Cu(II)-mediated generation of triarylamine radical cations and their dimerization. An easy route to tetraarylbenzidines

Article abstract of DOI:10.1021/jo800349n

(Chemical Equation Presented) Triphenylamine (TPA) derivatives react with Cu²⁺ in acetonitrile to give TPA radical cations which undergo dimerization and deprotonation reactions to yield tetraphenylbenzidines (TPB). Synthetic utility of this reaction is demonstrated using several triphenylamine derivatives, and yields in excess of 80% are obtained in most cases. Involvement of the amine radical cations in these reactions was confirmed by ESR and absorption spectroscopic studies. A mechanism consistent with all observations is proposed. This study also revealed a very good correlation between the free energy change for radical cation formation and product yields.

Full text of DOI:10.1021/jo800349n

Cu(II)-Mediated Generation of Triarylamine Radical Cations and
Their Dimerization. An Easy Route to Tetraarylbenzidines
Kesavapillai Sreenath, Chettiyam Veettil Suneesh, Venugopal K. Ratheesh Kumar, and
Karical R. Gopidas*
Photosciences and Photonics, Chemical Sciences and Technology DiVision, National Institute for
Interdisciplinary Science and Technology, TriVandrum 695 019, India
gopidaskr@rediffmail.com
ReceiVed February 12, 2008
Triphenylamine (TPA) derivatives react with Cu2+ in acetonitrile to give TPA radical cations which
undergo dimerization and deprotonation reactions to yield tetraphenylbenzidines (TPB). Synthetic utility
of this reaction is demonstrated using several triphenylamine derivatives, and yields in excess of 80%
are obtained in most cases. Involvement of the amine radical cations in these reactions was confirmed by
ESR and absorption spectroscopic studies. A mechanism consistent with all observations is proposed.
This study also revealed a very good correlation between the free energy change for radical cation formation
and product yields.
SCHEME 1. Ullmann Synthesis of TPB Derivatives
Introduction
Tetraphenylbenzidine (TPB) and its derivatives have received
considerable attention because of their applications as photo-
conductors and hole-transporting layers in organic light-emitting
diodes, organic solar cells, organic field effect transistors, and
xerography.1-7 In each of these applications, materials of
electronic grade purity are required for optimal device perfor-
mance. At present, TPB derivatives are most generally prepared
by the Ullmann condensation reaction between diphenylamine
and an unactivated aryl halide in the presence of a copper
catalyst (metal, alloy, or salt) as shown in Scheme 1.
High temperatures and extended reaction times are often
required, and the yields are generally unpredictable because of
side reactions such as dehalogenation and homocoupling.
Several attempts were made to moderate the Ullmann reaction
conditions. For example, Goodbrand et al.8a and Lu et al.8b
employed 1,10-phenanthroline ligand along with cuprous chlo-
ride and KOH in aromatic hydrocarbon solvents (toluene or
xylene) at refluxing temperatures to obtain good yields of TPB
* To whom correspondence should be addressed. Fax: 91-471-2490186.
(1) (a) Thelakkat, M. Macromol. Mater. Eng. 2002, 287, 442. (b)
Thelakkat, M.; Fink, R.; Haubner, F.; Schmidt, H.-W. Macromol. Symp.
1997, 125, 157.
(2) Adachi, C.; Tokito, S.; Tsutsui, T.; Saito, S. Jpn. J. Appl. Phys. 1988,
27, L269.
(3) Adachi, C.; Nagai, K.; Tamoto, N. Appl. Phys. Lett. 1995, 66, 2679.
(4) Handa, S.; Wietasch, H.; Thelakkat, M.; Durrant, J. R.; Haque, S.
A. Chem. Commun. 2007, 17, 1725.
(5) Yang, Y.; Xue, S.; Liu, S.; Huang, J.; Shen, J. Appl. Phys. Lett. 1996,
69, 377.
(6) Saragi, T. P. I.; Fuhrmann-Lieker, T.; Salbeck, J. AdV. Funct. Mater.
2006, 16, 966.
(7) Theys, R. D.; Sosnovsky, G. Chem. ReV. 1997, 97, 83.
