C4-Selective C-H Borylation of Pyridinium Derivatives Driven by Electron Donor-Acceptor Complexes

Visible-Light-Driven Tandem Cyclization of o-Hydroxyaryl Enaminones: Access to 3-(α-Arylsulfonamido)trifluoroethyl Chromones

A visible-light-driven intermolecular tandem α-amidotrifluoroethylation/cyclization of enaminones using a previously unreported N-trifluoroethylaminopyridinium salt was achieved in the absence of transition metal catalysts or bases. Notable features of this synthetic method include mild conditions, high selectivity, excellent functional group compatibility, and satisfactory yields. Preliminary mechanistic studies indicate that the reaction proceeds via a radical pathway, involving an in situ generated N-trifluoroethyl radical, followed by a 1,2-H shift.

Ligand-Controlled Regioselective Dearomative Vicinal and Conjugate Hydroboration of Quinolines

A dearomative strategy to regioselectively modify arenes using a "diene" synthon within aromatic rings provides access to highly functionalized heterocycles from abundant aromatic feedstocks and represents an alternative synthetic approach besides traditional cross-coupling and C-H functionalization methodologies. In this study, we present an efficient method for selectively introducing boron onto quinolines through dearomative hydroboration using easily accessible and stable phosphine-ligated borane complexes. The vicinal 5,6- and conjugate 5,8-hydroborated products could be obtained regioselectively by modifying the phosphine ligand. Drawing inspiration from diverse organoboron transformations, these borane building blocks were diversified by a range of downstream functionalizations, providing modular pathways for the skeletal modifications of quinolines to access a variety of challenging functionalized heterocycles.

Advances in Pyridine C-H Functionalizations: Beyond C2 Selectivity

The pyridine core is a crucial component in numerous FDA-approved drugs and Environmental Protection Agency (EPA) regulated agrochemicals. It also plays a significant role in ligands for transition metals, alkaloids, catalysts, and various organic materials with diverse properties, making it one of the most important structural frameworks. However, despite its significance, direct and selective functionalization of pyridine is still relatively underdeveloped due to its electron-deficient nature and the strong coordinating ability of nitrogen. Among the variety of synthetic transformation, direct functionalization of C-H bond is straightforward and atom economical approach and it's advantageous for late-stage functionalization of pyridine containing drugs. In recent years, innovative strategies for regioselective C-H functionalization of pyridines and azines have emerged, offering numerous benefits such as high regioselectivity, mild conditions, and enabling transformations that were challenging with traditional methods. This review emphasizes the latest advancements in meta and para-C-H functionalization of pyridines through various approaches, including pyridine phosphonium salts, photocatalytic methods, temporary de-aromatization, Minisci-type reactions, and transition metal-catalyzed C-H activation techniques. We discuss the advantages and limitations of these current methods and aim to inspire further progress in this significant field.

Regioselective Homolytic C2-H Borylation of Unprotected Adenosine and Adenine Derivatives via Minisci Reaction

A Minisci-type borylation of unprotected adenosine, adenine nucleotide, and adenosine analogues was successfully achieved through photocatalysis or thermal activation. Despite the challenges posed by the presence of two potential reactive sites (C2 and C8) in the purine motif, the unique nucleophilic amine-ligated boryl radicals effortlessly achieved excellent C2 site selectivity and simultaneously avoided the formation of multifunctionalized products. This protocol proved effective for the late-stage borylation of some important biomolecules as well as a few antiviral and antitumor drug molecules, such as AMP, cAMP, Vidarabine, Cordycepin, Tenofovir, Adefovir, GS-441524, etc. Theoretical calculations shed light on the site selectivity, revealing that the free energy barriers for the C2-Minisci addition are further lowered through the chelation of additive Mg2+ to N3 and furyl oxygen. This phenomenon has been confirmed by an IGMH analysis. Preliminary antitumor evaluation, derivation of the C2-borylated adenosine to other analogues with high-value functionalities, along with the CuAAC click reactions, suggest the potential application of this methodology in drug molecular optimization studies and chemical biology.

Visible-Light-Induced Alkoxypyridylation of Alkenes Using N-Alkoxypyridinium Salts as Bifunctional Reagents

Considering the ubiquitous presence of pyridine moieties in pharmaceutical compounds, it holds immense value to develop practical and straightforward methodologies for accessing heterocyclic aromatic hydrocarbons. In recent years, N-alkoxypyridinium salts have emerged as convenient radical precursors, enabling the generation of the corresponding alkoxy radicals and pyridine through single-electron transfer. Herein, we present the first report on visible-light-mediated intermolecular alkoxypyridylation of alkenes employing N-alkoxylpyridinium salts as bifunctional reagents with an exceptionally low catalyst loading (0.5 mol %).

