Protonated cyclopropanes. VII. Evidence for edge protonation

Behavior of protonated cyclopropyl intermediates during polyalphaolefin synthesis: Mechanism and predicted product distribution

“…On this catalyst, double bonds migrated in a step‐wise manner. Additionally, skeletal isomerization to methyl substituted olefins occurred, presumably through a protonated cyclopropyl intermediate . During skeletal isomerization, the methyl groups formed at nearly random positions along the carbon chain, regardless of the double bond location in the starting olefin, and the catalyst was inactive towards linear alkanes under the conditions of our experiments.…”

“…We think the intermediates in our previous experiments opened by 1,2 hydride shifts that led always (or almost always) to a tertiary rather than a secondary carbenium ion. The acidic proton associated with such cyclopropyl intermediates appears to protonate either edge‐wise on one of the carbon–carbon bonds or on a corner carbon of the ring and is rapidly mobile. Calculations have indicated that corner‐protonated species are slightly more stable than edge‐protonated species and that proton migration occurs rapidly through the edge‐protonated intermediate .…”

Not all acid sites are created equal: Alkene isomerization and dimerization occur on separate sites on Amberlyst®15 catalyst

“…[ 16 ] The extra proton is rapidly mobile around the perimeter of the ring, binding alternately at corner carbons and along edges. [ 17–20 ] Each carbon alpha to the ring interacts with both the ring carbons on its opposite side of the ring. When these ring carbons are protonated, they serve as leaving groups, and each alpha carbon can form two new protonated cyclopropanes, as shown in Scheme 4.…”