Electrophilic nitration of triphenylamines as a route to high oxidation potential electrocatalysts. Polynitration, nitrodebromination, and bromine dance

Article abstract of DOI:10.1016/j.tetlet.2006.08.139

The nitration of triphenylamine derivatives is facile, readily leading to an introduction of up to three nitro groups at room temperature. The nitration of 4,4′,4″-tribromotriphenylamine results in nitrodebromination on one or more rings and bromination a

Full text of DOI:10.1016/j.tetlet.2006.08.139

Tetrahedron Letters 47 (2006) 7667–7669
Electrophilic nitration of triphenylamines as a route to
high oxidation potential electrocatalysts. Polynitration,
nitrodebromination, and bromine dance
Xin Wu, Mufaro A. Dube and Albert J. Fry*
Chemistry Department, Wesleyan University, Middletown, CT 06459, United States
Received 26 July 2006; revised 26 August 2006; accepted 31 August 2006
Abstract—The nitration of triphenylamine derivatives is facile, readily leading to an introduction of up to three nitro groups at room
temperature. The nitration of 4,40,400-tribromotriphenylamine results in nitrodebromination on one or more rings and bromination
at other sites.
Ó 2006 Elsevier Ltd. All rights reserved.
Our discovery that b-aryl-a-silyl esters can be anodically
oxidized to afford unstable a-carbonyl carbocations1 has
aroused our interest in the possible synthetic applica-
tions of such cations. However, the fact that that
oxidation of these silyl esters requires quite positive
potentials1 means that relatively few nucleophiles can
be used in such applications. The electrocatalytic oxida-
tion of these and related silanes, if feasible, would
by definition occur at a lower potential. a-Silyl esters,
however, will require electrocatalysts exhibiting more
positive oxidation potentials than that of 4,40,400-
tribromotriphenylamine (1b), the best known catalyst
of this type. Schmidt and Steckhan and co-workers
synthesized a series of polybromotriphenylamines and
found that the oxidation potentials of these substances
increase with the increasing degree of bromine substitu-
tion.2 These substances are attractive as potential elect-
rocatalysts for organosilanes: they are readily soluble in
a variety of organic solvents and afford stable cation
radicals when all para-positions are substituted to steri-
cally block dimerization of the cation radical.3 Indeed,
we recently reported the use of an electrode-bound di-
bromotriarylamine for the electrocatalytic oxidation of
a benzyl silane.4 The catalytic oxidation of a-silyl esters
will require more positive oxidation potentials than are
accessible with these substances. Since the electron-with-
drawing effect of the nitro group is considerably greater
than that of bromine, our interest turned to the synthesis
of nitrotriphenylamines, particularly those containing
bromine substituents to improve solubility and further
increase their redox potentials. Here we report the nitra-
tion of several triarylamines and the phenomena which
take place upon nitration of 1b under even very mild
conditions.5
1a, U = W = X = H = V = Y = Z = H
b, U = W = X = Br; V = Y = Z = H
c, U = NO2; W = X = V = Y = Z = H
d, U = W = NO2; X = V = Y = Z = H
e, U = W = X = NO2; V = Y = Z = H
f, W = X = U = CH3; Y = V = Z = H
g, Z = NO2; V = Y = H; U = W = X = CH3
h, V = Y = NO2; Z = H; U = W = X = CH3
i, V = Y = Z = NO2; U = W = X = CH3
j, U = W = CH3; X = Y = V = Z = H
k, X = V = Z = NO2; U = W = CH3; Y = H
l, X = Y = W = Br; Z = U = NO2; V = H
m, X = W = Br; Z = U = NO2; Y = V = H
n, X = Y = Br; U = W = NO2; V = Z = H
o, X = Br; U = W = NO2; V = Y = Z = H
p, X = Y = W = Br; U = V = Z = H
Z
X
W
N
Y
V
U
q, X = W = Br; U = NO2; U = Y = Z = H
The nitration of triphenylamine (1) can be effected using
Cu(NO3)2 in Ac2O for 1–2 h at room temperature. The
activating effect of the central nitrogen is quite striking,
since even with a relatively small excess of nitrating
agent (Cu(NO3)2/1a = 2:1) a mixture of mono-, di-,
and trinitro 4-substituted triphenylamines (1c–e) is ob-
tained, which can be separated by chromatography.6
Of these three, the only one likely to prove to be a stable
electrocatalyst is 1e because the other two have
*
Corresponding author. Tel./fax: +1 860 685 2622; e-mail:
afry@wesleyan.edu
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.08.139

