UNEXPECTED ELECTROCHEMICAL REDUCTION OF FLUORANTHENE IN THE SOLVENTS DME AND HMPA - NEW LIGHT ONTO THE MECHANISM OF HYDROGENATION TO PRODUCE TETRAHYDROFLUORANTHENE

The nonalternant aromatic hydrocarbon fluoranthene (Ar-0) has been red uced, either chemically with Na or Li or by electrolysis, to the radic al anion Ar-.- in the three solvents THF (tetrahydrofuran), DME (dimet hoxyethane) and HMPA (hexamethylphosphoric triamide), The W-VIS absorp tion spectrum of the orange-brown Ar-.- is quite similar in the three solvents and in all instances addition of H+-H2O has resulted in quant itative electron-back-donation along with H-2 evolution and recovery o f unchanged fluoranthene Ar-0, Thus the usual Birch-type reduction to a dihydro derivative is totally inefficient in the cases under investi gation, The two-electron reduction has also been achieved in these thr ee solvents, The greenish-yellow dianion Ar2- produced in THF exhibits characteristic UV-VIS absorption patterns, disproportionates with Ar- 0 and reacts with H+-H2O only to evolve H-2. With DME or HMPA a blood- red species is produced whose absorption spectrum is virtually the sam e and quite different from that observed in THF, In both solvents addi tion of H+-H2O leads to tetrahydrofluoranthene as a main reaction prod uct but disproportionation is not observed at all in HMPA and this is not compatible with a regular dianion Ar2-. Reaction with D+-D2O inste ad 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 indicat e that the dianionic blood-red species is most likely a complex writte n as [Ar-.-...(.-)Solvent], in which the Ar-.- moiety is bound to a so lvated electron localized on a solvent molecule.