Synthesis of 20-iodochlorophyll derivatives and their properties including reactivity, electronic absorption, and self-aggregation

Abstract

Methyl 20-iodopyropheophorbides-a possessing several substituents at the 3-position were prepared by treatment of the corresponding 20-unsubstituted chlorophyll-a derivatives with iodine and [bis(trifluoroacetoxy)iodo]benzene in chloroform and water at room temperature. The sterically demanding 20-iodination was clean, efficient, and regioselective without alteration of the 3-substituents, and also highly reproducible. The resulting 20-iodides were so reactive that they were readily transformed into 20-acetylchlorin (Pd-catalyzed cross-coupling) and 20-unsubstituted chlorin (acidic deiodination). Electronic absorption spectra in dichloromethane showed that the 20-iodination shifted the maxima to longer wavelengths in their monomeric states. Zinc complexes of 31-hydroxy-20-iodo-131-oxochlorins self-aggregated in non-polar organic solvents to give red-shifted and broadened absorption bands, similarly as in the corresponding 20-unsubstituted molecules, which are good models of the main light-harvesting antenna systems in photosynthetic green bacteria.