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Rothemund synthesis of porphyrins is conveniently achieved using p-toluenesulfonic acid-catalysed reaction between pyrrole and aldehyde with azeotropic removal of the water formed in the reaction. These conditions are suitable for moderate-scale reactions and involve an easy work-up. In this way, 5,10,15,20-tetraheptylporphyrin (2), 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrin (3), 5,10,15,20-tetra-tert-butylporphyrin (4) and 5,10,15,20-[pyromellitoyl(tetrakis-o-oxyethoxyphenyl)]por-phyrin (5) were prepared in moderate to good yields.

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.

The successful use of dipyrromethanecarbinols in rational routes to porphyrinic macrocycles requires catalysis conditions that enable irreversible condensation, thereby avoiding substituent scrambling and formation of undesired porphyrin products. Previously, successful conditions of trifluoroacetic acid (TFA) (30 mM) in acetonitrile were identified following a lengthy survey of TFA and BF₃-etherate catalysis in diverse solvents. In this study, focus was placed on the acid catalyst by examining 17 acids in CH₂Cl₂, the traditional solvent for two-step, one-flask porphyrin syntheses. In the self-condensation of the carbinol derived from 1-(4-methylbenzoyl)-5-phenyldipyrromethane, porphyrin yields of 9–55% were obtained from the various acids, compared to 20% under TFA catalysis in acetonitrile. A number of catalytic conditions that produce little to no porphyrin in reactions of pyrrole + benzaldehyde afforded good yields of porphyrin and the suppression of scrambling in reactions of dipyrromethanecarbinols. The four best acid catalysts (InCl₃, Sc(OTf)₃, Yb(OTf)₃, and Dy(OTf)₃) initially identified were then examined with dipyrromethanecarbinols bearing challenging substituents (alkyl, pyridyl, or no substituent). The greatest improvement was obtained with the pyridyl substrates. Selected reactions performed on a preparative scale (115 to 460 mg of isolated porphyrin) verified the results of the analytical-scale experiments and revealed the more facile isolation of the porphyrin from reactions performed in CH₂Cl₂ rather than acetonitrile. This study provides alternatives to the use of TFA/acetonitrile that offer advantages in terms of yield and isolation of the porphyrin without sacrificing suppression of scrambling. Furthermore, the finding that poor catalysts for the benzaldehyde + pyrrole reaction can be excellent catalysts for dipyrromethanecarbinols provides guidance for the identification of other catalysts for use with reactive precursors in porphyrin-forming reactions.

A set of 45 acids or acid combinations was examined in the condensation of pyrrole + benzaldehyde (10 mM each in CH₂Cl₂) leading to meso-tetraphenylporphyrin (TPP). Initial screening experiments identified suitable acid catalysts and the optimal concentration (in terms of TPP formation) for each acid. Subsequent experiments followed the time course of reactions using the best conditions identified in the screening experiments. The reaction course was followed by monitoring reactions from 1 min to 24 h for the yield of TPP (by UV-vis and HPLC), the yields of N-confused tetraphenylporphyrin and tetraphenylsapphyrin (by HPLC), the quantity of unreacted benzaldehyde (by TLC), and the oligomer composition (by laser desorption mass spectrometry). Diverse acids (Brønsted or Lewis; soluble or insoluble) were found to provide yields of TPP ranging up to ~50%. Only 10 acids gave no TPP. In addition, N-confused tetraphenylporphyrin was found to be a ubiquitous byproduct, whereas tetraphenylsapphyrin was not widely observed. MgBr₂-etherate and CuCl₂ each catalyzed porphyrinogen formation and resulted in porphyrin metalation following oxidation and neutralization of the reaction mixture, thereby providing direct, one-flask syntheses of magnesium and copper porphyrins. Observations concerning the reaction course obtained from prior studies of TFA or BF₃-etherate catalysis have been found to be quite general across a broad range of acid catalysts. Collectively, these results show that many acids have potential utility in porphyrin syntheses.

The stability towards acidolysis of intermediates used in the preparation of core-modified porphyrinic macrocycles was examined. Experiments were performed using analogs of 5-phenyldipyrromethane in which one of the two pyrrole rings was modified (XPM). The XPMs utilized in these studies had X = 2-furyl (OPM), 2-thienyl (SPM), 2-selenyl (SePM), or 3-pyrrolyl (^NCPPM). The XPMs possess a potentially labile linkage to a heteroatom-modified ring as well as a potentially labile linkage to a pyrrole ring. The stability of the two types of linkages was examined under acid catalysis conditions commonly used in the preparation of porphyrinic macrocycles. The methodology employed enabled characterization of the stability of the XPM (GC analysis), the yield of porphyrin (UV-vis absorption), and the composition of porphyrinic species (laser-desorption mass spectrometry, LD-MS) formed upon acidolysis of the XPM. These experiments showed that (1) the linkage to the heteroatom-modified ring is fairly stable in the presence of a linkage to pyrrole, and (2) the linkage to pyrrole is much more stable in the XPMs than is the case in 5-phenyldipyrromethane. Additional experiments with analogs of 5-phenyldipyrromethane having both rings modified (X₂M), where X = 2-furyl (O₂M), 2-thienyl (S₂M), or 2-selenyl (Se₂M) confirmed that linkages to furyl, thienyl, and selenyl rings are stable towards acidolysis. Taken together, the observations concerning stability of various linkages provide a foundation for understanding the types and yields of core-modified porphyrinic macrocycles formed in reactions with reactants that are not direct precursors to the isolated products.

The synthesis of biodiesel is attracting tremendous attention both in academic and industrial fields in recent years. The use of acid heterogeneous catalyst for the production of biodiesel from low-cost raw materials with high free fatty acid is one of the research hotspots. This work reports the synthesis of Fe₃O₄@C@SiO₂-SO₃H/CH₃ (SFCS) via layer-by-layer coating of carbonated polydopamine (PDA) and thiol-SiO₂ on the Fe₃O₄ core, followed by the oxidation and acidification of thiol groups into sulfonic acid groups. TEM-EDX, BET, XRD, FTIR, and TGA were employed to characterize the structure and composition of SFCS. The esterification of oleic acid and methanol was photothermally actuated by near infrared light and the sulfonic acid groups on SFCS surface catalyzed the esterification with highest conversion of 93.2% of oleic acid. The SFCS can be recycled by magnet and reused.