Deamination of endo- and exo-bicyclo[3.2.1]octan-3-ylamines and their derivatives

Abstract

Bicyclo[3.2.1] octan-3-ylamines have been deaminated in acetic acid by nitrous acid and via their N-phenyltriazenes; their ethyl N-nitrosocarbamates have also been solvolysed in ethanol. The exo-isomers give mainly unrearranged substitution, some elimination, and very little rearrangement. The unrearranged substitution, is derived from both the solvent (the external nucleophile, either acetic acid or ethanol) and the internal nucleophile liberated in the deaminative fragmentation step (water from the nitrous acid reaction, aniline from the triazene, and ethyl carbonate from the nitrosocarbamate). It is of predominantly retained configuration in all three reactions with the solvent, and, as demonstrated in the nitrosocarbamate solvolysis, with the internal nucleophile. The endo-isomers give mainly elimination, some unrearranged substitution, and appreciable rearrangement. The solvent-derived unrearranged substitution is with predominant inversion of configuration in all three reactions whereas that from the internal nucleophile, established by the nitrosocarbamate solvolysis, is predominantly with retention. Rearrangement from both endo- and exo-compounds is best explained in terms of hydride shift from the first formed carbonium ions (in nitrogen-separated complex ion-pairs with hydrogen-bonded anions) produced in the deaminative fragmentation. This gives rearranged classical bicyclo[3.2.1]octan-2-yl carbonium ions which, in competition with nucleophilic capture and proton loss, undergo further stepwise rearrangemnt to a common unsymmetrical non-classical carbonium ion. The non-classical cation and its classical precursors (which, from exo- and endo- substrates, differ in the location of the counter-anion) give rise to substitution products derived from solvent and the internal nucleophile. The non-classical carbonium ion also gives some tricyclo[3.2.1.0^2,7] octane. The high yields of internal substituion products from both endo- and exo-compounds rule out long lived intermediates such as diazonium ions.