Transition-Metal-Free Aryl-Aryl Cross-Coupling: C-H Arylation of 2-Naphthols with Diaryliodonium Salts

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.

A Strongly Ambiphilic Ferrocene-Based Cyclic (Alkyl)(amino)carbene - Specific Decomposition to an Enamine by a 1,2-Phenyl Shift

The recently described crystalline cyclic (alkyl)(amino)carbene with a 1,1'-ferrocenylene (fc) backbone fc(CPh2-C-NMes) (A, Mes=mesityl) is highly reactive due to its particularly pronounced ambiphilicity and is thermally not stable in solution due to an intramolecular insertion of the divalent carbon atom into a methyl C-H bond of the Mes substituent. The closely related congener fc(CPh2-C-N-p-C6H4-tBu) (1) cannot undergo such an insertion reaction. Nevertheless, 1 is too short-lived for isolation due to a rapid 1,2-shift of a phenyl group, furnishing the isomeric cyclic enamine fc[C(Ph)=C(Ph)-N-p-C6H4-tBu] (1') in a specific decomposition process unprecedented for CAACs. Trapping of 1 was possible with carbon monoxide, elemental selenium and with [CuBr(SMe2)], respectively affording the aminoketene 1=C=O, the selenoamide 1=Se and the homoleptic CuI complex [Cu(1)2][CuBr2]. 1 is an even stronger ambiphile than A according to NMR spectroscopic data. Similar to A, 1 does not react with H2, because the experimentally observed intramolecular process is kinetically more favourable according to DFT results.

Electrochemical Dearomatizing Methoxylation of Phenols and Naphthols: Synthetic and Computational Studies

The electrochemical oxidative dearomatizing methoxylation of phenols and naphthols was developed. It provides an alternative route for the preparation of methoxycyclohexadienones, important and versatile synthetic intermediates, that eliminates the need for stoichiometric high-energy chemical oxidants and generates hydrogen as a sole by-product. The reaction proceeds in a simple constant current mode, in an undivided cell, and it employs standardized instrumentation. A collection of methoxycyclohexadienones derived from various 2,4,6-tri-substituted phenols and 1-substituted-2-naphthols was obtained in moderate to excellent yields. These include a complex derivative of estrone, as well as methoxylated dearomatized 1,1'-bi-2-naphthols (BINOLs). The mechanism of the reaction was subject to profound investigations using density functional theory calculations. In particular, the reactivity of two key intermediates, phenoxyl radical and phenoxenium ion, was carefully examined. The obtained results shed light on the pathway leading to the desired product and rationalize experimentally observed selectivities regarding a side benzylic methoxylation and the preference for the functionalization at the para over the ortho position. They also uncover the structure-selectivity relationship, inversely correlating the steric bulk of the substrate with its propensity to undergo the side-reaction. Moreover, the loss of stereochemical information from enantiopure BINOL substrates during the reaction is rationalized by the computations.

Csp2-H functionalization of phenols: an effective access route to valuable materials via Csp2-C bond formation

In the vast majority of top-selling pharmaceutical and industrial products, phenolic structural motifs are highly prevalent. Non-functionalized simple phenols serve as building blocks in the synthesis of value-added chemicals. It is worth mentioning that lignin, being the largest renewable biomass source of aromatic building blocks in nature, mainly consists of phenolic units, which enable the production of structurally diverse phenols. Given their remarkable applicability in the chemical value chain, many efforts have been devoted to increasing the molecular complexity of the phenolic scaffold. Among the key techniques, direct functionalization of Csp2-H is a powerful tool, enabling the construction of new Csp2-C bonds in an economical and atomic manner. Herein we present and summarize the large plethora of direct Csp2-H functionalization methods that enables scaffold diversification of simple, unprotected phenols, leading to the formation of new Csp2-C bonds. In this review article, we intend to summarize the contributions that appeared in the literature mainly in the last 5 years dealing with the functionalization of unprotected phenols, both catalytic and non-catalytic. Our goal is to highlight the key findings and the ongoing challenges in the stimulating and growing research dedicated to the development of new protocols for the valorization of phenols.

Metal-Free S-Arylation of Phosphorothioate Diesters and Related Compounds with Diaryliodonium Salts

We developed a direct metal-free S-arylation of phosphorothioate diesters using diaryliodonium salts. The method allows for the preparation under simple conditions of a broad range of S-aryl phosphorothioates, including complex molecules (e.g., dinucleotide or TADDOL derivatives), as well as other related organophosphorus compounds arylated at a chalcogen. The reaction proceeds with a full retention of the stereogenic center at the phosphorus atom, opening convenient access to P-chiral products. The mechanism of the reaction was established using DFT calculations.

Metal-free S-arylation of 5-mercaptotetrazoles and 2-mercaptopyridine with unsymmetrical diaryliodonium salts

Herein, we demonstrate the application of unsymmetrical iodonium salts towards S-arylation of heterocyclic thiols (especially tetrazole-5-thiols and pyridine-2-thiol) under metal-free conditions, affording a diverse range of di(hetero)aryl thioethers in moderate to good yields. A detailed study on the effects of counter-anions and the auxiliary of iodonium salts was conducted. Suitable auxiliary selection of the unsymmetrical iodonium salt offers flexibility for a wide range of aryl moieties and its incorporation into S-arylation. The DFT study supports the experimental observations of chemoselective arylation.

