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Porphyrins and their analogues demonstrate great applications in various scientific fields such as catalysis, supramolecular chemistry, biomimetic models for photosynthesis, and medical applications such as phototherapeutic agents for photodynamic therapy. Introduction of vicinal alkyne units at the periphery extends these properties by modulating the already unusual electronic structure. It also increases their chemical reactivity: substitution makes these porphyrins susceptible to Bergman cyclization in the presence of heat or light to generate a novel class of highly conjugated picenoporphyrins. The discovery of photochemical Bergman cyclization as a novel approach to drive enediyne reactivity in a controlled manner prompted us to design enediyne molecules with strongly absorbing chromophores and low thermal barriers to cyclization. For this purpose, we have recently synthesized 2,3-dialkynylporphyrins and their halo-analogues in good to excellent yields (55-90%). The terminal alkyne units on these porphyrins, coupled with the double bond at the pyrrole backbone, behave as an “enediyne” motif, and therefore undergoes Bergman cycloaromatization thermally and photochemically to produce the highly conjugated picenoporphyrins.
Crystal Structure of Dibromo-picenoporphyrin
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