Double friedel-crafts acylation reactions on the same ring of a metallocene: Synthesis of a 2,5-diacetylphospharuthenocene

The synthetic outcome of the Friedel-Crafts acylation of 1′,2′,3,3′,4,4′,5′-heptamethylphospharuthenocene reflects the nature of the acylating agent, with alkanoyl anhydride/ trifluoromethanesulfonic acid (TfOH) reagents giving monosubstitution at the phospholyl ring, whereas alkanoyl chloride/AlCl₃ gives 2,5-disubstitution. DFT calculations indicate that this unusual double acylation can be facilitated by the intervention of the phosphorus atom at an early stage in the reaction trajectory, with the acyl group being delivered from the phosphorus atom into the ring 2- or 2,5-positions. Pathfinders: Double electrophilic acylation of the phospholyl ring occurs upon treatment of a phospharuthenocene with the classical Friedel-Crafts acylating agent MeCOCl/AlCl₃ (see scheme). Theoretical calculations suggest that two pathways, mediated by either the ruthenium centre or the phosphorus atom, are available to the incoming acylium ion.