Pentacyclododecylkationen (29 -32)
Rearrangements of Pentacyclododecyl CationsPentacyclododecyl cations (29 -32) have been investigated as models for distorted 2-norbornyl cations. When 30 and 32 were generated by solvolysis (brosylates 8 and 20) or by nitrous acid deamination (amines 10 and 21), Wagner-Merrwein rearrangement occurred competitively with solvent capture. Both epimeric precursors, 3x and 3n, reacted with high exo-selectivity. In contrast, endo-precursors 2n (12,13) produced major quantities of endoproducts. Retentive displacement at 2n ist attributed to intervention of the bridged ion 29, generated independently by protonation of the hexacyclododecane 28. A rapid, degenerate alkyl shift in 30 was detected by means of a deuterium label (-32) suggests that the bridged structure 31 is not significantly lower in energy than 30. -Thus, pentacyclododecyl cations show several features uncommon to a norbornyl system: (1) relatively slow Wagner-Meerwein rearrangement, (2) fast endo,endo-3,2-shifts, and (3) abundant formation of 29 (a 7-bridged norbornyl cation), all explicable in terms of the distorted norbornyl geometry. -Less nucleophilic media led to fragmentation of 30 and/or 32. In trifluoroacetic acid the tetracyclododecenyl cation 64 was formed and trapped selectively. The dehydration of 7 on alumina (275°C) provided a convenient route to the alkcne 17. Minor products (47, 50, 51) arose via competitive fragmentation-elimination paths.0 VCH Verlagsgesellschaft mbH, D