Energy transannular strain

It must be emphasized diat angle strain, torsional strain, and transannular strain are simply terms invented by chemists to categorize structural features in rings which lead to increased energy (strain). The molecules diemselves adopt structures which are the most stable and have die greatest amount of bonding energy. Nevertheless, strain is an important feature in molecules to keep in mind since it is manifested by altered chemical reactivity. 

There is also a boat-like conformation, 49C, but the flat part of the seven-membered ring has diminished transannular strain because the hydrogen atoms are a little further apart. The strain energies for chair cycloheptane and boat cycloheptane are close, and one does not greatly predominate over the other. There are several other conformations for cycloheptane, as there are for cyclohexane, because the size and flexibility of the ring has increased however, at this point, no other conformations for cycloheptene will be discussed. 

Figure 11.8 Computational results of the [3.2.0] system 22, its complex 24, and transition state 25. (a) Tricyclic boranes at the fourth through sixth homologated stages contain inverted pyramidalized boron centers arising to reduce transannular interactions and angle strain, (b) Complex formation with the ylide 1 is endothermic, (c) Transannular strain energy is partially relieved at the transition state.

The boat-boat (417) and the twist-boat-boat (418) have low torsional strains but severe non-bonded repulsions, which, as usual, are transferred to internal angle strains. However, heteroatoms can modify these repulsions and certain transannular interactions can drastically reduce them. Even so, the boat-boat family is relatively unimportant as its energy is calculated to be quite high (12 kj mol-1) in cyclooctane. It probably serves as an intermediate for certain conformational interconversions of the boat-chair, especially when the twist-boat-chair pseudorotation itinerary is of high energy. In cyclooctane the boat-boat and its twisted partner have nearly the same energies and are not separated by a significant barrier. 

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