Regioselective Synthesis of 1,2,3,4-Tetrasubstituted Arenes by Vicinal Functionalization of Arynes Derived from Aryl(Mes)iodonium Salts**

Herein, the synthesis of 1,2,3,4-tetrasubstituted benzenoid rings, motifs found in pharmaceutical, agrochemical, and natural products, is described.^[1] In the past, the regioselective syntheses of such compounds have been a significant challenge. This work reports a method using substituted arynes derived from aryl(Mes)iodonium salts to access a range of densely functionalized 1,2,3,4-tetrasubstituted benzenoid rings. Significantly, it was found that halide substituents are compatible under these conditions, enabling post-synthetic elaboration via palladium-catalyzed coupling. This concise strategy is predicated on two regioselective events: 1) ortho- deprotonation of aryl(Mes)iodonium salts to generate a substituted aryne intermediate, and 2) regioselective trapping of said arynes, thereby improving previously reported reaction conditions to generate arynes at room temperature and in shorter reaction times. Density functional theory (DFT) computations and linear free energy relationship (LFER) analysis suggest the regioselectivity of deprotonation is influenced by both proximal and distal ring substituents on the aryne precursor. A competition experiment further reveals the role of arene substituents on relative reactivity of aryl(Mes)iodoniums as aryne precursors.     

Copper-Catalyzed One-Pot Arylation and Cyclization of Diaryliodonium Salts Derived from o-Iodoanilines for Indolo[2,3-b]indoles Syntheses

One-pot synthesis of various indolo[2,3-b]indoles has been accomplished from a copper-catalyzed transformation of easily accessible 2-(substituted-amino)aryl)(mesityl)iodonium salts and indole derivatives. Reaction offers great functional group toleration, providing a series of indolo[2,3-b]indole derivatives in good yields. Additionally, intramolecular cyclization of 2-(substituted-amino)aryl)(mesityl)iodonium salts in the presence of copper-catalyst provided direct access to benzoxazole derivatives.     

Olefin Bifunctionalization: A Visible‐light Photoredox‐catalyzed Aryl Alkoxylation of Olefins

Olefin bifunctionalization is a facile route to obtain complex molecules from abundant and commercially available olefin feedstocks. Visible light together with a catalytic amount of tris(bipyridine)ruthenium salt catalyzes the aryl alkoxylation of styrenes with aryl diazonium salts in alcohol solvents via a photoredox process. The scope of this proposed reaction with respect to various aryl diazonium salts and styrenes has been investigated.     

Catalyst-Free Arylation of Tertiary Phosphines with Diaryliodonium Salts Enabled by Visible Light

The visible-light-induced arylation of tertiary phosphines with aryl(mesityl)iodonium triflates to produce the quaternary phosphonium salts occurs under mild, metal, and catalyst-free conditions. Photo-excited EDA complexes between diaryliodonium salts and phosphines supposedly enable this transformation, which is difficult to achieve through the traditional ground-state reactions. Demonstrating high functional group tolerance, broad scope, and complete selectivity of the aryl group transfer, the method is particularly compatible with sterically congested phosphines, which are challenging under metal-based catalytic methods.     

One-pot synthesis of 1-alkyl-1H-indazoles from 1,1-dialkylhydrazones via aryne annulation

The reaction of readily accessible 1,1-dialkylhydrazones with commercially available o-(trimethylsilyl)aryl triflates provides a direct one-step route to pharmaceutically important 1-alkylindazoles. The products are obtained in high yields by one-pot NCS-chlorination/aryne annulation or Ac(2)O-acylation/deprotection/aromatization protocols.     

Synthesis of 2,3-dihydroindoles, indoles, and anilines by transition metal-free amination of aryl chlorides

Aliphatic and aromatic amines react with 2- and 3-chlorostyrene in the presence of potassium tert-butoxide to give N-substituted 2,3-dihydroindoles in good yields. The combination of this domino-amination protocol with a suitable dehydrogenation reaction gives access to pharmacologically interesting indoles in a one-pot procedure. Overall product yields of N-substituted indoles >50% are obtained by this method starting from commercially available substrates. In addition to the intramolecular base-promoted amination of aromatic C-Cl bonds, metal-free intermolecular aminations of aryl chlorides with primary and secondary amines are described. The use of potassium tert-butoxide as base allows the synthesis of various anilines in good to excellent yields. Due to the formation of aryne intermediates, either N-substituted anilines or meta-substituted anilines are produced with excellent selectivities.     

