The roles of templates in the syntheses of porphyrin oligomers

Abstract

4,4′-Bipyridyl is an efficient template for the formation of cyclic porphyrin dimer from a diethynyl porphyrin monomer; s-tri(4-pyridyl)triazine promotes the formation of cyclic trimer from the same starting material to a lesser degree whereas meso-tetra(4-pyridyl)porphyrin is not a practical template for the preparation of cyclic porphyrin tetramer from monomer. The factors responsible for these differences in template efficiency were elucidated by analysing the product distributions from coupling linear porphyrin dimer and tetramer in the presence of these and other ligands. The 4,4′-bipyridyl template increases the kinetic effective molarity for the cyclisation of linear dimer by a factor of 20. meso-Tetra(4-pyridyl)porphyrin is a good template for the cyclisation of linear porphyrin tetramer; it is also a good template for the preparation of cyclic tetramer from linear dimer and this is the best preparative route to cyclic tetramer. There are three roles for the meso-tetra(4-pyridyl)porphyrin template in the latter case: (i) as a negative template it prevents linear dimer cyclising to cyclic dimer, (ii) as a linear template it promotes the formation of linear tetramer from two linear dimers, and (iii) as a cyclisation template it accelerates the closure of linear tetramer to cyclic tetramer. All three roles were detected under optimum conditions but the second role is not significant when TMEDA is present during coupling.