Design, synthesis, structural analysis and atropoisomerisation studies of polynucleating ligands based on porphyrins bearing catechol units

Abstract

New polynucleating ligands based on a combination of a meso-tetrakis(phenyl)porphyrin core and four catecholamido units connected at the ortho positions of the phenyl moieties have been prepared. All four atropoisomers of the methoxy-protected meso-tetrakis(o-catecholamidophenyl)porphyrin 2 were obtained in high yields, however, the separation of isomers, although achievable, was found to be extremely tedious and did not allow the preparation of large quantities of the different atropoisomers. This difficulty was overcome by first separating the atropoisomers of the precursor meso-tetrakis(o-aminophenyl)porphyrin 3 and then by a quantitative condensation reaction of the separated isomers with the acyl chloride derivative of methyl-protected catechol 6. All four atropoisomers were subsequently deprotected in quantitative yields using BBr₃. Although following this route the protected 2_α4 and 2_α3β atropoisomers were obtained on the gram scale, for the other two 2_αβαβ and 2_α2β2 isomers, due to separation difficulties, only low quantities could be synthesised. After a systematic kinetic study of the atropoisomerisation process, the 2_αβαβ isomer was obtained in large quantities by thermal isomerisation of the random mixture of isomers of 2 in toluene at 110 °C in the presence of a large excess of trifluoroacetic acid, whereas the 2_α2β2 isomer was isolated under the same conditions but in the absence of acid. All four atropoisomers were characterised by classical 1- and 2-D NMR spectroscopy. Furthermore, the 1_αβαβ, 1_α4, 2_αβαβ, 2_α4 and 3_αβαβ atropoisomers were also characterised in the solid state by X-ray diffraction on single crystals.