Chemical Transformations of Mono‐ and Bis(buta‐1,3‐dien‐1‐yl)porphyrins: A New Synthetic Approach to Mono‐ and Dibenzoporphyrins
β-Butadienyl- and β,β′-dibutadienylporphyrins were prepared by the Wittig reaction of β-formyl- and β,β′-diformyl-meso-tetraphenylporphyrins with allylic phosphorus ylide. Subsequent treatment of β-butadienylporphyrin with dienophiles afforded the corresponding Diels–Alder adducts. In the absence of dienophiles, β-butadienylporphyrin underwent electrocyclization, followed by oxidation, to give monobenzoporphyrin in good yield. Similarly, adjacent and opposite dibenzoporphyrins were successfully synthesized from adjacent and opposite β,β′-dibutadienylporphyrins, respectively. This is the first report of electrocyclization of β-butadienylporphyrins. The structures of mono- and dibenzoporphyrin nickel complexes, as well as of a Diels–Alder adduct, were determined by single-crystal X-ray diffraction; a strong distortion from the planarity of the porphyrin core was observed. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)