Synthetic metalloporphyrins

EXPERIMENTAL APPROACHES TO STUDYING ORR CATALYSIS BY SYNTHETIC METALLOPORPHYRINS... 

Oxidizing enzymes use molecular oxygen as the oxidant, but epoxidation with synthetic metalloporphyrins needs a chemical oxidant, except for one example Groves and Quinn have reported that dioxo-ruthenium porphyrin (19) catalyzes epoxidation using molecular oxygen.69 An asymmetric version of this aerobic epoxidation has been achieved by using complex (7) as the catalyst, albeit with moderate enantioselectivity (Scheme 9).53... 

In the course of research on catalysis by synthetic metalloporphyrins, Tagliatesta and coworkers reported a unique cyclodimerization reaction of aryl acetylenes to give 2-aryl naphthalenes (6, Scheme 9.2) [5]. Ru(CO) and RhCl complexes effectively promote this reaction, with the latter catalyst (8) providing generally superior yields and selectivities for a small range of substrates (Table 9.1). As a synthetic method, Tagliatesta s cyclodimerization reaction is most remarkable for its efficiency. Yields of up to 78% were observed at a substrate/catalyst ratio of 5700 1. Successful recycling of recovered catalyst was also demonstrated. 

In the last decade, transition metal complexes (e.g. metalloporphyrins) have been used to catalyze epoxidation. These entities can reproduce and mimic all reactions catalyzed by heme-enzymes (cytochromes P-450)54. Synthetic metalloporphyrins are analogous to the prosthetic group of heme-containing enzymes which selectively catalyze various oxidation reactions. The metallo complexes of Fe, Co, Cr, Mn, Al, Zn, Ru, etc. possessing porphyrin ligands have been mostly studied55 -57. Porphyrin ligands (4) are planar and can possess several redox states of the central metallic ions and hence they can exist as oxo metals. 

Among the oxidation catalyzed by heme enzymes, alkane hydroxylation is the hardest process to mimic with synthetic metalloporphyrins (7). This finding is also true for myoglobin mutants, even though the mutants are able to afford... 

A class of synthetic metalloporphyrins which are conveniently studied in aqueous solution derives from coordination compounds of the quadridentate ligand a,(S,7,5-tetra(4-pyridyl)-porphine. Although a method of synthesis for the zinc complex has been published,1 the yield is often poor and the method unreliable. If the same reaction is allowed to proceed under... 

The problems connected with the high cost and low stability of peroxidases, which limits their potential applications in processes of industrial interest, can be possibly overcome, at least in part, by the use of heme-peptide complexes as small-size peroxidase analogs. In this context, we will consider only the heme-peptide complexes more strictly related to the peroxidases, and will not deal with the extensive literature about catalytic oxidations by synthetic metalloporphyrins. It is difficult to reproduce in a synthetic molecule the peculiar features of complex enzymes such as... 

As in the case of horseradish peroxidase, several synthetic metalloporphyrins in the presence of H2O2 have been found to be potent catalysts for the chemiluminescent oxidation of luminol or isoluminol. The microperoxidases, mainly MP8 and MPll, have been shown to act as functional peroxidase enzyme models. " However, they are readily inactivated within one min of catalytic turnover, and incorporation into a molecular sieve... 

Synthetic metalloporphyrins as biomimetic catalysts (cytochrome P450 models) in the oxidative activation of drugs 04MI33. 

The mechanism of oxygen activation by P-450 has been extensively studied (7c, 5a, b) however, cmcial intermediates in the catalytic cycle are so unstable that it is not possible to examine their structures and reactivities. Accordingly, model systems for P-450 with synthetic metalloporphyrins have illuminated the enzymic reaction mechanism. For instance, the so-called picket fence porphyrin, /ncjo-tetra(a,a,a,a-pivalamidophenyl)porphyrin (TpivPP) Fe(II), prepared by Collman and co-workers, forms a stable complex with O2 6a, b). An X-ray... 

Although the mechanism of hydroxylation of C-H bonds by P-450 is surrounded by controversy [8], it is very likely that hydroxylation of activated C-H bonds catalyzed by synthetic metalloporphyrin complexes proceeds through radical intermediates. 

Studies using synthetic metalloporphyrins (Figure 1.10) as models for cytochrome P450 have afforded important insights into the nature of the enzymatic processes . Indeed, each of the intermediates shown in Scheme 1.1 has been independently identified by model studies using S5mthetic analogs, especially meso-tetraaryl porphyrins . ... 

For general reviews on the use of synthetic metalloporphyrins as heme-enzyme models, see refs. 153 and 154. 

Over the last three decades it has become known that certain synthetic metalloporphyrins can also act as oxidative catalysts thus mimicking the action of cytochrome P450. Transformations concerning substrates epoxidation and hydroxylation have been widely studied [ 19,55,56]. 

Synthetic metalloporphyrins have received a lot of recent attention as mimics of numerous enzymes. In addition, 10 models have been developed for peroxidases and particularly, ligninases. Metalloporphyrins have also found utility as model systems for studies of the oxidative metabolism of drugs.A detailed study of the metabolism of lidocaine has been reported, as have preliminary studies on the use of metalloporphyrins as chemical mimics of cytochrome P450 systems (Scheme 29.24). ... 

The first synthetic metalloporphyrins were found to be oxidatively labile. Few catalytic turnovers were seen, due to the rapid destruction of the porphyrin macrocycle. It has been shown that introduction of halogens onto the aryl groups (of mesotetraarylporphyrins) and... 

Since the analogues of the heme prosthetic group containing enzymes (e.g. cytochrome P450, peroxidases, chloroperoxidases and catalase) have been implicated in biological oxidation reactions (70, 77), then their use as biomimetic, synthetic metalloporphyrins in the area of alkane/alkene oxidation chemistry has shown that... 

By the beginning of the 1970s, the majority of electrochemical studies on synthetic metalloporphyrins was being carried out in nonaqueous media using the technique of cyclic voltammetry. However, most utilized instrumentation was still homemade and only a handful of laboratories were actually making the measurements. An overview of the situation at this period is provided in several independent reviews [2, 6, 7, 9, 21]. 

The present review describes the electrochemistry of synthetic metalloporphyrins in nonaqueous media. This work does not include metaUoporphyrin electrochemistry in aqueous media and discusses only briefly the electrochemistry of some iron porphyrins in real biological systems. We have not included the electrochemistry of porphyrin-Kke complexes such as metallocorroles, metalloporphycenes and metallochlorins, or the electrochemistry of pigments of biological relevance, such as vitamin B12 derivatives. For a coverage of these topics, the reader is referred to recent major reviews in The Porphyrin Handbook. 

Oxidations catalyzed by iron porphyrin complexes are not limited to hydroxylation of alkanes and epoxidation of alkenes. Many types of reactions catalyzed by heme enzymes such as P-450, peroxidases, and catalases have been modeled by synthetic metalloporphyrin complexes. 

---