Dehydroxylative Alkylation of α-Hydroxy Carboxylic Acid Derivatives via a Spin-Center Shift

HDAC inhibitor SAHA triggers the production of previously undescribed sesquiterpenes and undergoes biotransformation by the fungus Robillarda sessilis from Verbena officinalis L

Cultivation of the endophytic fungus Robillarda sessilis XL-308 with SAHA resulted in the discovery of two previously undescribed cyclonerane sesquiterpenes (1 and 2), one previously unreported norcadinane type sesquiterpene (3), and seven suberanilohydroxamic acid (SAHA) derivatives (4-10). The identification of previously undescribed compounds was confirmed through NMR spectroscopic analyses, quantum chemistry calculations, and single-crystal X-ray crystallography. Among them, compounds 1 and 2 were products of the silenced genome in XL-308 that activated by SAHA. Concurrently, SAHA underwent biotransformation because of its moderate toxicity towards this fungus. It was hypothesized that the hydroxamic acid group in SAHA triggering the self-defense mechanism of XL-308 and converting SAHA into a less toxic derivative 5.

Photocatalytic Amine-Promoted Selective Hydrochlorination and sp3 C-O Acylation of Alkyne-Tethered Methyl Ethers with Aldehydes

We present a photoredox and alkylamine-assisted approach for the selective hydrochlorination and acylation of sp3 C-O bonds in alkynyl methyl ethers using aldehydes. This method leverages a cascade of radical processes─including chlorine radical addition, hydrogen atom transfer, in situ imine radical addition, and spin-center shift─to enable selective hydrochlorination of alkynes and the spontaneous cleavage of sp3 C-O bonds. The transformation accommodates a broad range of internal alkyne-tethered ethers and aldehydes, providing an efficient and streamlined pathway to chloro-alkenyl ketones. Utilizing only a photocatalyst, chloride, and propylamine under light irradiation, this strategy offers a practical and complementary alternative to previous sp3 C-O cleavage protocols.

Remote Spin-Center Shift Enables Activation of Distal Benzylic C─O and C─N Bonds

A spin-center shift (SCS) is a radical process that commonly involves a 1,2-radical shift along with the elimination of an adjacent leaving group by a two-electron ionic movement. The conventional SCS process is largely limited to 1,2-radical translocation, while a remote SCS event involving 1,n-radical translocation over a greater distance to enable distal bond functionalization remains largely underexplored. Herein, we report the boryl radical-promoted distal deoxygenation and deamination of free benzylic alcohols and simple benzylic amines, respectively, through a remote SCS event. The reaction was initiated by the addition of a 4-dimethylaminopyridine (DMAP)-boryl radical to the carbonyl oxygen atom of a benzoate or benzamide. Then, radical translocation took place across the aromatic ring to promote benzylic C─O or C─N bond cleavage. The resulting radical intermediate subsequently coupled with various alkenes to afford a wide range of alkylated products. The proposed mechanistic pathway was supported by experimental investigations.

Photocatalyst-free formate-mediated C-O cleavage by the EDA complex and SCS strategy for the synthesis of diaryl 1,4-diketone in air

Under mild visible light conditions, formates facilitate C-O cleavage via the EDA complex and SCS strategy, yielding α-carbonyl alkyl radicals. These radicals then react with olefins under air conditions, leading to the synthesis of diaryl 1,4-dicarbonyl compounds. Mechanistic studies reveal that α-formyloxy ketone is generated in situ by the reaction between α-brominated acetophenone and formates, followed by the formation of the EDA complex. Additionally, formates also serve as a single-electron reducing reagent in the reaction.

A Ketone Synthesis via Cu(I)-Catalyzed Regioselective Coupling of 2-Pyridylthioesters with Grignard Reagents: In Quest of Straightforward Access to Pharmaceuticals

For commercial production of SGLT2 inhibitors, cryogenic conditions (-78 °C) were required at the key C-glycosidation step, which has restricted their supply to the world market. To address the challenge, reported herein is a new synthetic method based on a new ketone synthesis by means of copper(I)-catalyzed coupling of 2-pyridylthioesters with Grignard reagents. The facile transformation from the ketones to the final APIs was realized under mild conditions due to the use of readily cleavable acetyl protecting groups.

C(sp3)-F Bond Activation by Lewis Base-Boryl Radicals via Concerted Electron-Fluoride Transfer

Selective C-F bond activation through a radical pathway in the presence of multiple C-H bonds remains a formidable challenge, owing to the extraordinarily strong bond strength of the C-F bond. By the aid of density functional theory calculations, we disclose an innovative concerted electron-fluoride transfer mechanism, harnessing the unique reactivity of Lewis base-boryl radicals to selectively activate the resilient C-F bonds in fluoroalkanes. This enables the direct abstraction of a fluorine atom and subsequent generation of an alkyl radical, thus expanding the boundaries of halogen atom transfer reactions.

