On the Brassard's rule of regioselectivity in Diels–Alder reactions between haloquinones and polar dienes

Abstract

The [4 + 2] polar Diels–Alder reaction between nucleophilic dienes and haloquinones proceeds with remarkably high regioselectivity. In the present work, we simulated diene–dienophile bimolecular encounters in a model stoichiometric reaction between haloquinones and Brassard's diene, using Monte Carlo sampling; the energetics of the stacking modes was then studied through DFT calculations. QTAIM analysis for the theoretical electron density was applied to describe contacts between π-systems, foreseeing chemically-productive paths for pre-cycloaddition assemblies associated with the experimental regioisomer. Upon thermally-triggered cycloaddition, Brassard's rule of selectivity manifests: only certain stacked microstates consolidate asynchronously into cycloadducts. The halogen atom tunes electron density at the reactive centers, as confirmed by NMR and calculations. Unidimensional energy landscapes were derived to capture energetic profiles of the studied reaction. The model systems were synthesized with high regioselectivity, delving further into the role of halogen substituents.