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We have found that the combination of Oxone® with an organoiodine compound, i.e., 2-iodobenzoic acid (2-IB), selectively yields p-quinones from monomethoxyarenes under mild conditions. In this reaction system, an organoiodine compound is immediately oxidized by Oxone® to generate cyclic hypervalent iodine (III) species in situ, which serves as the specific mediator for the selective p-quinone synthesis, preventing o-quinone formation.

An efficient protocol for C–H condensation of hypervalent iodine compounds toward arenes in fluoroalcohols has been applied to the recyclable preparation of mesityl iodonium(III) salts. The electrophilicities of [hydroxy(tosyloxy)iodo]mesitylene (MesI(OH)OTs) and iodomesitylene diacetate (MesI(OAc)2) are suitably enhanced in 2,2,2-trifluoroethanol. A series of nucleophilic aromatic compounds react smoothly with MesI(OH)OTs and MesI(OAc)2 or in situ hypervalent iodine(III) species, generated from iodomesitylene, to provide the target mesityl iodonium(III) salts in good yields at room temperature with broad functional group tolerance. This C–H condensation strategy merits high para-regioselectivities during the diaryliodonium(III) salt formation, but the major limitation in the case of low-reactive aromatic substrates is byproduct formation resulting from the self-condensation of the nucleophilic mesitylene ring in MesI(OH)OTs and MesI(OAc)2.

It was determined that diaryliodonium(III) triflates bearing a trimethoxybenzene (TMP) auxiliary are more reactive than the reported selective aryl-transfer iodonium salts in the N-arylation of benzimidazoles and other types of azole compounds under catalytic conditions. The TMP-iodonium(III) salts can thus effectively facilitate the reaction at 50 °C or below, producing the corresponding N-arylated biaryls without the formation of TMP-derived coupling byproducts. Utilization of this TMP reagent under mild conditions would prevent the underlying problem of participation of the auxiliary group in the coupling reactions, which is observed while using the iodonium(III) salts that require elevated temperatures.

Quinone monoacetals (QMAs) were found to be convenient substrates for a unique arylation reaction of enol silyl ethers. This arylation process of QMAs typically proceeds through the acetal activation of the QMAs by a hydrogen-bond donor solvent, such as a fluoroalcohol, for the initiating step. The silyl transfer from silyl enol ethers to the carbonyl oxygen of the QMAs appears to be involved in the C–C coupling step, followed by QMA aromatization. By this method, valuable α-aryl carbonyl compounds containing o-phenol moieties can be obtained directly under mild conditions without lactone formation.

The excellent oxidizing behavior of the μ-oxo-bridged phenyliodine trifluoroacetate 1 is revealed during the phenolic oxidations mediated by hypervalent iodine(III) reagents. The use of the μ-oxo-bridged compound 1 instead of PhI(OAc) 2 (PIDA) and PhI(OCOCF 3 ) 2 (PIFA) during the oxidative cyclization of phenols involving carbon–oxygen, carbon–nitrogen, and carbon–carbon bond formations could produce spirocyclized cyclohexadienones in comparable or somewhat better yields. Thus, we have concluded that the unique reagent 1 is a promising alternative to PIDA and PIFA, and the use of reagent 1 as a reasonable choice is recommended for the hypervalent iodine(III)-mediated phenolic oxidations as well as other transformations.

We first introduced 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) and 2,2,2-trifluoroethanol (TFE) as unique solvents for reactions involving hypervalent iodine-mediated phenolic oxidations in the 1980s, in which the fluoroalcohols have been successfully utilized as stabilizing solvents of the reactive cationic intermediates, generated in situ by the action of phenyliodine(III) diacetate (PIDA) and phenyliodine bis(trifluoroacetate) (PIFA). This pioneering study produced a breakthrough in hypervalent iodine chemistry, and many synthetic applications that enable a variety of transformations have appeared utilizing this unique medium. For example, the single-electron-transfer (SET) oxidation ability of PIFA toward phenyl ethers has been discovered for the first time in HFIP and TFE, taking advantage of the unique acid-like behaviors to stabilize the aromatic cation radicals. More recently, the catalytic strategy of hypervalent iodine reagents has found extensive applications with the aid of the fluoroalcohols to produce highly reactive catalytic species under mild conditions. The fluoroalcohols are now widely used as versatile solvents not only in hypervalent iodine chemistry, but also in other organic syntheses. This manuscript for the Special Issue deals with the background of the hypervalent iodine chemistry and a new topic concerning the utility of fluoroalcohols for the synthesis of diaryliodonium(III) salts.

In 1994, we first determined the single-electron-transfer (SET) oxidation ability of phenyliodine(III) bis(trifluoroacetate) (PIFA) toward phenyl ethers, affording the corresponding aromatic cation radicals. Since then, hypervalent iodine(III) has been utilized as a selective and efficient SET oxidizing agent that enables a variety of direct C–H functionalizations of aromatic rings in electron-rich arenes under mild conditions. We have now extended the original method to work in a series of heteroaromatic compounds such as thiophenes, pyrroles, and indoles. The investigations and results obtained since the start of this century are summarized in this article.

We have found that the use of the PhI(OAc)2/Et4N+Br− combination in water can significantly enhance its oxidation ability and oxidize a wide range of alcohols 1 to carbonyl compounds 2 in good to excellent yields. This clean aqueous oxidation method shows no detectable racemization processes, and even an enolizable ketone 2m could be obtained in an optically pure form from the corresponding chiral alcohol 1m. Utilization of the recyclable reagent 3 as a more practical alternative to PhI(OAc)2 is also successful in these reactions.