Hyperconjugation ground-state effects

Ground state /-effects of silicon may be responsible for the elongated C(alkyl)-O(ester) bond in n.s-3-trimelhylsilylcyclohexyl p-nitrobenzoate 59 relative to the silicon-free derivative. It is suggested that the ground state /-effect could be due either to homohyperconjugation, 60, or to inductively enhanced C—C hyperconjugation where the trimethylsilyl substituent increases the importance of the resonance form 61 relative to the silicon-free derivative. 

Ground-state and excited-state reactions of chiral Meldrum s acid derivatives 39 with the enone function have been reviewed with an emphasis on the facial selectivity in the C=C bond (Figure 2) <1996H(42)861>. Top-face preference, even when it is sterically more hindered than bottom-face attack, is supported by hyperconjugation no —r c=c 39a, whereas bottom-face preference is dominated by steric effects in the sofa conformation of the molecule 39. The trajectory of the attacking reagent plays a balancing role. 

In addition to the stabilization of /1-carbocations, the /1-effect of silicon can also be observed in the ground states of neutral molecules. Lambert and Singer58 studied compounds of the type 42 where hyperconjugation should be enhanced by increasing the electron-accepting properties of the substituent X (MeO < Me < H < CN). o-tt overlap in this system gives the resonance structure 43 shown in equation 19. 

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. 

The issues that remain under discussion are (1) the relative importance of the acceptor (Felkin-Ahn) or donor (Cieplak) hyperconjugation capacity of a substituents (2) the relative importance of electrostatic effects and (3) the role of reactant pyramidalization in transmitting the substituent effects. Arguments have been offered regarding the importance of electrostatic effects in all the systems we have discussed. Consideration of electrostatic effects appears to be important in the analysis of stereoselective reduction of cyclic ketones. Orbital interactions (hyperconjugation) are also involved, but whether they are primarily ground state (e.g., reactant pyramidalization) or transition state (e.g., orbital stabilization) effects is uncertain. 

Hyperconjugative interactions in ground state from X-ray geometries Laube, T., Ha, T. K. (1988). Detection of hyperconjugative effects in experimentally determined structures of neutral molecules. Journal of the American Chemical Society, 110, 5511-5517. 

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