Ruthenium-Catalyzed Enantioselective C-H Functionalization: A Practical Access to Optically Active Indoline Derivatives

Sulfoximine Aided Ru(II)-Catalyzed Asymmetric Double C(sp2)-H Hydroarylations of Olefins

Presented here is a sulfoximine-directed Ru(II)-catalyzed asymmetric intramolecular double C(sp2)-H hydroarylation of olefins. This process provides a diastereoselective and enantiospecific synthetic route to highly substituted tetrahydrobenzodifurans. Notably, the reaction accommodates labile functional groups and is scalable to gram quantities.

Synthesis of indolines via palladium-catalyzed [4 + 1] annulation of (2-aminophenyl)methanols with sulfoxonium ylides

A facile strategy for the synthesis of valuable indolines has been developed, involving a palladium(II)/Brønsted acid co-catalyzed annulation of readily available (2-aminophenyl)methanols and sulfoxonium ylides. This protocol allows for the direct utilization of the OH group as a leaving group, tolerates alkyl and aryl groups on the N atom of the aniline moiety, operates under mild reaction conditions, and exhibits good efficiency.

H8-BINOL-Derived Chiral η6-Benzene Ligands: New Opportunities for the Ruthenium-Catalyzed Asymmetric C-H Activation

Given the tremendous success of (p-cymene)RuII-catalyzed C-H activation over the past 20 years, the community has long been aware that the development of chiral η6-benzene (Ben) ligands should be a potent strategy for achieving the attractive but incredibly underdeveloped ruthenium(II)-catalyzed asymmetric C-H activation. However, it has rarely been achieved due to the severe difficulty in developing proper chiral Ben ligands. In particular, designing chiral Ben ligands by connecting a benzene fragment to a chiral framework including benzene rings remained an unsolved challenge until this effort. Here we present a novel class of axially chiral Ben ligands derived from readily available (S)-5,5',6,6',7,7',8,8'-octahydro-1,1'-bi-2-naphthol ((S)-H8-BINOL) in 4-8 steps. Notably, when coordinated with ruthenium, such chiral Ben ligand containing three benzene rings only forms one of the three possible isomeric BenRuII complexes. The related chiral BenRuII catalysts could effectively catalyze the asymmetric C-H activation of N-sulfonyl ketimines with alkynes, affording a range of chiral spirocyclic sultams in up to 99 % yield with up to >99 % ee. These catalysts are expected to find broad applications in future.

DLPNO-CCSD(T) and DFT study of the acetate-assisted C-H activation of benzaldimine at [RuCl2(p-cymene)]2: the relevance of ligand exchange processes at ruthenium(II) complexes in polar protic media

To gain mechanistic insights into the acetate-assisted cyclometallations of arylimines promoted by [RuCl2(p-cymene)]2 in polar protic media, DFT geometry optimizations (with M06 and ωB97X-D3 functionals and the cc-pVDZ-PP[Ru] basis set) followed by DLPNO-CCSD(T)/CBS energy evaluations were performed using benzaldimine as a model substrate and methanol as the solvent (with CPCM or SMD models). The calculation results show that coordination of the imine to an acetate ruthenium precursor is followed by anion (chloride or acetate) dissociation as the rate-determining step of the process. H-Bonding of two explicit MeOH to the anion reduces the calculated activation energy to ca. 23 kcal mol-1, in good agreement with the experimental half-life at room temperature. Subsequent AMLA/CMD C-H activation of the intermediate cationic complexes is a faster, reversible process. Alternative reaction pathways involving neutral diacetate ruthenium complexes offer AMLA/CMD transition state structures of lower energy but are precluded due to higher energy barriers for the initial ligand exchange processes at ruthenium. Solvent assistance accelerates the final chloride/acetate exchange processes on the cycloruthenate intermediates, particularly when compression in the condensed phase is taken into consideration. The performance of six DFT functionals (with the aug-pVTZ-PP[Ru] basis set) was assessed using the DLPNO-CCSD(T)/CBS reference energies. Neutral diacetate ruthenium complexes were incorrectly predicted as being kinetically relevant when using hybrid DFT methods (PBE0-D3(BJ), M06-2X or ωB97M-V). Good agreement between the calculated barrier heights and our benchmark energy results was obtained by using double-hybrid DFT methods. PWPB95 with D3(BJ) or D4 dispersion energy corrections was found to be the most accurate (ΔG≠ MUE of ca. 1 kcal mol-1). This study may aid our understanding of and help with further experimental investigations of synthetically useful carboxylate-assisted C-H bond functionalizations involving (N,C)-cyclometallated (p-cymene)Ru(II) intermediate complexes in sustainable polar protic solvents.

