Construction of electrocatalytic electrodes bearing the triphenylamine nucleus covalently bound to carbon. A halogen dance in protonated aminotriphenylamines

Article abstract of DOI:10.1021/ol052558n

The triarylamine nucleus has been attached to a carbon fiber electrode by diazotization of an aminotriphenylamine followed by electrochemical reduction. The resulting electrodes can electrocatalyze the oxidation of organic substrates. In acid, 4-amino-4′,4″-dibromotriphenylamine undergoes dismutation into a mixture of amines containing from 0 to 3 bromine atoms.

Full text of DOI:10.1021/ol052558n

ORGANIC
LETTERS
2006
Vol. 8, No. 3
411-414
Construction of Electrocatalytic
Electrodes Bearing the Triphenylamine
Nucleus Covalently Bound to Carbon.
A Halogen Dance in Protonated
Aminotriphenylamines
Brian T. Mayers and Albert J. Fry*
Chemistry Department, Wesleyan UniVersity, Middletown, Connecticut 06459
afry@wesleyan.edu
Received October 21, 2005
ABSTRACT
The triarylamine nucleus has been attached to a carbon fiber electrode by diazotization of an aminotriphenylamine followed by electrochemical
reduction. The resulting electrodes can electrocatalyze the oxidation of organic substrates. In acid, 4-amino-4
undergoes dismutation into a mixture of amines containing from 0 to 3 bromine atoms.
′,4′′-dibromotriphenylamine
A major theme in modern electrochemistry is the modifica-
tion of the surfaces of electrodes to make them more selective
toward certain components of solutions or otherwise enhance
their redox properties. For example, many groups have
attached enzymes or other biomolecules, redox-active groups,
or other functional groups to electrodes to enhance their
reactivity and specificity.1,2 In such studies, the surface-
modified electrode is often able to catalyze redox processes
at potentials lower than those necessary at unmodified
electrodes, permitting the use of milder conditions. A variety
of methods have been described for attaching materials to
electrode surfaces.2 A concern in many such procedures is
eventual loss of the coating by, for example, solubilization,
desorption, or hydrolysis of the linking group. Pinson,
Save´ant, and co-workers have described an elegant, general,
and efficient procedure for covalent bonding of aryl groups
to an electrode by electrochemical reduction of an aryldia-
zonium ion at the electrode.3 Aryl radicals are produced and
bind efficiently to the electrode in this process. The bonding
apparently involves true chemical bonds; consequently, the
resulting modified electrode is highly stable against loss of
the compactly packed monolayer of aryl groups. A variety
of aryl groups have been attached to carbon, silicon, and
metal surfaces by this procedure.4 An obvious use of such
electrodes is for electrocatalysis. We report the first example
of an electrode modified in this fashion which can effect
electrocatalytic oxidation of organic substrates.
The radical cation of 4,4′,4′′-tribromotriphenylamine (1)
is a versatile oxidant which has been shown to be an efficient
electrocatalyst, e.g., for deprotection of dithioacetals to
acetals or aldehydes, depending on electrolysis conditions,
and benzylic ethers to aldehydes.5 Steckhan developed
polymers containing the triarylamine nucleus.5c Our proce-
dure for synthesizing the necessary aminotriphenylamine
(1) (a) Bartlett, P. N.; Cooper, J. M. J. Electroanal. Chem. 1993, 362, 1.
(b) Bianco, P. ReV. Mol. Biotech. 2002, 82, 393. (c) Gorton, L. Elec-
troanalysis 2000, 7, 23.
(2) (a) Wrighton, M. S. Science 1986, 231, 32. (b) Downard, A. J.
Electroanalysis 2000, 12, 1085. (c) Murray, R. W. Technique Chem. 1992,
22, 1. (d) Wang, J. Analytical Electrochemistry, 2nd ed.; Wiley-VCH: New
York,, 2000; pp 118, 121.
(3) Allongue, P.; Delamar, M.; Desbat, B.; Fagebaume, O.; Hitmi, R.;
Pinson, J.; Saveant, J.-M. J. Am. Chem. Soc. 1997, 119, 201.
(4) (a) Pinson, J.; Podvorica, F. Chem. Soc. ReV. 2005, 34, 429. (b)
Coulon, E.; Pinson, J. J. Org. Chem. 2002, 67, 8518.
10.1021/ol052558n CCC: $33.50
© 2006 American Chemical Society
Published on Web 01/13/2006

