Bromination of Unsaturated Dodecahedranes—En Route to C₂₀ Fullerene

Abstract

As part of a study to achieve selective oligo(poly)bromination-ultimately perbromination-of the dodecahedral C(20) 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 C(20)H(x)Br(10)). 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 CH(2)N(2). 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 C(20)H(4), "tetrahydro-C(20) 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-beta-HBr/cis-beta-Br(2) eliminations in bromododecahedranes with [Fe(2)(CO)(9)], P(2)F/[FeCp(2)] and [Fe(tmeda)Cp*Cl] (in situ protection) are presented.

C20 Carbon Clusters: Fullerene–Boat–Sheet Generation, Mass Selection, Photoelectron Characterization

“…[33] Replacement of the strongly bound hydrogen atoms by weakly bound chlorine or even more weakly bound bromine atoms, dehalogenation by electron-impact ionization of the highly to extremely strained "halogen-balls" C 20 Cl 20 or C 20 Br 20 , mass-selection of the C 20 À ion and PE spectroscopic characterization of 1 had surfaced as "state-of-the-art" strategies. [10,11,13] The synthetic part presented in the preceding papers [23,34] was successful to the extent that through "forcing" to "brute-force" halogenations of 1 the hydrogen-free C 20 Cl 16 dienes 11 and the hydrogen-poor C 20 H 0-3 Br 14-12 tri-/ tetraenes 12 became reproducibly accessible. Both materials survived sublimation at 200-250 8C/10 À4 Torr unchanged and, unlike the parents 7-9 were oxygen-insensitive, and could be handled without precautions.…”

“…[23] The neccessary protection against dimerization is provided by four allylic CÀH bonds which themselves are flanked by two eclipsed CÀH bonds. At this point (1989) synthesis of the bare olefins 6-10, in which all C=C double bonds enjoyed allylic protection, became a vigorously pursued goal.…”

Addition Reactions: Polar Addition

“…18 The extent of substitution to be attained in unsaturated and brominated dodecahedranes through electrophilic and radical bromination have been elucidated as part of an effort directed at the C 20 fullerene. 19 Full geometric optimization of endo-tricyclo [3.2.1.0 2,4 ]oct-6-ene (endo-TCO) by ab initio and DFT methods has demonstrated the endo-pyramidalization of the double bond and non-equivalence of its two faces, with the exo face having regions with much higher electron density (qi, HOMO) and more negative electrostatic potential. The endo-TCO-Br 2 system was investigated at the B3LYP/6-311+G * * level and the endo-TCO· · ·Br 2 (exo) molecular complex was found to be considerably lower in energy than its endo-TCO· · ·Br 2 (endo) stereoisomer, suggesting a high preference for the exo-facial selectivity.…”

“…This study contributed to defining the relationship between these reactions and those catalysed by metal triflates. 36 The acid-induced cyclization of unsaturated thioacetals (19) gives anti-Markovnikov products (20), apparently involving sulfur elimination and readdition. 37…”

Thermochemical stabilities, electronic structures, and optical properties of C56X10 (X = H, F, and Cl) fullerene compounds

“…By the halogenation on the smaller fullerenes, three kinds of exohedral derivatives are synthesized and characterized unambiguously. Examples comprise the C 20 Br n H 202n , 27,28 C 50 Cl 10 , 29,30 C 58 F 18 , and C 58 F 17 CF 3 . 31 Inspired by the successful synthesis of C 50 Cl 10 , Yang et al 32 investigated the electronic properties, static polarizability, and hyperpolarizability using finite-field (FF) approach.…”