Trimethoxyphenyl (TMP) as a Useful Auxiliary for in situ Formation and Reaction of Aryl(TMP)iodonium Salts: Synthesis of Diaryl Ethers

Copper-catalyzed N-arylation of 1,2,4-oxadiazin-5(6H)-ones by diaryliodonium salts

Herein, we developed a new synthetic approach for the preparation of N-arylated 1,2,4-oxadiazin-5(6H)-ones by direct arylation with diaryliodonium salts. The reaction with symmetrical diaryliodonium salts using CuI as a catalyst proceeded in toluene in the presence of DIPEA at 60 °C with the formation of the desired products in isolated yields of 46 to 97% (20 examples). The use of more readily available unsymmetrical diaryliodonium salts required higher reaction temperatures (up to 100 °C) to achieve similar yields. The only limitation observed in reaction was with an ortho-substituted iodonium salt. In all other cases, the developed approach allowed the preparation of a broad range of N-arylated 1,2,4-oxadiazin-5(6H)-ones under mild conditions utilizing a cheap and readily available catalyst.

Iodoarene Activation: Take a Leap Forward toward Green and Sustainable Transformations

Constructing chemical bonds under green sustainable conditions has drawn attention from environmental and economic perspectives. The dissociation of (hetero)aryl-halide bonds is a crucial step of most arylations affording (hetero)arene derivatives. Herein, we summarize the (hetero)aryl halides activation enabling the direct (hetero)arylation of trapping reagents and construction of highly functionalized (hetero)arenes under benign conditions. The strategies for the activation of aryl iodides are classified into (a) hypervalent iodoarene activation followed by functionalization under thermal/photochemical conditions, (b) aryl-I bond dissociation in the presence of bases with/without organic catalysts and promoters, (c) photoinduced aryl-I bond dissociation in the presence/absence of organophotocatalysts, (d) electrochemical activation of aryl iodides by direct/indirect electrolysis mediated by organocatalysts and mediators acting as electron shuttles, and (e) electrophotochemical activation of aryl iodides mediated by redox-active organocatalysts. These activation modes result in aryl iodides exhibiting diverse reactivity as formal aryl cations/radicals/anions and aryne precursors. The coupling of these reactive intermediates with trapping reagents leads to the facile and selective formation of C-C and C-heteroatom bonds. These ecofriendly, inexpensive, and functional group-tolerant activation strategies offer green alternatives to transition metal-based catalysis.

Synthesis of N-Aryl Carbamates from Aryl(TMP)iodonium Salts via C-N Coupling

Modular C-N coupling is a desirable way to construct N-aryl carbamates, which are privileged scaffolds in active pharmaceutical ingredients. However, there are no broadly applicable metal-free methods for theN-arylation of carbamates. Herein, we describe a metal-free approach that uses aryl(TMP)iodonium salts as arylation reagents for cyclic carbamates by exploiting the metal-like reactivity of iodine(III). The scope includes 25 examples, including 17 different aryl(TMP)iodonium salts and 9 different carbamates, in yields ranging between 55 and 97% (75% avg).

Transition-metal-free dibenzoxazepinone synthesis by hypervalent iodine-mediated chemoselective arylocyclizations of N-functionalized salicylamides

We have developed transition-metal-free synthetic methodologies for dibenzoxazepinones utilizing salicylamides as starting materials and employing two distinct types of successive hypervalent iodine-mediated arylocyclizations. This synthetic protocol encompasses selective phenol O-arylation of salicylamides with diaryliodonium salts, followed by electrophilic aromatic amination utilizing chemically or electronically generated hypervalent iodine reagents in the second stage of the process.

Recent advances in transition-metal-free arylation reactions involving hypervalent iodine salts

Diaryliodonium salts have become widely recognized as arylating agents in the last two decades. Both, symmetrical and unsymmetrical forms of these salts serve as effective electrophilic arylating reagents in various organic syntheses. The use of diaryliodoniums in C-C and carbon-heteroatom bond formations, particularly under metal-free conditions, has further enhanced the popularity of these reagents. In this review, we concentrate on various arylation reactions involving carbon and other heteroatoms, encompassing rearrangement reactions in the absence of any metal catalyst, and summarize advancements made in the last five years.

