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Electrochemistry Of 9,9' Spirobifluorene Derivatives: Electrosynthesis Of Stereoisomeric 2,3-bis(2'-
Published 1997 · Chemistry, Medicine
Abstract 2,2′-diacetyl-9,9′-spirobifluorene 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 E10 = −1.75 V and E20 = −1.90 V vs sce (saturated calomel electrode) respectively. A third reduction step occurs with Ep = −2.40 V, at v = 50 mV −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 CO 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.