Metalloporphyrin reactions

Cytochrome P-450 can perform both intramolecular and intermolecular nitrene insertions with 106 in water solution [181]. However, the intermediate metallo-nitrene (108) also hydrolyzed to some extent so the enzyme performed a hydroxylation as well as an amidation. Although we have also performed an intermolecular amidation of a steroid, using such a metalloporphyrin reaction with 106 to form 112 [182], we have not yet extended it to the kinds of directed functionalizations described above with either benzophenones or chlorinations. 

Lamy, J., and J. Lamy, eds. Invertebrate Oxygen-Binding Proteins. New York Dekker, 1981. Lavallee, D. K. Kinetics and mechanisms of metalloporphyrin reactions. Coord. Chem. Rev. 61... 

Oxo-hydroxo tautomerism as a useful tool in oxygenation reactions catalyzed by water-soluble metalloporphyrins 98CC2167. 

Photoinduced electron transfer reactions in supramolecularmodel systems based on metalloporphyrins 97YGK557. 

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. 

Chlorins are also accessible by carbene additions to C-C double bonds on the periphery of metalloporphyrins. The most effective reaction on a preparative scale is the addition of ethyl diazoacetate in refluxing benzene to copper octaethylporphyrin (4) or meso-tetraphenylpor-phyrin in the presence of copper(I) iodide,100108b 110 which gives a diastereomcric mixture of chlorins, e.g. 5. 

Metalloporphyrin redox reactions. L. E. Bennett, Prog. Inorg. Chem., 1973,18,1-176 (750). 

The oxidation states and reversible redox reactions of metalloporphyrins. J. H. Furhrhop, Struct. Bonding (Berlin), 1974,18,1-67 (221). 

Catalysis by metalloporphyrins of reactions involving oxidation by molecular oxygen and oxygen-containing compounds. N. S. Enikolopyan, K. A. Bogdanova, L. V. Karmilova and K. A. Askarov, Russ. Chem. Rev. (Engl. Transl.), 1985,54,215 (188). 

One family of porphyrin complexes that will be treated in the review, even though they do not contain metal-carbon bonds, are metalloporphyrin hydride and dihydrogen complexes. As in classical organometallic chemistry, hydride complexes play key roles in some reactions involving porphyrins, and the discovery of dihydrogen complexes and their relationship to metal hydrides has been an important advance in the last decade. 

The above-described structures are the main representatives of the family of nitrogen ligands, which cover a wide spectrum of activity and efficiency for catalytic C - C bond formations. To a lesser extent, amines or imines, associated with copper salts, and metalloporphyrins led to good catalysts for cyclo-propanation. Interestingly, sulfinylimine ligands, with the chirality provided solely by the sulfoxide moieties, have been also used as copper-chelates for the asymmetric Diels-Alder reaction. Amide derivatives (or pyridylamides) also proved their efficiency for the Tsuji-Trost reaction. 

To select the metal to be incorporated into the substrate porphyrin unit, the following basic properties of metalloporphyrins should be considered. The stability constant of MgPor is too small to achieve the usual oligomeric reactions and purification by silica gel chromatography. The starting material (Ru3(CO)i2) for Ru (CO)Por is expensive and the yield of the corresponding metalation reaction is low. Furthermore, the removal of rutheniirm is difficult, and it is likewise difficult to remove the template from the obtained ruthenium CPOs. Therefore, ZnPor is frequently used as a substrate in this template reaction, because of the low prices of zinc sources (zinc acetate and/or zinc chloride), the high yield in the metalation reaction, the sufficient chemical stability of the ZnPor under con-... 

The qualitative stabihty constants of metalloporphyrins are summarized in Table 1. The metals classified in class I produce the most stable metalloporphyrins and the demetalation reaction does not proceed smoothly even under concentrated sulfuric acid condition. The incorporated metals classified in classes 11 and III are removed using mild acids such as hydrochloric acid. Calcium classified in class V is removed by (the pH of) water. The mixed cyclization reactions afford the heterometalated CPO, and the acid treatment of the CPO obtained produces the CPO containing 2HPor moiety. Further treatment of the metal salt classified in a class lower than that of the unremoved metal(s), which is classified in a class higher in Table 1, produces another heterometalated CPO. Representative examples are summarized in Fig. 5 [25]. The initial cyclization reaction is carried out by using Ru (CO)Por 6, the metal of which is classified in class 1, and ZnPor 5, clas-... 

Table 1. Demetallation reaction conditions for metalloporphyrins based on the qualitative stability constant...

Of considerable interest was the demonstration that metalloporphyrins and the like can be used as nonmetallic catalysts in electrochemical reactions, nourishing hopes that in the future, expensive platinum catalysts could be replaced. Starting in 1968, dimensionally stable electrodes with a catalyst prepared from the mixed oxides of titanium and ruthenium found widespread use in the chlorine industry. 

Fuhrhop, J.-H. The Oxidation States and Reversible Redox Reactions of Metalloporphyrins. Vol. 18, pp. 1-67. 

Secondly, Fig. 5 shows that the polymeric rate constants parallel values of heterogeneous rate constants that have been observed for the electrochemical reactions of solutions of the corresponding dissolved porphyrin monomers. (The slope of the line is 0.5). This re-emphasizes what was said above, that measurements of electron hopping in polymers can give rate constants that are meaningful in the context of the metalloporphyrin s intrinsic electron transfer chemistry. 

Iodorhodium(III) porphyrins generally lead to alkylrhodium(III) porphyrins (Scheme 42)398>. This is also true for the reaction with ethyl diazoacetate in the presence of HOAc or an alcohol, and the insertion product 412 (M = Rh) could not be detected, in contrast to the corresponding cobalt porphyrin. A mechanistic scheme, which includes the diverse reaction modes of metalloporphyrins towards diazo compounds, has been proposed by Callot 393,398). 

Metal-oxenoid (oxo metal) species and metal-nitrenoid (imino metal) species are isoelectronic and show similar reactivity both species can add to olefins and be inserted into C—H bonds. Naturally, the study of nitrene transfer reactions began with metalloporphyrins, which were originally used as the catalysts for oxene transfer reactions. 

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