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

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.

Progress in organocatalysis with hypervalent iodine catalysts

Hypervalent iodine compounds as environmentally friendly and relatively inexpensive reagents have properties similar to transition metals. They are employed as alternatives to transition metal catalysts in organic synthesis as mild, nontoxic, selective and recyclable catalytic reagents. Formation of C-N, C-O, C-S, C-F and C-C bonds can be seamlessly accomplished by hypervalent iodine catalysed oxidative functionalisations. The aim of this review is to highlight recent developments in the utilisation of iodine(III) and iodine(V) catalysts in the synthesis of a wide range of organic compounds including chiral catalysts for stereoselective synthesis. Polymer-, magnetic nanoparticle- and metal organic framework-supported hypervalent iodine catalysts are also described.

Lewis Base Catalyzed Dioxygenation of Olefins with Hypervalent Iodine Reagents

1,2-Diols are extremely useful building blocks in organic synthesis. Hypervalent iodine reagents are useful for the vicinal dihydroxylation of olefins to give 1,2-diols under metal-free conditions, but strongly acidic promoters are often required. Herein, we report a catalytic vicinal dioxygenation of olefins with hypervalent iodine reagents using Lewis bases as catalysts. The conditions are mild and compatible with various functional groups.

Organoiodine-Catalyzed Enantioselective Intramolecular Oxy­aminations of Alkenes with N-(Fluorosulfonyl)carbamate

Organoiodine-catalyzed enantioselective intramolecular oxyaminations were realized by the use of benzyl N-(fluorosulfonyl)carbamate as the exogenous nitrogen source. The method allows access to enantioenriched lactones and oxazolines, starting from γ,δ- and δ,ε-unsaturated esters and N-allyl amides, respectively.

Aminolactonization of Unactivated Alkenes Catalyzed by Aryl Iodine

A one-step protocol of the aryl iodine-catalyzed aminolactonization of unactivated alkenes under oxidation conditions was first reported to efficiently construct diverse amino lactones in a short time using HNTs₂ as the compatible nitrogen source. In addition, we investigated the influence of the reaction rate based on the structure of the iodoarene precatalyst, which revealed the selective adjustment effect on aminolactonization and oxylactonization. Finally, preliminary experiments verified the feasibility of asymmetric aminolactonization catalyzed by a chiral iodoarene precatalyst.

Organoiodine-Catalyzed Enantioselective Intermolecular Oxyamination of Alkenes

Metal-free, catalytic enantioselective intermolecular oxyamination of alkenes is realized by use of organoiodine(I/III) chemistry. The protocol is applicable toward aryl- and alkyl-substituted alkenes with high enantioselectivity and electronically controlled regioselectivity. The oxyaminated products can be easily deprotected in one step to reveal free amino alcohols in high yields without loss of enantioselectivity. A key to our success is the discovery of a virtually unexplored chemical entity, N-(fluorosulfonyl)carbamate, as a bifunctional N,O-nucleophile.

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.

Recent Application of Chiral Aryliodine Based on the 2-Iodo­resorcinol Core in Asymmetric Catalysis

Chiral iodoarenes have been steadily increasing in importance in recent years, especially in enantioselective synthesis and catalysis. Since the development of the concept of chiral iodine(I/III) catalysis, the use of various chiral aryliodine frameworks has been explored in this area. This short review gives an overview of the use of chiral hypervalent iodine(I/III) reagents based on the 2-iodoresorcinol core with two attached two lactic side chains bearing ester or amide groups for the catalytic enantioselective dearomatization of phenol compounds, asymmetric oxidation of alkenes, and enantioselective α-functionalization of carbonyl compounds highlighting the excellent reactivities in terms of yield and enantioselectivity.

1 Introduction

2 Enantioselective Dearomatization of Phenol Derivatives

3 Asymmetric Oxidation of Alkenes

4 Enantioselective α-Functionalization of Carbonyl Compounds

5 Conclusion and Outlook

Enantioselective Aryl-Iodide-Catalyzed Wagner-Meerwein Rearrangements

We report a strategy for effecting catalytic, enantioselective carbocationic rearrangements through the intermediacy of alkyl iodanes as stereodefined carbocation equivalents. Asymmetric Wagner-Meerwein rearrangements of β-substituted styrenes are catalyzed by the C₂-symmetric aryl iodide 1 to provide access to enantioenriched 1,3-difluorinated molecules possessing interesting and well-defined conformational properties. Hammett and kinetic isotope effect studies, in combination with computational investigations, reveal that two different mechanisms are operative in these rearrangement reactions, with the pathway depending on the identity of the migrating group. In reactions involving alkyl-group migration, intermolecular fluoride attack is product- and enantio-determining. In contrast, reactions in which aryl rearrangement occurs proceed through an enantiodetermining intramolecular 1,2-migration prior to fluorination. The fact that both pathways are promoted by the same chiral aryl iodide catalyst with high enantioselectivity provides a compelling illustration of generality across reaction mechanisms in asymmetric catalysis.

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.

