| Abstract: | NMR spectroscopy has been used to investigate the ring inversions of the unsaturated seven membered ring system in a total of 20 benzocycloheptene derivatives with 1, 2 and 3 pairs of geminal substituents. For all compounds the inversion of the ring at ? 80°C is ‘frozen’ and at this temperature only one conformation is present in detectable quantity, presumably that of the chair form. The free activation enthalpies ΔG≠ for the chair inversions lie between 9·9 and 13·7 kcal/mole. For disubstituted and tetrasubstituted benzocycloheptenes the ΔG≠ values vary according to the positions of the ligands: for disubstituted derivatives ΔG≠ is largest for the 5-position and smallest for the 3-position. For the tetrasubstituted derivatives the inversion of the ring—compared to that in the comparable dimethyl derivatives—is made more difficult when the ligands are in the 3,6- or 3,7- positions, but is facilitated when in the 3,5- or 4,6- positions. The effect observed in the 3,5- and 4,6- substituted rings is due to transanular repulsion of synaxial substituents, which leads to a flattening of the ring. Such a repulsion does not occur when the ligands are in the 3,6- positions. On the other hand, when the ligands are in 3,7- positions the transanular repulsion leads to a stronger puckering of the chair; the inversion could be hindered by this. For benzocycloheptene the activation energies for the inversions between chair, boat and twist (S, W, T) conformations were determined from model calculations. The best route for the inversion of the chair is the version way S → W via the transitional conformation V45 and V56. The calculated activation energy for this (14·6 kcal/mole) agrees well with the experimentally determined value (13 ± 1·5 kcal/mole). For the pseudorotation W → T a slightly lower calculated value of 11·1 kcal/mole was found. |
