Insights into some Diels–Alder cycloadditions via the electrostatic potential and the reaction force constant

We have investigated computationally a series of 18 Diels–Alder cycloadditions, involving four dienes (butadiene, 2-hydroxybutadiene, cyclopentadiene and cyclohexadiene) and variously-substituted ethylenes and acetylenes as dienophiles. It is demonstrated that the respective molecular electrostatic potentials can be used to provide insight into the initial modes of interaction of the dienes and dienophiles, focusing particularly upon the synchronicities and nonsynchronicities of the processes. This is shown to complement predictions based upon the profiles of the reaction force constants κ(ξ) in the transition regions along the intrinsic reaction coordinates ξ: one κ(ξ) minimum ∼ synchronicity, a minimum and a shoulder ∼ moderate nonsynchronicity, two minima ∼ strong nonsynchronicity. Temporal aspects of the reactions can also be examined through κ(ξ).