Asymmetric Synthesis of Chiral 1,2-Bis(Boronic) Esters Featuring Acyclic, Non-Adjacent 1,3-Stereocenters

Regio- and Enantioselective Rhodium-Catalyzed Allylic Arylation of Racemic Allylic Carbonates with Arylboronic Acids

Rhodium-catalyzed regio- and enantioselective allylic arylation of racemic alkyl- and aryl- substituted allylic carbonates with arylboronic acids using commercially available BIBOP ligand is reported. This reaction proceeds at room temperature without base or other additive to deliver allylic arylation products in excellent yields, regio- and enantioselectivity (up to 95 % yield, >20 : 1 b/l, >99 % ee). Rh/BIBOP is disclosed as an efficient catalytic system for allylic substitution reaction.

Asymmetric synthesis of atropisomers featuring cyclobutane boronic esters facilitated by ring-strained B-ate complexes

The strain-release-driven reactions of bicyclo[1.1.0]butanes (BCBs) have received significant attention from chemists. Notably, 1,2-migratory reactions enabled by BCB-derived B-ate complexes effectively complement the reactions initiated by common BCBs. The desired products are particularly valuable for late-stage transformations due to the presence of the C-B bond. However, asymmetric reactions mediated by BCB-derived boronate complexes have progressed slowly. In this study, we develop an asymmetric synthesis of atropisomers featuring cis-cyclobutane boronic esters facilitated by 1,2-carbon or boron migration of ring-strained B-ate complexes, achieving high enantioselectivity. The reaction is compatible with various aryl, alkenyl, alkyl boronic esters and B2pin2, and shows good compatibility with natural product derivatives. Mechanistic studies are conducted to understand stereoselective control in the dynamic kinetic asymmetric transformations (DYKATs). The target products can undergo a series of transformations, further demonstrating the practicality of this methodology.

Organocatalytic Enantioselective Synthesis of [5.7]-Fused ε-Sultam N,O-Heterocycles via (3 + 2)-Annulation of Seven-Membered Cyclic N-Sulfonylimines with γ-Hydroxy-α,β-Unsaturated Ketones

A highly stereoselective protocol for the (3 + 2)-annulation of biphenyl-bridged seven-membered cyclic N-sulfonylimines with γ-hydroxy-α,β-unsaturated ketones was developed. The reactions afforded a wide range of chiral [5.7]-fused ε-sultams bearing N-adjacent 1,3-stereocenters in excellent yields (93-98% yields) and high enantio/diastereoselectivities (up to >99% ee, >20:1 d.r.) and two other examples with alkoxyl groups were obtained in 52-61% yields, 95% ee, and >20:1 d.r. by utilizing organocatalysis with quinine-derived squaramides.

Catalytic Selective Functionalization of Poly(organoborons)†

Organoborons are commonly used building blocks for rapidly increasing molecular complexity. Although significant progress has been made in the selective functionalization of mono‐organoborons, the site‐selective functionalization of poly(organoborons) has attracted substantial interest in organic synthesis, pharmaceuticals, and agrochemicals due to the presence of multiple potential reaction sites. This review discusses various activation modes of the target C–B bond, with diverse transformations being achieved in both a selective and efficient manner. Recent advances in the catalytic selective transformations of 1,n‐diboronates through ionic and radical pathways are highlighted. Furthermore, we summarize the existing challenges and future research directions in this field.

Key Scientists

In 1993, Suzuki, Miyaura and coworkers developed a pioneering example of selective arylation towards cis‐1,2‐bis(boryl) alkenes, marking the inception of this field. The Morken group has made significant contributions to the asymmetric diboration of alkenes and realized elegant catalytic functionalization of these compounds since 2004. In 2016, Fernández and colleagues achieved the selective arylation of the internal C–B bond of tri(boronates). Since 2019, the Aggarwal group has developed efficient Giese‐type addition and selective arylation at the more substituted C–B bond of 1,2‐bis(boronic) esters through photoredox catalysis. The controllable regiodivergent alkynylation of 1,3‐bis(boronic) esters was developed by Gao and coworkers in 2023. Recently, Qin conducted elegant research on the programmable late‐stage functionalization of bridge‐substituted bicyclo[1.1.1]pentane (BCP) bis‐boronates. Since 2013, catalytic stereoselective transformations have been developed by several groups, including those led by Morken and Chen. This review summarizes the latest and most significant developments in this field since 1993.

