Preferred Conformations from Conjugation in the a Framework

We have already seen in Section 2.2.3.3 how conformation can be affected by anomeric interactions, which can lead electronegative substituents to be axial at the 2-position in tetrahydropyranyl rings, and sometimes cause a chain of atoms to adopt a seemingly more hindered gauche conformation 2.81-2.83, 2.85 and 2.86 rather than the more usual zigzag arrangement. Similar hyperconjugative interactions in neutral molecules between two a bonds, one a a donor and the other a a acceptor, can lead them to adopt conformations in which the stereoelectronic effect overrides the purely steric effect. 

C—H bonds with the vicinal C—F bonds. Hyperconjugation will be energy-lowering with an interaction diagram like that in Fig. 2.14, but with the low-lying antibonding orbital taking the place of the empty 

With a more powerful donor like an Si—C bond, the preference for the donor and the acceptor bonds to be anti can be seen in cyclohexyl esters carrying a f3 silyl group. The equilibrium proportion of the alcohol is in favour of the normal diequatorial isomer 2.153 (R = H), but with esters (R = acyl) the equilibrium shifts to favour the diaxial conformation 2.154. Furthermore, the equilibrium constant correlates with how good the carboxylate ion is as a leaving group (pK of RO drops).182 

---