Iodine(III) Reagents in Radical Chemistry

Photoinduced remote regioselective radical alkynylation of unactivated C-H bonds

A method for the remote regioselective alkynylation of unactivated C(sp³)-H bonds in diverse aliphatic amides by photogenerated amidyl radicals has been developed. The site-selectivity is dominated via a 1,5-hydrogen atom transfer (HAT) process of the amide. Mild reaction conditions and high regioselectivity are demonstrated in this methodology.

Diradical Generation via Relayed Proton-Coupled Electron Transfer

Diradical generation followed by radical-radical cross-coupling is a powerful synthetic tool, but its detailed mechanism has yet to be established. Herein, we proposed and confirmed a new model named relayed proton-coupled electron transfer (relayed-PCET) for diradical generation, which could open a door for new radical-radical cross-coupling reactions. Quantum mechanics calculations were performed on a selected carbene-mediated diradical cross-coupling reaction model and a designed model, and the exact electronic structural changes during the radical processes have been observed for the first time.

Hypervalent Iodine-Mediated Late-Stage Peptide and Protein Functionalization

Hypervalent iodine compounds are powerful reagents for the development of novel transformations. As they exhibit low toxicity, high functional group tolerance, and stability in biocompatible media, they have been used for the functionalization of biomolecules. Herein, we report recent advances up to June 2021 in peptide and protein modification using hypervalent iodine reagents. Their use as group transfer or oxidizing reagents is discussed in this Minireview, including methods targeting polar, aromatic, or aliphatic amino acids and peptide termini.

Hypervalent iodine promoted the synthesis of cycloheptatrienes and cyclopropanes

A new strategy is reported for intramolecular Buchner-type reactions using PIDA as a promotor. Traditionally, the Buchner reaction is achieved via Rh-carbenoids derived from Rh^II catalysts with diazo compounds. Herein, the first metal-free Buchner-type reaction to construct highly strained cycloheptatriene- and cyclopropane-fused lactams is presented. The advantage of these transformations is in their mild reaction conditions, simple operation, broad functional group compatibility and rapid synthetic protocol. In addition, scaled-up experiments and a series of follow-up synthetic procedures were performed to clarify the flexibility and practicability of this method. DFT calculations were carried out to clarify the mechanism.

A new strategy is reported for intramolecular Buchner-type reactions using PIDA as a promotor.

Cu-catalyzed [2 + 2 + 1] cascade annulation of vinyl iodonium salts with elemental sulfur/selenium for the modular synthesis of thiophenes and selenophenes

The Cu-catalyzed [2 + 2 + 1] regioselective synthesis of thiophenes/selenophenes was realized from aryl-vinyl iodonium salts and elemental sulfur/selenium.

The polymerization of carvacrol catalyzed by Mn-porphyrins: obtaining the desired product guided by the choice of solvent, oxidant, and catalyst

Less polar solvents could modulate the catalytic activity of Mn(iii)-porphyrins in carvacrol's oxidation leading to polymer/oligomer formation instead of thymoquinone formation.

Updates on hypervalent-iodine reagents in metal-free organic synthesis

Hypervalent iodine (HVI) chemistry is a rapidly growing subdomain of contemporary organic chemistry because of its enormous synthetic applications. The high nucleofugality of the phenyliodonio group (I+Ph) and their radical...

Iodonitrene: a direct metal-free electrophilic aminating reagent

Iodonitrene is a new type of reactive electrophilic aminating reagent that opens up opportunities for new discoveries.

Hypervalent iodine(iii)-mediated oxidative dearomatization of 2H-indazoles towards indazolyl indazolones

We accomplished a [bis(trifluoroacetoxy)iodo]benzene mediated oxidative dearomatization of 2H-indazoles, obtaining a new family of N-1 indazolyl indazolone derivatives in good to excellent yields through C–N and C–O bond formations.

Metal- and base-free regioselective cascade sulfonylation-cyclization of 1,5-dienes via the insertion of sulfur dioxide: access to pyrrolinones

A metal- and base-free regioselective cascade sulfonylation–cyclization reaction of linear substrates, 1,5-dienes, has been developed to synthesize sulfonylated pyrrolinones.

