Enantioselective Oxidative Rearrangements with Chiral Hypervalent Iodine Reagents

Chemoselective cycloisomerization of O-alkenylbenzamides via concomitant 1,2-aryl migration/elimination mediated by hypervalent iodine reagents

As an ambident nucleophile, controlling the reaction selectivities of nitrogen and oxygen atoms in amide moiety is a challenging issue in organic synthesis. Herein, we present a chemodivergent cycloisomerization approach to construct isoquinolinone and iminoisocoumarin skeletons from o-alkenylbenzamide derivatives. The chemo-controllable strategy employed an exclusive 1,2-aryl migration/elimination cascade, enabled by different hypervalent iodine species generated in situ from the reaction of iodosobenzene (PhIO) with MeOH or 2,4,6-tris-isopropylbenzene sulfonic acid. DFT studies revealed that the nitrogen and oxygen atoms of the intermediates in the two reaction systems have different nucleophilicities and thus produce the selectivity of N or O-attack modes.

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.

Aryl iodine-catalysed divergent synthesis of isobenzofuranones and isocoumarins via oxidative 1,2-aryl migration/elimination

The divergent synthesis of isobenzofuranones and isocoumarins was realized from the reaction of 2-alkenyl benzoic acids and mCPBA in the presence of catalytic aryl iodine and (±)-10-camphorsulfonic acid (CSA). The organocatalytic oxidative reaction is assumed to undergo a cascade process involving lactonization, 1,2-aryl migration and elimination enabled by a modified Koser reagent generated in situ.

A General Method for α-Oxyacylation of Vinyl Ketones Using Koser's Reagent

A direct general method for the preparation of α-oxyacylated vinyl ketones using Koser's hypervalent iodine reagent is reported. A variety of acyloxy groups from long-chain aliphatic, aromatic, α,β-unsaturated carboxylic acids have been installed efficiently for the first time. The oxyacylated adducts were used for the preparation of densely functionalized chiral δ-lactones and cyclopentenes using carbene organocatalysis.

Chiral Ligands in Hypervalent Iodine Compounds: Synthesis and Structures of Binaphthyl-Based λ3 -Iodanes

Several novel binaphthyl-based chiral hypervalent iodine(III) reagents have been prepared and structurally analysed. Various asymmetric oxidative reactions were applied to evaluate the reactivities and stereoselectivities of those reagents. Moderate to excellent yields were observed; however, very low stereoselectivities were obtained. NMR experiments indicated that these reagents are very easily hydrolysed in either chloroform or DMSO solvents leading to the limited stereoselectivities. It is concluded that the use of chiral ligands is an unsuccessful way to prepare efficient stereoselective 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.

Enantioselective Construction of Tertiary Fluoride Stereocenters by Organocatalytic Fluorocyclization

1,1-Disubstituted styrenes with internal oxygen and nitrogen nucleophiles undergo oxidative fluorocyclization reactions with in situ generated chiral iodine(III)-catalysts. The resulting fluorinated tetrahydrofurans and pyrrolidines contain a tertiary carbon–fluorine stereocenter. Application of a new 1-naphthyllactic acid-based iodine(III)-catalyst allows the control of tertiary carbon–fluorine stereocenters with up to 96% ee. Density functional theory calculations are performed to investigate the details of the mechanism and the factors governing the stereoselectivity of the reaction.

Hypervalent iodine reagent-mediated reactions involving rearrangement processes

Hypervalent iodine reagents have been extensively employed in various types of oxidative organic reactions including oxidative coupling/cyclization, bifunctionalization of olefins and cyclopropane, C-H functionalization, and oxidative rearrangement reactions. In this review, the developments of the exclusive hypervalent iodine-mediated reactions involving oxidative rearrangement processes, including [1,2]-migration, Hofmann rearrangement, Beckmann rearrangement, ring contraction, ring expansion, [3,3]-sigmatropic/iodonium-Claisen rearrangement and some miscellaneous rearrangements, have been summarized.

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.

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.

Cobalt-Catalyzed E-Selective Isomerization of Alkenes with a Phosphine-Amido-Oxazoline Ligand

An efficient method to access (E)-trisubstituted alkenes is reported via cobalt-catalyzed isomerization of 1,1-disubstituted alkenes using a phosphine-amido-oxazoline ligand. The reaction could also convert mono- and 1,2-disubstituted alkenes to (E)-internal alkenes with benzylic selectivity. This protocol is atom-economy and operationally simple and uses readily available starting materials with good functional tolerance. This catalytic system could be scaled up to gram scale smoothly with a catalyst loading of 0.1 mol %.

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.

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.

Iodine-Mediated Oxidative Rearrangement of α,β-Unsaturated Diaryl Ketones: A Facile Access to 1,2-Diaryl Diketones

A metal-free oxidative rearrangement was explored for the synthesis of 1,2-diaryl diketones by utilizing α,β-unsaturated diaryl ketones and I2/TBHP in good to high yields. The reaction proceeds via oxidative aryl migration, followed by C-C bond cleavage. A simple and high-yielding protocol was developed for the synthesis of a wide range of 1,2-diaryl diketones, which are the backbone for a variety of medicinally important molecules.

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.

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.

Synthesis of spirocyclic scaffolds using hypervalent iodine reagents

Hypervalent iodine reagents have been developed as highly valuable reagents in synthetic organic chemistry during the past few decades. These reagents have been identified as key replacements of various toxic heavy metals in organic synthesis. Various synthetically and biologically important scaffolds have been developed using hypervalent iodine reagents either in stoichiometric or catalytic amounts. In addition, hypervalent iodine reagents have been employed for the synthesis of spirocyclic scaffolds via dearomatization processes. In this review, various approaches for the synthesis of spirocyclic scaffolds using hypervalent iodine reagents are covered including their stereoselective synthesis. Additionally, the applications of these reagents in natural product synthesis are also covered.

A survey of chiral hypervalent iodine reagents in asymmetric synthesis

The recent years have witnessed a remarkable growth in the area of chiral hypervalent iodine chemistry. These environmentally friendly, mild and economic reagents have been used in catalytic or stoichiometric amounts as an alternative to transition metals for delivering enantioenriched molecules. Varieties of different chiral reagents and their use for demanding asymmetric transformations have been documented over the last 25 years. This review highlights the contribution of different chiral hypervalent iodine reagents in diverse asymmetric conversions.

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.