An Admix Approach To Determine Counter Anion Effects on Metal-Free Arylation Reactions with Diaryliodonium Salts

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.

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.

Metal-Free Synthesis of Benzisoxazolones Utilizing ortho-Ester and ortho-Cyano-Functionalized Diaryliodonium Salts with Protected Hydroxylamines

Herein, we report the development of metal-free one/two-pot procedures for the synthesis of benzo[c]isoxazol-3(1H)-one (benzisoxazolone) heterocycles by designing diaryliodonium salts featuring ortho-ester or nitrile functional groups. These react smoothly with protected hydroxylamines under mild conditions to produce N-arylhydroxylamine intermediates, which readily cyclize to give benzisoxazolone derivatives under acidic conditions. This metal-free process maintains the weak N-O bond, tolerates a wide range of diaryliodonium salts and protected hydroxylamines with diverse functional/protecting groups, thereby overcoming the challenges associated with previous transformations. The protocol expands the reaction scope and broadens the chemical space of the fused isoxazolone backbones to include unprecedented five-membered heteroaryl-fused isoxazolones in high yields. This method is also applicable to gram-scale synthesis, and the resulting benzisoxazolones can be effectively derivatized at the N-position to afford valuable compounds.

Low-Cost, Safe, and Anion-Flexible Method for the Electrosynthesis of Diaryliodonium Salts

An electrochemical approach toward the synthesis of diaryliodonium salts based on anodic C-I coupling between aryl iodides and arenes is presented. In contrast to previous protocols, our method requires no chemical oxidants, strong acids, or fluorinated solvents. A further advantage is that by use of the appropriate supporting electrolyte, the counterion of choice can be introduced, which is time- and cost-saving as compared to postsynthesis ion exchange. This "anion-flexibility" is particularly interesting when considering the pronounced effect of the counterion on the reactivity of diaryliodonium species in aryl transfer reactions. The scope of our method comprises 24 examples with isolated yields of up to 99%. Scalability was demonstrated by the synthesis on a gram scale. Furthermore, it was shown that the diaryliodonium-containing post-electrolysis solution can be used without further workup as a reactive medium for O-arylation reactions. Finally, a series of para-substituted diaryliodonium compounds was studied using linear sweep voltammetry on a microelectrode and analyzed with respect to the influence of the electronic structure on the redox behavior.

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.

gem-Difluoroalkylation of Cyclic Ethers Enabled by Cobalt-Catalyzed C(sp3)-H Oxidation under Mild Conditions

Fluorinated rings have emerged as privileged structural modules in the fields of drug discovery and materials research. The incorporation of fluorine atoms into aromatic rings or heterocycles can lead to significant improvements in the physicochemical and biological properties of small molecules, making them valuable components in the design of new drugs and functional materials. Herein, we presented a cobalt-catalyzed C-H oxidation/gem-difluorination cascade reaction of readily available cyclic ethers with difluoroenoxysilanes, affording a series of gem-difluorinated analogues with moderate to high yields. The obtained products as versatile fluoroalkyl building blocks were showcased through divergent-oriented transformations.

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.

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.

BODIPY(aryl)iodonium salts in the efficient synthesis of diversely functionalized BODIPYs and selective detection of serum albumin

BODIPY(aryl)iodonium salts were readily accessible from the high-yielding reaction of BODIPY with iodoarenes or hydroxyl(tosyloxy)iodoarenes in the presence of m-CPBA. The prepared BODIPY(aryl)iodonium salts bearing substituents of varied electronic nature were utilized for the direct syntheses of thiocyanate, azide, amine and acrylate functionalized BODIPYs and β,β'-bis-BODIPYs. The regioselective syntheses of α-piperidinyl and β-piperidinyl substituted BODIPYs were achieved through the reaction of BODIPY(aryl)iodonium salts with piperidine in the absence and presence of copper(I). Expeditious and high yielding (79-82%) synthesis of β,β'-bis-BODIPYs was also developed through the palladium-catalyzed reductive coupling of the easily accessible BODIPY(aryl)iodonium salts. Some of the indole-appended BODIPYs and bis-BODIPYs displayed strong absorption in the visible region (∼610 nm). The BODIPY(aryl)iodonium salts also showed significant binding with serum albumin and were observed to be selective serum protein sensors with estimated limits of detection as low as 7 μg mL-1 in some cases.

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.

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.

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...

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.

