Formation of N-chloramine : silver ion complexes, isolation of a stable nitrenium ion and the silver ion-induced rearrangement of N-chlorocyclobutylamines

Several secondary N-chloramines were characterized by both proton and carbon NMR revealing predictable chemical shift effects. The addition of silver ion to some of these N-chloramines produced N-chloramine: silver ion complexes that were studied using proton NMR. From this NMR study, it is clear that the site of silver complexation in these compounds occurs predominately at the nitrogen unshared electrons. Precipitation of silver chloride from one of these complexes left behind a crystalline material that was stable at ambient temperature to atmospheric moisture and air. Subsequent characterization by X-ray crystallography revealed a central nitrogen with two sigma bonds and a tetrafluoroborate counterion, hence a nitrenium ion. Proton and carbon NMR of this nitrenium ion were also obtained. N-Chloramines, which undergo rearrangement following the introduction of silver ion, are thought to do so through the intermediacy of nitrenium ions. Specifically, certain N_b-chloro-N_b-cyclobutyltryptamines and N_b-chloro-1-spiro-1'- cyclobutyl-1,2,3,4-tetrahydro-β-carbolines were utilized in the preparation of 2,3,5,6,11,11b-hexahydro-1H-indolizino[8,7-b]indole. In both cases reaction was initiated with silver tetrafluoroborate. Subsequent addition of either concentrated hydrochloric add or sodium cyanoborohydride, respectively, provided the desired tetracycle in yields ranging from 16-66%.