Unexpected electrochemical reduction of fluoranthene in the solvents DME and HMPA: new light onto the mechanism of hydrogenation to produce tetrahydrofluoranthene

The non-alternant aromatic hydrocarbon fluoranthene (Ar0) has been reduced, either chemically with Na or Li or by electrolysis, to the radical anion Ar˙– in the three solvents THF (tetrahydrofuran), DME (dimethoxyethane) and HMPA (hexamethylphosphoric triamide). The UV–VIS absorption spectrum of the orange–brown Ar˙– is quite similar in the three solvents and in all instances addition of H+–H2O has resulted in quantitative electron-back-donation along with H2 evolution and recovery of unchanged fluoranthene Ar0. Thus the usual Birch-type reduction to a dihydro derivative is totally inefficient in the cases under investigation. The two-electron reduction has also been achieved in these three solvents. The greenish-yellow dianion Ar2– produced in THF exhibits characteristic UV–VIS absorption patterns, disproportionates with Ar0 and reacts with H+–H2O only to evolve H2.With DME or HMPA a blood-red species is produced whose absorption spectrum is virtually the same and quite different from that observed in THF. In both solvents addition of H+–H2O leads to tetrahydrofluoranthene as a main reaction product but disproportionation is not observed at all in HMPA and this is not compatible with a regular dianion Ar2–.Reaction with D+–D2O instead of H+–H2O has shown that hydrogenation involves radical abstraction of H atoms from the solvent in both cases; this sheds new light onto the reaction mechanism. Furthermore, several other experiments indicate that the dianionic blood-red species is most likely a complex written as [Ar˙–⋯˙– Solvent], in which the Ar˙– moiety is bound to a solvated electron localized on a solvent molecule.