Asymmetric Synthesis with Hypervalent Iodine Reagents

Recent Progress in Synthetic Applications of Hypervalent Iodine(III) Reagents

Hypervalent iodine(III) compounds have found wide application in modern organic chemistry as environmentally friendly reagents and catalysts. Hypervalent iodine reagents are commonly used in synthetically important halogenations, oxidations, aminations, heterocyclizations, and various oxidative functionalizations of organic substrates. Iodonium salts are important arylating reagents, while iodonium ylides and imides are excellent carbene and nitrene precursors. Various derivatives of benziodoxoles, such as azidobenziodoxoles, trifluoromethylbenziodoxoles, alkynylbenziodoxoles, and alkenylbenziodoxoles have found wide application as group transfer reagents in the presence of transition metal catalysts, under metal-free conditions, or using photocatalysts under photoirradiation conditions. Development of hypervalent iodine catalytic systems and discovery of highly enantioselective reactions using chiral hypervalent iodine compounds represent a particularly important recent achievement in the field of hypervalent iodine chemistry. Chemical transformations promoted by hypervalent iodine in many cases are unique and cannot be performed by using any other common, non-iodine-based reagent. This review covers literature published mainly in the last 7-8 years, between 2016 and 2024.

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.

Asymmetric Direct/Stepwise Dearomatization Reactions Involving Hypervalent Iodine Reagents

A remarkable growth in hypervalent iodine-mediated oxidative transformations as stoichiometric reagents as well as catalysts has been well-documented due to their excellent properties, such as mildness, easy handling, high selectivity, environmentally friendly nature, and high stability. This review aims at highlighting the asymmetric oxidative dearomatization reactions involving hypervalent iodine compounds. The present article summarizes asymmetric intra- and intermolecular dearomatization reactions using chiral hypervalent iodine reagents/catalysts as well as hypervalent iodine-mediated dearomatization reactions followed by desymmetrization.

Selective Azidooxygenation of Alkenes Enabled by Photo-induced Radical Transfer Using Aryl-λ3-azidoiodane Species

The photolytic radical-induced vicinal azidooxygenation of synthetically important and diverse functionalized substrates including unactivated alkenes is reported. The photoredox-catalyst/additive-free protocol enables intermolecular oxyazidation by simultaneously incorporating two new functionalities; C-O and C-N across the C=C double bond in a selective manner. Mechanistic investigations reveal the intermediacy of the azidyl radical facilitated via the photolysis of λ³-azidoiodane species and cascade proceeding to generate an active carbon-centered radical. The late-stage transformations of azido- and oxy-moieties were amply highlighted by assembling high-value drug analogs and bioactive skeletons.

Enantio- and Regioselective Palladium(II)-Catalyzed Dioxygenation of (Aza-)Alkenols

An oxidative Pd-catalyzed intra-intermolecular dioxygenation of (aza-)alkenols has been reported, with total regioselectivity. To study the stereoselectivity, different chiral ligands as well as different hypervalent-iodine compounds have been compared. In particular, by using a C-6 modified pyridinyl-oxazoline (Pyox) ligand and hypervalent iodine bearing an aromatic ring, an excellent enantio- and diastereoselectivity has been achieved.

C-N Axial Chiral Hypervalent Iodine Reagents: Catalytic Stereoselective α-Oxytosylation of Ketones

A simple synthesis of a library of novel C−N axially chiral iodoarenes is achieved in a three‐step synthesis from commercially available aniline derivatives. C−N axial chiral iodine reagents are rarely investigated in the hypervalent iodine arena. The potential of the novel chiral iodoarenes as organocatalysts for stereoselective oxidative transformations is assessed using the well explored, but challenging stereoselective α‐oxytosylation of ketones. All investigated reagents catalyse the stereoselective oxidation of propiophenone to the corresponding chiral α‐oxytosylated products with good stereochemical control. Using the optimised reaction conditions a wide range of products was obtained in generally good to excellent yields and with good enantioselectivities.

Hexafluoroisopropanol-Enabled Copper-Catalyzed Asymmetric Halogenation of Cyclic Diaryliodoniums for the Synthesis of Axially Chiral 2,2'-Dihalobiaryls

An efficient asymmetric halogenation of cyclic diaryliodonium salts is demonstrated, which gives access to a wide range of axially chiral 2,2'-dihalobiaryls in good to excellent yields and with excellent enantioselectivities. The use of CuX with chiral bisoxazoline ligand and tetrabutylammonium halides in the unique solvent of hexafluoroisopropanol (HFIP) led to the best results in the process. The axially chiral 2,2'-dihalobiaryls can be transformed into a number of enantiopure chiral ligands that could be potentially useful in asymmetric catalysis.

