The electrochemical oxidation of five 2,5-diaryl-1,4-dithiins with ary
l=p-anisyl (BAD), p-tolyl (BTD), phenyl (DPD), p-chlorophenyl (BCD) an
d p-nitrophenyl (END) has been studied in MeCN-CH2Cl2 (1:1) by cyclic
voltammetry (CV), derivative cyclic voltammetry (DCV) and linear sweep
voltammetry (LSV) as well as constant current coulometry and product
analyses. All compounds were found to undergo two quasi-reversible one
-electron transfers to the radical cations and the dications. The form
al potential of the first redox couple and the life-time of the radica
l cation were found to decrease when the aryl group became more electr
on donating. The radical cation of END was non-reactive on the timesca
le of slow-scan CV, whereas the radical cation of BAD was so reactive
that it was impossible to outrun the follow-up reaction even at a scan
rate of 1000 V s(-1). The number of electrons determined in the prese
nce of 2,6-lutidine, added to prevent acid-catalyzed conversion of sub
strate, was between 1.1 and 1.5 except for END, the radical cation of
which catalyzed the oxidation of some unknown species in the solution.
The major products from preparative electrolysis were the correspondi
ng 2,2'-dimers, which were isolated in yields up to 20%. A detailed me
chanistic and kinetic analysis of the dimerization, in the presence of
2,6-lutidine, involving the simultaneous fitting of theoretical data
and experimental data obtained by LSV and DCV on a timescale covering
several orders of magnitude, demonstrated that the process was of the
radical cation-radical cation type and allowed for the determination o
f rate and equilibrium constants, k(1) and K-1. In addition, the value
s of the heterogeneous electron transfer rate constant, k(8), and the
transfer coefficient, alpha, could be determined this way. The values
of k(1) were found to vary between 2.2x10(6) M(-1) s(-1) (BAD) and 8.7
x 10(2) M(-1) s(-1) (BCD). The Hammett-plot, log k(1) vs. sigma(+), w
as linear indicating the importance of resonance stabilization for the
dicationic transition state. The formal potentials for the reversible
oxidation of the 2,2'-dimers to the radical cations were 70-120 mV hi
gher than those for the monomers although AMI calculations predicted t
he ionization potentials of the dimers to be slightly lower than those
for monomers. It is suggested that differences in solvation more than
counterbalance the purely electronic effects.