1,8-Bis(diphenylmethylium)naphthalenediyl dication as an organic oxidant: Synthesis of benzidines via self-coupling of N,N-Dialkylanilines

Article abstract of DOI:10.1021/ol048058w

(Chemical equation presented) 1,8-Bis(diphenylmethylium)naphthalenediyl dications promoted oxidative coupling of various N,N-dialkylanilines to afford the corresponding para-coupled bisanilines, benzidines, in good to excellent yield. The dications were readily prepared from a cyclic ether precursor, derived from 1,8-dibromonaphthalene, via deoxygenation with a silylating reagent in 1,1,1,3,3,3-hexafluoro-2-propanol under anhydrous conditions.

Full text of DOI:10.1021/ol048058w

ORGANIC
LETTERS
2004
Vol. 6, No. 24
4563-4565
1,8-Bis(diphenylmethylium)naphthalenediyl
Dication as an Organic Oxidant:
Synthesis of Benzidines via
Self-Coupling of N,N-Dialkylanilines
Terunobu Saitoh, Suguru Yoshida, and Junji Ichikawa*
Department of Chemistry, Graduate School of Science, The UniVersity of Tokyo,
Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
junji@chem.s.u-tokyo.ac.jp
Received September 23, 2004
ABSTRACT
1,8-Bis(diphenylmethylium)naphthalenediyl dications promoted oxidative coupling of various N,N-dialkylanilines to afford the corresponding
para-coupled bisanilines, benzidines, in good to excellent yield. The dications were readily prepared from a cyclic ether precursor, derived
from 1,8-dibromonaphthalene, via deoxygenation with a silylating reagent in 1,1,1,3,3,3-hexafluoro-2-propanol under anhydrous conditions.
Triarylmethyliums, readily prepared from the corresponding
alcohols1 or halides,2 constitute a class of versatile carbo-
cations with functions such as hydride and alkoxy abstrac-
tion,3 Lewis acid catalysis,4 and others.5 Recently, dicationic
systems in which two triarylmethylium ions are linked by a
carbon skeleton such as 2,2′-biphenylene,6 [1,1′-binaphtha-
lene]-2,2′-diyl,7 and 1,8-naphthalenediyl8 groups have been
prepared and exhibited unique electrochemical properties.
These dications undergo electrochemical reduction, leading
to the corresponding neutral compounds via C-C bond
formation between the two carbocation centers. Among these,
the two cationic centers are closest in the dications with a
1,8-naphthalenediyl tether, which resulted in an especially
high oxidation potential compared with those of other
dications and triarylmethyliums (monocations) examined
using cyclic voltammogram analysis. Despite their potential
as an oxidant, there have been no reports on their application
in organic synthesis. In this communication, we report a
convenient synthetic method for 1,8-bis(diarylmethylium)-
naphthalenediyl dications 4 and their function as a new
organic oxidant in the coupling reaction of aniline derivatives.
(1) Lambert, J. B.; Schulz, W. J., Jr.; McConnell, J. A.; Schilf, W. J.
Am. Chem. Soc. 1988, 110, 2201.
(2) Fukui, K.; Ohkubo, K.; Yamabe, T. Bull. Chem. Soc. Jpn. 1969, 42,
312.
(3) (a) Pearson, A. J.; Zettler, M. W. J. Am. Chem. Soc. 1989, 111, 3908.
(b) Rubio, A.; Liebskind, L. S. J. Am. Chem. Soc. 1993, 115, 891.
(4) (a) Mukaiyama, T.; Tamura, M.; Kobayashi, S. Chem. Lett. 1986,
1017. (b) Kobayashi, S.; Murakami, M.; Mukaiyama, T. Chem. Lett. 1985,
1535. (c) Mukaiyama, T.; Kobayashi, S.; Shoda, S. Chem. Lett. 1984, 1529.
(d) Bochkov, A. F.; Kochetkov, N. K. Carbohydr. Res. 1975, 39, 355.
(5) Encyclopedia of Reagents for Organic Synthesis; Paquette, L. A.,
Ed.; John Wiley & Sons, Ltd.: Chichester, 1995.
(6) (a) Carey, K. A.; Clegg, W.; Elsegood, M. R. J.; Golding, B. T.;
Hill, M. N. S.; Maskill, H. J. Chem. Soc., Perkin Trans. 1 2002, 2673. (b)
Suzuki, T.; Nishida, J.; Tsuji, T. Chem. Commun. 1998, 2193. (c) Suzuki,
T.; Nishida, J.; Tsuji, T. Angew. Chem., Int. Ed. Engl. 1997, 36, 1329.
(7) Suzuki, T.; Yamamoto, R.; Higuchi, H.; Hirota, E.; Ohkita, M.; Tsuji,
T. J. Chem. Soc., Perkin Trans. 2 2002, 1937.
(8) (a) Wang, H.; Webster, C. E.; Perez, L. M.; Hall, M. B.; Gabba¨ı, F.
P. J. Am. Chem. Soc. 2004, 126, 8189 and references therein. See also: (b)
Kawai, H.; Nagasu, T.; Takeda, T.; Fujiwara, K.; Tsuji, T.; Ohkita, M.;
Nishida, J.; Suzuki, T. Tetrahedron Lett. 2004, 45, 4553. (c) Suzuki, T.;
Nagasu, T.; Kawai, H.; Fujiwara, K.; Tsuji, T. Tetrahedron Lett. 2003, 44,
6095.
10.1021/ol048058w CCC: $27.50
© 2004 American Chemical Society
Published on Web 11/02/2004

