exhibits a reversible couple at 0.75 V.) The magnitude of
the current decreased during the first few scans and then
remained constant but even after a number of such scans
the anodic current remained substantially greater than at an
unmodified electrode. This electrode bearing several different
triarylamino groups is designated here as electrode A.
Electrocatalysis. The cyclic voltammogram of 1,2-bis-
(trimethylsilyl)-1,2-diphenylethane (5) in acetonitrile at an
unmodified electrode is shown as a solid line in Figure 2.
with 3,4-dimethoxybenzyl alcohol, which is converted to the
corresponding aldehyde by the electrocatalyst.5a
It was not possible to separate the mixture of amines 4
before diazotization because of the ready oxidation of these
substances in air. The presence of tri- and monobromo-
substituted triphenylamine components presumably accounts
for the broadness of the anodic peak in Figure 1 and the
observed peak potential of ca. 0.9 V; the parent substance 1
has an oxidation potential of ca. 0.75 V. A second method
was therefore devised for attachment of the 4,4′-dibromot-
riphenylamine nucleus to carbon. 4-Nitrotriphenylamine (2)
was reduced with tin and hydrochloric acid, diazotized, and
electrochemically attached to a carbon fiber electrode surface
as previously described for the mixture of amines 4. Pinson
has shown that it is possible to carry out a variety of chemical
reactions on the aryl rings in these modified electrodes.4 The
electrode was therefore immersed for 2 min in a 1 M solution
of Br2 in chloroform to afford electrode B (Scheme 2, second
equation). Bromination at positions ortho to the central
nitrogen atom of the triarylamine nucleus is presumably
sterically blocked in this modified electrode. The oxidation
potential of such electrodes is ca. 0.7 V, i.e., 0.2 V less
positive than those prepared from 4 (Figure 3). Apparently,
Figure 2. Cyclic voltammetryin 0.1 M TBAHFP/acetonitrile: (a)
solid line, 1,2-bis[trimethylsilyl-1,2-diphenylethane (5) (1.5 mM)
at an unmodified carbon electrode; (b) dotted line, a modified
electrode A in the solvent without 5; (c) dashed line, same as (b)
but in the presence of 1.5 mM 5.
An anodic peak is observed at ca. +1.0 V for the direct
oxidation of 5. The dotted line represents the behavior of
electrode A in a 0.1 M solution of tetrabutylammonium
hexafluorophosphate (TBAHFP) in acetonitrile. The dashed
line represents the voltammetric response after addition of
5 (1.5 mM) to the solution. A severalfold increase in the
anodic current is observed at potentials in the range 0.8-
0.9 V, affording clear evidence for catalytic oxidation of 5
by the attached triarylamine moiety. It should be noted that
because the carbon fibers we use are larger and less smooth
than typical electrodes for analytical voltammetry, these
voltammograms are not as well-defined as those at smaller
electrodes. Preparative-scale electrolysis at +0.8 V of 5 (0.3
g) in methanol containing solid NaHCO3 at a larger carbon
electrode (1 cm2) modified in the same fashion afforded a
65:35 mixture of 1,2-dimethoxy-1,2-diphenylethane (6) and
1,1-dimethoxy-2,2-diphenylethane (7) in 95% yield (Scheme
3). Addition of NaHCO3 ensured that 7 is a primary product
Figure 3. Cyclic voltammetry at carbon fiber electrode B (l ) 1
cm) in acetonitrile/0.1 M TBAHFP.
this is because only two bromine atoms are introduced by
this method. This electrode again showed electrocatalytic
behavior, even in a solution of only 1.5 mM 5 (the limit of
its solubility) (Figure 4). These experiments demonstrate the
feasibility of chemical attachment of the triarylamine nucleus
to carbon electrodes. The procedure used here will permit
tuning the redox potential of the catalytic electrode by
varying the number and electronic nature of the groups on
the triphenylamine nucleus.
The formation of amines 4a-e upon reduction of the
dibromide 3 calls for comment. The literature contains a
number of so-called halogen dance reactions in which a
halobenzene is converted into a mixture of substances
containing anywhere from 0 to 3 halogen atoms. Many such
processes have involved intermediate aryl carbanions.10
Certain of these have been electrochemically induced.10c,d
Some halogen dances involve reaction of an aryl halide with
Scheme 3. Electrocatalytic Oxidation of
1,2-Bis(trimethylsilyl)-1,2-diphenylethane at Electrode A
(9) Porter, J. M.; Xuan, X.; Blackman, B.; Hsu, D.; Fry, A. J. Tetrahedron
Lett. 1997, 38, 7147.
(10) (a) Bunnett, J. F. Acc. Chem. Res. 1972, 5, 139. (b) Duan, X.-F.;
Zheng, Z.-B. Heterocycles 2005, 65, 2005. (c) Mubarak, M. S.; Peters, D.
G. J. Electroanal. Chem. 1997, 435, 47. (d) Mubarak, M. S.; Peters, D. G.
J. Org. Chem. 1996, 61, 8074.
and not formed by acid-promoted pinacolic rearrangement
of 6. These are the same products in the same ratios as from
the direct electrolysis.7 We have discussed the formation of
7 previously.9 Similar electrocatalytic behavior was observed
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