UNEXPECTED ELECTROCHEMICAL REDUCTION OF FLUORANTHENE IN THE SOLVENTS DME AND HMPA - NEW LIGHT ONTO THE MECHANISM OF HYDROGENATION TO PRODUCE TETRAHYDROFLUORANTHENE
Sg. Boue et al., UNEXPECTED ELECTROCHEMICAL REDUCTION OF FLUORANTHENE IN THE SOLVENTS DME AND HMPA - NEW LIGHT ONTO THE MECHANISM OF HYDROGENATION TO PRODUCE TETRAHYDROFLUORANTHENE, Perkin transactions. 2, (8), 1996, pp. 1691-1697
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.