Synthesis of covalently-linked dimeric porphyrins

Abstract

DOLPHIN. Can. J. Chem. 56,1710(1978. Dimeric porphyrins covalently linked directly at their periphery (P,Pr-biporphyrins) or via a hydrocarbon chain (CH?),, where tt = 1-8, have been synthesized from the corresponding P,P'-linked bispyrroles. JOHN B. PAINE 111 et DAVID DOLPHIN. Can. J. Chem. 56,1710Chem. 56, (1978 Utilisant ies bispyrroles correspondants lies en P.P' cornme produits d e depart, on a synthetise les porphyrines dimkres liees d'une f a~o n covalente directernent & leur piriphirie (P.P' biporphyrines) ou par I'intermediaire d'une chaine hydrocar-bonee (CH?),, ou t~ = 1-8.[Traduit par le joul-nal]Since our first report (1) of covalently-linked dimeric porphyrins, joined by amide linkages, a number of similar dimeric porphyrins with amide, ester, or ether linkages have been reported (2-10). That all of these systems employed the coupling of porphyrins, through these functional groups, results from the 'ready' availability of the porphyrin precursors and the ease of formation of amide, ester, and ether linkages. However, once formed, such linkages have numerous disadvantages in that they generally lower the solubility (of systems which naturally have low solubilities), and increase the reactivity, and thereby decrease the stability. In addition the presence of extraneous functional groups complicates mechanistic and spectral studies on such systems.Whereas all previous syntheses of dimeric porphyrins consist of joining two preformed porphyrin entities in the final steps, our approach consists of constructing the covalent linkjrsr, and then building a porphyrin onto either end. To avoid the disadvantages of the linkages discussed above a synthesis has been developed where hydrocarbon chainlinked bispyrroles 1 are converted into the corresponding dimer porphyrins 7 where the hydrocarbon chain plays a passive role both chemically and electronically.Experimentally, the sequence depicted in Scheme 1 is facile to perform. The preparation of the bispyrroles 1 depends upon the length of the linking chain. When 17 = 0 the P,Pf-bipyrrole was prepared by Ullmann coupling (11) of the corresponding P-iodopyrrole 8 (R = I). For 11 = 1 the P,P'-dipyrromethane was prepared from 9 (R = H ) and acidified methanolic formaldehyde. For n > 5 acylation of 8 (R = H) or 9 (R = H ) with a diacid chloride (in methylene chloride -nitromethane a t 0°C with excess stannic chloride) gave the diketo bispyrroles 13 which were reduced to 1 with an excess of diborane. The diacid chlorides of succinic and glutaric acids could only be mono substituted, presumably owing to the propensity of the intermediate pyrrolyl keto acid chlorides 10c to cyclise to the five-and six-membered en01 lactones, which upon work u p gave the pyrrolyl keto acids 10a. These after esterification to the ethyl esters lob were reduced by diborane in ethyl acetate -tetrahydrofuran to lla, which were selectively saponified (carboxylalkyl sidechain esters saponify much faster than nuclear carboxylate esters which are vinylogous carbamates) to llb. These two carboxy...

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