Pagodanes isomerization

The standard enthalpy of formation of dodecahedrane remains unmeasured. However, standard enthalpies of formation of a bis-carbomethoxy derivative of the isomeric pagodane, and pagodane are available in the literature H.-D. Beckhaus, C. Riichardt, D. R. Lagerwall, L. A. Paquette,... 

Scheme 4.17 presents a couple of other strange-looking cationic species which were discovered in studies in a related field. In connection with the problem of dodecahedrane synthesis via the isomerization of pagodane 34 (cf. data in Schemes 4.10 and 4.11), Olah s and Prinzbach s groups engaged in studies of the behavior of pagodane derivatives under superacid conditions. Their hope was to force the cationic isomerization of 34 to 3. Despite all attempts, this route was unworkable. As a reward for these apparently futile efforts they were able to observe the unexpected formation of a very stable cationic species, the pagodane dication 55 (Scheme 4.17). The pattern of its NMR spectra combined with the nature of its quenching adduct 56, and the theoretical analysis of possible alternatives, enabled the authors to ascribe to this dication the unprecedented four-center/two-electron delocalized bis-homoaromatic structure.

The [2 + 2] cyclodimerization of benzene has been studied theoretically (Engelke et al., 1984) and used practically in the synthesis of polycyclic hydrocarbons such as 69 (Fessner et al., 1983). This represents a first step in the synthesis of the undecacyclic hydrocarbon pagodane (70), which in turn can be isomerized to give dodecahedrane (Fessner et al., 1987). 

A novel approach to highly symmetrical pentagonal dodecahedrane which involved gas phase isomerization of [l.l.l.l]pagodane over 0.1% Pt/Al203 under an atmosphere of H2 at 360 °C was provided by Prinzbach, Schleyer, Roth and coworkers . Dodecahedrane was obtained in 8% yield (equation 27). The isomerization probably involves carbocationic intermediates. 

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