Electrochemistry of 9,9′-spirobifluorene derivatives: 2-acetyl- and 2,2′-diacetyl-9,9′-spirobifluorene. Preparation of stereoisomeric 2,3-bis(9,9′-spirobifluoren-2-yl)butane-2,3-diols

Abstract

2-Acetyl- and 2,2′-diacetyl-9,9′spirobifluorene 1 and 2 were studied by cyclic voltammetry in dimethylformamide. The corresponding anion radicals show remarkable persistency in aprotic DMF. The (apparent) standard potentials are E°=–1.77 V (SCE) and E°=–1.75 V for the (quasi-reversible) reduction of 1 and 2 to the anion radicals, respectively. Preparative electrolysis of 1 in DMF–Et₄NClO₄(0.1 mol dm^–3), with excess acetic acid as proton donor, furnished alcohol 3 and the diastereoisomeric pinacols 5 and 6, which were isolated and characterized. The diastereoisomeric excess, de, as evaluated (NMR) on the electrolyzed solution was only slightly in favour of the (±) compound. Spectroscopic properties of compounds 1–6 are, inter alia, the ¹³C NMR chemical shift for the spiro-carbon at δ= 65.9 (TMS), and the fragmentation patterns in the mass spectra, with the 100% relative abundance of the molecular-ion M˙⁺ in the case of the aromatic ketones 1 and 2. Some comments on the influence of conformations of 5 and 6 and of the presence of an intramolecular hydrogen-bond in some intermediates and products are also presented.