Planar-chiral arene ruthenium complexes: synthesis, separation of enantiomers, and application for catalytic C-H activation

Heating tert-butyl-tetraline with [(p-cymene)RuCl2]2 produces the racemic complex [(arene)RuCl2]2, which can be separated into enantiomers by chromatography of its diastereomeric adducts with chiral phosphine ligand. The resolved chiral complex catalyzes C-H activation of N-methoxy-benzamides and their annulation with N-vinyl-pivaloyl amide giving dihydroisoquinolones in 50-80% yields and with 40-80% enantiomeric excess.

Asymmetric Ruthenium-Catalyzed C-H Activation by a Versatile Chiral-Amide-Directing Strategy

A versatile and readily available chiral amide directing group has been developed for the ruthenium(II)-catalyzed asymmetric C-H activation. Asymmetric C-H activation of the related chiral benzamides with various olefins, aldehydes and propargylic alcohols has been accomplished with high stereoselectivities, affording a series of chiral products including 3,4-dihydroisocoumarins (up to 96 % ee), isocoumarins (up to 92 % ee), phthalides (up to 99 % ee), chiral bicyclo[2.2.1]heptanes (>20 : 1 dr), 4-alkylidene-3,4-dihydroisocoumarins (up to 97 % ee) and allenes (>20 : 1 dr). Importantly, our methodologies enabled concise syntheses of many biologically active compounds and natural products (e.g., Montroumarin, Cyclosporone E, Cyclosporone Q, Concentricolide, Chuangxinol, and Eleutherol).

Rhodium-Catalyzed Two-Fold, Regioselective and Enantioselective C-H Activation: an Efficient Strategy to Chiral Single-Benzene-Based Fluorophores

A Rh-catalyzed two-fold, regioselective and enantioselective C-H activation via chiral transient directing group strategy has been demonstrated in moderate to good yields with commendable enantioselectivities. The newly synthesized chiral fluorophores exhibit favorable photophysical properties, including large Stokes shifts, good fluorescence quantum yields, aggregation-induced emission in aqueous solution, and intense emission and circularly polarized luminescence in the solid state, indicating great potential applications as chiral fluorescent probes or optoelectronic materials.

Continuous-Flow Enantioselective Hydroacylations under Heterogeneous Chiral Rhodium Catalysts

Transition metal-catalyzed enantioselective C-H bond functionalizations have become efficient methods for the synthesis of complex optically active molecules. Heterogeneous catalysts for this chemistry remain largely unexplored despite the advantages they offer in terms of ease of separation and reuse of catalysts. Herein, we report the development of heterogeneous chiral Rh catalysts for continuous-flow enantioselective hydroacylations. Heterogeneous catalysts could be prepared simply by mixing supports and Rh complexes. The prepared catalysts exhibited excellent activity and enantioselectivity affording optically active ketones in quantitative yields with 99 % ee's. Under the optimized reaction conditions, a turnover number >300 was achieved without the leaching of Rh species. The catalysts exhibited a wide substrate scope and in sequential-flow reactions with other heterogeneous catalysts, the syntheses of biologically active molecules and functional materials were demonstrated.

Synthesis of N-Fused Polycyclic Indole Derivatives via Ru(II)-Catalyzed C-H Bond Activation and Intramolecular Hydroarylation

A new synthesis of N-fused tetracyclic indole derivatives and their related polycyclic analogues has been developed based on ruthenium(II)-catalyzed C-H activation and intramolecular hydroarylation. A series of polycyclic indoles with a 3-formyl group have been prepared in good to high yields. Various aliphatic and aromatic amines have been studied to form a transient directing group with the aldehyde for the catalytic process. A significant impact of the structures of the aromatic amines was identified, and 1-naphthylamine was shown to enable the catalytic process. DFT computations were performed to gain further insight into the role of the transient directing groups.

