Benzene formation from prismane

As noted above, there are no measurements on the enthalpy of formation of prismane, per se, but experimental studies of the rearrangement enthalpy of hexamethylprismane to hexamethylbenzene have been reported". But the two relevant numerical values, 382 and (345 3) kJ mol" respectively, are markedly different from each other, and from the ca 570 kJmol" suggested by modern calculational theory" for the parent species. While the measured rearrangement enthalpy" of hexakis(trifluoromethyl)prismane to hexakis(trifluo-romethyl)benzene is ca 135 kJ mol" smaller than the above hexamethyl results, so be it CF3 substitution is significantly different from that of Me". But it eludes us why there is... 

In the most thoroughly studied series, it was found that hexamethylprismane gives hexamethyl Dewar benzene with log k = 14.50 — 33 00l23RT and hexamethyl-benzene with log A = 13.94 — 33 200/2.3/ T (Oth, 1968). Hexamethylbenzvalene was detected as a steady-state intermediate in the NMR spectrum of the reaction suggesting that the portion of the reaction leading from the prismane to the benzene involves rate-determining formation of the benzvalene. If the observations with the parent system are extrapolatable from to this, hexamethylbenzvalene must give hexamethylbenzene directly. From the heats of reaction derived from calorimetry and the activation enthalpies, an enthalpy surface for the interconversion can be constructed (Scheme 7.7). 

Of interest is the fact that the relative heats of formation of the permethyl isomers are almost identical to those calculated by MINDO 3 for the parent molecules. More recent, higher level (G2) calculations utilizing isodesmic reactions have provided very different relative heats of formation. The absolute values are benzene 19.6 kcal/mol Dewar benzene 95 kcal/mol prismane 131 kcal/mol benzvalene 90.5 kcal/mol and 3,3 -biscyclopropenyl 138 kcal/mol. The experimental value for the heat of formation of benzene is 19.8 kcal/mol, and the experimental difference in enthalpy between benzvalene and benzene is 67.5 kcal/mol. The latter value is reproduced by the calculation to within 4 kcal/mol. These calculations along with the experimental activation data from the above provides the enthalpy surface of Scheme 7.8. 

The formation of the permethyl Dewar benzene (and presumably the parent compound) from the prismane appears to be a symmetry forbidden retro 2 + 2 cyclization perhaps proceeding via a biradical which can also give the benzvalene via a 1,2-shift of the cyclopropane (Scheme 7.9). 

The Photochemical synthesis of alkyl substituted benzene takes advantage of various pathways to provide one or more of the valence bond isomers or a positional isomer of the starting material is illustrated for 1,2,4-tiibutyl benzene, shown below. The formation of the 1,3,5-isomer results from cycloreversion of the prismane depicted or arises via the benzvalene to deld the least sterically congested product. By an analogus sequence o-xylene can gives nv and p-isomer. 

---