DFT-Based Reaction Pathway Analysis of Hexadiene Cyclization via Carbenium Ion Intermediates:  Mechanistic Study of Light Alkane Aromatization Catalysis

Abstract

We conducted density functional theory calculations to identify the complete cyclization mechanism (ring formation and ring expansion) for protonated hexadiene in the gas-phase as a precursory means of studying aromatization of light alkanes in acidic zeolite catalysts. We identify the rate-determining step to consist of ring expansion from a methylcyclopenta carbenium precursor to a stable cyclohexa carbenium intermediate, exhibiting an activation barrier of 9.6 kcal/mol and proceeding through a bicyclic intermediate starting from a secondary cyclopentyl carbocation. This pathway for ring closure was preferred over tertiary precursor expansion and direct cyclization to a cyclohexyl carbocation. Expecting carbocation intermediates to be represented by alkoxide species near Brønsted acid sites, we calculated the relative stability of primary, secondary, and tertiary alkoxide analogues to cyclopentyl carbocation intermediates involved in ring expansion and notice a reversal of stability relative to gas-phase carbocation stabilities (i.e., primary > secondary > tertiary stability). However, on the basis of the notion that transition state stability depends heavily on carbocation character of the transition state, even in alkoxide-based chemistry, we conclude that ring expansion would nevertheless constitute the rate-determining step in a zeolite-catalyzed mechanism.

Reaction Mechanism for Direct Cyclization of Linear C5, C6, and C7 Alkenes over H‐ITQ‐13 Zeolite Investigated Using Density Functional Theory

“…Thus, the similar intersection structure of H‐ITQ‐13 (diameter of 6.72 Å) to H‐ZSM‐5 (diameter of 6.36 Å) would enable the formation of π‐complex and alkoxide complex possible with respect to C 5 , C 6 and C 7 linear alkenes. It has been reported that these linear alkoxides and alkenes are most probably cyclized to conventional 5‐membered ring (5‐MR), 6‐MR and 7‐MR …”

Ring contraction mechanisms in substituted cyclohexylium cations

“…1) [7,9]. These can be described as appearing either flat or puckered about the charge center, with the third having the CH 2 a to the charge center lying out-of-plane of the ring.…”

“…Ring contraction reactions of cations have been studied using numerous modeling methods including semi-empirical [4,5], Hartree-Fock and density functional theory (DFT) [6,7]. Vrček et al [8] as part of a study on the biogenesis of steroids, used Møller-Plesset techniques to determine the reaction profile for the rearrangement of the model 1-(2-propyl)cyclopentyl cation to 1,2-dimethyl-1-cyclohexyl cation.…”

Thermochemical and Kinetic Study of the Carbocation Ring Contraction of Cyclohexylium to Methylcyclopentylium

“…3 More recent DFT work by Joshi et al 14 suggests that the mechanism of 1 is somewhat more complicated than earlier proposals. 10 They found three low-energy conformations for I. One of these structures was found to directly form a secondary methylcyclopentylium cation through a cyclopropane transition state structure.…”