Palladium-Catalyzed Enantioselective Intramolecular Heck Dearomative Annulation of Indoles with N-Tosylhydrazones

An elegant Pd(dba)2-catalyzed enantioselective Heck dearomative annulation of indoles and N-tosylhydrazones for the straightforward assembly of structurally diverse optically active indoline scaffolds containing the quaternary carbon centers at the C2 position has been developed. The tandem protocol, which utilized a Pd(dba)2/BINOL-based phosphoramidite ligand as the catalytic system, proceeded smoothly through successive oxidative addition, intramolecular carbon palladation, migratory insertion, and β-elimination sequences, leading to the chiral indoline derivatives in moderate to excellent yields, with excellent enantioselectivities and diastereoselectivities. In addition, the synthetic practicability of the catalytic system was underlined by a scaled-up experiment and the late-stage derivatization of the products, thus highlighting the potential applications in synthetic chemistry, medicinal chemistry, and material science.

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.

Synthesis of 1-Aryltetralins via Re2O7/HReO4 Mediated Intramolecular Hydroarylations

Here, we describe highly efficient intramolecular hydroarylations mediated by Re2O7/HReO4. Styrene derivatives of different electronic properties have been activated to effect a challenging intramolecular hydroarylation for the facile access to various substituted 1-aryltetralin structures. This method is characterized by mild reaction conditions, broad substrate scope, high chemical yields, and 100% atom economy. The potential synthetic application of this methodology was exemplified by the efficient total synthesis of an isoCA-4 analogue.

Synthesis of Naphthalimides through Tandem Pd(II)-Catalyzed C(sp3)-H Oxidation and Diels-Alder Reaction Using a Transient Directing Group Strategy

Naphthalimides have found extensive applications in materials science and pharmaceuticals. It is still highly desirable to develop efficient methods for the synthesis of naphthalimides with structural diversity. In this work, we developed a new approach for the synthesis of naphthalimides via a tandem reaction of o-methylbenzaldehydes and maleimides. The tandem reaction involves Pd(II)-catalyzed benzylic C(sp³)-H oxidation using an amino acid as the transient directing group and Diels-Alder reaction. The subsequent dehydration forms naphthalimides. The reaction introduces the imide moiety and constructs a benzene ring simultaneously, allowing for easy access to a range of naphthalimides with a variety of substituents.

Palladium-Catalyzed ortho-C-H Alkoxycarbonylation of Aromatic Aldehydes via a Transient Directing Group Strategy

Transient directing groups (TDGs) can be a powerful strategy for directly functionalizing C-H bonds of aldehydes. We report a palladium-catalyzed o-C-H alkoxycarbonylation of benzaldehydes using a catalytic amount of aromatic amine to form a transient imine that plays the role of a monodentate TDG. The reaction conditions were applied to a broad range of aldehydes, and the corresponding 2-formyl benzoates were used as direct precursors for the synthesis of phthalides and 1-isoindolinones.

Enantioselective C-H Bond Functionalization Involving Arene Ruthenium(II) Catalysis

The p-Cymene ruthenium(II) complex is one of the most widely used catalysts in C-H activation. However, enantioselective C-H activation promoted by arene ruthenium(II) complexes has not been realized until recently. The revealed strategies include intramolecular nitrene C-H insertion, the use of chiral transient directing groups, chiral carboxylic acid, relay catalysis, and chiral arene ligands. In this minireview, these advances are summarized and discussed in the hope of spurring further developments.

