Walk rearrangements energies

Thus it may be that 14 actually results from a walk rearrangement (vide infra) with a very low activation energy, due to the alkoxycarbonyl group. 

Rate Constants and Gibbs Activation Energies of Walk Rearrangements and Electrocyclic Ring Openings in Ester-Substituted and Nitrile-Substituted Dimethylbicyclopentene Derivatives (25)... 

Walk rearrangements have been observed in many carbo- and heterocyclic norcaradiene cycloheptatriene systems upon thermal as well as photochemical excitation. Table 4 contains selected examples. In this connection, the very different thermal stability of ll,ll-dimethyl-l,6-methano[10]annulene and its radical anion is worth mentioning (59). The neutral hydrocarbon rearranges to 7,7-dimethyl-l,2-benzocycloheptatriene at temperatures between 150 and 190°C [Table 4, entry 3, X = C(CH3)2 log A = UA Ea = 35.9 kcal/mol]. The corresponding rearrangement of the radical anion produced by reduction of the hydrocarbon with potassium occurs at -110°C already. The activation energy is lowered here by about 25 kcal/mol over that of the hydrocarbon. The norcaradiene walk has also gained some synthetic importance, for example, in the preparation of the unsaturated bicycle 44, a precursor of heptalene (60). 

Figure 14. Stereochemistry of the photochemically induced walk rearrangement in homo-tropylium cations. Gibbs activation energy of the thermal endo - exo isomerization is in kcal/mol (124B, 127).

Based on ab initio SCF calculations (114), the difference in the activation energies required for the two walk rearrangements might result entirely from these ground state differences between the bicyclo-[3.1.0]hexenyl and homotropylium cations. 

---