Porphyrin-Based Catalysis

Improvement in the stereoselectivity of the oxidation of ds-stilbene was observed by increasing the number of substituents on the aryl groups of the porphyrin ligand, pointing to an enhanced preference for a concerted pathway. In general, it should be noted, however, that frans-alkenes are poor substrates for these catalysts [57, 62]. 

Enhanced epoxidation rates were observed using a Mn-porphyrin complex 8 in which the carboxylic acid and imidazole groups are both linked to the ligand 

Recently, Hulsken et al. have employed STM to probe the mechanism by which manganese porphyrin complexes achieve epoxidation of alkenes with dioxygen [69]. Importantly, this study demonstrates the positive role of solid-support immobilization in precluding the formation of inactive oxido-bridged catalyst dimers (a common deactivation pathway). 

---

Porphyrin-based self-assembled molecular squares 389 can form mesoporous thin films in which the edge of a square, thus the size of the cavity, can be adjusted by appropriate choice of substituents [8]. Fibers that form coil-coiled aggregates with distinct, tunable helicity are built from crown ethers bearing porphyrins 390 [9]. In addition to the porphyrin applications discussed in Sections 6.3.2.2 and 6.4, dendrimer metalloporphyrins 391 to be applied in catalysis [10] and the water-soluble dendritic iron porphyrin 319 modelling globular heme proteins [11] can be mentioned. 

Rhodium-based catalysis suffers from the high cost of the metal and quite often from a lack of stereoselectivity. This justifies the search for alternative catalysts. In this context, ruthenium-based catalysts look rather attractive nowadays, although still poorly documented. Recently, diruthenium(II,II) tetracarboxylates [42], polymeric and dimeric diruthenium(I,I) dicarboxylates [43], ruthenacarbor-ane clusters [44], and hydride and silyl ruthenium complexes [45 a] and Ru porphyrins [45 b] have been introduced as efficient cyclopropanation catalysts, superior to the Ru(II,III) complex Ru2(OAc)4Cl investigated earlier [7]. In terms of efficiency, electrophilicity, regio- and (partly) stereoselectivity, the most efficient ruthenium-based catalysts compare rather well with the rhodium(II) carboxylates. The ruthenium systems tested so far seem to display a slightly lower level of activity but are somewhat more discriminating in competitive reactions, which apparently could be due to the formation of less electrophilic carbenoid species. This point is probably related to the observation that some ruthenium complexes competitively catalyze both olefin cyclopropanation and olefin metathesis [46], which is at variance with what is observed with the rhodium catalysts. 

In summary, the mechanism of reductive nitrosylation of Co porphyrins differs completely from the one observed for Fe analogues. It seems that the main reason for such a different reactivity behavior of Fe and Co porphyrins can be accounted for in terms of the h h instability of the [Co (P)(NO) (H2O)] intermediate. Although the electronic structure of the latter complex may have some contribution from the [Co (P)(NO )(H20)] structure, its instabUity prevents any further reaction with nucleophiles. This is in contrast to the mechanism of reductive nitrosylation of Fe(III) porphyrins, where general base catalysis involving a relatively stable [Fe (P)NO ] complex was observed. 

Most of the catalysts employed in PEM and direct methanol fuel cells, DMFCs, are based on Pt, as discussed above. However, when used as cathode catalysts in DMFCs, Pt containing catalysts can become poisoned by methanol that crosses over from the anode. Thus, considerable effort has been invested in the search for both methanol resistant membranes and cathode catalysts that are tolerant to methanol. Two classes of catalysts have been shown to exhibit oxygen reduction catalysis and methanol resistance, ruthenium chalcogen based catalysts " " and metal macrocycle complexes, such as porphyrins or phthalocyanines. ... 

Catalysis by cobalt porphyrins and Schiff s base complexes 387... 

At least two systems can be cited as catalysts of peroxide oxidation the first are the iron (III) porphyrins (44) and the second are the Gif reagents (45,46), based on iron salt catalysis in a pyridine/acetic acid solvent with peroxide reagents and other oxidants. The author s opinion is that more than systems for stress testing these are tools useful for the synthesis of impurities, especially epoxides. From another point of view, they are often considered as potential biomimetic systems, predicting drug metabolism. Metabolites are sometimes also degradation impurities, but this is not a general rule, because enzymes and free radicals have different reactivity an example is the metabolic synthesis of arene oxides that never can be obtained by radical oxidation. 

Based on the entire set of data on the important catalytic performance of this class of organic compounds in chemical and biological systems, the far-reaching conclusion can be made that biomimics will provide a foundation for catalysis chemists to solve future applied tasks of catalysis. In particular, it is the author s opinion that the unique properties of porphyrins will open new perspectives for chemical technology in the creation of highly effective chemical production. 

Two routes for the synthesis of a four-terminal wire compound based on a central porphyrin unit are presented in Scheme 10.25 [13b], In one route, the tetrabro-mide 75 was treated with potassium thioacetate, which resulted in the substitution product 76. Alternatively, reaction of aldehyde 77 with pyrrole 78 at room temperature using mixed acid catalysis conditions provided 76 after oxidation with DDQ. 

---