Ruthenium(II)/Imidazolidine Carboxylic Acid-Catalyzed C-H Alkylation for Central and Axial Double Enantio-Induction

Enantioselective C-H activation has surfaced as a transformative toolbox for the efficient assembly of chiral molecules. However, despite of major advances in rhodium and palladium catalysis, ruthenium(II)-catalyzed enantioselective C-H activation has thus far largely proven elusive. In contrast, we herein report on a ruthenium(II)-catalyzed highly regio-, diastereo- and enantioselective C-H alkylation. The key to success was represented by the identification of novel C2-symmetric chiral imidazolidine carboxylic acids (CICAs), which are easily accessible in a one-pot fashion, as highly effective chiral ligands. This ruthenium/CICA system enabled the efficient installation of central and axial chirality, and featured excellent branched to linear ratios with generally >20 : 1 dr and up to 98 : 2 er. Mechanistic studies by experiment and computation were carried out to understand the catalyst mode of action.

Ru(II)-Catalyzed Hydroarylation of in situ Generated 3,3,3-Trifluoro-1-propyne by C-H Bond Activation: A Facile and Practical Access to β-Trifluoromethylstyrenes

In this study, a practical and straightforward synthesis of β-(E)-trifluoromethylstyrenes by ruthenium-catalyzed C-H bond activation was developed. The readily available and inexpensive 2-bromo-3,3,3-trifluoropropene (BTP), a non-ozone depleting reagent, was used as a reservoir of 3,3,3-trifluoropropyne. With this approach, the monofunctionalization of a panel of heteroarenes was possible in a safe and scalable manner (23 examples, up to 87 % yield). Mechanistic investigations and density functional theory (DFT) calculations were also conducted to get a better understanding of the mechanism of this transformation. These studies suggested that 1) a cyclometallated ruthenium complex enabled the transformation, 2) this complex exhibited high efficiency in this transformation compared to the commercially available [RuCl2 (p-cymene)]2 and 3) the mechanism proceeded through a bis-cyclometallated ruthenium intermediate for the carboruthenation step.

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.

HFIP in Organic Synthesis

1,1,1,3,3,3-Hexafluoroisopropanol (HFIP) is a polar, strongly hydrogen bond-donating solvent that has found numerous uses in organic synthesis due to its ability to stabilize ionic species, transfer protons, and engage in a range of other intermolecular interactions. The use of this solvent has exponentially increased in the past decade and has become a solvent of choice in some areas, such as C-H functionalization chemistry. In this review, following a brief history of HFIP in organic synthesis and an overview of its physical properties, literature examples of organic reactions using HFIP as a solvent or an additive are presented, emphasizing the effect of solvent of each reaction.

Chemodivergent Synthesis of Indeno[1,2-b]indoles and Isoindolo[2,1-a]indoles via Mn(III)-Mediated or Electrochemical Intramolecular Radical Cross-Dehydrogenative Coupling

Chemodivergent synthesis of indeno[1,2-b]indoles and isoindolo[2,1-a]indoles from the same starting materials involving radical cross-dehydrogenative couplings have been developed. Mn(OAc)₃·2H₂O selectively promoted an intramolecular radical C-H/C-H dehydrogenative coupling reaction to provide indeno[1,2-b]indoles, while an intramolecular radical C-H/N-H dehydrogenative coupling reaction could proceed via electrochemistry to deliver isoindolo[2,1-a]indoles. Plausible mechanisms of the chemodivergent reactions were proposed.

Chiral Arene Ligand as Stereocontroller for Asymmetric C-H Activation

Development of chiral ligands is the most fundamental task in metal-catalyzed asymmetric synthesis. In the last 60 years, various kinds of ligands have been sophisticatedly developed. However, it remains a long-standing challenge to develop practically useful chiral η⁶ -arene ligands, thereby seriously hampering the asymmetric synthesis promoted by arene-metal catalysts. Herein, we report the design and synthesis of a class of readily tunable, C₂ -symmetric chiral arene ligands derived from [2.2]paracyclophane. Its ruthenium(II) complexes have been prepared and successfully applied in the enantioselective C-H activation to afford a series of axially chiral isoquinolones (up to 99 % yield and 96 % ee). This study not only lays chemists' longstanding doubts about whether it is possible to use chiral arene ligands to stereocontrol ruthenium(II)-catalyzed asymmetric C-H activation, but also opens up a new avenue to achieve asymmetric C-H activation.

