Stereochemistry of the electrophilic ring opening of cyclopropanes by deuterium(1+) ion. Evidence for an unsymmetrical, nonrotating, corner-protonated cyclopropane

Abstract

It could be argued that opening occurs both from corner-and edge-protonated species, with perhaps the 5 % erythro acetate found in the opening of 1 (the product of nucleophilic retention) arising from the latter intermediate via an open carbonium ion. We think it unlikely that if opening were occurring competitively from both 6 and 8, the stereochemical results would remain constant over such a wide range of solvents and reaction conditions.

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.…”

Nucleophilic step of ring-opening reactions of cyclopropanes with electrophiles. Electronic substituent effects on stereoselectivity of reactions of some 1-arylbicyclo[4.1.0]heptanes with mercuric salts

“…The ring opening of cyclopropanes by electrophiles occurs in the direction of the more stable carbocation with either retention or inversion of configuration at the site of the electrophilic attack, depending on the nature and configuration of the ring substituents, whereas the stereochemistry of the nucleophilic step takes place with complete or strongly predominant inversion of configuration. [1][2][3][4] Recently it has been shown5 that the ring opening of phenylcyclopropane la with mercuric salts occurs, according to expectation,1-2-4•6 by attack of the electrophile on the least-substituted carbon, in the direction of the benzylic carbon. However, the stereochemistry of the nucleophilic step was highly variable, ranging from almost complete inversion to markedly predominant retention of configuration depending on the type of mercuric salt and on the solvent.…”