Figure-Eight Tetrathiaoctaphyrin and Dihydrotetrathiaoctaphyrin

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Abstract
The acid‐catalyzed condensation of pyrrole and 2,5‐bis(p‐tolylhydroxymethyl)thiophene yields two novel giant heteroporphyrins: 5,10,15,20,25,30,35,40‐octa(p‐tolyl)‐41,43,45,47‐tetrathia[36]octaphyrin(1.1.1.1.1.1.1.1) (S4OP) and 5,10,15,20,25,30,35,40‐octa(p‐tolyl)‐dihydro‐41,43,45,47‐tetrathia[38]octaphyrin(1.1.1.1.1.1.1.1) (S4OPH2). These tetrathiaoctaphyrins are interconvertible through dihydrogenation or dehydrogenation processes. Both compounds possess helical figure‐eight geometry with two thiophene rings for S4OP or two pyrrole rings for S4OPH2 in the ribbon‐crossing center as shown by NOESY experiments. The synthesis of the figure‐eight tetrathiaoctaphyrin implies a prearrangement process to form helical M or P dithiatetrapyrromethanes. The P–P or M–M condensation leads to the figure‐eight molecule in competition with an intramolecular ring closure. Two dithiaporphyrin‐like pockets of S4OP or S4OPH2 behave as independent proton acceptors. The stepwise process yields symmetric or asymmetric cationic species that depends on whether an even or odd number of NH protons are added. Dihydrotetrathiaoctaphyrin contains 38 π electrons in its conjugation pathway, which corresponds to the formal [4 n+2] Hückel type π‐electron formulation that is consistent with a modest diatropic ring current effect in their 1H NMR spectra. The formal 4 n type π‐electron formulation for tetrathiaoctaphyrin accounts for the residual paratropic shifts. A figure‐eight conveyor‐belt‐like movement of the whole macrocyclic ring without a racemization step is proposed to account for the dynamic properties of S4OP. The molecule shuttles between two degenerate configurations. A S4OP–S4OPH2 couple may be considered as a molecular element which, while preserving the overall figure‐eight geometry, “chooses” pyrrole or thiophene rings as spacers as a function of the macrocyclic oxidation state.

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