moiety for subsequent attachment to an electrode is shown
in Scheme 1. 4-Nitrotriphenylamine (2) was synthesized by
ascertained by pyrolysis of the dry salt to afford 4-fluoro-
4′,4′′-dibromotriphenylamine as the major product.8
Electrode Modification. Pinson has shown that it is
possible to attach aryl radicals to a variety of forms of
carbon.4 Carbon fibers were chosen here because of their
relatively high surface area.2d The triphenylamine moiety was
attached to the carbon electrode as shown in Scheme 2. A
Scheme 1. Halogen Dance in 4-Aminotriphenylamines
Scheme 2. Construction of Modified Electrodes Bearing an
Attached Triphenylamine Nucleus
carbon fiber electrode consisting of a strand of pressed
graphite fibers was chosen for attachment of the tripheny-
lamine moiety. Cyclic voltammetry of an aqueous solution
of the mixture of diazonium salts showed the expected1
irreversible peak at -0.37 V vs Ag0.1 M AgNO3 (the
potential of this electrode is +0.35 V vs SCE). The electrode
was then immersed in a solution of the diazonium salts,
which were reduced and the corresponding aryl radical
attached to the electrode, at controlled potential (by a single
potential step from -0.2 to -0.6 V) until the current had
decreased to zero. After repeated washing with organic
solvents, the cyclic voltammogram shown in Figure 1 was
reaction of triphenylamine with Cu(NO3)2 in acetic anhydride
at room temperature, a reagent system which we have used
previously to nitrate reactive arenes.6 Compound 2 could
readily be obtained in pure form (54% crude yield; 41%
isolated yield), free of the corresponding dinitro (8%) and
trinitro (38%) congeners, by medium-pressure liquid chro-
matography. Addition of 2 to a solution of Br2 in CH2Cl2 at
room temperature afforded the corresponding dibromide (3)7
(99% yield; homogeneous by TLC and capillary chroma-
tography). Reduction of 3 by tin in acetic acid at reflux,
followed by treatment with 0.1 M NaOH, afforded a mixture
of 4-aminotriphenylamine (4a) (4%). 4-amino-4′-bromot-
riphenylamine (4b) (6%), 4-amino-4′,4′′-dibromotripheny-
lamine (4c) (42%), an isomer of 4c, probably 4-amino-3,4′-
dibromotriphenylamine (4d) (13%), and 4-amino-3,4′,4′′-
tribromotriphenylamine (4e) (32%). When the reaction was
carried out at 75 °C, 4c was formed in 87% yield, together
with tribromide 4e (12%) and monobromide 4b (1%). (We
will return to this dismutation process later). The amines are
too sensitive in air to permit chromatographic separation of
4c from the other amines. Diazotization of the mixture of
amines produced at 75 °C with NaNO2 in aqueous HBF4 at
0 °C afforded a mixture of the solid diazonium tetrafluo-
roborates, which could be isolated by filtration of the cold
solution. The yield of diazonium species is about 65%,
Figure 1. Repetitive cyclic voltammetry on carbon fiber electrode
A (l ) 1 cm) in acetonitrile/0.1 M TBAHFP: solid line, scan 1;
dashed line, scan 2; dotted line, scan 3.
(5) (a) Brinkhaus, K. H. G.; Steckhan, E.; Schmidt, W. Acta Chem. Scand.
1983, 37B, 499. (b) Schmidt, W.; Steckhan, E. Angew. Chem., Int. Ed. Engl.
1978, 17, 673. (c) Wend, R.; Steckhan, E. Electrochim. Acta 1997, 42,
2027.
(6) Yamato, T.; Fujita, K.; Kamimura, H.; Tashiro, M.; Fry, A. J.; Simon,
J.; Ochterski, J. Tetrahedron Lett. 1995, 51, 9851.
(7) Eberson, L.; Hartshorn, M. P.; Svensson, J. A. Acta Chem. Scand.
1997, 51, 279.
obtained by repetitive scanning from 0 to +1.2 V. An
oxidation wave was observed at ca. +0.9 V. (Compound 1
(8) Roe, A. Org. React. 1949, 5, 193.
Org. Lett., Vol. 8, No. 3, 2006
412

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
Org. Lett., Vol. 8, No. 3, 2006
413

Scheme 4. Mechanism of the Halogen Dance
Figure 4. Cyclic voltammetry in 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) squares, a modified electrode
B in the solvent without 5; (c) dashed line, same as (b) but in the
presence of 1.5 mM 5.
strong Lewis acids.11 The dismutation to afford 4a-e that
takes place upon Sn/HCl reduction of 3 seems to be related
to the latter. A likely mechanism is shown in Scheme 4. The
reaction presumably takes place on the protonated aminot-
riphenylamine. Protonation at the 4-amino group (probably
the most basic site) does not offer an obvious path for
halogen loss. On the other hand, protonation at the 4′-carbon
atom is assisted by electron donation from the central
nitrogen, as is clear from the high reactivity of this site
toward electrophilic substitution (recall that nitration of
triphenylamine, even at room temperature, affords almost
40% of the trinitro derivative). Loss of Br+ from the
protonated amine would then lead to monobromide 4b and,
upon repetition of the sequence, 4a. Halogenation by Br+
would afford tribromide 4e and the isomeric dibromide 4d.
Acknowledgment. This work was supported by the
National Science Foundation (CHE-0411699) and by Wes-
leyan University.
Supporting Information Available: Experimental pro-
cedures for synthesis of compounds and modification of
electrode surfaces. This material is available free of charge
via the Internet at http://pubs.acs.org.
(11) (a) Olah, G. A.; Tolgyesi, W. S.; Dear, R. E. A. J. Org. Chem.
1962, 27, 3441. (b) Olah, G. A.; Tolgyesi, W. S.; Dear, R. E. A. J. Org.
Chem. 1962, 27, 3455.
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