Syntheses of strapped, capped and bent porphyrins
Creators: name: Wijesekera, Tilak Panini
Publisher: name: University of British Columbia
Year: 2010

description: The doming of the porphyrin macrocycle, and the accompanied movement of the central iron atom in and out of the plane, have previously been suggested to be important in the mechanism of reversible oxygen binding and cooperativity of heme proteins. The objective of this work was to develop a synthetic route for the construction of bent porphyrins that would serve as simple models for the biological systems. This thesis describes the syntheses of the porphyrins of the type 109 containing a diagonally attached carbon chain, short enough to cause distortion of the porphyrin. The introduction of the short strap prior to cyclization was mandatory since the porphyrin is extremely stable in its planar configuration. Therefore, in the first step of the synthesis, the strap obtained as its terminal dicarboxylic acid, was linked to two moles of the β-unsubstituted pyrrole 66 by two simultaneous Friedel-Crafts acylation reactions followed by the diborane reduction of the ketonic groups. The chain linked bis pyrrole ethyl ester 89 so obtained was converted to the synthetically useful bis formylpyrrole 93 via the bis pyrrole benzyl ester 90, the bis carboxypyrrole 91 and the bis α-unsub-stituted pyrrole 92. The two formyl groups of 93 were subsequently protected by converting this to the dicyanovinyl derivative 94. Two monochlorinations at the α -methyl groups of 94 (using 2 equivalents of sulfuryl chloride) followed by the condensation of the bis α -chloromethyl derivative with two equivalents of the α -unsubstituted pyrrole 79 produced the dipyrromethane dimer 96. The saponification of the two ester groups and the deprotection of the formyl groups were effected in a single step by the use of strong aqueous alkali. The α -formyl- α'-carboxydipyrromethane dimer 107 so obtained was subjected to thermal decarboxylation (in refluxing dimethyl-formamide) and the resulting α -formyl- α' -unsubstituted dipyrromethane dimer 108 was cyclized under extremely high dilution using toluene-p-sulfonic acid to produce the porphyrin 109. Once the synthesis of the porphyrin 109a (n=ll) was successfully accomplished, the syntheses of the porphyrins with shorter carbon chains were attempted. It was possible to synthesize the 10 and 9-carbon strapped porphyrins (109b and 109c respectively) but the cyclization of 108d (n=8) did not produce a porphyrin. An alternative synthetic route to strapped porphyrins 109 is also described. The branch off point was the bis formyl-pyrrole 93. This was converted to the bis cyanoacrylate 115 and used to produce the dipyrromethane dimer 117 in a manner similar to that used to prepare compound 96 (the α-free pyrrole used was 80) . Compound 117 was converted to the prophyrin precursor 108 via 118 and 119 and subjected to acid-catalyzed cyclization as previously described. The next phase of this work dealt with the construction of the capped porphyrin 143. The key synthetic intermediate was durene-bis-pentanoic acid 130 which was prepared from bis(chloromethyl) durene via two malonate syntheses. The durene-bis-pentanoic acid was carried through the same reaction sequence as previously described (the dicyanovinyl-ethyl ester route) for the simple dicarboxylic acids to give the capped porphyrin 143. The presence of a short strap had a rhodofying effect on the electronic absorption spectra of the porphyrins. A definite trend was observed in going from the capped porphyrin 143 to the porphyrins of progressively shorter chains (109a ￫ 109b ￫ 109c). This change was attributed to distortion of the macrocycle. ¹H NMR spectra exhibited large upfield shifts of the chain methylene proton resonances and also of the durene methyl proton resonance due to the influence of the diamagnetic ring current of the porphyrin. [See thesis for diagrams].