Electrooxidative Activation of B-B Bond in B2 cat2 : Access to gem-Diborylalkanes via Paired Electrolysis

Electrochemical deconstruction of alkyl substituted boron clusters to produce alkyl boronate esters

Closo-Hexaborate (closo-B6H62-) can engage in nucleophilic substitution reactions with a wide variety of alkyl electrophiles. The resulting functionalized boron clusters undergo oxidative electrochemical deconstruction, selectively cleaving B-B bonds while preserving B-C bonds in these species. This approach allows the conversion of multinuclear boron clusters into single boron site organoboranes. Trapped boron-based fragments were isolated from the electrochemical cluster deconstruction process, providing further mechanistic insights into the developed reaction.

Asymmetric Paired Electrolysis: Enantioselective Alkylation of Sulfonylimines via C(sp3)-H Functionalization

Enantioselective transformation of ubiquitous C(sp3)-H bonds into three-dimensional chiral scaffolds is of longstanding interest to synthetic chemists. Herein, an asymmetric paired electrolysis enables a highly efficient and sustainable approach to the enantioselective alkylation of sulfonylimines via C(sp3)-H functionalization. In this protocol, anodic oxidation for benzylic radical formation and Lewis acid-catalyzed sulfonylimine reduction on the cathode were seamlessly cross-coupled (up to 88 % yield). Enantioenriched chiral amines containing a tetrasubstituted carbon stereocenter are accessed with high enantioselectivity (up to 96 % ee). Mechanistic studies suggest that the amine generated in situ could serve as a base to deprotonate phenols and decrease the oxidation potential of the reaction, allowing phenols with lower potentials to be preferentially oxidized.

Highly Selective Boron-Wittig Reaction: A Practical Method to Synthesize Trans-Aryl Alkenes

Olefins play an essential role in synthetic chemistry, serving not only as important synthons but also as key functional groups in numerous bio-active molecules. Consequently, there has been considerable interest in the development of more powerful methods for olefins. While the Wittig reaction stands as a prominent choice for olefin synthesis due to its simplicity and the ready availability of raw materials, its limitation lies in the challenge of controlling cis-trans selectivity, hampering its broader application. In this study, a novel Boron-Wittig reaction has been developed utilizing gem-bis(boryl)alkanes and aldehydes as starting materials. This method enables creating favourable intermediates, which possess less steric hindrance, and leading to trans-olefins via intramolecular O-B bonds elimination. Notably, synthesis studies have validated its good efficacy in modifying bioactive molecules and synthesizing drug molecules with great trans-selectivity. Furthermore, the reaction mechanism was elucidated based on intermediate trapping experiments, isotope labelling studies, and kinetic analyses.

Selective Cleavage of Cβ-O-4 Bond for Lignin Depolymerization via Paired-Electrolysis in an Undivided Cell

The cleavage of C-O bonds is one of the most promising strategies for lignin-to-chemicals conversion, which has attracted considerable attention in recent years. However, current catalytic system capable of selectively breaking C-O bonds in lignin often requires a precious metal catalyst and/or harsh conditions such as high-pressure H2 and elevated temperatures. Herein, we report a novel protocol of paired electrolysis to effectively cleave the Cβ-O-4 bond of lignin model compounds and real lignin at room temperature and ambient pressure. For the first time, "cathodic hydrogenolysis of Cβ-O-4 linkage" and "anodic C-H/N-H cross-coupling reaction" are paired in an undivided cell, thus the cleavage of C-O bonds and the synthesis of valuable triarylamine derivatives could be simultaneously achieved in an energy-effective manner. This protocol features mild reaction conditions, high atom economy, remarkable yield with excellent chemoselectivity, and feasibility for large-scale synthesis. Mechanistic studies indicate that indirect H* (chemical absorbed hydrogen) reduction instead of direct electron transfer might be the pathway for the cathodic hydrogenolysis of Cβ-O-4 linkage.

