Polar effects in reactions of carbon-centered radicals with diazonium salts: free-radical diazo coupling

A Halogen-Atom Transfer (XAT)-Based Approach to Indole Synthesis Using Aryl Diazonium Salts and Alkyl Iodides

Indoles are among the most important N-heterocycles in pharmaceuticals. Here, we present an alternative to the classic Fischer indole synthesis based on the radical coupling between aryl diazoniums and alkyl iodides. This iron-mediated strategy features a double role for the aryl diazoniums that sequentially activate the alkyl iodides through halogen-atom transfer and then serve as radical acceptors. The process operates under mild conditions and enables the preparation of densely functionalized indoles.

Synthetic Route to Phenyl Diazenes and Pyridazinium Salts from Phenylazosulfonates

The synthesis of pyridazinium salts was achieved from readily available phenylazosulfonates in a single reaction step. The reaction proceeds via the formation of short-lived phenyldiazenes, which-owing to the strongly acidic conditions-are partially protonated. The phenyldiazenes then undergo a rapid cycloaddition to furans to give pyridazinium salts via elimination of water. The fact that the pyridazinium synthesis shows a low sensitivity toward oxygen, although phenyldiazenes occur as intermediates, can be explained by the very fast cycloaddition step and the partial protonation of the phenyldiazene.

Visible Light Promoted, Catalyst-Free Radical Carbohydroxylation and Carboetherification under Mild Biomimetic Conditions

Metal and catalyst-free carbohydroxylations and carboetherifications at room temperature have been achieved by a combination of beneficial factors including high aryl diazonium concentration and visible light irradiation. The acceleration of the reaction by visible light irradiation is particularly remarkable against the background that neither the aryldiazonium salt nor the alkene show absorptions in the respective range of wavelength. These observations point to weak charge transfer interactions between diazonium salt and alkene, which are nevertheless able to considerably influence the reaction course. As highly promising perspective, many more aryldiazonium-based radical arylations might benefit from simple light irradiation without requiring a photocatalyst or particular additive.

Radical Arylation of Triphenyl Phosphite Catalyzed by Salicylic Acid: Mechanistic Investigations and Synthetic Applications

A straightforward and scalable methodology to synthesize diphenyl arylphosphonates at 20 °C within 1-2 h is reported using inexpensive SA as the catalytic promoter of the reaction. Mechanistic investigations suggest that the reaction proceeds via radical-radical coupling, consistent with the so-called persistent radical effect. The reaction tolerated a wide range of functional groups and heteroaromatic moieties. The synthetic usefulness and the unique reactivity of the obtained phosphonates were demonstrated in different one-step transformations.

Photoredox-Catalyzed Synthesis of Alkylaryldiazenes: Formal Deformylative C-N Bond Formation with Alkyl Radicals

Diazenes are valuable compounds that have found broad applicability because of their optical and biological properties. We report the synthesis of alkylaryldiazenes via formal, photoredox-catalyzed, deformylative C-N bond formation. The procedure employs dihydropyridines for the generation of alkyl radicals, which are then trapped by diazonium salts and reduced to the corresponding diazenes. Control experiments were performed to confirm the involvement of radicals in the mechanism. The reaction can be carried out at room temperature and employs readily available reagents; the mild conditions allowed the use of highly functionalized substrates. There was no observed tautomerization of the diazenes to the corresponding arylhydrazones.

Modular Synthesis of Alkylarylazo Compounds via Iron(III)-Catalyzed Olefin Hydroamination

A novel Fe-catalyzed olefin hydroamination with aryldiazo sulfones for accessing alkylarylazo compounds has been successfully developed. Aryldiazo sulfones are used as radical acceptors, and N-N double bonds will be regenerated when an arene sulfonyl group leaves. The reaction features mild reaction conditions and a broad substrate scope, allowing access to many azo compounds that would be difficult or perhaps impossible to access using other methods.

Biaryl synthesis with arenediazonium salts: cross-coupling, CH-arylation and annulation reactions

The rich legacy of arenediazonium salts in the synthesis of unsymmetrical biaryls, built around the seminal works of Pschorr, Gomberg and Bachmann more than a century ago, continues to make important contributions at various evolutionary stages of modern biaryl synthesis. Based on in-depth mechanistic analysis and design of novel pathways and reaction conditions, the scope of biaryl synthesis with arenediazonium salts has enormously expanded in recent years through applications of transition metal/photoredox-catalysed cross-coupling, thermal/photosensitized radical chain CH-arylation of (hetero)arenes and arylative radical annulation reactions with alkynes. These recent developments have provided facile synthetic access to a wide variety of unsymmetrical biaryls of pharmaceutical, agrochemical and optoelectronic importance with green scale-up options and created opportunities for late-stage modification of peptides, nucleosides, carbon nanotubes and electrodes, the details of which are captured in this review.

