Template Effect of Pyridinium Salts in the Synthesis of Crown-meso-Tetraphenylporphyrin

Using pyridine in the presence of pyridinium salts as a reaction medium was suggested for the synthesis of crown-meso-tetraphenylporphyrin from pyrrole and podand dialdehyde. We expected that the absence of a free acid in the reaction mixture would minimize the linear polymerization rate of the reactants. Use of the proposed reaction medium has almost doubled the yield as compared to the original published synthetic procedure for the target porphyrin and facilitated purification due to a smaller amount of polymeric impurities. Quantum chemical simulations have demonstrated that the podand precursor could form H-bonded complexes with protonated pyridine and thus become pre-organized for the formation of a porphyrinic macrocycle. The same reaction with benzaldehyde did not produce even trace amounts of the porphyrin. Obviously, this is caused by the higher complexation energy for the podand-pyridinium system compared to benzaldehyde-pyridinium (calculated values are 36 and 24 kcal/mol, respectively).