Investigation of acid cocatalysis in syntheses of tetraphenylporphyrin

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Abstract
The condensation of pyrrole and benzaldehyde has generally been carried out with trifluoroacetic acid (TFA)(20-50 mM), BF3-etherate (1 mM), or more recently with BF3-etherate in the presence of a salt. Differences in the reaction course with TFA or BF3-etherate prompted studies of the combined use of BF3-etherate and TFA. We found that the reaction of pyrrole + benzaldehyde (10 mM each) cocatalyzed by TFA (15 mM) and BF3-etherate (0.3 mM) provided tetraphenylporphyrin (TPP) in yields of 50–55%, compared with 40% or 26%, respectively, from optimal catalysis by TFA (20 mM) or BF3-etherate (1 mM) individually. Examination of the oligomer composition (LD-MS), yield of TPP (UV-vis), yield of N-confused TPP (HPLC), and level of unreacted aldehyde (TLC) in the cocatalytic reaction indicated a reaction course that contained features of those observed with each acid individually. Cocatalysis also was observed with methanol (50 mM) and BF3-etherate (1.0 mM), which gave TPP in ~40% yield. The beneficial effect of an added salt in BF3-etherate catalyzed reactions was reexamined by comparisons of reactions with NaCl/BF3-etherate versus BF3-etherate alone in terms of the oligomer composition, yield of TPP, yield of N-confused TPP, level of unreacted aldehyde, reversibility of the reaction, inactivation of the acid, and formation of TPP via intermediate oligomers. The studies strongly suggest that the presence of salt facilitates the addition of benzaldehyde to pyrrolic units (which is the limiting step with catalysis by BF3-etherate alone), thereby affording better utilization of the aldehyde and a giving a commensurate increase in the yield of TPP.