Pd(II)-Catalyzed Atroposelective C-H Allylation: Synthesis of Enantioenriched N-Aryl Peptoid Atropisomers

Atropisomeric Foldamers

This concept article explores the emerging role of atropisomerism in foldamer chemistry, a field focussed on oligomers that adopt well-defined conformations through non-covalent interactions. Atropisomerism introduces a novel dimension to foldamer design by restricting rotational freedom around single bonds to dictate molecular shape with precision. Despite the prevalence of atropisomeric bonds in organic synthesis, their application within foldamers remains underexplored. Here, we discuss key developments in both backbone and sidechain atropisomerism, and suggest future directions for atropisomeric foldamers in the context of a recent surge in atropselective synthetic methods. We propose that judicious use of atropisomerism may serve as a transformative tool in the construction of shape-defined macromolecules.

Atroposelective catalysis

Atropisomeric compounds-stereoisomers that arise from the restricted rotation about a single bond-have attracted widespread attention in recent years due to their immense potential for applications in a variety of fields, including medicinal chemistry, catalysis and molecular nanoscience. This increased interest led to the invention of new molecular motors, the incorporation of atropisomers into drug discovery programmes and a wide range of novel atroposelective reactions, including those that simultaneously control multiple stereogenic axes. A diverse set of synthetic methodologies, which can be grouped into desymmetrizations, (dynamic) kinetic resolutions, cross-coupling reactions and de novo ring formations, is available for the catalyst-controlled stereoselective synthesis of various atropisomer classes. In this Review, we generalize the concepts for the catalyst-controlled stereoselective synthesis of atropisomers within these categories with an emphasis on recent advancements and underdeveloped atropisomeric scaffolds beyond stereogenic C(sp2)-C(sp2) axes. We also discuss more complex systems with multiple stereogenic axes or higher-order stereogenicity.

Pd(II)-Catalyzed atroposelective C-H olefination: synthesis of enantioenriched N-aryl peptoid atropisomers

Herein, we reported the synthesis of enantioenriched N-aryl peptoid atropisomers via Pd(II)-catalyzed atroposelective C-H olefination using the easily accessible L-pyroglutamic acid (L-pGlu-OH) as the chiral ligand. A series of optically active N-aryl peptoid atropisomers were obtained in synthetically useful yields with high enantioselectivities.

Palladium-Catalyzed Enantioselective C-H Olefination to Access Planar-Chiral Cyclophanes by Dynamic Kinetic Resolution

Planar-chiral cyclophanes have received increasing attention for drug discovery and catalyst design. However, the catalytically asymmetric synthesis of planar-chiral cyclophanes has been a longstanding challenge. We describe the first Pd(II)-catalyzed enantioselective C-H olefination of prochiral cyclophanes. The low rotational barrier of less hindered benzene ring in the substrates allows the reaction to proceed through a dynamic kinetic resolution. This approach exhibits broad substrate scope, providing the planar-chiral cyclophanes in high yields (up to 99 %) with excellent enantioselectivities (up to >99 % ee). The ansa chain length scope studies reveal that the chirality of the cyclophanes arises from the bond rotation constraint of the benzene ring around the macrocycle plane, rather than the C-N axis. The C-H activation approach is also applicable to the late-stage modification of bioactive molecules and pharmaceuticals.

Stereodynamic Strategies to Induce and Enrich Chirality of Atropisomers at a Late Stage

Enantiomers, where chirality arises from restricted rotation around a single bond, are atropisomers. Due to the unique nature of the origins of their chirality, synthetic strategies to access these compounds in an enantioselective manner differ from those used to prepare enantioenriched compounds containing point chirality arising from an unsymmetrically substituted carbon center. In particular stereodynamic transformations, such as dynamic kinetic resolutions, thermodynamic dynamic resolutions, and deracemizations, which rely on the ability to racemize or interconvert enantiomers, are a promising set of transformations to prepare optically pure compounds in the late stage of a synthetic sequence. Translation of these synthetic approaches from compounds with point chirality to atropisomers requires an expanded toolbox for epimerization/racemization and provides an opportunity to develop a new conceptual framework for the enantioselective synthesis of these compounds.

Palladium-Catalyzed Regioselective C3-Allylic Alkylation of 2-Aryl Imidazopyridines with MBH Carbonates

Imidazopyridine is an important framework that constitutes several pharmaceutical drugs and biologically active molecules. Herein, we present the palladium-catalyzed regioselective C3-allylic alkylation of 2-aryl imidazopyridines with MBH carbonates. This strategy furnishes a broad spectrum of C3-allylated imidazopyridines, and their structures have been unequivocally established using X-ray analysis. Besides, the reaction can be easily scaled up on a gram scale, and the ensuing product can be smoothly manipulated into synthetically useful entities.

Construction of Non-Biaryl Atropisomeric Amide Scaffolds Bearing a C-N Axis via Enantioselective Catalysis

The significant scaffold offered by atropisomeric amides with a C–N chiral axis has been extensively utilized for pharmaceuticals, agricultural science, and organic syntheses. As a result, the field of atropisomer synthesis has attracted considerable interest within chemistry communities. To date, a range of catalytic atroposelective approaches has been reported for the efficient construction of these challenging scaffolds. However, greatly concise and highly useful methodologies for the synthesis of these atropisomeric compounds, focusing on transition-metal, chiral amine, and phosphoric acid catalysis reactions, etc., are still desirable. Hence, it is indispensable to succinctly and systematically present all such reports by means of disclosing the mechanistic analysis and application, as well as the challenges and issues associated with the establishment of these atropisomers. In this review, we summarize the development of catalytic asymmetric synthetic strategies to access non-biaryl atropisomers rotating around a C–N chiral axis, including the reaction methods, mechanism, late-stage transformations, and applications.

Palladium-Catalyzed C-H Functionalization of Aryl Acetamides and Benzoquinones: Synthesis of Substituted Aryl Quinones

An efficient synthesis of aryl-substituted quinones via Pd(II)-catalyzed C-H functionalization of less expensive and abundant benzoquinones with aryl acetamides is demonstrated. An auxiliary ligand N,N-bidentate-directing group 8-aminoquinoline plays a crucial role in the success of the reaction. A broad range of substituted phenyl acetamides including commercially available drug molecules were examined and also found to be highly compatible with quinones. The aryl-substituted quinones were also easily converted into aryl-substituted hydroquinone derivatives. A plausible reaction mechanism was proposed to account for the selective distal C-H bond functionalization.

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

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

Modification of [2.2]paracyclophane through cobalt-catalyzed ortho-C–H allylation and acyloxylation

The first cobalt-catalyzed ortho-C–H allylation and acyloxylation of [2,2]paracyclophanes are reported.