The Carbocation Rearrangement Mechanism, Clarified

Abstract

The role of protonated cyclopropane (PCP(+)) structures in carbocation rearrangement is a decades-old topic that continues to confound. Here, quantum-chemical computations (PBE molecular dynamics, PBE and CCSD optimizations, CCSD(T) energies) are used to resolve the issue. PCP(+) intermediates are neither edge-protonated nor corner-protonated (normally) but possess "closed" structures mesomeric between these two. An updated mechanism for hexyl ion rearrangement is presented and shown to resolve past mysteries from isotope-labeling experiments. A new table of elementary-step barrier heights is provided. The mechanism and barrier heights should be useful in understanding and predicting product distributions in organic reactions, including petroleum modification.

Dynamic behavior of rearranging carbocations – implications for terpene biosynthesis

“…The shorter mechanism was found to involve nine discrete chemical steps and had an overall predicted activation barrier of 33 kcal/mol, while the longer pathway involved 16 steps with an overall barrier of 37 kcal/mol. Similar results have been obtained for other complex carbocations: the same group used molecular dynamics to explore the homocubyl cation’s rearrangement behavior [ 68 ], and East et al used “rising-temperature” molecular dynamics to determine the carbocation branching behavior of molecules relevant to petroleum chemistry [ 69 – 71 ].…”

Kinetics of Zeolite-Catalyzed Heptane Hydroisomerization and Hydrocracking with Cbmc-Modeled Adsorption Terms: Zeolite Beta as a Large Pore Base Case

“…Again, reactions forming geminal dimethyl isomers formally involve conversion of tertiary carbenium ions to secondary carbenium ions. As mentioned, the higher barriers for type B isomerization are explained by a higher energy closed-primary meso-PCP transition state [43,44]. It is worth summarizing the above by noting that, except for the ethyl shift reaction, within each isomerization class the fitted values for activation enthalpies appear to follow the order, in terms of carbenium ions formally involved: sec ?…”

“…Type A isomerization reactions lead to shifting of alkyl groups through a corner-protonated cyclopropane (PCP) transition state to form isomeric alkanes with an equal number of branches. In type B isomerization, the branching degree changes, which involves a higher barrier [40][41][42], due to an asymmetric transition state described by Sandbeck et al [43] as closed primary meso-PCP and by Rey et al as an asymmetric edge-protonated CP [44,45].…”

On the curing and degradation of bisphenol A diglycidyl ether and epoxidized soybean oil compounds cured with itaconic and succinic acids

“…The opening of oxirane rings generates nucleophilic and electrophilic sites. However, primary carbocations' formation is not favorable and induces rearrangement forming a stable state 29 . The nucleophilic sites propagate the cross‐linking with dicarboxylic acids, which is more favorable to ITA.…”