Investigation of acid cocatalysis in syntheses of tetraphenylporphyrin

✍ Scribed by G. Richard Geier III; Jennifer A. Riggs; Jonathan S. Lindsey

• Publisher: John Wiley and Sons
• Year: 2001
• Tongue: English
• Weight: 219 KB
• Volume: 05
• Category: Article
• ISSN: 1088-4246
• DOI: 10.1002/jpp.380

✦ Synopsis

The condensation of pyrrole and benzaldehyde has generally been carried out with trifluoroacetic acid (TFA)(20-50 mM), BF~3~-etherate (1 mM), or more recently with BF~3~-etherate in the presence of a salt. Differences in the reaction course with TFA or BF~3~-etherate prompted studies of the combined use of BF~3~-etherate and TFA. We found that the reaction of pyrrole + benzaldehyde (10 mM each) cocatalyzed by TFA (15 mM) and BF~3~-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 BF~3~-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 BF~3~-etherate (1.0 mM), which gave TPP in ~40% yield. The beneficial effect of an added salt in BF~3~-etherate catalyzed reactions was reexamined by comparisons of reactions with NaCl/BF~3~-etherate versus BF~3~-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 BF~3~-etherate alone), thereby affording better utilization of the aldehyde and a giving a commensurate increase in the yield of TPP.