Double Group Transfer Reactions: Role of Activation Strain and Aromaticity in Reaction Barriers

Double group transfer (DGT) reactions, such as the bimolecular automerization of ethane plus ethene, are known to have high reaction barriers despite the fact that their cyclic transition states have a pronounced in-plane aromatic character,, as indicated by NMR spectroscopic parameters. To arrive at a way of understanding this somewhat paradoxical and incompletely understood phenomenon of high-energy aromatic transition states, we have explored six archetypal DGT reactions using density functional theory (DFT) at the OLYP/TZ2P level. The main trends in reactivity are rationalized using the activation strain model of chemical reactivity. In this model, the shape of the reaction profile Delta E(zeta) and the height of the overall reaction barrier Delta E-not equal = Delta E-(zeta=zeta(TS)) is interpreted in terms of the strain energy Delta E-strain(zeta) associated with deforming the reactants along the reaction coordinate zeta plus the interaction energy Delta E-int(zeta) between these deformed reactants: Delta E(zeta) = Delta E-strain(zeta) + Delta E-int(zeta). We also use an alternative fragmentation and a valence bond model for analyzing the character of the transition states.