From Diazonium Halides

The coupling of diazonium halides with suitable halogenated activated methylene groups is an excellent method of synthesis of a variety of acyl and aroyl hydrazidoyl halides. This method has been used by Favrel and by Dieckmann and Platz to synthesize the acetyl derivative (IV). 

Similarly, carboxyl and carbalkoxy hydrazidoyl chlorides are readily synthesized by this method (5.6-i7,24,37-39,40,49-5i.w.soj or example, coupling of 2-nitrobenzdiazonium chloride with ethyl chloroacetoacetate affords the carbethoxyhydrazidoyl chloride V ( ). 

The free carboxylic acid VI is obtained from chloromalonic acid and the corresponding diazonium halide 

In a similar manner the aldehyde VII is obtained from chloromalonalde-hyde and benzenediazonium chloride ( ). 

The reaction of 4-nitrobenzdiazonium chloride (VIII) with diazomethane yields the formhydrazidoyl chloride (IX) in 81 % yield ( ). 

---

The diazonium salts 145 are another source of arylpalladium com-plexes[114]. They are the most reactive source of arylpalladium species and the reaction can be carried out at room temperature. In addition, they can be used for alkene insertion in the absence of a phosphine ligand using Pd2(dba)3 as a catalyst. This reaction consists of the indirect substitution reaction of an aromatic nitro group with an alkene. The use of diazonium salts is more convenient and synthetically useful than the use of aryl halides, because many aryl halides are prepared from diazonium salts. Diazotization of the aniline derivative 146 in aqueous solution and subsequent insertion of acrylate catalyzed by Pd(OAc)2 by the addition of MeOH are carried out as a one-pot reaction, affording the cinnamate 147 in good yield[115]. The A-nitroso-jV-arylacetamide 148 is prepared from acetanilides and used as another precursor of arylpalladium intermediate. It is more reactive than aryl iodides and bromides and reacts with alkenes at 40 °C without addition of a phosphine ligandfl 16]. 

Aryl Halides from Diazonium Ion Intermediates. Replacement of diazonium groups by halides is a valuable alternative to direct halogenation for the preparation of aryl halides. Aryl bromides and chlorides are usually prepared by a reaction using the appropriate Cu(I) salt, which is known as the Sandmeyer reaction. Under the classic conditions, the diazonium salt is added to a hot acidic solution of the cuprous halide.99 The Sandmeyer reaction occurs by an oxidative addition reaction of the diazonium ion with Cu(I) and halide transfer from a Cu(III) intermediate. 

Aryldiazonium salts react with bis(dibenzylideneacetone)palladium to form arylpalladium salts and nitrogen. Therefore, diazonium salts may be employed to catalytically arylate alkenes under mild conditions. Since many aryl halides are made from diazonium salts this variation could even be more convenient than using aryl halides. The reaction proceeds in good to excellent yields in nonaqueous solvents, using sodium acetate as the base at room temperature with terminal alkenes and cyclopentene." Internal alkenes usually give poor yields, however. 

Aromatic nitriles generally cannot be prepared from the unreactive aryl halides (Sec. 25.5). Instead they are made from diazonium salts by a reaction we shall discuss later (Sec. 23.13). Diazonium salts are prepared from aromatic... 

As already mentioned, aromatic nitriles are made, not from the unrcactive aryl halides, but from diazonium salts (Sec. 23.13). 

The advantages of the synthesis of aryl halides from diazonium salts will be discussed in detail in Sec. 25.3. Aryl fluorides and iodides cannot generally be prepared by direct halogenation. Aryl chlorides and bromides can be prepared by direct halogenation, but, when a mixture of 0- and p isomers is obtained, it is difficult to isolate the pure compounds because of their similarity in boiling point. Diazonium salts ultimately go back to nitro compounds, which are usually obtainable in pure form. 

PREPARATION OF ARYL HALIDES 1. From diazonium salts. Discussed in Secs. 23.12 and 25.3. 

The preparation of aryl halides from diazonium salts is more important than direct halogenation for several reasons. First of all, fluorides and iodides, which can seldom be prepared by direct halogenation, can be obtained from the diazonium salts. Second, where direct halogenation yields a mixture of ortho and para isomers, the ortho isomer, at least, is difficult to obtain pure. On the other hand, the ortho and para isomers of the corresponding nitro compounds, from which the diazonium salts ultimately come, can often be separated by fractional distillation (Sec. 11.7). For example, the o- and />bromotoluenes boil only three degrees apart 182° and 185°. The corresponding o- and -nitrotoluenes, however, boil sixteen degrees apart 222° and 238°. 

From diazonium salt-bismuth halide complexes (method J)... 

Both dialkyl- and diaryltrihaloantimony compounds are known, although only a few dialkyl compounds have been described. The trichlorides have been obtained by the chlorination of either dialkylchlorostibines (169) or tetraalkyldistibines (170) with sulfuryl chloride. Dimethyltrichloroantimony [7289-79-4], C2H(5Cl3Sb, is dimeric in the solid state but is monomeric in solution (171). The dimer exists in two different forms, covalent and ionic (172). The covalent form contains bridging chlorine atoms the ionic form possesses the structure [(CH3)4Sb] [SbQJ. The diaryl compounds, both symmetrical and unsymmetrical, are best prepared from a diazonium halide and an aryldihalostibine ... 

Primary amines are not usually made by reduction of amides (15) but by other reductive processes which are minor variations on this scheme. For unbranched amines (16) we can reduce cyanides. This method is especially suitable for benzylic amines since aryl cyanides (17) can be made from diazonium salts (see Chapter 2), and for the homologous amines (18) since cyanide ion reacts easily with benzyl halides. 

Side reactions that occur in this process with diazonium halides and nitrates prevent the diazotization being effected in nitric or hydrochloric acid solution it is necessary to do this in sulfuric acid solution, and the sparing solubility of the sulfates of aromatic amines necessitates certain departures from the usual diazotization procedures. 

---