Anodic properties of silafluorenes

Three 1,3-diphenyl ring-substituted silafluorenes (I–III) and one ring-unsubstituted derivative, 1,1-diphenylsilafluorene (IV) were characterized electrochemically by cyclic voltammetry measurements in dichloromethane-acetonitrile (1:4, v/v)-lithium perchlorate at both Pt and glassy carbon (GC) anodes. Compounds I–III show at least two irreversible oxidation waves, the first being at a lower potential (around 1.4 V) and the second around 1.75 V whereas IV exhibits only one irreversible oxidation wave at 1.85 V (vs. Ag/AgCl). Electrolysis at a C (graphite) anode under controlled potential conditions led to the expulsion of the silicon moiety in all cases. Introducing phenyl groups on the silafluorene ring at the 1 and 3 positions in I–III afforded 1,3,5-triphenylbenzene (1) as the sole aromatic hydrocarbon product. However, ‘naked’ compound IV showed less selectivity, producing a mixture of products involving biphenyl, substituted biphenyls (and additional unidentified aromatic hydrocarbon). The silicon fragments formed mostly linear and cyclic siloxanes on both C and Pt anodes. However at Pt the reaction was less selective than at C and produced in addition “intermediate” products containing a single C–Si bond (out of the two in the parent silafluorene ring) due to only one C–Si bond cleavage.