Enantio- and Z-Selective δ-Hydroarylation of Aryl-Substituted 1,3-Dienes via Rh-Catalyzed Conjugate Addition

Metal-catalyzed enantioselective conjugate arylations of electron-poor alkenes are highly selective processes for C(sp2)-C(sp3) bond formation. δ-Selective hydroarylations of electron-poor 1,3-dienes are less well developed and reactions that deliver high enantioselectivity while giving single alkene isomer products are elusive. Here we report the Rh-catalyzed δ-arylation of aryl-substituted 1,3-dienes that gives nearly exclusive Z-1,4-addition products (generally with >95 : 5 positional and geometrical selectivity). This remote functionalization provides access to chiral diarylated alkenes from readily available precursors poised for further functionalization, including in the synthesis of bioactive molecules. Mechanistic studies suggest that protonolysis of a Rh-allyl intermediate generated by diene insertion into a Rh-aryl is the turnover limiting step and occurs by an inner-sphere proton transfer pathway.

Chiral Diene Ligands in Asymmetric Catalysis

Asymmetric catalysis has emerged as a general and powerful approach for constructing chiral compounds in an enantioselective manner. Hence, developing novel chiral ligands and catalysts that can effectively induce asymmetry in reactions is crucial in modern chemical synthesis. Among such chiral ligands and catalysts, chiral dienes and their metal complexes have received increased attention, and a great progress has been made over the past two decades. This review provides comprehensive and critical information on the essential aspects of chiral diene ligands and their importance in asymmetric catalysis. The literature covered ranges from August 2003 (when the first effective chiral diene ligand for asymmetric catalysis was reported) to October 2021. This review is divided into two parts. In the first part, the chiral diene ligands are categorized according to their structures, and their preparation methods are summarized. In the second part, their applications in asymmetric transformations are presented according to the reaction types.

Ruthenium catalyzed β-selective alkylation of vinylpyridines with aldehydes/ketones via N2H4 mediated deoxygenative couplings

Umpolung (polarity reversal) tactics of aldehydes/ketones have greatly broadened carbonyl chemistry by enabling transformations with electrophilic reagents and deoxygenative functionalizations. Herein, we report the first ruthenium-catalyzed β-selective alkylation of vinylpyridines with both naturally abundant aromatic and aliphatic aldehyde/ketones via N₂H₄ mediated deoxygenative couplings. Compared with one-electron umpolung of carbonyls to alcohols, this two-electron umpolung strategy realized reductive deoxygenation targets, which were not only applicable to the regioselective alkylation of a broad range of 2/4-alkene substituted pyridines, but also amenable to challenging 3-vinyl and steric-embedded internal pyridines as well as their analogous heterocyclic structures.

Ruthenium-catalyzed β-selective alkylation of vinylpyridines with carbonyls (both aromatic and aliphatic ketones/aldehydes) via N₂H₄ mediated deoxygenative couplings was achieved.

Enantioselective Synthesis of Chiral Indane Derivatives by Rhodium-Catalyzed Addition of Arylboron Reagents to Substituted Indenes

Rhodium-catalyzed asymmetric addition of arylboron reagents to indene derivatives proceeded to give 2-arylindanes in good yields with high enantioselectivity. Deuterium-labeling experiments indicated that the present reaction involved a 1,4-Rh shift from an initially formed benzylrhodium to an arylrhodium intermediate before protonation leading to the corresponding addition product. The asymmetric addition was also successful for acenaphthylene, which has a similar skeleton to indene, where it was found that the benzylrhodium intermediate underwent direct protonation without the 1,4-Rh shift.

Enantioselective Synthesis of Functionalized Arenes by Nickel-Catalyzed Site-Selective Hydroarylation of 1,3-Dienes with Aryl Boronates

A catalytic method for the site-selective and enantioselective synthesis of functionalized arenes by the intermolecular hydroarylation of terminal and internal 1,3-dienes with aryl pinacolato boronates is reported. The reactions are promoted by 5.0 mol % of a readily available monodentate phosphoramidite-Ni complex in ethanol, affording a variety of enantioenriched products in up to 96 % yield and 99:1 er. Mechanistic studies indicate that Ni-allyl formation is irreversible and related to the nature of the arylboronate.

