Strain transannular

Instead, transannular strain between the silylether and one of the Cp ligands leads to a product 77 from a boat-like insertion transition structure (76 bottom). 

Torsional strain and van der Waals repulsions between hydrogen atoms across rings (transannular strain) cause the small instabilities of these higher cycloalkanes. 

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. 

Transannular strain (Section 6.6) A type of strain that occurs in larger rings because of steric crowding of atoms on the opposite sides of the ring. Also called cross-ring strain. 

Internal strain in cyclic systems 93,95) incorporates angle strain (Baeyer strain), torsional strain (Pitzer strain) and transannular strain. In the case of nitrogen inversion, large effects are expected to arise from angle strainb>, the nature of the cyclic system imposing a value of the angle 0... 

With larger bicyclo[n. 1. OJalkyl cyclopropyl derivatives such as 8 and 9, their solvolytic behavior follows from that of the simple alkyl-substituted cyclopropyl derivatives. With smaller bicyclo[n.l.0]alkyl cyclopropyl derivatives such as 10 and 11, however, where a trans-a y cation cannot be accommodated in the ring, the order of reactivity is reversed. In both the [6.1.0] and [3.1.0] examples mentioned above, the rates are given relative to cyclopropyl tosylate. The much higher reactivity of the endo-[3.1.Qi] system (11) over the endo-[6.1.0] system (9) reflects the stability of the almost strain-free cyclohexenyl allylic cation versus the cyclononenyl allylic cation which possesses both torsional and transannular strain. 

Pitzer strain The strain that results from eclipsed conformations, as opposed to transannular strain. 

Transannular strain The strain that occurs due to the interaction of the substituents on Cl and C3 or Cl and C4, when a gauche conformation has been assumed by the molecule. It results from there being insufficient internal space to accommodate the groups in question. 

However, on top of the inclusion phenomena, cyclophanes have more interesting properties to offer. In his first publication on cyclophanes, Cram [13] had already expressed his opinion about some peculiarities to be expected in cyclophanes, which he outlined as follows a) electronic interaction between aromatic rings placed face to face , b) the resulting influence on substitution reactions in the aromatic rings by transannular electronic effects, c) intramolecular charge transfer complexes and d) ring strain, steric strain and transannular strain. These effects have been studied on the parent compounds in detail [3]. 

---