Catalyst-Free Visible-Light-Promoted Cyclization of Aldehydes: Access to 2,5-Disubstituted 1,3,4-Oxadiazole Derivatives

Eco-friendly approaches to 1,3,4-oxadiazole derivatives: A comprehensive review of green synthetic strategies

This review article offers an environmentally benign synthesis of 1,3,4-oxadiazole derivatives, with a focus on sustainable methodologies that have minimal impact on the environment. These derivatives, known for their diverse applications, have conventionally been associated with synthesis methods that utilize hazardous reagents and produce significant waste, thereby raising environmental concerns. The green synthesis of 1,3,4-oxadiazole derivatives employs renewable substrates, nontoxic catalysts, and mild reaction conditions, aiming to minimize the environmental impact. Innovative techniques such as catalyst-based, catalyst-free, electrochemical synthesis, green-solvent-mediated synthesis, grinding, microwave-mediated synthesis, and photosynthesis are implemented, providing benefits in terms of scalability, cost-effectiveness, and ease of purification. This review emphasizes the significance of sustainable methodologies in the synthesis of 1,3,4-oxadiazole and boots for continued exploration in this research domain.

Visible Light-Induced Reactions of Diazo Compounds and Their Precursors

In recent years, visible light-induced reactions of diazo compounds have attracted increasing attention in organic synthesis, leading to improvement of existing reactions, as well as to the discovery of unprecedented transformations. Thus, photochemical or photocatalytic generation of both carbenes and radicals provide milder tools toward these key intermediates for many valuable transformations. However, the vast majority of the transformations represent new reactivity modes of diazo compounds, which are achieved by the photochemical decomposition of diazo compounds and photoredox catalysis. In particular, the use of a redox-active photocatalysts opens the avenue to a plethora of radical reactions. The application of these methods to diazo compounds led to discovery of transformations inaccessible by the classical reactivity associated with carbenes and metal carbenes. In most cases, diazo compounds act as radical sources but can also serve as radical acceptors. Importantly, the described processes operate under mild, practical conditions. This Review describes this subfield of diazo compound chemistry, particularly focusing on recent advancements.

Alkoxy Diazomethylation of Alkenes by Photoredox-Catalyzed Oxidative Radical-Polar Crossover

Herein, we present the discovery and development of the first photoredox-catalyzed alkoxy diazomethylation of alkenes with hypervalent iodine reagents and alcohols. This multicomponent process represents a new disconnection approach to diazo compounds and is featured by a broad scope, mild reaction conditions, and excellent selectivity. Key to the process was the generation of diazomethyl radicals, which engaged alkenes and alcohols in an inter- and intramolecular fashion by a photoredox-catalyzed oxidative radical-polar crossover leading to unexplored β-alkoxydiazo compounds. The synthetic utility of such diazo compounds was demonstrated with a series of transformations involving C-H, N-H, and O-H insertions as well as in the construction of complex sp3-rich heterocycles.

Advances in heterocycle synthesis through photochemical carbene transfer reactions

Heterocyclic skeletons are commonly found in various bioactive molecules and pharmaceutical compounds, making them crucial in areas such as medicinal chemistry, materials science, and the realm of natural product synthesis. In recent years, the rapid advancements of visible light methodologies in organic synthesis have shown promising potential for the development of light-induced carbene transfer reactions. This is particularly significant as most organic molecules do not absorb visible light. Free carbene, known for its high activity, is frequently utilized for insertion reactions or cyclopropanation reactions. This review focuses on the photochemical strategy for the construction of heterocyclic skeletons, specifically highlighting the methods that employ visible light-promoted carbene transfer reactions.

Photoredox-Catalyzed Cascade sp2 C-H Bond Functionalization to Construct Substituted Acridine with Diarylamine and Hypervalent Iodine(III) Reagents

A photocatalyzed cascade double C-C formation via sp² C-H bond activation of diarylamines with hypervalent iodine diazo reagents was developed. A variety of diarylamines and hypervalent iodine(III) reagents were tolerated well, and a range of substituted acridines with yields ranging from moderate to excellent was provided efficiently. The protocol introduces diazo groups onto diarylamines and enables subsequent late-stage assembly point functionalization with the diazonium structure, forming two new C-C bonds in a sequential fashion.

Catalytic Synthesis of Cyclopropenium Cations with Rh-Carbynoids

Herein, we report the first catalytic one-step synthesis of cyclopropenium cations (CPCs) with readily available alkynes and hypervalent iodine reagents as carbyne sources. Key to the process is the catalytic generation of a novel Rh-carbynoid that formally transfers monovalent cationic carbynes (:+C-R) to alkynes via an oxidative [2+1] cycloaddition. Our process is able to synthesize a new type of CPC substituted with an ester group that underpins the regioselective attack of a broad range of carbon and heteroatomic nucleophiles, thus providing a new platform for the synthesis of valuable cyclopropenes difficult or not possible to make by current methodologies.

Synthesis of 2-Imino-1,3,4-oxadiazolines from Acylhydrazides and Isothiocyanates via Aerobic Oxidation and a DMAP-Mediated Annulation Sequence

In this work, an efficient synthesis of 2-imino-1,3,4-oxadiazolines from acylhydrazides and isothiocyanates is described. In the presence of 4-dimethylaminopyridine (DMAP) and molecular oxygen, various 2-imino-1,3,4-oxadiazolines were produced in good to high yields. The developed method showed a broad substrate scope and was effective on the gram scale. On the basis of the mechanistic studies and previous literature, it was proposed that the mechanism consists of an aerobic oxidation of acylhydrazides facilitated by DMAP and isothiocyanates, followed by a DMAP-mediated annulation of the in situ generated acyldiazenes with isothiocyanates.

Synthesis of [1,2,3]Triazolo-[1,5-a]quinoxalin-4(5H)-ones through Photoredox-Catalyzed [3 + 2] Cyclization Reactions with Hypervalent Iodine(III) Reagents

An efficient synthesis of a variety of [1,2,3]triazolo-[1,5-a]quinoxalin-4(5H)-ones via a [3 + 2] cyclization reaction by photoredox catalysis between quinoxalinones and hypervalent iodine(III) reagents is reported. A range of quinoxalinones and hypervalent iodine(III) reagents were tolerated well. This cyclization reaction allows access to structurally diverse [1,2,3]triazolo-[1,5-a]quinoxalin-4(5H)-ones in moderate to good yields.

Visible light photocatalysis in the synthesis of pharmaceutically relevant heterocyclic scaffolds

Visible light and photoredox catalysis have emerged as a powerful and long-lasting tool for organic synthesis, demonstrating the importance of a variety of chemical bond formation methods.

Diazotrifluoroethyl Radical: A CF3-Containing Building Block in [3 + 2] Cycloaddition

We present herein a visible-light-induced [3 + 2] cycloaddition of a hypervalent iodine(III) reagent with α-ketoacids for the construction of 5-CF₃-1,3,4-oxadiazoles that are of importance in medicinal chemistry. The reaction proceeds smoothly without a photocatalyst, metal, or additive under mild conditions. Different from the well-established trifluorodiazoethane (CF₃CHN₂), the diazotrifluoroethyl radical [CF₃C(·)N₂], a trifluoroethylcarbyne (CF₃CĊ:) equivalent and an unusual CF₃-containing building block, is involved in the present reaction system.