Diastereoselective Homocoupling of Benzylic C(sp3)-H Bonds Enabled by Halogen Transfer

A transition-metal- and harsh-oxidant-free strategy for diastereoselective homocoupling of benzylic α-boryl carbanions has been developed. Central to this methodology is the ability of the halogen transfer reagent to seamlessly integrate halogenation and substitution within a compatible process. Additionally, this methodology is also applicable to the homocoupling of diarylmethanes and alkylheteroarenes. Substrates bearing oxidatively sensitive functional groups were well-tolerated. Preliminary studies suggest that the hydrogen bond between two boryl groups contributes to the high diastereoselectivities.

Merging Organocatalysis with 1,2-Boronate Rearrangement: A Lewis Base-Catalyzed Asymmetric Multicomponent Reaction

Catalytic asymmetric multicomponent 1,2-boronate rearrangements provide a practical approach for synthesizing highly valuable enantioenriched boronic esters. When applied to alkenyl or heteroaryl boronates, these reactions have relied mainly on transition-metal catalysis. Herein, we present an organocatalytic, Lewis base-catalyzed asymmetric multicomponent 1,2-boronate rearrangement, involving indoles, boronic esters, and Morita-Baylis-Hillman carbonates, leading to enantioenriched, highly substituted indole and indoline derivatives. Using cinchona alkaloid-based catalysts, high selectivity has been achieved, enabling expansion of the chemical space around pharmaceutically relevant indole and indoline derivatives.

Chemoselective and Stereoselective Allylation of Bis(alkenyl)boronates

Bis(alkenyl)boronates react with optically active Ir(π-allyl) species in a process that involves allylation of the more substituted olefin and 1,2-metalate shift of the less substituted olefin. The method constructs valuable enantioenriched tertiary allylic boronic esters with high chemoselectivity, enantioselectivity and diastereoselectivity. Allylic functionalization reactions transform the 1,3-stereodiad to 1,5- and 1,6-stereochemical relationships.

Stereodivergent Construction of 1,5/1,7-Nonadjacent Tetrasubstituted Stereocenters Enabled by Pd/Cu-Cocatalyzed Asymmetric Heck Cascade Reaction

The construction of chiral motifs containing nonadjacent stereocenters in an enantio- and diastereoselective manner has long been a challenging task in synthetic chemistry, especially with respect to their stereodivergent synthesis. Herein, we describe a protocol that enables the enantio- and diastereoselective construction of 1,5/1,7-nonadjacent tetrasubstituted stereocenters through a Pd/Cu-cocatalyzed Heck cascade reaction. Notably, a C=C bond relay strategy involving the shift of the π-allyl palladium intermediate was successfully applied in the asymmetric construction of 1,7-nonadjacent stereocenters. The current method allows for the efficient preparation of chiral molecules bearing two privileged scaffolds, oxindoles and non-natural α-amino acids, with good functional group tolerance. The full complement of the four stereoisomers of products bearing 1,5/1,7-nonadjacent stereocenters could be readily accessed by a simple combination of two chiral metal catalysts with different enantiomers.

PolyBorylated Alkenes as Energy-Transfer Reactive Groups: Access to Multi-Borylated Cyclobutanes Combined with Hydrogen Atom Transfer Event

While polyborylated alkenes are being recognized for their elevated status as highly valuable reagents in modern organic synthesis, allowing efficient access to a diverse array of transformations, including the formation of C-C and C-heteroatom bonds, their potential as energy-transfer reactive groups has remained unexplored. Yet, this potential holds the key to generating elusive polyborylated biradical species, which can be captured by olefins, thereby leading to the construction of new highly-borylated scaffolds. Herein, we report a designed energy-transfer strategy for photosensitized [2+2]-cycloadditions of poly-borylated alkenes with various olefins enabling the regioselective synthesis of diverse poly-borylated cyclobutane motifs, including the 1,1-di-, 1,1,2-tri-, and 1,1,2,2-tetra-borylated cyclobutanes. In fact, these compounds belong to a family that presently lacks efficient synthetic pathways. Interestingly, when α-methylstyrene was used, the reaction involves an interesting 1,5-hydrogen atom transfer (HAT). Mechanistic deuterium-labeling studies have provided insight into the outcome of 1,5-hydrogen atom transfer process. In addition, the polyborylated cyclobutanes are then demonstrated to be useful in selective oxidation processes resulting in the formation of cyclobutanones and γ-lactones.

