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Dodecahedrane-The Chemical Transliteration of Plato's Universe (A Review) LEO A

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Dodecahedrane-The Chemical Transliteration of Plato's Universe (A Review) LEO A Proc. NoL Acad, Sci. USA Vol. 79, pp. 4495-4500, July 1982 Review Dodecahedrane-The chemical transliteration of Plato's universe (A Review) LEO A. PAQUETTE Evans Chemical Laboratories, The Ohio State University, Columbus, Ohio 43210 Communicated by Daniel E. Koshlond, Jr., April 26, 1982 ABSTRACT The development ofchemical interest in three of deviate the greatest from normalcy, this molecule should pos- Plato's five convex polyhedra is described from an historical per- sess the greatest strain energy per carbon atom. In contrast, spective. The successful synthesis of 1,16-dimethyldodecahedrane dodecahedrane is characterized by minimal angle strain, al- and its structural characteristics are outlined. Finally, an account though unparalleled high levels oftorsional strain exist because ofrecent work leading to the still more aesthetically appealing and ofthe precise eclipsing ofall the C-H bonds. The chemist's fas- ultrasymmetric parent dodecahedrane is given. cination with 1, 2, and 3 is fueled further by the aesthetically delightful topologies ofthese structures, the presence ofcavities Men oflearningin ancient Greece exhibited particular fondness ofincreasing size (perhaps reaching a practical lower limit in 3), for, and interest in, five exquisitely symmetric convex poly- the spectral consequences of 4, 8, or 20 symmetry-equivalent hedra, each composed of totally equivalent vertices and edges. methine units, the absence of structurally allied substances in In Plato's thinking, tetrahedrons constituted the smallest com- nature, and much more. ponents of the element fire and pentagonal dodecahedra com- Despite dire theoretical predictions about the intrinsic insta- prised the building blocks ofheavenly matter; in between were bility of tetrahedrane, there has been no shortage of attempts cubes for earth, octahedra for air, and icosahedra for water. to synthesize this fascinating molecule. Although the parent Much later, in the 17th century, the astronomer Johannes Kep- hydrocarbon remains unknown, Maier et aL succeeded in 1978 ler made extraordinary use of these five regular bodies in his in preparing the tetra-tert-butyl derivative 5(1, 2). The critical model of the universe. steps in the synthesis (Scheme I), which depends very heavily That the heritage of this wonder should inevitably pass into on light-induced reactions, are the photoisomerization and de- the realm of synthetic organic chemistry comes as no surprise. carbonylation of 4. The colorless crystalline 5 is amazingly re- However, the requisite transliteration into hydrocarbon equiv- sistant to oxygen, moisture, and heat (mp 1350( dec). At the alents necessarily excludes octahedrane and icosahedrane for melting temperature, isomerization to tetra-tert-butylcyclo- practical reasons. Ofthe remaining three, 1 belongs to Td point butadiene occurs. The extraordinary stability of 5 has been group with its 24 identity operations, 2 has full Oh symmetry, and 3, with its 'h character and 120 identity operations, is the hvi -hv&-I. II + 0 /a -c hVi hv *0 I 2 5 4 a, Br2; b, KOH; c, tert-BuLi. Scheme I attributed to the fact that rupture of any framework bond is opposed by increased steric interaction between the pendant tert-butyl groups. The most notable chemical property of5 un- covered to date is its penchant for oxidation to the isomerized radical cation in the presence of many reagents. The tactical elaboration of cubane was masterfully achieved by Eaton and Cole (3) at the University of Chicago as early as 3 1964. Their protocol (Scheme H) took good advantage of the facility with which 2-bromocyclopentadienone dimerizes to 6. most highly symmetric hydrocarbon possible. All three mole- Following photocyclization of the dimer, 2-fold Favorskii ring cules clearly are structurally unusual, and their properties are contraction was achieved with hot potassium hydroxide. Once not predictable with certainty a priori from current theories of dicarboxylic acid 7 was in hand, it was a simple matter to remove chemical bonding. Because the facial angles of tetrahedrane the carboxyl groups by conventional methods. 4495 Downloaded by guest on October 3, 2021 4496 Review: Paquette Proc. NatL Acad, Sci. USA 79 (1982) Br f-t Y0 6%A Lto0 ]. 