Asymmetric Ru/Cinchonine Dual Catalysis for the One-Pot Synthesis of Optically Active Phthalides from Benzoic Acids and Acrylates

Herein, we report the asymmetric Ru/cinchonine dual catalysis that provides straightforward access to enantioselective synthesis of C-3 substituted phthalides via tandem C-H activation/Michael addition cascade. The use of readily accessible and less expensive [RuCl₂(p-cym)]₂ and cinchonine catalyst for the one-pot assembly of chiral phthalides greatly overcomes the present trend of using highly sophisticated catalysts. The developed method provides access to both enantiomers of a product using pseudoenantiomeric cinchona alkaloids as catalysts streamlining the synthesis of phthalide in both the optically active forms.

DFT Case Study on the Comparison of Ruthenium-Catalyzed C-H Allylation, C-H Alkenylation, and Hydroarylation

Density functional calculations at the B3LYP-D3+IDSCRF/TZP-DKH(-dfg) level of theory have been performed to understand the mechanism of ruthenium-catalyzed C-H allylation reported in the literature in depth. The plausible pathway consisted of four sequential processes, including C-H activation, migratory insertion, amide extrusion, and recovery of the catalyst, in which C-H activation was identified as the rate-determining step. The amide extrusion step could be promoted kinetically by trifluoroacetic acid since its mediation lowered the free-energy barrier from 32.1 to 12.2 kcal/mol. Additional calculations have been performed to explore other common pathways between arenes and alkenes, such as C-H alkenylation and hydroarylation. A comparison of the amide extrusion and β-H elimination steps established the following reactivity sequence of the leaving groups: protonated amide group > β-H group > unprotonated amide group. The suppression of hydroarylation was attributed to the sluggishness of the Ru-C protonation step as compared to the amide extrusion step. This study can unveil factors favoring the C-H allylation reaction.

Application of a transient directing strategy in cyclization reactions via C–H activation

This review introduces seven types of cyclization reactions via C–H activation using a transient directing strategy.

Nickel(0)-catalysed linear-selective hydroarylation of 2-aminostyrenes with arylboronic acids by a bifunctional temporary directing group strategy

We report a nickel(0)-catalyzed linear-selective hydroarylation of 2-aminostyrenes with arylboronic acids using a bifunctional temporary directing group strategy. In the presence of a catalytic amount of commercially available 3,5-dibromosalicylaldehyde, an...

Monodentate Transient Directing Group-Assisted Palladium-Catalyzed Direct ortho-C-H Iodination of Benzaldehydes for Total Synthesis of Hernandial

The first palladium-catalyzed direct o-C-H iodination of benzaldehydes was successfully developed with the assistance of commercially available 2,5-bis(trifluoromethyl)aniline as the optimal monodentate transient directing group (MonoTDG). Moderate to excellent yields and good selectivity were achieved for a broad substrate scope under mild conditions. More importantly, the synthetic application was demonstrated by a concise two-step total synthesis of the natural product hernandial, which was accomplished by merging this new MonoTDG-assisted C-H iodination and subsequent copper-catalyzed cross-coupling.

Transient Directing Group Strategy as a Unified Method for Site Selective Direct C4-H Halogenation of Indoles

A unified method for direct C4-H halogenation of indoles has been accomplished with the assistance of anthranilic acids as suitable transient directing groups. Exclusive site selectivity (one out of five potential reactive sites) as well as good functional group tolerance was obtained to install three kinds of halogen atoms (Cl, Br and I, respectively) by using inexpensive N-halosuccinimides (NXS) as halogen sources under mild conditions. Taking advantage of the rich functional groups in the product, a diversity of nitrogen-containing heterocycles were facily constructed via one-step late-stage derivations.

Synthesis of Ruthenium Catalysts with a Chiral Arene Ligand Derived from Natural Camphor

A ruthenium complex with a chiral arene ligand [(camphor–arene)RuCl2]2 was synthesized by the reaction of RuCl3·nH2O with a chiral diene which was obtained from natural camphor in three steps. This complex catalyzed the asymmetric hydrogenation of acetophenone (64–85% ee), but decomposed in catalytic reactions involving C–H activation of 2-phenylpyridine or benzoic acid derivatives.

