Palladium-Catalyzed Enantioselective Intermolecular Coupling of Phenols and Allylic Alcohols

Stereoselective Synthesis of 1,n-Dicarbonyl Compounds Through Palladium-Catalyzed Ring Opening/Isomerization of Densely Substituted Cyclopropanols

We report a highly diastereoselective protocol for the synthesis of 1,4- and 1,5-dicarbonyl compounds from densely substituted cyclopropanols. The methodology involves a palladium-catalyzed ring opening reaction followed by a "metal-walk" and oxidation of a remote hydroxyl group. The methodology represents a new application of cyclopropanols as initiation sites for chain walking remote functionalization. Importantly, this approach provides a straightforward access to highly valuable succinaldehyde derivatives bearing vicinal quaternary and tertiary stereocenters as single diastereomers.

Immobilization of Trifluoromethyl-Substituted Pyridine-Oxazoline Ligand and Its Application in Asymmetric Continuous Flow Synthesis of Benzosultams

This study presents an improved synthetic route to ligand (S)-4-(tert-butyl)-2-(5-(trifluoromethyl)pyridin-2-yl)-4,5-dihydrooxazole and its application as a highly active and enantioselective catalyst in the addition of arylboronic acids to cyclic N-sulfonylketimines. Immobilization of such a ligand was achieved using a commercially available starting material and a PS-PEG TentaGel S NH2 support, resulting in a stable heterogeneous catalyst. Although the anchored catalyst exhibited a slight reduction in enantioselectivity and a 4-fold decrease in reaction rate, it displayed remarkable stability, enabling 10 consecutive reaction cycles. Furthermore, the successful transition to a continuous flow system demonstrated even higher turnover numbers compared to batch arrangements. These findings provide valuable insights into the development of efficient flow reactors for continuous synthesis of benzosultams, further advancing the field of asymmetric catalysis.

Mechanistic Insights on the Selectivity of the Tandem Heck-Ring-Opening of Cyclopropyldiol Derivatives

The preparation of a new class of alkenyl cyclopropyl diols, easily available through a copper-catalyzed carbometalation reaction of cyclopropenes, has enabled the study of key mechanistic aspects of the tandem Heck-cyclopropane ring-opening reaction. Utilizing these substrates containing two distinct hydroxyl groups allowed us to examine parameters affecting the reaction outcome and selectivity. The combination of these experimental results with detailed DFT studies shed light on the mechanism governing the regio- and stereoselectivity of the cyclopropane ring-opening. A thorough investigation displayed the dual roles fulfilled by the hydroxyl group during the reaction, which is key to this remarkable transformation. In addition to its mechanistic implication, the reaction granted access to various lactones possessing up to four stereocenters as a single diastereomer, conveniently prepared in only two catalytic steps from easily accessible achiral cyclopropenes.

Switchable Oxidative Reactions of N-allyl-2-Aminophenols: Palladium-Catalyzed Alkoxyacyloxylation vs an Intramolecular Diels-Alder Reaction

The Pd(II)-catalyzed reaction of N-allyl-2-aminophenols in the presence of PhI(OCOR)₂ as the oxidant resulted in an alkoxyacyloxylation process, with the formation of functionalized dihydro-1,4-benzoxazines. The reaction performed in the absence of palladium catalyst switched to an intramolecular Diels–Alder reaction (IMDA) pathway, which was the result of an oxidative dearomatization of the 2-aminophenol, nucleophilic addition, and Diels–Alder reaction cascade, highlighting the role of the oxidant as both a nucleophilic donor and an oxidizing agent.

Asymmetric Palladium-Catalyzed Oxycarbonylation of Terminal Alkenes: Efficient Access to β-Hydroxy Alkylcarboxylic Acids

A novel Pd^II -catalyzed enantioselective oxycarbonylation of alkenes has been established. The ligand with an ethyl group at the C-6 position of Pyox plays a significant role in the intermolecular oxypalladation process, leading to high reactivity and excellent enantioselective control. Compared to the conventional methods, the reaction itself features alkenes as easily prepared starting materials, mild and operationally simple reaction conditions, and insensitivities to air and water. Moreover, this method allows for broad alkene substrate scope, excellent regio- and enantioselectivities, scalabilities and a wide array of applications, and provides a useful route for the convenient and straightforward synthesis of chiral β-hydroxy alkylcarboxylic acids/esters.

Effects of ligands on the migratory insertion of alkenes into rhodium-oxygen bonds

Migratory insertions of olefins into metal-oxygen bonds are elementary steps of important catalytic processes, but well characterised complexes that undergo this reaction are rare, and little information on the effects of ancillary ligands on such reactions has been gained. We report a series of alkoxo alkene complexes of rhodium(i) that contain a range of bidentate ligands and that undergo insertion of the alkene. Our results show that complexes containing less electron-donating ancillary ligands react faster than their counterparts containing more electron-donating ancillary ligands, and that complexes possessing ligands with larger bite angles react faster than those with smaller bite angles. External added ligands had several effects on the reactions, including an inhibition of olefin isomerisation in the product and acceleration of the displacement of the product from complexes of ancillary ligands with small bite angles. Complementary computational studies help elucidate the details of these insertion processes.

