Electrochimica Acta, Vol.42, No.13-14, 2257-2264, 1997
Electrochemistry of 9,9’-Spirobifluorene Derivatives - Electrosynthesis of Stereoisomeric 2,3-bis(2’-Acetyl-9,9’-Spirobifluoren-2-Yl)Butane-2,3-Diols and of 1-(2’-Acetyl-9,9’-Spirobifluoren-2-Yl)Ethanol and Redox Properties of Polyacetylated Spirobifluorenes
2,2’-diacetyl-9,9’-spirobifluore 3a, an axially dissymmetric compound, when dissolved in virtually aprotic solvent DMF, is characterised by several reduction peaks in the voltammetric experiments, using a glassy carbon electrode; the first two electron transfers are reversible, with standard potentials E-l(0) = -1.75 V and E-2(0) = -1.90 V vs sce (saturated calomel electrode) respectively. A third reduction step occurs with E-p = -2.40 V, at v = 50 mV s(-1). The first two steps are pertinent to the formation of the anion radical A(.-) and of the dianion diradical (-.)A(-.) of 3a (A) respectively. Controlled potential electrolysis of 3a on a rvc electrode in DMF-Et4NClO4 (0.1 M), with acetic acid 1:1 mol 3a/mol acid added, furnishes the two diastereomeric keto-alcohols 4 and the six diastereomeric keto-pinacols 5-7. This result is obtained if the electrolysis is stopped after 1 F/mol 3a was passed. If the electrolysis is continued until both C=O groups are reduced, a mixture of diastereomeric pinacols 8 is obtained. Spectroscopic properties of compounds 5-7 are sensitive to the various spatial arrangements in the isomers. Some aspects of the redox properties of 3a, as related to the spiro-structure of the molecule, are also discussed. Redox properties of tri- and tetra-acetylated spirobifluorenes 3b and 3c in virtually aprotic conditions, as determined through cyclic voltammetry, are in turn related to those of the diacetylated derivative 3a : compounds [3a, b, c] are the first examples of spirocompounds bearing multiple redox-active substituents.