Phlorins

Oxidized porphyrin chromophores are also chemically and biologically important. Oxo-phlorins (7) possess a carbonyl group at one of the methine positions nominally, they are... 

Chlorins (2) are undoubtedly the most important dihydroporphyrins, since the chlorin chromophore is found in chlorophylls and some bacteriochlorophylls and, as the magnesium complex, is the catalyst in photosynthesis. The method of choice for formation of trans-chlorins involves reduction of iron porphyrins with sodium in boiling isopentyl alcohol (57JCS3461), but methods involving photochemical reduction of tin(IV) porphyrins, isomerization of phlorins, reduction of metalloporphyrins with sodium anthracenide followed by protonolysis, heating with sodium ethoxide, and photoreductions of zinc(II) porphyrins in the presence of ascorbic acid have also been employed. The best method for formation of c/s-chlorins (note that all natural chlorophylls possess the trans arrangement) appears to... 

There are relatively few examples of nucleophilic attack upon the porphyrin system, electrophilic substitutions being far more common. One well substantiated example of nucleophilic attack was described by Woodward (62MI30700). In this example the sterically hindered porphyrin (59) suffers nucleophilic attack at the 15-position to form a phlorin cation (60), the driving force for the reaction presumably arising from release of strain accompanied by the change from sp2 (planar) to sp3 (tetrahedral) of the C-15 atom. 

A famous use of electrochemical reduction of porphyrin systems is that used to convert a chlorophyll a derivative, chlorin-e6 trimethyl ester (64), into a chlorophyll b derivative, rhodin-g7 trimethyl ester (65) (71LA(749)109). Electrolysis of chlorin-e6 trimethyl ester gave the chlorin-phlorin which was photolyzed in dioxane/water to give the trans-d o (Scheme 7) this was simply transformed into rhodin-g7 trimethyl ester (65) to accomplish the first ever interconversion of a chlorophyll a series pigment into one of the companion b series. 

There are several porphyrin derivatives of gold(III) such as the tetraphenyl-porphyrinatogold(III) cation, [Au(TPP)]+ (95), which forms a square pyramidal complex [AuC TPP)] with chloride.620- 2 Reduction of the latter complex gives the phlorin derivative... 

There are several other nonaromatic hydroporphyrins known.2 14 Phlorins (5,22-dihydropor-phyrins) are stable only in acidic media (El/2 —0.04 V vs. SCE, pH2/HCl) but easily oxidized to porphyrins under neutral to basic conditions (El/2 = — 0.5 to — 0.7 V vs. SCE) unless the methylene bridge is blocked with substituents. Their spectra are characterized by two bands at 430nm (e 105) and 650 nm (broad). The former is accompanied by a band at 370nm (s 104), and both red-shift by 20 nm on metallation. The latter shifts to 750 nm on protonation and to more than 800 nm on metallation. Phlorins are at the same oxidation level as chlorins, and are the intermediates of photochemical conversions of porphyrins to chlorins. 

Chlorexolone as diuretic, 1, 174 Chlorides synthesis, 1, 448 Chlorin, 4, 370 metal chelates, 4, 391 Chlorin, dihydroxy-, 4, 393 Chlorin e6, 4, 404 trimethyl ester, 4, 398 synthesis, 4, 416 Chlorination pyridazines, 3, 20, 21 Chlorine trifluoride bonding, 1, 564 Chlorin-phlorin, 4, 398 Chlorins, 4, 378 absorption spectra, 4, 389 formation, 4, 394 molecular structure, 4, 385 oxidation, 4, 395 Chlorisondamine chloride as hypertensive agent, 1, 176 Chlormethiazoles metabolism, 1, 235 Chlormethiuron against ectoparasites veterinary use, 1, 217 Chlormezanone as antidepressant, 1, 169 Chlorocruoroheme, 4, 380 Chlorofucin conformation, 7, 703 Chloronium iodide, biphenylene-reactions, 1, 566... 

A ring localized hydrogenation can be a consequence of the disproportionation of photochemically formed 7t-anion radicals. Such a process was found [243] to proceed in aqueous solutions containing two gold(III) porphyrins Au(Por)Cl, where Por = TMPyP or TPyP, and electron donors (EDTA or TEA), jt-radicals abstract protons from HaO+ ions (pH < 4) and give the parent porphyrin complexes Au(Por)Cl and reduced phlorins Au(H2Por)Cl. 

Saturation at a meso position, interrupting cyclic conjugation, is met with isoporphyrin (VI), phlorin (VII), /(-chlorophlorin (VIII), porphodimethene (IX), porphomethene (X) and porphyrinogen (XI). 

Photoreductions of free base porphyrins and metal complexes with a variety of reducing agents, including ascorbic acid, glutathione, ethylene-diaminetetraacetic acid (EDTA), and acetoacetate, have been studied mainly from a mechanistic point of view (75MI6). Unstable hydroporphyrins of phlorin type VI are formed, which subsequently isomerize to chlorins, bacteriochlorins, or isobacteriochlorins. 

Catalytic hydrogenation of the isobacteriochlorin 39 carried out with Pt/C in ethyl acetate at ambient temperature occurred at the meso position, yielding phlorin-type 40, which subsequently was isomerized to 41 in formic acid at 70°C (80AG(E)143). 

Hydroboration of Zn-OEP gave a 5 1 mixture of cis- and trans-OEC (12), indicating two competing reductions in which the trans isomer is formed via a phlorin intermediate, rather than by direct attack of the boron hydride at the peripheral double bond (69TL1145). 

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