Anion Chelation via Double Chalcogen Bonding: The Case of a Bis-telluronium Dication and Its Application in Electrophilic Catalysis via Metal-Chloride Bond Activation

Telluronium cations have long been known to engage their counteranions via secondary interactions. Yet, this property has rarely been exploited for anion binding. Motivated by such an application, we have now synthesized a bis-telluronium dication ([3]²⁺) that was obtained as a tetrafluoroborate salt by reaction of 2,7-di-tert-butyl-9,9-dimethylxanthene-4,5-diboronic acid with phenoxatellurine difluoride and BF₃·OEt₂. As confirmed by the formation of Te-(μ-BF₄)-Te bridges in the structure of [3][BF₄]₂, [3]²⁺ functions as a bidentate Lewis acid toward anions. [3][BF₄]₂ has also been converted into the more exposed [3][BArF₂₄]₂ ([BArF₂₄]^- = [B(3,5-(CF₃)₂C₆H₃)₄]^-). The latter, which readily ionizes Ph₃CCl, displays a chloride anion binding constant that exceeds that of a monofunctional model compound by almost 4 orders of magnitude. The unique properties of this new bis-telluronium dication are further highlighted by its ability to activate Ph₃PAuCl and cis-(Ph₃P)₂PtCl₂, leading to catalytic systems highly active in the cycloisomerization of propargylamide or enyne substrates.

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.

Redox-controlled chalcogen-bonding at tellurium: impact on Lewis acidity and chloride anion transport properties

Our interests in the chemistry of atypical main group Lewis acids have led us to devise strategies that augment the affinity of chalcogen-bond donors for anionic guests. In this study, we describe the oxidative methylation of diaryltellurides as one such strategy along with its application to the synthesis of [Mes(C₆F₅)TeMe]⁺ and [(C₆F₅)₂TeMe]⁺ starting from Mes(C₆F₅)Te and (C₆F₅)₂Te, respectively. These new telluronium cations have been evaluated for their ability to complex and transport chloride anions across phospholipid bilayers. These studies show that, when compared to their neutral Te(ii) precursors, these Te(iv) cations display both higher Lewis acidity and transport activity. The positive attributes of these telluronium cations, which originate from a lowering of the tellurium-centered σ* orbitals and a deepening of the associated σ-holes, demonstrate that the redox state of the main group element provides a convenient handle over its chalcogen-bonding properties.

The oxidative alkylation of diorganotellurides enhances the chalcogen-bond donor properties of the tellurium center, an effect manifested in the enhanced chloride anion affinity and transport properties of the resulting telluronium cations.

Digging the Sigma-Hole of Organoantimony Lewis Acids by Oxidation

The development of group 15 Lewis acids is an area of active investigation that has led to numerous advances in anion sensing and catalysis. While phosphorus has drawn considerable attention, emerging research shows that organoantimony(III) reagents may also act as potent Lewis acids. Comparison of the properties of SbPh₃ , Sb(C₆ F₅ )₃ , and SbAr^F ₃ with those of their tetrachlorocatecholate analogues SbPh₃ Cat, Sb(C₆ F₅ )₃ Cat, and SbAr^F ₃ Cat (Cat=o-O₂ C₆ Cl₄ , Ar^F =3,5-(CF₃ )₂ C₆ H₃ ) demonstrates that the Lewis acidity of electron deficient organoantimony(III) reagents can be readily enhanced by oxidation to the +V state-as verified by binding studies, organic reaction catalysis, and computational studies. The results are rationalized by explaining that oxidation of the antimony center leads to a lowering of the accepting σ* orbital and a deeper carving of the associated σ-hole.

DBU-Promoted Cu(OAc)•H2O-Catalysed Coupling Reactions of Aryl Iodides and Sodium Azide

An efficient and simple protocol for the synthesis of aryl azides by the coupling of aryl iodides with sodium azide, in good to excellent yields in DMSO at 95 °C under catalysis by Cu(OAc)2-H2O and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), has been established. The optimised loadings of Cu(OAc)2-H2O and DBU were 10 mol% and 15 mol% respectively.

Imide arylation with aryl(TMP)iodonium tosylates

Herein, we describe the synthesis of N-aryl phthalimides by metal-free coupling of potassium phthalimide with unsymmetrical aryl(TMP)iodonium tosylate salts. The aryl transfer from the iodonium moiety occurs under electronic control with the electron-rich trimethoxyphenyl group acting as a competent dummy ligand. The yields of N-aryl phthalimides are moderate to high and the coupling reaction is compatible with electron-deficient and sterically encumbered aryl groups.

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.