Intramolecular Electron Transfer in Bipyridinium Disulfides

Reductive cleavage
of disulfide bonds is an important step in many
biological and chemical processes. Whether cleavage occurs stepwise
or concertedly with electron transfer is of interest. Also of interest
is whether the disulfide bond is reduced directly by intermolecular
electron transfer from an external reducing agent or mediated intramolecularly
by internal electron transfer from another redox-active moiety elsewhere
within the molecule. The electrochemical reductions of 4,4′-bipyridyl-3,3′-disulfide
(1) and the di-N-methylated derivative
(22+) have been studied in acetonitrile.
Simulations of the cyclic voltammograms in combination with DFT (density
functional theory) computations provide a consistent model of the
reductive processes. Compound 1 undergoes reduction directly
at the disulfide moiety with a substantially more negative potential
for the first electron than for the second electron, resulting in
an overall two-electron reduction and rapid cleavage of the S–S
bond to form the dithiolate. In contrast, compound 22+ is reduced at less negative potential than 1 and at the dimethyl bipyridinium moiety rather than at the
disulfide moiety. Most interesting, the second reduction of the bipyridinium
moiety results in a fast and reversible intramolecular two-electron
transfer to reduce the disulfide moiety and form the dithiolate. Thus,
the redox-active bipyridinium moiety provides a low energy pathway
for reductive cleavage of the S–S bond that avoids the highly
negative potential for the first direct electron reduction. Following
the intramolecular two-electron transfer and cleavage of the S–S
bond the bipyridinium undergoes two additional reversible reductions
at more negative potentials.