Unusual reactivities of acridine derivatives in catalytic hydrogenation. A combined experimental and theoretical study

Hydrogenation of acridine derivatives over Rh/Al2O3, has been studied. Strong influence of the pyrrolidino-substituent on the reduction pathway and reductions products was found. When, the reaction was performed on the unsubstituted acridine nucleus full conversion was obtained in 8h. Under the same conditions, when a pyrrolidine substituent was settled on the central ring of acridine (9-position), a pure product was obtained with the two lateral carbocycles reduced whereas the central heterocyclic ring was not reduced. When the pyrrolidine substituent was at the 1-position, pure partially reduced central heterocyclic ring was obtained, but the compound was rapidly re-oxidized by air. In order to clarify such substituent effect, theoretical calculations were performed. Considering the energies and thermodynamic values of each intermediate and product as well as the interaction with the catalyst surface, the selectivity and diversity in the reduced product formation were partially explained and different reaction pathways were drawn according to the substitution pattern on the acridine scaffold.