to a common product under identical catalytic conditions. A combination of experimental and computational studies were used to probe the operative mechanism. A variety of concepts key to understanding the process of reversible C-C bond formations were investigated, including the effect of electronic and steric parameters on the C-C bond-cleavage step.

Transition-Metal-Catalyzed Enantioselective Synthesis of Indoles from 2-Alkynylanilines

Optically active indole derivatives are ubiquitous in natural products and are widely recognized as privileged components in pharmacologically relevant compounds. Therefore, developing catalytic asymmetric approaches for constructing indole derivatives is highly desirable. In this short review, we summarize methods for the transition-metal-catalyzed enantioselective synthesis of indoles from 2-alkynyl­anilines.

1 Introduction

2 Aminometalation-Triggered Asymmetric Cross-Coupling Reactions/Insertion

2.1 Asymmetric Cross-Coupling Reactions

2.2 Asymmetric Insertion of C=O, C=C and C≡N Bonds

3 Asymmetric Relay Catalysis

4 Conclusion

Palladium-catalyzed enantioselective domino ring-opening/Hiyama coupling of cyclobutanones: development and application to the synthesis of (+)-herbertene-1,14-diol

A palladium-catalyzed enantioselective domino ring-opening/Hiyama coupling of cyclobutanones is reported, providing chiral 1-indanones bearing a versatile alkenyl group.

Palladium-catalysed alkenyl and carbonylative C-C bond activation of cyclobutanones

A palladium catalysed C-C bond activation of cyclobutanones for the construction of alkenyl and carbonylated indanones has been developed. The in situ generated σ-alkylpalladium intermediate VIA C-C bond cleavage of cyclobutanone could be trapped with N-tosylhydrazones and carbon monoxide, respectively. The reactions were carried out under mild conditions with excellent functional group tolerance.

Recent Strategies for Carbon-Halogen Bond Formation Using Nickel

Nickel catalysis has demonstrated the capability of performing a broad range of synthetically challenging transformations over the last decade. Though recent literature has focused on the formation of C-C and C-N bonds, a variety of breakthroughs in the field of C-X bond generation have also been reported. A diverse range of strategies using nickel have been developed, in an effort to expand the scope and synthetic utility of these halogenation methods. This Minireview will cover six emerging strategies in this field including: oxidatively induced C-X reductive elimination, triflate-to-halogen exchange reactions, directed C-H halogenation, non-directed electrophilic C-H halogenation of arenes, enantioselective α-fluorination of carbonyl containing compounds, and 1,2-difunctionalization-halogenation reactions. The final section has been split into two parts: nickel-catalyzed hydrohalogenation and nickel-catalyzed carbohalogenation reactions.

Site-Selective C-C Cleavage of Benzocyclobutenones Enabled by a Blocking Strategy Using Nickel Catalysis

Controlling the chemo- and regioselectivity of transition-metal-catalyzed C-C activation remains a great challenge. The transformations of benzocyclobutenones (BCBs) usually involve the cleavage of C1-C2 bond. In this work, an unprecedented highly selective cleavage of C1-C8 bond with the insertion of alkynes is achieved by using blocking strategy via Ni catalysis, providing an efficient method for synthesis of 1,8-disubstituted naphthalenes. Notably, the blocking group could be readily removed after the transformation.

Cycloisomerization of Carbamoyl Chlorides in Hexafluoroisopropanol: Stereoselective Synthesis of Chlorinated Methylene Oxindoles and Quinolinones

Hexafluoroisopropanol (HFIP) was employed as an additive for the generation of 3-(chloromethylene)oxindoles via the chloroacylation of alkyne-tethered carbamoyl chlorides. This reaction avoids the use of a metal catalyst and accesses products in high yields and stereoselectivities. Additionally, this reaction is scalable and proved amenable to a series of product derivatizations, including the synthesis of nintedanib. The reactivity of alkene-tethered carbamoyl chlorides with hexafluoroisopropanol (HFIP) was harnessed towards the synthesis of 2-quinolinones.

The Discovery of Multifunctional Chiral P Ligands for the Catalytic Construction of Quaternary Carbon/Silicon and Multiple Stereogenic Centers

