Porphyrins oxidative ring-opening

The catabolism of tryptophan seems to have many steps in common with the formation of nicotinamide. The amino nitrogen is not attacked initially, but instead the indole ring is opened up by tryptophan pyrrolase (an iron-porphyrin enzyme). Just as in other cases of oxidative ring opening (cf. Chapt. X-7), the double bond is split and each new end receives one oxygen atom. In this way N-formylkynurenine is formed it is then hydrolyzed to formate and kynurenine. 

In another example, a polymer-supported chromium porphyrin complex was supported on ArgoGel Cl and then employed for the ring-opening polymerization of 1,2-cyclohexene oxide and C02 [95], This complex showed higher activity than a C02-soluble equivalent, and the solid nature of the catalyst meant that recycling of the catalyst was much easier. 

Epoxidation reactions are important because epoxides are useful intermediates -ring opening allows subsequent reactions at one or both of the carbon atoms, and this is particularly attractive if asymmetric epoxidations can be accomplished (3.1.1.3). This is an increasingly important area for Ru-catalysed oxidations, and a review [2] emphasises in particular mechanistic aspects of such processes. There are also earlier brief reviews of the topic [4, 7,18], in particular by porphyrin-based systems [17-22],... 

Verdoheme Excess nitrite oxidizes porphyrin ring Ring opened Fe(III) Absent Green... 

The ligand group can be introduced either on the meso or on the /5-pyrrole position of the porphyrin ring, but the synthesis of the meso-functionalized derivatives is easier and has been more widely exploited. Balch (50-53) reported that the insertion of trivalent ions such as Fe(III) (32) and Mn(III) (33) into octaethyl porphyrins functionalized at one meso position with a hydroxy group (oxophlorins) leads to the formation of a dimeric head-to-tail complex in solution (Fig. 11a) (50,51). An X-ray crystal structure was obtained for the analogous In(III) complex (34), and this confirmed the head-to-tail geometry that the authors inferred for the other dimers in solution (53) (Fig. lib). The dimers are stable in chloroform but open on addition of protic acids or pyridine (52). The Fe(III) octaethyloxophlorin dimer (52) is easily oxidized by silver salts. The one-electron oxidation is more favorable than for the corresponding monomer or p-oxo dimer, presumably because of the close interaction of the 7r-systems in the self-assembled dimer. 

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