Copper-Catalyzed Asymmetric Functionalization of Vinyl Radicals for the Access to Vinylarene Atropisomers

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.

Creation of a Chiral All-Carbon Quaternary Center Induced by CF3 and CH3 Substituents via Cu-Catalyzed Asymmetric Conjugate Addition

Cu-catalyzed asymmetric construction of a chiral quaternary center bearing CH3 and CF3 groups was achieved with high to excellent enantioselectivity using our originally developed ligands. The asymmetric conjugate addition of Me3Al to β-CF3-substituted enones and unsaturated ketoesters proceeded efficiently. The use of unsaturated ketoesters gives optically active furanones in high yields with high enantioselectivities. The perfluoroalkyl-substituted enone does not seem to be favorable in the present reaction.

Red-light-mediated copper-catalyzed photoredox catalysis promotes regioselectivity switch in the difunctionalization of alkenes

Controlling regioselectivity during difunctionalization of alkenes remains a significant challenge, particularly when the installation of both functional groups involves radical processes. In this aspect, methodologies to install trifluoromethane (-CF3) via difunctionalization have been explored, due to the importance of this moiety in the pharmaceutical sectors; however, these existing reports are limited, most of which affording only the corresponding β-trifluoromethylated products. The main reason for this limitation arises from the fact that -CF3 group served as an initiator in those reactions and predominantly preferred to be installed at the terminal (β) position of an alkene. On the contrary, functionalization of the -CF3 group at the internal (α) position of alkenes would provide valuable products, but a meticulous approach is necessary to win this regioselectivity switch. Intrigued by this challenge, we here develop an efficient and regioselective strategy where the -CF3 group is installed at the α-position of an alkene. Molecular complexity is achieved via the simultaneous insertion of a sulfonyl fragment (-SO2R) at the β-position. A precisely regulated sequence of radical generation using red light-mediated photocatalysis facilitates this regioselective switch from the terminal (β) position to the internal (α) position. Furthermore, this approach demonstrates broad substrate scope and industrial potential for the synthesis of pharmaceuticals under mild reaction conditions.

Asymmetric Sulfonylation from a Reaction of Cyclopropan-1-ol, Sulfur Dioxide, and 1-(Alkynyl)naphthalen-2-ol

Asymmetric sulfonylation from a reaction of cyclopropan-1-ol, sulfur dioxide, and 1-(alkynyl)naphthalen-2-ol in the presence of a catalytic amount of organocatalyst at room temperature is developed. Axially chiral (S)-(E)-1-(1-(alkylsulfonyl)-2-arylvinyl)naphthalen-2-ols are generated in moderate to good yields with excellent enantioselectivity and regioselectivity under mild conditions. During this transformation, γ-keto sulfinate generated in situ from cyclopropan-1-ol and sulfur dioxide acts as the key intermediate.

Catalytic Asymmetric Difluoroalkylation Using In Situ Generated Difluoroenol Species as the Privileged Synthon

A robust and practical difluoroalkylation synthon, α,α-difluoroenol species, which generated in situ from trifluoromethyl diazo compounds and water in the presence of dirhodium complex, is disclosed. As compared to the presynthesized difluoroenoxysilane and in situ formed difluoroenolate under basic conditions, this difluoroenol intermediate displayed versatile reactivity, resulting in dramatically improved enantioselectivity under mild conditions. As demonstrated in catalytic asymmetric aldol reaction and Mannich reactions with ketones or imines in the presence of chiral organocatalysts, quinine-derived urea, and chiral phosphoric acid (CPA), respectively, this relay catalysis strategy provides an effective platform for applying asymmetric fluorination chemistry. Moreover, this method features a novel 1,2-difunctionalization process via installation of a carbonyl motif and an alkyl group on two vicinal carbons, which is a complementary protocol to the metal carbene gem-difunctionalization reaction.

Reversible Radical Addition Guides Selective Photocatalytic Intermolecular Thiol-Yne-Ene Molecular Assembly

In modern organic chemistry, harnessing the power of multicomponent radical reactions presents both significant challenges and extraordinary potential. This article delves into this scientific frontier by addressing the critical issue of controlling selectivity in such complex processes. We introduce a novel approach that revolves around the reversible addition of thiyl radicals to multiple bonds, reshaping the landscape of multicomponent radical reactions. The key to selectivity lies in the intricate interplay between reversibility and the energy landscapes governing C-C bond formation in thiol-yne-ene reactions. The developed approach not only allows to prioritize the thiol-yne-ene cascade, dominating over alternative reactions, but also extends the scope of coupling products obtained from alkenes and alkynes of various structures and electron density distributions, regardless of their relative polarity difference, opening doors to more versatile synthetic possibilities. In the present study, we provide a powerful tool for atom-economical C-S and C-C bond formation, paving the way for the efficient synthesis of complex molecules. Carrying out our experimental and computational studies, we elucidated the fundamental mechanisms underlying radical cascades, a knowledge that can be broadly applied in the field of organic chemistry.

