DFT-based reaction pathway analysis of hexadiene cyclization via carbenium ion intermediates: Mechanistic study of light alkane aromatization catalysis

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.