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X. Wu et al. / Tetrahedron Letters 47 (2006) 7667–7669
unblocked para positions. Unfortunately, this com-
pound is rather insoluble in most solvents. It was of
interest, therefore, to nitrate triphenylamines substituted
at one or more para positions by other groups. The
nitration of 4,40,400-trimethyltriphenylamine (1f) for a
few minutes at room temperature afforded a mixture
of 2,20-dinitro- and 2,20,200-trinitro derivatives (1h and
1i), separable by flash chromatography (90:10 hexane/
EtOAc). By varying the conditions, it is possible to ob-
tain reaction mixtures in which either the mono-, di-, or
trinitro derivative (1g–i) constitutes the principal prod-
uct (94–95% purity), though each is always accompanied
by small amounts of the other congeners (see Supple-
mentary data). In a like fashion, the reaction of 4,40-
dimethyltriphenylamine (1j) with excess nitrating agent
readily affords 2,20,400-trinitro-4,40-dimethyltriphenyl-
amine (1k). These substances are much more soluble
in common solvents than 1d.
positions para or ortho to the central nitrogen atom.
Bromination of 1d also proceeds normally. Tribromide
1a, however, reacts very differently under the same
nitrating conditions. It is notable that in each of the
three major products 1l–n each of the rings exhibits a
different substitution pattern. Products 1l–n correspond
to the replacement of one or more bromine atoms by the
nitro groups; 1l and 1n correspond as well to intermole-
cular transfer of bromine. The first of these processes,
nitrodebromination (Eq. 1) has been observed with a
number of reactive aromatic systems, for example,
bromine-containing furans, thiophenes, and pyrroles,7
phenols,8a halophenylethers,8b and porphyrins.9,10
Compound 1m exhibits perhaps the simplest pattern:
one of its nitro groups arises by the replacement of a
bromine atom and the other by nitration ortho to the
central nitrogen atom, suggesting that the attack at the
ipso and ortho positions of 1a has about the same activa-
tion energy. In compounds 1l and 1n, one of the rings
contains two bromine atoms; the source of the second
bromine atom is presumably the ‘Br+’ species released
in the second stage of the nitrodebromination. In this
sense, the bromine transfer is related to certain acid-pro-
moted ‘halogen dance’ reactions.2,11–13 Not surprisingly,
the second bromine is introduced only onto the rings
carrying only a weakly deactivating bromine substitu-
ent, and not those bearing the strongly deactivating ni-
tro group. To test for the liberation of Br+, a 4:1
mixture of tribromoamine (1b) and parent amine 1a
was subjected to the usual nitration conditions. The
products consisted of a 60:40 mixture of 2,4,40-tri-
bromo-400-nitrotriphenylamine (1p) and 4,40-dibromo-
400-nitrotriphenylamine (1q). These products (and the
absence of 1a and 1b) can only be explained by the
release of free Br+ into the medium. In principle,
nitrodebromination could also take place by acid-
catalyzed debromination (Eq. 2) followed by the nitra-
tion of the debrominated material. This was ruled out
by stirring 1b overnight in acetic acid; no change was
observed. Finally, the high reactivity of these triphenyl-
amines is notable. The intrinsic reactivity of this system
may be so high that nitration is encounter-controlled,
even after the introduction of one or more nitro groups.
It is likely that the reaction proceeds via an initial single
electron transfer (SET) mechanism.14
The polynitro derivatives described thus far, or at any
rate those fully substituted at the para positions, ought
to be good electrocatalysts for processes requiring rather
positive redox potentials. The triarylamines bearing
more than three electron-withdrawing groups would be
even more desirable, especially since the alkyl groups re-
duce the oxidation potentials of 1i and 1k to a degree.
We have not observed 1d, 1i, and 1k to exhibit heat or
shock sensitivity. However, we have been reluctant to
attempt to nitrate these substances any further, particu-
larly since much more vigorous conditions would be re-
quired to do so. It appeared, though, that the nitration
of 4,40,400-tribromotriphenylamine (1b) could afford sub-
stances potentially bearing as many as six electron-with-
drawing groups (three nitro and three bromo). The
nitration of 1b, however, took a very different course
(Fig. 1). It was reacted at room temperature for 4 h with
sufficient Cu(NO3)2 to introduce three nitro groups. The
complex mixture that resulted could be separated only
by a tedious procedure involving both flash column
and preparative thin layer chromatographies. The prod-
ucts isolated, apart from several other substances
formed in 1–5% yields, which could not be separated
in pure form, consisted of 2,4,40-tribromo-20,400-dinitro-
triphenylamine (1l) (20%), 4,40-dibromo-2,400-dinitro-
triphenylamine (1m) (10%), and 2,4-dibromo-40,400-
dinitrotriphenylamine (1n) (22%) (Fig. 1). The bromina-
tion of 4,40-dinitrotriphenylamine (1d) with 10 equiv of
Br2 at 0 °C also afforded 1n (45%), together with the
known4 monobromide (1o) (15%).
Br
ArNO2
+
Br+
+
ð1Þ
+
ArBr + NO2
ArBr + H+
NO2
H
Br+
+
ð2Þ
ArH
+
Triphenylamine (1a) and the di- and tri-p-tolyl deriva-
tives 1f and 1j undergo the expected nitration at the
NO2
Br
Br
Br
NO2
Br
Br
Br
Br
Cu(NO)3)2(1.5 eq.)
N
N
N
N
+
+
NO2
Br
Br
NO2
0 °C - r.t., 4hr
NO2
NO2
1n, 22 %
NO2
Br
1a
1l, 20 %
1m, 10 %
Figure 1. Nitration of 4,40,400-tribromotriphenylamine.

