Octa-alkyl porphyrins

One of the variables in the structures of the porphyrins present in heme proteins is the presence or absence of vinyl substituents on the periphery of the macrocycle. For example, b hemes have vinyl substituents whereas c hemes do not. Because of the sensitivity of such vinyl substituents during synthetic transformations, it has often been desirable to use octa-alkyl porphyrins in model studies of the spectroscopic properties of heme systems. The development of improved methods for the preparation of octa-alkyl porphyrins has likewise increased the availability of such porphyrins for model studies (20, 21). To assess the effect that replacement of the two vinyl substituents in protoporphyrin IX with alkyl (ethyl) groups has on the MCD properties of the heme system, an extensive and systematic study of the MCD properties of mesoheme IX-reconstituted myoglobin and horseradish peroxidase in comparison with the spectra of the native protoheme-bound proteins has been carried out (22). The structures of these two porphyrins are shown in Figure 3. 

Tamiaki and coworkers developed novel synthetic methods which allowed transformations of octa-alkyl porphyrins, in particular of octaethyl-porphyrin [88]. Figure 17 shows the structure of the fully synthetic BChl mimics that were synthesized using these methods by Balaban and coworkers (16-22) [86,87] while compoimds 23 and 24 were synthesized by Tamiaki s group [88]. 

Khene S, Cammidge AN, Cook MJ, Nyokong T (2007) Electrochemical and photophysical characterization of non-peripherally-octa alkyl substituted dichlorotin(IV) phthalocyanine and tetrabenzotriazaporphyrin compounds. J Porphyrins Phthalocyanines 11(10) 761-770... 

H. J. Callot, New syntheses of octa-alkyl- and octaisoalkyl-porphyrins, Angew. Chem. 94, 297... 

Note that other, related octasubstituted porphyrins have been prepared but little information was given concerning their mesomorphism. The zinc /3-octa(decyl)porphyrin (the eight chains were pure alkyl chains) was found to display a mesophase (probably columnar) between 117°C and 177 °C, which is in the same temperature range as the ether analogs. It also showed conductivity properties of the order of magnitude to that of (22). No mesomorphism was detected for porphyrins with eight alkoxy chains. ... 

Figure 13.19. 5,15-Diaryl octa-alkyl pcaphyrin HOMO has an Aia character. The tetraaryl porphyrin HOMO has an A2a character.

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