Recyclable Hypervalent Iodine Reagents in Modern Organic Synthesis

Hypervalent iodine (HVI) reagents have gained much attention as versatile oxidants because of their low toxicity, mild reactivity, easy handling, and availability. Despite their unique reactivity and other advantageous properties, stoichiometric HVI reagents are associated with the disadvantage of generating non-recyclable iodoarenes as waste/co-products. To overcome these drawbacks, the syntheses and utilization of various recyclable hypervalent iodine reagents have been established in recent years. This review summarizes the development of various recyclable non-polymeric, polymer-supported, ionic-liquid-supported, and metal–organic framework (MOF)-hybridized HVI reagents.

1 Introduction

2 Polymer-Supported Hypervalent Iodine Reagents

2.1 Polymer-Supported Hypervalent Iodine(III) Reagents

2.2 Polymer-Supported Hypervalent Iodine(V) Reagents

3 Non-Polymeric Recyclable Hypervalent Iodine Reagents

3.1 Non-Polymeric Recyclable Hypervalent Iodine(III) Reagents

3.2 Recyclable Non-Polymeric Hypervalent Iodine(V) Reagents

3.3 Fluorous Hypervalent Iodine Reagents

4 Ionic-Liquid/Ion-Supported Hypervalent Iodine Reagents

5 Metal–Organic Framework (MOF)-Hybridized Hypervalent Iodine Reagents

6 Conclusion

Amphiphilic Iodine(III) Reagents for the Lipophilization of Peptides in Water

We report the functionalization of cysteine residues with lipophilic alkynes bearing a silyl group or an alkyl chain using amphiphilic ethynylbenziodoxolone reagents (EBXs). The reactions were carried out in buffer (pH 6 to 9), without organic co-solvent or removal of oxygen, either at 37 °C or room temperature. The transformation led to a significant increase of peptide lipophilicity and worked for aromatic thiols, homocysteine, cysteine, and peptides containing 4 to 18 amino acids. His6 -Cys-Ubiquitin was also alkynylated under physiological conditions. Under acidic conditions, the thioalkynes were converted into thioesters, which could be cleaved in the presence of hydroxylamine.

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.

Advances in Synthetic Applications of Hypervalent Iodine Compounds

The preparation, structure, and chemistry of hypervalent iodine compounds are reviewed with emphasis on their synthetic application. Compounds of iodine possess reactivity similar to that of transition metals, but have the advantage of environmental sustainability and efficient utilization of natural resources. These compounds are widely used in organic synthesis as selective oxidants and environmentally friendly reagents. Synthetic uses of hypervalent iodine reagents in halogenation reactions, various oxidations, rearrangements, aminations, C-C bond-forming reactions, and transition metal-catalyzed reactions are summarized and discussed. Recent discovery of hypervalent catalytic systems and recyclable reagents, and the development of new enantioselective reactions using chiral hypervalent iodine compounds represent a particularly important achievement in the field of hypervalent iodine chemistry. One of the goals of this Review is to attract the attention of the scientific community as to the benefits of using hypervalent iodine compounds as an environmentally sustainable alternative to heavy metals.

PIFA-mediated ethoxyiodination of enamides with potassium iodide

The combination of PIFA with KI in ethanol triggers the regioselective ethoxyiodination of a broad range of enamides with excellent yields and diastereoselectivities.

An alternative to the Sandmeyer approach to aryl iodides

Iodoarenes are important synthons for a wide range of organic transformations. Here we report a general strategy to prepare singly iodinated electron-rich aromatic compounds through the intermediacy of diaryliodonium salts. This process, which incorporates a phase separation that greatly simplifies product purification, is an attractive replacement for the Sandmeyer approach to iodoarenes that are otherwise difficult to access.