Transient Directing Groups in Metal-Organic Cooperative Catalysis

The direct functionalization of C-H bonds is among the most fundamental chemical transformations in organic synthesis. However, when the innate reactivity of the substrate cannot be utilized for the functionalization of a given single C-H bond, this selective C-H bond functionalization mostly relies on the use of directing groups that allow bringing the catalyst in close proximity to the C-H bond to be activated and these directing groups need to be installed before and cleaved after the transformation, which involves two additional undesired synthetic operations. These additional steps dramatically reduce the overall impact and the attractiveness of C-H bond functionalization techniques since classical approaches based on substrate pre-functionalization are sometimes still more straightforward and appealing. During the past decade, a different approach involving both the in situ installation and removal of the directing group, which can then often be used in a catalytic manner, has emerged: the transient directing group strategy. In addition to its innovative character, this strategy has brought C-H bond functionalization to an unprecedented level of usefulness and has enabled the development of remarkably efficient processes for the direct and selective introduction of functional groups onto both aromatic and aliphatic substrates. The processes unlocked by the development of these transient directing groups will be comprehensively overviewed in this review article.

Investigating the exceptional adducts of alkoxides with Ru(II)-arene cations in alkyl alcohol solution using electrospray ionization mass spectrometry

RATIONALE

Exploring the formation mechanism of the exceptional adducts of alkoxides with Ru(II)-arene cations in alkyl alcohol solution using electrospray ionization mass spectrometry (ESI-MS) is crucial for further understanding the physicochemical properties of Ru(II)-arene complexes in solution.

METHODS

All mass spectra were collected with an AB SCIEX TripleTOF 5600⁺ mass spectrometer in positive mode. Theoretical calculations were carried out using the density functional theory method at the B3LYP level with a hybrid basis set consisting of 6-31G(d,p) and LanL2DZ in the Gaussian 03 program.

RESULTS

When ruthenated [1₅ ]paracyclophanes (Ru-[1₅ ]PCPs) and Ru(II)-arene dimers were dissolved in alkyl alcohol solvents, the adducts of alkoxides with Ru(II)-arene cations were observed under positive ion mode ESI-MS, which resulted from the coordination of alkyl alcohol molecules with the Ru(II)-arene cations followed by the deprotonation of O-H bonds of the coordinated alcohols. Furthermore, the number of alkoxides binding to Ru-[1₅ ]PCPs was regulated by the number of ruthenium atoms. Attributed to good solubility and small steric hindrance, the signal intensity of the adducts of methoxides with Ru(II)-arene cations was the strongest among the three alkyl alcohols used in this study.

CONCLUSIONS

The characteristic adducts of alkoxides with Ru(II)-arene cations were pervasively present in positive ion mode ESI-MS of nine Ru(II)-arene complexes dissolved in alkyl alcohol solvents. Taking into consideration the solubility and signal response, methanol is the most suitable solvent for the ESI-MS experiments with Ru(II)-arene complexes among the solvents studied, where almost only the diagnostic adducts of methoxides with Ru(II)-arene cations are present.

Highly Enantioselective Iridium(I)-Catalyzed Hydrocarbonation of Alkenes: A Versatile Approach to Heterocyclic Systems Bearing Quaternary Stereocenters

We report a versatile, highly enantioselective intramolecular hydrocarbonation reaction that provides a direct access to heteropolycyclic systems bearing chiral quaternary carbon stereocenters. The method, which relies on an iridium(I)/bisphosphine chiral catalyst, is particularly efficient for the synthesis of five-, six- and seven-membered fused indole and pyrrole products, bearing one and two stereocenters, with enantiomeric excesses of up to >99 %. DFT computational studies allowed to obtain a detailed mechanistic profile and identify a cluster of weak non-covalent interactions as key factors to control the enantioselectivity.