Visible-light-driven C(sp2)-H arylation of phenols with arylbromides enabled by electron donor-acceptor excitation

We have developed a catalyst-free visible-light-driven C(sp²)-H arylation of unprotected phenols with arylbromides to give 2-arylated phenols. This reaction proceeds through the excitation of an electron donor-acceptor complex between a phenolate and an arylbromide, electron transfer, and debrominative C(sp²)-C(sp²) coupling.

Ru-Catalyzed Selective Catalytic Methylation and Methylenation Reaction Employing Methanol as the C1 Source

Methanol can be employed as a green and sustainable methylating agent to form C-C and C-N bonds via borrowing hydrogen (BH) methodology. Herein we explored the activity of the acridine-derived SNS-Ru pincer for the activation of methanol to apply it as a C1 building block in different reactions. Our catalytic system shows great success toward the β-C(sp³)-methylation reaction of 2-phenylethanols to provide good to excellent yields of the methylated products. We investigated the mechanistic details, kinetic progress, and temperature-dependent product distribution, which revealed the slow and steady generation of in situ formed aldehyde, is the key factor to get the higher yield of the β-methylated product. To establish the environmental benefit of this reaction, green chemistry metrics are calculated. Furthermore, dimerization of 2-naphthol via methylene linkage and formation of N-methylation of amine are also described in this study, which offers a wide range of substrate scope with a good to excellent yield.

Copper-Catalyzed Synthesis of Axially Chiral Biaryls with Diaryliodonium Salts as Arylation Reagents

NOBIN and BINAM derivatives harboring biaryl frameworks are recognized as a class of important atropisomers with versatile applications. Here, we present an efficient synthetic route to access such compounds through copper-catalyzed domino arylation of N-arylhydroxylamines or N-arylhydrazines with diaryliodonium salts and [3,3]-sigmatropic rearrangement. This reaction features mild conditions, good substrate compatibility, and excellent efficiency. The practicality of this protocol was further extended by the synthesis of biaryl amino alcohols.

Benzene-Free Synthesis of Multisubstituted Catechol via Oxidative Dearomatic Reorganization

A benzene-free synthesis of multisubstituted catechol via an oxidative dearomatic reorganization is reported. This reaction tolerated a wide spectrum of functionalities, which could be applied in the synthesis of an electron-deficient arene-conjugated catechol that is difficult to access via biomimetic oxidative coupling. In addition, a diversification-oriented transformation that leveraged the versatile catechol afforded a series of functionality-rich products.

Total Synthesis of Mallotusinin

The total synthesis of mallotusinin, which bears a tetrahydroxydibenzofuranoyl (THDBF) bridge between the 2-oxygen and 4-oxygen of glucose on corilagin with a 3,6-O-(R)-hexahydroxydiphenoyl (HHDP) bridge, is described. The key features of the total synthesis are: 1) improvements of our previously reported method to synthesize corilagin; 2) establishment of the THDBF skeleton via an unusual intramolecular S_N Ar reaction of an HHDP analogue, and 3) the application of a two-step bislactonization strategy for a HHDP bridge construction into the 2,4-O-THDBF bridge. Oxidative phenol coupling of 1,2,4-orthoacetyl-3,6-di-(4-O-benzylgalloyl)-α-d-glucopyranose and the orthoester cleavage of the coupling product without the pyranose-furanose ring transformation are key reactions for the improved synthesis of corilagin, which enabled the adequate supply of a corilagin precursor that was required to develop the mallotusinin synthesis. These established methods are expected to help develop the synthesis of other ellagitannins with a bridge between the two oxygens of corilagin.

The Diaryliodonium(III) Salts Reaction With Free-Radicals Enables One-Pot Double Arylation of Naphthols

The chemoselective reaction of the C- followed by the O-centered naphthyl radicals with the more electron-deficient hypervalent bond of the diaryliodonium(III) salts is described. This discovered reactivity constitutes a new activation mode of the diaryliodonium(III) salts which enabled a one-pot doubly arylation of naphthols through the sequentialC s p 2 -C s p 2 /O-C s p 2 bond formation. The naphthyl radicals were generated in the reaction by the tetramethylpiperidinyl radical (TMP·) which resulted from the homolytic fragmentation of the precursor TMP2O. Experimental and DFT calculations provided a complete panorama of the reaction mechanism.

Aryl radical-induced desulfonylative ipso-substitution of diaryliodonium salts: an efficient route to sterically hindered biarylamines

By using vicinal aryl sulfonamide substituted diaryliodonium salts, a cascade of desulfonylation/aryl migration was promoted by triethylamine in the synthesis of sterically hindered biarylamines, which operated via a radical-induced reaction pathway. The products were readily converted into a variety of important synthons. Furthermore, coupling reactions of N-methyl biarylamine and 1,6-dibromopyrene provided a potentially attractive molecule in OLEDs.

K2S2O8-HFIP synergistically promoted para-selective sp3 C-H bond diarylation of glycine esters

A metal-free K₂S₂O₈-HFIP synergistically promoted double Friedel-Crafts alkylation between a glycine derivative and N-substituted aniline was developed to efficiently synthesize diarylmethane derivatives with high para-selectivity. The reaction proceeded smoothly in the absence of any metal and ligand, and exhibited a good tolerance of functional groups.

Aqueous synthesis of Mn-doped CuInSe2 quantum dots to enhance the performance of quantum dot sensitized solar cells

Mn-Doped QDs extended light absorption by altering the bandgap and facilitated rapid electron injection and charge separation, which together result in enhanced overall power conversion efficiency (PCE).