Pentagonal dodecahedranes. Novel substitution patterns— MS fragmentation and unsaturation

Abstract

For two- to six-fold functionalised dodecahedranes (7–10) the chances for selective variation of their substitution have been explored, as part of a program directed at homododecahedranes and at highly unsaturated dodecahedranes, ultimately C₂₀ fullerene. With 1,6-dimethyl ester 7 several side chain transformations next to the very bulky dodecahedral cage were effected (1,6-bismethylene derivatives 25–31). In changing environments, Barton-type halogenative/hydrogenative decarboxylations (15–17, 38, 49, 59, 60, 77) as well as various nucleophilic substitutions (18, 20, 23, 39, 61–64) were achieved, mostly with good to high efficiency and retention of the substitution patterns. For the cage olefins 8/9 and the diepoxide 10, front-side cis-1,2-addition faced only slight competition in the reactions with HBr and CF₃CO₂H, but was only a minor pathway in the reaction with Br₂. In the latter case, by a sequence of Br⁺ addition/HBr elimination steps, up to nine vicinally placed bromine substituents were implanted upon the C₂₀ skeleton. The fate of variously functionalised dodecahedranes upon electron impact was studied—the competition between external (C–X/Y) and internal (C–C) bond cleavage was found to be typically dependent on the nature and relative orientation of the functionalities (X,Y) involved. There is good evidence that ions between m/z 242 (C₂₀H₂) and 256 (C₂₀H₁₆) resulting from the elimination of the respective Br, CO₂CH₃, CO₂NH₂, OCOCF₃ substituents represent unsaturated dodecahedranes with up to nine CC double bonds.