Rhodium(I)-Catalyzed Asymmetric Hydroarylative Cyclization of 1,6-Diynes to Access Atropisomerically Labile Chiral Dienes

Axially chiral open-chained olefins are an underexplored class of atropisomers, whose enantioselective synthesis represents a daunting challenge due to their relatively low racemization barrier. We herein report rhodium(I)-catalyzed hydroarylative cyclization of 1,6-diynes with three distinct classes of arenes, enabling highly enantioselective synthesis of a broad range of axially chiral 1,3-dienes that are conformationally labile (ΔG≠ (rac)=26.6-28.0 kcal/mol). The coupling reactions in each category proceeded with excellent enantioselectivity, regioselectivity, and Z/E selectivity under mild reaction conditions. Computational studies of the coupling of quinoline N-oxide system reveal that the reaction proceeds via initial oxidative cyclization of the 1,6-diyne to give a rhodacyclic intermediate, followed by σ-bond metathesis between the arene C-H bond and the Rh-C(vinyl) bond, with subsequent C-C reductive elimination being enantio-determining and turnover-limiting. The DFT-established mechanism is consistent with the experimental studies. The coupled products of quinoline N-oxides undergo facile visible light-induced intramolecular oxygen-atom transfer, affording chiral epoxides with complete axial-to-central chirality transfer.

Copper-Catalyzed Fluoroalkylphosphorothiolation of Alkynes for the Synthesis of (E)-β-Fluoroalkyl Vinyl Phosphorothioates

A convenient copper-catalyzed three-component radical-based fluoroalkylphosphorothiolation of terminal alkynes with (iPrO)2P(O)SH and fluoroalkylation reagent for the synthesis of a variety of (E)-β-fluoroalkyl vinyl phosphorothioates with excellent regioselectivity and stereoselectivity has been developed. All the starting materials used in this reaction are highly stable and readily available. Thus, this process features with mild reaction conditions, simple operation and good functional group tolerance (>40 examples). Furthermore, this modular reaction system allows the late-stage functionalization of bioactive molecules.

Coordinating activation strategy enables 1,2-alkylamidation of alkynes

The radical 1,2-difunctionalization reaction of alkynes has been evolved into a versatile approach for expeditiously increasing the complexity of the common feedstock alkyne. However, intermolecular 1,2-carboamidation with general alkyl groups is an unsolved problem. Herein, we show that a coordinating activation strategy could act as an efficient tool for enabling radical 1,2-alkylamidation of alkynes. With the employment of diacyl peroxides as both alkylating reagents and internal oxidants, a large library of β-alkylated enamides is constructed in a three-component manner from readily accessible amides and alkynes. This protocol exhibits broad substrate scope with good functional group compatibility and is amenable for late-stage functionalization of natural molecules and biologically compounds.

Transition-metal-catalyzed atroposelective synthesis of axially chiral styrenes

Axially chiral styrenes, a type of atropisomer analogous to biaryls, have attracted great interest because of their unique presence in natural products and asymmetric catalysis. Since 2016, a number of methodologies have been developed for the atroposelective construction of these chiral skeletons, involving both transition metal catalysis and organocatalysis. In this feature article, we aim to provide a comprehensive understanding of recent advances in the asymmetric synthesis of axially chiral styrenes catalyzed by transition metals, integrating scattered work with different catalytic systems together. This feature article is cataloged into five sections according to the strategies, including asymmetric coupling, enantioselective C-H activation, central-to-axial chirality transfer, asymmetric alkyne functionalization, and atroposelective [2+2+2] cycloaddition.

An Axially Chiral Styrene-Phosphine Ligand for Pd-Catalyzed Asymmetric N-Alkylation of Indoles

An efficient palladium-catalyzed enantioselective direct N-alkylation of indoles using a novel type of axially chiral styrene-phosphine ligand SJTU-PHOS-1 was developed. This reaction demonstrated good functional group compatibility and a wide range scope of substrates in mild conditions. Moreover, the DFT calculations expounded the coordination mode of the metal catalyst and the axially chiral styrene-phosphine ligand in the enantioselectivity control.

Stereoselective Sulfa-Michael/Aldol Reaction Promoted by an Axially Chiral Styrene-Based Organocatalyst

Herein, we describe a stereoselective sulfa-Michael/aldol cyclization reaction promoted by a rationally designed novel axially chiral styrene-based organocatalyst. A variety of highly substituted tetrahydrothiophenes featuring an alkyne-substituted quaternary stereogenic center are obtained in good yields, excellent stereoselectivities, and exclusive trans selectivities. This process tolerates a broad range of alkynyl-substituted acrylamides under mind conditions. The utility of this approach is highlighted in its excellent asymmetric introduction, scalability, and attractive product diversification.

Organocatalytic Synthesis of Furan-Embedded Styrene Atropisomers

Herein, we report the first synthesis of furan-embedded styrene atropisomers via the reaction between 1-(aryl-ethynyl)-naphthalen-2-ol and γ-hydroxyenone. The reaction proceeds through in situ furan formation from γ-hydroxyenone. The styrene derivatives were obtained in moderate to good yields with high diastereoselectivities with a catalytic amount of PTSA. Few applications such as triazole formation and cross-coupling reactions have been demonstrated. A preliminary catalytic asymmetric version was also reported.