(8) (a) Goodbrand, H. B.; Hu, N.-X. J. Org. Chem. 1999, 64, 670. (b)
Lu, J.; Hlil, A. R.; Sun, Y.; Hay, A. S.; Maindron, T.; Dodelet, J.-P.; D’Iorio,
M. Chem. Mater. 1999, 11, 2501. (c) Liu, Y.-H.; Chen, C.; Yang, L.-M.
Tetrahedron Lett. 2006, 47, 9275. (d) Low, P. J.; Paterson, M. A. J.; Goeta,
A. E.; Yufit, D. S.; Howard, J. A. K.; Cherryman, J. C.; Tackley, D. R.;
Brown, B. J. Mater. Chem. 2004, 14, 2516. (e) Low, P. J.; Paterson, M. A.
J.; Yufit, D. S.; Howard, J. A. K.; Cherryman, J. C.; Tackley, D. R.; Brook,
R.; Brown, B. J. Mater. Chem. 2005, 15, 2304. (f) Wang, X.; Chen, Z.;
Ogino, K.; Sato, H.; Strzelec, K.; Miyata, S.; Luo, Y.; Tan, H. Macromol.
Chem. Phys. 2002, 203, 739.
10.1021/jo800349n CCC: $40.75 © 2008 American Chemical Society
Published on Web 03/22/2008
J. Org. Chem. 2008, 73, 3245-3251
3245

Sreenath et al.
derivatives. Ligands such as 1,4-diazabuta-1,3-diene,8c 18-
crown-6,8d,e or tri-tert-butylphosphine8f along with Cu salts and
a base (K2CO3 or t-BuOK) were employed by others. All these
modifications could not eliminate the requirements of high
reaction temperatures (125-150 °C) and long reaction times
(3-45 h). Palladium-catalyzed aromatic amination reactions
developed by Buchwald and Hartwig appear promising, but
these have not been applied for the synthesis of TPB deriva-
tives.9 In this context, new synthetic methodologies, which
involve mild reaction conditions and cheap chemicals, are very
desirable. Herein we report a simple and highly efficient method,
which involves the dimerization of triarylamine radical cations,
for the synthesis of TPB and several of its derivatives. It is to
be mentioned here that dimerization of electrochemically
generated triphenylamine radical cations to TPB derivatives were
studied previously by several groups,10 but the electrochemical
method has not yet evolved as synthetically useful for the
preparation of TPB derivatives.
in the experimental section to get good to excellent yields of
tetraphenylbenzidines. Structures of the starting TPA and
product TPB derivatives along with isolated yields are shown
in Table 1.
TABLE 1. Starting Amines, Products, and Yields of Cu2+-Initiated
Triarylamine Dimerizations
Results and Discussion
Recently, we have introduced a very simple and effective
method for the generation of aromatic amine radical cations.11
The reaction involved mixing of the aromatic amines with 1
equiv of Cu(ClO4)2 in acetonitrile (ACN) solution. The amine
radical cation is generated as a result of single electron transfer
from the amine to Cu2+ as in eq 1.
amine + Cu2+ f amine•+ + Cu+
(1)
We have shown that a large number of aromatic amines
undergo this reaction. Taking N,N-dialkylanilines as examples,
we have recently shown that this method can be effectively
explored for generating the amine radical cations and studying
their reactivity patterns.12 In the present paper, we demonstrate
that several triphenylamine derivatives are capable of generating
their radical cations upon reaction with Cu2+ in acetonitrile and
that this reaction is synthetically useful for the preparation of
tetraphenylbenzidine derivatives. The reaction conditions are
shown in Scheme 2.
SCHEME 2. Synthesis of Tetraarylbenzidines
In a typical experiment, the triarylamine was mixed with Cu-
(ClO4)2 in acetonitrile and stirred at room temperature for 12
h. The solution turns deep blue upon addition of Cu(ClO4)2,
and completion of the reaction is indicated by change of color
to pale yellow. Reaction mixture was worked up as described
a Yields based on reacted starting material.
It can be seen from Table 1 that the reaction gives good yields
even when sensitive groups such as -CHO, -COOCH3, or
-SCN are present in the substrate. As mentioned previously,
TPB derivatives of the type shown in Table 1 are generally
prepared by Ullmann reaction, which generally fails with
sensitive substrates. The method presented here is obviously
better than the above approaches. It may, however, be noted
that electron-withdrawing substituents such as -CHO and
(9) (a) Wolfe, J. P.; Wagaw, S.; Marcoux, J.-F.; Buchwald, S. L. Acc.
Chem. Res. 1998, 31, 805. (b) Hartwig, J. F. Acc. Chem. Res. 1998, 31,
852. (c) Hartwig, J. F. Angew. Chem., Int. Ed. 1998, 37, 2046.
