Anion Metathesis of Diaryliodonium Tosylate Salts with a Solid-Phase Column Constructed from Readily Available Laboratory Consumables

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.

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.

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.

Copper-Catalyzed C–H Arylation of Fused-Pyrimidinone Derivatives Using Diaryliodonium Salts

Copper-catalyzed Csp2–Csp2 bond forming reactions through C–H activation are still one of the most useful strategies for the diversification of heterocyclic moieties using various coupling partners. A catalytic protocol for the C–H (hetero)arylation of thiazolo[5,4-f]quinazolin-9(8H)-ones and more generally fused-pyrimidinones using catalyst loading of CuI with diaryliodonium triflates as aryl source under microwave irradiation has been disclosed. The selectivity of the transfer of the aryl group was also disclosed in the case of unsymmetrical diaryliodonium salts. Specific phenylation of valuable fused-pyrimidinones including quinazolinone are provided. This strategy enables a rapid access to an array of various (hetero)arylated N-containing polyheteroaromatics as new potential bioactive compounds.

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.

Lewis Acidity Scale of Diaryliodonium Ions toward Oxygen, Nitrogen, and Halogen Lewis Bases

Equilibrium constants for the associations of 17 diaryliodonium salts Ar₂I⁺X^- with 11 different Lewis bases (halide ions, carboxylates, p-nitrophenolate, amines, and tris(p-anisyl)phosphine) have been investigated by titrations followed by photometric or conductometric methods as well as by isothermal titration calorimetry (ITC) in acetonitrile at 20 °C. The resulting set of equilibrium constants K_I covers 6 orders of magnitude and can be expressed by the linear free-energy relationship lg K_I = s_I LA_I + LB_I, which characterizes iodonium ions by the Lewis acidity parameter LA_I, as well as the iodonium-specific affinities of Lewis bases by the Lewis basicity parameter LB_I and the susceptibility s_I. Least squares minimization with the definition LA_I = 0 for Ph₂I⁺ and s_I = 1.00 for the benzoate ion provides Lewis acidities LA_I for 17 iodonium ions and Lewis basicities LB_I and s_I for 10 Lewis bases. The lack of a general correlation between the Lewis basicities LB_I (with respect to Ar₂I⁺) and LB (with respect to Ar₂CH⁺) indicates that different factors control the thermodynamics of Lewis adduct formation for iodonium ions and carbenium ions. Analysis of temperature-dependent equilibrium measurements as well as ITC experiments reveal a large entropic contribution to the observed Gibbs reaction energies for the Lewis adduct formations from iodonium ions and Lewis bases originating from solvation effects. The kinetics of the benzoate transfer from the bis(4-dimethylamino)-substituted benzhydryl benzoate Ar₂CH-OBz to the phenyl(perfluorophenyl)iodonium ion was found to follow a first-order rate law. The first-order rate constant k_obs was not affected by the concentration of Ph(C₆F₅)I⁺ indicating that the benzoate release from Ar₂CH-OBz proceeds via an unassisted S_N1-type mechanism followed by interception of the released benzoate ions by Ph(C₆F₅)I⁺ ions.