Stepwise Asymmetric Allylic Substitution-Isomerization Enabled Mimetic Synthesis of Axially Chiral B,N-Heterocycles

Synthesis of Alkene Atropisomers with Multiple Stereogenic Elements via Catalytic Asymmetric Rearrangement of 3-Indolylmethanols

Catalytic enantioselective preparation of alkene atropisomers with multiple stereogenic elements and discovery of their applications have become significant but challenging issues in the scientific community due to the unique structures of this class of atropisomers. We herein report the first catalytic atroposelective preparation of cyclopentenyl[b]indoles, a new kind of alkene atropisomers, with stereogenic point and axial chirality via an unusual rearrangement reaction of 3-indolylmethanols under asymmetric organocatalysis. Notably, this novel type of alkene atropisomers have promising applications in developing chiral ligands or organocatalysts, discovering antitumor drug candidates and fluorescence imaging materials. Moreover, the theoretical calculations have elucidated the possible reaction mechanism and the non-covalent interactions to control the enantioselectivity. This approach offers a new synthetic strategy for alkene atropisomers with multiple stereogenic elements, and represents the first catalytic enantioselective rearrangement reaction of 3-indolylmethanols, which will advance the chemistry of atropisomers and chiral indole chemistry.

Copper-catalysed asymmetric annulation of yne-allylic esters with amines to access axially chiral arylpyrroles

The construction of atropisomers with 1,2-diaxes, while maintaining high enantiocontrol, presents a significant challenge due to the dynamic nature of steric hindrance at ortho-aryl substituents. Although various catalytic asymmetric methods have been developed for accessing axially chiral arylpyrroles, the synthesis of axially chiral arylpyrroles with 1,2-diaxes in a catalytic asymmetric manner has remained rare. Herein, the authors report the synthesis of diverse axially chiral arylpyrroles with 1,2-diaxes, and C-C and C-N axes through copper-catalysed asymmetirc [4 + 1] annulation of yne-allylic esters with arylamines via a remote stereocontrol strategy. This approach provides facile access to a broad range of heterobiaryl atropisomers (67 examples) in excellent enantioselectivities, each bearing one or two C-C/C-N axes, demonstrating its versatility and efficiency. The utility of this methodology is further highlighted by the transformation of the product into chiral phosphine ligand, and chiral thioureas for the use in asymmetric catalysis.

Copper-catalysed asymmetric hydroboration of alkenes with 1,2-benzazaborines to access chiral naphthalene isosteres

Bioisosteric replacement has emerged as a clear strategy for drug-structure optimization. Naphthalene is the core element of many chiral pharmaceuticals and drug candidates. However, as a promising isostere of naphthalene, the chiral version of 1,2-benzazaborine has rarely been explored due to the lack of efficient synthetic methods. Here we describe a copper-catalysed enantioselective hydroboration of alkenes with 1,2-benzazaborines. The method provides a general platform for the atom-economic and efficient construction of diverse chiral 1,2-benzazaborine compounds (more than 60 examples) that bear a 2-carbon-stereogenic centre or allene skeleton in high yields and excellent enantioselectivities. Three 1,2-benzazaborine analogues of bioactive chiral naphthalene-containing molecules have been prepared, and a series of transformations around chiral 1,2-benzazaborines have also been developed. Notably, the hydroboration process of this study reveals that the identity of 1,2-benzazaborine plays an essential role in the rate-determining step and catalyst resting state.

Enantioselective Synthesis of Axially Chiral Sulfone-Containing Styrenes Based on Ion-Exchange Strategy

A novel ion exchange strategy has been developed to enable the asymmetric construction of axially chiral sulfone-containing styrenes. This approach provides a practical synthesis pathway for various axially chiral sulfone-containing styrenes with good yields, exceptional enantioselectivities, and nearly complete E/Z selectivities. Additionally, the reaction mechanism is elucidated in detail through density functional theory (DFT) calculations.

Photocatalytic Z/E isomerization unlocking the stereodivergent construction of axially chiral alkene frameworks

The past century has witnessed a large number of reports on the Z/E isomerization of alkenes. However, the vast majority of them are still limited to the isomerization of di- and tri-substituted alkenes. The stereospecific Z/E isomerization of tetrasubstituted alkenes remains to be an underdeveloped area, thus lacking in a stereodivergent synthesis of axially chiral alkenes. Herein we report the atroposelective synthesis of tetrasubstituted alkene analogues by asymmetric allylic substitution-isomerization, followed by their Z/E isomerization via triplet energy transfer photocatalysis. In this regard, the stereodivergent synthesis of axially chiral N-vinylquinolinones is achieved efficiently. Mechanistic studies indicate that the benzylic radical generation and distribution are two key factors for preserving the enantioselectivities of axially chiral compounds.