(10) (a) Seo, E. T.; Nelson, R. F.; Fritsch, J. M.; Marcoux, L. S.; Leedy,
D. W.; Adams, R. N. J. Am. Chem. Soc. 1966, 88, 3498. (b) Creason, S.
C.; Wheeler, J.; Nelson, R. F. J. Org. Chem. 1972, 37, 4440. (c) Nelson,
R. F.; Feldberg, S. W. J. Phys. Chem. 1969, 73, 2623.
(11) Sumalekshmy, S.; Gopidas, K. R. Chem. Phys. Lett. 2005, 413, 294.
(12) Kirchgessner. M.; Sreenath, K.; Gopidas, K. R. J. Org. Chem. 2006,
71, 9849.
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Cu(II)-Mediated Generation of Triarylamine Radical Cations
-NO2 exert a negative effect on this reaction. When the TPA
contains one -CHO or -NO2 group the reactions did not go
to completion. Nearly 30% of the starting materials were
recovered, and the yields reported in Table 1 are based on the
amount of starting materials reacted. When two -CHO or -NO2
groups were present, TPB formation did not take place, and
most of the starting materials were recovered unchanged.
Dimerization of electrochemically generated TPA radical
cation was studied in great detail and a mechanism for the
dimerization was proposed previously.10 Using cyclic voltam-
metry and spectroelectrochemical measurements (ESR, UV/vis/
NIR), Dunsch and co-workers have clearly established the
reaction pathways of several closely related, electrochemically
generated triarylamine radical cations.13 Electrochemical be-
haviors of several amino substituted TPA derivatives were
recently studied by Chiu et al.14 In Scheme 3 we have proposed
a mechanism in line with those proposed in the electrochemical
work. The first step in Scheme 3 is the formation of TPA radical
cation by reaction with Cu2+. Dimerization of the radical cation
would lead to the dication H2TPB2+, which undergoes elimina-
tion of two protons to give TPB. The TPB generated is a better
electron donor compared to TPA and hence it can donate an
electron either to TPA•+ or to Cu2+ and generate TPB•+, which
is the end product of the reaction. Steps 2 and 3 in Scheme 3
are identical to those in electrochemical dimerization. In the
electrochemical reaction, oxidation of TPB to TPB•+ (step 4)
occurs at the electrode, which is kept at a higher potential. In
the electrochemical reaction as well as in the chemical oxidation
by Cu2+ reported here, the end product of is TPB•+. We
observed that TPB•+ can be neutralized by triethylamine (TEA)
or potassium carbonate during workup to give good yields of
TPB.
FIGURE 1. Plot of product yield vs Cu2+ concentration for TPA/
Cu2+ system.
determined for other TPA derivatives in Table 1, and we simply
used 1.5 equiv of Cu2+ in all these cases.
In order to clearly establish the similarity between the
electrochemical and Cu2+ mediated dimerization reactions, we
have carried out spectroscopic and kinetic investigations on the
TPA/Cu(ClO4)2 system in acetonitrile. The first step is to
identify if radical cations are involved in these reactions and
this is best done using ESR spectroscopy. We have recorded
ESR spectra of all the TPA derivatives listed in Table 1 in the
presence of Cu2+. All TPA derivatives which gave benzidines
as products in this reaction exhibited ESR signals, and some
examples are given in Figure 2 (see the Supporting Information
(SI) for remaining ESR spectra).
The paramgnetic Cu2+ shows a ESR signal around 3000 G.
When 1 equiv of a TPA derivative was added, the ESR signal
at 3000 G completely disappeared and new signals correspond-
ing to radical cations of the TPA appeared immediately near
3250 G. This clearly indicates that radical cations of the TPAs
are formed as a result of electron transfer from the amine to
the paramagnetic Cu2+. The ESR signals decayed with time and
the spectra presented in Figure 2 were collected during the first
3 min of mixing. In all cases, we got almost identical single
line ESR spectra. Stamires and Turkevich have previously
observed unresolved single line ESR spectrum for TPA•+ at
relatively high concentrations.15 The lack of hyperfine structure
was attributed to interactions of the radical cations with each
other. In the case of the diformyl and dinitro derivatives (entries
9 and 10 in Table 1), ESR spectra corresponding to amine
radical cations were not observed and the signal due to Cu2+
did not disappear.
