Benziodoxole Triflate as a Versatile Reagent for Iodo(III)cyclization of Alkynes

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).

Three-component N-alkenylation of azoles with alkynes and iodine(III) electrophile: synthesis of multisubstituted N-vinylazoles

A stereoselective N-alkenylation of azoles with alkynes and iodine(III) electrophile is reported. The reaction between various azoles and internal alkynes is mediated by benziodoxole triflate as the electrophile in a trans-fashion, affording azole-bearing vinylbenziodoxoles in moderate to good yields. The tolerable azole nuclei include pyrazole, indazole, 1,2,3-triazole, benzotriazole, and tetrazole. The iodanyl group in the product can be leveraged as a versatile synthetic handle, allowing for the preparation of hitherto inaccessible types of densely functionalized N-vinylazoles.

(Indol-3-yl)(DMIX)Iodonium Salts: Novel Electrophilic Indole Reagents

A new umpolung approach to the C3-H functionalization of indoles with diverse nucleophiles based on the intermediate formation of I(III) reagents is described. The 3,5-dimethylisoxazol-4-yl auxiliary allows for selective indole transfer under catalyst-free conditions, which was impossible using previously reported reagents. Combining the mildness of transition-metal-free conditions and the high reactivity of hypervalent iodine reagents, this protocol tolerates various functional groups and provides access to indoles that are difficult to prepare conventionally.

Structure-reactivity analysis of novel hypervalent iodine reagents in S-vinylation of thiols

The transition-metal free S-vinylation of thiophenols by vinylbenziodoxolones (VBX) constituted an important step forward in hypervalent iodine-mediated vinylations, highlighting the difference to vinyliodonium salts and that the reaction outcome was influenced by the substitution pattern of the benziodoxolone core. In this study, we report several new classes of hypervalent iodine vinylation reagents; vinylbenziodazolones, vinylbenziodoxolonimine and vinyliodoxathiole dioxides. Their synthesis, structural and electronic properties are described and correlated to the S-vinylation outcome, shedding light on some interesting facets of these reagents.

Aryl-, Akynyl-, and Alkenylbenziodoxoles: Synthesis and Synthetic Applications

Hypervalent iodine reagents are in high current demand due to their exceptional reactivity in oxidative transformations, as well as in diverse umpolung functionalization reactions. Cyclic hypervalent iodine compounds, known under the general name of benziodoxoles, possess improved thermal stability and synthetic versatility in comparison with their acyclic analogs. Aryl-, alkenyl-, and alkynylbenziodoxoles have recently received wide synthetic applications as efficient reagents for direct arylation, alkenylation, and alkynylation under mild reaction conditions, including transition metal-free conditions as well as photoredox and transition metal catalysis. Using these reagents, a plethora of valuable, hard-to-reach, and structurally diverse complex products can be synthesized by convenient procedures. The review covers the main aspects of the chemistry of benziodoxole-based aryl-, alkynyl-, and alkenyl- transfer reagents, including preparation and synthetic applications.

Hypervalent iodine(iii)-mediated ring-expansive difluorination of alkynylcyclopropanes en route to the synthesis of difluorinated alkylidenecyclobutanes

Reported herein is a hypervalent iodine(iii)-mediated ring-expansive difluorination of alkynylcyclopropanes featuring a Wagner–Meerwein-type rearrangement to access a variety of difluorinated alkylidenecyclobutanes.

Regio- and Stereoselective Synthesis of Enol Carboxylate, Phosphate, and Sulfonate Esters via Iodo(III)functionalization of Alkynes

β-Iodo(III)enol carboxylates, phosphates, and tosylates can be efficiently synthesized through regio- and stereoselective iodo(III)functionalization of alkynes. The combination of chlorobenziodoxole and silver salt has proven to generate a versatile cationic iodine(III) electrophile to activate alkynes and engage various carboxylic acids, triethyl phosphate, and p-toluenesulfonic acid as nucleophiles. The β-iodo(III)enol esters serve as starting materials for the synthesis of multisubstituted alkenes through sequential cross-coupling of the C-I(III) and C-O bonds.