Unexpected ester and phosphonate radical generation by hypervalent iodine compounds for synthesizing 6-phenanthridine derivatives

An unprecedented elimination of diazomethane process of a diazo group-bearing hypervalent iodine reagent has been described.

Recent advances in the synthesis of indolizine and its derivatives by radical cyclization/cross-coupling

Indolizine is a nitrogen-containing heterocycle that has a variety of potential biological activities, and some indolizine derivatives with excellent fluorescence properties can even be used as organic fluorescent molecules for biological and material applications. Thus, many approaches for their synthesis have been developed. Among them, radical-induced synthetic approaches are receiving increasing attention owing to their unique advantages, such as efficient heterocycle construction, efficient C-C or C-X bond construction, and high atom- and step-economy. This review systematically examines the current and latest synthetic strategies using radical species or radical intermediates for synthesizing indolizines and their derivatives. This review is classified into two parts based on the type of building blocks used for indolizine ring construction and the type of radical trigger for indolizine derivative construction. We anticipate that this review will provide a deep understanding of this topic, and ultimately help researchers to develop novel approaches for the synthesis of indolizine and its derivatives.

An Expanded SET Model Associated with the Functional Hindrance Dominates the Amide-Directed Distal sp3 C-H Functionalization

The mechanistic understanding of catalytic radical reactions currently lags behind the flourishing development of new types of catalytic activation. Herein, an innovative single electron transfer (SET) model has been expanded by using the nonadiabatic crossing integrated with the rate-determining step of 1,5-hydrogen atom transfer (HAT) reaction to provide the control mechanism of radical decay dynamics through calculating excited-state relaxation paths of a paradigm example of the amide-directed distal sp³ C-H bond alkylation mediated by Ir-complex-based photocatalysts. The stability of carbon radical intermediates, the functional hindrance associated with the back SET, and the energy inversion between the reactive triplet and closed-shell ground states were verified to be key factors in improving catalytic efficiency via blocking radical inhibition. The expanded SET model associated with the dynamic behaviors and kinetic data could guide the design and manipulation of visible-light-driven inert bond activation by the utilization of photocatalysts bearing more or less electron-withdrawing groups and the comprehensive considerations of kinetic solvent effects and electron-withdrawing effects of substrates.

Stereoselective Synthesis of Cyclobutanes by Contraction of Pyrrolidines

Here we report a contractive synthesis of multisubstituted cyclobutanes containing multiple stereocenters from readily accessible pyrrolidines using iodonitrene chemistry. Mediated by a nitrogen extrusion process, the stereospecific synthesis of cyclobutanes involves a radical pathway. Unprecedented unsymmetrical spirocyclobutanes were prepared successfully, and a concise, formal synthesis of the cytotoxic natural product piperarborenine B is reported.

Cross-dehydrogenative N-N couplings

The relatively high electronegativity of nitrogen makes N-N bond forming cross-coupling reactions particularly difficult, especially in an intermolecular fashion. The challenge increases even further when considering the case of dehydrogenative N-N coupling reactions, which are advantageous in terms of step and atom economy, but introduce the problem of the oxidant in order to become thermodynamically feasible. Indeed, the oxidizing system must be designed to activate the target N-H bonds, while at the same time avoid undesired N-N homocoupling as well as C-N and C-C coupled side products. Thus, preciously few intermolecular hetero N-N cross-dehydrogenative couplings exist, in spite of the central importance of N-N bonds in organic chemistry. This review aims at analyzing these few rare cases and provides a perspective for future developments.

Ring Expansion Fluorination of Unactivated Cyclopropanes Mediated by a New Monofluoroiodane(III) Reagent

Herein, we report a new strategy for carbon-carbon bond scission and intramolecular ring expansion fluorination of unactivated cyclopropanes, which was accomplished with a new hypervalent fluoroiodane(III) reagent 1. This novel method delivers medicinally relevant 4-fully substituted fluoropiperidines in moderate to high yields with excellent regio- and diastereoselectivity. Reagent 1, which has an N-acetylbenziodazole framework, was readily synthesized via three steps in 76 % overall yield and was characterized by NMR spectroscopy and X-ray crystallography. Owing to the presence of a secondary I⋅⋅⋅O bonding interaction between the λ³ -iodane atom and the carbonyl oxygen of the acetyl group of the N-acetylbenziodazole framework, 1 has excellent stability and can be stored at ambient temperature for 6 months without any detectable decomposition. Density functional theory calculations and experimental studies showed that the reaction proceeds via a carbocation intermediate that readily combines with a fluoride ion to generate the product.