Nickel(II)-Catalyzed Addition of Aryl-, Alkenyl-, and Alkylboronic Acids to Alkenylazaarenes

A nickel(II)-catalyzed addition of aryl-, alkenyl-, and alkylboronic acids to alkenylazaarenes was presented. This reaction exhibited high efficiency (up to 93% yield), a broad substrate scope (seven types of heterocycles), and good functional group compatibility. The resulting products can be further transformed to many useful building blocks. Finally, the preliminary studies suggested that the adjacent N atom of the heterocycles was essential for the high reactivity.

A Transient-Directing-Group Strategy Enables Enantioselective Reductive Heck Hydroarylation of Alkenes

Metal-coordinating directing groups have seen extensive use in the field of transition-metal-catalyzed alkene functionalization; however, their waste-generating installation and removal steps limit the efficiency and practicality of reactions that rely on their use. Inspired by developments in asymmetric organocatalysis, where reactions rely on reversible covalent interactions between an organic substrate and a chiral mediator, we have developed a transient-directing-group approach to reductive Heck hydroarylation of alkenyl benzaldehyde substrates that proceeds under mild conditions. Highly stereoselective migratory insertion is facilitated by in situ formation of an imine from catalytic amounts of a commercially available amino acid additive. Computational studies reveal an unusual mode of enantioinduction by the remote chiral center in the transient directing group.

Copper-catalyzed asymmetric silyl addition to alkenyl-substituted N-heteroarenes

Asymmetric conjugate addition of PhMe₂SiBPin to a wide range of N-heteroaryl alkenes proceeded in the presence of a copper catalyst coordinated with a chiral phosphoramidite ligand to afford useful β-silyl N-heteroarenes in high yields and ees.

Rh-Catalyzed Highly Enantioselective Synthesis of Aliphatic Sulfonyl Fluorides

Rh-catalyzed, highly enantioselective (up to 99.8% ee) synthesis of aliphatic sulfonyl fluorides was accomplished. This protocol provides a portal to a class of novel 2-aryl substituted chiral sulfonyl fluorides, which are otherwise extremely difficult to access. This asymmetric synthesis has the feature of mild conditions, excellent functional group compatibility, and wide substrate scope (51 examples) generating a wide array of structurally unique chiral β-arylated sulfonyl fluorides for sulfur(VI) fluoride exchange (SuFEx) click reaction and drug discovery.

Enantioselective Conjugate Addition of Aryl Halides and Triflates to Electron-Deficient Olefins via Nickel- and Rhodium-Catalyzed Sequential Relay Reactions

Asymmetric conjugate addition of aryl halides or aryl triflates to electron-deficient olefins was realized by sequential Miyaura borylation and Hayashi-Miyaura conjugate addition in one pot. A nickel-catalyzed borylation of aryl halides or triflates and a rhodium-chiral diene complex catalyzed enantioselective conjugate addition was executed as a pair of relay reactions as a more efficient and greener protocol.

Kinetic Resolution and Dynamic Kinetic Resolution of Chromene by Rhodium-Catalyzed Asymmetric Hydroarylation

A highly efficient kinetic resolution and dynamic kinetic resolution of chromene is reported for the first time and they procced by a rhodium-catalyzed asymmetric hydroarylation pathway. This new approach offers versatile access to various chiral 2,3-diaryl-chromanes containing vicinal stereogenic centers, as well as the recovered chiral flavenes, in high yields with excellent ee values (s factor up to 532). Particularly noteworthy is that this strategy can be further extended to the establishment of a dynamic version of the kinetic resolution of chromene acetals and allows complete access to chiral isoflavanes and α-aryl hydrocoumarins.

Rhodium-catalyzed asymmetric addition of arylboronic acids to 2H-chromenes leading to 3-arylchromane derivatives

The Rh-catalyzed enantioselective addition of arylboronic acids to 2H-chromenes proceeded to give 3-arylchromanes with high enantioselectivity.

