Use of Montmorillonite clay for the synthesis of linear tetrapyrroles and their cyclization to Uroporphyrinogens

Effects of diverse acid catalysts on the reaction course in the two-step one-flask synthesis of meso-tetraphenylporphyrin

A set of 45 acids or acid combinations was examined in the condensation of pyrrole + benzaldehyde (10 mM each in CH 2 Cl 2) 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. MgBr 2-etherate and CuCl 2 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 BF 3-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.

New Route to ABCD-Porphyrins via Bilanes

A new strategy for preparing porphyrins that bear up to four different meso-substituents (ABCD-porphyrins) relies on two key reactions. One key reaction entails a directed synthesis of a 1-protected 19-acylbilane by acid-catalyzed condensation at high concentration (0.5 M) of a 1-acyldipyrromethane and a 9-protected dipyrromethane-1-carbinol (derived from a 9-protected 1-acyldipyrromethane). Three protecting groups (X) were examined, including thiocyanato, ethylthio, and bromo, of which bromo proved most effective. The bilanes were obtained in 72-80% yield, fully characterized, and examined by 15N NMR spectroscopy. The second key reaction entails a one-flask transformation of the 1-protected 19-acylbilane under basic, metal-templating conditions to give the corresponding metalloporphyrin. The reaction parameters investigated for cyclization of the bilane include solvent, metal salt, base, concentration, temperature, atmosphere, and time. The best conditions entailed the 1-bromo-19-acylbilane at 100 mM in toluene containing DBU (10 mol equiv) and MgBr2 (3 mol equiv) at 115 degrees C exposed to air for 2 h, which afforded the magnesium porphyrin in 65% yield. The magnesium porphyrin is readily demetalated to give the free base porphyrin. A stepwise procedure (which entailed treatment of the 1-(ethylthio)-19-acylbilane to oxidation, metal complexation, desulfurization, carbonyl reduction, and acid-catalyzed condensation) was developed but was much less efficient than the one-flask process. The new route to ABCD-porphyrins retains the desirable features of the existing "2 + 2" (dipyrromethane + dipyrromethane-1,9-dicarbinol) method, such as absence of scrambling, yet has significant advantages. The advantages include the absence of acid in the porphyrin-forming step, the use of a metal template for cyclization, the ability to carry out the reaction at high concentration, the lack of a quinone oxidant, avoidance of use of dichloromethane, and the increased yield of macrocycle formation to give the target ABCD-metalloporphyrin.

Highly efficient synthesis of linear pyrrole oligomers by twofold Heck reactions

The twofold Heck reaction of the vinylpyrroles 3a and 3b with the iodobenzenes 4a-c led to the linear pyrrole oligomers 5, 6, and 7. The synthesis of both symmetrical and unsymmetrical oligomers, such as 10a and 10b, was also accomplished by a Heck reaction of 8 and 9 and by a Heck reaction of 3a and 11 followed by a Wittig reaction and a second Heck reaction with 8. The pentacyclic oligomers 14 and 19 were prepared by a twofold Heck reaction of 13 with 4 and by a twofold Heck reaction of 15 with 16 followed by a Wittig reaction and a twofold Heck reaction with 8.