A survey of chiral hypervalent iodine reagents in asymmetric synthesis

Riboflavin Photocatalyzed Dearomative Spiro-Etherification of Naphthols

The dearomative spiro-etherification of naphthols is achieved using catalytic amounts of riboflavin tetracetate (RFTA) as a photosensitizer and molecular oxygen as a terminal oxidizing agent under blue light (440 nm) irradiation in the presence of acid. The presence of acid increases the photooxidation power of RFTA and facilitates the dearomatization reaction.

Toward a General Protocol for Catalytic Oxidative Transformations Using Electrochemically Generated Hypervalent Iodine Species

A simple catalytic electrosynthetic protocol for oxidative transformations mediated by hypervalent iodine reagents has been developed. In this protocol, electricity drives the iodine(I)/iodine(III) catalytic cycle enabling catalysis with in situ generated hypervalent iodine species, thereby eliminating chemical oxidants and the inevitable chemical waste associated with their mode of action. In addition, no added electrolytic salts are needed in this process. The developed method has been validated using two different hypervalent iodine-mediated transformations: (i) the oxidative cyclization of N-allylic and N-homoallylic amides to the corresponding dihydrooxazole and dihydro-1,3-oxazine derivatives, respectively, and (ii) the α-tosyloxylation of ketones. Both reactions proceeded smoothly under the developed catalytic electrosynthetic conditions without reoptimization, featuring a wide substrate scope and excellent functional group tolerance. In addition, scale-up to gram-scale and catalyst recovery were easily achieved maintaining the high efficiency of the process.

Two-step continuous-flow synthesis of 6-membered cyclic iodonium salts via anodic oxidation

We describe a multi-step continuous-flow procedure for the generation of six-membered diaryliodonium salts. The accompanying scalability and atom economy are significant improvements to existing batch methods. Benzyl acetates are submitted to this two-step procedure as highly available and cheap starting materials. An acid-catalyzed Friedel-Crafts alkylation followed by an anodic oxidative cyclization yielded a defined set of cyclic iodonium salts in a highly substrate-dependent yield.

Enantioselective iodolactonization to prepare ε-lactone rings using hypervalent iodine

Despite the rapid growth of enantioselective halolactonization reactions in recent years, most are effective only when forming smaller (6,5,4-membered) rings. Seven-membered ε-lactones, are rarely formed with high selectivity, and never without conformational bias. We describe the first highly enantioselective 7-exo-trig iodolactonizations of conformationally unbiased ε-unsaturated carboxylic acids, effected by an unusual combination of a bifunctional BAM catalyst, I₂, and I(iii) reagent (PhI(OAc)₂:PIDA).

We describe the first highly enantioselective 7-exo-trig iodolactonizations of conformationally unbiased ε-unsaturated carboxylic acids, effected by an unusual combination of a bifunctional BAM catalyst, I₂, and I(iii) reagent (PhI(OAc)₂:PIDA).

Structure Elucidation of In Situ Generated Chiral Hypervalent Iodine Complexes via Vibrational Circular Dichroism (VCD)

The structure of in situ generated chiral aryl-λ³ -iodanes obtained under oxidative reaction conditions was not yet observable with experimental techniques and their proposed structures are purely based on DFT calculations. Herein, we establish vibrational circular dichroism (VCD) spectroscopy as an experimental technique to verify DFT-calculated chiral iodane structures. Based on a chiral triazole-substituted iodoarene catalyst, we were able to elucidate a yet undescribed cationic chiral iodane as the most populated intermediate under oxidative conditions with a significant intramolecular N-I-interaction and no significant interactions with tosylate or m-chlorobenzoic acid as potential anionic ligands. Instead, aggregation of these substrates was found, which resulted in the formation of a non-coordinating anionic hydrogen bonded complex. The importance of VCD as a crucial experimental observable is further highlighted by the fact that our initial structural proposal, that was purely based on DFT calculations, could be falsified.

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.

Chiral Hypervalent Iodine Catalysis Enables an Unusual Regiodivergent Intermolecular Olefin Aminooxygenation

A novel iodide-catalyzed intermolecular aminooxygenation strategy is described here. Amide is used as the O- and N- source to probe for regiocontrol strategies. Notably, simple additives can be selectively introduced to achieve regiodivergent oxyamination processes for electronically activated alkenes while being regio-complementary for unactivated alkenes. Our preliminary data demonstrates that this regiocontrol strategy based on nucleophile can also be applied in asymmetric processes using chiral hypervalent iodine catalysis.