Borate-mediated aryl polyfluoroalkoxylation under transition-metal-free conditions

We describe the transition-metal-free coupling for polyfluoroalkoxy arenes using polyfluoroalkoxy borates, which serve as counterions to diaryliodonium salts and transferring mediators of polyfluoroalkoxy groups. This strategy demonstrates high functional group compatibility owing to the low nucleophilicity of the borate mediator, thus offering a practical approach for synthesizing diverse polyfluoroalkoxy arenes.

Arylation of Diethyl Acetamidomalonate with Diaryliodonium Salts En Route to α-Arylglycines

Diethyl acetamidomalonate (DEAM) has been widely used for the synthesis of α-amino acids via C-alkylation under basic conditions followed by hydrolysis/decarboxylation. In contrast, the C-arylation of this reagent remains undeveloped. Herein, we report a novel strategy for the synthesis of racemic α-arylglycines based on the selective arylation of DEAM with diaryliodonium salts under mild, transition metal-free conditions. The reaction features good functional group tolerance and easy scalability and is applicable to the chemoselective C-H-modification of arenes including approved drugs, thus enabling a straightforward approach to complex α-arylglycines that would be challenging to make otherwise.

Silver-Catalyzed Coupling of Unreactive Carboxylates: Synthesis of α-Fluorinated O-Aryl Esters

α-Fluorinated aryl esters pose a challenge in synthesis via O-arylation of α-fluorinated carboxylates owing to their low reactivities. This limitation has been addressed by combining a silver catalyst with aryl(trimethoxyphenyl)iodonium tosylates to access α-fluorinated aryl esters. We envision that the catalytic system involves high-valent aryl silver species generated via the oxidation of silver(I) salt. The present method provided a synthetic protocol for various α-fluorinated aryl esters including fluorinated analogs of drug derivatives.

Synthesis and Evaluation of diaryl ether modulators of the leukotriene A4 hydrolase aminopeptidase activity

Activation of the aminopeptidase (AP) activity of leukotriene A4 hydrolase (LTA4H) presents a potential therapeutic strategy for resolving chronic inflammation. Previously, ARM1 and derivatives were found to activate the AP activity using the alanine-p-nitroanilide (Ala-pNA) as a reporter group in an enzyme kinetics assay. As an extension of this previous work, novel ARM1 derivatives were synthesized using a palladium-catalyzed Ullmann coupling reaction and screened using the same assay. Analogue 5, an aminopyrazole (AMP) analogue of ARM1, was found to be a potent AP activator with an AC50 of 0.12 μM. An X-ray crystal structure of LTA4H in complex with AMP was refined at 2.7 Å. Despite its AP activity with Ala-pNA substrate, AMP did not affect hydrolysis of the previously proposed natural ligand of LTA4H, Pro-Gly-Pro (PGP). This result highlights a discrepancy between the hydrolysis of more conveniently monitored chromogenic synthetic peptides typically employed in assays and endogenous peptides. The epoxide hydrolase (EH) activity of AMP was measured in vivo and the compound significantly reduced leukotriene B4 (LTB4) levels in a murine bacterial pneumonia model. However, AMP did not enhance survival in the murine pneumonia model over a 14-day period. A liver microsome stability assay showed metabolic stability of AMP. The results suggested that accelerated Ala-pNA cleavage is not sufficient for predicting therapeutic potential, even when the full mechanism of activation is known.

Auxiliary strategy for the general and practical synthesis of diaryliodonium(III) salts with diverse organocarboxylate counterions

Diaryliodonium(III) salts are versatile reagents that exhibit a range of reactions, both in the presence and absence of metal catalysts. In this study, we developed efficient synthetic methods for the preparation of aryl(TMP)iodonium(III) carboxylates, by reaction of (diacetoxyiodo)arenes or iodosoarenes with 1,3,5-trimethoxybenzene in the presence of a diverse range of organocarboxylic acids. These reactions were conducted under mild conditions using the trimethoxyphenyl (TMP) group as an auxiliary, without the need for additives, excess reagents, or counterion exchange in further steps. These protocols are compatible with a wide range of substituents on (hetero)aryl iodine(III) compounds, including electron-rich, electron-poor, sterically congested, and acid-labile groups, as well as a broad range of aliphatic and aromatic carboxylic acids for the synthesis of diverse aryl(TMP)iodonium(III) carboxylates in high yields. This method allows for the hybridization of complex bioactive and fluorescent-labeled carboxylic acids with diaryliodonium(III) salts.