Development and Mechanistic Interrogation of Interrupted Chain-Walking in the Enantioselective Relay Heck Reaction

The formation of alkyl-palladium complexes via the nucleopalladation of alkenes is the entry point for a wide range of diverse reactions. One possibility is that the intermediate alkyl-Pd complexes can undergo a "chain-walking" event, to allow for remote functionalization through various termination processes. However, there are few methods to selectively interrupt the chain-walking process at a prescribed location. Herein, we demonstrate that a variety of homoallylic protected amines undergo an interrupted enantioselective relay Heck reaction to give enantioenriched allylic amine products. The selectivity of this process can be diverted to exclusively yield the ene-amide products by virtue of changing the nature of the amine protecting group. To rationalize this observation, we combine experiment and computation to investigate the mechanism of the chain-walking process and termination events. Isotopic labeling experiments and the computed reaction pathways suggest that the system is likely under thermodynamic control, with the selectivity being driven by the relative stability of intermediates encountered during chain-walking. These results illustrate that the chain-walking of alkyl-palladium complexes can be controlled through the alteration of thermodynamic processes and provides a roadmap for exploiting these processes in future reaction development.

Ru-catalyzed isomerization of ω-alkenylboronates towards stereoselective synthesis of vinylboronates with subsequent in situ functionalization

The stereoselective preparation of synthetically versatile vinylboronates from ω-alkenylboronates is achieved through a ruthenium-catalyzed isomerization reaction. A variety of di- and trisubstituted vinylboronates were conveniently produced and could be used as a new starting point for subsequent in situ remote functionalization through either a sequential Ru/Pd or Ru/Cu double catalytic system.

A regio- and stereoselective ruthenium-catalyzed isomerization of ω-alkenyl boronates into stereodefined di- and trisubstituted alkenylboronate derivatives is reported.

Palladium-catalyzed enantioselective alkenylation of alkenylbenzene derivatives

A regioselective and enantioselective palladium-catalyzed relay Heck alkenylation of alkenylbenzene derivatives to construct remote stereocenters is disclosed. Various β-substituted styrenes were readily obtained in moderate yields with good to excellent levels of enantioselectivity. This strategy provides rapid access to enantioenriched δ, ε, ζ, and η-alkenyl aryl compounds from simple starting materials. Mechanistic studies suggest that termination of the relay reaction is controlled by affinity of the arene for the Pd complex during migration.

Palladium-Catalyzed Enantioselective Alkenylation of Enelactams Using a Relay Heck Strategy

In this report, a palladium-catalyzed redox-relay Heck process to access optically active alkenylated α,β-unsaturated lactams is described. Under mild reaction conditions, electron-deficient alkenyl triflates and electron-rich alkenyl iodonium salts undergo enantioselective and site-selective coupling with enelactams to deliver the products in high yields and excellent enantioselectivities. Furthermore, the products allow facile access to natural products such as (+)-calvine and (+)-2-epicalvine in addition to the bioactive molecule aza-goniothalamin.

Synthesis of bicyclic ethers by a palladium-catalyzed oxidative cyclization-redox relay-π-allyl-Pd cyclization cascade reaction

Bicyclic ether scaffolds are found in a variety of natural products and are of interest in probe and drug discovery. A palladium-catalyzed cascade reaction has been developed to provide efficient access to these scaffolds from readily available linear diene-diol substrates. A Pd redox-relay process is used strategically to transmit reactivity between an initial oxypalladative cyclization and a subsequent π-allyl-Pd cyclization at remote sites. The reaction affords a variety of bicyclic ether scaffolds with complete diastereoselectivity for cis-ring fusion.

Enantioselective Markovnikov Addition of Carbamates to Allylic Alcohols for the Construction of α-Secondary and α-Tertiary Amines

Herein we describe the development of a Pd-catalyzed enantioselective Markovnikov addition of carbamates to allylic alcohols for the construction of α-tertiary and α-secondary amines. The reaction affords a range of β-amino alcohols, after reduction of the aldehyde in situ, which contain a variety of functional groups in moderate yields and moderate to good enantioselectivities. These products can be readily oxidized to β-amino acids, valuable building blocks for the synthesis of biologically active compounds. Mechanistic studies indicate that the C-N bond formation occurs via a syn amino-palladation mechanism, an insight which may guide future reaction development given the limited number of enantioselective syntheses of α-tertiary amines.

