Photochemical Organocatalytic Functionalization of Pyridines via Pyridinyl Radicals

Trinuclear Borane (B3H7)-Mediated Selective C4-H Alkylation and Phosphonation of Quinolines and Tetrahydroquinolines

Herein, we report a method for the regioselective alkylation and phosphonation of quinoline C4-H via a B3H7-mediated nucleophilic addition of Turbo Grignard reagents and phosphine oxide anions to quinolines bearing different substituents, affording the 4-alkyl and 4-phosphoryl quinolines and tetrahydroquinolines after one-pot oxidation or reduction. The results indicate that coordination of the B3H7 group can activate substrates toward a potential 1,4-dearomative addition and subtly control the regioselectivity by preventing the 1,2-dearomative addition.

Polysubstituted Pyridines from 1,4-Oxazinone Precursors

This study describes a general method for the preparation of 1,4-oxazin-2-one intermediates from acetylene dicarboxylate and β-amino alcohol precursors. Oxazinones prepared in this manner were employed in a tandem cycloaddition/cycloreversion reaction sequence with a model alkyne (phenyl acetylene) to give substituted pyridine products. Fundamental reactivity and selectivity studies are complemented by the synthesis of the polycyclic ergot alkaloid natural product xylanigripone A.

Optically Gated Dissociation of a Heptazinyl Radical Liberates H• through a Reactive πσ* State

Using trianisole heptazine (TAHz) as a monomeric analogue for carbon nitride, we performed ultrafast pump-photolysis-probe transient absorption (TA) spectroscopy on the intermediate TAHzH• heptazinyl radical produced from an excited state PCET reaction with 4-methoxyphenol (MeOPhOH). Our results demonstrate an optically gated photolysis that releases H• and regenerates ground state TAHz. The TAHzH• radical signature at 520 nm had a lifetime of 7.0 ps, and its photodissociation by the photolysis pulse is clearly demonstrated by the ground state bleach recovery of the closed-shell neutral TAHz. This behavior has been previously predicted as evidence of a dissociative πσ* state. For the first time, we experimentally demonstrate photolysis of the TAHzH• heptazinyl radical through a repulsive πσ* state. This is a critical feature of the proposed reaction mechanisms involving water oxidation and CO2 reduction.

Photo-induced carboxylation of C(sp2)-S bonds in aryl thiols and derivatives with CO2

Aryl thiols have proven to be a useful class of electron donors and hydrogen atom sources in photochemical processes. However, the direct activation and functionalization of C(sp2)-S bonds in aryl thiols remains elusive in the field of photochemistry. Herein, a photochemical carboxylation of C(sp2)-S bonds in aryl thiols with CO2 is reported, providing a synthetic route to important aryl carboxylic acids. Moreover, different kinds of aryl thiol derivatives, benzeneselenol and diphenyl diselenide also show moderate-to-high reactivity in this transformation. Mechanistic studies, including DFT calculations, suggest that the in situ generated carbon dioxide radical anion (CO2•-) and disulfide might be the key intermediates, which undergo radical substitution to yield products. This reaction features mild and catalyst-free conditions, good functional group tolerance and wide substrate scope. Furthermore, the efficient degradation of polyphenylene sulfide highlights the usefulness of this methodology.

Catalytic Generation of Pyridyl Radicals via Electron Donor-Acceptor Complex Photoexcitation: Synthesis of 2-Pyridylindole-Based Heterobiaryls

We report the catalytic generation of pyridyl radicals through photoexcitation of the electron donor-acceptor (EDA) complex, which enables the C2-selective heteroarylation of indole under ambient conditions. In this manifold, catalytic triarylamine and chloropyridine aggregate into an EDA complex in the presence of an inorganic base, making readily available chloropyridines good precursors for the generation of diverse pyridyl radicals. Given the broad reaction scope, this catalytic EDA complex protocol provides robust access to heterobiaryl scaffolds that are widely present in biologically important molecules.

Unified ionic and radical C-4 alkylation and arylation of pyridines

C-H Functionalization of pyridines is an efficient strategy to access pyridine derivatives occurring in pharmaceuticals, agrochemicals, and materials. Nucleophilic additions to pyridiniums via both ionic and radical species have proven particularly useful. However, these reactions suffer from poor regioselectivity. By identifying an enzyme-mimic pocket-type urea activation reagent, we report a general platform for pyridine C-4 functionalization. Both ionic and radical nucleophiles can be incorporated to achieve the alkylation and arylation. Notably, the highly regioselective C-4 radical arylation is disclosed for the first time. The broad scope of nucleophiles and pyridines renders this platform applicable to the late-stage functionalization of drug-like molecules and the preparation of complex biologically important molecules.

p-Diarylboryl Halothiophenols as Multifunctional Catalysts via Photoactive Intramolecular Frustrated Lewis Pairs

p-Diarylboryl halothiophenols are developed and unequivocally characterized. Their photophysical properties and catalytic performance are unveiled by experimental and theoretical investigations. This novel class of triarylboranes behaves as a Brønsted acid to generate the corresponding borylthiophenolate that can absorb visible light to undergo intramolecular charge transfer to form a radical pair consisting of a boron radical anion and thiyl radical, which acts as a single-electron reductant while engaging in hydrogen atom transfer to regenerate the parent borylthiophenol. The synthetic relevance of this mode of action is demonstrated by the establishment of unique catalysis that integrates three different yet tunable functions in a single catalytic cycle, thereby allowing borylthiophenols to solely promote the assembly of sterically congested 1,2-diols and 1,2-aminoalcohol derivatives via radical-radical cross-coupling.