Bifunctional iminophosphorane catalysed enantioselective sulfa-Michael addition of alkyl thiols to alkenyl benzimidazoles

The first enantioselective sulfa-Michael addition of alkyl thiols to alkenyl benzimidazoles, enabled by a bifunctional iminophosphorane (BIMP) organocatalyst, is described. The iminophosphorane moiety of the catalyst provides the required basicity to deprotonate the thiol nucleophile while the chiral scaffold and H-bond donor control facial selectivity. The reaction is broad in scope with respect to the thiol and benzimidazole reaction partners with the reaction proceeding in up to 98% yield and 96 : 4 er.

Conjugate Addition-Enantioselective Protonation of N-Aryl Glycines to α-Branched 2-Vinylazaarenes via Cooperative Photoredox and Asymmetric Catalysis

An enantioselective protonation strategy has been successfully applied to the synthesis of chiral α-tertiary azaarenes. With a dual catalytic system involving a chiral phosphoric acid and a dicyanopyrazine-derived chromophore (DPZ) photosensitizer that is mediated by visible light, a variety of α-branched 2-vinylpyridines and 2-vinylquinolines with N-aryl glycines underwent a redox-neutral, radical conjugate addition-protonation process and provided valuable chiral 3-(2-pyridine/quinoline)-3-substituted amines in high yields with good to excellent enantioselectivities (up to >99% ee). An application of this methodology to a two-step synthesis of the enantiomerically pure medicinal compound pheniramine (Avil) is also presented.

Enantioselective Intermolecular Pd-Catalyzed Hydroalkylation of Acyclic 1,3-Dienes with Activated Pronucleophiles

We report a highly enantioselective Pd-PHOX-catalyzed intermolecular hydroalkylation of acyclic 1,3-dienes. Meldrum's acid derivatives and other activated C-pronucleophiles, such as β-diketones and malononitriles, react with a variety of aryl- and alkyl-substituted dienes in ≤20 h at room temperature. The coupled products, obtained in up to 96% yield and 97.5:2.5 er, are easily transformed into useful chemical building blocks for downstream synthesis.

Ni-catalysed regioselective 1,2-diarylation of unactivated olefins by stabilizing Heck intermediates as pyridylsilyl-coordinated transient metallacycles

We report a Ni-catalysed diarylation of unactivated olefins in dimethylpyridylvinylsilane by intercepting Heck C(sp3)-NiX intermediates, derived from aryl halides, with arylzinc reagents. This approach utilizes a modifiable pyridylsilyl moiety as a coordinating group that plays a dual role of intercepting oxidative addition species to promote Heck carbometallation, and stabilizing the Heck C(sp3)-NiX intermediates as transient metallacycles to suppress β-hydride elimination, and facilitate transmetalation/reductive elimination. This method affords 1,2-diarylethylsilanes, which can be readily oxidized to 1,2-diarylethanols that occur as structural motifs in 3-aryl-3,4-dihydroisocoumarin and dihydrostilbenoid natural products.

Cu-Catalyzed Enantioselective Boron Addition to N-Heteroaryl-Substituted Alkenes

Catalytic enantioselective Cu-B(pin) (pin = pinacolato) addition to N-heteroaryl-substituted alkenes followed by protonation promoted by phosphine-Cu complexes is presented. The resulting alkylboron products that contain a N-heteroaryl moiety are afforded in up to 97% yield and 99:1 enantiomeric ratio. The highly versatile C-B(pin) bond can be converted to a range of useful functional groups, delivering a variety of enantiomerically enriched building blocks that are otherwise difficult to access. The utility of this method is further demonstrated by application to a fragment synthesis of biologically active molecule U-75302. Preliminary mechanistic studies revealed that the adjacent N atom of the heterocycles plays a unique role in high reactivity and enantioselectivity.

Highly enantioselective catalytic synthesis of chiral pyridines

General methods to prepare chiral pyridine derivatives are greatly sought after due to their significance in medicinal chemistry. Here, we report highly enantioselective catalytic transformations of poorly reactive β-substituted alkenyl pyridines to access a wide range of alkylated chiral pyridines. The simple methodology involves reactivity enhancement via Lewis acid (LA) activation, the use of readily available and highly reactive Grignard reagents, and a copper-chiral diphosphine ligand catalyst. Apart from allowing the introduction of different linear, branched, cyclic, and functionalised alkyl chains at the β-position of alkenyl pyridines, the catalytic system also shows high functional group tolerance.

