Active oxygen metallo-porphyrin

Generation of an Active Oxygen Metallo-Porphyrin Intermediate... 

This reaction of the oxygen molecule activated by a heme enzyme is somewhat reminiscent of the photooxygenation of metallo-porphyrins mentioned in V.l (Formulas XII—XV). 

Much work has been reported on oxygenation reactions catalyzed by metallo-porphyrins in homogeneous medium. Initially, relatively high turnover numbers can be obtained. The solubility of such complexes, however, is rather limited and only a few solvents are suitable reaction media. Inactive micro-oxo-complexes, which are catalytically inactive are easily formed. The active entities in homogeneous conditions are also able to catalyse the self-destruction of the catalyst. Compared with the natural enzyme, these metallo-porphyrin complexes show a decreased regio-, stereo-, and positional selectivity (12,17,18). 

In order to generate stereochemical restrictions near the active site of the metallo-porphyrin, to avoid cluster formation of the complex, and eliminate oxidative degradation in reaction conditions, large substituents have been attached to the porphyrin (17,18). Examples of pocket-, picked-fence-, capped-and strapped- porphyrin complexes are shown in Fig. 2. In this way, the selectivity of most oxygenation reactions is enhanced and the life-time of the catalysts is considerably increased. However, as a result of the advanced synthesis methods used, such catalysts become extremely expensive. 

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