Bromination of Unsaturated Dodecahedranes-En Route to C20 Fullerene

As part of a study to achieve selective oligo(poly)bromination—ultimately perbromination—of the dodecahedral C20 skeleton, the extent and direction of the ionic bromination of dodecahedrene and 1,16‐dodecahedradiene were explored. Along sequences of Br+ additions/deprotonations and allylic rearrangements, up to ten hydrogen atoms were substituted (traces of C20HxBr10). Tetrabromododecahedrenes obtained under defined conditions in up to 50 % total yield with three and four allylic bromine substituents protecting the extremely bent CC bonds, proved highly unreactive even towards oxygen but reacted rapidly with CH2N2. Upon electron impact ionization (MS) of the newly secured oligo(poly)bromododecahedra(e)nes, sequential loss of the substituents ended generally in polyunsaturated dodecahedranes (in the extreme C20H4, “tetrahydro‐C20 fullerenes”). Only subsequently did skeletal fragmentations occur. From X‐ray crystal‐structure analyses, more information was obtained on the structural response of the dodecahedral skeleton to the strain induced by the voluminous substituents. As Appendix, the forcing radical bromination of 1,6‐dibromododecahedrane and exploratory cis‐β‐HBr/cis‐β‐Br2 eliminations in bromododecahedranes with [Fe2(CO)9], P2F/[FeCp2] and [Fe(tmeda)Cp*Cl] (in situ protection) are presented. The extent of substitution to be achieved in unsaturated and brominated dodecahedranes through electrophilic and radical bromination, the utility of the respective oligobromides for the generation of C20Hx hydrofullerenes, and the in situ protection of the latter are discussed as part of a project directed at the C20 fullerene.