Enantioselective Iodine(III)-Mediated Synthesis of α-Tosyloxy Ketones: Breaking the Selectivity Barrier

Novel Brønsted Acid Catalyzed C-C Bond Activation and α-Alkylation of Ketones

A novel approach for the α-alkylation of ketones was developed using Brønsted acid-catalyzed C-C bond cleavage. Both aromatic and aliphatic ketones reacted smoothly with 2-substituted 1,3-diphenylpropane-1,3-diones to afford α-alkylation products with high yields and with excellent regioselectivity, in which the 1,3-dicarbonyl group acted as a leaving group in the presence of the catalyst TfOH. Mechanism experiments showed that the β-C-C bond cleavage of diketone and the shift of the equilibrium towards the enol formation from ketone are driving forces that induce the desired products.

Synthesis of Chiral Iodoaniline-Lactate Based Catalysts for the α-Functionalization of Ketones

A family of chiral iodoaniline-lactate based catalysts with C1 and C2 symmetry were efficiently synthesized. Comparisons between the reactivity and selectivity between the new and previously reported catalysts are made. The new catalysts promoted the α-oxysulfonylation of ketones in shorter reaction times and with higher yields of up to 99%. A scope for the oxysulfonylation reaction is presented, forming a variety of reported and novel products with enantioselectivities of up to 83%.

Catalyst-Substrate Helical Character Matching Determines the Enantioselectivity in the Ishihara-Type Iodoarenes Catalyzed Asymmetric Kita-Dearomative Spirolactonization

Catalyst design has traditionally focused on rigid structural elements to prevent conformational flexibility. Ishihara's elegant design of conformationally flexible C₂-symmetric iodoarenes, a new class of privileged organocatalysts, for the catalytic asymmetric dearomatization (CADA) of naphthols is a notable exception. Despite the widespread use of the Ishihara catalysts for CADAs, the reaction mechanism remains the subject of debate, and the mode of asymmetric induction has not been well established. Here, we report an in-depth computational investigation of three possible mechanisms in the literature. Our results, however, reveal that this reaction is best rationalized by a fourth mechanism called "proton-transfer-coupled-dearomatization (PTCD)", which is predicted to be strongly favored over other competing pathways. The PTCD mechanism is consistent with a control experiment and further validated by applying it to rationalize the enantioselectivities. Oxidation of the flexible I(I) catalyst to catalytic active I(III) species induces a defined C₂-symmetric helical chiral environment with a delicate balance between flexibility and rigidity. A match/mismatch effect between the active catalyst and the substrate's helical shape in the dearomatization transition states was observed. The helical shape match allows the active catalyst to adapt its conformation to maximize attractive noncovalent interactions, including I(III)···O halogen bond, N-H···O hydrogen bond, and π···π stacking, to stabilize the favored transition state. A stereochemical model capable of rationalizing the effect of catalyst structural variation on the enantioselectivities is developed. The present study enriches our understanding of how flexible catalysts achieve high stereoinduction and may serve as an inspiration for the future exploration of conformational flexibility for new catalyst designs.

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.

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.

Sulfur(iv)-mediated umpolung α-heterofunctionalization of 2-oxazolines

The α-umpolung of carbonyl compounds significantly expands the boundaries of traditional carbonyl chemistry. Despite various umpolung methods available today, reversing the inherent reactivity of carbonyls still remains a substantial challenge. In this article, we report the first use of sulfonium salts, in lieu of well-established hypervalent iodines, for the carbonyl umpolung event. The protocol enables the incorporation of a wide variety of heteroatom nucleophiles into the α-carbon of 2-oxazolines. The success of this investigation hinges on the following factors: (1) the use of sulfoxides, which are abundant, structurally diverse and tunable, and easily accessible, ensures the identification of a superior oxidant namely phenoxathiin sulfoxide for the umpolung reaction; (2) the "assembly/deprotonation" protocol previously developed for rearrangement reactions in our laboratory was successfully applied here for the construction of α-umpoled 2-oxazolines.

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.

