Porphyrins aromaticity

FIGURE 3.30. Reaction of iron(0) and iron(I) pophyrins with n-, s-, and r-butyl bromides. The chart shows the various porphyrins and their symbolic designations. iron porphyrins, aromatic anion radical, lines best-fitting parabolas through the aromatic anion radicals data. Dashed lines outer-sphere curves obtained by use of the Morse curve model (Section 3.2.2). Adapted from Figure 4 in reference 47b, with permission from the American Chemical Society. 

Metal-mediated cross-coupling reactions involving mcw-haloporphyrins enable the fabrication of porphyrin arrays that exhibit exceptional electronic interactions between their constituent porphyrinic building blocks. Because mew-halopor-phyrins derive from direct halogenation of the porphyrinic aromatic macrocycle, porphyrins bearing unsubstituted meso positions are important synthetic precursors to these supramolecular, multichromophoric systems. 

Thallium(III) acetate reacts with alkenes to give 1,2-diol derivatives (see p. 128) while thallium(III) nitrate leads mostly to rearranged carbonyl compounds via organothallium compounds (E.C. Taylor, 1970, 1976 R.J. Ouelette, 1973 W. Rotermund, 1975 R. Criegee, 1979). Very useful reactions in complex syntheses have been those with olefins and ketones (see p. 136) containing conjugated aromatic substituents, e.g. porphyrins (G. W. Kenner, 1973 K.M. Smith, 1975). 

Iron Porphyrins. Porphyrias (15—17) are aromatic cycHc compouads that coasist of four pyrrole units linked at the a-positions by methine carbons. The extended TT-systems of these compounds give rise to intense absorption bands in the uv/vis region of the spectmm. The most intense absorption, which is called the Soret band, falls neat 400 nm and has 10. The TT-system is also responsible for the notable ring current effect observed in H-nmr spectra, the preference for planar conformations, the prevalence of electrophilic substitution reactions, and the redox chemistry of these compounds. Porphyrins obtained from natural sources have a variety of peripheral substituents and substitution patterns. Two important types of synthetic porphyrins are the meso-tetraaryl porphyrins, such as 5,10,15,20-tetraphenylporphine [917-23-7] (H2(TPP)) (7) and P-octaalkylporphyrins, such as 2,3,7,8,12,13,17,18-octaethylporphine [2683-82-1] (H2(OEP)) (8). Both types can be prepared by condensation of pyrroles and aldehydes (qv). 

Porphyrin, 5,10,15,20-tetraphenyl-, 4, 386 Porphyrin, vinyl-synthesis, 4, 278, 279 Porphyrin coenzymes in biochemical pathways, 1, 258-260 Porphyrinogen, mcso-tetraaryl-synthesis, 4, 230 Porphyrinogens, 4, 378, 394 pyrazoles, 5, 228 synthesis, 4, 231 Porphyrins, 4, 377-442 acetylation, 4, 395 aromatic ring current, 4, 385 basicity, 4, 400 biosynthesis, reviews, 1, 99... 

The 7/NMR spectrum displays signals of shielded protons = - 2.35, integral level 1) and of deshielded ones = 10.45 and 9.39, integral levels 1 1). This reflects a ring current due to aromaticity as described for annulenes and porphyrins in section 2.5.2. To conclude, the reaction involves an oxidative cyclisatlon of 2,5-bis(2-pyrrolylmethyl)-17/-pyrrole 2 with 47/-trlazole-3,5-dlaldehyde 3 to the corresponding 2,3-diazaporphyrin 4, following the 3-t-l pathway of porphyrin synthesis. Two non-equivalent tautomers may exist these are the diaza[ 18]annulene 4a and the tetraaza[18]annulene 4b. 

Ibers used the Paal-Knorr furan synthesis to prepare a key intermediate for the synthesis of novel porphyrin-like aromatic macrocycles. Bis yrolyljfuran 27 was available in good yield via the acid catalyzed condensation of diketone 26. ... 

F. Aromatic and Heteroaromatic Large Rings 1. Porphyrins and Related Compounds... 

The porphyrin ring system (the parent compound 1 is also known as porphin) consists of four pyrrole-type subunits joined by four methine ( = CH-) bridges to give a macrotetracycle. The macrocycle contains 227i-electrons from which 1871-electrons form a delocalized aromatic system according to Huckel s 4n + 2 rule for aromaticity. The aromaticity of the porphyrin determines the characteristic physical and chemical properties of this class of compounds. The aromatic character of porphyrins has been confirmed by determination of their heats of combustion.1"3 X-ray investigations4 of numerous porphyrins have shown the planarity of the nucleus which is a prerequisite for the aromatic character. 

The high stability of porphyrins and metalloporphyrins is based on their aromaticity, so that porphyrins are not only most widespread in biological systems but also are found as geoporphyrins in sediments and have even been detected in interstellar space. The stability of the porphyrin ring system can be demonstrated by treatment with strong acids, which leave the macrocycle untouched. The instability of porphyrins occurs in reduction and oxidation reactions especially in the presence of light. The most common chemical reactivity of the porphyrin nucleus is electrophilic substitution which is typical for aromatic compounds. 

The reduced symmetry of the chromophore, which still contains 187t-electrons and is therefore an aromatic system, influences the electronic spectrum which shows a bathochromic shift and a higher molar extinction coefficient of the long-wavelength absorption bands compared to the porphyrin, so that the photophysical properties of the chlorins resulting from this structural alteration render them naturally suitable as pigments for photosynthesis and also make them of interest in medical applications, e.g. photodynamic tumor therapy (PDT).2... 

Whereas in porphyrins, chlorins and bacteriochlorins the 1871-aromatic perimeter includes two nitrogens excluding their lone pairs, the.1871 perimeter of the isobacteriochlorin includes three nitrogens, one with its electron lone pair as part of the aromatic perimeter. The electron pair of the /J,/J -C —C double bond of ring D of isobacteriochlorin is not involved in the cyclic conjugation path. 

All the known porphyrin isomers are typical benzoid aromatic compounds which show distinctly porphyrin-like characteristic electronic absorption spectra.13 Also the complexation properties for metal ions, NH tautomerism and the NMR spectra are quite similar to the parent porphyrin structure. 

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