Sapphyrin isomer

The approach makes use of a bipyrroledicarbaldehyde 54 and a tripyrranedicarboxylic acid 55 which are condensed with subsequent decarboxylation in the presence of acid and oxygen. The presence of oxygen, as in many other syntheses of porphinoid macrocycles, is necessary to adjust the oxidation level of the chromophore which is in sapphyrins a 2271-aromatic cyclically conjugated system. Many sapphyrins 56 with different substitution patterns have been synthesized according to this general scheme. As in all McDonald-type condensations at least one of the components has to be symmetric because otherwise mixtures of constitutional isomers would be formed. 

Figure 5.5.2 Structures 5.85a and 5.85b showing the two observed isomers of the tetraligated zinc(II) complex of sapphyrin...

The second sapphyrin analog reported by Sessler and coworkers is derived directly from macrocycle 6.26. Here, subjecting the alkyne-containing macrocycle 6.26a to Lindlar reduction conditions was found to afford a near quantitative conversion to the partially reduced macrocycle 6.27a (Scheme 6.3.3). As inferred from H NMR spectroscopic analysis, the specific structure of this reduction product is the tra 5-alkene 6.27. This compound may formally be regarded as being a true isomer of pentaazasapphyrin, and is thus referred to as [22]sapphyrin-(2.1.0.0.1) or [22]pentaphyrin- 2.1.0.0.1). 

A separate single crystal X-ray structural analysis revealed that the partially reduced macrocycle 6.27b also adopts a near-planar conformation in the solid state (Figure 6.3.4). It also confirmed the anticipated trans-stereochemistry about the meso-C = C bond in that one of the protons was found to be directed into the core of the macrocycle, while the other points outward. Taken together these solid-state structural findings are thus consistent with the proposal that 6.27 is a true isomer of sapphyrin. 

Porphyrin has fourfold symmetry, with the pyrrole rings connected by alternate meso bridges. About eight isomers can be sketched down by changing the sequence of the subunits, of which porphycene (37), hemiporphycene, corrphycene, and isoporphycene have been synthesized to date. Expanded porphyrins are, by definition, those possessing more than 16 atoms in the smallest circuit of the macrocycle. Sapphyrin (39) is the common expanded porphyrin derived by the insertion of an additional pyrrole ring into the macrocycle [40]. 

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