Study on Pyridine-Boryl Radical-Promoted, Ketyl Radical-Mediated Carbon-Carbon Bond-Forming Reactions

Ketyl radicals are synthetically versatile reactive species, but their applications have been hampered by harsh generation conditions employing highly reducing metals. Recently, the pyridine-boryl radical received wide attention as a promising organic reductant because of its mildness as well as convenience in handling. While probing the utility of the pyridine-boryl radical, our group observed facile pinacol coupling reactivity that had not been known at that time. This serendipitous finding was successfully rendered into a practical synthesis of tetraaryl-1,2-diols in up to 99% yield within 1 h. Subsequently, upon examinations of various reaction manifolds, a diastereoselective ketyl-olefin cyclization was accomplished to produce cycloalkanols such as trans-2-alkyl-1-indanols. Compared to the previous methods, the stereocontrolling ability was considerably enhanced by taking advantage of the structurally modifiable boryl group that would be present near the bond-forming site. In this full account, our synthetic efforts with the O-boryl ketyl radicals are disclosed in detail, covering the discovery, optimization, scope expansion, and mechanistic analysis, including density functional theory (DFT) calculations.

Mechanisms for radical reactions initiating from N-hydroxyphthalimide esters

Due to their ease of preparation, stability, and diverse reactivity, N-hydroxyphthalimide (NHPI) esters have found many applications as radical precursors. Mechanistically, NHPI esters undergo a reductive decarboxylative fragmentation to provide a substrate radical capable of engaging in diverse transformations. Their reduction via single-electron transfer (SET) can occur under thermal, photochemical, or electrochemical conditions and can be influenced by a number of factors, including the nature of the electron donor, the use of Brønsted and Lewis acids, and the possibility of forming charge-transfer complexes. Such versatility creates many opportunities to influence the reaction conditions, providing a number of parameters with which to control reactivity. In this perspective, we provide an overview of the different mechanisms for radical reactions involving NHPI esters, with an emphasis on recent applications in radical additions, cyclizations and decarboxylative cross-coupling reactions. Within these reaction classes, we discuss the utility of the NHPI esters, with an eye towards their continued development in complexity-generating transformations.

Nickel-Catalyzed Three-Component Alkylarylation of Alkenyl N-Heteroarenes

A nickel-catalyzed three-component alkylarylation of alkenyl N-heteroarenes with α-bromocarboxylates and aryl boronic acids is reported. The protocol provides a new method to access a variety of N-heteroarene substituted diarylalkanes in moderate to good yields. It features mild reaction conditions, cheap nickel catalyst, readily available substrates, and broad substrate scope.

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

This report describes the unprecedented electrooxidation of a solvent (e.g., DMF)-ligated B₂ cat₂ complex, whereby a solvent-stabilized boryl radical is formed via quasi-homolytic cleavage of the B-B bond in a DMF-ligated B₂ cat₂ radical cation. Cyclic voltammetry and density functional theory provide evidence to support this novel B-B bond activation strategy. Furthermore, a strategy for the electrochemical gem-diborylation of gem-bromides via paired electrolysis is developed for the first time, affording a range of versatile gem-diborylalkanes, which are widely used in synthetic society. Notably, this reaction approach is scalable, transition-metal-free, and requires no external activator.

Diboron(4)-Catalyzed Remote [3+2] Cycloaddition of Cyclopropanes via Dearomative/Rearomative Radical Transmission through Pyridine

Ring structures such as pyridine, cyclopentane or their combinations are important motifs in bioactive molecules. In contrast to previous cycloaddition reactions that necessitated a directly bonded initiating functional group, this work demonstrated a novel through-(hetero)arene radical transmission concept for selective activation of a remote bond. An efficient, metal-free and atom-economical [3+2] cycloaddition between 4-pyridinyl cyclopropanes and alkenes or alkynes has been developed for modular synthesis of pyridine-substituted cyclopentanes, cyclopentenes and bicyclo[2.1.1]hexanes that are difficult to access using known methods. This complexity-building reaction was catalyzed by a very simple and inexpensive diboron(4) compound and took place via dearomative/rearomative processes. The substrate scope was broad and more than 100 new compounds were prepared in generally high yields. Mechanistic experiments and density function theory (DFT) investigation supported a radical relay catalytic cycle involving alkylidene dihydropyridine radical intermediates and boronyl radical transfer.