Sulfur-Based Chiral Iodoarenes: An Underexplored Class of Chiral Hypervalent Iodine Reagents

Chiral hypervalent iodine reagents are active players in modern stereoselective organic synthesis. Structurally diverse chiral hypervalent iodine reagents have been synthesised and extensively studied, but hypervalent iodine reagents containing chiral sulfur stereogenic centre are scarce and their synthesis is challenging. A small library of iodoarenes containing chiral sulfinamide and chiral sulfoximine moieties has been synthesised using commercially available reagents. The oxidation of the chiral iodoarene precursors to iodine(III) reagents was cumbersome due to facile overoxidation of the sulfoxide moiety and hence loss of chirality under various oxidation conditions. Oxidation of chiral sulfonimidoyl derivatives to the corresponding hypervalent iodine reagents was successful and led to novel sulfur-based chiral iodine(III) reagents.

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.

Effect of noncovalent interactions in ion pairs on hypervalent iodines: inversion of regioselectivity in sulfonyloxylactonization

The noncovalent interactions between the sulfonyloxy group and the cationic nitrogen-containing heterocyclic moiety substituted in hypervalent iodines caused specific regioselectivity in the sulfonyloxylactonization of 2-vinyl benzoic acids.

Iodoarene-Catalyzed Oxyamination of Unactivated Alkenes to Synthesize 5-Imino-2-Tetrahydrofuranyl Methanamine Derivatives

Reported here is the room-temperature metal-free iodoarene-catalyzed oxyamination of unactivated alkenes. In this process, the alkenes are difunctionalized by the oxygen atom of the amide group and the nitrogen in an exogenous HNTs₂ molecule. This mild and open-air reaction provided an efficient synthesis to N-bistosyl-substituted 5-imino-2-tetrahydrofuranyl methanamine derivatives, which are important motifs in drug development and biological studies. Mechanistic study based on experiments and density functional theory calculations showed that this transformation proceeds via activation of the substrate alkene by an in situ generated cationic iodonium(III) intermediate, which is subsequently attacked by an oxygen atom (instead of nitrogen) of amides to form a five-membered ring intermediate. Finally, this intermediate undergoes an S_N2 reaction by NTs₂ as the nucleophile to give the oxygen and nitrogen difunctionalized 5-imino-2-tetrahydrofuranyl methanamine product. An asymmetric variant of the present alkene oxyamination using chiral iodoarenes as catalysts also gave promising results for some of the substrates.

Catalytic Enantioselective Synthesis of Difluorinated Alkyl Bromides

We report an iodoarene-catalyzed enantioselective synthesis of β,β-difluoroalkyl bromide building blocks. The transformation involves an oxidative rearrangement of α-bromostyrenes, utilizing HF-pyridine as the fluoride source and m-CPBA as the stoichiometric oxidant. A catalyst decomposition pathway was identified, which, in tandem with catalyst structure-activity relationship studies, facilitated the development of an improved catalyst providing higher enantioselectivity with lower catalyst loadings. The versatility of the difluoroalkyl bromide products was demonstrated via highly enantiospecific substitution reactions with suitably reactive nucleophiles. The origins of enantioselectivity were investigated using computed interaction energies of simplified catalyst and substrate structures, providing evidence for both CH-π and π-π transition state interactions as critical features.

Hypervalent Iodine(III)-Catalysed Enantioselective α-Acetoxylation of Ketones

An enantioselective catalytic synthesis of α-acetoxylated ketones through I(I)/I(III) catalysis using a resorcinol/lactamide-based chiral iodoarene is reported. Catalyst turnover by in situ generation of the active iodine(III) derivative is achieved by oxidation with mCPBA in the presence of acetic acid. The prior transformation of ketones to easily accessible acetyl enol ethers is beneficial and yields up to 97 % with enantioselectivities up to 88 % ee are obtained using only low catalyst loadings of only 5 mol % under mild reaction conditions.

One-Pot Synthesis and Conformational Analysis of Six-Membered Cyclic Iodonium Salts

Two one-pot procedures for the construction of carbon-bridged diaryliodonium triflates and tetrafluoroborates are described. Strong Brønsted acids enable the effective Friedel-Crafts alkylation with diversely substituted o-iodobenzyl alcohol derivatives, providing diphenylmethane scaffolds, which are subsequently oxidized and cyclized to the corresponding dibenzo[b,e]iodininium salts. Based on NMR investigations and density functional theory (DFT) calculations, we could verify the so-far-undescribed existence of two stable isomers in cyclic iodonium salts substituted with aliphatic side chains in the carbon bridge.