Chiral pyridines are valuable building blocks in medicinal chemistry applications. Here, the authors report the copper-catalysed Lewis acid-assisted asymmetric alkylation of β-substituted alkenyl pyridines with Grignard reagents affording chiral pyridines with excellent enantioselectivity.

Chiral Pincer Carbodicarbene Ligands for Enantioselective Rhodium-Catalyzed Hydroarylation of Terminal and Internal 1,3-Dienes with Indoles

Catalytic enantioselective addition of N-heteroarenes to terminal and internal 1,3-dienes is reported. Reactions are promoted by 5 mol % of Rh catalyst supported by a new chiral pincer carbodicarbene ligand that delivers allylic substituted arenes in up to 95% yield and up to 98:2 er. Mechanistic and X-ray evidence is presented that supports that the reaction proceeds via a Rh(III)-η³-allyl.

A nordehydroabietyl amide-containing chiral diene for rhodium-catalysed asymmetric arylation to nitroolefins

A non-covalently interacting, phellandrene derived, nordehydroabietyl amide-containing chiral diene was developed in rhodium catalysed asymmetric arylation.

Copper-catalyzed borylative coupling of vinylazaarenes and N-Boc imines

Diastereoselective Cu-catalyzed three-component couplings of vinylazaarenes, B₂(pin)₂, and N-Boc imines are described.

Enantioselective synthesis of bicyclo[3.n.1]alkanes by chiral phosphoric acid-catalyzed desymmetrizing Michael cyclizations

2,2-Disubstituted cyclic 1,3-diketones containing a tethered electron-deficient alkene undergo chiral phosphoric acid-catalyzed desymmetrizing Michael cyclizations to give bridged bicyclic products in high enantioselectivities. Both bicyclo[3.2.1]octanes and bicyclo[3.3.1]nonanes are accessible using this methodology.

Applications of Bartoli indole synthesis

In 1989, the reaction of vinyl magnesium halides with ortho-substituted nitroarenes leading to indoles was discovered. This reaction is now frequently reported as the "Bartoli reaction" or the "Bartoli indole synthesis" (BIS). It has rapidly become the shortest and most flexible route to 7-substituted indoles, because the classical indole syntheses generally fail in their preparation. The flexibility of the Bartoli reaction is great as it can be extended to heteroaromatic nitro derivatives and can be run on solid support. This review will focus on the use of the Bartoli indole synthesis as the key step in preparations of complex indoles, which appeared in the literature in the last few years.

Rhodium-catalyzed asymmetric arylation of allyl sulfones under the conditions of isomerization into alkenyl sulfones

The reaction of 3-sulfolene with arylboronic acids in the presence of a chiral diene-rhodium catalyst under highly basic conditions (10 equiv of KOH) gave high yields of 3-arylsulfolanes with high enantioselectivity, where 3-sulfolene is in equilibration with 2-sulfolene by base-catalyzed isomerization and the more reactive 2-sulfolene undergoes the rhodium-catalyzed asymmetric arylation.

Computational modelling of the enantioselectivity in the asymmetric 1,4-addition of phenylboronic acid to a bulky, doubly pro-chiral maleimide catalyzed by a Rh/chiral diene complex

Computational chemistry is a powerful tool for understanding chemical reactions used for the synthesis of chiral compounds.

Computational modelling of the enantioselectivity in the asymmetric 1,4-addition reaction catalyzed by a Rh complex of a S-chiral disulfoxide

Computational chemistry is a powerful tool for understanding chiral catalysis and can also aid future catalyst design.

A second-generation ligand for the enantioselective rhodium-catalyzed addition of arylboronic acids to alkenylazaarenes

A 2,4,6-trialkylanilide-containing chiral diene has been identified as a superior ligand for the enantioselective rhodium-catalyzed arylation of alkenylazaarenes with arylboronic acids.