Reactions of Aryl Diazonium Salts

Problem 18.31 Using CgH, C HjCHj, via diazonium salts and other needed reagents, prepare a) o-chloroto-luene, (i) m-chlorotoluene, (c) 1,3,5-tribromobenzene, d) m-bromochlorobenzene, (e) / -iodotoluene, (/) p-dini-trobenzene and (g) p-cyanobenzoic acid. Do not repeat the synthesis of intermediate products. 

The —NO2 is used to block the para position and it also directs meta, so that chlorination will occur only ortho to CHj. 

The acetylated —NH2 is used to direct Cl into its ortho position, which is meta to CH, it is then removed, (c) Aniline is rapidly and directly tribrominated and the NH2 removed. 

The aryl diazonium cation, ArNj, is a weak electrophile that is nevertheless capable of attacking aromatic rings with strongly activating groups. Diarylazo compounds are thereby produced with no loss of N2. 

Problem 18.32 Explain the following conditions used in coupling reactions (a) excess of mineral acid during diazotization of arylamines, (b) weakly acidic medium for coupling with ArNH2, (c) weakly basic solution for coupling with ArOH.  

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A reaction of aryl diazonium salts that does not involve loss of nitrogen takes place when they react with phenols and arylamines Aryl diazonium ions are relatively weak elec trophiles but have sufficient reactivity to attack strongly activated aromatic rings The reaction is known as azo coupling two aryl groups are joined together by an azo (—N=N—) function... 

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... 

It is noted that the reaction of aryl diazonium salts with CuCN to give benzonitrile derivatives is also called the Sandmeyer reaction. It is usually conducted in neutral solution to avoid liberation of HCN. 

The profitable conversion of thiouronium salts into dialkyl thioethers (see Section 4.1) is less successful for synthesis of diaryl thioethers. For example, l-chloro-4-nitrobenzene reacts with bis-thiouronium salts of the type (H2N)2CS(CH2) SC(NH2)2+ under soliddiquid and liquidrliquid conditions to produce the desired bis-thioethers, ArS(CH2) SAr, (20-35%), together with the diaryl sulphide, Ar2S (5-15%). Higher yields of the diaryl sulphide are observed under liquiddiquid conditions whereas, under solidiliquid conditions, the diaryl disulphide, (ArS)2, (20%) is also formed [56], Diaryl disulphides are the sole products (>65%) from the stoichiometric reaction of aryl diazonium salts with benzyltriethylammonium tetrathiomolybdate [57],... 

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. 

Replacement of diazonium groups by halide is a valuable alternative to direct halogenation for preparation of aryl halides. Aryl bromides and chlorides are usually prepared by reaction of aryl diazonium salts with the appropriate Cu(I) salt, a process 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.92 The Sandmeyer reaction is formulated as proceeding by an oxidative addition reaction of the diazonium ion with Cu(I) and halide transfer from the Cu(III) intermediate. 

These reactions are related to the reaction of aryl diazonium salts with iodide yielding iodoaryls, the mechanism of which seems to be a one-electron transfer (radical) reaction and not a nucleophilic displacement. Just as iodide is easily oxi- zed to iodine by the aryl diazonium cation, 2.4.6-triphenyl-X -phosphorin is oxidized to the radical cation 58. 

Reaction mann reactions (Figure 16.4-16.6). Regarding the preparation of aromatic nitriles, this reaction presents an alternative to the Sandmeyer reaction of aryl diazonium salts with CuCN (cf. Figure 5.53). 

The Pd )-catalyzed reaction of aryl diazonium salts with mono-substituted alkenes [1] was found to be an interesting alternative to the well-known Pd - catalyzed arylhalide alkene coupling (Heck type reaction) or the copper mediated reaction of aryl diazonium salts with alkenes (Meerwein arylation) [2], The reaction can be run without isolation of the diazonium salt in presence of only 0.5 to 1 mol% of the Palladium catalyst in a one pot procedure, in high yield and under nuld conditions. The resulting styrene is reduced in a subsequent hydrogenation step with an in situ generated heterogeneous Pd-catalyst. The combination of three reaction steps without isolation of intermediates and the virtually complete recovery of the Pd-metal at the end of the reaction sequence makes this process [4] extremely efficient. 

A reaction of aryl diazonium salts that does not involve loss of nitrogen takes place when they react with phenols and arylamines. Aryl diazonium ions are relatively weak... 

Modifications of the classical method are mainly concerned with the decomposition of the diazonium salt. Thermal [74], photochemical [75], and electrochemical [76] processes were found to be effective for the generation of the radical intermediate. Ti(III)-catalyzed Pschorr reaction of aryl diazonium salts and phenols gave symmetrical biaryls in excellent yield [77]. 

The photocatalyzed Meerwein arylation Classic reaction of aryl diazonium salts, in particular, direct C—H arylation ofheteroarenes 13AG(E)4734. Prebiotic chemistry in eutectic solutions at the water—ice matrix 12CSR5404. 

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