Thermally Induced Carbohydroxylation of Styrenes with Aryldiazonium Salts

The radical carbohydroxylation of styrenes with aryldiazonium salts has been achieved under mild thermal conditions. A broad range of aryldiazonium salts was tolerated, and the reaction principle based on a radical-polar crossover mechanism could be extended to carboetherification as well as to a two-step, metal-free variant of the Meerwein arylation leading to stilbenes.

A family of low molecular-weight, organic catalysts for reductive C-C bond formation

Hydrazines form a new family of low molecular-weight reducing agents for diazonium salts. Using only small amounts of hydrazine catalyst, the coupling of diazonium salts to a variety of reactive partners has been achieved, without the requirement for either metal adjuvants or irradiation with visible or ultraviolet light. The generality of the concept proposed herein as well as its advantages in the preparative scale is outlined and discussed.

Visible-light-mediated α-arylation of enol acetates using aryl diazonium salts

Visible light mediates efficiently the α-arylation of enol acetates by aryl diazonium salts under mild conditions using [Ru(bpy)(3)]Cl(2) as a photoredox catalyst. The broad scope of the reaction toward various diazonium salts and enol acetates was explored. The application of this reaction in the concise synthesis of 2-substituted indoles was demonstrated.

Continuous-flow synthesis of monoarylated acetaldehydes using aryldiazonium salts

Anilines and ethyl vinyl ether can be used as precursors for a process that is the synthetic equivalent of the α-arylation of acetaldehyde enolate. The reaction manifests a high level of functional group compatibility, allowing the ready preparation of a number of synthetically valuable compounds.

An investigation of the reduction in aqueous acetonitrile of 4-methoxybenzenediazonium ion by the tetrakis(acetonitrile)Cu(i) cation catalysed by hydrogenphosphate dianion

In aqueous acetonitrile containing a phosphate buffer, 4-methoxybenzenediazonium ion is reduced by one or more of the partially aquated cations derived from tetrakis(acetonitrile)Cu(I) cation in this medium. Investigation of the reaction mechanism indicates the rate determining step to be the association of the diazonium ion with the hydrogenphosphate dianion to give an adduct which then undergoes reduction by Cu(I). The reaction gives a range of products which have been identified and quantified by GC. One of these, 4-methoxyphenol was unexpected in the reducing conditions; its presence could be explained by the disproportionation of a 4-methoxyphenylcopper(II) complex giving bis(4-methoxyphenyl)copper(III) which reacts with water to produce the phenol and an equivalent amount of methoxybenzene. A scheme is proposed which accounts for all the observed products and computer modelling gives a satisfactory description of the distributions of the five major products as functions of the relative proportions of the reactants for dilute conditions and those where the reductant is in excess. When the diazonium ion is in excess, the behaviour of the model and the experimental reactant accountability suggest the occurrence of additional reactions which give products unobserved by GC.

Oxidative and Reductive Carbodiazenylation of Nonactivated Olefins

Procedures for the carbodiazenylation of nonactivated olefins with a wide range of aryldiazonium salts have been developed. The azo compounds obtained can serve as valuable precursors for beta-arylamines (carboamination products), beta-amino acids, ketones, and various heterocyclic structures.

Reductive Carbodiazenylation of Nonactivated Olefins via Aryl Diazonium Salts

[Structure: see text] The reduction of aryl diazonium salts in the presence of nonactivated olefins provides rapid entry to carbodiazenylation products. The regioselective functionalization of double bonds is achieved in a one-pot process starting from aniline derivatives. Carboamination of olefins is possible via a two-step carbodiazenylation-hydrogenation sequence in which one aniline equivalent is recovered.

One-Pot Four-Component Reaction: Aqueous TiCl3/PhN2+-Mediated Alkyl Radical Addition to Imines Generated in Situ

Ti(III)-induced free-radical decomposition of a phenyldiazonium salt, followed by phenyl radical iodine-atom abstraction from alkyl iodides, leads to a one-pot selective alkyl radical addition to the C-atom of imines generated in situ under aqueous acidic conditions. [reaction: see text]

Direct electroreductive preparation of indolines and indoles from diazonium salts

[reaction: see text] Indolines and indoles are prepared for the first time by direct electrochemical reduction of arenediazonium salts.

Electron Transfer from a Cyclic Diaryliodine Species to Arenediazonium Salts. Evidence for the Intermediacy of 9-I-2 Structures in Iodine Atom Transfer Reactions(1)

2,2'-Diiodobiphenyl (1) reacts with arenediazonium hexafluorophosphates (ArN(2)(+)PF(6)(-)) in acetonitrile to form biphenyleneiodonium salt (4) and iodoarenes (ArI) as major products. The reaction follows a free-radical-chain mechanism and involves a cyclic diaryliodine, 9-I-2 species (3), which is trapped as the iodonium salt (4) by a single-electron transfer (SET) to the diazonium salt. The results show that diaryliodine intermediates formed by the addition of phenyl radicals to iodoarenes can have sufficient lifetimes to allow trapping by bimolecular processes.