Dodecahedrane

Dodecahedrane is a chemical compound (C20H20) first synthesised by Leo Paquette of Ohio State University in 1982, primarily for the "aesthetically pleasing symmetry of the dodecahedral framework". In this molecule, each vertex is a carbon atom that bonds to three neighbouring carbon atoms. Each carbon atom is bonded to a hydrogen atom as well. It has Ih symmetry (just like that other molecular sphere fullerene) evidenced by its proton NMR spectrum in which all hydrogen atoms appear at a single chemical shift of 3.38 ppm. Dodecahedrane is one of the platonic hydrocarbons, the others being cubane and tetrahedrane and does not occur in nature.

Total synthesis
For over 30 years several research groups have actively pursued the total synthesis of dodecahedrane. A review article published in 1978 just dealt with the different strategies that existed up to then. The first attempt was initiated in 1964 by R.B. Woodward with the synthesis of the compound triquinacene which was thought to be able to simply dimerize to dodecahedrane. Other groups joined in the race, for example that of Philip Eaton and Paul von Ragué Schleyer

Paquette's 1982 organic synthesis takes about 29 steps with raw materials cyclopentadiene (2 equivalents 10 carbon atoms), dimethyl acetylenedicarboxylate (4 carbon atoms) and allyltrimethylsilane (2 equivalents, 6 carbon atoms). In the first leg of the procedure two molecules of cyclopentadiene 1 are coupled together by reaction with elemental sodium (forming the cyclopentadienyl complex) and iodine to dihydrofulvalene 2. Next up is a tandem Diels-Alder reaction with dimethyl acetylenedicarboxylate 3 with desired sequence pentadiene-acetylene-pentadiene as in symmetrical adduct 4. An equal amount of asymmetric pentadiene-pentadiene-acetylene compound is formed and discarded. Organic reduction with lithium aluminium hydride gives diol 5 and reaction with sulfene gives the mesylate 6 as efficient leaving groups which is then heated with sodium iodide in hexamethylphosphoramide to the tetraene 7. This compound is then oxidized to diester 8.

In the next step of the sequence iodine is temperarily introduced via a iodolactonization of the diacid of 8 to dilactone 9. The ester group is cleaved next by methanol to the halohydrin 10, the alcohol groups converted to ketone groups in 11 by Jones oxidation and the iodine groups reduced by a zinc-copper couple in 12.

The final 6 carbon atoms are inserted in a nucleophilic addition to the ketone groups of the carbanion 14 generated from allyltrimethylsilane 13 and n-butyllithium. In the next step the vinyl silane 15 reacts with peracetic acid in acetic acid in a radical substitution to the dilactone 16 followed by an intramolecular friedel-Crafts alkylation with phosphorus pentoxide to diketone 17. This molecule contains all required 20 carbon atoms and is also symmetrical which facilitates that the construction of the remaining 5 carbon-carbon bonds Reduction of the double bonds in 17 to 18 is accomplished with hydrogenation with palladium on carbon and that of the ketone groups to alcohol groups in 19 by sodium borohydride. Replacement of hydroxyl by chlorine in 21 via nucleophilic aliphatic substitution takes place through the dilactone 20 (tosyl chloride). The first C-C bond forming reaction is a kind of Birch alkylation (lithium, ammonia) with the immediate reaction product trapped with chloromethyl phenyl ether, the other chlorine atom in 22 is simply reduced. This temporary appendix will in a later stage prevent unwanted enolization. The newly formed ketone group then forms another C-C bond by photochemical Norrish reaction to 23 whose alcohol group is unduced to eliminate with TsOH to alkene 24.

The double bond is reduced with hydrazine and sequential diisobutylaluminum hydride reduction and pyridinium chlorochromate oxidation of 25 forms the aldehyde 26. A second Norrish reaction then adds another C-C bond to alcohol 27 and having served its purpose the phenoxy tail is removed in several steps: a Birch reduction to diol 28, oxidation with pyridinium chlorochromate to ketoaldehyde 29 and a reverse Claisen condensation to ketone 30. A third Norrish reaction produces alcohol 31 and a second dehydration 32 and another reduction 33 at which point the synthesis is left completely without functional groups. The missing C-C bond is put in place by hydrogen pressurized dehydrogenation with palladium on carbon at 250°C.