Copper with diazonium ions

In aqueous solution, instead of undergoing reaction (c), the aryl radicals can dimerize to give biaryls (d) or react with diazonium ions to give azo compounds (e), copper(i) chloride... 

In the context of copper catalysis in important synthetic dediazoniations of arene-diazonium ions, Starkey s group (Bolth et al., 1943, Whaley and Starkey, 1946) isolated blue pyridine complexes with the constitution ArCu(C5H5N)3 by adding copper powder to pyridine solutions of ArNjBF4. However, it is unlikely that arylcopper is a relevant intermediate in these reactions (see Sec. 8.6). 

Introduction of Other Nucleophiles Using Diazonium Ion Intermediates. Cyano and azido groups are also readily introduced via diazonium intermediates. The former involves a copper-catalyzed reaction analogous to the Sandmeyer reaction. Reaction of diazonium salts with azide ion gives adducts that smoothly decompose to nitrogen and the aryl azide.56... 

Scheme 11.6 gives some examples of the various substitution reactions of aryl diazonium ions. Entries 1 to 6 are examples of reductive dediazonization. Entry 1 is an older procedure that uses hydrogen abstraction from ethanol for reduction. Entry 2 involves reduction by hypophosphorous acid. Entry 3 illustrates use of copper catalysis in conjunction with hypophosphorous acid. Entries 4 and 5 are DMF-mediated reductions, with ferrous catalysis in the latter case. Entry 6 involves reduction by NaBH4. 

Meerwein Arylation Reactions. Aryl diazonium ions can also be used to form certain types of carbon-carbon bonds. The copper-catalyzed reaction of diazonium ions with conjugated alkenes results in arylation of the alkene, known as the Meerwein arylation reaction.114 The reaction sequence is initiated by reduction of the diazonium ion by Cu(I). The aryl radical adds to the alkene to give a new (3-aryl radical. The final step is a ligand transfer that takes place in the copper coordination sphere. An alternative course is oxidation-deprotonation, which gives a styrene derivative. 

The Pschorr reaction is a method of synthesis of phenanthrenes from diazotized Z-2-aminostilbenes. A traditional procedure involves heating with a copper catalyst. Improved yields are often observed, however, if the diazonium ion is treated with iodide ion. Suggest a mechanism for the iodide-catalyzed reaction. 

Substituted l,2,4-thiadiazol-5-yl diazonium tetrafluoroborates (314) react with potassium halides in acetonitrile in the absence of catalysts to afford the corresponding 5-halogeno compounds (315) in excellent yields.169 The isomeric 5-phenyl-l,2,4-thiadiazol-3-yl diazonium salts, however, resist substitution by this procedure, except with iodide ions by taking advantage of the catalytic effect of copper... 

One cannot distinguish between the analogous copper intermediates involved in oxidative electron-transfer and ligand-transfer reactions. In each the ionization of the ligand to copper(II) has an important role in the formation of carbonium ion intermediates. A reaction analogous to the copper-catalyzed decomposition of peroxides is the copper-promoted decomposition of diazonium salts (178). The diazonium ion and copper(I) afford aryl radicals which can undergo ligand-transfer oxidation with copper(II) halides (Sandmeyer reaction) or add to olefins (Meerwein reaction). 

Antongst other confounds which function as accelerators with diazonium salts are thiourea dioxide (formamldine sulfinic acid), and p-tolylhydrazine. whilst copper is the most efficient of the metal ions, catalytic qviantities giving a large Increase in R, some other transition metals have a marked effect. These include titanic sulfate and vanadyl sulfate. 

Alternatively in aromatic compounds, the chlorine can be introduced by reacting the diazonium ion with copper(I) chloride ... 

The formation of aryl chlorides from arenediazonium ions on reaction with copperfi) chloride is known as the Sandmeyer reaction and also involves the intermediacy of aryl radicals [192]. Copper salts are similarly involved in the reactions of diazonium ions with cyanide, nitrite, and sulfur dioxide. The uses of arenediazonium salts in synthetic reactions forming carbon-carbon, carbon-sulfur, and carbon-boron bonds have been summarized recently [193]. 

In the Sandmeyer reactions involving copper, electron-transfer-mediated radical reactions are certainly involved. In HCI, cuprous chloride (CuCl) is in equifib-rium with the dichlorocuprate ion CuCl2 (Frg. 14.56).The dichlorocuprate ion can transfer an electron to the diazonium ion, which can then lose molecular nitrogen to give a phenyl radical. In turn, the phenyl radical can pluck a chlorine atom from a copper chloride molecule to give the chlorobenzene and a molecule of cuprous chloride. The mechanism for the formation of chlorobenzene in the Sandmeyer reaction is shown in Figure 14.57. 

Replacement of diazonium groups by halide is a very valuable synthetic alternative to direct halogenation for preparation of aryl halides. There are three broad types of procedures decomposition of aryl diazonium-halide adducts with expulsion of nitrogen, copper-mediated redox processes, and thermal processes proceeding via aryl radicals. The first type of process is probably involved in the reaction of aryl diazonium salts with iodide ion. Smooth high-yield transformation takes place in the absence of any metal catalyst. The mechanism of the reaction... 

Aryl bromides and iodides are usually prepared from diazonium salts by a copper-catalyzed process, a reaction commonly known as the Sandmeyer reaction. Under the classic conditions of the Sandmeyer reaction, the diazonium salt is added to a hot acidic solution of the cuprous halide.It is also possible to convert anilines to aryl halides by generating the diazonium ion in situ. Reaction of anilines with alkyl nitrites and cuprous halides in acetonitrile gives good yields of aryl bromides by a copper-mediated process which is mechanistically similar to that occurring under the usual Sandmeyer conditions. Diazonium salts can also be converted to... 

Copper(I) cyanide reacts with aryl diazonium ions to give aryl nitriles. 

Acylamino groups also are useful activating groups and have the advantage that the amino groups obtained after hydrolysis of the acyl function can be removed from an aromatic ring by reduction of the corresponding diazonium salt with hypophosphorous acid, preferably in the presence of copper(I) ions. 

In the photometric determination of copper, a coupling product formed between the diazonium salt from 2-amino-pyridine and resorcinol, or 4-(2-pyridinylazo)-l,3-benzenediol 21, has been used. Here the formed copper complex under acetate buffer exhibits an absorption peak at 520 nm, which is measured photometrically <2003KPU28>. Similarly for photometric determination of iron(ll), a coupling product formed between the diazonium salt of 2-amino-4,6-dihydroxypyrimidine and 8-hydroxyquinoline, or 6-hydroxy-2-(8-hydroxy-7-quinolinyl)azo-4(l//)-pyrimidinone 22, has been used. This reagent forms a blue complex with iron(n) ions with an absorption maximum at 625 nm that does not interfere with the presence of other metals <2003KD95>. 

It can be useful to replace an amino group with hydrogen, since this can offer a route to compounds difficult to prepare by other direct methods. The most reliable means of achieving this is by conversion to the diazonium salt and subsequent reaction with phosphinic acid (hypophospho-rous acid, H3PO2), catalysed by copper(I) salts. A free-radical mechanism is proposed, in which copper(I) ion acts as a one-electron reducing agent and initiates a chain reaction (Scheme 8.13). 

In the Sandmeyer reaction, the cold diazonium salt solution is run into a solution of the copper(I) halide dissolved in the halogen acid. The complex, which usually separates, is decomposed to the aryl halide by heating the reaction mixture. The mechanism (Scheme 8.16) involves generation of an aryl radical by electron transfer from Cu(I), which then reacts with the halide ion. 

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