5,10,15,20-Tetraphenyl-p-benziporphyrin, synthesized by M. Stępień, is a novel carbaporphyrinoid with a p-phenylene ring embedded in the tripyrrolic framework.
p-Benziporphyrin may be considered to have two adjacent CH units in the macrocyclic core. The macrocycle is planar except for the tilt of the p-phenylene moiety. meta-Benziporphyrin, reported in earlier papers, lacks the macrocyclicaromaticity typical of regular porphyrins because the m-phenylene fragment cannot participate in the overall conjugation. In the para isomer, however, the macrocyclic aromaticity is feasible (a quinoid canonical structure can be drawn) and is indeed observed using 1H NMR. The outer and inner p-phenylene signals resonate respectively at 7.7 and 2.3 ppm. Interestingly at higher temperatures the two p-phenylene signals coalesce into a singlet at 5.2 ppm.The macrocycle is very flexible with the phenylene ring seesawing in the tripyrrolic brace. The macrocycle acts as a ligand for transition metals, where an agostic interaction can be observed while metals did not present a tendency to form M-C bonds (Cd(II), Zn(II)). But the reaction of p-benziporphyrin with palladium(II) chloride, performed by B. Szyszko, gave unexpected product of a catalytic contraction of six membered carbocycle with formation of a cyclopentadienporphyrinato palladium(II) complex.
Further studies on this topic, conducted with K. Kupietz, provided an observation of the contraction under gold(III) coordination. This reaction creates a direct Au-C(sp2) bond giving a gold(III) complex of true carbaporphyrin.
Exploration of the reactivity of p-phenylene dicarbinol with pyrrole and benzaldehyde in conditions characteristic for porphyrin synthesis gave A,D-di-p-benzihexaphyrin(18.104.22.168.1.1).
The resulting molecule bears four mesityl and two phenyl substituents. The bulk of the mesityl groups assists the formation of expanded macrocycles and additionally prevents molecular aggregation. Structurally, this new system combines the features of expanded porphyrins and benziporphyrins, the phenylene-containing porphyrinoids. Because of the non-planarity of the macrocycle, its p system is torsionally distorted As a consequence of the unusual arrangement of the phenylene rings, the surface of the p system is single-sided, and corresponds to the Mobius band with a single half twist.
Szyszko, B.; Latos-Grażyński, L.; Szterenberg, L. A Facile Palladium-Mediated Contraction of Benzene to Cyclopentadiene: Transformations of Palladium(II) p-Benziporphyrin. Angew. Chem. Int. Ed. 2011,50, 6587.
Szyszko, B.; Kupietz, K.; Szterenberg, L.; Latos-Grażyński, L. Gold(III)-Mediated Contraction of Benzene to Cyclopentadiene: From p-Benziporphyrin to Gold(III) True Tetraarylcarbaporphyrin. Chem. Eur. J. 2014, 20, 1376.
Stępień, M., Latos-Grażyński, L., Sprutta, N., Chwalisz, P., Szterenberg, L. Expanded Porphyrin with a Split Personality: A Hückel-Möbius Aromaticity Switch. Angew. Chem. Int. Ed. 2007, 46, 7869.
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