Hyperconjugation double

There are several structural types of delocalization, summarized in Table 5.1. The stabilization of the dimethylalkylidine cation (3) is an example of double hyperconjugation. ... 

The stabilization of a positive charge by 5-silicon is believed to be through double hyperconjugation, represented by the resonance structures shown in equation 22. 

The stabilization energies provided by the three isodesmic reactions provide different information. Effects of substituents in axial cations, which are described in Figure 4.1a, include a complicated interplay of many factors such as hybridization, inductive, and field effects, which are still present in these species even when double hyperconjugation (interaction of donor and acceptor through the sigma bridge) is minimized. 

Alabugin, I. V., Manoharan, M. (2004). Effect of Double Hyperconjugation on the Apparent Donor Ability of o-Bonds Insights from the Relative Stability of 5-Substituted Cyclohexyl Cations. Journal of Organic Chemistry, 69, 9011-9024. 

A balanced description of relative donor ability of o-bonds can be accomplished if the donor and acceptor sites are not directly connected, e.g. by comparing the relative total energies and geometries of two conform-ers ( hyperconjomers ) of 5-substituted cyclohexyl cations. In these systems, interaction between the bond and the empty p-orbital is transmitted via the double hyperconjugation mechanism (see Chapter 8). Such description partially cancels the non-hyperconjugative contributions such as the inductive and field effects (Figure 5.26). ... 

The diaxial preference is further strengthened in 1,4-difluorocyclohexane whereas the ee-isomer is more stable in the respective 1,3-dihalocyclohexanes. This difference suggests that the unusual conformational preference of 1,4-isomer is likely to be stereoelectronic (note the unfavorable double hyperconjugation pattern where two o CF acceptors compete for the same donor - more details in Chapter 9). The conformational preferences in 1,2-difluorocyclohexane can be understood as an extension of the conformational preferences of 1,2-difluoroethane (the gauche effect). 

Figure 8.17 Effect of negative double hyperconjugation on the stability of aryl anions.

Figure 8.18 Effects of double hyperconjugation on basicity of 4-substituted piperidines.

The remarkable 5-effect on the transition state of the solvolysis was ascribed to the occurrence of double hyperconjugation (through bond coupling), i.e. 269A 269B -o-... 

---