Propene conformational stability

A similar study was done with methyl acrylate as the dienophile.28 The uncatalyzed and catalyzed TSs are shown in Figure 6.7. As with propenal, the catalyzed reaction is quite asynchronous with C(2)-C(3) bonding running ahead of C(l)-C(6) bonding. In this system, there is a shift from favoring the exo-s-cis TS in the thermal reaction to the endo-s-trans TS in the catalyzed reaction. A large component in this difference is the relative stability of the free and complexed dienophile. The free dienophile favors the s-cis conformation, whereas the BF3 complex favors the s-trans conformation. 

The two conformations of l-fluoro-2-propene which we shall compare are the cis and gauche conformations shown below along with the dominant stabilizing donor-acceptor interactions which obtain in each case. 

Yet another application of the frontier orbitals lies in the field of the relative stability of isomers and conformers. Let us consider propene. 

The conformation of propene is influenced by hyperconjugation. The methyl substiment has an overall stabilizing effect (2.7kcal) on the double bond, as can be concluded from the less negative heat of hydrogenation compared to ethene (see Section 3.1.1). This stabilization arises from a-ir interactions. The major effect is a transfer of electron density from the methyl a C-H bonds to the empty tt orbital. 

Fig. 1.32. Interactions between two hydrogen s orbitals and carbon 2p, orbitals stabilize the eclipsed conformation of propene.

---