Hyperconjugation: intermediates and transition states in replacement and elimination

Abstract

A review is given of the development of experimental evidence for hyperconjugation in the form of electron-release to an unsaturated or electron-deficient centre from saturated H-C and CC bonds. This type of electron-release can contribute regioselectively to the reactivity of organic molecules, and need not involve breakage of the H-C or CC bond; under these circumstances, H-C and CC bonds make similar contributions to the observed effects of alkyl and derived groups, the relative contributions varying with reaction and solvent. In the limit, however, this form of electron-release can result in breakage of the hyperconjugating bond, and in such circumstances (as in transition states leading to proton-loss or 1,2-elimination), H-C bonds become much more effective than CC bonds because of the greater solvation of incipient protons than of incipient alkyl cations. Evidence for the corresponding hyperconjugation by H-O bonds comes especially, but not exclusively, from the relative rates of electrophilic aromatic replacements affected by H-0 and R-O groups. Recent experiments on the effects of these groups on nucleophilic replacements in aromatic side-chains show that the electron-releasing power of the H-O group is quite variable, and suggest that the impact of H-O hyperconjugation is more dependent on the reaction and the solvent than that of H-C hyperconjugation has been shown to be. Reaction pathways in the hydrolysis of R0.C6H2Br2.CHBr2 and RO.C6H3Br.CHBr2, which can lead to products both of replacement and of elimination, are discussed, and isotope effects in H-C and H-O hyperconjugation are compared.

Quantum chemical studies of non-covalent interactions between the ethyl cation and rare gases

“…Because the hyperconjugation controls the stabilization of C 2 H 5 + , this effect shows itself sensitive to the changes in the pÁÁÁH and CÁÁÁH contacts [71,72]. These bond lengths vary between 0.0199 and 0.0371 Å as well as between 0.0011 and 0.0130 Å ; these profiles are not in line with the interaction strengths, obviously at this stage of the study regarding only the interatomic distances.…”

From Catalytic Mechanism to Rational Design of Reversible Covalent Inhibitors of Serine and Cysteine Hydrolases

“…Formation of a TC is accompanied by considerable donorÀacceptor charge transfer, as discussed below. A different interpretation of the relative stability of carbanions and their isoelectronic analogues was based on hyperconjugation [64,65] and the anomeric effect. [66][67][68][69] Neither approach alone can provide a comprehensive understanding of the reactions of hydroxyl and sulfhydryl anions with carbonyl species.…”

Orbital Control of Photochemical Rearrangement of 4-Aryl-1,1-dicyano-1-butenes through the Hyperconjugative Substitution on the Linker Chain

“…Hyperconjugation is ubiquitous in many molecular systems and important to explain atypical conformational preferences. It is usually defined as the stabilization through σ–π* interaction, but secondary hyperconjugation such as σ–σ* interaction is often included. − Such interaction is able to explain the preference for the staggered conformation in ethane and other peculiar conformer preferences (e.g., the gauche effect) and has been utilized as a tool for controlling the selectivity in various (thermal) reactions. − The effects of hyperconjugation have also been discussed in some photochemical reactions. Enhanced cyclization of fluorinated ketones (Norrish Type II reaction) and syn-selective Paternò-Büchi reaction of 5-substituted adamantanones have been rationalized by the hyperconjugative stabilization in the biradical intermediate.…”