Copper salts reactions with aryl diazonium

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. 

In these reactions, diazonium salts are cleaved to aryl radicals, in most cases with the assistance of copper salts. Reactions 14-17 and 14-18 may also be regarded as belonging to this category with respect to the attacking compound. For nucleophilic substitutions of diazonium salts, see 13-17-13-20. 

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... 

Diazonium salts react with oximes to give aryl oximes, which are easily hydrolyzed to aldehydes (R = H) or ketones." A copper sulfate-sodium sulfite catalyst is essential. In most cases higher yields (40-60%) are obtained when the reaction is used for aldehydes than for ketones. In another method for achieving the conversion ArNj —> ArCOR, diazonium salts are treated with R4Sn and CO with palladium acetate as catalyst. In a different kind of reaction, silyl enol ethers of aryl ketones, Ar C(OSiMe3)=CHR, react with sohd diazonium fluoroborates, ArNj BF4, to give ketones, ArCHRCOAr. " This is, in effect, an arylation of the aryl ketone. 

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... 

The most satisfactory method of preparation of a copper(i) cyanide solution is to dissolve the copper(i) cyanide (90 g, 1 mol) in a solution of sodium cyanide (125 g, 2.5 mol) (CAUTION) in 600 ml of water. If it is desired to avoid the preparation of solid copper(i) cyanide, the following procedure may be adopted. Copper(i) chloride, prepared from 35 g of copper(n) sulphate pentahydrate as described under 22 above, is suspended in 60 ml of water contained in a 500-ml round-bottomed flask, which is fitted with a mechanical stirrer. A solution of 18.5 g of sodium cyanide (96-98%) in 30 ml of water is added and the mixture is stirred. The copper(i) chloride passes into solution with considerable evolution of heat. As the copper(i) cyanide is usually employed in reactions with solutions of aryl diazonium salts it is usual to cool the resulting copper(i) cyanide solution in ice. 

A second method for preparing aryl halides is the Sandmeyer reaction. During a Sandmeyer reaction, a diazonium salt reacts with copper (I) bromide, copper (I) chloride, or potassium iodide to form the respective aryl halide. The diazonium salt is prepared from aniline by reaction with nitrous acid at cold temperatures. 

Replacement of the Diazonium Group by Chloride, Bromide, and Cyanide The Sandmeyer Reaction Copper salts (cuprous salts) have a special affinity for diazonium salts. Cuprous chloride, cuprous bromide, and cuprous cyanide react with arenediazonium salts to give aryl chlorides, aryl bromides, and aryl cyanides. The use of cuprous salts to replace arenediazonium groups is called the Sandmeyer reaction. The Sandmeyer reaction (using cuprous cyanide) is also an excellent method for attaching another carbon substituent to an aromatic ring. 

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. 

Generation of an aryl radical and an aryl cation from a diazonium salt are easy processes. Both species are very reactive and are readily trapped by a wide variety of nucleophiles. Conversion of an amino group into a nitro group involves reaction of a diazonium fluoroborate with aqueous sodium nitrite solution in the presence of copper powder (Scheme 8.22). 

A diazonium salt reacts with copper(I) chloride or copper(I) bromide to form an aryl chloride or aryl bromide, respectively. This is called the Sandmeyer reaction. It provides an alternative to direct chlorination and bromination of an aromatic ring using CI2 or Br2 and a Lewis acid catalyst. 

Sandmeyer reaction a solution containing an aryl diazonium salt is treated with copper(l) chloride to give an aryl chloride. 

Meenvein reaction [1, 166, before references]. The arylation of olefinic com pounds by diazonium halides with copper salt catalysis was discovered by Meerweit (1939).8 Cupric chloride has been usually employed. Cleland,9 however, prefers cuprous bromide (MCB reagent grade) and recommends that the salt (light green) be washed with acetone until the washings are colorless and then with benzene and then with hexane. The resulting solid is dried at 120° and is only faintly colored. He... 

Treatment of diazonium salts with cuprous, Cu(I), salts generates aryl halides. When 398 reacts with CuCl (cuprous chloride) or CuBr (cuprous bromide), the products are chlorobenzene or bromobenzene via what is probably a radical reaction.29l jhis conversion is known as the Sandmeyer reaction. 2 The use of copper powder rather than cuprous salts for this transformation is often called the Gattermann reaction. 93,292b,c Aryl iodides are also produced from diazonium salts by reaction with potassium iodide (KI) but the actual reactive species may be l3-.294,295 Treatment of aniline derivative 403 with sodium nitrite and HCl followed by treatment with KI, for example, gave a 89% yield of 404.Aryl nitriles are generated under Sandmeyer conditions using cuprous cyanide (CuCN), as in the conversion of 405 to benzonitrile derivative 407 via diazonium chloride, 406. 

A related process that involves diazonium salts is the Pschorr reaction, hich also couples aryl diazo-nium compounds to other aromatic rings. This diazonium salt coupling can he done under acidic conditions, but addition of copper powder usually promotes the radical process. Aryl amines generate aryl diazonium salts upon treatment with nitrous acid.l An example is the reaction of 233 to give an aryl diazonium salt, which cyclized in the presence of copper to give thaliporphine (234) in 43% yield. Kupchan called this transformation an improved Pschorr reaction. 68... 

When Pschorr reported more than a century ago on the first intramolecular homolytic aromatic substitution [25], he showed that biaryls could be readily prepared by intramolecular homolytic aromatic substitution using reactive aryl radicals and arenes as radical acceptors. The aryl radicals were generated by treatment of arene-diazonium salts with copper(l) ions. Today, this reaction and related processes are referred to as Pschorr reactions. It was later found that radical biaryl synthesis could be conducted without copper salts by photochemical or thermal generation of the aryl radical from the corresponding diazonium salt [26], Moreover, the reduction of aryl diazonium salts offers another route to generate reactive aryl radicals. Hence, electrochemistry [27], titanium(lll) ions [28], Fe(II)-salts [29], tet-rathiafulvalene [30] and iodide [31] have each been used successfully for the reduction of diazonium salts to generate the corresponding aryl radicals [32]. As an example, the iodide-induced cycUzation of diazonium salt 6 to phenanthrene derivative 7 is presented in Scheme 13.3 [31]. For further information on the... 

Nesmeyanov has extended this reaction to the synthesis of aryl derivatives of the heavier B-elements, but the yields are often low. Often the double salts of the metal halide with the diazonium salt are first isolated and then decomposed by a metal such as copper or zinc, e.g. 

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