Porphins separation

It has been recently demonstrated that the simplest of the cobalt porphyrins (Co porphine) adsorbed on a pyrolytic graphite electrode is also an efficient electrocatalyst for reduction of 02 into 1120.376 The catalytic activity was attributed to the spontaneous aggregation of the complex on the electrode surface to produce a structure in which the cobalt-cobalt separation is small enough to bridge and activate 02 molecules. The stability of the catalyst is quite poor and largely improved by using porphyrin rings with mew-substitu-tion.377-380 Flowever, as the size of the mew-substituents increases the four-electron reduction efficiency decreases. 

Dih symmetrical porphyrins such as porphin itself,7 H2TPP and H2OEP9 are conveniently prepared from pyrrole and aldehyde by cyclocondensations (Scheme 1). Addition of Zn11 ion often improves the yield.10 The product is often contaminated with chlorin by-product, which can be converted to porphyrin by oxidants such as DDQ,10 or separated by chromatography.11 Bulky substituents tend to lower the yield, and meso o-substituted phenyl groups give rise to atrope isomers. 

However, the use of chloroform as a solvent has certain disadvantages, in particular a concentration dependence of spectra in this solvent has been noted 172>. Furthermore, many porphyrins, including porphine itself, are only sparingly soluble in chloroform, and this has lead to wide use of trifluoroacetic acid (TFA) as a solvent, in which these compounds exist as di-cations. Protonation of two of the nitrogen atoms results in an increased separation between the chemical shifts of the internal and external protons. Thus, in deuterio-TFA the tneso protons of the octaethylporphine, 41, are shifted even further downfield to t —0.98, while the imino protons, whose increased shielding more than compensates for the positive charge on the nitrogen atoms, are shifted upheld to t 14.65 m.i72). 

The effects of the differences in solubility due to the side chains, and the differences in pK at which the protons are added at the pyrrolenine nitrogens, may be combined to bring about the separation of closely related porphins. A method based on a combination of these properties was first systematically applied and used with great skill by Willstatter (158). The Willstatter method consists of the solution of the porphins in ether and extraction with aqueous hydrochloric acid of different concentrations. The HCl numbers are characteristic physical constants of the porphins. The HCl number is defined as that per cent of aqueous hydrochloric acid which will extract two-thirds of the porphin from w ater-saturated ether into an equal volume of the aqueous layer. The ether solution is made up to contain 20 milligrams per cent porphin or a saturated solution containing less. 

As can be seen from Table IV isomers cannot be separated by the HCl method since their HCl numbers are almost, if not identical. The separation of copro from protoporphin is readily accomplished because the HCl numbers differ appreciably from one another indeed, it is the method of choice in analyzing tissues for naturally occurring porphins. Uroporphin I at pH 4 is practically insoluble in acetic or ether and can thus be separated from the other porphins. 

In the synthesis of porphins from dipyrroles this separation technic is applied systematically to isolate a number of possible products from a mixture. The ether solution containing the porphins is extracted with 0.01% ammonium hydroxide and the dicarbonic acid porphins enter the aqueous solution. The monocarbonic acid porphins are then extracted from the ether with small amounts of 10% sodium hydroxide. The third... 

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