Diversification of Complex Diaryl Ethers via Diaryliodonium Intramolecular Aryl Rearrangement

In this study, we present an efficient site-selective O-arylation method applicable to a broad range of complex arenes involving intramolecular aryl rearrangement. The reaction was facilitated by diaryliodonium salts bearing vicinal trifluoromethanesulfonate (OTf) groups. The procedure was initiated with selective C-H bond activation of arenes, which were then converted into diaryl ethers through nucleophilic aromatic substitution (SNAr). This synthetic method successfully affords complex diaryl ether derivatives, showcasing its practicality for the diversification of functionalized arenes and pharmaceutical agents.

Metal-free and atom-efficient protocol for diarylation of selenocyanate by diaryliodonium salts

We developed an atom- and reaction mass efficient strategy for the preparation of diarylselenides using iodonium salts as reactants. The developed approach allows the obtaining of diarylselenides from the corresponding trimethoxyphenyl-substituted iodonium salts via a two-step one-pot reaction sequence. The proposed metal-free methodology is based on the involvement of both iodonium aryl groups for diarylation.

Reaction of Diaryliodonium Salts with Potassium Alkyl Xanthates as an Entry Point to Accessing Organosulfur Compounds

Preparation of S-aryl xanthates via transition-metal-catalyzed or S_NAr reactions is complicated by their further transformations under the utilized conditions. In contrast, S-arylation of potassium O-alkyl xanthates with diaryliodonium salts proceeds under mild conditions, enabling access to substituted S-aryl xanthates. The method exhibits good functional group tolerance and can be applied to the late-stage C-H functionalization of drug molecules. Divergent transformations of the resulting S-aryl xanthates provide rapid access to a range of medicinal chemistry-relevant organosulfur compounds.

Synthesis of Cyclic and Acyclic ortho-Aryloxy Diaryliodonium Salts for Chemoselective Functionalizations

Two regioselective, high-yielding one-pot routes to oxygen-bridged cyclic diaryliodonium salts and ortho-aryloxy-substituted acyclic diaryliodonium salts are presented. Starting from easily available ortho-iodo diaryl ethers, complete selectivity in formation of either the cyclic or acyclic product could be achieved by varying the reaction conditions. The complimentary reactivities of these novel ortho-oxygenated iodonium salts were demonstrated through a series of chemoselective arylations under metal-catalyzed and metal-free conditions, to deliver a range of novel, ortho-functionalized diaryl ether derivatives.

Metal-Free Regioselective N2-Arylation of 1H-Tetrazoles with Diaryliodonium Salts

We describe a simple, metal-free regioselective N²-arylation strategy for 5-substituted-1H-tetrazoles with diaryliodonium salts to access 2-aryl-5-substituted-tetrazoles. Diaryliodonium salts with a wide range of both electron-rich and previously challenged electron-deficient aryl groups are applicable in this method. Diversely functionalized tetrazoles are tolerable also. We have devised a one-pot system to synthesize 2,5-diaryl-tetrazoles directly from nitriles. The synthetic utility of this method is furthered extended to late-stage arylation of two biologically active molecules.

Organocatalytic Synthesis of Phenols from Diaryliodonium Salts with Water under Metal-Free Conditions

The metal-free synthesis of phenols from diaryliodonium salts with water was developed by using N-benzylpyridin-2-one as an organocatalyst. In this process, sterically congested, functionalized, and heterocycle-containing iodonium salts were smoothly converted to the desired products, and the clofibrate and mecloqualone derivatives were also synthesized in high yields. In addition, the gram-scale experiment was successfully carried out with 10 mmol of a sterically congested substrate.

Metal-free S-arylation of 5-mercaptotetrazoles and 2-mercaptopyridine with unsymmetrical diaryliodonium salts

Herein, we demonstrate the application of unsymmetrical iodonium salts towards S-arylation of heterocyclic thiols (especially tetrazole-5-thiols and pyridine-2-thiol) under metal-free conditions, affording a diverse range of di(hetero)aryl thioethers in moderate to good yields. A detailed study on the effects of counter-anions and the auxiliary of iodonium salts was conducted. Suitable auxiliary selection of the unsymmetrical iodonium salt offers flexibility for a wide range of aryl moieties and its incorporation into S-arylation. The DFT study supports the experimental observations of chemoselective arylation.

