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

Abstract

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 Ar^2– 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 Ar^2–.

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.