Transition-state aromaticity and its relationship with reactivity in pericyclic reactions.

The influence of transition-state aromaticity on the barrier heights of concerted pericyclic reactions is summarized herein. To this end, selected representative examples ranging from fundamental processes such as Diels-Alder or Alder-ene reactions to double-group transfer reactions or 1,3-dipolar cycloadditions involving metal complexes are presented. It is found that while more synchronous processes tend to exhibit greater aromatic character in their transition states, this increased aromaticity does not necessarily correlate with lower activation barriers. State-of-the-art computational methods on reactivity, such as the combined activation strain model (ASM)-energy decomposition analysis (EDA) method, reveal that factors other than aromaticity govern the barrier heights of these pericyclic reactions.