The development of highly effective chiral ligands is a key topic in enhancing the catalytic activity and selectivity in metal-catalyzed asymmetric synthesis. Traditionally, the difficulty of ligand synthesis, insufficient accuracy in controlling the stereoselectivity, and poor universality of the systems often become obstacles in this field. Using the concept of nonequivalent coordination to the metal, our group has designed and synthesized a series of new chiral catalysts to access various carbon/silicon and/or multiple stereogenic centers containing products with excellent chemo-, diastereo-, and enantioselectivity.In this Account, we summarize a series of new phosphine ligands with multiple stereogenic centers that have been developed in our laboratory. These ligands exhibited good to excellent performance in the transition-metal-catalyzed enantioselective construction of quaternary carbon/silicon and multiple stereogenic centers. In the first section, notable examples of the design and synthesis of new chiral ligands by non-covalent interaction-based multisite activation are described. The integrations of axial chirality, atom-centered chirality, and chiral anions and multifunctional groups into a single scaffold are individually highlighted, as represented by Ar-BINMOLs and their derivative ligands, HZNU-Phos, Fei-Phos, and Xing-Phos. In the second, third, and fourth sections, the enantioselective construction of quaternary carbon stereocenters, multiple stereogenic centers, and silicon stereogenic centers using our newly developed chiral ligands is summarized. These sections refer to detailed reaction information in the chiral-ligand-controlled asymmetric catalysis based on the concept of nonequivalent coordination with multisite activation. Accordingly, a wide array of transition metal and main-group metal catalysts has been applied to the enantioselective synthesis of chiral heterocycles, amino acid derivatives, cyclic ketones, alkenes, and organosilicon compounds bearing one to five stereocenters.This Account shows that this new model of multifunctional ligand-controlled catalysts exhibits excellent stereocontrol and catalytic efficiency, especially in a stereodivergent and atom-economical fashion. Furthermore, a brief mechanistic understanding of the origin of enantioselectivity from our newly developed chiral catalyst systems could inspire further development of new ligands and enhancement of enantioselective synthesis by asymmetric metal catalysis.

Rhodium-catalyzed enone carbonyl directed C–H activation for the synthesis of indanones containing all-carbon quaternary centers

Rhodium(iii)-catalyzed enone carbonyl directed C–H activation/annulation of α-aroyl ketene dithioacetals with diazo compounds has been realized for the synthesis of β-quaternary indanones.

Palladium-catalyzed gaseous CO-free carbonylative C–C bond activation of cyclobutanones

A palladium-catalyzed carbonylative C–C bond activation reaction of cyclobutanones is reported, and it affords a variety of indanones bearing ester or amide groups using phenyl formate and benzene-1,3,5-triyl triformate as CO surrogates.

Rhodium(II)-catalyzed multicomponent assembly of α,α,α-trisubstituted esters via formal insertion of O-C(sp3)-C(sp2) into C-C bonds

The direct cleavage of C(CO)−C single bonds, delivering otherwise inaccessible compounds, is a significant challenge. Although the transition metal-catalyzed insertion of functional groups into C(CO)−C bonds has been studied, strained ketone substrates or chelating assistance were commonly required. In this article, we describe a rhodium(II)-catalyzed three-component reaction of 1,3-diones, diazoesters, and N,N-dimethylformamide (DMF), leading to an unusual formal insertion of O–C(sp³)–C(sp²) into unstrained C(CO)−C bonds. This procedure provides a rapid entry to a gamut of otherwise inaccessible α,α,α-trisubstituted esters/amide from relatively simple substrates in a straightforward manner. 55 examples of highly decorated products demonstrate the broad functional group tolerance and substrate scope. The combination of control experiments and isotope-labeling reactions support that O, C(sp³), and C(sp²) units derive from 1,3-diones, diazoesters, and DMF, respectively.

The direct cleavage of C(CO)−C single bonds is usually restricted to strained ketone substrates or to chelating assistance. Here, the authors show a rhodium(II)-catalyzed three-component reaction of 1,3-diones, diazoesters, and DMF, leading to an unusual formal insertion of O–C(sp³)–C(sp²) into unstrained C(CO)–C bonds.

Metal-Catalyzed Approaches toward the Oxindole Core

The oxindole scaffold is a privileged structural motif that is found in a variety of bioactive targets and natural products. Moreover, derivatives of the oxindole structure are widely present in a number of biologically relevant compounds and are key intermediates in the synthesis of diverse natural products and pharmaceuticals. Therefore, novel methods to obtain oxindoles remain of high priority in synthetic organic chemistry.Over the past several decades, novel transition-metal-catalyzed methodologies have been applied toward the synthesis of a variety of heterocycles. A detailed mechanistic understanding facilitates the disruption of traditional catalytic pathways to access useful synthetic intermediates. The strategies employed have generally revolved around the generation of high-energy organometallic intermediates, which undergo cyclization reactions through domino processes. Domino cyclization methodologies are therefore attractive, as they allow facile access to functionalized oxindoles containing all-carbon quaternary centers or tetrasubstituted olefins with high chemo- and stereoselectivities. Furthermore, these developed synthetic strategies can often be easily applied in the syntheses of other related scaffolds.In this Account, we discuss the three unique strategies that our group has leveraged for the synthesis of valuable oxindole scaffolds. The first section in this Account outlines the use of an initial oxidative addition to a C(sp²)-X bond, followed by a migratory insertion, yielding a neopentyl species amenable to a variety of subsequent functionalizations. From this reactive neopentyl metal species, we have reported C-X reductive eliminations, anionic capture cascade reactions, and intramolecular C-H functionalization processes. The second section of this Account summarizes our group's findings on 1,2-insertions of a metal-nucleophile species across an unsaturation, generating a reactive organometallic intermediate; subsequent reactions with tethered electrophiles form the desired heterocyclic core. We have explored a wide array of transition metal-catalyzed strategies using this approach, including rhodium-catalyzed conjugate additions, an asymmetric copper-catalyzed borylcupration, and a palladium(II)-catalyzed chloropalladation protocol. The final section of this Account details the use of dual-metal catalysis to perform a cyclization through a C-H functionalization-allylation domino reaction. Throughout this Account, we provide details of mechanistic studies that better enabled our understanding of the domino processes.Overall, our group has developed methods exploiting the unique reactivity of palladium, nickel, copper, rhodium, and ruthenium catalysts to develop methods toward a wide array of oxindole scaffolds. On the basis of the utility, diversity, and applicability of the strategies developed, we believe that they will prove to be highly useful in the syntheses of other important targets and inspire further development and mechanistic understanding of various metal-catalyzed processes.