Unexpected Reduction of a Coordinated Diazapyridinophane Ligand Bound to Chromium(III) Ion Leading to Delocalization of the Unpaired Electron across Two Isolated Pyridine Units

In the tetraazamacrocyclic ligand N,N'-dimethyl-2,11-diaza-[3.3](2,6)pyridinophane (L-N4 Me2 ), the two pyridine units are separated from each other by sp3 -hybridized triatomic bridges. Such electronically isolated pyridine moieties are considerably less prone to reductions than di- or triimines. A detailed structural, magnetic, and spectroscopic investigation of the complexes [Cr(L-N4 Me2 )(OAc)2 ] and [Cr(L-N4 Me2 )(OAc)2 ](PF6 ), in combination with theoretical calculations, reveals that the reduced complex must be described as a chromium(III) ion coordinated to the anionic radical ligand (L-N4 Me2 )⋅- rather than a low-spin chromium(II) ion bound to closed-shell ligands. Thus, it is, to the best of our knowledge, only the second example of a stable and structurally characterized metal complex containing a reduced isolated pyridine unit. The stability is attributed to the delocalization of the unpaired electron across the two pyridine units, mediated by their interaction to the metal ion.

Al2 O3 -coated BiVO4 Photoanodes for Photoelectrocatalytic Regioselective C-H Activation of Aromatic Amines

Photoelectrochemistry is becoming an innovative approach to organic synthesis. Generally, the current photoelectrocatalytic organic transformations suffer from limited reaction type, low conversion efficiency and poor stability. Herein, we develop efficient and stable photoelectrode materials using metal oxide protective layer, with a focus on achieving regioselective activation of amine compounds. Notably, our photoelectrochemistry process is implemented under mild reaction conditions and does not involve any directing groups, transition metals or oxidants. The results demonstrate that beyond photocatalysis and electrocatalysis, photoelectrocatalysis exhibits high efficiency, remarkable repeatability and good functional group tolerance, highlighting its great potential for applications.

A High-Nuclearity Copper Sulfide Nanocluster [S-Cu50] Featuring a Double-Shell Structure Configuration with Cu(II)/Cu(I) Valences

This work represents an important step in the quest for creating atomically precise binary semiconductor nanoclusters (BS-NCs). Compared with coinage metal NCs, the preparation of BS-NCs requires strict control of the reaction kinetics to guarantee the formation of an atomically precise single phase under mild conditions, which otherwise could lead to the generation of multiple phases. Herein, we developed an acid-assisted thiolate dissociation approach that employs suitable acid to induce cleavage of the S-C bonds in the Cu-S-R (R = alkyl) precursor, spontaneously fostering the formation of the [Cu-S-Cu] skeleton upon the addition of extra Cu sources. Through this method, a high-nuclearity copper sulfide nanocluster, Cu50S12(SC(CH3)3)20(CF3COO)12 (abbreviated as [S-Cu50] hereafter), has been successfully prepared in high yield, and its atomic structure was accurately modeled through single-crystal X-ray diffraction. It was revealed that [S-Cu50] exhibits a unique double-shell structural configuration of [Cu14S12]@[Cu36S20], and the innermost [Cu14] moiety displays a rhombic dodecahedron geometry, which has never been observed in previously synthesized Cu metal, hydride, or chalcogenide NCs. Importantly, [S-Cu50] represents the first example incorporating mixed Cu(II)/Cu(I) valences in reported atomically precise copper sulfide NCs, which was unambiguously confirmed by XPS, EPR, and XANES. In addition, the electronic structure of [S-Cu50] was established by a variety of optical investigations, including absorption, photoluminescence, and ultrafast transient absorption spectroscopies, as well as theoretical calculations. Moreover, [S-Cu50] is air-stable and demonstrates electrocatalytic activity in ORR with a four-electron pathway.

