Aryldiazonium salts addition reactions

Owing to their particular interest two individual reactions will now be discussed separately. The reaction of methoxycarbonylhydrazine and 3-bromo-2,4-pentanedione affords, in addition to the expected pyrazole (608), a pyrazolium salt (609), the structure of which was established by X-ray crystallography (74TL1987). Aryldiazonium salts have been used instead of arylhydrazines in the synthesis of pyrazolines (610) and pyrazoles (611) (82JOC81). These compounds are formed by free radical decomposition of diazonium salts by titanium(n) chloride in the presence of a,/3-ethylenic ketones. 

The addition of aryl radicals, generated by chemical reduction of aryldiazonium salts, onto arenes in the Gomberg-Hey reaction is well established [163]. The addition of these radicals to alkenes in the Meerwein reaction is also well known [164], Aryl o-radicals generated by electrochemical reduction of aryl halides take part in similar reactions. Good yields of the products are obtained when the intermediate phenyl radical can react in an intramolecular manner. The addition step is then fast and competes successfully with further electron transfer to form the phenyl car-banion, followed by protonation. 

These reactions probably occur by the nucleophilic addition of the aryl anion to one of the CO groups of pentacarbonyliron followed by the addition of electrophiles. Pentacarbonyliron also causes carbonylation of electrophilic aryldiazonium salts [20]. 

Two important syntheses of aromatic phosphonic and phosphinic acids are based on classical aromatic chemistry. The use of aryldiazonium salts has the distinct advantage over Friedel-Crafts reactions in that the entering phosphorus is placed in a position of certainty and, additionally, only one step is required in the final stage, whereas two are sometimes needed in the Friedel-Crafts procedure. 

Being much more stable than their aliphatic counterparts, aryldiazonium ions can undergo addition reactions in which nitrogen is not lost. Reactions of aryldiazonium salts... 

This reaction has been modified by using alkyl nitrite to produce the aryldiazonium salt. In addition, an ionic analogue of Meerwein arylation has been developed in which an aryl cation is generated photochemically. ... 

The Heck reaction can be used to couple alkenyl, aryl, allyl (the intramolecular Heck reaction on aUyUc substrates is called a palladaene reaction),f ° t f benzyl, methyl, alkoxycarbonyhnethyl, alkynyl, certain alkyl, and silyl fragments to a variety of alkenes. The nature of the leaving group greatly affects the reaction rate aryl iodides react faster than bromides, and aryl chlorides are notoriously unreactive unless special catalysts or ligands and elevated temperatures are used to enhance the reaction rate. This has been taken to indicate that the oxidative addition of the haloarene (haloalkene) to pal-ladium(O) is the rate-determining step. " It has been shown that the Heck reaction can be performed with aryldiazonium salts, A -nitroso-Af-arylacetamides, and hy-pervalent iodo compounds " at room temperature. 

Aryl halides are frequently prepared from the corresponding aryldiazonium salts by diazotation procedures. However, diazonium salts can be subjected directly to very mild Heck arylation conditions, which deliver coupled products (entry 19). Preferably, the reaction is executed in nonaqueous solvents such as acetonitrile, acetone, or methylene chloride with sodium acetate as base and with palladiumbis(dibenzylideneacetone) as catalyst. Alternatively, a combination of the amine and f-butyl nitrite, in a mixture of acetic acid and monochloroacetic acid, can provide the desired product directly, which makes the isolation of a diazonium salt unnecessary (entry 20). " It is also possible to use aromatic acid anhydrides as oxidative addition precursors (entry 21). Clearly, anhydrides are very interesting starting materials for a number of Heck reactions due to price and absence of halide salt formation. 

Mastrorilli et al. described a combined ESI HRMS and F NMR mechanistic study of this reaction with palladacycle complex 10 (Scheme 7.5) as the precatalyst [17], from which the authors postulated the formation of the true catalytic species in a first step. It is suggested that palladacycle 10 reacts with potassium trifiuorophenylborate 8a to give Pd(0) intermediate 117 (Entry 3, Table 7.3), which starts the catalytic cycle. This species undergoes oxidative addition of the aryldiazonium salt to give the cationic aryl-palladium(II) complexes 120, which contain azobenzene 11 as a ligand (Entry 4, Table 7.3). In this study, no intermediate was detected corresponding to transmetalation species. 

The general catalytic cycle for the Heck-Matsuda reaction using acyclic olefins starts with the oxidative addition of the Pd(0) catalyst A to the aryldiazonium salt and sequential elimination of nitrogen to produce the cationic palladium species C. This intermediate is very electrophilic and it promptly... 

The proposed general catalytic cycle for this reaction encompasses a mix of conventional intermediates with tmconventional ones. The process presumably starts with the oxidative addition of the aryldiazonium salt to produce the cationic aryl palladium B. As the substrate has two coordinating... 

It should be mentioned that O-methylation of the ketone in 24 was necessary to avoid competition between the Heck-Matsuda arylation and the nucleophilic addition of the enol to the aryldiazonium salt, a process known as the Japp-Klingemann reaction. Indeed, when 24 was submitted to the standard Heck-Matsuda reacticm conditions, the azo compound 25 was obtained in quantitative yield (Scheme 9). Azo compounds are easily detected due to the formation of highly colored products via this reaction. 

Today, the GBH reactions are performed by using aryldiazonium tetrafluoroborate salts. An aromatic amine is diazotized at about -10 to 0 C in the presence of at least 3 equivalents of concentrated hydrochloric acid by adding the solution of sodium nitrite [61]. To the aryldiazonium chloride, 50% aqueous solution of tetrafluoroboric acid is added with additional cooling to precipitate the corresponding aryldiazonium... 

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