Para-Selective Cu-Catalyzed C–H Aryloxylation of Electron-Rich Arenes and Heteroarenes

(Indol-3-yl)(DMIX)Iodonium Salts: Novel Electrophilic Indole Reagents

A new umpolung approach to the C3-H functionalization of indoles with diverse nucleophiles based on the intermediate formation of I(III) reagents is described. The 3,5-dimethylisoxazol-4-yl auxiliary allows for selective indole transfer under catalyst-free conditions, which was impossible using previously reported reagents. Combining the mildness of transition-metal-free conditions and the high reactivity of hypervalent iodine reagents, this protocol tolerates various functional groups and provides access to indoles that are difficult to prepare conventionally.

Diaryliodonium(III) Salts in One-Pot Double Functionalization of C–IIII and ortho C–H Bonds

Since the 1950s, diaryliodonium(III) salts have been demonstrated to participate in various arylation reactions, forming aryl–heteroatom and aryl–carbon bonds. Incorporating the arylation step into sequential transformations would provide access to...

Heteroaryliodonium(III) Salts as Highly Reactive Electrophiles

In recent years, the chemistry of heteroaryliodonium(III) salts has undergone significant developments. Heteroaryliodonium(III) salts have been found to be useful synthetic tools for the transfer of heteroaryl groups under metal-catalyzed and metal-free conditions for the preparation of functionalized heteroarene-containing compounds. Synthetic transformations mediated by these heteroaryliodonium(III) salts are classified into two categories: (1) reactions utilizing the high reactivity of the hypervalent iodine(III) species, and (2) reactions based on unique and new reactivities not observed in other types of conventional diaryliodonium salts. The latter feature is of particular interest and so has been intensively investigated in recent decades. This mini-review therefore aims to summarize the recent synthetic applications of heteroaryliodonium(III) salts as highly reactive electrophiles.

Site-selective aromatic C-H λ3-iodanation with a cyclic iodine(iii) electrophile in solution and solid phases

An efficient and site-selective aromatic C-H λ³-iodanation reaction is achieved using benziodoxole triflate (BXT) as an electrophile under room temperature conditions. The reaction tolerates a variety of electron-rich arenes and heteroarenes to afford the corresponding arylbenziodoxoles in moderate to good yields. The reaction can also be performed mechanochemically by grinding a mixture of solid arenes and BXT under solvent-free conditions. The arylbenziodoxoles can be used for various C-C and C-heteroatom bond formations, and are also amenable to further modification by electrophilic halogenation. DFT calculations suggested that the present reaction proceeds via a concerted λ³-iodanation-deprotonation transition state, where the triflate anion acts as an internal base.

Connecting remote C-H bond functionalization and decarboxylative coupling using simple amines

Transition metal-catalyzed C–H functionalization and decarboxylative coupling are two of the most notable synthetic strategies developed in the last 30 years. Herein, we connect these two reaction pathways using bases and a simple Pd-based catalyst system to promote a para-selective C–H functionalization reaction from benzylic electrophiles. Experimental and computational mechanistic studies suggest a pathway involving an uncommon Pd-catalyzed dearomatization of the benzyl moiety followed by a base-enabled rearomatization through a formal 1,5-hydrogen migration. This reaction complements “C–H activation” strategies that convert inert C–H bonds into C–metal bonds prior to C–C bond formation. Instead, this reaction exploits an inverted sequence and promotes C–C bond formation prior to deprotonation. These studies provide an opportunity to develop general para-selective C–H functionalization reactions from benzylic electrophiles and show how new reactive modalities may be accessed with careful control of reaction conditions.

Diaryliodonium Salt-Mediated Intramolecular C-N Bond Formation Using Boron-Masking N-Hydroxyamides

Intramolecular aromatic C-N bond formation reactions using electron-rich aromatic tethered boron-masking N-hydroxyamide as substrate were realized. These new C-N bond formation reactions involve the in situ generation of a diaryliodonium salt by treatment with hypervalent iodine, deborylation by base treatment, spontaneous N → O acyl migration, cyclization, reductive elimination, elimination of benzoic acid, and tautomerization to indole formation. Hereby, we obtained highly functionalized electron-rich indoles and quinoline in practical yields.

