Porphyrin, capped strapped

Figure 9 A few schematic examples of mononuclear Fe(II) -porphyrin derivatives used to prevent the irreversible oxidation to oxygen-bridged binuclear Fe(III) derivatives. (a) Picket-fence complexes (b) strapped complexes (c) capped complexes (d) picnic-basket complexes (e) pocket complexes...

The more sophisticated pocket and picnic basket porphyrins are also capable of mimicking properties such as 02/C0 binding selectivity in which the (normally high) CO affinity is reduced because of steric interactions between the CO molecule (which adopts a linear Fe C=0 conformation) and the capping or strapping moiety. Selectivity for small molecules such as 02 is also enhanced, relative to larger bases. 

As the formation of five-coordinate complexes with nitrogenous bases largely depends on the size of the host cavity, single-face-hindered porphyrins have been developed pocket [40], cyclophane [41-44], bridged [45], crowned [46], capped [47-53], strapped [54-60] and hybrid [61], In these compounds, the size and the chemical nature of the superstructure can be conveniently varied by changing the number of methylene groups and the aromatic substituent incorporated. 

The second strategy was adopted by Dolphin and coworkers [59] and Baldwin et al. [56, 57]. Here the porphyrin cyclization was achieved during the final step. The synthesis of capped porphyrins (16) was an adaptation of the Rothemund synthesis. All four aldehyde groups destined to form the meso positions are provided from the pyromellitoyl derivative obtained in two steps from salicylaldehyde (Scheme 4). Dolphin adopted another strategy based on the cyclization of a bis(dipyrrylmethane) in which the dipyrrylmethane units are linked at the pyrrolic -positions by a pure hydrocarbon strap. In numerous examples of these compounds the straps contain a great variety of bulky blocking groups (Scheme 5). 

This approach has been used by many groups to produce a wide variety of architecturally different model porphyrins, e.g. picket-fence, capped, cyclophane, crowned, strapped, basket-handle, etc., which are discussed below. 

Recently the capped porphyrin approach has been extended to prepare a modified Q-capped porphyrin in which a pyridine is covalently boimd to two opposite meio-aromatic rings of the parent C2-capped porphyrin, forming some kind of strap ... 

Baldwin adapted his strapped porphyrin synthesis to prepare a system 199 with a benzene ring above one face (Scheme 59) Although structurally similar to the C2-capped porphyrin 160, the absence of the two extra linkages resulted in a floppy strap which did not prevent six-coordination by ligands such as 1-MeIm or pyridine and which did not prevent p-oxo bridge formation. An even more bulky strap, incorporating a naphthalene ring as in 200, was no more successful. 

Early studies using porphyrin model compounds attempted to mimic reversible oxygen binding in myoglobin. This involved the synthesis of porphyrins with a variety of "straps", "caps", "tail bases", "picket fences" and "basket handles" that served to provide a degree of supramolecular control over the way oxygen bound to the iron cation. ... 

Fig. 7.20 A variety of strapped and capped hemoprotein model porphyrins. [From D. Lexa and J.M. Saveart, Proceedings of the International Symposium on Redox Mechanisms and the Importance of Interfacial Properties in Molecular Biology, 3rd Meeting, G. Dryhurst and N. Katsumi (eds). Plenum, New York (1988).

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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