SCHEME 3. Mechanism Showing the Formation of TPB•+
from TPA
We have monitored the dimerization reaction using absorption
spectroscopy. TPA do not absorb above 370 nm in acetonitrile
solution. Upon addition of 1.5 equiv of Cu(ClO4)2, an intense
absorption band in the 600-800 nm region is formed and as a
result the solution turned blue. Absorption spectrum of TPA/
Cu2+ system in acetonitrile immediately after mixing is shown
in Figure 3a (λmax ) 680 nm). Lewis and Lipkin had generated
TPA•+ by photoionization of TPA in rigid matrix at low
temperature.16 TPA•+ thus generated exhibited absorption
maximum at 660 nm. Hasegawa had shown that TPA•+
generated by the chemical oxidation of TPA with acid zeolites
absorbed at 678 nm.17 Hall obtained TPA•+ by reacting TPA
with silica-alumina catalyst and reported its absorption maxi-
According to Scheme 3, the product TPB inhibits the reaction
by reducing part of Cu2+ to Cu+ or converting part of TPA•+
back to TPA. This means that more than one equivalent of Cu2+
is required to drive the reaction to completion. In order to
identify the optimum amount of Cu2+, we have carried out the
dimerization of TPA at different concentrations of Cu2+. As
shown in Figure 1, the product yield reached a maximum at
1.5 equiv of Cu2+. Optimum concentrations of Cu2+ were not
(13) (a) Rapta, P.; Rohde, D.; Hartmann, H.; Dunsch, L. Tetrahedron
Lett. 2006, 47, 7587. (b) Rapta, P.; Zeika, O.; Rohde, D.; Hartmann, H.;
Dunsch, L. ChemPhysChem 2006, 7, 863.
(14) Chiu, K. Y.; Su, T. X.; Li, J. H.; Lin, T.-H.; Liou, G.-S.; Cheng,
S.-H. J. Electroanal. Chem. 2005, 575, 95.
(15) Stamires, D. N.; Turkevich, J. J. Am. Chem. Soc. 1963, 85, 2557.
(16) Lewis, G. N.; Lipkin, E. J. Am. Chem. Soc. 1942, 64, 2801.
(17) Hasegawa, H. J. Phys. Chem. 1962, 66, 834.
J. Org. Chem, Vol. 73, No. 8, 2008 3247

Sreenath et al.
FIGURE 2. (a) ESR spectrum of 0.1 M solution of Cu(ClO4)2 in ACN; (b-d) ESR spectra of 1 mmol of triarylamine with 1 mmol of Cu(ClO4)2
in ACN.
FIGURE 3. Time-dependent changes in the absorption spectrum of a solution containing TPA (1 × 10-4 M) and Cu(ClO4)2 (1.5 × 10-4 M) in
ACN. Spectrum “n” was taken after 24 h. Insets show the time dependences of absorptions at 680 and 1270 nm.
mum as 660 nm.18 Dollish and Hall generated TPA•+ by reacting
TPA with iodine and showed that this species absorbs around
660 nm.19 Based on these reports and our own previous
findings,10,11 we assign the 680 nm band in Figure 2a to TPA•+.
The absorption due to TPA•+ was monitored as a function
of time and this is shown in Figure 3b-n. It can be seen that
the absorption due to TPA•+ decreased with time along with
concomitant increase in the absorptions in the 400-580 nm and
900-1600 nm regions. Presence of the isosbestic points in
Figure 3 suggest that the species responsible for the new
absorptions originate from TPA•+. This is also clear from the
decay at 680 nm and the matching growth at 1270 nm, shown
as insets in Figure 2. The final spectrum (Figure 3n) was very
stable (more than 24 h) and is assigned to TPB•+ based on
literature reports and our own results.10,11,15-20 For example,
Seo et al. observed that electrolysis of TPA or TPB solutions
at +1.15 V gave the same species having absorption at 480
nm, and this was identified as TPB•+.10a Dollish and Hall19
generated TPA•+ by oxidizing TPA with silica-alumina catalyst
and observed that the initial absorption band at 660 nm
assignable to TPA•+ decreased with time leading to the
formation a new band at 480 nm. This band was assigned to
TPB•+, arising from a π-π* transition. Recent studies have
also established the existence of a long wavelength band in the
(18) Hall, W. K. J. Catalysis 1962, 1, 53.
(19) Dollish, F. R.; Hall, W. K. J. Phys. Chem. 1965, 69, 2127.
(20) Rapta, P.; Fa´ber, R.; Dunsch, L.; Neudeck, A.; Nuyken, O.
Spectrochim. Acta 2000, 56 A, 357.
