From diazonium ions

In the context of this section it is important that Ruchardt and Tan (1970 a) found that (solid) benzenediazonium fluoroborate gave benzyne adducts with potassium acetate in the presence of aryne trapping agents such as tetracyclone or anthracene. This is, however, not the case if water is present (Cadogan, 1971). As a consequence of these observations, Cadogan et al. (1971) simplified the formation of arynes from diazonium ions by converting aniline or its substitution products into arynes in a... 

Table 10-2. Ratios of the Sandmeyer products from diazonium ions with different substituents (after Hanson et al., 1991)a).

Stronger reducing agents than Cu1 can be used for reactions that are related to the classical Meerwein reaction. Tim salts not only catalyze the formation of aryl radicals from diazonium ions but, as shown by Citterio and Vismara (1980) and Cit-terio et al. (1982 a), in stoichiometric proportions they also reduce the primary aryl-ethane radical to the arylethyl anion, which is finally protonated by the solvent SH (Scheme 10-61). This method is the subject of a contribution to Organic Syntheses (Citterio, 1990), in which 4-(4 -chlorophenyl)buten-2-one is obtained in 65-75% yield from 4-chlorobenzenediazonium chloride and but-3-en-2-one. 

Phenols from Diazonium Ion Intermediates. Aryl diazonium ions can be converted to phenols by heating in water. Under these conditions, there is probably formation of a phenyl cation. 

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. 

Elimination to give alkenes is a major reaction-pathway in the deamination of simple amines. The relative unimportance of elimination for amino groups equatorially attached to six-membered rings has been attributed to the fact that, in the supposed transformation from diazonium ion to carbonium ion, the developing carbon p-or-... 

Convincing evidence that vinyl cations, possibly resulting from diazonium ion precursors, are the actual intermediates of the acid-catalysed decomposition of l-aryl-3-vinyl triazene derivatives 112 (equation 20) has been discussed by Jones and Miller (1967). 

Whiting and Huisgen found identical products from diazonium ions formed in different reaction paths when ion pair dissociation or exchange could reasonably be supposed to have occurred prior to formation of the carbonium ion. A rather different type of study has been carried out by Miller and Stedronsky (1966) (ef. White and Elliger, 1967) who have sought to generate the same diazonium ion pair in the reaction of diphenyldiazomethane with jj-nitrobenzoic acid and in the rearran -ment of N-benzhydryl-N-nitroso-p-nitrobenzamide. 

In previous sections the factors complicating comparisons between carbonium ions formed from diazonium ions with those formed in other solvolytic processes have been emphasized these are, chiefly, ion pair phenomena and conformational control of rearrangements and elimination. Some of the attempts to exclude or take explicit account of these factors are now described. 

The most detailed comparison of carbonium ions from diazonium ion and ester precursors is due to Diaz and Winstein (1966) who measured the relative rates of ion pair collapse and dissociation in the reaction of diphenyldiazomethane, DDM, with benzoic acids and in the solvolysis of benzhydryl benzoates (scheme 9). 

Cohen, T., Dietz, A. G., Jr., Miser, J. R. A simple preparation of phenols from diazonium ions via the generation and oxidation of aryl radicals by copper salts. J. Org. Chem. 1977,42, 2053-2058. 

Decomposition of aliphatic diazonium salts often yields a more complicated mixture of products (see Table 8) than the one formed in solvolysis. Streitwieser explains this difference that in the first case the reaction proceeds not only through carbocations but also directly from diazonium ions the latter react un-... 

One of the best leaving groups is molecular nitrogen from diazonium ions. Diazonium salts are usually prepared by nitrosation of primary aliphatic or aromatic amines ... 

Carbonium ions derived from diazonium ions are very reactive species capable of alkylating any functional group on the enzyme. 

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