Oxygen lone electron pairs, stabilizing interactions

The interaction diagrams for the above conformations are identical with that of methyl vinyl ether (Fig. 30) except that the oxygen lone pair AO is replaced by an unoccupied carbon 2p AO. With this in mind we conclude that the transoid conformation of the cation, Ts, will be more stable than the cisoid conformation, Cs, since the <(>j—Pz two electron stabilizing interaction is greater for the Ts conformation. 

Consider the reaction of acetoxycyclopentadiene with ethylene. In the conformation shown, the oxygen sp2 lone pair (cf. p. 200) has the correct symmetry to interact with the ethylene n orbital in the transition state leading to the anti isomer. This is a four-electron combination, so it is destabilizing. However, at the same time, the oxygen p lone pair interacts favorably with the ethylene n. Furthermore, a rotation of the ace-toxy group around the OC bond will diminish the overlap of the sp2 lone pair with k and increases its overlap with n. Both lone pairs can then interact with n the stabilizing interaction dominates and the anti isomer should be preferred. Experimentally, Winstein et al.32 isolated only this isomer. 

One interesting possibility emerges from the likelihood that an n-71 interaction between an oxygen lone pair of LBA and jr electrons of the terminal carbon-carbon double bond of the substrates stabilizes the transition state of the cyclization or the initial protonation step. The transition-state assembly proposed on the basis of this assumption and the steric repulsion would clearly lead to predominant approach of (i )-LBA to the si face of the terminal isoprenyl group (Fig. 4). 

It is also tempting to propose a stabilizing interaction between the electron cloud associated with the oxygen atom lone pairs and the electron-depleted carbon atom of the iminium unit. This suggestion in this case is supported by single-crystal X-ray analysis (Fig. 5.6), although other catalysts of this general... 

Box based his explanation (8, 14) of the reverse anomeric effect on the lone-pair interaction model analogous to that presented earlier by Finch and Nagpurkar vide supra). The observed conformational behavior is a result of an interplay of stabilizing and destabilizing interactions. The former consist of attractive, stabilizing interactions between the lone pairs of the 0(1) oxygen and the electron deficient r, or n, orbitals of a substituent, which are favorable in 76e, as shown in Figure 9. The normal steric requirements of... 

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