3248 J. Org. Chem., Vol. 73, No. 8, 2008

Cu(II)-Mediated Generation of Triarylamine Radical Cations
FIGURE 5. Plot of 1/At - 1/A0 vs time for TPA/Cu2+ system.
FIGURE 4. Effect of addition of increasing amounts of TEA to
solution of spectrum “n” in Figure 3.
The mechanism presented in Scheme 3 is based on the reports
dealing with the electrochemical oxidations of triphenylamine.
In electrochemical reactions, high concentrations of the radical
species are generated at the electrodes and radical-radical
coupling is the preferred pathway.10b In the present case, where
the reaction occurs in bulk solution another mechanism involv-
ing addition of TPA•+ to TPA (radical-parent coupling) also
needs to be considered. The coupled product can undergo
electron transfer with Cu2+ to generate H2TPB2+. A distinction
between the two reaction pathways could be made using kinetic
experiments. The radical-radical coupling is second order with
respect to TPA•+ whereas the radical-parent pathway is first-
order each in TPA•+ and TPA. The absorption due to TPA•+
occurs in a region where TPA or other intermediates in Scheme
3 do not interfere. This gives us an opportunity to determine
the order of the reaction by monitoring the absorption due to
TPA•+. The time dependence of the absorbance due to TPA•+
at 680 nm plotted as per second-order kinetics is shown in Figure
5. The straight line is a fit to second-order kinetics. Good match
of the data to the fit strongly supported the radical-radical
coupling mechanism shown in Scheme 3. The rate constant
1000-1600 nm region for TPB•+, which is assigned to the
intervalence charge-transfer excitation.21 TPB•+ consists of a
neutral TPA unit and a TPA•+ unit. The intervalence charge-
transfer band arises as a result of photoexcited electron transfer
from the neutral TPA moiety to the TPA•+. These reports are
in agreement with the assignments made here. We observed
that TPB•+ could also be generated by the reaction of Cu2+ with
TPB and the spectrum obtained was identical to Figure 3n,
which further confirms the assignment.
The absorptions due to TPB•+ at 400-580 nm and at 900-
1600 nm were very stable for several hours. Upon addition of
excess TEA these bands disappeared leading to the formation
of a new absorption below 400 nm (Figure 4). Based on a
comparison with authentic sample, the new band was assigned
to TPB. We observed that TPB•+ can also be quenched by
aqueous K2CO3, although the mechanism of quenching is not
very clear. Thus, K2CO3 was employed in the workup procedure.
Although we have proposed a few intermediates for the
reaction in Scheme 3, the spectroscopic studies (Figures 2 and
3) could identify only TPA•+ and TPB•+. The isosbestic points
in Figure 3 suggest that TPA•+ is converted smoothly into
TPB•+, and there are no observable intermediates in this
transformation. Formation of TPB•+ from TPA•+, however, has
to involve dimerization of the radical cation (radical-radical
coupling) to the dicationic species H2TPB2+, deprotonation of
H2TPB2+ to TPB and oxidation of TPB to TPB•+ as shown in
Scheme 3. Most probably, H2TPM2+ and TPB are not observed
in Figure 3 because the deprotonation and electron-transfer
reactions are very fast compared to the dimerization reaction.
In general, radical cations and dications are highly acidic
compared to neutral molecules, and hence deprotonation of H2-
TPB2+ to neutral TPB molecule is expected to be very fast.
The TPB thus formed is a better electron donor (Eox ) 0.69V
vs SCE) than TPA (Eox ) 0.92 V vs SCE). Hence, a spontaneous
electron exchange reaction between the two can take place,
leading to the formation of TPB•+.10a If Cu2+ is present in
excess, electron transfer from TPB to Cu2+ can also occur
spontaneously, leading to the formation of TPB•+.
obtained for the dimerization of TPA was 9.3 × 10-4 M-1 s-1
.
The value obtained is several orders smaller than that reported
by Creason et al.10b (k ) 1.2 × 103 M-1 s-1) for the
electrochemical dimerization. The large value in the electro-
chemical work is due to the large concentration of the ion
radicals at the electrode surface. It is to be mentioned here that
the kinetic experiments were carried out only in the case of
TPA. For other derivatives shown in Table 1, particularly those
with electron-withdrawing substituents, reaction of the amines
with Cu2+ was slow. Thus, formation of radical cations
interfered with its decay and determination of dimerization rate
constants required prior knowledge about the rate of formation
of the amine radical cations. Stopped-flow experiments to
determine these rates are being planned.
