Porphyrin picnic-basket

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...

A picnic-basket porphyrin is reminiscent of the lacunar cyclidenes mentioned earlier, in which an alkyl chain tied the ends of the cyclidene together and formed a pocket for the complexation of dioxygen. Cobalt complexes of the picnic-basket porphyrins 31 bind dioxygen reversibly at room temperature with high oxygen affinities (84). The oxygen affinity increases as the basket size decreases (as determined by the length... 

Dramatic shape selectivities in competitive olefin epoxidation was observed with picnic basket metalloporphyrins312 313 designed to exclude bulky axial ligands on one sterically protected porphyrin face. When oxidized with PhIO in acetonitrile in the presence of the rigid p-xylyl-strapped porphyrin, cis-2-octene reacted selectively versus ds-cyclooctene or 2-methyl-2-pentene, giving >1000 reactivity ratios.313,314 Some immobilized manganese(III) porphyrins proved to be as efficient as their homogeneous equivalents in epoxidation with PhIO.151,315... 

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. 

Figure 5, Exchange experiments with gases in picnic-basket porphyrin 11 (26).

Collman, J.P, X. Zhang, R.T. Hembre, and J.I. Brauman (1990). Shape-selective olefin epoxidation catalyzed by manganese picnic basket porphyrins. J Am. Chem. Soc. 112, 5356-5357. 

The normally observed reversal of reaction (2) is inhibited in the strongly acidic media, probably through the reaction of O2 with to give HO2, and thus a clean source of [Ru(bpy)3] is available photochemically. Ruthenium picnic basket porphyrins may bind N2 or O2 reversibly. Infrared measurement of (160 i6q suggests that the oxygen may be described as a coordinated superoxide ion. Correlation between the binding of dioxygen and the Ru(III/II) potential was observed. 

Fig. 2 Dioxygen-binding myoglobin analogues as synthesized by Collman and coworkers.[ (A) Picket-fence porphyrin. (B) Pocket porphyrin. (C) Picnic-basket porphyrin.

Cytochrome P450 is one of nature s oxidative workhorses, and it is involved in a wide variety of biological reactions, one of the most important being the detoxification of foreign bodies within the liver. A wide variety of approaches was developed to design and construct synthetic model systems of the cytochrome P450 family.They vary from the earlier mentioned "capped" and "picnic basket" porphyrins, over supramolecular cytochrome P450 models, to dendritic porphyrins. ... 

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