A simple method for efficient synthesis of tetrapyridyl-porphyrin using Adler method in acidic ionic liquids

Abstract

We investigated the preparation of tetraphenylporphyrin (TPP) using the several acidic ionic liquids, [HC₄im][X] (X⁻ = CF₃SO₃⁻, ClO₄⁻, Cl⁻, CF₃CO₂⁻, and BF₄⁻), as acid catalytic media. For such ionic liquids, the anion (X⁻) of [HC₄im][X] is related to the acidity of the ionic liquid, and affects porphyrin formation. This synthetic method using acidic ionic liquids can also be applied to other meso-substituted phenyl porphyrins and 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine (TPyP), which has 4-pyridyl moieties at four meso positions. In [HC₄im][CF₃CO₂], TPyP could be obtained in 11% yield, and the [HC₄im][CF₃CO₂] could be reused at least 3 times without any loss of its catalytic activity. The TPyP synthesis methodology using acidic ionic liquids can remove the ionic liquids from TPyP by easy filtration in contrast to the traditional Alder method, which needs vacuum distillation or liquid–liquid extraction for removing propionic acid. Our proposed porphyrin preparation method using the acidic ionic liquids potentially have wide applications to various useful porphyrin analogues.