Alcohols as Alkyl Electrophiles for Deoxygenative Heck Reaction Enabled by Excited State Pd Catalysis

Here, we present a general method for the photoinduced Pd-catalyzed deoxygenative Heck reaction of vinyl arenes with ortho-iodophenyl-thionocarbonate derived from alcohols. Mechanistic studies reveal that the deoxygenation involves a 5-endo-trig cyclization and fragmentation process, with radical addition identified as the rate-determining step in this transformation. This one-pot procedure demonstrates excellent selectivity for less hindered hydroxyl groups in diols, facilitating late-stage functionalization of complex molecules and scalability to gram-scale synthesis. The protocol highlights significant synthetic potential and can be extended to the cascade 1,1-difunctionalization of isocyanides and the intermolecular radical cascade cyclization of N-arylacrylamides.

Radical Replacement Process for Ligated Boryl Radical-Mediated Activation of Unactivated Alkyl Chlorides for C(sp3)-C(sp3) Bond Formation

The ligated boryl radical (LBR) has emerged as a potent tool for activating alkyl halides in radical transformations through halogen-atom transfer (XAT). However, unactivated alkyl chlorides still present an open challenge for this strategy. We herein describe a new activation mode of the LBR for the activation of unactivated alkyl chlorides to construct a C(sp3)-C(sp3) bond. Mechanistic studies reveal that the success of the protocol relies on a radical replacement process between the LBR and unactivated alkyl chloride, forming an alkyl borane intermediate as the alkyl radical precursor. Aided with the additive K3PO4, the alkyl borane then undergoes one-electron oxidation, generating an alkyl radical. The incorporation of the radical replacement activation model to activate unactivated alkyl chlorides significantly enriches LBR chemistry, which has been applied to activate alkyl iodides, alkyl bromides, and activated alkyl chlorides via XAT.

Visible Light-Induced 1,2-Diphenyldisulfane-Mediated Defluoroborylation of Polyfluoroarenes

A green and practical protocol of defluoroborylation of polyfluoroarenes with stable and readily accessible NHC-borane was developed, using 1,2-diphenyldisulfane as a hydrogen atom transfer (HAT) and single electron transfer (SET) reagent precursor under visible-light irradiation, leading to the concise formation of value-added fluorinated organoboron scaffolds. Mechanism studies revealed the method underwent a boryl radical addition reaction with polyfluoroarene, followed by successive single electron transfer pathways and defluorination of the C-F bond to offer the targeted product. This unprecedented platform relies on 1,2-diphenyldisulfane and base without using expensive photocatalysts, highlighting the methodology has promising application value to prepare borylated polyfluoroarene compounds.

Boryls, their compounds and reactivity: a structure and bonding perspective

Boryls and their compounds are important due to their diverse range of applications in the fields of materials science and catalysis. They are an integral part of boron chemistry, which has attracted tremendous research interest over the past few decades. In this perspective, we provide an in-depth analysis of the reaction chemistry of boryl compounds from a structure and bonding perspective. We discuss the reactivity of boryls in various transition metal complexes and diborane(4) compounds towards different substrate molecules, with a focus on their nucleophilic and electrophilic properties in various reaction processes. Additionally, we briefly discuss the reactivity of boryl radicals. Our analysis sheds new light on the unique properties of boryls and their potential for catalytic applications.

Photochemical Deoxygenative Hydroalkylation of Unactivated Alkenes Promoted by a Nucleophilic Organocatalyst

The direct utilization of simple and abundant feedstocks in carbon-carbon bond-forming reactions to embellish sp3 -enriched chemical space is highly desirable. Herein, we report a novel photochemical deoxygenative hydroalkylation of unactivated alkenes with readily available carboxylic acid derivatives. The reaction displays broad functional group tolerance, accommodating carboxylic acid-, alcohol-, ester-, ketone-, amide-, silane-, and boronic ester groups, as well as nitrile-containing substrates. The reaction is operationally simple, mild, and water-tolerant, and can be carried out on multigram-scale, which highlights the utility of the method to prepare value-added compounds in a practical and scalable manner. The synthetic application of the developed method is further exemplified through the synthesis of suberanilic acid, a precursor of vorinostat, a drug used for the treatment of cutaneous T-cell lymphoma. A novel mechanistic approach was identified using thiol as a nucleophilic catalyst, which forms a key intermediate for this transformation. Furthermore, electrochemical studies, quantum yield, and mechanistic experiments were conducted to support a proposed catalytic cycle for the transformation.