A Triazole-Substituted Aryl Iodide with Omnipotent Reactivity in Enantioselective Oxidations

A widely applicable triazole‐substituted chiral aryl iodide is described as catalyst for enantioselective oxidation reactions. The introduction of a substituent in ortho‐position to the iodide is key for its high reactivity and selectivity. Besides a robust and modular synthesis, the main advantage of this catalyst is the excellent performance in a plethora of mechanistically diverse enantioselective transformations, such as spirocyclizations, phenol dearomatizations, α‐oxygenations, and oxidative rearrangements. DFT‐calculations of in situ generated [hydroxy(tosyloxy)iodo]arene isomers give an initial rational for the observed reactivity.

Asymmetric [N–I–N]+ halonium complexes

The first examples of unrestrained asymmetric silver(i) and halonium complexes have been prepared and characterised.

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.

Carbohydrate-Based Chiral Iodoarene Catalysts: A Survey through the Development of an Improved Catalyst Design

Iodoarene catalysts can be applied in versatile reactions, for instance in the construction of complex chiral molecules via dearomatization of simple aromatic compounds. Recently, we reported the synthesis of the first carbohydrate-based chiral iodoarene catalysts and their application in asymmetric catalysis. Here we describe the synthesis of some new and improved catalysts. An account on how we got to the improved catalyst design, as well as the X-ray structure of one of the carbohydrate-based iodoarenes, is given.

A Continuum from Halogen Bonds to Covalent Bonds: Where Do λ3 Iodanes Fit?

The intrinsic bonding nature of λ 3 -iodanes was investigated to determine where its hypervalent bonds fit along the spectrum between halogen bonding and covalent bonding. Density functional theory with an augmented Dunning valence triple zeta basis set ( ω B97X-D/aug-cc-pVTZ) coupled with vibrational spectroscopy was utilized to study a diverse set of 34 hypervalent iodine compounds. This level of theory was rationalized by comparing computational and experimental data for a small set of closely-related and well-studied iodine molecules and by a comparison with CCSD(T)/aug-cc-pVTZ results for a subset of the investigated iodine compounds. Axial bonds in λ 3 -iodanes fit between the three-center four-electron bond, as observed for the trihalide species IF 2 − and the covalent FI molecule. The equatorial bonds in λ 3 -iodanes are of a covalent nature. We explored how the equatorial ligand and axial substituents affect the chemical properties of λ 3 -iodanes by analyzing natural bond orbital charges, local vibrational modes, the covalent/electrostatic character, and the three-center four-electron bonding character. In summary, our results show for the first time that there is a smooth transition between halogen bonding → 3c–4e bonding in trihalides → 3c–4e bonding in hypervalent iodine compounds → covalent bonding, opening a manifold of new avenues for the design of hypervalent iodine compounds with specific properties.

Hypervalent iodine reactions utilized in carbon–carbon bond formations

This review covers recent developments of hypervalent iodine chemistry in dearomatizations, radicals, hypervalent iodine-guided electrophilic substitution, arylations, photoredox, and more.

Synthesis, characterisation, and reactivity of novel pseudocyclic hypervalent iodine reagents with heteroaryl carbonyl substituents

New heteroatom-containing pseudocyclic iodanes are being introduces as alternatives to the Koser reagent.

Difluorinative ring expansions of benzo-fused carbocycles and heterocycles are achieved withp-(difluoroiodo)toluene

Fluorination of exocyclic alkenes and allenes withp-TolIF₂gives ring-expanded β,β-difluoridesviaa 1,2-phenyl shift.

Asymmetric iodine catalysis-mediated enantioselective oxidative transformations

The implementation of chiral iodine catalysis has tremendously been developed in the field of asymmetric synthesis over the past decade. It enables the stereoselective creation of C-O as well as C-C, C-N and C-X (X = halogen) bonds through oxidative transformations. Thanks to the low toxicity and ease of handling of iodine compounds, this strategy offers many advantages over classical metal-catalyzed oxidations with chiral ligands. The approaches rely on iodine(i/iii) or (-i/+i) catalysis by using a chiral aryliodine or ammonium iodide respectively in combination with a suitable terminal oxidant. As such, the design of iodine compounds with central, axial or even planar chirality has allowed us to achieve high enantioselectivities. The goal of this review is to cover the different chiral iodine compound-catalyzed oxidative transformations including α-functionalization of carbonyl compounds, dearomatization of phenol derivatives and difunctionalization of alkenes which should demonstrate that iodine catalysis has now found its place in the realm of asymmetric organocatalysis.