THE TETRATHIAPORPHYCENE REDOX SYSTEM - EL ECTROCHEMICAL REDUCTION OF A 20-PI CYCLOPHANE TO ITS DIATROPIC 22-PI DIANION

The reductive carbonyl coupling (McMurry reaction) of 5,5'-diformyl-2, 2'-bithiophene affords the fourfold sulfur bridged [20]annulene 5 and its [30]annulene homologue 10 in 8 and 3% yields. Coupling of 5,5''-di formyl-2,2':5',2''-terthiophene produces structurally related macrocyc les, albeit in very low yields. As shown by X-ray crystallographic inv estigation, the bridged annulenes 5 and 10 are non-planar cyclophanes exhibiting transannular electronic interaction. The sulfur bridged [20 ] annulene 5 constitutes the central molecule of the tetrathiaporphyce ne redox system emcompassing the dicationic tetrathiaporphycene 3, the annulene 5, the 22pi dianion 8 and the two intermediate radical ion s pecies. Compound 5 is reduced in one two-electron step giving the diat ropic 22-pi dianion 8 which is characterized by cyclovoltammetry, coul ometry, spectroelectrochemistry and H-1-NMR spectroscopy. By contrast, 5 fails to undergo oxidation with formation of the dicationic 18pi te trathioporphycene.