Metal-Free Reductive Coupling of para-Quinone Methides with 4-Cyanopyridines Enabled by Pyridine-Boryl Radicals: Access to Pyridylated Diarylmethanes with Anti-Cancer Activity

Reported herein is a streamlined protocol to produce pyridylated diarylmethanes through pyridine-boryl radical induced reductive coupling between para-quinone methides (p-QMs) and 4-cyanopyridines using bis(pinacolato)diboron (B₂ pin₂ ) as a templated reagent. The metal-free process is characterized by an operationally simple approach, excellent chemoselectivity (1,2- vs. 1,6-selectivity), and a broad substrate scope with good functional group compatibility. The mechanistic studies provided important insights into the reductive cross-coupling process between diarylmethyl radical and pyridine-boryl radical. Moreover, part of the obtained pyridylated diarylmethane products were screened against a panel of cancer cell lines, and 3 v was confirmed to significantly inhibit the proliferation of head and neck squamous cell carcinoma (HNSCC) cells. This method offers a platform for the preparation of new lead compounds with antitumor activity.

Iodoperfluoroalkylation of unactivated alkenes via pyridine-boryl radical initiated atom-transfer radical addition

The pyridine/bis(pinacolate)diboron combination has been found to be able to initiate the iodoperfluoroalkylation of unactivated alkenes with perfluoroalkyl iodides. Theoretical calculations and control experiments indicate that the atom transfer radical addition mechanism is responsible for the formation of iodoperfluoroalkylation products. This metal-free and photo-free strategy is applicable to a wide range of perfluoroalkyl iodides and unactivated alkenes with good functional group tolerance. Further applications in iodoperfluoroalkylation of organic semiconductor-relevant or bioactive molecules demonstrate the synthetic potential of this method.

Direct decarboxylative Giese reactions

The quest to find milder and more sustainable methods to generate highly reactive, carbon-centred intermediates has led to a resurgence of interest in radical chemistry. In particular, carboxylic acids are seen as attractive radical precursors due their availability, low cost, diversity, and sustainability. Moreover, the corresponding nucleophilic carbon-radical can be easily accessed through a favourable radical decarboxylation process, extruding CO₂ as a traceless by-product. This review summarizes the recent progress on using carboxylic acids directly as convenient radical precursors for the formation of carbon-carbon bonds via the 1,4-radical conjugate addition (Giese) reaction.

Bromine-radical-induced Csp2–H difluoroalkylation of quinoxalinones and hydrazones through visible-light-promoted Csp3–Br bond homolysis

Bromine radicals derived from photo-induced Csp3–Br bond homolysis can mediate H abstraction/imine radical formation from quinoxalinones and hydrazones, which in turn quench the in situ-generated difluoroalkyl radicals to furnish the products.

Metal-to-Ligand Ratio-Dependent Chemodivergent Asymmetric Synthesis

Chemodivergent asymmetric synthesis was achieved by tuning the metal-to-ligand ratio in an organometallic catalytic system. Using N-(aroyloxy)phthalimide as the precursor of either an oxygen-centered aroyloxy radical or a nitrogen-centered phthalimidyl radical, enantioselective oxocyanation or aminocyanation of alkenes was achieved separately through a dual photoredox and copper catalysis. The metal-to-ligand ratio can exert chemoselective control while retaining the high enantiopurity of divergent products. Both reactions proceed efficiently with catalyst loading as low as 0.2 mol % and can be performed on a gram scale without loss of chemoselectivity or enantioselectivity. Chemodivergent asymmetric 1,5-aminocyanation or 1,5-oxocyanation of vinylcyclopropane can also be realized by this protocol. Mechanistic investigations involving electron paramagnetic resonance (EPR) experiments were performed to shed light on the stereochemical and chemodivergent results.