An examination of Table 1 shows that the dimerization
reaction is slow or incomplete when the TPAs contain electron-
withdrawing groups. In the case of the dinitro derivative, the
radical cation was not formed and the reaction did not proceed
at all. These factors can be attributed to the free energy changes
associated with the radical cation formation. Free energy change
for the electron-transfer reaction (1) can be calculated using the
standard expression
(21) (a) Lambert, C.; No¨ll, G. J. Am. Chem. Soc. 1999, 121, 8434. (b)
Nelsen, S. F. Chem. Eur. J. 2000, 6, 581. (c) Lambert, C.; Noll, G. J. Chem.
Soc., Perkin Trans. 2 2002, 2039. (d) Littleford, R. E.; Paterson, M. A. J.;
Low, P. J.; Tackley, D. R.; Jayes, L.; Dent, G.; Cherryman, J. C.; Brown,
B.; Smith, W. E. Phys. Chem. Chem. Phys. 2004, 6, 3257. (e) Low, P. J.;
Paterson, M. A. J.; Puschmann, H.; Goeta, A. E.; Howard, J. A. K.; Lambert,
C.; Cherrymann, J. C.; Tackley, D. R.; Leeming, S.; Brown, B. Chem. Eur.
J. 2004, 10, 83.
∆G° ) Eox - Ered - e2/dꢀ
(2)
J. Org. Chem, Vol. 73, No. 8, 2008 3249

Sreenath et al.
TABLE 2. Eox for TPA Derivatives and ∆G0 Associated with
mechanism is confirmed by spectroscopic and kinetic studies.
Finally, the yields of the TPB derivatives are correlated to the
free energy change associated with the formation of radical
cations.
Radical Cation Formation
triarylaminea
E
ox vs SCE
∆G0
R1 ) H, R2 ) H
0.92
0.90
0.88
0.76
1.02
1.05
1.08
1.16
1.21
1.35
-0.210
-0.230
-0.250
-0.370
-0.101
-0.080
-0.050
+0.030
+0.080
+0.220
R1 ) H, R2 ) CH3
R1 ) CH3, R2 ) CH3
R1 ) OCH3, R2 ) OCH3
R1 ) H, R2 ) SCN
R1 ) H, R2 ) COOCH3
R1 ) H, R2 ) CHO
R1 ) H, R2 ) NO2
Experimental Section
1. General Methods. Absorption spectra were recorded on a
1
UV-vis-NIR spectrophotometer. The H and 13C NMR spectra
were recorded in CDCl3 on a FT-NMR (300 MHz) spectrometer,
and the chemical shifts, δ, are referred to TMS. The FAB mass
spectra were recorded on a spectrometer. Redox potentials of Cu-
(II) and all the TPA derivatives were measured using a cyclic
voltammetric analyzer, with a conventional three-electrode system.
The ESR spectra were recorded using an ESR spectrometer
operating at X-band frequencies and having a 100 kHz field
modulation. Cu(ClO4)2 and triphenylamine were commercial samples.
Other triphenylamine derivatives were prepared by the reactions
of primary or secondary amines with aryl halides as per reported
procedures.8 All experiments were carried out at room temperature
(25 ( 1 °C), unless otherwise mentioned.
R1 ) CHO, R2 ) CHO
R1 ) NO2, R2 ) NO2
a R1 and R2 as in Scheme 2.
where Eox is the oxidation potential of the TPA derivatives, Ered
is the reduction potential of Cu2+, d is the center-to-center
distance between TPA and Cu(ClO4)2 hexahydrate in the
collision complex, and ꢀ is the dielectric constant () 37) of
ACN. Eox values for all the TPA derivatives were measured
using cyclic voltammetry and these are given in Table 2. Ered
of Cu2+ in ACN is + 0.952 V vs SCE. In fact, this value is
approximately 1 V more positive than Ered in water, and the
gain in energy by changing the solvent to ACN is attributed to
formation of the stable tetrakis(acetonitrile)copper(I) complex
immediately following the reduction of Cu2+ to Cu+.22 Reaction
(1) takes place only in acetonitrile, and part of the driving force
for the reaction comes form the free energy of formation of
[Cu(CH3CN)4]+ complex. Hence, reaction (1) should be modi-
fied as shown in (3).