The Synthesis of Functionalized 3-Aryl- and 3-Heteroaryloxazolidin-2-ones and Tetrahydro-3-aryl-1,3-oxazin-2-ones via the Iodocyclocarbamation Reaction: Access to Privileged Chemical Structures and Scope and Limitations of the Method

3-Aryl- and 3-heteroaryloxazolidin-2-ones, by virtue of the diverse pharmacologic activities exhibited by them after subtle changes to their appended substituents, are becoming increasingly important and should be considered privileged chemical structures. The iodocyclocarbamation reaction has been extensively used to make many 3-alkyl-5-(halomethyl)oxazolidin-2-ones, but the corresponding aromatic congeners have been relatively underexplored. We suggest that racemic 3-aryl- and 3-heteroaryl-5-(iodomethyl)oxazolidin-2-ones, readily prepared by the iodocyclocarbamation reaction of N-allylated N-aryl or N-heteroaryl carbamates, may be useful intermediates for the rapid preparation of potential lead compounds with biological activity. We exemplify this point by using this approach to prepare racemic linezolid, an antibacterial agent. Herein, we report the results of our systematic investigation into the scope and limitations of this process and have identified some distinguishing characteristics within the aryl/heteroaryl series. We also describe the first preparation of 3-aryloxazolidin-2-ones bearing new functionalized C-5 substituents derived from conjugated 1,3-dienyl and cumulated 1,2-dienyl carbamate precursors. Finally, we describe the utility of the iodocyclocarbamation reaction for making six-membered tetrahydro-3-aryl-1,3-oxazin-2-ones.

Hypervalent Iodine(III) Compounds as Biaxial Halogen Bond Donors

“Hypervalent” iodine(III) derivatives have been established as powerful reagents in organic transformations, but so far only a handful of studies have addressed their potential use as halogen-bonding noncovalent Lewis acids. In contrast to “classical” halogen-bond donors based on iodine(I) compounds, iodine(III) salts feature two directional electrophilic axes perpendicular to each other. Herein we present the first systematic investigation on biaxial binding to such Lewis acids in solution. To this end, hindered and unhindered iodolium species were titrated with various substrates, including diesters and diamides, via ¹H NMR spectroscopy and isothermal titration calorimetry. Clear evidence for biaxial binding was obtained in two model systems, and the association strengths increased by 2 orders of magnitude. These findings were corroborated by density functional theory calculations (which reproduced the trend well but underestimated the absolute binding constants) and a cocrystal featuring biaxial coordination of a diamide to the unhindered iodolium compound.

Chiral Hypervalent Iodines: Active Players in Asymmetric Synthesis

Asymmetric organocatalytic oxidations have been witnessed to an impressive development in the last years thanks to the establishment of important chiral hypervalent iodines(III/V). Many different approaches involving both stoichiometric and catalytic versions have provided a fundamental advance in this area within asymmetric synthesis. The easily handing, nontoxic, mild, environmentally friendly (green oxidants), and high stability that are features of these reagents have been applied to many reactions and also have allowed exploring further unprecedented enantioselective transformations. The intention of the present review is thus to highlight as a whole the many approaches utilized up to date to prepare chiral iodines(III/V), as well as their reactivity in a comprehensive manner.

Alkyliodines in High Oxidation State: Enhanced Synthetic Possibilities and Accelerated Catalyst Turn-Over

In contrast to aryliodine(III) compounds, which have matured into a particularly attractive class of oxidants in modern synthesis, the synthetic potential of related alkyliodine(III) derivatives has remained widely underestimated. This is surprising since several unique synthetic possibilities arise directly from the low stability of their central carbon-iodine bond. In this respect, these high-oxidation-state iodine compounds resemble environmentally benign variants of the prominent metal counterparts such as those derived from palladium, nickel and copper. This Concept article summarizes the general reactivity trends in alkyliodine(III) chemistry and discusses selected examples of their strategic use as highly reactive, transient species in organic synthesis and homogeneous catalysis.

Alkene Difunctionalization Using Hypervalent Iodine Reagents: Progress and Developments in the Past Ten Years

Hypervalent iodine reagents are of considerable relevance in organic chemistry as they can provide a complementary reaction strategy to the use of traditional transition metal chemistry. Over the past two decades, there have been an increasing number of applications including stoichiometric oxidation and catalytic asymmetric variations. This review outlines the main advances in the past 10 years in regard to alkene heterofunctionalization chemistry using achiral and chiral hypervalent iodine reagents and catalysts.