Enantioselective N-Alkylation of Indoles via an Intermolecular Aza-Wacker-Type Reaction

The development of an intermolecular and enantioselective aza-Wacker reaction is described. Using indoles as the N-source and a selection of alkenols as the coupling partners selective β-hydride elimination toward the alcohol was achieved. This strategy preserves the newly formed stereocenter by preventing the formation of traditionally observed enamine products. Allylic and homoallylic alcohols with a variety of functional groups are compatible with the reaction in high enantioselectivity. Isotopic-labeling experiments support a syn amino-palladation mechanism for this new class of aza-Wacker reactions.

Asymmetric Aza-Wacker-Type Cyclization of N-Ts Hydrazine-Tethered Tetrasubstituted Olefins: Synthesis of Pyrazolines Bearing One Quaternary or Two Vicinal Stereocenters

We have developed an asymmetric aza-Wacker-type cyclization of N-Ts hydrazine-tethered tetrasubstituted olefins, affording optically active pyrazolines bearing chiral tetrasubstituted carbon stereocenters. This reaction is tolerant to a broad range of substrates under mild reaction conditions, giving the desired chiral products with high enantioselectivities. Generation of two vicinal stereocenters on the C═C double bonds was also achieved with high selectivities, a process which has been rarely studied for Wacker-type reactions. A mechanistic study revealed that this aza-Wacker-type cyclization undergoes a syn-aminopalladation process. It was also found that for substrates bearing two linear alkyl substituents on the outer carbon atom of the olefin, both of which are larger than a methyl group, the alkyl substituent that is cis to the intranucleophilic group participates more readily in β-hydride elimination. When one of the two alkyl substituents on the outer carbon atom of the olefin is a methyl group, β-hydride elimination proceeds selectively at the methylene side, thus both diastereomers can be prepared via switching the configuration of the olefin. Furthermore, the product can be converted to a pharmaceutical compound in high yields over three steps.

Palladium-Catalyzed Enantioselective Relay Heck Arylation of Enelactams: Accessing α,β-Unsaturated δ-Lactams

In this Communication, we describe the construction of chiral α,β-unsaturated δ-lactams, widely used as pharmacophores, in high yields and excellent enantioselectivities using an oxidative relay Heck arylation reaction. This strategy also allows facile access to 7-substituted α,β-unsaturated ε-lactam products and δ-lactams containing a tetrasubstituted nitrogen-bearing stereocenter.

Formation of Chiral Allylic Ethers via an Enantioselective Palladium-Catalyzed Alkenylation of Acyclic Enol Ethers

This report details a palladium-catalyzed process to access highly functionalized, optically active allylic aryl ethers. A number of electron-deficient alkenyl triflates underwent enantioselective and site-selective coupling with acyclic aryl enol ethers in the presence of a chiral palladium catalyst. This transform provides chiral allylic ether products in high yields and excellent enantiomeric ratios, furnishing a unique disconnection to incorporate heteroatoms at a stereocenter. Finally, the applicability of the products to target synthesis was demonstrated through the formation of a chiral allylic alcohol and the generation of a flavone-inspired product.

Renaissance of pyridine-oxazolines as chiral ligands for asymmetric catalysis

Oxazoline-containing ligands have been widely employed in numerous asymmetric catalytic reactions. Pyridine-oxazoline-type ligands, a class of hybrid ligands, were designed earlier than bisoxazoline and phosphine-oxazoline ligands; however, their unique properties have only been discovered recently. Pyridine-oxazoline-type chiral ligands are rapidly becoming popular for use in asymmetric catalysis, especially for several new and efficient asymmetric methodologies. Several types of challenging asymmetric reactions have been discovered recently using pyridine-oxazoline-type ligands showing their special properties and potential for future application in a wide range of new catalytic methodologies. This review provides an overview of this field, with the aim of highlighting both ligand design and synthetic methodology development.

Direct Synthesis of Secondary Benzylic Alcohols Enabled by Photoredox/Ni Dual-Catalyzed Cross-Coupling

An operationally simple, mild, redox-neutral method for the cross-coupling of α-hydroxyalkyltrifluoroborates is reported. Utilizing an Ir photocatalyst, α-hydroxyalkyl radicals are generated from the single-electron oxidation of the trifluoroborates, and these radicals are subsequently engaged in a nickel-catalyzed C-C bond-forming reaction with aryl halides. The process is highly selective, functional group tolerant, and step economical, which allows the direct synthesis of secondary benzylic alcohol motifs.

Enantioselective Synthesis of Chromenes via a Palladium-Catalyzed Asymmetric Redox-Relay Heck Reaction

A palladium-catalyzed asymmetric redox-relay Heck reaction of 4H-chromenes and arylboronic acids has been successfully developed. The reaction proceeded in moderate to good yields with good to high enantioselectivities. The resulting product is an advanced intermediate of bio-active compound BW683C.

Palladium-catalyzed oxidative amination of homoallylic alcohols: sequentially installing carbonyl and amino groups along an alkyl chain

A novel Pd-catalyzed intermolecular oxidative amination of homoallylic alcohols affording valuable β-amino acetones under mild conditions has been developed.