2. General Procedure for Preparation of Tetraphenylben-
zidines. A solution of Cu(ClO4)2 hexahydrate (222 mg, 0.60 mmol)
in ACN (10 mL) was added to a solution of the triarylamine (0.4
mmol) in ACN (20 mL) at room temperature, and the mixture was
stirred for 12 h. Solid K2CO3 (500 mg) and water (1 mL) were
added, and stirring was continued for 0.5 h. Filtered and the solids
were washed twice with chloroform. The combined organic extracts
were filtered through a short pad of alumina. The solvent was
removed and the residue so obtained was recrystallized form
dichloromethane-hexane mixture to give pure products. Yields
shown in Table 1 are those of pure products. For entries 6-8 in
Table 1, the organic residues were chromatographed over neutral
alumina. Elusion with a mixture of chloroform-hexane (1:9) gave
the unreacted starting materials. Further elution with chloroform-
hexane (1:4) gave the pure products.
amine + Cu2+ CH3CN8 amine•+ + [Cu(CH3CN)4]+ (3)
N,N,N′,N′-Tetraphenylbiphenyl-4,4′-diamine(1):8c yield 81%;
1H NMR (CDCl3) δ 6.99-7.45 (m, 28H); 13C NMR (CDCl3) δ
122.76, 124.04, 124.24, 127.25, 129.21, 134.66, 146.66, 147.65;
MS (FAB) m/z 488 (M+). Anal. Calcd for C36H28N2: C, 88.49; H,
5.78; N, 5.73. Found: C, 88.02; H, 6.21; N, 5.87.
Under these conditions, it is advisable to replace Ered in eq 2
with the reduction peak potential (Ep) obtained by linear sweep
voltammetry. For the reduction of Cu2+ in ACN, we obtained
Ep as +1.082. The Coulombic term in eq 2 was calculated
assuming d ) 8 Å. Using these values, the ∆G° associated with
the radical cation formation was calculated for all TPA
derivatives shown in Table 1 and the values are presented in
Table 2.
Analysis of Tables 1 and 2 showed that there is good
correlation between the ∆G° values and product yields. For the
first six entries in Table 2, ∆G° values are negative and yields
of TPB are very good. For the mononitro derivative (entry 7),
the ∆G° value is slightly positive and the reaction did not go
to completion. In the case of diformyl and dinitro derivatives,
TPB derivatives were not formed because the ∆G° for formation
of radical cations were positive. This shows that a correlation
exists between ∆G° of radical cation fromation and yields of
TPB in the systems studied.
N,N′-Diphenyl-N,N′-di(p-tolyl)biphenyl-4,4′-diamine(2):8f yield
1
86%; H NMR (CDCl3) δ 2.32 (s, 6H), 7.07-7.42 (m, 26H); 13C
NMR (CDCl3) δ 20.82, 122.33, 123.52, 123.72, 125.00, 127.15,
129.12, 129.93, 132.83, 134.36, 145.11, 146.82, 147.87; MS (FAB)
m/z 516 (M+). Anal. Calcd for C38H32N2: C, 88.34; H, 6.24; N,
5.42. Found: C, 88.08; H, 6.50; N, 5.32.
N,N,N′,N′-Tetra(p-tolyl)biphenyl-4,4′-diamine(3):8d yield 94%;
1H NMR (CDCl3) δ 2.15 (s, 12H), 6.83-6.94 (m, 20H), 7.25 (d,
4H); 13C NMR (CDCl3) δ 21.01, 123.15, 124.68, 127.24, 130.05,
132.48, 134.15, 145.51, 147.17; MS (FAB) m/z 544 (M+). Anal.
Calcd for C40H36N2: C, 88.20; H, 6.66; N, 5.14. Found: C, 87.81;
H, 7.16; N, 5.45.
N,N,N′,N′-Tetrakis(4-methoxyphenyl)biphenyl-4,4′-diamine
(4):21a yield 87%; 1H NMR (CDCl3) δ 3.78 (s, 12H), 6.81 (d, 8H),
6.96 (m, 4H), 7.05 (d, 8H), 7.34 (d, 4 H); 13C NMR (CDCl3) δ
55.45, 114.62, 121.08, 126.37, 126.84, 132.99, 140.92, 147.35,
155.67; MS (FAB) m/z 608 (M+). Anal. Calcd for C40H36N2O4:
C, 78.92; H, 5.96; N, 4.60. Found: C, 78.86; H, 6.45; N, 4.64.