Ligand-Controlled, Nickel-Catalyzed Stereodivergent Construction of 1,3-Nonadjacent Stereocenters

In contrast to the asymmetric synthesis of molecules with a single stereocenter or 1,2-adjacent stereocenters, the simultaneous construction of acyclic 1,3-nonadjacent stereocenters via a single catalyst in an enantioselective and diastereoselective manner remains a formidable challenge. Here, we demonstrate the enantioselective and diastereodivergent construction of 1,3-nonadjacent stereocenters through Ni-catalyzed reductive cyclization/cross-coupling of alkene-tethered aryl bromides and α-bromoamides, which represents the major remaining stereochemical challenge of cyclization/difunctionalization of alkenes. Using Ming-Phos as ligand, a diverse set of oxindoles containing 1,3-nonadjacent stereocenters were obtained with high levels of enantio- and diastereoselectivity. Mechanistic experiments and density functional theory calculations indicate that magnesium salt plays a key role in controlling the diastereoselectivity. Furthermore, another set of complementary stereoisomeric products were constructed from the same set of starting materials using Ph-Phox as ligand.

Simultaneous Stereoinvertive and Stereoselective C(sp3)-C(sp3) Cross-Coupling of Boronic Esters and Allylic Carbonates

With increasing interest in constructing more three-dimensional entities, there has been growing interest in cross-coupling reactions that forge C(sp3)-C(sp3) bonds, which leads to additional challenges as it is not just a more difficult bond to construct but issues of stereocontrol also arise. Herein, we report the stereocontrolled cross-coupling of enantioenriched boronic esters with racemic allylic carbonates enabled by iridium catalysis, leading to the formation of C(sp3)-C(sp3) bonds with single or vicinal stereogenic centers. The method shows broad substrate scope, enabling primary, secondary, and even tertiary boronic esters to be employed, and can be used to prepare any of the four possible stereoisomers of a coupled product with vicinal chiral centers. The new method, which combines the simultaneous enantiospecific reaction of a chiral nucleophile with the enantioselective reaction of a chiral electrophile in a single process, offers a solution for stereodivergent cross-coupling of two C(sp3) fragments.

Synergistic Pd/Cu-Catalyzed 1,5-Double Chiral Inductions

Much attention has been focused on the catalytic asymmetric creation of single chiral centers or two adjacent stereocenters. However, the asymmetric construction of two nonadjacent stereocenters is of significant importance but is challenging because of the lack of remote chiral induction models. Herein, based on a C═C bond relay strategy, we report a synergistic Pd/Cu-catalyzed 1,5-double chiral induction model. All four stereoisomers of the target products bearing 1,5-nonadjacent stereocenters involving both allenyl axial and central chirality could be obtained divergently by simply changing the combination of two chiral catalysts with different configurations. Control experiments and DFT calculations reveal a novel mechanism involving 1,5-oxidative addition, contra-thermodynamic η3-allyl palladium shift, and conjugate nucleophilic substitution, which play crucial roles in the control of reactivity, regio-, enantio-, and diastereoselectivity. It is expected that this C═C bond relay strategy may provide a general protocol for the asymmetric synthesis of structural motifs bearing two distant stereocenters.

Regioselective Alkynylation and Alkenylation at the More Hindered C-B Bond of 1,2-Bis(Boronic) Esters

Selective transformations at the more sterically hindered sites of organic molecules represent a frontier in the ability to precisely modify molecules. The lack of effective synthetic methods stands in stark contrast to the large number of encumbered sites encountered in molecules of interest. Here, we demonstrate that 1,2-bis(boronates) undergo selective alkynylation and alkenylation at the more sterically hindered C-B bond. Our preliminary mechanistic studies disclosed that this reaction can proceed through two convergent pathways involving direct coupling of sterically encumbered site versus 1,2-boron migratory coupling. Notably, this method facilitated convenient access to alkenyl and alkynyl boron products, which can be diversified by an array of transformations.