Iodine(III)-Mediated Contraction of 3,4-Dihydropyranones: Access to Polysubstituted γ-Butyrolactones

Functionalized γ-butyrolactones are privileged structures in the field of medicinal chemistry; they are found in numerous natural products and synthetic compounds with diverse biological activities. The oxidative ring contraction of 3,4-dihydropyran-2-one derivatives represents a promising yet underappreciated strategy to access these compounds. To the best of our knowledge, very few examples of this strategy have been reported, with limited investigation of the influence of stereogenic centers on the starting dihydropyranones. We investigated the iodine(III)-mediated contraction of a representative set of dihydropyranone derivatives. The method gives rapid access to functionalized γ-butyrolactones in good yields. The reaction scope was investigated, and the method was found to support various levels of substituents, even enabling access to sterically congested quaternary centers. The stereoselectivity was investigated using chiral substrates and a chiral iodine(III) reagent.

Difluorination of Functionalized Aromatic Olefins Using Hypervalent Iodine/HF Reagents

The hypervalent iodine/HF reagent consisting of PhIO and HF·py was found to be effective for fluorination of functionalized aromatic olefins bearing synthetically important carbonyl and hydroxyl groups. Fluorination of 1,3-diphenyl-2-propen-1-one with PhIO/HF·py reagent in CH₂Cl₂ at room temperature gave 3,3-difluoro-1,2-diphenyl-1-propanone in high yield. Other α-aryl-α,β-unsaturated ketones underwent the fluorination to yield aryl 2,2-difluoroethyl ketone derivatives in good to high yields. Catalytic fluorination of α-aryl-α,β-unsaturated ketones using a p-TolI/HF·py/mCPBA reagent system also worked well. Moreover, the fluorination of cinnamyl alcohol derivatives by PhIO/HF·py reagent proceeded smoothly to afford 2-aryl-3,3-difluoro-1-propanols in moderate yields.

Pd-catalyzed versus uncatalyzed, PhI(OAc)2-mediated cyclization reactions of N6-([1,1'-biaryl]-2-yl)adenine nucleosides

In this work we have assessed reactions of N⁶-([1,1'-biaryl]-2-yl)adenine nucleosides with Pd(OAc)₂ and PhI(OAc)₂, via a Pd^II/Pd^IV redox cycle. The substrates are readily obtained by Pd/Xantphos-catalyzed reaction of adenine nucleosides with 2-bromo-1,1'-biaryls. In PhMe, the N⁶-biarylyl nucleosides gave C6-carbazolyl nucleoside analogues by C-N bond formation with the exocyclic N⁶ nitrogen atom. In the solvent screening for the Pd-catalyzed reactions, an uncatalyzed process was found to be operational. It was observed that the carbazolyl products could also be obtained in the absence of a metal catalyst by reaction with PhI(OAc)₂ in 1,1,1,3,3,3-hexafluoroisopropanol (HFIP). Thus, under Pd catalysis and in HFIP, reactions proceed to provide carbazolyl nucleoside analogues, with some differences. If reactions of N⁶-biarylyl nucleoside substrates were conducted in MeCN, formation of aryl benzimidazopurinyl nucleoside derivatives was observed in many cases by C-N bond formation with the N1 ring nitrogen atom of the purine (carbazole and benzimidazole isomers are readily separated by chromatography). Whereas Pd^II/Pd^IV redox is responsible for carbazole formation under the metal-catalyzed conditions, in HFIP and MeCN radical cations and/or nitrenium ions can be intermediates. An extensive set of radical inhibition experiments was conducted and the data are presented.

Iodine(III)-mediated synthesis of chiral α-substituted ketones: recent advances and mechanistic insights

The development of iodine(III)-mediated synthetic transformations has received growing interest, in particular to mediate enantioselective processes. In this class, the α-tosyloxylation of ketone derivatives using iodine(III) is a particularly powerful one, as it yields α-tosyloxy ketones – versatile chiral precursors that enable rapid access to numerous α-chiral ketones through nucleophilic displacement. Despite years of research from numerous groups, the enantioselectivities for this transformation have remained modest. Using quantum chemical calculations, we have uncovered a possible rational for the lack of selectivity. With these computational insights, we have developed an alternative experimental strategy and achieved unprecedented levels of selectivities. Applying this newfound knowledge, we have recently developed a new method to access α-halo ketones from non-ketonic precursors.

Chiral Hypervalent Iodine(III) Catalyst Promotes Highly Enantioselective Sulfonyl- and Phosphoryl-oxylactonizations

An efficient enantioselective hypervalent iodine promoted oxylactonization of 4-pentenoic acids has been achieved using stoichiometric or a catalytic amount of chiral aryl-λ³-iodane. This reaction provides straightforward access to a wide range of sulfonyloxy- and phosphoryloxy-γ-butyrolactones in respectable yields with moderate to excellent enantioselectivities.

Enantioselective Oxidative Rearrangements with Chiral Hypervalent Iodine Reagents

A stereoselective hypervalent iodine‐promoted oxidative rearrangement of 1,1‐disubstituted alkenes has been developed. This practically simple protocol provides access to enantioenriched α‐arylated ketones without the use of transition metals from readily accessible alkenes.