Copper-Catalyzed Selective Diphenylation of Carboxylic Acids with Cyclic Diaryliodonium Salts

Organohypervalent heterocycles

This review summarizes the structural and synthetic aspects of heterocyclic molecules incorporating an atom of a hypervalent main-group element. The term "hypervalent" has been suggested for derivatives of main-group elements with more than eight valence electrons, and the concept of hypervalency is commonly used despite some criticism from theoretical chemists. The significantly higher thermal stability of hypervalent heterocycles compared to their acyclic analogs adds special features to their chemistry, particularly for bromine and iodine. Heterocyclic compounds of elements with double bonds are not categorized as hypervalent molecules owing to the zwitterionic nature of these bonds, resulting in the conventional 8-electron species. This review is focused on hypervalent heterocyclic derivatives of nonmetal main-group elements, such as boron, silicon, nitrogen, carbon, phosphorus, sulfur, selenium, bromine, chlorine, iodine(III) and iodine(V).

Cyclic diaryliodonium salts: applications and overview

Owing to the recent renewed interest and groundbreaking advances in hypervalent chemistry, cyclic diaryliodonium salts have had a myriad of unique applications in the past decade. Their numerous properties, such as an efficient dual arylation mechanism, straightforward one-pot synthesis compatibility, wide substrate scope, and functionalization tolerance, have made them appropriate starting materials for many bioactive compounds. Fluorenes, thiophenes, carbazoles, phenanthrenes, and many other useful cyclic bioactive molecules that are essential for pharmaceutical synthesis can be readily accessed from cyclic diaryliodonium salts. Particular focus has been given to the high optical activity and good enantiomeric excess of the products that facilitate the easy formation of many difficult-to-obtain optical isomers, such as atropisomers. This review aims to compile and summarize all the recent advances in synthesizing methodologies to prepare the important compounds where cyclic diaryliodonium salt is an integral part of the methodologies and would hopefully provide a good foundation for further research on this topic.

Recent Progress in Cyclic Aryliodonium Chemistry: Syntheses and Applications

Hypervalent aryliodoumiums are intensively investigated as arylating agents. They are excellent surrogates to aryl halides, and moreover they exhibit better reactivity, which allows the corresponding arylation reactions to be performed under mild conditions. In the past decades, acyclic aryliodoniums are widely explored as arylation agents. However, the unmet need for acyclic aryliodoniums is the improvement of their notoriously low reaction economy because the coproduced aryl iodides during the arylation are often wasted. Cyclic aryliodoniums have their intrinsic advantage in terms of reaction economy, and they have started to receive considerable attention due to their valuable synthetic applications to initiate cascade reactions, which can enable the construction of complex structures, including polycycles with potential pharmaceutical and functional properties. Here, we are summarizing the recent advances made in the research field of cyclic aryliodoniums, including the nascent design of aryliodonium species and their synthetic applications. First, the general preparation of typical diphenyl iodoniums is described, followed by the construction of heterocyclic iodoniums and monoaryl iodoniums. Then, the initiated arylations coupled with subsequent domino reactions are summarized to construct polycycles. Meanwhile, the advances in cyclic aryliodoniums for building biaryls including axial atropisomers are discussed in a systematic manner. Finally, a very recent advance of cyclic aryliodoniums employed as halogen-bonding organocatalysts is described.

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.

Palladium-Catalyzed C(sp3)-H Biarylation of 8-Methyl Quinolines with Cyclic Diaryliodonium Salts to Access Functionalized Biaryls and Fluorene Derivatives

Herein, we have developed the cyclic diaryliodonium salts as biarylating agents in the C(sp³)-H functionalization using 8-methyl quinoline as the intrinsic directing group. The oxidant-free reaction produces a vast array of the biarylated products with iodo functionality that can be further functionalized. Additionally, intramolecular C(sp³)-H functionalization in a stepwise manner under palladium-catalyzed conditions produced the fluorene derivatives in excellent yields.

Color-tunable emissive heptagon-embedded polycyclic aromatic dicarboximides

Heptagon-embedded polycyclic aromatic dicarboximides were developed as new push-pull fluorescent dyes through palladium-catalysed [4+3] annulation followed by nucleophilic substitution. The introduction of a seven-membered ring in these push-pull systems can efficiently modulate the optical properties leading to an enhancement of the fluorescence quantum yields up to 0.93 with color tunable emission covering the visible-NIR spectrum.

Dibenzopleiadiene-embeded polyaromatics via [4 + 3] annulative decarbonylation/decarboxylation

A novel and efficient sequential cross-coupling/annulation strategy is developed to construct structurally and optoelectronically diverse class of dibezopleiadiene-embeded polyaromatics.