N-Cyanation of Primary and Secondary Amines with Cyanobenzio-doxolone (CBX) Reagent

An efficient electrophilic N-cyanation of amines with a stable and less-toxic cyanobenziodoxole reagent towards the synthesis of cyanamides is disclosed. This synthetically practicable strategy allows the construction of a wide variety of cyanamides under very mild and simple conditions with a broad functional group compatibility, and showcases a huge potential in late-stage modification of complex molecules.

Selective Azidooxygenation of Alkenes Enabled by Photo-induced Radical Transfer Using Aryl-λ3-azidoiodane Species

The photolytic radical-induced vicinal azidooxygenation of synthetically important and diverse functionalized substrates including unactivated alkenes is reported. The photoredox-catalyst/additive-free protocol enables intermolecular oxyazidation by simultaneously incorporating two new functionalities; C-O and C-N across the C=C double bond in a selective manner. Mechanistic investigations reveal the intermediacy of the azidyl radical facilitated via the photolysis of λ³-azidoiodane species and cascade proceeding to generate an active carbon-centered radical. The late-stage transformations of azido- and oxy-moieties were amply highlighted by assembling high-value drug analogs and bioactive skeletons.

Radical Perfluoroalkylation of Arenes via Carbanion Intermediates

The use of sodium dithionite with perfluoroalkyl iodides under basic conditions facilitates the direct perfluoroalkylation of arenes with pendant benzylic electron-withdrawing groups. This occurs via attack of the arene on the electrophilic perfluoroalkyl radical, through the donation of electron density from a benzylic anion. The substrate scope was expanded beyond benzylic nitriles with cyclic substrates bearing electron-withdrawing groups at the benzylic position-enforcing donation of electron density to the aromatic ring and enabling attack on the perfluoroalkyl radical.

Deciphering the Keys for High Enantioselectivity in Hypervalent Iodine-Catalyzed 1,2-Difunctionalization: Improved Synthesis of Ishihara-Muñiz Precatalysts

In the course of upscaling the synthesis of enantiopure aryliodine precatalysts, we detected an unreported meso form of the catalysts for the first time. A new scalable route was developed to avoid epimerization of the lactamide arms, providing syntheses of the precatalysts that are both more effective and much less time-consuming. The catalysts obtained with these synthetic procedures have been employed in some published reactions, reaching the maximum ee ever reported.

Cyclic Diaryl λ3 -Bromanes as Original Aryne Precursors

Despite the widespread application of hypervalent iodines, the corresponding λ³ -bromanes are less explored. Herein we report a general, safe, and high-yielding strategy to access cyclic diaryl λ³ -bromanes. These unique compounds feature reactivity that is appealing and complementary to that of λ³ -iodanes, generating arynes under mild reaction conditions and in the presence of a weak base. Accordingly, formal meta-selective transition-metal-free C-O and C-N couplings may be achieved. Mechanistic studies unambiguously support the aryne generation mechanism.

Reactions promoted by hypervalent iodine reagents and boron Lewis acids

Understanding the role of boranes in hypervalent iodine chemistry will open up new reactivities which can be utilised in organic synthesis. Due to similar reactivities, λ3-iodanes have presented themselves as viable alternatives for many transformations dominated by transition metals whilst mitigating some of the associated drawbacks of metal systems. As showcased by recent reports, boranes can adopt a dual role in hypervalent iodine chemistry that surpasses mere activation of the hypervalent iodine reagent. Increased efforts to harness this potential with diverse boranes will uncover exciting reactivity with high applicability across various disciplines including adoption in the pharmaceutical sciences. This review will be relevant to the wider synthetic community including organic, inorganic, materials, and medicinal chemists due to the versatility of hypervalent iodine chemistry especially in combination with borane activation or participation. We aim to highlight the development of hypervalent iodine compounds including their structure, bonding, synthesis and utility in metal-free organic synthesis in combination with Lewis acidic boranes.

