Competition of Secondary versus Tertiary Carbenium Routes for the Type B Isomerization of Alkenes over Acid Zeolites Quantified by Ab Initio Molecular Dynamics Simulations

The skeletal isomerization of alkenes catalyzed by zeolites involves secondary and tertiary carbenium ions for which respective reactivity cannot be easily assessed by standard theoretical approaches. Thanks to ab initio molecular dynamics, starting from 4-methyl-hex-1-ene (a monobranched C-7 alkene), we identify and compare two mechanistic routes for skeletal isomerization: (i) a type B isomerization transforming a secondary carbenium into a tertiary carbenium (conventional route), and (ii) a two-step route involving an intramolecular 1,3 hydride-shift producing a tertiary carbenium, followed by a type B isomerization between two tertiary carbenium ions. We find that, in the case of the secondary cation, the relevant species from a kinetic point of view is the corresponding pi-complex. The transition states found for type B isomerization reactions are edge-protonated cyclopropanes (edge-PCP) that exhibit similar stabilities and structures. The transition state for the 1,3-hydride shift is an edge-type PCP with one elongated C-C bond that is more stable than the one found for type B isomerization. From this analysis, we deduce relevant kinetic constants and quantify the respective contribution of both pathways to the global reaction rate. Although the secondary carbenium ions are poorly stable species, we show that they can hold a significant part of the reaction flux. Finally, we discuss, in detail, our kinetic and mechanistic insights with previous kinetic modeling data reported in the literature.