Tailed MnIII-tetraarylporphyrins bearing an axial ligand and/or a carboxylic group: self-consistent catalysts for H2O2 or NaOCl alkene epoxidation

Abstract

Mn^III tetraarylporphyrins bearing either a heterocyclic nitrogen base axial ligand or a carboxylic acid group, or both, covalently bonded to the porphyrin through a single flexible chain, have been synthesized. Their basic frame is that of the robust tetrakis(2, 6-dichlorophenyl)porphyrin, and the chains are connected by ether or amido linkages to the ortho-positions of one meso-aryl group. Catalytic efficiency was tested in alkene epoxidations at O °C under aqueous CH₂Cl₂ two-phase conditions in the presence of NaOCl (pH 10. 5) or 30% H₂O₂(pH 4. 5) as oxygen donors (ODs). Compound 4 bearing an imidazole ligand showed satisfactory catalytic activity in the presence of both ODs, whereas Mn^III-porphyrins 5a–d bearing a covalently bonded carboxylate group are suitable for the activation of H₂O₂, and proved to be very efficient in the presence of an externally added axial ligand. Catalyst 6, featuring an imidazole ligand and a carboxylate group bonded on the 2, 6-positions of the same meso-aryl group, is particularly efficient in alkene epoxidations promoted by 30% H₂O₂(initial rates up to 500 turnovers/min, overall turnovers up to 1200). A possible reaction mechanism for epoxidation catalysed by the bis-tailed Mnⁱⁱⁱ-porphyrins is discussed.