Regioselectivity in the Nitration of Eugenol Is Independent of Inorganic Reagents: An Experimental and Theoretical Investigation with Synthetic and Mechanistic Implications

In this study, we reinvestigated the straightforward nitration of eugenol using traditional reagents and bismuth nitrate. NMR analysis of the obtained products revealed that the regioselectivity of eugenol nitration was independent of the inorganic nitrating reagent used, consistently resulting in the formation of 6-nitroeugenol. This contradicts previous literature reports because the elusive synthesis of 5-nitroeugenol using Bi(NO₃)₃·5H₂O was not achievable through straightforward methods; instead, this isomer could only be prepared via the well-established three-step synthesis. Theoretical investigations using DFT calculations, considering both the dielectric constant of the medium and explicit water molecules, substantiated this regioselectivity. It was found that hydration water played a critical role in the formation of a Zundel cation, shifting the thermodynamic equilibrium toward the exclusive production of 6-nitroeugenol. These results imply that all biological studies involving eugenol derivatives synthesized via direct nitration with Bi(NO₃)₃·5H₂O should be reviewed, as they dealt with 6-substituted eugenol derivatives rather than the previously assumed 5-substituted eugenol.