Photoredox-mediated hydroalkylation and hydroarylation of functionalized olefins for DNA-encoded library synthesis

DNA-encoded library (DEL) technology features a time- and cost-effective interrogation format for the discovery of therapeutic candidates in the pharmaceutical industry. To develop DEL platforms, the implementation of water-compatible transformations that facilitate the incorporation of multifunctional building blocks (BBs) with high C(sp3) carbon counts is integral for success. In this report, a decarboxylative-based hydroalkylation of DNA-conjugated trifluoromethyl-substituted alkenes enabled by single-electron transfer (SET) and subsequent hydrogen atom termination through electron donor-acceptor (EDA) complex activation is detailed. In a further photoredox-catalyzed hydroarylation protocol, the coupling of functionalized, electronically unbiased olefins is achieved under air and within minutes of blue light irradiation through the intermediacy of reactive (hetero)aryl radical species with full retention of the DNA tag integrity. Notably, these processes operate under mild reaction conditions, furnishing complex structural scaffolds with a high density of pendant functional groups.

Visible-Light-Mediated Alkylation of Thiophenols via Electron Donor-Acceptor Complexes Formed between Two Reactants

A metal-free, photocatalyst-free, photochemical system was developed for the direct alkylation of thiophenols via electron donor-acceptor (EDA) complexes (K_EDA = 145 M^-1) between two reactants, N-hydroxyphthalimide esters as acceptors and thiophenol anions as donors, in the presence of a tertiary amine. The EDA complexes in the reaction system have a broad range of visible-light absorption (400-650 nm) and can trigger the reaction effectively under sunlight.

Pyridine-Mediated B–B Bond Activation of (RO)2B–B(OR)2 for Generating Borylpyridine Anions and Pyridine-Stabilized Boryl Radicals as Useful Boryl Reagents in Organic Synthesis

Significant developments have been achieved in recent years toward the utilization of (RO)2B–B(OR)2 for exploring transition-metal-free organic transformations in organic synthesis. Among the various combinations of Lewis bases with diborons developed so far, pyridine derivatives are simple, commercially available, and cheap compounds to expand the synthetic utility of diborons by generating borylpyridine anions and pyridine-stabilized boryl radicals via B–B bond cleavage. These borylpyridine species mediate a series of transformations in both a catalytic and stoichiometric manner for C–X activation (X = halogen, CO2H, NR2) and concomitant C-borylation, hydroboryl­ation, C–C bond formation, and reduction reactions.

1 Introduction

2 Reaction Pathway for B–B Bond Cleavage of Diborons with Electron-Deficient Pyridines

3 Pyridine-Mediated B–B Bond Activation of (RO)2B–B(OR)2 for Application in Organic Synthesis

3.1 Dehalogenative C-Borylation

3.2 Desulfonative C-Borylation

3.3 Decarboxylative C-Borylation

3.4 Deaminative C-Borylation

3.5 Hydroborylation

3.6 C–C Bond Formation

3.7 Pyridine Functionalization

3.8 Deoxygenation and N-Borylation Reactions

4 Conclusions

A convenient pinacol coupling of diaryl ketones with B2pin2via pyridine catalysis

A practical pinacol coupling of diaryl ketones is developed by employing a stable diboron reagent and pyridine Lewis base catalyst. The operationally simple process affords the desired diols with excellent efficiency in up to 99% yield within 1 hour.

Mechanistic insights into the dearomative diborylation of pyrazines: a radical or non-radical process?

The mechanisms of the dearomative diborylation of pyrazines were investigated via a combination of density functional theory calculations and experimental studies.

Biomimetic Total Syntheses of Ergot Alkaloids via Decarboxylative Giese Coupling

Biomimetic total syntheses of Festuclavine and Pyroclavine were achieved by a sequential radical coupling. The key steps include intramolecular decarboxylative Giese reaction to form the central C ring and 4-nitrobenzenesulfonyl (Ns)-directed indole C4-H olefination to introduce the indole C4 component. In addition, D-ring formation was completed by decarboxylative alkenylation and intramolecular S_N2 reaction.

