10.1021/jo951643d
The study investigates the semi-π analogue of double hyperconjugation, known as "hyperconjugation/conjugation," in 4-isopropylidenecyclohexyl mesylate (4-OMs) and compares it with the saturated analogue, trans-4-isopropylcyclohexyl mesylate (5-OMs). The researchers found that the unsaturated substrate 4-OMs reacts only four times faster than the saturated substrate 5-OMs in 97% trifluoroethanol, indicating no significant through-bond interaction of the double bond with the reactive center. This is attributed to less than ideal overlap of the γ,δ π orbitals with the R, σ orbitals. However, when an electron-rich tin atom is attached to the 4-position, it provides a large rate enhancement and changes the mechanism to carbocation formation through double hyperconjugation. The study concludes that the π bond does not effectively stabilize positive charge through two stages of conjugation in the studied system, suggesting that the hyperconjugation/conjugation mode may not be a viable mechanism under the given conditions.