Ritter-type iodo(iii)amidation of unactivated alkynes for the stereoselective synthesis of multisubstituted enamides

The Ritter reaction, Brønsted- or Lewis acid-mediated amidation of alkene or alcohol with nitrile via a carbocation, represents a classical method for the synthesis of tertiary amides. Although analogous reactions through a vinyl cation or a species alike may offer a route to enamide, an important synthetic building block as well as a common functionality in bioactive compounds, such transformations remain largely elusive. Herein, we report a Ritter-type trans-difunctionalization of alkynes with a trivalent iodine electrophile and nitrile, which affords β-iodanyl enamides in moderate to good yields. Mediated by benziodoxole triflate (BXT), the reaction proves applicable to a variety of internal alkynes as well as to various alkyl- and arylnitriles. The benziodoxole group in the product serves as a versatile handle for further transformations, thus allowing for the preparation of various tri- and tetrasubstituted enamides that are not readily accessible by other means.

A one-pot successive cyclization-alkylation strategy for the synthesis of 2,3-disubstituted benzo[b]thiophenes

In this study, a new environmentally benign iodine-mediated one-pot iodocyclization/alkylation strategy for the synthesis of benzo[b]thiophene derivatives starting from 2-alkynylthioanisoles was developed. The synthesis of a diverse population of 2,3-disubstituted benzo[b]thiophenes was achieved in high yields by employing moderate reaction conditions using 1,3-dicarbonyl substrates as the nucleophile and various substituted propargyl alcohols as both the cyclization precursor and the alkylating agent. This method resulted in the formation of a series of complex structures obtained in a single step. Additionally, a strategy was devised for the one pot iodocyclization/oxidation of propargyl alcohols into carbonyl functionalized benzo[b]thiophene structures. These green one-pot reaction processes were designed to reduce wastes and byproducts while generating a complex substitution pattern on the benzo[b]thiophene structure. The reported methodologies may be used to synthesize more functionalized benzo[b]thiophene structures that can be used in both biomedical and organic electronic applications.

Iodo(III)-Meyer-Schuster Rearrangement of Propargylic Alcohols Promoted by Benziodoxole Triflate

Benziodoxole triflate (BXT), a cyclic iodine(III) electrophile, has been found to promote a rearrangement of propargylic alcohols into α,β-unsaturated ketones bearing an α-λ³-iodanyl group. This iodo(III)-Meyer-Schuster rearrangement proceeds under mild conditions and tolerates a variety of functionalized propargylic alcohols, thus complementing previously reported halogen-intercepted Meyer-Schuster rearrangement. The α-λ³-iodanylenones can be utilized for facile Pd-catalyzed cross-coupling for the synthesis of multisubstituted enones.

2-Iodosylbenzoic acid activated by trifluoromethanesulfonic anhydride: efficient oxidant and electrophilic reagent for preparation of iodonium salts

2-Iodosylbenzoic acid in the presence of trifluoromethanesulfonic anhydride is an efficient oxidant and electrophilic reagent useful for preparation of the corresponding alkenyl and aryliodonium salts.

Transition metal-free and regioselective vinylation of phosphine oxides and H-phosphinates with VBX reagents

A series of phosphine oxides and H-phosphinates were vinylated in the presence of the iodine(iii) reagents vinylbenziodoxolones (VBX), providing the corresponding alk-1-enyl phosphine oxides and alk-1-enyl phosphinates in good yields with complete chemo- and regioselectivity. The vinylation proceeds in open flask under mild and transition metal-free conditions.

Electrophilic Vinylation of Thiols under Mild and Transition Metal-Free Conditions

The iodine(III) reagents vinylbenziodoxolones (VBX) were employed to vinylate a series of aliphatic and aromatic thiols, providing E-alkenyl sulfides with complete chemo- and regioselectivity, as well as excellent stereoselectivity. The methodology displays high functional group tolerance and proceeds under mild and transition metal-free conditions without the need for excess substrate or reagents. Mercaptothiazoles could be vinylated under modified conditions, resulting in opposite stereoselectivity compared to previous reactions with vinyliodonium salts. Novel VBX reagents with substituted benziodoxolone cores were prepared, and improved reactivity was discovered with a dimethyl-substituted core.