Domino lignin depolymerization and reconnection to complex molecules mediated by boryl radicals

Lignin has been demonstrated as a source of complex molecules via a boryl-mediated domino degradation/reconnection process.

A theoretical mechanistic study of IrIII/CuI-metallaphotoredox catalyzed asymmetric radical decarboxylative cyanation

The radical mechanism and origin of the high enantioselectivity of the photoredox-mediated Ir^III/Cu^I dual-catalyzed asymmetric decarboxylative cyanation have been theoretically disclosed.

The photoredox alkylarylation of styrenes with alkyl N-hydroxyphthalimide esters and arenes involving C-H functionalization

The In(OTf)₃-promoted three-component photoredox alkylarylation of styrenes with alkyl NHP esters and arenes to access alkylated arene derivatives through C-C bond cleavage and C-H functionalization is reported. By utilizing visible-light photoredox catalysis, alkyl N-hydroxyphthalimide esters serving as alkyl carbon-centered radicals and a wide range of arenes (e.g., indoles, pyrrole, and electron-rich arenes) as nucleophiles were used to enable the introduction of various alkyl groups and aryl groups across the C[double bond, length as m-dash]C bonds with excellent selectivity and functional group tolerance.

Photoredox-catalyzed stereoselective alkylation of enamides with N-hydroxyphthalimide esters via decarboxylative cross-coupling reactions

Stereoselective β-C(sp2)-H alkylation of enamides with redox-active N-hydroxyphthalimide esters via a photoredox-catalyzed decarboxylative cross-coupling reaction is demonstrated. This methodology features operational simplicity, broad substrate scopes, and excellent stereoselectivities and functional group tolerance, affording a diverse array of geometrically defined and synthetically valuable enamides bearing primary, secondary or tertiary alkyl groups in satisfactory yields.

Nickel-Catalyzed Reductive Arylalkylation via a Migratory Insertion/Decarboxylative Cross-Coupling Cascade

Reported is a nickel-catalyzed reductive arylalkylation of unactivated alkenes tethered to aryl iodides with redox active N-hydroxyphthalimide esters as the alkyl source through successful merging of migratory insertion and decarboxylative cross-coupling in a cascade. This new method avoids the use of pregenerated organometallic reagents and thus enables the synthesis of diverse benzene-fused carbo- and heterocyclic compounds with high tolerance of a wide range of functional groups.

Nickel-catalyzed allylic defluorinative alkylation of trifluoromethyl alkenes with reductive decarboxylation of redox-active esters

Herein, we report a nickel-catalyzed allylic defluorinative alkylation of trifluoromethyl alkenes through reductive decarboxylation of redox-active esters. The present reaction enables the preparation of functionalized gem-difluoroalkenes with the formation of sterically hindered C(sp3)-C(sp3) bonds under very mild reaction conditions, while tolerating many sensitive functional groups and requiring minimal substrate protection. Therefore, this method provides an efficient and convenient approach for late-stage modification of biologically interesting molecules.

Perfluoroalkylative pyridylation of alkenes via 4-cyanopyridine-boryl radicals

A metal- and photo-free method for the perfluoroalkylative pyridylation of alkenes has been developed.

A metal-free and photo-free method for the perfluoroalkylative pyridylation of alkenes has been developed via a combination of computational and experimental studies. Density functional theory calculations and control experiments indicate that the homolysis of R_f–X (X = Br, I) bonds by the 4-cyanopyridine-boryl radicals in situ generated from 4-cyanopyridine and B₂pin₂ is the key step. Sequential addition of R_f radicals to alkenes and the selective cross-coupling of the resulting alkyl radicals and 4-cyanopyridine-boryl radicals gives alkene difunctionalization products with a quaternary carbon center. This method exhibits a broad substrate scope and good functional group compatibility.

Isolable diboryl radicals acting as highly efficient reaction intermediates under mild conditions

Activation of the B–B bond in diborane with dimesitylpyridyl boranes afforded the stable diboryl radicals in moderate yields.