Unraveling Phenanthrenoid Dimerization in <i>Juncus acutus</i>: A DFT‐Guided Exploration of Radical‐Coupling Reaction Mechanisms

Abstract Juncus acutus is a plant found near ponds and brackish marshes, producing secondary metabolites like phenanthrenoid dimers with antibacterial, antioxidant, and anti‐inflammatory activities. Despite this, little is known about the detailed mechanisms behind their dimerization reactions. In this study, we used Density Functional Theory to explore the reaction mechanisms of phenanthrene and dihydrophenanthrene species in radical coupling, both in gas‐phase and in solution. The dimerization reactions, initiated by the OH radical, result in the formation of two phenanthrenoid dimers (homodimer and heterodimer). The OH radical abstracts a hydrogen atom from the hydroxyl group, leading to the formation of radicals and water. Our analysis of electronic spin density highlighted the preferred reactive sites that drive regioselective dimer formation. Furthermore, the study identifies a triplet π ‐ π stacked intermediate preceding the diketone formation on the singlet potential energy surface, revealing a two‐state reactivity mechanism. The diketone dimers are then converted via a water‐mediated keto‐enol tautomerization into their dienolic forms, completing the reaction pathway. This work offers insights into the complex dimerization processes of phenanthrenoids in nature, using computational methods to shed light on poorly understood molecular mechanisms in plant organisms.