Asymmetric paired oxidative and reductive catalysis enables enantioselective alkylarylation of olefins with C(sp3)-H bonds

Enantioselective transformations of hydrocarbons to three-dimensional chiral molecules remain a significant challenge in synthetic chemistry. This study uses asymmetric paired oxidative and reductive catalysis to promote the enantioselective alkylarylation of olefins through the functionalization of C(sp3)-H bonds in alkanes. This asymmetric photoelectrocatalytic approach enables the facile construction of a wide range of enantioenriched α-aryl carbonyls with excellent enantioselectivity (up to 96% ee) from readily accessible starting materials. Notably, aryl bromides, aryl iodides, and even aryl chlorides were compatible with the developed catalytic system. Mechanistic studies reveal that alkanes and electrophiles are simultaneously activated on the electrodes.

Photoredox/Cu-Catalyzed Decarboxylative C(sp3)-C(sp3) Coupling to Access C(sp3)-Rich gem-Diborylalkanes

gem-Diborylalkanes are highly valuable building blocks in organic synthesis and pharmaceutical chemistry due to their ability to participate in multi-step cross-coupling transformations, allowing for the rapid generation of molecular complexity. While progress has been made in their synthetic metholodology, the construction of β-tertiary and C(sp3)-rich gem-diborylalkanes remains a synthetic challenge due to substrate limitations and steric hindrance issues. An approach is presented that utilizes synergistic photoredox and copper catalysis to achieve efficient C(sp3)-C(sp3) cross-coupling of alkyl N-hydroxyphthalimide esters, which can easily be obtained from alkyl carboxylic acids, with diborylmethyl species, providing a series of C(sp3)-rich gem-diborylalkanes with 1°, 2°, and even 3° β positions. Furthermore, this approach can also be applied to complex medicinal compounds and natural products, offering rapid access to molecular complexity and late-stage functionalization of C(sp3)-rich drug candidates. Mechanistic experiments revealed that diborylmethyl Cu(I) species participated in both the photoredox process and the key C(sp3)-C(sp3) bond-forming step.

Vinylcyclopropane-Cyclopentene (VCP-CP) Rearrangement Enabled by Pyridine-Assisted Boronyl Radical Catalysis

An unprecedented VCP-CP (vinylcyclopropane-cyclopentene) rearrangement approach has been established herein by virtue of the pyridine-boronyl radical catalyzed intramolecular ring expansions. This metal-free radical pathway harnesses readily available catalysts and unactivated vinylcyclopropane starting materials, providing an array of cyclopentene derivatives chemoselectively under relatively mild conditions. Mechanistic studies support the idea that the boronyl radical engages in the generation of allylic/ketyl radical species, thus inducing the ring opening of cyclopropanes and the following intramolecular cyclization processes.

Photoelectrochemical Fe/Ni cocatalyzed C-C functionalization of alcohols

The simultaneous activation of reactants on the anode and cathode via paired electrocatalysis has not been extensively demonstrated. This report presents a paired oxidative and reductive catalysis based on earth-abundant iron/nickel cocatalyzed C-C functionalization of ubiquitous alcohols. A variety of alcohols (i.e., primary, secondary, tertiary, or unstrained cyclic alcohols) can be activated at very low oxidation potential of (~0.30 V vs. Ag/AgCl) via photoelectrocatalysis coupled with versatile electrophiles. This reactivity yields a wide range of structurally diverse molecules with broad functional group compatibility (more than 50 examples).

Dinuclear gold-catalyzed divergent dechlorinative radical borylation of gem-dichloroalkanes

The enormous and widespread use of organoboronic acids has prompted the development of innovative synthetic methodologies to meet the demands on structural diversity and functional group tolerance. The existing photoinduced defunctionalization radical borylation, typically focused on the conversion of one C-X bond (X= Br, I, or other leaving group) into only one C-B bond. Herein, we disclose a divergent radical dechloroborylation reaction enabled by dinuclear gold catalysis with visible light irradiation. A wide range of structurally diverse alkyl boronic, α-chloroboronic, and gem-diboronic esters can be synthesized in moderate to good yields (up to 92%). Its synthetic robustness is further demonstrated on a preparative scale and applied to late-stage diversification of complex molecules. The process hinges on a C-Cl bond relay activation in readily available gem-dichloroalkanes through inner-sphere electron transfer, overcoming the redox potential limits of unreactive alkyl chlorides.

