Is aromaticity loss necessary for transition-metal promoted arene–alkene cycloadditions?

Abstract

Computed gas-phase reaction profiles for Diels–Alder reactions, nucleus-independent chemical shifts, and bonding analyses for substituted and metal complexed, η²-, η⁴-, η⁶-coordinated arenes reveal that dearomatization is necessary for arenes to exhibit ene- and diene-like reactivity. Substituted arenes and η⁶-arenes exhibit high free energy barriers towards cycloaddition reactions, but strongly dearomatized η²- and η⁴-arenes can display low free energy barriers (∼20 kcal mol⁻¹ or less) for [4 + 2] cycloaddition reactions.