Pd-Catalyzed C(sp3)-H Biarylation via Transient Directing Group Strategy

Here, we describe a highly selective Pd-catalyzed C(sp³)-H biarylation of 2-methylbenzaldehydes using cyclic diaryliodonium salts as arylation reagents. The key strategy is the employment of tert-leucine as a bidentate transient directing group for the proximity-driven metalation to achieve reactivity and selectivity in C-H activation. Various functionalized biaryls bearing both aldehyde and iodine functional groups were prepared successfully, which could be further transformed into a wide range of compounds with potential applications in pharmaceutical chemistry and materials science.

Palladium-Catalyzed Direct Mono- or Polyhalogenation of Benzothiadiazole Derivatives

An unprecedented method for the palladium (II)-catalyzed direct halogenation of benzothiadiazole derivatives is reported here, which would allow the rational tuning of their electronic properties by further transformations. The key strategy is the combination of the double functions of benzothiadiazole as both an important moiety of functional materials and modifiable directing group. Various mono/polychlorinated, brominated, or iodinated benzothiadiazole products could be prepared efficiently, which can be converted into a wide range of valuable polyaromatic compounds with potential applications in material chemistry.

Synthesis of Tribenzo[b,d,f]azepines via Cascade π-Extended Decarboxylative Annulation Involving Cyclic Diaryliodonium Salts

Various functionalized tribenzo[b,d,f]azepines were prepared efficiently with the readily available 2-aminobenzoic acids and cyclic hypervalent diaryliodonium reagents as starting materials under Pd(II) catalysis. The key of this step-economical protocol is that the carboxylic acid functionality was employed as both a traceless directing group for the N-H activation/arylation and a functional handle for the tandem π-extended decarboxylative annulation.

Cascade π-Extended Decarboxylative Annulation Involving Cyclic Diaryliodonium Salts: Site-Selective Synthesis of Phenanthridines and Benzocarbazoles via a Traceless Directing Group Strategy

A novel cascade π-extended decarboxylative annulation (PEDA) involved with cyclic diaryliodonium salts is described. Via fine-tuning of the reaction conditions, the Pd(II)-catalyzed site-selective N1/C2 or C2/C3 annulation of commercially available indole-2-carboxylic acids can be achieved, affording valuable phenanthridines or benzocarbazoles, respectively. The key strategy is the carboxylic acid functionality being employed as both a traceless directing group for the ortho C-N or C-C coupling and a reactive group for the cascade π-extended decarboxylative annulation in a highly step economical manner.

Two-in-One Strategy for the Pd(II)-Catalyzed Tandem C-H Arylation/Decarboxylative Annulation Involved with Cyclic Diaryliodonium Salts

We report here a two-in-one strategy for the Pd(II)-catalyzed tandem C-H arylation/decarboxylative annulation between readily available cyclic diaryliodonium salts and benzoic acids. The carboxylic acid functionality can be used as both a directing group for the ortho-C-H arylation and the reactive group for the tandem decarboxylative annulation. By a step-economical double cross-coupling annulation procedure, the privileged triphenylene frameworks were efficiently constructed, which have potential applications in material chemistry.

Visible light driven, nickel-catalyzed aryl esterification using a triplet photosensitiser thioxanthen-9-one

The nickel-catalyzed esterification of carboxylic acids with aryl bromides using thioxanthen-9-one as a photosensitizer provided aryl esters with excellent yields.

Highly Selective Palladium-Catalyzed Arene C-H Acyloxylation with Benzothiadiazole as a Modifiable Directing Group

An efficient protocol for the palladium-catalyzed direct arene C-H acyloxylation of benzothiadiazole-arene derivatives is reported for the first time. The key strategy is the employment of benzothiadiazole as a modifiable directing group. The highly selective mono- or bis-acyloxylation can be achieved by tuning the reaction conditions, affording various acyloxylated benzothiadiazole derivatives, which could offer a rational tailoring of their electronic properties and be applied in organic electronic and optoelectronic materials. Finally, by diversity-oriented modification of the benzothiadiazole directing group, the acyloxylated products can be readily converted into various valuable functionalized (hetero)biaryls.

Site-selective synthesis of functionalized dibenzo[f,h]quinolines and their derivatives involving cyclic diaryliodonium salts via a decarboxylative annulation strategy

An unprecedented site-selective synthesis of dibenzo[f,h]quinolines and their derivatives was reported via an acid directed decarboxylative annulation cascade.

Synthesis of Functionalized Biaryls and Poly(hetero)aryl Containing Medium-Sized Lactones with Cyclic Diaryliodonium Salts

A novel one-pot procedure is described for the transition-metal catalyzed sequential difunctionalization of diaryliodonium reagents. Reaction of commercially available anthranilic acid derivatives with readily available cyclic diaryliodonium salts followed by a Sonogashira coupling afforded various alkyne substituted biaryls in good to excellent yields. The functionalized biaryls were then utilized for the rapid and efficient one-pot synthesis of novel poly(hetero)aryl containing 10-membered lactones which are potential G-quadruplex binders and telomerase inhibitors.