Photoinduced Cobalt Catalysis for the Reductive Coupling of Pyridines and Dienes Enabled by Paired Single-Electron Transfer

Selective hydroarylation of dienes has potential to provide swift access to useful building blocks. However, most existing methods rely on dienes stabilised by an aromatic group and transmetallation or nucleophilic attack steps require electron-rich aryl coupling partners. As such, there are few examples which tolerate wide-spread heteroarenes such as pyridine. Whilst allylic C-H functionalisation could be considered an alternative approach, the positional selectivity of unsymmetrical substrates is hard to control. Here, we report a general approach for selective hydropyridylation of dienes under mild conditions using metal catalysed hydrogen-atom transfer. Photoinduced, reductive conditions enable simultaneous formation of a cobalt-hydride catalyst and the persistent radical of easily-synthesised pyridyl phosphonium salts. This facilitates selective coupling of dienes in a traceless manner at the C4-position of a wide-range of pyridine substrates. The mildness of the method is underscored by its functional-group tolerance and demonstrated by applications in late-stage functionalisation. Based on a combination of experimental and computational studies, we propose a mechanistic pathway which proceeds through non-reversible hydrogen-atom transfer (HAT) from a cobalt hydride species which is uniquely selective for dienes in the presence of other olefins due to a much higher relative barrier associated with olefin HAT.

Direct Decarboxylative C-2 Alkylation of Azoles through Minisci-Type Coupling

This note discusses the application of a Minisci-type reaction for the direct alkylation of azoles with carboxylic acids as radical precursors. Different reaction conditions were investigated to achieve high yield of the desired products, focusing on acid strength and solvent screening. Moreover, the reactivity of imidazoles with various carboxylic acids was investigated, showing good yield for most cases. The study reveals the potential of this approach for late-stage functionalization in drug discovery.

Salicylaldehyde as an SET-HAT Bifunctional Photocatalyst for the Intermolecular Transalkylation of Phthalimide

Salicylaldehyde works as an efficient photocatalyst for the intermolecular transalkylation of phthalimide. The well-designed dimethyl N-hydroxyphthalimide ester proves to be a good alkylation reagent. It inhibits the competing intramolecular alkylation of alkylating reagent, enabling the site-specific synthesis of N-substituted phthalimide.

Recent Advances in C-H Functionalisation through Indirect Hydrogen Atom Transfer

The functionalisation of C-H bonds has been an enormous achievement in synthetic methodology, enabling new retrosynthetic disconnections and affording simple synthetic equivalents for synthons. Hydrogen atom transfer (HAT) is a key method for forming alkyl radicals from C-H substrates. Classic reactions, including the Barton nitrite ester reaction and Hofmann-Löffler-Freytag reaction, among others, provided early examples of HAT. However, recent developments in photoredox catalysis and electrochemistry have made HAT a powerful synthetic tool capable of introducing a wide range of functional groups into C-H bonds. Moreover, greater mechanistic insights into HAT have stimulated the development of increasingly site-selective protocols. Site-selectivity can be achieved through the tuning of electron density at certain C-H bonds using additives, a judicious choice of HAT reagent, and a solvent system. Herein, we describe the latest methods for functionalizing C-H/Si-H/Ge-H bonds using indirect HAT between 2018-2023, as well as a critical discussion of new HAT reagents, mechanistic aspects, substrate scopes, and background contexts of the protocols.

C-H Functionalization of Pyridines via Oxazino Pyridine Intermediates: Switching to para-Selectivity under Acidic Conditions

para-Selective C-H functionalization of pyridines holds a significant value but remains underdeveloped. Site-switchable C-H functionalization of pyridines under easily tunable conditions expedites drug development. We recently reported a redox-neutral dearomatization-rearomatization strategy for meta-C-H functionalization of pyridines via oxazino pyridine intermediates. Here, we demonstrate that these oxazino pyridine intermediates undergo highly para-selective functionalization simply by switching to acidic conditions. A broad scope of para-alkylated and arylated pyridines is prepared through radical as well as ionic pathways. These mild and catalyst-free methods are applied to the late-stage para-functionalization of drugs using pyridines as the limiting reagents. Consecutive meta,para-difunctionalization of pyridines is also achieved with complete regiocontrol relying on the pH-dependent reactivity of oxazino pyridines.

Bifunctional 1-Hydroxypyrene Photocatalyst for Hydrodesulfurization via Reductive C(Aryl)-S Bond Cleavage

We developed the visible-light-induced hydrodesulfurization of alkyl aryl thioethers via the reductive cleavage of the C(aryl)-S bond using 1-hydroxypyrene as a Brønsted acid-reductant bifunctional photocatalyst. The hydrodesulfurization reaction proceeded under simple reaction conditions (1-hydroxypyrene and Et₃N in THF under purple LED illumination); this reaction did not require chemicals commonly used for hydrodesulfurization, such as hydrosilanes, transition metal catalysts, and/or stoichiometric amounts of metal reagents. Detailed mechanistic studies based on control experiments, spectroscopic measurements, and computational studies revealed that the cleavage of the C(aryl)-S bond and the formation of the C(aryl)-H bond proceeded via the formation of the ion pair between the radical anion of alkyl aryl thioether and Et₃N⁺H, resulting in the generation of a sulfur radical. In addition, the 1-hydroxypyrene catalyst was regenerated via hydrogen atom transfer (HAT) from Et₃N.