A General Method to Access Sterically Encumbered Geminal Bis(boronates) via Formal Umpolung Transformation of Terminal Diboron Compounds

General methods for the preparation of geminal bis(boronates) are of great interest due to their widespread applications in organic synthesis. While the terminal gem-diboron compounds are readily accessible, the construction of the sterically encumbered, internal analogues has remained a prominent challenge. Herein, we report a formal umpolung strategy to access these valuable building blocks. The readily available 1,1-diborylalkanes were first converted into the corresponding α-halogenated derivatives, which then serve as electrophilic components, undergoing a formal substitution with a diverse array of nucleophiles to form a series of C-C, C-O, C-S, and C-N bonds. This protocol features good tolerance to steric hindrance and a wide variety of functional groups and heterocycles. Notably, this strategy can also be extended to the synthesis of diaryl and terminal gem-diboron compounds, therefore providing a general approach to various types of geminal bis(boronates).

Diborodichloromethane as Versatile Reagent for Chemodivergent Synthesis of gem-Diborylalkanes

The development of boron reagents is crucial for synthetic chemistry. Herein, we present a scalable and practical synthesis of diborodichloromethane (DBDCM) through the reaction of trichloromethyllithium with bis(pinacolato)diboron (B2 pin2 ). The resulting DBDCM reagent serves as a basic synthetic unit for the construction of various structurally diverse gem-diborylalkanes through controllable C-Cl functionalizations. Moreover, we have developed consecutive tetra-functionalizations of DBDCM for the construction of diverse tertiary and quaternary carbon containing molecules. The use of isotopically enriched 13 C-chloroform and 10 B2 pin2 enables the synthesis of isotopically enriched 13 C-DBDCM and 10 B-DBDCM reagents, which are beneficial for the convenient synthesis of carbon-13 and boron-10 molecules.

Rapid and Practical Synthesis of gem-Dibromoalkanes from Aldehydes by Tribromide Reagent

gem-Dibromoalkanes are important synthetic building block in organic chemistry, but their preparation is still troublesome. Herein, we have developed a simple and practical protocol for the synthesis of gem-dibromoalkanes from aldehydes using tetrabutylammonium tribromide and triphenyl phosphite. A variety of alkyl and aromatic aldehydes can be transformed into the corresponding products within 10 minutes. This protocol is also applicable to alcohols, and the configuration of chiral alcohol is inverted during the process with excellent enantiopurity.

Selective C(sp3)-H arylation/alkylation of alkanes enabled by paired electrocatalysis

We report a combination of electrocatalysis and photoredox catalysis to perform selective C(sp3)-H arylation/alkylation of alkanes, in which a binary catalytic system based on earth-abundant iron and nickel is applied. Reaction selectivity between two-component C(sp3)-H arylation and three-component C(sp3)-H alkylation is tuned by modulating the applied current and light source. Importantly, an ultra-low anodic potential (~0.23 V vs. Ag/AgCl) is applied in this protocol, thus enabling compatibility with a variety of functional groups (>70 examples). The robustness of the method is further demonstrated on a preparative scale and applied to late-stage diversification of natural products and pharmaceutical derivatives.

Photocatalytic and Electrochemical Borylation and Silylation Reactions

Due to their high versatility borylated and silylated compounds are inevitable synthons for organic chemists. To escape the classical hydroboration/hydrosilylation paradigm, chemists turned their attention to more modern and green methods such as photoredox chemistry and electrosynthesis. This account focuses on novel methods for the generation of boryl and silyl radicals to forge C-B and C-Si bonds from our group.