Flow Synthesis of Diaryliodonium Triflates

Recent Progress in Synthetic Applications of Hypervalent Iodine(III) Reagents

Hypervalent iodine(III) compounds have found wide application in modern organic chemistry as environmentally friendly reagents and catalysts. Hypervalent iodine reagents are commonly used in synthetically important halogenations, oxidations, aminations, heterocyclizations, and various oxidative functionalizations of organic substrates. Iodonium salts are important arylating reagents, while iodonium ylides and imides are excellent carbene and nitrene precursors. Various derivatives of benziodoxoles, such as azidobenziodoxoles, trifluoromethylbenziodoxoles, alkynylbenziodoxoles, and alkenylbenziodoxoles have found wide application as group transfer reagents in the presence of transition metal catalysts, under metal-free conditions, or using photocatalysts under photoirradiation conditions. Development of hypervalent iodine catalytic systems and discovery of highly enantioselective reactions using chiral hypervalent iodine compounds represent a particularly important recent achievement in the field of hypervalent iodine chemistry. Chemical transformations promoted by hypervalent iodine in many cases are unique and cannot be performed by using any other common, non-iodine-based reagent. This review covers literature published mainly in the last 7-8 years, between 2016 and 2024.

A novel non-chemically amplified resist based on polystyrene-iodonium derivatives for electron beam lithography

To break the resolution limitation of traditional resists, more work is needed on non-chemically amplified resists (non-CARs). Non-CARs based on iodonium salt modified polystyrene (PS-I) were prepared with controllable molecular weight and structure. The properties of the resist can be adjusted by the uploading of iodonium salts on the polymer chain, the materials with a higher proportion of iodonium salts show better lithography performance. By comparing contrast curves and quality of the lithographic patterns, the optimum developing condition of 4-methyl-2-pentanone and ethyl alcohol (v:v = 1:7) was selected. The high-resolution stripes of 15 nm half-pitch (HP) can be achieved by PS-I0.58in e-beam lithography (EBL). PS-I0.58shows the advanced lithography performance in the patterns of 16 nm HP and 18 nm HP stripes with low line edge roughness (3.0 nm and 2.4 nm). The resist shows excellent potential for further pattern transfer, the etch selectivity of resist PS-I0.58to the silicon was close to 12:1. The lithographic mechanism of PS-I was investigated by experimental and theoretical calculation, which indicates the polarity of materials changes results in the solubility switch. This work provides a new option and useful guidelines for the development of high-resolution resist.

Dehydrogenative Electrochemical Synthesis of N-Aryl-3,4-Dihydroquinolin-2-ones by Iodine(III)-Mediated Coupling Reaction

Electrochemically generated hypervalent iodine(III) species are powerful reagents for oxidative C-N coupling reactions, providing access to valuable N-heterocycles. A new electrocatalytic hypervalent iodine(III)-mediated in-cell synthesis of 1H-N-aryl-3,4-dihydroquinolin-2-ones by dehydrogenative C-N bond formation is presented. Catalytic amounts of the redox mediator, a low supporting electrolyte concentration and recycling of the solvent used make this method a sustainable alternative to electrochemical ex-cell or conventional approaches. Furthermore, inexpensive, readily available electrode materials and a simple galvanostatic set-up are applied. The broad functional group tolerance could be demonstrated by synthesizing 23 examples in yields up to 96 %, with one reaction being performed on a 10-fold higher scale. Based on the obtained results a sound reaction mechanism could be proposed.

A field guide to flow chemistry for synthetic organic chemists

Flow chemistry has unlocked a world of possibilities for the synthetic community, but the idea that it is a mysterious "black box" needs to go. In this review, we show that several of the benefits of microreactor technology can be exploited to push the boundaries in organic synthesis and to unleash unique reactivity and selectivity. By "lifting the veil" on some of the governing principles behind the observed trends, we hope that this review will serve as a useful field guide for those interested in diving into flow chemistry.

Continuous flow technology-a tool for safer oxidation chemistry

The advantages and benefits of continuous flow technology for oxidation chemistry have been illustrated in tube reactors, micro-channel reactors, tube-in-tube reactors and micro-packed bed reactors in the presence of various oxidants.

Diaryliodonium(III) Salts in One-Pot Double Functionalization of C–IIII and ortho C–H Bonds

Since the 1950s, diaryliodonium(III) salts have been demonstrated to participate in various arylation reactions, forming aryl–heteroatom and aryl–carbon bonds. Incorporating the arylation step into sequential transformations would provide access to...

