Mediated electron transfer from lithium investigated voltammetrically in tetrahydrofuran: why are some mediators more effective reducing reagents than others?

A study of a range of aromatic molecules is investigated electrochemically to determine what makes an effective reducing mediator. With the aim of developing a better understanding of electron transfers (ETs) mediated from lithium in functional group reduction, a series of single ET reactions are reported in this paper. Typical reaction conditions involved the use of aromatic mediators such as naphthalene, anthracene, 4,4′-di-tert-butyl-1,1′- biphenyl (DBB) with lithium metal in tetrahydrofuran (THF) at -78 °C. The results of these experiments showed that some mediators were more effective reducing reagents than others. Cryoelectrochemical procedures are used to mimic the conditions of the SET reactions in order to investigate the exact nature and role of the mediator formed upon ET. It is demonstrated that electro-generated and stabilized radical anions of anthracene at -78 °C mediate the reduction of organic substrates, whereas the more reactive dianion is quickly protonated and therefore unable to act as an ET reagent; direct electrochemical reduction of the sulfide, phenyl 3-phenylpropylsulfide (RSPh) gives the thiol, thiophenol, and propyl benzene whereas mediated reduction gives the dimer, diphenyl disulfide and propyl benzene. The possibility to selectively reduce a substrate with either a single electron or with two electrons is possible by using either the radical anion (mediated) or via the direct electroreduction. DBB and naphthalene (both single electron accepting species only) have been found to be the most effective reducing reagents in this study. Anthracene and other two-electron accepting species only showed effective reducing ability when a stoichiometric amount of lithium was used therefore preventing the 'over-reduction' to the dianion. Copyright © 2007 John Wiley and Sons, Ltd