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I first came across dodecahedrane in comments below What dodecahedral molecule is Linus Pauling likely holding in this photograph? Does it have 40 carbon atoms?

Wikipedia's Dodecahedrane; Derivatives; Hydrogen substitution says:

Substitution of all 20 hydrogens by fluorine atoms yields the relatively unstable perfluorododecahedrane $\ce{C20F20}$, which was obtained in milligram quantities. Trace amounts of the analogous perchlorododecahedrane $\ce{C20Cl20}$ were obtained, among other partially chlorinated derivatives, by reacting $\ce{C20H20}$ dissolved in liquid chlorine under pressure at about 140°C and under intense light for five days. Complete replacement by heavier halogens seems increasingly difficult due to their larger size. Half or more of the hydrogen atoms can be substituted by hydroxyl groups to yield polyols, but the extreme compound $\ce{C20(OH)20}$ remained elusive as of 200613 Amino-dodecahedranes comparable to amantadine have been prepared, but were more toxic and with weaker antiviral effects.

Annelated dodecahedrane structures have been proposed.

13Fabian Wahl et al. (2006)Towards Perfunctionalized Dodecahedranes—En Route to C20 Fullerene (also [readable here)

The abstract of Wahl et al. (2006):

“One-pot” substitution of the twenty hydrogen atoms in pentagonal dodecahedrane ($\ce{C20H20)}$ by OH, F, Cl, and Br atoms is explored. Electrophilic insertion of oxygen atoms with DMDO and TFMDO as oxidizing reagents ended, far off the desired $\ce{C20(OH)20}$, in complex polyol mixtures (up to $\ce{C20H10(OH)10}$ decols, a trace of $\ce{C20H(OH)19}$?). Perfluorination was successful in a NaF matrix but (nearly pure) $\ce{C20F20}$ could be secured only in very low yield. “Brute-force” photochlorination (heat, light, pressure, time) provided a mixture of hydrogen-free, barely soluble $\ce{C20Cl16}$ dienes in high yield and $\ce{C20Cl20}$ as a trace component. Upon electron-impact ionization of the $\ce{C20Cl16}$ material sequential loss of the chlorine atoms was the major fragmentation pathway furnishing, however, only minor amounts of chlorine-free $\ce{C20+}$ ions. “Brute-force” photobrominations delivered an extremely complex mixture of polybromides with $\ce{C20HBr13}$ trienes as the highest masses. The MS spectra exhibited exclusive loss of the Br substituents ending in rather intense singly, doubly, and triply charged $\ce{C20H_{4–0}^{+(2+)(3+)}}$ ions. The insoluble ∼$\ce{C20HBr13}$ fraction ($\ce{C20Br14}$ trienes as highest masses) obtained along a modified bromination protocol, ultimately allowed the neat mass selection of $\ce{C20−}$ ions. The $\ce{C20Cl16}$ dienes and $\ce{C20H_{0–3}Br_{14–12} tri-/tetraenes}$, in spite of their very high olefinic pyramidalization, proved resistant to oxygen and dimerization (polymerization) but added $\ce{CH2N2}$ smoothly. Dehalogenation of the respective cycloaddition products through electron-impact ionization resulted in $\ce{C22–24H_{4–8}^{+(2+)}}$ ions possibly constituting bis-/tris-/tetrakis-methano-$\ce{C20}$ fullerenes or partly hydrogenated $\ce{C22}$, $\ce{C23}$, and $\ce{C24}$ cages.

Since Wikipedia lists nothing after Wahl et al. (2006) I'd like to ask:

Question: Can all 20 H atoms of pentagonal dodecahedrane be substituted with high yield and a wide range of substituents? What limits full substitution?

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Full substitution has been obtained, but only for very limited cases. The WP article cites successful full substitution only with fluorine and chlorine atoms. Neither heavier halogens nor small groups such as hydroxyl or amino were substituted fully in the syntheses that were carried out. This is summarized in the question and will not be repeated here.

A significant stumbling block to full substitution is steric hindrance. While the pentagonal angles are an almost perfect fit for "natural" Bond angles in tetrahedrally bonded carbon, the forced planarity of the rings leads to ligands being eclipsed relative to each other and thus creates steric repulsion between these. Isolated five-membered rings in organic compounds are generally puckered unless they are fully conjugated and aromatically coupled, as in pyrrole. The steric repulsions are small for hydrogen atoms but become worse with larger atoms or groups, explaining the decreasing success seen with heavier halogens and the lack of full substitution even with simple multi atomic groups. It would seem, therefore, that the fully methylated compound seemingly displayed by Linus Pauling would be very difficult to establish in real life.

Dodecahedrane has been dehydrogenated to make $\ce{C_{20}}$, which is perforce the smallest fullerene made up of faces with at least five sides.

The synthesis of the C20 fullerene C20 in 2000, from brominated dodecahedrane,[1] may have demoted C20H20 to second place.

Cited references

  1. Prinzbach, H.; Weiler, A.; Landenberger, P.; Wahl, F.; Wörth, J.; Scott, L. T.; Gelmont, M.; Olevano, D.; von Issendorff, B. Gas-phase production and photoelectron spectroscopy of the smallest fullerene, C20. Nature 2000, 407 (6800), 60–63. DOI: 10.1038/35024037.
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