Halides from diazonium ions

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. 

Because a neutral molecule is eliminate4 rather than an anion, there is no electrostatic attraction (ion pairing) between the products of the dissociation step. As a result, the carbocations generated by diazonium-ion decomposition frequently exhibit somewhat different behavior from those generated from halides or sulfonates under solvolytic conditions. ... 

This seeming unreactivity of vinyl halides in solvolytic processes and the lack of availability of more reactive precursors, such as sulfonate esters, until recently has discouraged early attempts at mechanistic investigations of vinyl cations generated by solvolyses. However, vinyl cations have been generated via vinyl diazonium ions derived from various precursors. 

Treatment of an aromatic amine with nitrous acid gives dia-zonium compounds. These couple with phenols to give dyes. Displacement of nitrogen from a diazonium compound by a halide or cyanide ion takes place with copper catalysis and is a useful synthetic method. 

This is the first step, i.e. formation of the initial carbonium ion. These ions may be formed in many different ways protonation of an alcohol loss of nitrogen from a diazonium ion addition of a proton to an alkene or the interaction of a Lewis acid with a halide. 

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. 

There is evidence that carbocations do play a role in the reaction of some diazonium ions. For example, the products of nucleophilic addition to f-butyl carbocations are essentially the same whether the carbocations are produced from aliphatic amines or from solvolysis of the corresponding alkyl halide. The decomposition of the diazonium ion to form a carbocation should become less favorable as the stability of the carbocation decreases, and the formation of an ethyl cation from ethyldiazonium ion in the gas phase has been calculated to be endothermic by 13.9kcal/mol. The decomposition of a 1° alkyldiazonium ion to N2 and a 1° carbocation in solution may be energetically more favorable, perhaps even exothermic, but it is questionable... 

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... 

Two methods for converting carboxylic acids to esters fall into the mechanistic group under discussion. One of these methods is the reaction of carboxylic acids with diazo compounds, especially diazomethane. The second is alkylation of car-boxylate anions by halides or sulfonates. The esterification of carboxylic acids with diazomethane is a very quick and clean reaction. The alkylating agent is the extremely reactive methyl diazonium ion, which is generated by proton transfer from the carboxylic acid to diazomethane. The collapse of the resulting ion pair with loss of nitrogen is extremely rapid ... 

The Sandmeyer reaction is formulated as proceeding by reduction of the diazonium ion by Cu(I) and halide transfer from copper. 

More recently, cationic intermediates have been observed in the Heck reactions of arene diazonium salts catalyzed by triolefinic macrocycle Pd(0) complexes [17,59], o-iodophenols and enoates to form new lactones [60], and o-iodophenols with olefins (the oxa-Heck reaction) [61 ]. In the first case ions were formed by oxidation of the analyte at the capillary, or by association of [NH4] or Na". In the two other cases ionization occurred through the more typical loss of a halide ligand. The oxa-Heck reaction provides a good example of how these experiments are typically performed and the type of information that can be obtained. The oxyarylations of olefins were performed in acetone, catalyzed by palladium, and required the presence of sodium carbonate as base. Samples from the reaction mixtures were diluted with acetonitrile and analyzed by ESI(+)-MS. Loss of iodide after oxidative addition of o-iodophenol to palladium afforded positively-charged intermediates. Species consistent with oxidative addition, such as [Pd(PPh3)2(C6H50)], and the formation of palladacycles of the type seen in Scheme 8 were observed. Based on this, a mechanism for the reaction was proposed (Scheme 8). 

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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