Chemistry

Saddling Up Porphyrins

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Science  27 Jul 2007:
Vol. 317, Issue 5837, pp. 429
DOI: 10.1126/science.317.5837.429c

The stereochemical purity of a polymer or supramolecular assembly can often be set by a comparatively small chiral enrichment of the molecular building blocks or their coordination partners in solution. Toyofuku et al. have harnessed this effect to amplify the chiral enrichment of an ensemble of porphyrin complexes. They had previously shown that on complexation with chiral acids, the interconverting saddle-shaped enantiomers of an aryl-substituted porphyrin locked into one favorable diastereomeric conformation, which was conserved when the chiral acids were displaced by the achiral coordinating partner acetic acid. When they instead formed coordination polymers by linking the porphyrins through complexation of tethered pyridyl substituents to Pt ions, they found that the addition of a chiral acid during assembly had a nonlinear amplification effect on the stereochemical outcome. An acid sample of 40% enantiomeric excess was sufficient to induce the highest observed optical purity of the assembly. By adding excess acetic acid and a phosphine ligand, they could then disassemble the polymer and obtain a fully enriched sample of the porphyrin-acetic acid saddles. — JSY

Angew. Chem. Int. Ed. 46, 10.1002/anie.200701668 (2007).

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