Formation of C-B, C-C, and C-X Bonds from Nonstabilized Aryl Radicals Generated from Diaryl Boryl Radicals

With the development of organoboron chemistry, boron-centered radicals have become increasingly attractive. However, their synthetic applications remain limited in that they have been used only as substrates for addition reactions or as initiators for catalytic reactions. We have achieved a new reaction pathway in which tetraarylborate salts are used as precursors for aryl radicals via boron radicals, by introducing a simple activation reagent. In addition, we carried out a diverse array of transformations involving these aryl radical precursors, which allowed the construction of new C-B, C-C, and C-X bonds in the presence of visible light.

Visible-Light Enabled Dehydroxylative Alkylation of α-Hydroxy Carboxylic Acid Derivatives via C-O Bond Cleavage

A novel visible-light photoredox strategy is reported for the efficient dehydroxylative alkylation of a wide array of α-hydroxy carboxylic acid derivatives using diaryl boron radical. The reaction features readily accessible starting materials, broad substrate scope with excellent functionality tolerance. Preliminary mechanistic studies reveal that the spin-center shift process is responsible for the C-O bond activation, which is promoted by the diaryl boron radical generated from bench-stable and commercially available tetraphenyl borate (NaBPh4 ).

Lewis Base-Boryl Radicals Enabled Borylation Reactions and Selective Activation of Carbon-Heteroatom Bonds

ConspectusThe past decades have witnessed tremendous progress on radical reactions. However, in comparison with carbon, nitrogen, oxygen, and other main group element centered radicals, the synthetic chemistry of boron centered radicals was less studied, mainly due to the high electron-deficiency and instability of such 3-center-5-electron species. In the 1980s, Roberts and co-workers found that the coordination of a Lewis base (amines or phosphines) with the boron center could form 4-center-7-electron boryl radicals (Lewis base-boryl radicals, LBRs) that are found to be more stable. However, only limited synthetic applications were developed. In 2008, Curran and co-workers achieved a breakthrough with the discovery of N-heterocyclic carbene (NHC) boryl radicals, which could enable a range of radical reduction and polymerization reactions. Despite these exciting findings, more powerful and valuable synthetic applications of LBRs would be expected, given that the structures and reactivities of LBRs could be easily modulated, which would provide ample opportunities to discover new reactions. In this Account, a summary of our key contributions in LBR-enabled radical borylation reactions and selective activation of inert carbon-heteroatom bonds will be presented.Organoboron compounds have shown versatile applications in chemical society, and their syntheses rely principally on ionic borylation reactions. The development of mechanistically different radical borylation reactions allows synthesizing products that are inaccessible by traditional methods. For this purpose, we progressively developed a series of NHC-boryl radical mediated chemo-, regio-, and stereoselective radical borylation reactions of alkenes and alkynes, by which a wide variety of structurally diverse organoboron molecules were successfully prepared. The synthetic utility of these borylated products was also demonstrated. Furthermore, we disclosed a photoredox protocol for oxidative generation of NHC-boryl radicals, which enabled useful defluoroborylation and arylboration reactions.Selective bond activation is an ideal way to convert simple starting materials to value-added products, while the cleavage of inert chemical bonds, in particular the chemoselectivity control when multiple identical bonds are present in similar chemical environments, remains a long-standing challenge. We envisaged that finely tuning the properties of LBRs might provide a new solution to address this challenge. Recently, we disclosed a 4-dimethylaminopyridine (DMAP)-boryl radical promoted sequential C-F bond functionalization of trifluoroacetic acid derivatives, in which the α-C-F bonds were selectively snipped via a spin-center shift mechanism. This strategy enables facile conversion of abundantly available trifluoroacetic acid to highly valuable mono- and difluorinated molecules. Encouraged by this finding, we further developed a boryl radical enabled three-step sequence to construct all-carbon quaternary centers from a range of trichloromethyl groups, where the three C-Cl bonds were selectively cleaved by the rational choice of suitable boryl radical precursors in each step. Furthermore, a boryl radical promoted dehydroxylative alkylation of α-hydroxy carboxylic acid derivatives was achieved, allowing for the efficient conversion of some biomass platform molecules to high value products.

1,2-Silylpyridylation Reaction of Aryl Alkenes with Silylboronate

A metal-free silyl-pyridylation of alkenes using silyl boronates and B₂pin₂ through a pyridine-mediated B-interelement activation has been demonstrated, which provides a practical strategy for a variety of C₄-silylalkylated pyridines. DFT calculations and control experiments show that the reaction proceeds through a silyl radical addition/radical-radical coupling sequence. This protocol features a broad substrate scope and excellent functional group compatibility, and thus it showcases great potential in the late-stage modification of bioactive molecules.