Horst Prinzbach

Gas-phase production and photoelectron spectroscopy of the smallest fullerene,C20

Fullerenes are graphitic cage structures incorporating exactly twelve pentagons. The smallest possible fullerene is thus C20, which consists solely of pentagons. But the extreme curvature and reactivity of this structure have led to doubts about its existence and stability. Although theoretical calculations have identified, besides this cage, a bowl and a monocyclic ring isomer as low-energy members of the C20 cluster family, only ring isomers of C20 have been observed so far. Here we show that the cage-structured fullerene C20 can be produced from its perhydrogenated form (dodecahedrane C20H 20) by replacing the hydrogen atoms with relatively weakly bound bromine atoms, followed by gas-phase debromination. For comparison we have also produced the bowl isomer of C20 using the same procedure. We characterize the generated C20 clusters using mass-selective anion photoelectron spectroscopy; the observed electron affinities and vibrational structures of these two C20 isomers differ significantly from each other, as well as from those of the known monocyclic isomer. We expect that these unique C20 species will serve as a benchmark test for further theoretical studies.

The azo-chromophor as component in photo [2+2]-cycloaddition reactions synthesis, structure and pe-analysis of planar-parallel bisazo-molecules

Abstract In order to test for a photo [2+2]-cycloaddition between two azo-units a tetracyclic bisazo-compound (7) has been constructed, for which the X-ray analysis confirms a planar-parallel arrangement of the N 2 -units and transannular N-N-distances of 2.877 A and 2.821 A, resp. In preliminary photoexperiments N 2 -elimination predominates in 7; in the N-oxides 10/11, however, after direct excitation (λ = 254 nm) N 2 /N 2 O-metathesis (13/14 and 15, resp.) is observed.

From Pagodanes to Dodecahedranes - Search for a Serviceable Access to the Parent (C20H20) Hydrocarbon**

Abstract By taking advantage of the novel SN2 pagodane → dodecahedrane routes a preparatively potent access to the parent pentagonal dodecahedrane (2) was explored. The one-pot catalytic procedure (Pd/C/H2) starting from an eightfold functionalized secopagodane (14, C20H12Br6 (CO2 CH3)2) excells in shortness but, rather erratic (33–53%), falls out of the competition. The longer route via 1,6-dicarboxyl/dibromo dodecahedranes (35, 22) turned out as first choice with a total yield of 74–76% based on the common pagodane precursor (13).

The azoxy(azo)-chromophore as π2-component in photo [2+2] cycloaddition reactions

Abstract Upon direct (sensitised) excitation selective metathesis is observed for the azoxy/ene 4 (with no intermediate being detectable at −50°C), quantitative [2+2] addition for the azo/ene 2. Thermally 4 undergoes uniform cycloaddition (t1/2(18°C) ca. 65 min) to the unusually stable 1,2,3-oxadiazolidine 3 (X-ray).

Molecular structures of a seco-dodecahedradiene and an iso-dodecahedrane

Abstract The olefinic carbon atoms on the closed side of seco-dodecahedradiene 1 are strongly pyramidalized (o = 35.3°). The transanular C1C2 bond of non-pentagonal isododecahedrane 2 is one of the longest CC bonds (1.691A) ever measured in cage structures.

1H-AZA- and oxa-heterocinyl-anions - planar-diatropic 10 π-systems

Abstract Independent of the different hetero-ring-elements, the 4-cyanosubstituted N-tert-butyl-/N-tosyl-azocinyl and oxocinyl anions ( 5a-c ) are planar-diatropic 10 π systems.

[6+6]-benzo/benzo - photocycloaddition reactions

Abstract Cyclobutane-formation between “face-to-race” oriented benzenoid chromophoric units has been observed in 1,6-diene- (B) (1b–d/4a–d), not, however, in 1,5-diene-derived (C) (7a–c) skeletons.

Biocatalysis in the chiral recognition of meso-diamides: An efficient route from cyclic olefinic hydrocarbons to optically pure diamino-polyols

Abstract meso-Diamino-di(tri,tetr)ols 1 – 8 were synthesized starting from cheap carbocyclic olefins and cis -diepoxy derivatives. Enantioselective hydrolysis of the corresponding bis(phenylacet)amides with penicillin amidase from E. coli (EC 3.5.1.11) was effected in good yields (73 – 90%) and with high optical purities (ee 91 − >97).

Cis-trioxa-tris-β-homobenzene as tridentate ligand : X-ray crystal structure analyses of [Li(C6H6O3)2 PF6] and [Ca(C6H6O3)3H2O(ClO4)2]

Abstract In the title complexes the cis-benzenetrioxide acts as tridentate ligand, allowing for octahedral and unusual tetracapped trigonal prismatic coordination (TECTP).

Unsaturated dodecahedranes—Synthesis of the highly pyramidalized, highly reactive C20H18 and C20H16 olefins

Methodological alternatives for the preparation of highly strained, highly pyramidalized dodecahedrene 2 (Estr=87.3 kcal mol−1; ϕ=43.5°, MM2) and 1,16-dodecahedradiene 3 (Estr=105.3 kcal mol−1; ϕ=42.9°, MM2) have been explored, protection/deprotection strategies have been tested—with the eye on their utilization for the generation of higher unsaturated dodecahedranes (e.g. 1,4, 16-triene 4, 1,4,10 (14),16-tetraene 5). For the acquisition of preparative quantities of monoene 2 the “P2F” catalyzed cis-β-elimination in bromododecahedrane, of diene 3 the FVP fragmentation of a “twofold protected” precursor (bis-furan adduct) have become the protocols of choice, which both profit from the recent synthetic advances along the pagodane → dodecahedrane scheme. Because of unusually effective steric protection the highly tilted C=C double bonds of 2 (λmax (CH3CN) = 254 nm, νC=C = 1658 cm−1, δC=C = 164.4) and 3 (δC=C = 170.5) enter into thermal stabilization pathways (dimerization, oligomerization) only at higher temperatures (for 2 ca. 50% consumption after 5 h at 100°C in a 3·10−3 molar toluene solution); extreme sensitivity to oxygen is primarily attributed to kinetically and thermodynamically promoted allylic hydrogen abstraction.