Decarboxylation of β-boryl NHPI esters enables radical 1,2-boron shift for the assembly of versatile organoborons

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.

Photogeneration of β-Borylsilyl Radical via Decarboxylation of N-Hydroxyphthalimide Esters

Gem-borylsilylalkanes are versatile intermediates in organic synthesis, and the traditional synthesis methods have mainly focused on metal reagents, the insertion reactions of diazo compounds, and hydrosilylation/hydroborylation reactions of unsaturated bonds. Herein, a novel, efficient gem-borylsilylalkanes synthesis via a radical approach is reported. This method introduced a β-gem-borylsilyl NHPI ester as the precursor of the β-borylsilyl radical that, coupling with radical acceptors under photo conditions, and the corresponding gem-borylsilylalkanes bearing unsaturated bonds, facilitate subsequent transformations.

A BN-Benzvalene

The synthesis and crystallographic characterization of BN-benzvalene, the first second-row heteroatom-containing benzvalene, is described. BN-benzvalenes are produced via photoexcitation of C5-aryl-substituted 1,2-azaborines under flow conditions. Mechanistic studies support a boron-specific, two-step photoisomerization pathway involving a BN-Dewar benzene intermediate, which is distinct from the photoisomerization pathway proposed in benzene and phospha- and silabenzenes for the formation of their respective benzvalene analogues.

Mechanochemical Synthesis of α-halo Alkylboronic Esters

α-halo alkylboronic esters, acting as ambiphilic synthons, play a pivotal role as versatile intermediates in fields like pharmaceutical science and organic chemistry. The sequential transformation of carbon-boron and carbon-halogen bonds into a broad range of carbon-X bonds allows for programmable bond formation, facilitating the incorporation of multiple substituents at a single position and streamlining the synthesis of complex molecules. Nevertheless, the synthetic potential of these compounds is constrained by limited reaction patterns. Additionally, the conventional methods often necessitate the use of bulk toxic solvents, exhibit sensitivity to air/moisture, rely on expensive metal catalysts, and involve extended reaction times. In this report, a ball milling technique is introduced that overcomes these limitations, enabling the external catalyst-free multicomponent coupling of aryl diazonium salts, alkenes, and simple metal halides. This approach offers a general and straightforward method for obtaining a diverse array of α-halo alkylboronic esters, thereby paving the way for the extensive utilization of these synthons in the synthesis of fine chemicals.

Three-component cascade carbopalladation/Heck cyclization/borylation: facile access to boryl-functionalized indenes

A mild Pd-catalyzed three-component cascade cyclization functionalization of o-iodostyrenes, internal alkynes and boron reagents is presented. The transformation is driven by a controlled reaction sequence of intermolecular carbopalladation, intramolecular Heck-type cyclization, and a borylation process to give versatile boryl-functionalized indene skeletons in a selective fashion. Significantly, (Bpin)2, (Bneop)2 and CH2(Bpin)2 as boron sources are all tolerated. Additionally, the synthetic utility of this approach is demonstrated by gram-scale synthesis and synthetic transformations.

N-Heterocyclic carbene catalytic 1,2-boron migrative acylation accelerated by photocatalysis

The transformation of organoboron compounds plays an important role in synthetic chemistry, and recent advancements in boron-migration reactions have garnered considerable attention. Here, we report an unprecedented 1,2-boron migrative acylation upon photocatalysis-facilitated N-heterocyclic carbene catalysis. The design of a redox-active boronic ester substrate, serving as an excellent β-boron radical precursor, is the linchpin to the success of this chemistry. With the established protocol, a wide spectrum of β-boryl ketones has been rapidly synthesized, which could further undergo various C─B bond transformations to give multifunctionalized products. The robustness of this catalytic strategy is underscored by its successful application in late-stage modification of drug-derived molecules and natural products. Preliminary mechanistic investigations, including several control experiments, photochemistry measurements, and computational studies, shed light on the catalytic radical reaction mechanism.

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.

Controllable Regiodivergent Alkynylation of 1,3-Bis(Boronic) Esters Activated by Distinct Organometallic Reagents

1,3-Bis(boronic) esters can be readily synthesized from alkylBpin precursors. Selective transformations of these compounds hold the potential for late-stage functionalization of the remaining C-B bond, leading to a diverse array of molecules. Currently, there are no strategies available to address the reactivity and, more importantly, the controllable regiodivergent functionalization of 1,3-bis(boronic) esters. In this study, we have achieved controllable regiodivergent alkynylation of these molecules. The regioselectivity has been clarified based on the unique chelation patterns observed with different organometallic reagents. Remarkably, this methodology effectively addresses the low reactivity of 1,3-bis(boronic) esters and bridges the gap in radical chemistry, which typically yields only the classical products formed via stable radical intermediates. Furthermore, the compounds synthesized through this approach serve as potent building blocks for creating molecular diversity.