Transition-Metal-Free O-Arylation of Alcohols and Phenols with S-Arylphenothiaziniums

Herein, we report the transition-metal-free O-arylation of alcohols and phenols with S-arylphenothiaziniums, which can be easily synthesized from boronic acids. Aryl substituents derived from arylboronic acids were selectively introduced into the hydroxy groups in alcohols and phenols, and a variety of aryl ethers were synthesized. This selectivity is supported by theoretical calculations.

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...

A New Scalable Synthesis of ELQ-300, ELQ-316, and other Antiparasitic Quinolones

The Endochin-Like Quinolone (ELQ) compound class may yield effective, safe treatments for a range of important human and animal afflictions. However, to access the public health potential of this compound series, a synthetic route needed to be devised that lowers costs and is amenable to large scale production. In the new synthetic route described here, a substituted β-keto ester, formed by an Ullmann reaction and subsequent acylation, is reacted with an aniline via a Conrad-Limpach reaction to produce 3-substituted 4(1H)-quinolones such as ELQ-300 and ELQ-316. This synthetic route, the first described to be truly amenable to industrial scale production, is relatively short (5 reaction steps), does not require palladium, chromatographic separation or protecting group chemistry, and may be performed without high vacuum distillation.

Recent Advances in Transition-Metal-Free Late-Stage C-H and N-H Arylation of Heteroarenes Using Diaryliodonium Salts

Transition-metal-free direct arylation of C-H or N-H bonds is one of the key emerging methodologies that is currently attracting tremendous attention. Diaryliodonium salts serve as a stepping stone on the way to alternative environmentally friendly and straightforward pathways for the construction of C-C and C-heteroatom bonds. In this review, we emphasize the recent synthetic advances of late-stage C(sp2)-N and C(sp2)-C(sp2) bond-forming reactions under metal-free conditions using diaryliodonium salts as arylating reagent and its applications to the synthesis of new arylated bioactive heterocyclic compounds.

Aryl(TMP)iodonium Tosylate Reagents as a Strategic Entry Point to Diverse Aryl Intermediates: Selective Access to Arynes

Arenes are broadly found motifs in societally important molecules. Access to diverse arene chemical space is critically important, and the ability to do so from common reagents is highly desirable. Aryl(TMP)iodonium tosylates provide one such access point to arene chemical space via diverse aryl intermediates. Here we demonstrate that controlling reaction pathways selectively leads to arynes with a broad scope of arenes and arynophiles (24 examples, 70% average yield) and efficient access to biologically active compounds.

Heteroaryliodonium(III) Salts as Highly Reactive Electrophiles

In recent years, the chemistry of heteroaryliodonium(III) salts has undergone significant developments. Heteroaryliodonium(III) salts have been found to be useful synthetic tools for the transfer of heteroaryl groups under metal-catalyzed and metal-free conditions for the preparation of functionalized heteroarene-containing compounds. Synthetic transformations mediated by these heteroaryliodonium(III) salts are classified into two categories: (1) reactions utilizing the high reactivity of the hypervalent iodine(III) species, and (2) reactions based on unique and new reactivities not observed in other types of conventional diaryliodonium salts. The latter feature is of particular interest and so has been intensively investigated in recent decades. This mini-review therefore aims to summarize the recent synthetic applications of heteroaryliodonium(III) salts as highly reactive electrophiles.

Diaryliodoniums Salts as Coupling Partners for Transition-Metal Catalyzed C- and N-Arylation of Heteroarenes

Owing to the pioneering works performed on the metal-catalyzed sp2 C–H arylation of indole and pyrrole by Sanford and Gaunt, N– and C-arylation involving diaryliodonium salts offers an attractive complementary strategy for the late-stage diversification of heteroarenes. The main feature of this expanding methodology is the selective incorporation of structural diversity into complex molecules which usually have several C–H bonds and/or N–H bonds with high tolerance to functional groups and under mild conditions. This review summarizes the main recent achievements reported in transition-metal-catalyzed N– and/or C–H arylation of heteroarenes using acyclic diaryliodonium salts as coupling partners.