Diazonium salts aryl, formation

When an aqueous solution of a diazonium salt is added to an alkaline solution of a phenol, coupling occurs with formation of an azo-compound (p. 188). If ho vc cr the ntiueous solution of the diazonium salt, t. . ., />-bromohenzene diazonium chloride, is mixed with an excess of an aromatic hydrocarbon, and aqueous sodium hydroxide then added to the vigorously stirred mixture, the diazotate which is formed, e.g., BrC,H N OH, dissolves in the hydrocarbon and there undergoes decomposition with the formation of nitrogen and two free radicals. The aryl free radical then reacts with the hydrocarbon to give a... 

Primary arylamines like primary alkylammes form diazonium ion salts on nitro sation Aryl diazonium 10ns are considerably more stable than their alkyl counterparts Whereas alkyl diazonium 10ns decompose under the conditions of their formation aryl diazonium salts are stable enough to be stored m aqueous solution at 0-5°C for a rea sonable time Loss of nitrogen from an aryl diazonium ion generates an unstable aryl cation and is much slower than loss of nitrogen from an alkyl diazonium ion... 

Reaction of aryl diazonium salts with iodide ion (Section 22 17) Adding po tassium iodide to a solution of an aryl diazonium ion leads to the formation of an aryl iodide... 

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... 

GOMBERG - BACHMANN - GRAEBE - ULLMANN Arylatwn Atykaryl bond formation via diazonium salts Carbazole synthesis by intramolecular aryl-aryl bond formation. 

Some observations are important for improvement of the yield and for the elucidation of the mechanism of the Meerwein reaction. Catalysts are necessary for the process. Cupric chloride is used in almost all cases. The best arylation yields are obtained with low CuCl2 concentrations (Dickerman et al., 1969). One effect of CuCl2 was detected by Meerwein et al. (1939) in their work in water-acetone systems. They found that in solutions of arenediazonium chloride and sodium acetate in aqueous acetone, but in the absence of an alkene, the amount of chloroacetone formed was only one-third of that obtained in the presence of CuCl2. They concluded that chloroacetone is formed according to Scheme 10-50. The formation of chloroacetone with CuCl2 in the absence of a diazonium salt (Scheme 10-51) was investigated by Kochi (1955 a, 1955 b). Some Cu11 ion is reduced by acetone to Cu1 ion, which provides the electron for the transfer to the diazonium ion (see below). 

Entries 7 and 8 illustrate conversion of diazonium salts to phenols. Entries 9 and 10 use the traditional conditions for the Sandmeyer reaction. Entry 11 is a Sandmeyer reaction under in situ diazotization conditions, whereas Entry 12 involves halogen atom transfer from solvent. Entry 13 is an example of formation of an aryl iodide. Entries 14 and 15 are Schiemann reactions. The reaction in Entry 16 was used to introduce a chlorine substituent on vancomycin. Of several procedures investigated, the CuCl-CuCl2 catalysis of chlorine atom transfer form CC14 proved to be the best. The diazonium salt was isolated as the tetrafluoroborate after in situ diazotization. Entries 17 and 18 show procedures for introducing cyano and azido groups, respectively. 

Figure 6.15 Aryl diazonium salts in C-P bond formation.

Diazonium salts are another useful source of free radicals, and the formation of the reactive species can be achieved by reductive electrolysis or direct treatment with diazonium tetrafluoroborate salts [39]. By this route, several aryl derivatives could be introduced onto the nanotube sidewalls [40]. Aryl groups bearing halogen or alkyne functionalities are particularly interesting as they can be further reacted in Pd-catalyzed coupling reactions (Suzuki, Heck) or in click chemistry reactions to create products with great potential in materials science [41]. 

Let us now consider the formation of aryl iodides from aryl diazonium salts and potassium iodide in methanol (Singh and Kumar 1972a, 1972b). Electron-donor substituents decelerate the process as compared with benzene diazonium (the substituent is hydrogen), whereas electron acceptor substituents accelerate it. Oxygen inhibits the reaction, and photoirradiation speeds it up. As the authors pointed out, in the case of 4-nitrobenzene diazonium, the reaction leads not only to 4-iodonitrobenzene but also to nitrobenzene, elemental iodine, and formaldehyde. All of these facts support the following sequence of events ... 