Recent discoveries on the structure of iodine(iii) reagents and their use in cross-nucleophile coupling

This perspective article discusses structural features of iodine(iii) compounds as a prelude to presenting their use as umpolung reagents, in particular as pertains to their ability to promote the selective coupling of two nucleophilic species via 2e^- oxidation.

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

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.

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.

Electrophilic Hypervalent Trifluoromethylthio-Iodine(III) Reagent

Herein we report the design and synthesis of hypervalent trifluoromethylthio-iodine(III) reagent 1 and the elucidation of its structure by NMR spectroscopy and X-ray crystallography. The trifluoromethylthiolation reactions of 1 with various nucleophiles were explored, and this compound was found to be a versatile electrophilic reagent for the transfer of a trifluoromethylthio group (-SCF₃). The hydrogen-bonding mode responsible for the activation of 1 by the solvent 1,1,1,3,3,3-hexafluoro-2-propanol was investigated both experimentally and computationally.

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.

Synthesis of enantiomerically pure 2-aryloxy carboxylic acids and their derivatives

Recent publications on the key preparation methods of the enantiomers of 2-aryloxy carboxylic acids are summarized and comparative analysis of the methods is given. The information is arranged according to the type of the starting compound, being classified into syntheses from enantiomerically pure chiral precursors and syntheses from prochiral precursors, which imply generation of an asymmetric centre in the substrate molecule. Data on the chemical resolution of racemic mixtures of the title compounds are addressed in a separate Section. Attention is focused on the preparation of practically valuable 2-aryloxy acids. Examples of biologically active derivatives of 2-aryloxy carboxylic acids are given.

The bibliography includes 121 references.

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.

Mechanism and Origins of Chemo- and Stereoselectivities of Aryl Iodide-Catalyzed Asymmetric Difluorinations of β-Substituted Styrenes

The mechanism of the aryl iodide-catalyzed asymmetric migratory geminal difluorination of β-substituted styrenes ( Banik et al. Science 2016, 353, 51 ) has been explored with density functional theory computations. The computed mechanism consists of (a) activation of iodoarene difluoride (ArIF2), (b) enantiodetermining 1,2-fluoroiodination, (c) bridging phenonium ion formation via SN2 reductive displacement, and (d) regioselective fluoride addition. According to the computational model, the ArIF2 intermediate is stabilized through halogen-π interactions between the electron-deficient iodine(III) center and the benzylic substituents at the catalyst stereogenic centers. Interactions with the catalyst ester carbonyl groups (I(III)+···O) are not observed in the unactivated complex, but do occur upon activation of ArIF2 through hydrogen-bonding interactions with external Brønsted acid (HF). The 1,2-fluoroiodination occurs via alkene complexation to the electrophilic, cationic I(III) center followed by C-F bond formation anti to the forming C-I bond. The bound olefin and the C-I bond of catalyst adopt a spiro arrangement in the favored transition structures but a nearly periplanar arrangement in the disfavored transition structures. Multiple attractive non-covalent interactions, including slipped π···π stacking, C-H···O, and C-H···π interactions, are found to underlie the high asymmetric induction. The chemoselectivity for 1,1-difluorination versus 1,2-difluorination is controlled mainly by (1) the steric effect of the substituent on the olefinic double bond and (2) the nucleophilicity of the carbonyl oxygen of substrate.

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.

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.

Chiral isoxazolidine-mediated stereoselective umpolung α-phenylation of methyl ketones

Asymmetric nucleophilic phenylation of N-alkoxyenamine with triphenylaluminium is achieved for the formation of enantioenriched α-phenyl ketones through a one-pot process.

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.

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.

Relative Rate Profiles of Functionalized Iodoarene Catalysts for Iodine(III) Oxidations

A series of rate studies were conducted to evaluate the steric and electronic properties that govern the reactivity of iodoarene amide catalysts in the α-oxytosylation of propiophenone. A meta-substituted benzamide catalyst emerged as the most reactive. This catalyst was employed in the α-oxytosylation of a series of substituted propiophenones, returning the α-tosyloxy ketone products in excellent isolated yield.