Stereoelectronic effect hyperconjugation

Key words Carbohydrate - Conformational analysis -Stereoelectronic effects - Hyperconjugation... 

This stereoelectronic effect also explains the contrasting regioselectivity of cis- and fra x-2-fluoro-4-f-butylcyclohexanone.235 As a result of a balance between its polar effect and hyperconjugation, the net effect of a fluoro substituent in acyclic systems is small. However, in 2-fluorocyclohexanones an unfavorable dipole-dipole interaction comes into play for the cis isomer and preferential migration of the fluoro-substituted carbon is observed. 

Stereoelectronic effects in chemical reactivity The bond-lengthening and -weakening influence of an antiperiplanar lone pair leads to strong stereoelectronic effects on chemical reactivity.97 In molecule 28a with lone-pair-bearing atom D adjacent to an A—B bond, a vicinal nD—s-cab hyperconjugative interaction can be associated (cf. Example 1.4 and Section 3.3.1) with a partial admixture of the alternative resonance structure 28b,... 

In the extreme carbocation limit of (3.163) and (3.164), the stereoelectronic secondary-hyperconjugation effects therefore blend seamlessly into ordinary pi-type conjugation phenomena (Section 3.3), the two extremes always being linked by electronic continuity. 

Unfortunately, intuitive predictions of reactivity on the basis of stereoelectronic effects are not always possible, because these effects are subtle and can easily be overridden by steric, inductive, or field effects, or by conformational changes during the reaction [58]. It must also be kept in mind that hyperconjugation in the transition state, and not in the ground state, will be have the largest effect on the reaction rate. 

Stereoelectronic effects can have a profound effect on the ground-state structure of molecules, and can often help to explain counter-intuitive conformational preferences or spectroscopic features. Their effect on the energy of transition states is, however, less straightforward to predict. As stated by the Curtin-Hammett principle [75] (Section 1.4), reactions will proceed via energetically unfavorable conformers if these are more reactive (as is often the case) than better stabilized conformers. In such instances ground-state stabilization of certain conformers or the weakening of bonds by hyperconjugation will not necessarily be predictive for the outcome of a reaction. 

Alabugin, I. V. Zeidan, T. A. Stereoelectronic effects and general trends in hyperconjugative acceptor ability of a bonds. J. Am. Chem. Soc. 

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. 

Due to the above differences, stereoelectronic effects in O- and S-heterocycles cannot be automatically transferred to the N-heterocycles and vice versa. Differences in hybridization were shown to be important for stereoelectronic hyperconjugative interactions. ° Not only is hybridization of a donor orbital related to its absolute energy, but hybridization determines the direction in which non-bonding orbitals are projected in space. Consequently, directionality of sp" hybrids can change considerably as a function of substitution. Furthermore, hybridization controls the relative size of the two lobes of a lone pair. The front and back lobes... 

This chapter outlines the variety of common stereoelectronic effects where donor and acceptor orbitals are separated by a single bond. With a few exceptions, the sigma bridge will not be directly considered as a contributor to the hyperconjugative interactions. It will be treated as simply a means for keeping the interacting vicinal orbitals in the parallel sidewise arrangement. 

Most stereoelectronic effects involve interactions of orbitals at least one of which corresponds to a o-bond. Consequently, they are classified under the umbrella of hyperconjugative interactions. However, also included are n-jt, p-jt, and conjugative effects that display similar stereoelectronic features. 

Orbital interactions are complex and depend on the combination of stabilizing delocalizations, sterics, and electrostatics. Again, in the spirit of the definition of stereoelectronic effects in the introduction, we will concentrate on stabilizing effects, such as hyperconjugation and conjugation. 

The authors analyzed the complexity of stereoelectronic effects in this system using NBO analysis of all vicinal interactions involved in electronic communication between the vinyl moiety and the substituted allyl group. The total hyperconjugative energies closely follow the overall conformational profiles. 

Another stereoelectronic effect not associated with hyperconjugative stabilization Cuevas, G., Martmez-Mayorga,... 

The remarkably high stability of (24-ax) (and (3-ax)) relative to (24-eq) (and (3-eq)) is probably the result of stereoelectronic effects. In particular, Uj hyperconjugative interaction [3] might be responsible for the preferred antiperiplanar (diaxial) orientation of the S=0 group and one of the lone pairs of electrons at sulfur (Scheme 3.39). In addition, the destabilizing antiperiplanm arrangement of lone pairs in (24-eq) (and (3-eq)), as illustrated in Scheme 3.39, is also expected to be an important factor [67]. 

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