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

Abstract

MnIII 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 MnIII-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.