N-Heterocyclic Carbene-Catalyzed Cyclization of Aldehydes with α-Diazo Iodonium Triflate: Facile Access to 2,5-Disubstituted 1,3,4-Oxadiazoles

Herein, we report a novel organocatalytic process for synthesis of complex 1,3,4-oxadiazoles from readily accessible aldehydes. By exploiting the nucleophilicity of the putative Breslow intermediate and the inherent electrophilicity of α-diazo iodonium triflate, we have found that N-heterocyclic carbene catalyst promotes efficient cyclization of various aldehydes and α-diazo iodonium triflates. The reaction proceeds under mild conditions with a wide range of functional group tolerance. The heterocyclic products can be readily further functionalized, rendering the protocol highly valuable.

Cross-Dehydrogenative N-N Coupling of Aromatic and Aliphatic Methoxyamides with Benzotriazoles

Nitrogen-nitrogen bond containing motifs are ubiquitous in bioactive compounds and organic materials. However, intermolecular hetero-selective N-H/N-H oxidative coupling reactions remain very challenging and largely unexplored. Here, we report an unprecedented, simple and hetero-selective cross-dehydrogenative N-N coupling of amides and benzotriazoles, utilizing only a hypervalent iodine species as the terminal oxidant. The scope and mechanistic investigations are discussed.

PhI(OAc)2-Mediated Dihalogenative Cyclization of 1,6-Enyne with Lithium Halide

The dihalogenative cyclization of 1,6-enyne with the assistance of PhI(OAc)₂ and lithium halide is presented. A plausible radical mechanism is proposed, which consists of addition of halogen radical to alkene, 5-exo-dig radical cyclization of enyne and halogenation via radical coupling. The alkenyl- and alkyl-halide groups in the resulted pyrrolidine products have been demonstrated to be facile handles for further transformations.

A benzene-bridged divanadium complex-early transition metal catalyst for alkene alkylarylation with PhI(O2CR)2via decarboxylation

The synthesis, structure and catalytic activity of a benzene-bridged divanadium complex were comprehensively studied. The reduction of (Nacnac)VCl2 (1) (Nacnac = (2,6-iPr2C6H3NCMe)2HC) supported by β-diketiminate with potassium graphite (KC8) by employing benzene as the solvent allows access to the benzene-bridged inverted-sandwich divanadium complex (μ-η6:η6-C6H6)[V(Nacnac)]2 (2a), which can catalyze alkene alkylarylation with hypervalent iodine(iii) reagents (HIRs) via decarboxylation to generate regioselectively diverse indolinones. Furthermore, the mild nature of this reaction was amenable to a wide range of functionalities on alkenes and HIRs. Mechanistic studies revealed a relay sequence of decarboxylative radical alkylation/radical arylation/oxidative re-aromatization.

Contemporary methods for generation of aryl radicals

The synthetic utility of aryl radicals has been established in the last century, however, their broad applications were hampered by ineffective generation methods. It was in the last decade, that a rapid development of various redox systems took place, thus triggering a renaissance of aryl radical chemistry. This tutorial review focuses on the start-of-the-art methods for generation of aryl radicals. Primarily, various light-induced systems, including photoredox catalysis, visible light transition metal catalysis, and chemistry of electron donor-acceptor complexes, are reviewed. The main current precursors of aryl radicals are evaluated together with the selected examples of their modern applications.

Synthesis, Characterization of Spirocyclic λ3 -Iodanes and Their Application to Prepare 4,1-Benzoxazepine-2,5-diones and 1,3-Diynes

Herein, a [3+2] cycloaddition of aza-oxyallylic cations with ethynylbenziodoxolones for synthesis of new λ³ -iodanes containing spirocyclic 4-oxazolidinone has been developed. This cyclic λ³ -iodanes display stability in air and excellent solubility in organic solvent. Using them as substrate, both the 4,1-benzoxazepine-2,5-diones and symmetrical 1,3-diynes derivatives were afforded in high yield under copper(I)-catalyzed conditions.