Arene diazonium salts

Yildirir and Okay (1991) recently described the mercapto-de-diazoniation of arene-diazonium salts by thioglycolic acid (HSCH2COOH) forming arylthioglycolic acids (Ar — SCH2COOH). 

One of the most readily available easily oxidized sulfides is tetrathiafulva-lene (TTF) and this material readily transfers electrons to good electrophiles such as arene diazonium salts. This reaction has been used to form the tricyclic precursor to the natural product aspidospermidine, stereospe-cifically (Scheme 1.16). Since water is very inert towards attack by free... 

Triazenes are disguised diazonium ions which can be released under very mild acidic conditions. Inspired by the use of triazenes in natural product synthesis by Nicolaou et al. [127] and the pioneering work of Moore et al. [128, 129] and Tour et al. [130] in the synthesis of triazenes on a solid support and the final detachment to give iodoarenes, a whole set of triazene-based linkers has been developed (Tab. 3.10) [131]. The arene diazonium salts generated from the triazene linkers offer diverse opportunities for multifunctional cleavage. Two linkers based on tria-... 

The mechanism by which a nucleophile displaces the diazonium group depends on the nucleophile. While some displacements involve phenyl cations, others involve radicals. Nucleophiles, e.g. CN , Cl and Br , replace the diazonium group if the appropriate cuprous salt is added to the solution containing the arene diazonium salt. The reaction of an arene diazonium salt with cuprous salt is known as a Sandmeyer reaction. 

The highly nucleophilic 3,4-dimethoxyfuran, when treated with arene diazonium salts in aqueous pyridine, undergoes 1,4-addition to afford pyridinium salts (51). The pyridine is easily displaced by nucleophiles including alcohols and phenols, thereby affording 5-alkoxy or 5-phenoxy derivatives of 3,4-dimethoxyfuran-2(5H)-one (52 Scheme 19) (78HCA1033). 

In 1990, Porta and co-workers [6] reported the first nucleophilic radical addition to aldimines promoted by aqueous TiCl3, based on a one-pot tricomponent reaction involving an aromatic amine, a generic aldehyde, and an arene-diazonium salt (Equation 14.15). 

Titanium plays a key role in this process owing to its reducing power in the Ti(III) state, it acts as radical initiator generating Ar- by decomposition of arene-diazonium salts (Equation 14.16) owing to its Lewis acid character in the Ti(IV) state, it promotes the formation of the inline [29] from a generic aldehyde and a primary aromatic amine in the aqueous medium (Equation 14.17) and increases... 

Scheme 2. Proposed mechanism forthe Heck reaaion with arene diazonium salts based on ESI(+)-MS data, nv zvalues are given for observed cationic species. Modified from reference (17).

More recently, cationic intermediates have been observed in the Heck reactions of arene diazonium salts catalyzed by triolefinic macrocycle Pd(0) complexes [17,59], o-iodophenols and enoates to form new lactones [60], and o-iodophenols with olefins (the oxa-Heck reaction) [61 ]. In the first case ions were formed by oxidation of the analyte at the capillary, or by association of [NH4] or Na". In the two other cases ionization occurred through the more typical loss of a halide ligand. The oxa-Heck reaction provides a good example of how these experiments are typically performed and the type of information that can be obtained. The oxyarylations of olefins were performed in acetone, catalyzed by palladium, and required the presence of sodium carbonate as base. Samples from the reaction mixtures were diluted with acetonitrile and analyzed by ESI(+)-MS. Loss of iodide after oxidative addition of o-iodophenol to palladium afforded positively-charged intermediates. Species consistent with oxidative addition, such as [Pd(PPh3)2(C6H50)], and the formation of palladacycles of the type seen in Scheme 8 were observed. Based on this, a mechanism for the reaction was proposed (Scheme 8). 

Scheme 9. Proposed mechanism for the Suzuki cross-coupling reaction of arene diazonium salts with potassium organotrifluoroborates based on ESl-MS investigation, m/z vaiues are given for cationic intermediates observed by ESl(+)-MS.

You met diazoniumsa Xs in Chapter 23. Arene diazonium salts are stable compounds, but alkyl diazonium salts, which are formed by protonation of diazo compounds, are not. They decompose rapidlyto carbocations— thiswas how the carboxylic acid got methylated at the beginning of... 

Primary arylamines rea t with nitrous acid, HNOg, to yield stable arene-diazonium salts, Ar-NsNX". This diazotization reaction is compatible with the presence of a wide variety of substituents on the aromatic ring. 

