Contracted and Expandedmeso-Alkynyl Porphyrinoids: from Triphyrin to Hexaphyrin

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Abstract
The boron trifluoride-catalyzed Rothemund condensation of triisopropylsilyl (TIPS) propynal 1 with 3,4-diethylpyrrole in dichloromethane, followed by oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) generates a mixture of products, including [15]triphyrin(1.1.3) H3, corrole H34, porphyrin H22, [24]pentaphyrin(1.1.1.1.1) H45, [28]hexaphyrin(1.1.1.1.1.1) H46, and two linear tripyrromethenes H27 and H28. We report the spectroscopic characteristics of these unusual chromophores, together with the crystal structures of triphyrin H3 (and its zinc complex ZnCl3), porphyrin H22 (and its metal complexes Zn2, Ni2 and Pt2), hexaphyrin H46, and tripyrromethene nickel(II) complex Ni7. When the condensation is catalyzed with trifluoroacetic acid, rather than boron trifluoride, the triphyrin H3 become the main product (26% yield). This novel macrocycle is linked with a TIPS-substituted exocyclic double bond. This CC bond makes an η2-interaction with the zinc center in ZnCl3 with C−Zn distances of 2.863 and 3.025 Å. The porphyrin H22 is severely ruffled, and its absorption spectrum is red-shifted and broadened compared with the analogous compound without ethyl substituents. The hexaphyrin H46 adopts a figure-of-eight conformation with virtual C2 symmetry in the solid state and C2 symmetry in solution on the NMR time scale. Oxidation with DDQ appears to convert this nonaromatic [28]hexaphyrin into an aromatic [26]hexaphyrin with a strongly red-shifted absorption spectrum, but the oxidized macrocyle is too unstable to isolate.

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