Recyclable synthesis of mesityl iodonium(III) salts

An efficient protocol for C–H condensation of hypervalent iodine compounds toward arenes in fluoroalcohols has been applied to the recyclable preparation of mesityl iodonium(III) salts. The electrophilicities of [hydroxy(tosyloxy)iodo]mesitylene (MesI(OH)OTs) and iodomesitylene diacetate (MesI(OAc)₂) are suitably enhanced in 2,2,2-trifluoroethanol. A series of nucleophilic aromatic compounds react smoothly with MesI(OH)OTs and MesI(OAc)₂ or in situ hypervalent iodine(III) species, generated from iodomesitylene, to provide the target mesityl iodonium(III) salts in good yields at room temperature with broad functional group tolerance. This C–H condensation strategy merits high para-regioselectivities during the diaryliodonium(III) salt formation, but the major limitation in the case of low-reactive aromatic substrates is byproduct formation resulting from the self-condensation of the nucleophilic mesitylene ring in MesI(OH)OTs and MesI(OAc)₂.     

Metal‐free Synthesis of Aryl Amines: Beyond Nucleophilic Aromatic Substitution

A mild and metal‐free approach to C?N coupling is described that employs diaryliodonium salt electrophiles and secondary aliphatic amine nucleophiles. This reaction results in direct ipso‐substitution of the iodonium moiety and unsymmetrical aryl(TMP)iodonium salts are primarily employed. Moreover, arene substituents and substitution patterns that currently pose a challenge to classical metal‐free methods are accommodated and the alicyclic amine nucleophiles used here are unprecedented in other contemporary metal‐free C?N coupling reactions.     

Synthesis and study of iodonium borate salts as photoinitiators

Iodonium butyltriphenylborate salts (A I⁺ Ar′Ph₃B⁻ R), (Bu) were found to be more efficient than iodonium tetraphenylborate salts (RPh) when used as photoinitiators for the polymerization of acrylates. Relative photodecomposition rates were also different. It was found from a study of the photoreaction of iodonium borate salts with a model monomer, methyl methacrylate, that iodonium butyltriphenylborate salts simultaneously produce a butyl radical from the borate anion and an aryl radical from the iodonium cation upon irradiation. Both radicals initiate polymerization. Iodonium tetraphenylborate salts were found to release an aryl radical, but only from the iodonium cation. Iodonium borate salts exhibit strong absorption below 300 nm with a tail absorption above 400 nm. Thus, iodonium butyltriphenyl borate salts are efficient photoinitiators even when used with visible light. When a photosensitizer such as 5,7-diiodo-3-butoxy-6-fluorone is employed, iodonium butyltriphenylborate salts are rendered much more efficient for visible light photopolymerization. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1667–1677, 1998     

Practical Enantioselective Arylation and Heteroarylation of Aldehydes with In Situ Prepared Organotitanium Reagents Catalyzed by 3‐Aryl‐H8‐BINOL‐Derived Titanium Complexes

A highly efficient and practical method for the catalytic enantioselective arylation and heteroarylation of aldehydes with organotitanium reagents, prepared in situ by the reaction of aryl‐ and heteroaryllithium reagents with ClTi(OiPr)₃, is described. Titanium complexes derived from DPP‐H₈‐BINOL ( 3 d ; DPP=3,5‐diphenylphenyl) and DTBP‐H₈‐BINOL ( 3 e ; DTBP=3,5‐di‐tert‐butylphenyl) exhibit excellent catalytic activity in terms of enantioselectivity and turnover efficiency for the transformation, providing diaryl‐, aryl heteroaryl‐, and diheteroarylmethanol derivatives in high enantioselectivity at low catalyst loading (0.2–2 mol %). The reaction begins with a variety of aryl and heteroaryl bromides through their conversion into organolithium intermediates by Br/Li exchange with nBuLi, thus providing straightforward access to a range of enantioenriched alcohols from commercially available starting materials. Various 2‐thienylmethanols can be synthesized enantioselectively by using commercially available 2‐thienyllithium in THF. The reaction can be carried out on a 10 mmol scale at 0.5 mol % catalyst loading, demonstrating its preparative utility.     