N,N′-Bis(p-carbomethoxyphenyl)-N,N′-di(phenyl)biphenyl-
Conclusion
In summary, we have shown that the reaction of tripheny-
lamine derivatives with Cu2+ in ACN generates radical cations
of the amines and that this reaction can be exploited for the
synthesis of tetraphenylbenzidine derivatives. A mechanism for
the formation of TPB derivatives is also proposed. The
1
4,4′-diamine (5): yield 83%; H NMR (CDCl3) δ 3.88 (s, 6H),
7.03 (d, 4H), 7.14-7.19 (m, 10H), 7.30-7.35 (m, 4H), 7.50 (d,
4H), 7.86 (d, 4H); 13C NMR (CDCl3) δ 52.71, 119.34, 121.39,
122.35, 124.81, 126.71, 128.75, 130.71, 131.94, 136.24, 146.54,
145.77, 151.77, 166.86; MS (FAB) m/z 605 (M + 1). Anal. Calcd
for C40H32N2O4: C, 79.45; H, 5.33; N, 4.63. Found: C, 79.01; H,
5.75; N, 4.90.
(22) (a) Parker, A. J. Pure Appl. Chem. 1981, 53, 1437. (b) Persson, I.
Pure Appl. Chem. 1986, 58, 1153. (c) Cox, B. G.; Jedral, W.; Palou, J. J.
Chem. Soc. Dalton Trans. 1988, 733. (d) Kamau, P.; Jordan, R. B. Inorg.
Chem. 2001, 40, 3879.
N,N′-Bis(4-nitrophenyl)-N,N′-diphenylbiphenyl-4,4′-diamine-
(6). 30% starting amine was recovered: yield based on reacted SM
3250 J. Org. Chem., Vol. 73, No. 8, 2008

Cu(II)-Mediated Generation of Triarylamine Radical Cations
81%; 1H NMR (CDCl3) δ 6.90 (d, 4H), 7.12-7.17 (m, 10H), 7.28-
7.33 (m, 4H), 7.48 (d, 4H), 7.97 (d, 4H); 13C NMR (CDCl3) δ
118.57, 125.45, 125.86, 126.41, 126.55, 128.16, 129.98, 137.10,
140.35, 144.98, 145.50, 153.23; MS (FAB) m/z 578 (M+). Anal.
Calcd for C36H26N2O4: C, 74.73; H, 4.53; N, 9.68. Found: C, 74.31;
H, 5.03; N, 9.51.
135.90, 145.67, 146.39, 149.65; MS (FAB) m/z 602 (M+). Anal.
Calcd for C38H26N4S2: C, 75.72; H, 4.35; N, 9.29. Found: C, 75.49;
H, 4.80; N, 9.14.
Acknowledgment. We thank the Council of Scientific and
Industrial Research (CSIR) and Department of Science and
Technology (DST), Government of India, for financial support.
K.S. and C.V.S. thank CSIR and V.K.R.K. thanks UGC for
their Research Fellowships. We thank Professor P. Sambasiva
Rao and Dr. B. Natarajan of the Department of Chemistry,
University of Pondicherry, for the ESR spectra. This is
contribution number NIIST-PPG-261.
N,N′-Bis(p-formylphenyl)-N,N′-diphenylbiphenyl-4,4′-di-
amine (7). 20% starting amine was recovered: yield based on
1
reacted SM 85%; H NMR (CDCl3) δ 7.07 (d, 4H), 7.16-7.25
(m,10H), 7.33-7.38 (m, 4H), 7.53 (d, 4H), 7.69 (d, 4H), 9.81(s,
2H); 13C NMR (CDCl3) δ 119.75, 125.23, 126.16, 126.34, 127.92,
129.34, 129.77, 131.29, 136.62, 145.39, 146.02, 153.11, 190.38;
MS (FAB) m/z 545 (M + 1). Anal. Calcd for C38H28N2O2: C, 83.80;
H, 5.18; N, 5.14. Found: C, 83.43; H, 5.61; N, 5.34.
N,N′-Diphenyl-N,N′-bis(4-thiocyanatophenyl)biphenyl-4,4′-di-
amine (8). 10% starting amine was recovered: yield based on
reacted SM 82%; 1H NMR (CDCl3) δ 7.01 (d, 4H), 7.08 (d, 10H),
7.23-7.33 (m, 8H), 7.42 (d, 4H); 13C NMR (CDCl3) δ 113.73,
121.96, 122.91, 123.41, 125.50, 126.71, 128.64, 130.63, 134.02,
Supporting Information Available: 1H NMR and 13C NMR
spectra for all compounds and ESR spectra. This material is
available free of charge via the Internet at http://pubs.acs.org.
JO800349N
J. Org. Chem, Vol. 73, No. 8, 2008 3251