Photoredox-Catalyzed Tandem Cyclization of Enaminones with N-Sulfonylaminopyridinium Salts toward the Synthesis of 3-Sulfonaminated Chromones

A photoredox-catalyzed intermolecular tandem sulfonamination/cyclization of enaminones was realized by using N-aminopyridinium salts as the sulfonaminated reagents without transition-metal catalysts or bases. The reaction exhibits a broad scope and good functional group tolerance, good yields, and regioselectivity. Preliminary mechanistic studies support the radical property of the reaction and the involvement of N-centered radical intermediates.

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.

Photocatalyst- and Transition Metal-Free Light-Induced Borylation Reactions

The increasing global warming concerns have propelled a surge in the demand for sustainable energy sources within the domain of synthetic organic chemistry. A particularly prominent area of research has been the development of mild synthetic strategies for generating heterocyclic compounds. Heterocyclic compounds containing boron have notably risen to prominence as pivotal reagents in a myriad of organic transformations, showcasing their wide-ranging applicability. This comprehensive review is aimed at collecting the literature pertaining to borylation reactions induced by light, specifically focusing on photocatalyst-free and transition metal-free methodologies. The central emphasis is on delving into selective mechanistic investigations. The amalgamation and analysis of these research insights elucidate the substantial potential inherent in eco-friendly approaches for synthesizing heterocyclic compounds, thus propelling the landscape of sustainable organic chemistry.

Nucleophilic C4-selective (hetero) arylation of pyridines for facile synthesis of heterobiaryls

The synthesis of heterobiaryl compounds holds significant value in organic chemistry due to their extensive range of applications. Herein, we report a highly efficient strategy for conducting C4-selective (hetero) arylation of pyridines using N-aminopyridinium salts. The reaction proceeds readily at room temperature in the presence of a base, thus eliminating the requirement for catalysts or oxidants. This method allows for the installation of various electron-rich (hetero) aryl groups on pyridines, resulting in the streamlined synthesis of highly valuable C4-(hetero) aryl pyridine derivatives, which are otherwise challenging to acquire via conventional methods. This simple and straightforward method will facilitate access to a range of heterobiaryl compounds thereby promoting their application in various scientific disciplines.

Unlocking the Potential of β-Fragmentation of Aminophosphoranyl Radicals for Sulfonyl Radical Reactions

Exploiting β-scission in aminophosphoranyl radicals for radical-mediated transformations has been a longstanding challenge. In this study, we investigated the untapped potential of β-fragmentation in aminophosphoranyl radicals by leveraging the unique properties of the P-N bond and the substituents of P(III) reagents. Our approach carefully considers factors such as cone angle and electronic properties of phosphine and employs density functional theory (DFT) calculations to probe structural and molecular orbital influence. We successfully induced β-fragmentation through N-S bond cleavage of aminophosphoranyl radicals under visible light and mild conditions, generating a range of sulfonyl radicals derived from pyridinium salts via the photochemical activity of electron donor-acceptor (EDA) complexes. This innovative synthetic strategy exhibits broad applicability, including late-stage functionalization, and paves the way for valuable sulfonyl radical-mediated reactions, such as alkene hydrosulfonylation, difunctionalization, and pyridylic C-H sulfonylation.

Selective multifunctionalization of N-heterocyclic carbene boranes via the intermediacy of boron-centered radicals

The selective difunctionalization of N-heterocyclic carbene (NHC) boranes with alkenes has been achieved via decatungstate and thiol synergistic catalysis. The catalytic system also allows stepwise trifunctionalization, leading to complex NHC boranes with three different functional groups which are challenging to prepare by other methods. The strong hydrogen-abstracting ability of the excited decatungstate enables the generation of boryl radicals from mono- and di-substituted boranes for realizing borane multifunctionalization. This proof-of-principle research provides a new chance for fabricating unsymmetrical boranes and developing boron-atom-economic synthesis.

Transition Metal-Free Aromatic C-H, C-N, C-S and C-O Borylation

Aromatic organoboron compounds are highly valuable building blocks in organic chemistry. They were mainly synthesized through aromatic C-H and C-Het borylation, in which transition metal-catalysis dominate. In the past decade, with increasing attention to sustainable chemistry, numerous transition metal-free C-H and C-Het borylation transformations have been developed and emerged as efficient methods towards the synthesis of aromatic organoboron compounds. This account mainly focuses on recent advances in transition metal-free aromatic C-H, C-N, C-S, and C-O borylation transformations and provides insights to where further developments are required.