One-Pot Synthesis and Applications of N-Heteroaryl Iodonium Salts

An efficient one-pot synthesis of N-heteroaryl iodonium triflates from the corresponding N-heteroaryl iodide and arene has been developed. The reaction conditions resemble our previous one-pot syntheses, with suitable modifications to allow N-heteroaryl groups. The reaction time is only 30 min, and no anion exchange is required. The obtained iodonium salts were isolated in a protonated form, these salts can either be employed directly in applications or be deprotonated prior to use. The aryl groups were chosen to induce chemoselective transfer of the heteroaryl moiety to various nucleophiles. The reactivity and chemoselectivity of these iodonium salts were demonstrated by selectively introducing a pyridyl moiety onto both oxygen and carbon nucleophiles in good yields.     

Metal‐Free Arylation of Oxygen Nucleophiles with Diaryliodonium Salts

Phenols and carboxylic acids are efficiently arylated with diaryliodonium salts. The reaction conditions are mild, metal free, and avoid the use of halogenated solvents, additives, and excess reagents. The products are obtained in good‐to‐excellent yields after short reaction times. Steric hindrance is very well tolerated, both in the nucleophile and diaryliodonium salt. The scope includes ortho‐ and halo‐substituted products, which are difficult to obtain by metal‐catalyzed protocols. Many functional groups are tolerated, including carbonyl groups, heteroatoms, and alkenes. Unsymmetric salts can be chemoselectively utilized to obtain products with hitherto unreported levels of steric congestion. The arylation has been extended to sulfonic acids, which can be converted to sulfonate esters by two different approaches. With recent advances in efficient synthetic procedures for diaryliodonium salts the reagents are now inexpensive and readily available. The iodoarene byproduct formed from the iodonium reagent can be recovered quantitatively and used to regenerate the diaryliodonium salt, which improves the atom economy.     

Arene diazonium saccharin intermediates: a greener and cost-effective alternative method for the preparation of aryl iodide

In the current protocol, the arene diazonium saccharin derivatives were initially produced from various substituted aromatic amines; subsequently, these intermediates were treated with a greener organic iodide for the preparation of the aryl iodide. We tried to choose low-cost, commercially available, biodegradable, recoverable, ecofriendly, and safe reagents and solvents. The arene diazonium saccharin intermediates could be stored in the liquid phase into a refrigerator for a long time with no significant loss activity. The outstanding merits of the current protocol (a) included the partial recovering of saccharin and tetraethylammonium salt, (b) reduce the use of solvents and the reaction steps due to eliminating separation and purification of intermediates, (c) good yield of the sterically hindered substrates, and (d) avoid the generation of heavy metal or corrosive waste.     

Copper‐Mediated Formation of Aryl,Heteroaryl, Vinyl and Alkynyl Difluoromethylphosphonates: A General Approach to Fluorinated Phosphate Mimics

A general and efficient access to aryl, heteroaryl, vinyl and alkynyl difluoromethylphosphonates is described. The developed methodology using TMSCF₂PO(OEt)₂, iodonium salts and a copper salt provided a straightforward manifold to reach these highly relevant products. The reaction proved to be highly functional group tolerant and proceeded under mild conditions, giving the corresponding products in good to excellent yields. This method represents the first general synthetic route to this important class of fluorinated scaffolds, which are well‐recognized as in vivo stable phosphate surrogates.     