Iridium/Acid Dual-Catalyzed Enantioselective Aza-ene-type Allylic Alkylation of Nitro Ketene Aminals with Racemic Allylic Alcohols

The enantioselective allylic alkylation of nitro ketene aminals with racemic allylic alcohols was realized by iridium/acid dual catalysis. An allyl group was installed on the α-position of nitro ketene aminals in a branched-selective manner in high efficiency with excellent enantioselectivities (93-99% ee). The protocol was applied to the late-stage modification of neonicotinoid insecticides, which directly furnished a novel neonicotinoid analogue with good insecticidal activity against Aphis craccivora (LC50 = 6.40 mg/L). On the basis of the control experiment, an aza-ene-type allylic alkylation reaction mechanism was proposed.

Palladium-Catalyzed Atroposelective Kinetic C-H Olefination and Allylation for the Synthesis of C-B Axial Chirality

The direct C-H functionalization of 1,2-benzazaborines, especially asymmetric version, remains a great challenge. Here we report a palladium-catalyzed enantioselective C-H olefination and allylation reactions of 1,2-benzazaborines. This asymmetric approach is a kinetic resolution (KR), providing various C-B axially chiral 2-aryl-1,2-benzazaborines and 3-substituted 2-aryl-1,2-benzazaborines in generally high yields with excellent enantioselectivities (selectivity (S) factor up to 354). The synthetic potential of this reaction is showcased by late-stage modification of complex molecules, scale-up reaction, and applications.

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.

Catalytic atroposelective synthesis of heterobiaryls with vicinal C-C and N-N diaxes via dynamic kinetic resolution

Reported herein is a highly efficient dynamic kinetic resolution protocol for the atroposelective synthesis of heterobiaryls with vicinal C-C and N-N diaxes. Atropisomers bearing vicinal diaxes mainly exist in o-triaryls, while that of biaryls is highly challenging in terms of the concerted rotation and deplanarization effects. The combination of C-C biaryl with N-N nonbiaryl delivers a novel class of vicinal-diaxis heterobiaryls. For their atroposelective synthesis, the dynamic kinetic resolution enabled by either quinine-catalyzed allylation or isothiourea-catalyzed acylation has been developed, allowing the preparation of a wide range of vicinal-axis heterobiaryls in good yields with excellent enantioselectivities. Atropisomerization experiments revealed that the C-C bond rotation led to diastereomers, and the N-N bond rotation offered enantiomers. Besides, this protocol could be extended to kinetic resolution by employing substrates with a more hindered axis.

Asymmetric Allylic Substitution-Isomerization for the Modular Synthesis of Axially Chiral N-Vinylquinazolinones

Axially chiral N-substituted quinazolinones are important bioactive molecules, which are presented in many synthetic drugs. However, most strategies toward their atroposelective synthesis are mainly limited to the axially chiral arylquinazolinone frameworks. The development of modular synthetic methods to access diverse quinazolinone-based atropisomers remains scarce and challenging. Herein, we report the regio- and atroposelective synthesis of axially chiral N-vinylquinazolinones via the strategy of asymmetric allylic substitution-isomerization. The catalysis system utilized both asymmetric transition-metal catalysis and organocatalysis to efficiently afford trisubstituted and tetrasubstituted N-vinylquinazolinone atropisomers, respectively. With the meticulous design of β-substituted allylic substrates, both Z- and E-tetrasubstituted axially chiral N-vinylquinazolinones were obtained in good yields and high enantioselectivities.

Construction of C-B axial chirality via dynamic kinetic asymmetric cross-coupling mediated by tetracoordinate boron

Catalytic dynamic kinetic asymmetric transformation (DyKAT) provides a powerful tool to access chiral stereoisomers from racemic substrates. Such transformation has been widely employed on the construction of central chirality, however, the application in axial chirality remains underexplored because its equilibrium of substrate enantiomers is limited to five-membered metalacyclic intermediate. Here we report a tetracoordinate boron-directed dynamic kinetic asymmetric cross-coupling of racemic, configurationally stable 3-bromo-2,1-azaborines with boronic acid derivatives. A series of challenging C-B axially chiral compounds were prepared with generally good to excellent enantioselectivities. Moreover, this transformation can also be extended to prepare atropisomers bearing adjacent C-B and C-C diaxes with excellent diastereo- and enantio-control. The key to the success relies on the rational design of a reversible tetracoordinate boron intermediate, which is supported by theoretical calculations that dramatically reduces the rotational barrier of the original C-B axis and achieves the goal of DyKAT.

Atroposelective Amination of Indoles via Chiral Center Induced Chiral Axis Formation

The construction of an N-C chiral axis for N-aryl indole derivatives is meaningful as they widely exist in functionalized molecules. This work provides a novel method for this purpose via amination of amino acid derivatives at the C2 position of the indole and chiral center induced chiral axis formation. The protocol of this transformation is easily accessible, not requiring metal or an organic chiral catalyst, endowing this method with great potential in the construction of axis chiral N-aryl indoles.