Zhu Jing, Gao Yang, Cao Shaokui*, Zhang
Wennan#, Wang Dongmei#, Cui Huifang#, Li Tianxuan# Received Apr. 8, 2002. Abstract Some symmetric and asymmetric
meso-hydroxyl substituted tetraphenylporphyrins were synthesized by using a modified Adler
method. The modification was to add an anhydride compound into the reaction system in
order to timely remove water produced in the reaction process. Through the modification,
the yields of four hydroxyl groups substituted meso-tetraphenylporphyrins (THPPs), the
symmetric products, were in the range of 14-22%, and the yield of the asymmetric product
containing only one hydroxyl group was higher than 10%. Meso-hydroxyl substituted
tetraphenylporphyrins and their derivatives are not only useful and highly sensitive
chromogenic agents in analytical chemistry [1,2], but also important
intermediates in the preparation of bio-mimetic catalyst in imitating photosynthetic
charge separation [3]. And also symmetric THPPs are useful precursors for
porphyrin-containing liquid crystal assemblies[4-8] and man-made oxygen carrier
with the similar function of heme [9,10]. 1 EXPERIMENTAL
Meso-tetra(2-hydroxyphenyl) porphyrin [T(2-HP),4b] Meso-tetra(2-hydroxyphenyl)-porphyrin 4b was prepared in a similar manner as described for the para isomer 4a with a yield of 20%. 1H-NMR (300MHZ, Acetone-d6, d in ppm): 8.95 (8H, pyrrole), 3.01 (4H, hydroxyl), 6.92~7.10 and 8.01~8.04 (16H, phenyl). IR (KBr tablet, cm-1): 3529.1, 1591.7, 1509.3, 1349.7, 1225.5, 1170.3 and 798.7. Elemental Analysis, found (calculated for C44H30N4O4) : C 77.44(77.86), H 4.76(4.44), N 8.30(8.33). Meso-tetra(3-hydroxyphenyl)-porphyrin [T(3-HP),4c] Meso-tetra(3-hydroxyphenyl)-porphyrin 4c was prepared in the similar way as 4a with yield of 14%. 1H-NMR (300MHZ, Acetone-d6, d in ppm): 8.90 (8H, pyrrole), 2.91 (4H, hydroxyl), 7.20~7.51 and 8.01~8.06 (16H, phenyl). IR (KBr tablet, cm-1): 3405.0, 1608.7, 1599.7, 1347.4, 1232.2. Elemental Analysis, found (calculated for C44H30N4O4): C 77.40 (77.86), H 4.80 (4.44), N 8.23(8.33). 5-(4-hydroxyphenyl)-10,15,20-triphenyl porphyrin [5-(4-HP)P, 4d] 5-(4-hydroxyphenyl)-10,15,20-triphenyl porphyrin 4d was prepared in the similar way as 4a with 120 mmol freshly distilled benzaldehyde, 40 mmol 4-hydroxy-benzaldehyde, 160 mmol freshly distilled pyrrole and 15 ml propionic anhydride. The mixture product was chromatographed on an alumina column and silica column with eluent of dichloromethane and chloroform. The elute was concentrated. The crude product was recrystallized giving purple crystalline 4d with a yield of 10% . 1H-NMR (300MHZ, Acetone-d6, d in ppm): 8.92 (8H, pyrrole), 2.80 (4H, hydroxyl), 7.28~7.31 and 8.04~8.08 (16H, phenyl). IR (KBr tablet, cm-1): 3504.4, 1611.0, 1513.0, 1349.5, 1261.1, 1170.6 and 798.7. UV/VIS (lmax, nm in EtOH): 419.5 (the soret band), 516.0, 556.0, 597.0 and 649.0 (the Q band). Elemental Analysis, found (calculated for C44H30N4O): C 82.69 (82.99), H5.91 (5.70), N 8.76 (8.80). 2 RESULT AND DISCUSSION The condensation of aldehyde with pyrrole exists an equilibrium of ring-chain formation. So the yields of the porphyrins are usually not very high. People have made some attempts to improve the yield of porphyrin, but there were some problems remain to be solved. The formation of each THPP molecule involves the condensation of four hydroxy-benzaldehydes with four pyrroles. Four water molecules were also formed at the same time. Therefore, based on the equilibrium shift principle, timely removing the produced water will promote the process of the condensation, thus result in a higher yield. With this consideration, we used propionic anhydride as a dewatering agent. Results showed an obvious improvement in the porphyrin yield. Propionic anhydride has a higher boiling temperature than propionic acid, hence may allow the reaction system to reflux at a higher temperature. According to the literature and our own experiences, an appropriate high reaction temperature is beneficial to the formation of porphyrin for the Alder condensation. In the process of Adler condensation of pyrrole with aldehyde, the hydroxyl groups on porphine ring easily form a hard black solid with acid. Therefore, when condensation is completed, a large amount of alcohol is necessary to be added at once under vigorous stirring. By adding alcohol, the strong interaction between the hydroxyl group on porphine ring and the carboxyl groups coming from propionic acid and anhydride was destroyed. The interaction was believed to take place in a short time and not strong at all. Through the addition of a large amount of alcohol with vigorous stirring, the above interaction will be destroyed and the hydroxyl porphyrin will be liberated, thus much higher purity product will be obtained. Otherwise, the isolation of product will be very difficult. After the addition of a large amount of alcohol, the reaction mixture was filtered. The selection of an appropriate washing solvent is very important for the purity of the product. Using chloroform as the washing agent did not give sufficient dissolution to the solid impurities, therefore the product was still quite impure. Using 95% alcohol instead, both of the product and impurities were well dissolved, resulting in failing in product isolation. A mixture of alcohol and propionic acid with 1:1 in volume was proved to be the best washing solvent in our experiment. By using this mixture as washing solvent, brilliant blue crystal product was obtained. For the modified Adler method, the optimum adding amount of anhyride was found to be a molar ratio of 2:3 to 1:1 to pyrrole. As shown in Table 1, with the increase of the anhydride, porphyrin yield was improved gradually while the molar ratio of anhydride to pyrrole was less than 2:3. Over 1:1 of the molar ratio of anhydride to pyrrole, further more amount of anhydride would have no effect on the increase of porphyrin yield. Table 1 The effect of anhydride addition on the yield of T(4-HP)
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