An old reaction patented by Lange [19] involves the coupling of naphthyl-diazonium salts with sodium sulfite to yield azonaphthalene. A more recent study of this reaction indicates that the first step of the reaction is the formation of an aryl-iyn-diazosulfonate which couples with another molecule of a diazonium salt and by a multicentered rearrangement ultimately affords an azo compound [20, 21]. The reaction is represented in Eq. (7). The validity of... 

Difluorotris(perfluoroalkyl)-25-phosphancs (Rf)3PF2 give stable salts with cesium fluoride, potassium fluoride, sodium fluoride or arenediazonium fluorides, i.e. Cat + [(RF)3PF3] , where Cal+ = Cs +. K+, Na +, ArN2+. The last are difficult to dissolve and they decompose at much lower temperatures (60-90 C) than tetrafluoroborates and hexafluorophosphates. which allows more complete precipitation of diazonium salts from water solutions and their conversion into aryl fluorides 6 without tar formation.6... 

Dediazoniation is an SN1 process for which the formation of an aryl cation is, at present, accepted as the first and rate-limiting step (Scheme 3).Ua.d.15il-e This aryl cation has been detected by ESR spectroscopy during the photodecomposition of arenediazonium tetra-fluoroborates at 77K.15f The nitrogen loss has been shown to be reversible, to some extent, by performing the reaction with a unsymmetrically 15N-labeled diazonium salt.151... 

The published research on the photochemical decomposition of di-azonium salts suggests that the two processes, a heterocyclic and a homolytic process, analogous to those of the thermal decomposition may occur. Various workers 36 187 have reported that phenols are formed when diazonium salts are photolyzed in water and aryl ethers result when an alcohol replaces water as the solvent. Homer and Stohr122 report that a process analogous to reductive deamination occurs in preference to ether formation results in alcohols. The importance of free radical intermediates in the photodecomposition, based on magnetic susceptibility measurements, has been stressed.25 Lee and his co-workers171 have recently suggested that in ethanol the photodecomposition of a diazonium salt occurs via a radical intermediate while in water an ionic process predominates. Thus, photodecomposition of a nitrobenzene diazonium chloride in water yielded both a nitrophenol and a chloronitrobenzene in ethanol, on the other hand, the major product of photolysis was the reduction product, nitrobenzene. 

Cuprous-catalyzed replacement reactions are called Sandmeyer reactions aryl chlorides, bromides, cyanides, and nitro compounds are prepared in this way formation of aryl iodides requires no catalyst, fluorides are obtained by heating diazonium fluoroborates (i.e., Schiemann reaction) benzenols are obtained by warming aqueous diazonium salt solutions. 

Although the bimolecular additions of aryl radicals to aromatic rings are not useful due to a lack of selectivity, the corresponding cyclizations are useful for constructing biaryl bonds. Equation (5) provides a generalized example of the Pschorr reaction.172 This cyclization of a diazonium salt is one of the classical methods for biaryl bond formation, and there are a variety of related reactions.138... 

Aryl iodides.1 DMSO is a particularly useful solvent for decomposition of aryl diazonium salts. Decomposition in the presence of KI (and I2 to minimize formation of iodinated biphenyls) results in aryl iodides in > 85% yield. 

An alternate entry to the narciclasine class of alkaloids has provided access to compounds related to isonarciclasine (263) (Scheme 24). In the event, the aryla-tion of p-benzoquinone with diazonium salts derived from the aryl amines 250 and 251 yielded the aryl-substituted benzoquinones 252 and 253, respectively (146). The selective hydroxylation of 252 and 253 with osmium tetraoxide provided the corresponding m-diols 254 and 255. Catalytic hydrogenation of 254 and 255 using Pd/C or Raney Ni and subsequent lactonization gave the triols 256 and 257 together with small amounts of the C-2 a-epimers 258 and 259. Aminolysis of 256 and 257 afforded the corresponding racemic tetrahydrophen-anthridones 260 and 261, whereas similar treatment of the a-epimers 258 and 259 led to the formation of ( )-isolycoricidine (262) and ( )-isonarciclasine (263), respectively. 

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