Recent developments in stereoselective organocatalytic oxyfunctionalizations

In this chapter, asymmetric at carbon oxidations using organocatalytic systems reported from 2012 up to 2018 have been illustrated. Asymmetric epoxidations and oxidation of heteroatom-containing molecules were not included. The processes selected encopass alpha-hydroxylation of carbonyl compounds, dihydroxylation and dioxygenation of alkenes, Baeyer-Villiger and oxidative desymmetrization reactions.

Synthesis of Polycyclic Cyclohexadienone through Alkoxy-Oxylactonization and Dearomatization of 3'-Hydroxy-[1,1'-biphenyl]-2-carboxylic Acids Promoted by Hypervalent Iodine

Alkox-oxylactonization and dearomatization of 3'-hydroxy-[1,1'-biphenyl]-2-carboxylic acid simultaneously promoted by hypervalent iodine have been developed using stoichiometric PhI(OAc)₂ or a catalytic amount of chiral aryl-λ³-iodane generated in situ. This reaction provides a concise method to synthesize diverse polycyclic cyclohexadienones as potential inhibitors of DNA polymerase under mild reaction conditions.

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.

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.

Catalytic, Enantioselective 1,2-Difluorination of Cinnamamides

The enantio- and diastereoselective synthesis of 1,2-difluorides via chiral aryl iodide-catalyzed difluorination of cinnamamides is reported. The method uses HF-pyridine as a fluoride source and mCPBA as a stoichiometric oxidant to turn over catalyst, and affords compounds containing vicinal, fluoride-bearing stereocenters. Selectivity for 1,2-difluorination versus a rearrangement pathway resulting in 1,1-difluorination is enforced through anchimeric assistance from a N- tert-butyl amide substituent.

Recent advances in hypervalent iodine(III)-catalyzed functionalization of alkenes

Hypervalent iodine(III) reagents have been well-developed and widely utilized in functionalization of alkenes, however, generally either stoichiometric amounts of iodine(III) reagents are required or stoichiometric oxidants such as mCPBA are employed to in situ generate iodine(III) species. In this review, recent developments of hypervalent iodine(III)-catalyzed functionalization of alkenes and asymmetric reactions using a chiral iodoarene are summarized.

Indanol-Based Chiral Organoiodine Catalysts for Enantioselective Hydrative Dearomatization

Rapid development in the last decade has rendered chiral organoiodine(I/III) catalysis a reliable methodology in asymmetric catalysis. However, due to the severely limited numbers of effective organoiodine catalysts, many reactions still give low to modest enantioselectivity. We report herein a solution to this issue through the introduction of a pivotal indanol scaffold to the catalyst design. Our catalyst architecture exhibits the advantage of high modularity and thereby expedites catalyst optimization. The catalyst was optimized for the challenging and highly sought-after hydrative dearomatization of 2-substituted phenols at the 4-position.

Catalytic Diastereo- and Enantioselective Fluoroamination of Alkenes

The stereoselective synthesis of syn-β-fluoroaziridine building blocks via chiral aryl iodide-catalyzed fluorination of allylic amines is reported. The method employs HF-pyridine as a nucleophilic fluoride source together with mCPBA as a stoichiometric oxidant, and affords access to arylethylamine derivatives featuring fluorine-containing stereocenters in high diastereo- and enantioselectivity. Catalyst-controlled diastereoselectivity in the fluorination of chiral allylic amines enabled the preparation of highly enantioenriched 1,3-difluoro-2-amines bearing three contiguous stereocenters. The enantioselective catalytic method was applied successfully to other classes of multifunctional alkene substrates to afford anti-β-fluoropyrrolidines, as well as a variety of 1,2-oxyfluorinated products.

Enantioselective dioxytosylation of styrenes using lactate-based chiral hypervalent iodine(III)

A series of optically active hypervalent iodine(III) reagents prepared from the corresponding (R)-2-(2-iodophenoxy)propanoate derivative was employed for the asymmetric dioxytosylation of styrene and its derivatives. The electrophilic addition of the hypervalent iodine(III) compound toward styrene proceeded with high enantioface selectivity to give 1-aryl-1,2-di(tosyloxy)ethane with an enantiomeric excess of 70-96% of the (S)-isomer.

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.

Catalytic 1,3-Difunctionalization via Oxidative C-C Bond Activation

Electronegative substituents arrayed in 1,3-relationships along saturated carbon frameworks can exert strong influence over molecular conformation due to dipole minimization effects. Simple and general methods for incorporation of such functional group relationships could thus provide a valuable tool for modulating molecular shape. Here, we describe a general strategy for the 1,3-oxidation of cyclopropanes using aryl iodine(I-III) catalysis, with emphasis on 1,3-difluorination reactions. These reactions make use of practical, commercially available reagents and can engage a variety of substituted cyclopropane substrates. Analysis of crystal and solution structures of several of the products reveal the consistent effect of 1,3-difluorides in dictating molecular conformation. The generality of the 1,3-oxidation strategy is demonstrated through the catalytic oxidative ring-opening of cyclopropanes for the synthesis of 1,3-fluoroacetoxylated products, 1,3-diols, 1,3-amino alcohols, and 1,3-diamines.