X. Wu et al. / Tetrahedron Letters 47 (2006) 7667–7669
4. Mayers, B. T.; Fry, A. J. Org. Lett. 2006, 8, 411.
7669
Acknowledgements
5. The structures of all new compounds were established by
1
1-D and COSY H NMR spectroscopy and both low and
high resolution mass spectrometries; see Supplementary
data.
This work was supported by the National Science Foun-
dation (Grant #CHE-041169) and Wesleyan University.
Mufaro Dube received a summer stipend from the How-
ard Hughes Medical Institute Undergraduate Science
Education Program. Professor John Hartwig generously
supplied a sample of 4,40-dimethyltriphenylamine (1j).
6. Nitration takes place at the 4-position of each ring, though
4-nitro compound 1c is accompanied by a trace (<1%) of
the 2-isomer.
7. Saldabols, N.; Liepins, E.; Popelis, J. Zh. Org. Khim. 1979,
15, 2547.
8. (a) Hartshorn, M. P.; Robinson, W. T.; Vaughan, J.;
White, J. M. Aust. J. Chem. 1985, 38, 575; (b) Clewley, R.;
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Smith, G.; Zojaji, M. Aust. J. Chem. 1997, 50, 1997.
10. Perrin, C. L. J. Org. Chem. 1971, 36, 420.
11. (a) Olah, G. A.; Tolgyesi, W. S.; Dear, R. E. A. J. Org.
Chem. 1962, 27, 3455; (b) Olah, G. A.; Tolgyesi, W. S.;
Dear, R. E. A. J. Org. Chem. 1962, 27, 3441.
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Supplementary data
The NMR and mass spectrometric data for the new
compounds and the experimental directions for their
synthesis. Supplementary data associated with this arti-
cle can be found, in the online version, at doi:10.1016/
j.tetlet.2006.08.139.
References and notes
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