in aqueous and nonaqueous solutions.18,19 In these studies,
attempts were made to detect DMA•+ using fast cyclic volta-
mmetry and optical spectrophotometry. These attempts were not
successful, and it was concluded that DMA•+ is very unstable
and undergoes dimerization by a mechanism similar to that
shown in Scheme 2. Under our experimental conditions, DMA•+
and DEA•+ were reasonably stable and could be characterized
by UV-vis spectroscopy. We also show here that the dialky-
laniline radical cation thus generated can lead to synthetically
useful reactions. Some examples are shown in Table 1. As
shown in Table 1, the radical cation produced dimerizes to give
benzidine derivatives in 83-86% yield. When a nucleophilic
reagent is present, the radical cation undergoes nucleophilic
substitution to give the para-substituted products in good yields.
See Supporting Information for experimental details and product
characterization.
Benzidine derivatives have received much interest recently
because of their applications in light-emitting diodes, field-effect
transistors, organic solar cells, and photoconductors.20-26 Oxida-
tion of dialkylanilines to radical cations and their subsequent
dimerization to benzidine derivatives has been achieved earlier
with several reagents, including Cu2+ 27-32
Jiang et al., for
.
example, reported the oxidative coupling of dialkylanilines in
62-85% yield using CuBr (1 equiv)/H2O2 (10 equiv).27 Oxida-
tive couplings using TiCl4 (1.5 equiv),28 ceric(IV)ammonium
nitrate (2 equiv),29 and organic oxidants such as 1,8-bis-
(diphenylmethylium)naphthalenediyl dication (1 equiv)30 are
recently reported. Most of these methods claim the intermediacy
of radical cations. The existence of radical cations is not proved
in any of these cases. The method presented here appears to be
better in terms of the yield, simplicity, and ease of workup
procedure. Most dialkylanilines have oxidation potentials less
than 0.9 V (vs SCE), and hence the reaction is expected to be
general for this class of compounds.
FIGURE 1. (A) Time-dependent changes observed in the absorption
spectrum of a DMA/Cu(ClO4)2 mixture. Spectra a-d were recorded at
1 min intervals, and e-j were recorded at 5 min intervals. (B) Effect
of addition of increasing amounts of TEA to solution in spectrum 1A-
(j) above.
carbonate. Figure 1B shows the disappearance of TMB•+ as a
result of TEA addition. The new band formed around 300 nm
is due to tetramethylbenzidine (confirmed by product isolation
and recording spectrum of isolated product). A solution of DEA
in acetonitrile also exhibited identical behavior as shown in
Figure 1 in the presence of 1 equiv of Cu(ClO4)2. The new
compound formed in this case was identified as tetraethylben-
zidine (TEB).
Under appropriate conditions, dialkylaniline radical cations
can be trapped using nucleophiles to give ring substitution
products in good yields as shown in Table 1. All these reactions
take place at room temperature, and special precautions, such
as inert atmosphere or dry solvents, are not required (detailed
reaction conditions, NMR, and GC-MS traces are given in the
Supporting Information). As is seen from Table 1, different types
Scheme 1 summarizes the observations made in Figure 1.
Figure 1A suggests that transformation of dialkylanilines
(PhNR2) to the tetraalkylbenzidines does not involve any stable
observable intermediates. This transformation, however, has to
involve dimerization, deprotonation, and electron donation as
shown in Scheme 2 (for DMA).
(18) Galus, Z.; White, R. M.; Rowland, F. S.; Adams, R. N. J. Am. Chem.
Soc. 1962, 84, 2065.
(19) 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.
(20) Johnson, G. E.; McGrane, K. M.; Stolka, M. Pure Appl. Chem. 1995,
67, 175.
(21) Noda, T.; Ogawa, H.; Noma, N.; Shirota, Y. J. Mater. Chem. 1999,
9, 2177.
(22) Horowitz, G. AdV. Mater. 1998, 10, 365.
(23) Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Science
1992, 258, 1474.
(24) Getautis, V.; Stanisauskaite, A.; Paliulis, O.; Uss, S.; Uss, V. J.
Prakt. Chem. 2000, 342, 58.
(25) Goodbrand, H. B.; Hu, N.-X. J. Org. Chem. 1999, 64, 670.
(26) O’Brien, D. F.; Burrows, P. E.; Forrest, S. R.; Koene, B. E.; Loy,
D. E.; Thompson, M. E. AdV. Mater. 1998, 10, 1108.
(27) Jiang, Y.; Xi, C.; Yang, X. Synlett 2005, 1381.
(28) Xi, C.; Jiang, Y.; Yang, X. Tetrahedron Lett. 2005, 46, 3909.
(29) Periasamy, M.; Jayakumar, K. N.; Bharathi, P. J. Org. Chem. 2000,
65, 3548.
(30) Saitoh, T.; Yoshida, S.; Ichikawa, J. Org. Lett. 2004, 6, 4563.
(31) Gerstner, P.; Rhode, D.; Hartmann, H. Synthesis 2002, 2487.
(32) Lopez-Cortes, J. G.; Penieres-Carrillo, G.; Ortega-Alfaro, M. C.;
Gutierrez-Perez, R.; Toscano, R. A.; Alvarez-Toledano, C. Can. J. Chem.
2000, 78, 1299.
Presence of a clear isosbestic point in Figure 1A suggests
that the dimerization to form the dihydrotetramethylbenzidine
dication is the slowest step in Scheme 2. In general, radical
cations and dications are highly acidic compared to neutral
molecules, and hence the second step in Scheme 2 is expected
to be very fast. The TMB thus formed is a better electron donor
(Eox ) 0.32 V vs SCE) than DMA (Eox ) 0.81 V vs SCE).17
Hence a very fast electron exchange reaction between the two
takes place, leading to the formation of the TMB cation radical.
If Cu2+ is present in excess, electron transfer from TMB to Cu2+
can also occur, leading to the formation of TMB•+.
Anodic oxidation of DMA to TMB•+ was studied previously
(17) Kavarnos, G. J.; Turro, N. J. Chem. ReV. 1986, 86, 401.
9850 J. Org. Chem., Vol. 71, No. 26, 2006