Azo-coupling reactions of aromatic compounds with arene diazonium salts, which are usually very fast, have been successfully carried out in... 

Mechanistic studies on systems related to commercial photoinitiators for polymerization include work on arene diazonium salts,sensitized photolysis of iodonium salts,and the use of excited states of ketones as electron donors in iodonium salt systems employed as photoinitiators. 

In Chapter 16, we will look at the reactions of substituted benzenes. First we will study reactions that change the nature of the substituent on the benzene ring and we will see how the nature of the substituent affects both the reactivity of the ring and the placement of any incoming substituent. Then we will look at three types of reactions that can be used to synthesize substituted benzenes other than those discussed in Chapter 15— reactions of arene diazonium salts, nucleophilic aromatic substitution reactions, and reactions that involve benzyne intermediates. You will then have the opportunity to design syntheses of compounds that contain benzene rings. 

We have seen that primary arylamines react with nitrous acid to form stable arene-diazonium salts (Section 16.12). Arenediazonium salts are useful to synthetic chemists because the diazonium group can be replaced by a wide variety of nucleophiles. This reaction allows a wider variety of substituted benzenes to be prepared than can be prepared solely from electrophilic aromatic substitution reactions. 

Pyrroles react with arene diazonium salts to give azo compounds, e.g. ... 

Still, there are base-free Mizoroki-Heck reactions, which are known to occur with arene diazonium salts. In this particular case, the absence of base is advantageous, as spontaneous decomposition of diazonium salts is known to be accelerated by nucleophiles and bases. High precatalyst loadings are required, as, for example, in the arylation of disubstited acrylates 10 (9 11, Scheme 2.2) [10] a base-free reaction might only be sustained for a few turnovers. 

When benzoin derivatives are irradiated in the presence of arene diazonium salts or aryl iodonium salts cationic species are formed. These species initiate ring-opening reactions of glycidyl ether in the presence of ketones or aldehydes. The method, however, also forms dioxolane and formation of the dioxolane predominates over homopolymerization. ... 

When Pschorr reported more than a century ago on the first intramolecular homolytic aromatic substitution [25], he showed that biaryls could be readily prepared by intramolecular homolytic aromatic substitution using reactive aryl radicals and arenes as radical acceptors. The aryl radicals were generated by treatment of arene-diazonium salts with copper(l) ions. Today, this reaction and related processes are referred to as Pschorr reactions. It was later found that radical biaryl synthesis could be conducted without copper salts by photochemical or thermal generation of the aryl radical from the corresponding diazonium salt [26], Moreover, the reduction of aryl diazonium salts offers another route to generate reactive aryl radicals. Hence, electrochemistry [27], titanium(lll) ions [28], Fe(II)-salts [29], tet-rathiafulvalene [30] and iodide [31] have each been used successfully for the reduction of diazonium salts to generate the corresponding aryl radicals [32]. As an example, the iodide-induced cycUzation of diazonium salt 6 to phenanthrene derivative 7 is presented in Scheme 13.3 [31]. For further information on the... 

In palladium-catalyzed carbonylations, aryl triflates are used regularly as substrates [145-151], while arene diazonium salts [152-156] and diaryl iodonium salts [157-162] are less commonly applied. 

Sabino, A.A., Machado, A.H.L., Correia, C.R.D., Eberlin, M.N. (2004) Probing the Mechanism of the Heck Reaction with Arene Diazonium Salts by Electrospray Mass and Tandem Mass Spectrometry. Angew. Chem. Int. Ed. 43 2514-2518. 

Arene diazonium salts are relatively stable in aqueous solutions below about 10 °C. At higher temperatures, or if allowed to dry, they may decompose explosively. 

Purpose, In this experiment you will learn the process of generating arene-diazonium salts in solution. The arenediazonium salt generated will be used... 

Gilman reaction dimethyl cuprate undergoes C-C bond coupling with methyliodide in the Gas Phase the preferred mechanism for Cu involves the formation of a T-shaped Cu transition state. Heck reaction key ionic intermediates of the Heck reaction of arene diazonium salts were intercepted and structurally characterized. Baylis-Hillman reaction protonated intermediates in the catalytic cyde of the Baylis-Hillman reaction between an activated alkene and an electrophile were intercepted and characterized, providing dired evidence for the currently accepted mechanism. 

Scheme 3.12 The catalytic cycle proposed by Matsuda and coworkers for the Heck reaction with arene diazonium salts.

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