N-Confused Tetraphenylporphyrin and Tetraphenylsapphyrin Formation in One-Flask Syntheses of Tetraphenylporphyrin

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Abstract
The yields of tetraphenylporphyrin (TPP), N-confused tetraphenylporphyrin (NC-TPP), and tetraphenylsapphyrin (TPS) were examined under diverse conditions in the one-flask room-temperature reaction of pyrrole and benzaldehyde. The conditions examined included TFA and BF3-etherate catalysis, acid concentrations, pyrrole and benzaldehyde concentrations, pyrrole:benzaldehyde ratios, the addition of salt, and reaction times. The quantities of TPP, NC-TPP, and TPS were determined by HPLC. The yield of NC-TPP mirrored that of TPP during systematic variation of reaction parameters but was consistently lower (≤7.5%). The rate of NC-TPP formation also resembled that of TPP. Excess pyrrole was not required to form NC-TPP. About 2-fold more NC-TPP was obtained with BF3-etherate compared with TFA catalysis. TPS was observed in only a few of the reactions examined. The yield of TPS peaked early and declined well before the maximum yields of TPP and NC-TPP were reached. The highest yield of TPS observed was 1.2%. TPS was produced in higher yields by TFA compared with BF3-etherate catalysis. Additional pigments with chromatographic properties spanning the range of TPP and NC-TPP were observed in very low yield but were not characterized. N-confused tetramesitylporphyrin was isolated from the reaction of mesitaldehyde and pyrrole under BF3-ethanol cocatalysis. Examination of the Adler reaction of benzaldehyde and pyrrole revealed the presence of NC-TPP and TPS in the filtrate but not in the crystalline TPP material. The substantial chromatographic differences between the porphyrins and N-confused porphyrins make for straightforward isolation of the purified porphyrin devoid of contaminating N-confused porphyrin. These studies indicate that the scope of conditions yielding NC-TPP is extremely broad.