THE TETRATHIAPORPHYCENE REDOX SYSTEM - ELECTROCHEMICAL 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''-diformyl-2,2':5',2''-terthiophene produces structurally related macrocycles, albeit in very low yields. As shown by X-ray crystallographic investigation, 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 tetrathiaporphycene redox system emcompassing the dicationic tetrathiaporphycene 3, the annulene 5, the 22pi dianion 8 and the two intermediate radical ion species. Compound 5 is reduced in one two-electron step giving the diatropic 22-pi dianion 8 which is characterized by cyclovoltammetry, coulometry, spectroelectrochemistry and H-1-NMR spectroscopy. By contrast, 5 fails to undergo oxidation with formation of the dicationic 18pi tetrathioporphycene.