Transition-Metal-Free Difunctionalization of Sulfur Nucleophiles**

Efficient protocols for accessing iodo-substituted diaryl and aryl(vinyl) sulfides have been developed using iodonium salts as reactive electrophilic arylation and vinylation reagents. The reactions take place under transition-metal-free conditions, employing odorless and convenient sulfur reagents. A wide variety of functional groups are tolerated in the S-diarylation, enabling the regioselective late-stage application of several heterocycles and drug molecules under mild reaction conditions. A novel S-difunctionalization pathway was discovered using vinyliodonium salts, which proceeds under additive-free reaction conditions and grants excellent stereoselectivity in the synthesis of aryl(vinyl) sulfides. A one-pot strategy combining transition-metal-free diarylation and subsequent reduction provided facile access to electron-rich thioanilines and a direct synthesis of a potential drug candidate derivative. The retained iodo group allows a wide array of further synthetic transformations. Mechanistic insights were elucidated by isolating the key intermediate, and the relevant energy profile was substantiated by DFT calculations.     

Copper‐Mediated Formation of Aryl,Heteroaryl, Vinyl and Alkynyl Difluoromethylphosphonates: A General Approach to Fluorinated Phosphate Mimics

A general and efficient access to aryl, heteroaryl, vinyl and alkynyl difluoromethylphosphonates is described. The developed methodology using TMSCF₂PO(OEt)₂, iodonium salts and a copper salt provided a straightforward manifold to reach these highly relevant products. The reaction proved to be highly functional group tolerant and proceeded under mild conditions, giving the corresponding products in good to excellent yields. This method represents the first general synthetic route to this important class of fluorinated scaffolds, which are well‐recognized as in vivo stable phosphate surrogates.     

Arylation with Unsymmetrical Diaryliodonium Salts: A Chemoselectivity Study

Phenols, anilines, and malonates have been arylated under metal‐free conditions with twelve aryl(phenyl)iodonium salts in a systematic chemoselectivity study. A new “anti‐ortho effect” has been identified in the arylation of malonates. Several “dummy groups” have been found that give complete chemoselectivity in the transfer of the phenyl moiety, irrespective of the nucleophile. An aryl exchange in the diaryliodonium salts has been observed under certain arylation conditions. DFT calculations have been performed to investigate the reaction mechanism and to elucidate the origins of the observed selectivities. These results are expected to facilitate the design of chiral diaryliodonium salts and the development of catalytic arylation reactions that are based on these sustainable and metal‐free reagents.     

Synthesis of (E)‐ and (Z)‐5‐(Bromomethylene)furan‐2(5H)‐one by Bromodecarboxylation of (E)‐2‐(5‐Oxofuran‐2(5H)‐ylidene)acetic Acid

(E)‐ and (Z)‐5‐(bromomethylene)furan‐2(5H)‐one have been prepared starting from the commercially available adduct between furan and maleic anhydride. A bromodecarboxylation reaction is a key step in the synthesis. The reaction gives the (E)‐ or (Z)‐5‐(bromomethylene)furan‐2(5H)‐one as the major product, dependent on the method used in the bromodecarboxylation.     

From Anilines to Aryl Ethers: A Facile,Efficient, and Versatile Synthetic Method Employing Mild Conditions

We have developed a simple and direct method for the synthesis of aryl ethers by reacting alcohols/phenols (ROH) with aryl ammonium salts (ArNMe₃⁺), which are readily prepared from anilines (ArNR′₂, R′=H or Me). This reaction proceeds smoothly and rapidly (within a few hours) at room temperature in the presence of a commercially available base, such as KO^tBu or KHMDS, and has a broad substrate scope with respect to both ROH and ArNR′₂. It is scalable and compatible with a wide range of functional groups.     

From Anilines to Aryl Ethers: A Facile,Efficient, and Versatile Synthetic Method Employing Mild Conditions

We have developed a simple and direct method for the synthesis of aryl ethers by reacting alcohols/phenols (ROH) with aryl ammonium salts (ArNMe₃⁺), which are readily prepared from anilines (ArNR′₂, R′=H or Me). This reaction proceeds smoothly and rapidly (within a few hours) at room temperature in the presence of a commercially available base, such as KO^tBu or KHMDS, and has a broad substrate scope with respect to both ROH and ArNR′₂. It is scalable and compatible with a wide range of functional groups.