Hyperconjugation stereoelectronic reactivity effects

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,... 

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. 

Houk has suggested that stereoelectronic effects have influence over the stereochemical course of hydroboration reactions of allylic alcohols [19]. Because borane is an electrophilic reagent, it exhibits a preference for electron-rich partners in hydroboration reactions. The more reactive conformer of an allylic alcohol is that in which the olefin avoids additional hyperconjugative interactions that would render it electron-deficient, such as jic=c ( c-x (allylic). Therefore, allylic hydroxy or alkoxy substituents tend to avoid the anti position with respect to the partially formed bonds (cf. transition structure 36). Altogether, both steric and electronic effects work in concert to support the predominance of transition structures 28 and 36,... 

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