Catalytic C-C Cleavage/Alkyne-Carbonyl Metathesis Sequence of Cyclobutanones

A ring-opening/alkyne-carbonyl metathesis sequence of alkyne-tethered cyclobutanones catalyzed by AgSbF₆ is realized for the first time to furnish multisubstituted naphthyl ketones under mild conditions. A range of substrates decorated with various substituents at different positions were all well accommodated. Preliminary mechanistic studies show that silver salt acted as a Lewis acid to facilitate both C-C cleavage of the cyclobutanone moiety and the subsequent metathesis between C═O and C≡C bonds.

Enantioselective Desymmetrization of Cyclobutanones: A Speedway to Molecular Complexity

Cyclobutanones hold a privileged role in enantioselective desymmetrization because their inherent ring strain allows for a variety of unusual reactions to occur. Current strategies include α-functionalization, rearrangement, and C-C bond activation to directly convert cyclobutanones into a wide range of enantiomerically enriched compounds, including many biologically significant scaffolds. This Minireview provides an overview of state-of-the-art methods that generate complexity from prochiral cyclobutanones in a single operation.

Nickel-Catalyzed Asymmetric Domino Ring Opening/Cross-Coupling Reaction of Cyclobutanones via a Reductive Strategy

Herein we demonstrate the successful application of reductive strategy in the asymmetric domino ring opening/cross-coupling reaction of prochiral cyclobutanones. Under the catalysis of a chiral nickel complex, various aryl iodide-tethered cyclobutanones were reacted with alkyl bromides as the electrophilic coupling partner, providing a variety of chiral indanones bearing a quaternary stereogenic center in highly enantioselective manner, which can be further converted to diverse benzene-fused cyclic compounds including indane, indene, dihydrocoumarin, and dihydroquinolinone. The preliminary mechanistic investigations support a mechanism involving Ni(I)-mediated enantiotopic C-C σ-bond activation of cyclobutanones as key elementary step in the catalytic cycle.

Enantioselective cyanation via radical-mediated C-C single bond cleavage for synthesis of chiral dinitriles

Ring-opening reaction via selective cleavage of C–C bond is known as a powerful strategy for construction of complex molecules. Complementary to the ionic process focusing on mostly small ring systems, radical-mediated C–C bond cleavage offers a solution for further diverse enantioselective functionalization benefited from its mild conditions, whereas such asymmetric transformations are still limited to three-membered rings so far. Herein, we describe radical-mediated ring-opening and enantioselective cyanation of four- and five-membered cycloketone oxime esters to access chiral 1,5- and 1,6-dinitriles. Employment of dual photoredox/copper catalysis is essential for the asymmetric ring-opening cyanation of cyclopentanone oxime esters. Both reactions proceed under mild conditions giving chiral dinitriles in high yields and enantioselectivity with low catalyst loading and broad substrate scope. The products dinitriles can be converted to valuable optically active diamides and diamines. Mechanistic studies indicate that the benzylic radical generated via C–C single bond cleavage is involved in the catalytic cycle.

Ring-opening reaction via selective cleavage of C–C bond is a powerful strategy to increase molecular complexity. Here the authors present the facile synthesis of chiral 1,5- and 1,6-dinitriles via the radical-mediated ring-opening and enantioselective cyanation of cycloketone oxime esters.

Enantioselective palladium/copper-catalyzed C-C σ-bond activation synergized with Sonogashira-type C(sp3)-C(sp) cross-coupling alkynylation

A highly enantioselective Pd/Cu-catalyzed tandem C–C bond activation/Sonogashira reaction was developed with the aid of new TFSi-Phos, which provided chiral alkynyl indanones in good yields and enantioselectivities.

The Sonogashira-type cross-coupling reaction is one of the most significant alkynylation transformations in organic chemistry. However, highly enantioselective alkynylation via the Sonogashira-type cross-coupling reaction is rather limited, mainly due to the difficulties in matching the stereoselective induction of chiral ligands with the combinational behavior of Pd/Cu-based bimetallic catalysts. We herein report novel enantioselective palladium/copper-catalyzed alkyl alkynylation of cyclobutanones with terminal alkynes via tandem C–C bond activation/Sonogashira-type cross coupling reaction, in which a novel chiral TADDOL-derived phosphoramidite ligand bearing fluorine and silicon-based bulky groups simplified as TFSi-Phos is found to be an efficient ligand for both C(sp²)–C(sp³) bond cleavage and new C(sp³)–C(sp) bond formation. A wide range of chiral alkynylated indanones bearing an all-carbon quaternary stereocenter are obtained efficiently with up to 97.5 : 2.5 er.