Bridgehead diazonium ions

Certain aliphatic diazonium species such as bridgehead diazonium ions and cyclo-propanediazonium ions, where the usual loss of N2 would lead to very unstable carbocations, have been coupled to aromatic substrates. ... 

Curtin and coworkers have observed the azo coupling of the bridgehead diazonium ion 6 with 2-naphthol at low temperature (2) in 50 % yield. 

Foster and Beauchamp found that the decomposition of the methanediazonium ion to a methyl cation and dinitrogen in the gas phase is endothermic to the extent of 158 kJ mol . This is indeed in dramatic contrast to its very rapid dediazonia-tion in solution. Obviously, this is due to nucleophilic participation of the solvent at the C-atom from the back side relative to the diazonio group. In aromatic diazonium ions this pathway cannot be considered for steric reasons. This assumption was already postulated by Bartlett and Knox in 1939. Curtin et al. (1962) were able to trap an alicyclic bridgehead diazonium ion in solution for the first time in 1962 (see Sect. 6.1). 

Other alkanediazonium ions, including the bridgehead bicyclo[2.2.1]heptane-l-diazonium ion 9, do not undergo azo coupling reactions but give only nucleophilic substitution productsIn addition to the examples given above, some alkene-diazonium ions generated from nitrosooxazolidones can also add azide ions ... 

Since a bridgehead carbonium ion would be expected to be shorter-lived than an a-phenethyl cation, the additional equilibration occurs at the stage of the correspondingly longer-lived diazonium ion pair. This work also provides the clearest demonstration that nitrogen may be lost from within an ion pair without causing its disruption. 

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