Cu(I)–PNF, an organic-based nanocatalyst, catalyzed C–O and C–S cross-coupling reactions

Peptide nanofiber has been prepared via a self-assembly protocol and decorated with Cu(I) to prepare a nanostructural catalyst. The catalytic activity of this prepared nanomaterial (Cu(I)–PNF) was examined in C–O and C–S cross-coupling reactions. Compared with conventional copper–ligand catalytic systems, CuNP–PNF has unique advantages such as water solubility, high efficiency, and low cost, which makes it a highly efficient and beneficial catalyst to reuse in cross-coupling reactions.

Reaching the south: metal-catalyzed transformation of the aromatic para-position

Regioselective functionalization of aromatic arenes has created a rapid insurgence in the modern era of organic chemistry. While the last few years have witnessed significant developments on site-selective ortho- and meta-C-H transformations, there existed very few reports on para-C-H functionalization. Recent advancements on template assisted protocols in para-C-H activation has emerged as a popular and convenient feat in this area. This review highlights the various protocols developed over the years for selective installation of suitable functional groups at the para-position of arenes thereby transforming them into value-added organic cores.

One-Pot Synthesis of Unsymmetric Diaryliodonium Salts from Iodine and Arenes

The first synthesis of unsymmetric diaryliodonium salts directly from iodine and arenes is presented. The methodology provides diaryliodonium salts with the trimethoxyphenyl (TMP) moiety as dummy group. The protocol avoids the customary use of iodoarenes, which can be both expensive and toxic. Excess reagents are not required, and the reactions are performed under mild conditions. O-Arylations with these TMP salts were demonstrated to be highly chemoselective.

Cu-Mediated C-H 18F-Fluorination of Electron-Rich (Hetero)arenes

This communication describes a method for the nucleophilic radiofluorination of electron-rich arenes. The reaction involves the initial C(sp2)-H functionalization of an electron-rich arene with MesI(OH)OTs to form a (mesityl)(aryl)iodonium salt. This salt is then used in situ in a Cu-mediated radiofluorination with [18F]KF. This approach leverages the stability and availability of electron-rich arene starting materials to enable mild late-stage radiofluorination of toluene, anisole, aniline, pyrrole, and thiophene derivatives. The radiofluorination has been automated to access a 41 mCi dose of an 18F-labeled nimesulide derivative in high (2800 ± 700 Ci/mmol) specific activity.

Charge-Transfer-Induced para-Selective sp2 C-H Bond Activation of Arenes by Use of a Hypervalent Iodine Compound: A Theoretical Study

The reaction mechanism of the C-H bond activation of toluene promoted by the hypervalent iodine compound TIPP-I(OH)OTs was investigated in detail by density functional theory calculations. Our calculations show that a plausible reaction pathway of the C-H bond activation of toluene contains two stages: (1) the ligand exchange process on TIPP-I(OH)OTs, involving the substitution of the hydroxyl group and tosyloxyl group with TfOH, and (2) the C-H bond activation of toluene promoted by the hypervalent iodine center with the assistance of the triflate anion. The second step is the rate-limiting step with a relatively low free energy barrier of 19.6 kcal mol^-1 in acetonitrile, which is in accord with the experimental fact that the reaction takes place at room temperature. Frontier molecular orbits and natural population analysis show that partial electron transfer from the toluene to the hypervalent iodine moiety occurs in the charge-transfer complex, resulting in the activation of the C-H bond at the para position of toluene. Further calculations show that this hypervalent iodine compound promoted C-H bond activation reaction will be effective if the substrate is electron-rich and a strong Brønsted acid is used.

Copper-Catalyzed Regioselective C-H Sulfonyloxylation of Electron-Rich Arenes with p-Toluenesulfonic Acid and Sulfonyloxylation of Aryl(mesityl)iodonium Sulfonates

Copper-catalyzed regioselective C-H sulfonyloxylation of electron-rich arenes with p-toluenesulfonic acid has been developed. Electron-rich benzene derivatives and heteroarenes can undergo this C-H sulfonyloxylation reaction to generate aryl tosylates. Furthermore, sulfonyloxylation of aryl(mesityl)iodonium sulfonates has also been investigated. Both aryl(mesityl)iodonium tosylates and triflates can react smoothly to get aryl sulfonates. The formed aryl sulfonates can be converted to phenols, as well as used as good partners of cross-coupling reactions.

Synthesis of Functionalized Dihydrobenzofurans by Direct Aryl C-O Bond Formation under Mild Conditions

A method for the synthesis of dihydrobenzofurans by a direct aryl C-O bond formation is described. A mechanistic pathway for the reaction, distinct from previously described similar transformations, allows for mild reaction conditions that are expected to be compatible with functionalized substrates.