Stereoselective Direct N-Trifluoropropenylation of Heterocycles with a Hypervalent Iodonium Reagent

The availability and synthesis of fluorinated enamine derivatives such as N-(3,3,3-trifluoropropenyl)heterocycles are challenging, especially through direct functionalization of the heterocyclic scaffold. Herein, a stereoselective N-trifluoropropenylation method based on the use of a bench-stable trifluoropropenyl iodonium salt is described. This reagent enables the straightforward trifluoropropenylation of various N-heterocycles under mild reaction conditions, providing trifluoromethyl enamine type moieties with high stereoselectivity and efficiency.

Regiodivergent C-H Arylation of Triphenylene Derivatives Controlled by Electronic Effects of Diaryliodonium Salts

A regiodivergent C-H arylation of triphenylene derivatives with diaryliodonium salts was developed. The regiodivergence was controlled by electronic effects of diaryliodonium salts. When the aryl(mesityl)iodonium salts bearing strong electron-donating groups at the para-position of aryl groups were used, the arylation reactions occurred ortho to amide groups. However, if the aryl(mesityl)iodonium salts bearing electron-withdrawing groups or weak electron-donating groups at the para-position of aryl groups were utilized, the arylation reactions occurred meta to amide groups.

Generation of aryl radicals by redox processes. Recent progress in the arylation methodology

Arylation methods based on the generation and use of aryl radicals have been a rapidly growing field of research in recent years and currently represent a powerful strategy for carbon – carbon and carbon – heteroatom bond formation. The progress in this field is related to advances in the methods for generation of aryl radicals. The currently used aryl radical precursors include aryl halides, aryldiazonium and diaryliodonium salts, arylcarboxylic acids and their derivatives, arylboronic acids, arylhydrazines, organosulfur(II, VI) compounds and some other compounds. Aryl radicals are generated under mild conditions by single electron reduction or oxidation of precursors induced by conventional reagents, visible light or electric current. A crucial role in the development of the radical arylation methodology belongs to photoredox processes either catalyzed by transition metal complexes or organic dyes or proceeding without catalysts. Unlike the conventional transition metal-catalyzed arylation methods, radical arylation reactions proceed very often at room temperature and have high functional group tolerance. Without claiming to be exhaustive, this review covers the most important advances of the current decade in the generation and synthetic applications of (het)aryl radicals. Examples of reactions are given and mechanistic insights are highlighted.

The bibliography includes 341 references.

Scalable electrochemical synthesis of diaryliodonium salts

Cyclic and acyclic diaryliodonium are synthesised by anodic oxidation of iodobiaryls and iodoarene/arene mixtures, respectively, in a simple undivided electrolysis cell in MeCN-HFIP-TfOH without any added electrolyte salts. This atom efficient process does not require chemical oxidants and generates no chemical waste. More than 30 cyclic and acyclic diaryliodonium salts with different substitution patterns were prepared in very good to excellent yields. The reaction was scaled-up to 10 mmol scale giving more than four grams of dibenzo[b,d]iodol-5-ium trifluoromethanesulfonate (>95%) in less than three hours. The solvent mixture of the large-scale experiment was recovered (>97%) and recycled several times without significant reduction in yield.

PhI(OTf)2 Does Not Exist (Yet)*

PhI(OTf)₂ has been used for the past 30 years as a strong I(III) oxidant for organic and inorganic transformations. It has been reported to be generated in situ from the reactions of either PhI(OAc)₂ or PhI=O with two equivalents of trimethylsilyl trifluoromethanesulfonate (TMS-OTf). In this report it is shown that neither of these reactions generate a solution with spectroscopic data consistent with PhI(OTf)₂ , with supporting theoretical calculations, and thus this compound should not be invoked as the species acting as the oxidant for transformations that have been associated with its use.

Reactions between Diazo Compounds and Hypervalent Iodine(III) Reagents

Site-selective "cut and sew" transformations employing diazo compounds and hypervalent iodine(III) compounds involve the departure of leaving groups, a "cut" process, followed by a reorganization of the fragments by bond formation, a "sew" process. Bearing controllable cleavage sites, diazo compounds and hypervalent iodine(III) compounds play a critical role as versatile reagents in a wide range of organic transformations because their excellent nucleofugality allows for a large number of unusual reactions to occur. In recent years, the combination of diazo compounds and hypervalent iodine(III) reagents has emerged as a promising tool for developing new and valuable approaches, and has met considerable success. In this Minireview, this combination is systematically illustrated with recent advances in the field, with the aim of elaborating the synthetic utility and potential of this concept as a powerful strategy in organic synthesis.