Site-selective aromatic C-H λ3-iodanation with a cyclic iodine(iii) electrophile in solution and solid phases

An efficient and site-selective aromatic C-H λ³-iodanation reaction is achieved using benziodoxole triflate (BXT) as an electrophile under room temperature conditions. The reaction tolerates a variety of electron-rich arenes and heteroarenes to afford the corresponding arylbenziodoxoles in moderate to good yields. The reaction can also be performed mechanochemically by grinding a mixture of solid arenes and BXT under solvent-free conditions. The arylbenziodoxoles can be used for various C-C and C-heteroatom bond formations, and are also amenable to further modification by electrophilic halogenation. DFT calculations suggested that the present reaction proceeds via a concerted λ³-iodanation-deprotonation transition state, where the triflate anion acts as an internal base.

Cyclization of ortho-ethynylbiaryls as an emerging versatile tool for the construction of polycyclic arenes

Cyclization and cycloisomerization of ortho-aryl(ethynyl)arenes provide an easy direct access to fused polycyclic aromatic carbo- and heterocycles. This methodology has demonstrated an impressive progress in the recent years. The goal of this review is to give a comprehensive outlook on the synthetic potential, scope, limitations, and mechanistic aspects of the cyclization reactions. The material is arranged according to the activation method that can be used to induce cyclization: pyrolysis, metal catalysis, electrophilic activation, radical induction, base catalysis. Particular attention is paid to the specificity of ortho-ethynylbiaryls with a heterocyclic central core.

The bibliography includes 257 references.

1-Iodobuta-1,3-diynes in Copper-Catalyzed Azide-Alkyne Cycloaddition: A One-Step Route to 4-Ethynyl-5-iodo-1,2,3-triazoles

Cu-catalyzed 1,3-dipolar cycloaddition of iododiacetylenes with organic azides using iodotris(triphenylphosphine)copper(I) as a catalyst was found to be an efficient one-step synthetic route to 5-iodo-4-ethynyltriazoles. The reaction is tolerant to various functional groups in both butadiyne and azide moieties. The synthetic application of 5-iodo-4-ethynyl triazoles obtained was also evaluated: the Sonogashira coupling with alkynes resulted in unsymmetrically substituted triazole-fused enediyne systems, while the Suzuki reaction yielded the corresponding 5-aryl-4-ethynyl triazoles.

Metal-Free Oxidative Cross Coupling of Indoles with Electron-Rich (Hetero)arenes

A new method for the synthesis of bi-heteroaryls is reported, based on the umpolung of indoles with benziodoxol(on)e hypervalent iodine reagents (IndoleBX). The oxidative coupling of IndoleBX with an equimolar amount of electron-rich benzenes, indoles, pyrroles, and thiophenes proceeded under mild transition-metal-free conditions. Functionalized non-symmetrical bi-indolyl heterocycles were accessed efficiently. Introduction of a new type of C2-substituted indole benziodoxole reagents further allowed extending the scope of the reaction to NH unprotected and C3-alkylated indoles. The obtained bi-heterocycles are important building blocks in synthetic and medicinal chemistry, and could be easily transformed into more complex heterocyclic systems.

Rhodium-catalyzed C-H functionalization of heteroarenes using indoleBX hypervalent iodine reagents

The C–H indolation of heteroarenes was realized using the benziodoxolone hypervalent iodine reagents indoleBXs. Functionalization of the C–H bond in bipyridinones and quinoline N-oxides catalyzed by a rhodium complex allowed to incorporate indole rings into aza-heteroaromatic compounds. These new transformations displayed complete regioselectivity for the C-6 position of bipyridinones and the C-8 position of quinoline N-oxides and tolerated a broad range of functionalities, such as halogens, ethers, or trifluoromethyl groups.

Iridium- and Rhodium-Catalyzed Directed C-H Heteroarylation of Benzaldehydes with Benziodoxolone Hypervalent Iodine Reagents

The C-H heteroarylation of benzaldehydes with indoles and pyrroles was realized using the benziodoxolone hypervalent iodine reagents indole- and pyrroleBX. Functionalization of the aldehyde C-H bond using either an o-hydroxy or amino directing group and catalyzed by an iridium or a rhodium complex allowed the synthesis of salicyloylindoles and (2-sulfonamino)benzoylindoles, respectively, with good to excellent yields (74-98%). This new transformation could be carried out under mild conditions (rt to 40 °C) and tolerated a broad range of functionalities, such as ethers, halogens, carbonyls, or nitro groups.