Ruthenium(II)/Chiral Carboxylic Acid Catalyzed Enantioselective C–H Functionalization of Sulfoximines

Ruthenium(II)-catalyzed enantioselective C–H functionalization reactions of sulfoximines with sulfoxonium ylides are described. The combination of [RuCl2(p-cymene)]2 and a pseudo-C 2-symmetric binaphthyl monocarboxylic acid furnished the S-chiral products in 76:24 to 92:8 er.

Transient directing ligands for selective metal-catalysed C–H activation

C-H activation is a 'simple-to-complex' transformation that nature has perfected over millions of years of evolution. Transition-metal-catalysed C-H activation has emerged as an expeditious means to expand the chemical space by introducing diverse functionalities. Notably, among the strategies to selectively cleave a particular C-H bond, the catalytic use of a small molecule as co-catalyst to generate a transient directing group, which provides a balance between step economy and chemical productivity, has gained immense attention in recent years. This allows one to convert a desired C-H bond irrespective of its geometrical or stereochemical configuration. This Review describes the various transient directing groups used in C-H activation and explains their mechanistic significance.

Ruthenium-Catalyzed Deuteration of Aromatic Carbonyl Compounds with a Catalytic Transient Directing Group

A novel ruthenium-catalyzed C-H activation methodology for hydrogen isotope exchange of aromatic carbonyl compounds is presented. In the presence of catalytic amounts of specific amine additives, a transient directing group is formed in situ, which directs selective deuteration. A high degree of deuteration is achieved for α-carbonyl and aromatic ortho-positions. In addition, appropriate choice of conditions allows for exclusive labeling of the α-carbonyl position while a procedure for the preparation of merely ortho-deuterated compounds is also reported. This methodology proceeds with good functional group tolerance and can be also applied for deuteration of pharmaceutical drugs. Mechanistic studies reveal a kinetic isotope effect of 2.2, showing that the C-H activation is likely the rate-determining step of the catalytic cycle. Using deuterium oxide as a cheap and convenient source of deuterium, the methodology presents a cost-efficient alternative to state-of-the-art iridium-catalyzed procedures.

Iridium-Catalyzed Hydroarylation via C-H Bond Activation

Hydroarylation reactions via C-H activation, which compensate for shortcomings of classical methods based on the Friedel-Crafts reaction, is one of the most attractive methods to synthesize substituted arenes. This Personal Account reviews our recent studies on iridium-catalyzed intermolecular hydroarylation of vinyl ethers, alkynes, bicycloalkenes, and 1,3-dienes, and intramolecular hydroarylation of m-allyloxyphenyl ketones, where asymmetric addition reactions are included. A cationic iridium catalyst, which is generated from chloroiridium [IrCl] and NaBAr^F ₄ [Ar^F =3,5-(CF₃ )₂ C₆ H₃ ], or a hydroxoiridium [Ir(OH)] complex is effective in catalyzing the hydroarylation depending on the substrates. 1,5-Cyclooctadiene (cod), chiral dienes, and conventional bisphosphines function as ligands controlling the high reactivity and selectivity of the catalysts in the hydroarylation. H/D exchange reaction of alkenes by use of a key intermediate of the hydroarylation reaction is also described.

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

Efficient Synthesis of Sulfur-Stereogenic Sulfoximines via Ru(II)-Catalyzed Enantioselective C-H Functionalization Enabled by Chiral Carboxylic Acid

Ru(II)-catalyzed enantioselective C-H functionalization involving an enantiodetermining C-H cleavage step remains undeveloped. Here we describe a Ru(II)-catalyzed enantioselective C-H activation/annulation of sulfoximines with α-carbonyl sulfoxonium ylides using a novel class of chiral binaphthyl monocarboxylic acids as chiral ligands, which can be easily and modularly prepared from 1,1'-binaphthyl-2,2'-dicarboxylic acid. A broad range of sulfur-stereogenic sulfoximines were prepared in high yields with excellent enantioselectivities (up to 99% yield and 99% ee) via desymmetrization, kinetic resolution, and parallel kinetic resolution. Furthermore, the resolution products can be easily transformed to chiral sulfoxides and key intermediates for kinase inhibitors.