Site-selective aromatic C-H λ3-iodanation with a cyclic iodine(iii) electrophile in solution and solid phases

An efficient and site-selective aromatic C-H λ³-iodanation reaction is achieved using benziodoxole triflate (BXT) as an electrophile under room temperature conditions. The reaction tolerates a variety of electron-rich arenes and heteroarenes to afford the corresponding arylbenziodoxoles in moderate to good yields. The reaction can also be performed mechanochemically by grinding a mixture of solid arenes and BXT under solvent-free conditions. The arylbenziodoxoles can be used for various C-C and C-heteroatom bond formations, and are also amenable to further modification by electrophilic halogenation. DFT calculations suggested that the present reaction proceeds via a concerted λ³-iodanation-deprotonation transition state, where the triflate anion acts as an internal base.

Reactions of PhIX2 I(iii) oxidants with heavy triphenyl pnictines

The reactions of [PhI(pyridine)2]2+, PhI(OAc)2 and PhI(OTf)(OAc) with Ph3As, Ph3Sb and Ph3Bi are described. The reactions of [PhI(pyridine)2]2+ with Ph3Sb and Ph3Bi afford dicationic Pn(v) complexes ligated by pyridine in one step. These were previously reported by Burford in multi-step syntheses. Reactions with PhI(OAc)2, which were already known for Sb and Bi giving Pn(v) diacetates, were confirmed to give the same type of compound for As. Reactions with PhI(OAc)(OTf) were less selective, resulting in the isolation of iodonium cations [Ph-I-Ph]+ for As and Bi, while Ph3Sb gave an oxobridged di-antimony species characteristic of the decomposition of a high valent triflate bound species.

Pushing the boundaries of C–H bond functionalization chemistry using flow technology

C–H functionalization chemistry is one of the most vibrant research areas within synthetic organic chemistry. While most researchers focus on the development of small-scale batch-type transformations, more recently such transformations have been carried out in flow reactors to explore new chemical space, to boost reactivity or to enable scalability of this important reaction class. Herein, an up-to-date overview of C–H bond functionalization reactions carried out in continuous-flow microreactors is presented. A comprehensive overview of reactions which establish the formal conversion of a C–H bond into carbon–carbon or carbon–heteroatom bonds is provided; this includes metal-assisted C–H bond cleavages, hydrogen atom transfer reactions and C–H bond functionalizations which involve an SE-type process to aromatic or olefinic systems. Particular focus is devoted to showcase the advantages of flow processing to enhance C–H bond functionalization chemistry. Consequently, it is our hope that this review will serve as a guide to inspire researchers to push the boundaries of C–H functionalization chemistry using flow technology.

Palladium-Catalyzed C8-Arylation of Naphthalenes through C-H Activation: A Combined Experimental and Computational Study

Herein, a direct C8-arylation reaction of 1-amidonaphthalenes is described. By using diaryliodonium salts as arylating agents, the palladium-catalyzed C-H activation reaction showed perfect C8 regioselectivity and a wide functional group tolerance. In most cases, the desired polyaromatic compounds were isolated in good to excellent yields. To explain the observed regioselectivity, DFT calculations were performed and highlighted the crucial role of the amide directing group. Finally, the utility of this method is showcased by the synthesis of benzanthrone derivatives.

Designing Microflowreactors for Photocatalysis Using Sonochemistry: A Systematic Review Article

Use of sonication for designing and fabricating reactors, especially the deposition of catalysts inside a microreactor, is a modern approach. There are many reports that prove that a microreactor is a better setup compared with batch reactors for carrying out catalytic reactions. Microreactors have better energy efficiency, reaction rate, safety, a much finer degree of process control, better molecular diffusion, and heat-transfer properties compared with the conventional batch reactor. The use of microreactors for photocatalytic reactions is also being considered to be the appropriate reactor configuration because of its improved irradiation profile, better light penetration through the entire reactor depth, and higher spatial illumination homogeneity. Ultrasound has been used efficiently for the synthesis of materials, degradation of organic compounds, and fuel production, among other applications. The recent increase in energy demands, as well as the stringent environmental stress due to pollution, have resulted in the need to develop green chemistry-based processes to generate and remove contaminants in a more environmentally friendly and cost-effective manner. It is possible to carry out the synthesis and deposition of catalysts inside the reactor using the ultrasound-promoted method in the microfluidic system. In addition, the synergistic effect generated by photocatalysis and sonochemistry in a microreactor can be used for the production of different chemicals, which have high value in the pharmaceutical and chemical industries. The current review highlights the use of both photocatalysis and sonochemistry for developing microreactors and their applications.

