Diazonium salts arenes

Azo coupling (Section 22 18) Formation of a compound of the type ArN=NAr by reaction of an aryl diazonium salt with an arene The arene must be strongly activated toward... 

The two main methods for the preparation of ar-yl halides, halogenation of arenes by electrophilic aromatic substitution and preparation by way of ar-yl diazonium salts, were described earlier and are reviewed in Table 23.2. A number of ar-yl halides occur naturally, some of which are shown in Figure 23.1. 

Arylamines are converted by diazotization with nitrous acid into arenediazonium salts, ArN2+ X-. The diazonio group can then be replaced by many other substituents in the Sandmeyer reaction to give a wide variety of substituted aromatic compounds. Aryl chlorides, bromides, iodides, and nitriles can be prepared from arenediazonium salts, as can arenes and phenols. In addition to their reactivity toward substitution reactions, diazonium salts undergo coupling with phenols and arylamines to give brightly colored azo dyes. 

Elegant evidence that free electrons can be transferred from an organic donor to a diazonium ion was found by Becker et al. (1975, 1977a see also Becker, 1978). These authors observed that diazonium salts quench the fluorescence of pyrene (and other arenes) at a rate k = 2.5 x 1010 m-1 s-1. The pyrene radical cation and the aryldiazenyl radical would appear to be the likely products of electron transfer. However, pyrene is a weak nucleophile the concentration of its covalent product with the diazonium ion is estimated to lie below 0.019o at equilibrium. If electron transfer were to proceed via this proposed intermediate present in such a low concentration, then the measured rate constant could not be so large. Nevertheless, dynamic fluorescence quenching in the excited state of the electron donor-acceptor complex preferred at equilibrium would fit the facts. Evidence supporting a diffusion-controlled electron transfer (k = 1.8 x 1010 to 2.5 X 1010 s-1) was provided by pulse radiolysis. 

Keumi et al. (1989) describe hydro-de-diazoniations of arenediazonium tetrafluo-roborates using chlorotrimethylsilane, (CH3)3SiCl), in tetrahydrofuran or tetra-hydrofuran/A A-dimethylformamide mixtures. Excellent yields were obtained with polycyclic arene derivatives such as 2-fluorene-, 2-fluorenone-, and 1-pyrenediazo-nium tetrafluoroborate and other similar diazonium salts. In a modification of this method 2-halogenofluorenones can be synthesized (see Sec. 10.6). 

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

The initial product is a salt of hydrazinesulfonic acid, which is converted to the hydrazine by acid treatment. Diazonium salts can also be reduced to arenes (14-24). N-Nitrosoamines can be denitrosated to secondary amines by a number of reducing agents, including H2 and a catalyst, BF3—THF—NaHC03, and NaBH4— TiCU, as well as by hydrolysis. ... 

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... 

Then the second arene undergoes electrophilic aromatic substitution, with the terminal N of the diazonium salt as the electrophilic atom. When nucleophilic arenes are added to diazonium salts, electrophilic aromatic substitution tends to take place instead of SnI substitution of the diazonium salt. 

The Gomberg coupling reaction of aryl diazonium salts with arenes is catalysed by quaternary ammonium salts [49], Although yields are variable, they are generally >50% [49, 50]. When aromatic solvents other than benzene are used, the appropriate biaryls are produced, e.g. 4-chlorobenzenediazonium tetrafluoroborate reacts with chlorobenzene to produce the 2,4 -, 3,4 - and 4,4 -dichlorobiphenyls in a ca. 67 18 15 ratio. 

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-... 

Base-promoted radical coupling between an aryl diazonium salt and an arene to form a diaryl compound. 

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). 

Support-bound triazenes, which can be prepared from resin-bound secondary aliphatic amines and aromatic diazonium salts [455], undergo cleavage upon treatment with acids, leading to regeneration of the aromatic diazonium salts. In cross-linked polystyrene, these decompose to yield nitrogen and, preferentially, radical-derived products. If the acidolysis of polystyrene-bound triazenes is conducted in the presence of hydrogen-atom donors (e.g. THF), unsubstituted arenes can be obtained (Entries 8 and 9, Table 3.47). In the presence of alkenes or alkynes and Pd(OAc)2, the initially formed diazonium salts undergo Heck reaction to yield vinylated or alkynylated arenes (Entry 10, Table 3.47). Similarly, unsubstituted arenes can be obtained by oxida-... 

The Meerwein arylation is at least formally related to the atom transfer method because a net introduction of an aromatic ring and a chlorine across a double bond is accomplished (Scheme 62). Facile elimination of HC1 provides an efficient route to the kinds of substituted styrenes that are frequently prepared by Heck arylations. Standard protocol calls for the generation of an arene diazonium chloride in situ, followed by addition of an alkene (often electron deficient because aryl radicals are nucleophilic) and a catalytic quantity of copper(II) chloride. It is usually suggested that the copper salt operates in a catalytic redox cycle, reducing the diazonium salt to the aryl radical as Cu1 and trapping the adduct radical as Cu11. 

The Pschorr Reaction allows the preparation of biaryl tricyclics by intramolecular substitution of one arene by an aryl radical. This radical is generated in situ from an aryl diazonium salt by copper catalysis. Although excess copper salts are used, the yield is normally moderate. 

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... 

The original reaction procedures have steadily been extended and improved, and an overwhelming number of catalyst systems are now known. Besides aryl halides, many additional substrates, for example aryl triflates, diazonium salts, sulfonyl and aroyl halides, carboxylic and phosphonic acids, and even arenes have been used as... 

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