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.

Manipulating d-Band Center of Nickel by Single-Iodine-Atom Strategy for Boosted Alkaline Hydrogen Evolution Reaction

Ni-based electrocatalysts have been predicted as highly potential candidates for hydrogen evolution reaction (HER); however, their applicability is hindered by an unfavorable d-band energy level (Ed). Moreover, precise d-band structural engineering of Ni-based materials is deterred by appropriative synthesis methods and experimental characterization. Herein, we meticulously synthesize a special single-iodine-atom structure (I-Ni@C) and characterize the Ed manipulation via resonant inelastic X-ray scattering (RIXS) spectroscopy to fill this gap. The complex catalytic mechanism has been elucidated via synchrotron radiation-based multitechniques (SRMS) including X-ray absorption fine structure (XAFS), in situ synchrotron radiation-based Fourier transform infrared (SR-FTIR) spectroscopy, and near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS). In particular, RIXS is innovatively applied to reveal the precise regulation of Ni Ed of I-Ni@C. Consequently, the role of such single-iodine-atom strategy is confirmed to not only facilitate the moderate Ed of the Ni site for balancing the adsorption/desorption capacities of key intermediates but also act as a bridge to enhance the electronic interaction between Ni and the carbon shell for forming a localized polarized electric field conducive to H2O dissociation. As a result, I-Ni@C exhibits an enhanced alkaline hydrogen evolution performance with an overpotential of 78 mV at 10 mA/cm2 and superior stability, surpassing the majority of the reported Ni-based catalysts. Overall, this study has managed to successfully tailor the d-band center of materials from the SRMS perspective, which has crucial implications for nanotechnology, chemistry, catalysis, and other fields.

Oxidation of Aldehydes into Carboxylic Acids by a Mononuclear Manganese(III) Iodosylbenzene Complex through Electrophilic C-H Bond Activation

The oxidation of aldehyde is one of the fundamental reactions in the biological system. Various synthetic procedures and catalysts have been developed to convert aldehydes into corresponding carboxylic acids efficiently under ambient conditions. In this work, we report the oxidation of aldehydes by a mononuclear manganese(III) iodosylbenzene complex, [Mn^III(TBDAP)(OIPh)(OH)]²⁺ (1), with kinetic and mechanistic studies in detail. The reaction of 1 with aldehydes resulted in the formation of corresponding carboxylic acids via a pre-equilibrium state. Hammett plot and reaction rates of 1 with 1°-, 2°-, and 3°-aldehydes revealed the electrophilicity of 1 in the aldehyde oxidation. A kinetic isotope effect experiment and reactivity of 1 toward cyclohexanecarboxaldehyde (CCA) analogues indicate that the reaction of 1 with aldehyde occurs through the rate-determining C-H bond activation at the formyl group. The reaction rate of 1 with CCA is correlated to the bond dissociation energy of the formyl group plotting a linear correlation with other aliphatic C-H bonds. Density functional theory calculations found that 1 electrostatically interacts with CCA at the pre-equilibrium state in which the C-H bond activation of the formyl group is performed as the most feasible pathway. Surprisingly, the rate-determining step is characterized as hydride transfer from CCA to 1, affording an (oxo)methylium intermediate. At the fundamental level, it is revealed that the hydride transfer is composed of H atom abstraction followed by a fast electron transfer. Catalytic reactions of aldehydes by 1 are also presented with a broad substrate scope. This novel mechanistic study gives better insights into the metal oxygen chemistry and would be prominently valuable for development of transition metal catalysts.

Hydride-Transfer Reaction to a Mononuclear Manganese(III) Iodosylarene Complex

Metal iodosylarene species have received interest because of their potential oxidative power as a catalyst. We present the first example of hydride-transfer reactions to a mononuclear manganese(III) iodosylbenzene complex, [Mn^III(TBDAP)(OIPh)(OH)]²⁺ (1; TBDAP = N,N-di-tert-butyl-2,11-diaza[3.3](2,6)pyridinophane), with dihydronicotinamide adenine dinucleotide (NADH) analogues. Kinetic studies show that hydride-transfer from the NADH analogues to 1 occurs via a proton-coupled electron transfer, followed by a rapid electron transfer.

PhI(OTf)2 Does Not Exist (Yet)*

PhI(OTf)₂ has been used for the past 30 years as a strong I(III) oxidant for organic and inorganic transformations. It has been reported to be generated in situ from the reactions of either PhI(OAc)₂ or PhI=O with two equivalents of trimethylsilyl trifluoromethanesulfonate (TMS-OTf). In this report it is shown that neither of these reactions generate a solution with spectroscopic data consistent with PhI(OTf)₂ , with supporting theoretical calculations, and thus this compound should not be invoked as the species acting as the oxidant for transformations that have been associated with its use.

Hypervalent organoiodine(v) metal-organic frameworks: syntheses, thermal studies and stoichiometric oxidants

Iodinated analogues of the highly porous IRMOF-9 and UiO-67 frameworks were prepared and post-synthetically oxidised with dimethyldioxirane (DMDO). Analysis by X-ray photoelectron spectroscopy (XPS) confirmed promotion to the iodine(v) state and detailed differential scanning calorimetry-thermal gravimetric analysis (DSC-TGA) showed the hypervalent metal-organic frameworks (MOFs) undergo exothermic elimination at ∼200 °C with XPS showing hypervalency is maintained. The hypervalent MOFs are active heterogeneous reagents in sulfoxidation and alcohol oxidation reactions. The crystallinity and porosity of the MOFs were maintained following post-synthetic oxidation, thermolysis and after the heterogeneous reactions, as shown by powder X-ray diffraction (PXRD) and gas adsorption analyses. This work showcases the unique ability MOFs hold for studying chemical reactions and the potential for hypervalent organoiodine MOFs as reuseable oxidants.

Synthesis, characterization and oxidizing strength of a nano-structured hypervalent iodine(v) compound: iodylbenzene nanofibers

In this study, disproportionation of freshly synthesized iodosylbenzene in boiling distilled water in the presence of triethylene glycol monomethyl ether was used to prepare ribbon-like iodylbenzene nanostructures with a wide length distribution (250 nm up to several microns) and a width in the range of 100–200 nm.

Noncovalent Halogen Bonding as a Mechanism for Gas-Phase Clustering

Gas-phase clustering of nonionizable iodylbenzene (PhIO₂) is attributed to supramolecular halogen bonding. Electrospray ionization results in the formation of ions of proton-charged and preferably sodium-charged clusters assignable to [H(PhIO₂) _n ]⁺, n = 1-7; [Na(PhIO₂) _n ]⁺, n = 1-6; [Na₂(PhIO₂) _n ]²⁺, n = 7-20; [HNa(PhIO₂) _n ]²⁺, n = 6-19; [HNa₂(PhIO₂) _n ]³⁺, n = 15-30; and [Na₃(PhIO₂) _n ]³⁺, n = 14-30. The largest cluster detected has a supramolecular mass of 7147 Da. Electronic structure calculations using the M06-2X functional with the 6-311++G(d,p) basis set for C, H, and O, and LANL2DZ basis set for I and Na predict 298 K binding enthalpies for the protonated and sodiated iodylbenzene dimers and trimers are greater than 180 kJ/mol. This is exceptionally high in comparison with other protonated and sodiated clusters with well-established binding enthalpies. Strongly halogen-bonded motifs found in the crystalline phases of PhIO₂ and its derivatives serve as models for the structures of larger gas-phase clusters, and calculations on simple model gas-phase dimer and trimer clusters result in similar motifs. This is the first account of halogen bonding playing an extensive role in gas-phase associations. Graphical Abstract ᅟ.