Visible-Light-Mediated Cross Dehydrogenative Coupling of Thiols with Aldehydes: Metal-Free Synthesis of Thioesters at Room Temperature

Pyridine-borane complex-catalysed thioesterification: the direct conversion of carboxylic acids to thioesters

Thioesters are a common class of biologically active fragments and synthetically useful building blocks. An attractive synthetic approach would be to use simple and bench-stable carboxylic acids as a coupling partner. Herein, we present a 4-bromo pyridine-borane complex as a catalyst for the direct coupling of carboxylic acids with thiols. A wide range of thioesters with good functional group compatibility could be prepared via this metal-free approach. The merit of this strategy is exemplified by the modification of carboxylic acid-containing drugs.

Photoredox-Enabled Deconstructive [5 + 1] Annulation Approach to Isoquinolones from Indanones in Water

We disclose a deconstructive [5 + 1] annulation protocol for the synthesis of isoquinolones through a nitrogen insertion into abundant indanones. This method exploits photoredox-catalyzed ring-opening of oxime esters. The reaction proceeds smoothly with water as the reaction medium and tolerates a range of functional groups on diverse thiophenols, amines, or indanones. Moreover, the representative isoquinolones exhibit promising antifungal activities.

Pushing Boundaries: What's Next in Metal-Free C-H Functionalization for Sulfenylation?

The synthesis of thioether derivatives has been explored widely due to the potential application of these derivatives in medicinal chemistry, pharmaceutical industry and material chemistry. Within this context, there has been an increasing demand for the environmentally benign construction of C-S bonds via C-H functionalization under metal-free conditions. In the present article, we highlight recent developments in metal-free sulfenylation that have occurred in the past three years. The synthesis of organosulfur compounds via a metal-free approach using a variety of sulfur sources, including thiophenols, disulfides, sulfonyl hydrazides, sulfonyl chlorides, elemental sulfur and sulfinates, is discussed. Non-conventional strategies, which refer to the development of thioether derivatives under visible light and electrochemically mediated conditions, are also discussed. The key advantages of the reviewed methodologies include broad substrate scope and high reaction yields under environmentally benign conditions. This comprehensive review will provide chemists with a synthetic tool that will facilitate further development in this field.

Rare Earth Amide-Catalyzed Direct Thioesterification of Aldehydes with Thiols under Mild Conditions

Direct thioesterification of aldehydes with thiols catalyzed by readily accessible rare earth/lithium amide RE[N(SiMe3)2]3(μ-Cl)Li(THF)3 is developed, which enables the preparation of a range of thioesters (31 examples) under room temperature and solvent-free conditions, without using any additive or external oxidant. This method provides a straightforward and atom-efficient approach for the thioester synthesis.

Unified Approach to Diverse Heterocyclic Synthesis: Organo-Photocatalyzed Carboacylation of Alkenes and Alkynes from Feedstock Aldehydes and Alcohols

We report an organo-photocatalyzed carboacylation reaction that offers a springboard to create chemical complexity in a diversity-driven approach. The modular one-pot method uses feedstock aldehydes and alcohols as acyl surrogates and commercially available Eosin Y as the photoredox catalyst, making it simple and affordable to introduce structural diversity. Several biologically relevant skeletons have been easily synthesized under mild conditions in the presence of visible light irradiation by fostering a radical acylation/cyclization cascade. The proposed reaction mechanism was further illuminated by a number of spectroscopic studies. Furthermore, we applied this protocol for the late-stage functionalization of pharmaceuticals and blockbuster drugs.

In situ generation of acyloxyphosphoniums for mild and efficient synthesis of thioesters

We present a novel approach for in situ generation of acyloxyphosphoniums by premixing iodobenzene dicarboxylates and triphenylphosphine, resulting in efficient thioester synthesis (up to 100% yield). Stable solid iodobenzene dicarboxylates, achieved via carboxylate exchange, serve as hypervalent iodine precursors. The resulting acyloxyphosphoniums allow convenient one-pot thioester synthesis under mild conditions. Our method demonstrates facile acyloxyphosphonium production from iodobenzene dicarboxylates and Ph3P, enabling diverse thioester preparation. ESI-MS analysis confirms acyloxyphosphonium ion formation, pivotal in acylation. This strategy holds potential for combinatorial thioester synthesis and broader nucleophile modification applications.

Synthesis of Thioesters from Aldehydes via N-Heterocyclic Carbene (NHC) Catalyzed Radical Relay

We have developed an efficient N-heterocyclic carbene (NHC)-catalyzed thioesterification of aldehydes using N-thiosuccinimides as the thiolation reagent. This organocatalyzed transition involves the generation of sulfur radicals by single electron transfer of the Breslow enolate (generated from aldehyde and NHC catalyst) with N-thiosuccinimides. This method offers facile access to various highly functionalized thioesters and exhibits good chemical yields and functional group tolerance.

Visible/solar-light-driven thiyl-radical-triggered synthesis of multi-substituted pyridines

A light-triggered synthesis of thio-functionalized pyridines is demonstrated using γ-ketodinitriles, thiols, and eosin Y as the photocatalyst. The reaction proceeds via the selective attack on one of the cyano groups by an in situ generated thiyl radical. The reaction also proceeds with nearly equal efficiency using direct sunlight. Large-scale synthesis and a few useful synthetic transformations of the substituted pyridines are also performed.

Synergistic Visible Light and Pd-Catalyzed C-H Alkylation of 1-Naphthylamines with α-Diazoesters

The combination of visible light irradiation and Pd-catalysis has been practically employed for the C-H alkylation reactions of naphthylamines and α-diazo esters, leading to the synthesis of α-naphthyl functionalized acetates via C-C bond construction under mild reaction conditions and under solvent-free conditions. The light irradiation has been proven to play a pivotal role in the reactions, probably by promoting the generation of active carbene species from α-diazo esters.

Visible-Light-Driven Thioesterification of Aryl Halides with Potassium Thiocarboxylates: Transition-Metal Catalyst-Free Incorporation of Sulfur Functionalities into an Aromatic Ring

Reactions of acceptor-substituted aryl iodides and bromides with potassium thiocarboxylates under white light irradiation allow for the preparation of S-aryl thioesters including synthetically versatile S-aryl thioacetates. This transition-metal and external photocatalyst-free method features extremely mild reaction conditions compared with those used in transition-metal-catalyzed protocols. Reactions proceed via the initial formation of an electron donor-acceptor (EDA) complex in the ground state, which was supported by UV-vis spectra. Electron paramagnetic resonance (EPR) spin-trapping experiments using phenyl-N-tert-butylnitrone (PBN) have revealed the radical nature of the reaction.

Exploring Eosin Y as a bimodular catalyst: organophotoacid mediated Minisci-type acylation of N-heteroarenes

Here we report Eosin Y as a bimodular catalyst for Minisci-type acylation reactions. The formation of organic exciplexes between photoexcited Eosin Y and N-heteroarenes was found to be a stabilizing factor for photoacid catalysis under optimized conditions. Spectroscopic investigations such as steady state fluorescence quenching and dynamic lifetime quenching experiments were employed to better understand the role of Eosin Y as both a photoredox catalyst and a photoacid. Feedstock aldehydes were employed as acyl radical precursors for engaging in C-C bond formation reactions with a variety of nitrogen containing heterocycles.