6 lhV cOc )H _ 0 d IO-OC O= Br I 4-d NW Br HOOC 2 7 a, NBS; b, Br2; c, (C2H5)3N; d, KOH, H20, A, and then H30+; e, SOC12; f, tert-BuOOH; g, triisopropylbenzene, A. Scheme II Presumably because oforbital symmetry constraints, 2 is also carbon atoms (26-28). From the outset, it was that impressively stable to heat. Only when cubane is kept at 2000C projected for 23 should derive from the cyclopentadienide anion (10), a sim- 2 weeks does one obtain cyclooctatetraene via cuneane (8) ple and inexpensive commodity. Following oxidative and semibullvalene (4). Silver(I) ion greatly accelerates this par- of 10 to coupling ticular isomerization (5); rhodium(I) leads 9,10-dihydrofulvalene, a domino Diels-Alder reaction alternatively to the (29-31) is carried out with dimethyl to syn dimer of cyclobutadiene (9) (6, 7). acetylenedicarboxylate give 11, a C2,,-symmetric molecule whose role it is to serve as the cornerstone of the spherical target molecule (Scheme Controlled reduction in the Ill). Ag (I) Rh(I) symmetry level ofthis intermediate -.Vl to diketo diester provide 12 which possesses only a simple C2 molecular axis is entirely possible by a hydroboration-oxidation sequence (32). In practice, however, a somewhat longer, more 8 2 efficient catercorner oxygenation sequence is preferred (26-28) because the unwanted The challenge offered by the synthesis ofthe pentagonal do- C. diketo diester is not produced decahedrane is of large proportions. The ultimate objective is concomitantly. to interlink 20 Bisspiroannulation of 12 with diphenyleyclopropylsulfonium methine units with 30 carbon-carbon bonds in ylide followed oxidation delivers the such a way that 12 multiply fused five-membered rings result. by hydrogen peroxide ax- ially symmetric spirolactone 13 which can be isomerized under Additionally, the topology of 3 is such that all ring fusions are strongly and cis-syn, a feature which causes acidic conditions subsequently fashioned into di- many partially constructed ore- lactone 14. It must be recognized that the continued mainte- cursors to suffer excessively high levels of nonbonded steric nance of C2 symmetry now reduces our analysis of the subse- strain and consequently to exhibit a pronounced tendency for quent chemistry of14 to the manipulation ofa pair ofsymmetry- transannular bonding, framework isomerization, and nonevelo- related ester carbonyl pentanoid cyclization (8, 9). It is groups. Furthermore, this intermediate chiefly for these reasons and already contains all requisite 20 carbon atoms and has ofthe other related factors that the several convergent approaches to 12 3 all-important cis-locked stereocenters properly in- which have been attempted by various groups have not suc- stalled. methine Although the chemical reactivity of 14 far less ceeded as yet. These include the formal combination oftwo proved C1o controllable than originally the con- units (10-18), capping of a C15 unit with a anticipated (33, 34), pivotal cyclopentane ring version to 15 can be accomplished quite (19-22), amalgamation of C,6-hexaquinacene with a C4 segment efficiently. (23, 24), and controlled Having arrived at 15 in a minimum of nine laboratory steps, Lewis acid-catalyzed isomerization of we an number to a more readily required equal craft the monoseco alcohol 20 accessible (CH)20 isomer (ref. 25; P. von R. Under the influence of in Schleyer, personal communication). (SchemeIV). lithium liquid ammonia, 15 experiences a splendid reduction-alkylation sequence to gen- These serious complications prompted consideration of a se- erate a dienolate which is directly methylated to yield 16 (35). rial synthetic approach to 3. Importantly, we hoped to overcome In this way, the endo orientation of the pair of groups the tactical disadvantages of nonconvergency by on carbonyl capitalizing is guaranteed and the methyl substituents serve as the symmetry ofthe target molecule throughout the sequence. 1H NMR invaluable Early in beacons. Light-induced ring closure at the ketone site 1981, my colleagues and I were pleased to be able to (ester are report our groups generally photoinactivatable), dehydration of successful acquisition ofthe 1, 16-dimethyl derivative the resulting tertiary alcohol, and diimide reduction delivers (23) in a number ofsteps fewer than the total count offramework the triseco ester 17. While this intermediate clearly possesses Downloaded by guest on October 3, 2021 Review: Paquette Proc. Natd. Acad. Sci. USA 79 (1982) 4497 H 3O0%IV!:: 0 No+ a b, c CH3.9- 10 11 12 d, e CH30 C |, .,% f-h '-VI I s o CH3 15 14 13 a, 12, then CH30OC=CCOOCH3; b, [(CH3)2CHCH]2-BH; c, Cr03; d, tA-SPh2, then H+; CH3 e, H22; f, P205, CH3SO3H; g, H0, Pd/C; h, NaCNBH3; i, HCI, CH30H. Scheme III many of the desirable structural features being sought, its op- efficiencyofthis process was appreciably lower (29%) than here- posed methylene groups remain unfunctionalized. After some tofore because of the well-established tendency of tertiary al- preliminary studies, the decision was made to address this po- dehydes to experience decarbonylation from their excited tentially complex issue at a later stage. Following reduction of states. Nevertheless, the important point here is that a five-ring 17 to the aldehyde level, 18 was photocyclized as before. The ketone (19) is produced upon which light may be impinged for CH30 CH30 CH34, 0 a b-d CI co3 is2 POP 3 CH3 15 16 17 e ,f OH 13 CH3H 0 b b, f OH3 CH3 20 19 18 a, Li/NH3, then CH31; b, hv; c, TsOH, C6H6, d, HN NH, H3OH; e, (i-Bu)2AIH; f, PCC CH2CI2. Scheme IV Downloaded by guest on October 3, 2021 AA.98 Review: Paquette Proc. Nad Acad. Set. USA 79 (1982) a b OH3 20 21 22 c lc CH3 io 23 a, TsOH, CAH6; b, H2NNH2, H202; c, CF3SO3H, 0H2C12.
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