Porphyrins as Photoredox Catalysts in Csp2-H Arylations: Batch and Continuous Flow Approaches

We have investigated both batch and continuous flow photoarylations of enol-acetates to yield different α-arylated aldehyde and ketone building blocks by using diazonium salts as the aryl-radical source. Different porphyrins were used as SET photocatalysts, and photophysical as well as electrochemical studies were performed to rationalize the photoredox properties and suggest mechanistic insights. Notably, the most electron-deficient porphyrin ( meso-tetra(pentafluorophenyl)porphyrin) shows the best photoactivity as an electron donor in the triplet excited state, which was rationalized by the redox potentials of excited states and the turnover of the porphyrins in the photocatalytic cycle. A two-step continuous protocol and multigram-scale reactions are also presented revealing a robust, cost-competitive, and easy methodology, highlighting the significant potential of porphyrins as SET photocatalysts.

Design and application of a modular and scalable electrochemical flow microreactor

Electrochemistry constitutes a mild, green and versatile activation method of organic molecules. Despite these innate advantages, its widespread use in organic chemistry has been hampered due to technical limitations, such as mass and heat transfer limitations which restraints the scalability of electrochemical methods. Herein, we describe an undivided-cell electrochemical flow reactor with a flexible reactor volume. This enables its use in two different modes, which are highly relevant for flow chemistry applications, including a serial (volume ranging from 88 μL/channel up to 704 μL) or a parallel mode (numbering-up). The electrochemical flow reactor was subsequently assessed in two synthetic transformations, which confirms its versatility and scale-up potential.

N-Arylation of DABCO with Diaryliodonium Salts: General Synthesis of N-Aryl-DABCO Salts as Precursors for 1,4-Disubstituted Piperazines

Employing DABCO as a substrate, aryl(mesityl)iodonium triflates are introduced as arylating agents for a tertiary sp³-nitrogen. Mild conditions and exceptional selectivity of the aryl group transfer allow unprecedented N-aryl-DABCO salts to be obtained, bearing substituents of different electronic natures. This metal-free methodology has no analogy among known transition-metal-based reactions. The utility of isolated N-aryl-DABCO salts is demonstrated for the preparation of flibanserin.

Visible-Light Photocatalytic Difluoroalkylation-Induced 1, 2-Heteroarene Migration of Allylic Alcohols in Batch and Flow

A convenient method for the preparation of sp³-rich heterocycles is reported. The method comprises a photocatalytic difluoroalkylation-induced 1,2-heteroarene migration of allylic alcohols. Here we describe for the first time the benefits of using flow to facilitate such migration reactions, including shorter reaction times, higher selectivities, and opportunities to scale the chemistry.

Advances in electro- and sono-microreactors for chemical synthesis

The anatomy of electrochemical flow microreactors is important to safely perform chemical reactions in order to obtain pure and high yielding substances in a controlled and precise way that excludes the use of supporting electrolytes. Flow microreactors are advantageous in handling unstable intermediates compared to batch techniques and have efficient heat/mass transfer. Electrode nature (cathode and anode) and their available exposed surface area to the reaction mixture, parameters of the spacer, flow rate and direction greatly affects the efficiency of the electrochemical reactor. Solid formation during reactions may result in a blockage and consequently decrease the overall yield, thus limiting the use of microreactors in the field of electrosynthesis. This problem could certainly be overcome by application of ultrasound to break the solids for consistent flow. In this review, we discuss in detail the aforementioned issues, the advances in microreactor technology for chemical synthesis, with possible application of sonochemistry to deal with solid formations. Various examples of flow methods for electrosynthesis through microreactors have been explained in this review, which would definitely help to meet future demands for efficient synthesis and production of various pharmaceuticals and fine chemicals.

Design of Trifluoroalkenyl Iodonium Salts for a Hypervalency-Aided Alkenylation-Cyclization Strategy: Metal-Free Construction of Aziridine Rings

The synthesis of fluorinated compounds and their use as pharmaceutical ingredients or synthetic building blocks have been in the focus of chemical and medicinal research. However, the efficient synthesis of trifluoromethylated nitrogen heterocycles is sometimes challenging. Herein, we disclose a simple aziridination process that relies on the use of amines and novel alkenyl iodonium reagents for the synthesis of strained, trifluoromethylated heterocycles. With the utilization of a newly designed and bench-stable but highly reactive hypervalent alkenyl iodonium species, these three-membered-ring heterocyclic compounds can be efficiently constructed from simple amines under mild conditions in the absence of transition-metal catalysts. The special reactivity of the new trifluoropropenyl synthon towards nucleophilic centers could be exploited in more general cyclization and alkenylation reactions in the future.

One-pot synthesis of diaryliodonium salts from arenes and aryl iodides with Oxone-sulfuric acid

A facile synthesis of diaryliodonium salts utilizing Oxone as versatile and cheap oxidant has been developed. This method shows wide applicability and can be used for the preparation of iodonium salts containing electron-donating or electron-withdrawing groups in good yields. In addition, this procedure can be applied to the preparation of symmetric iodonium salts directly from arenes via a one-pot iodination-oxidation sequence.