The bistriarylmethyliums mentioned above were previ-
ously prepared by treatment of the corresponding diols with
aqueous HBF4 or HClO4 in Ac2O or (CF3CO)2O.8 These
methods have a drawback in purification of the product
dications due to the low volatility of water, acid anhydrides,
and in situ-generated carboxylic acids. Therefore, the de-
velopment of a more convenient and practical method for
the dications is still required to isolate them in pure forms
so they can be used as reagents in synthetic organic reactions.
Our synthetic plan relied on the following two features: (1)
dications 4 would be generated from cyclic ether 3 via
deoxygenation, and (2) this process would proceed readily
in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) solvent (Scheme
1).9,10 Treatment of 3 with a strong silylating reagent was
Table 1. Coupling of Aniline 6a with Organic Oxidants
Scheme 1. Synthesis of
1,8-Bis(diphenylmethylium)naphthalene Dications 4a
deoxygenate it. HFIP successfully promoted deoxygenation
of 3 as expected, to afford the desired dication 4a in 94%
yield as dark red crystals, whereas dication 4a was not
obtained at all in dichloromethane, toluene, or benzene.
Dication 4b was also prepared from 3 on treatment with
TMSOTf instead of TMSClO4 under similar conditions,
although the dication proved to be unstable for use as a
reagent.
Recently, N,N′-perarylated derivatives of benzidine have
received much interest because of their tunable electric
conductivity, which finds diverse applications such as organic
light-emitting diodes, organic field-effect transistors, organic
solar cells, and organic photoconductors.13 Oxidative cou-
pling of aniline derivatives provides an efficient access to
benzidine derivatives. Although several methods for the
coupling reaction have been reported, most of them are low-
yielding processes.14 Among these methods, TiCl4-mediated
oxidative coupling seemed to be the most practical to afford
the benzidines, despite the requirement of a large excess of
starting anilines.13,14a These facts prompted us to try dication
4 as an oxidant in the coupling reaction of anilines.
N,N-Diethylaniline 6a was smoothly transformed to the
desired para-coupled bisaniline, N,N,N′,N′-tetraalkylbenzidine
7a in 98% yield on treatment with a small excess of 4a,
which was quantitatively recovered as acenaphthene 5 (Table
1).15,16 The C-C bond formation between the two carbo-
cation centers of 4a leading to 5 indicates that 4a acted as
an oxidant in the coupling reaction of 6a (Scheme 1).
a Reagents and conditions: (a) n-BuLi (2.2 equiv), Et2O, rt, 2 h.
(b) Ph2CO (2.4 equiv), reflux, 4 h. (c) CF3CO2H (0.1 equiv),
CH2Cl2, rt, 10 h. (d) TMSX (3.0 equiv) in toluene, (CF3)2CHOH
(HFIP), rt, 2 days.
expected to allow removal of the oxygen as a disiloxane
(R3SiOSiR3), leading to the desired dication under anhydrous
conditions. HFIP would assist the generation of the dications
and be easily removed afterward, due to its strong ionizing
power, low nucleophilicity, and high volatility.
Benzophenone was added to the dilithium species, pre-
pared by treating the known 1,8-dibromonaphthalene 1 with
n-BuLi, to afford the crude diol 2.11 On successive treatment
of its CH2Cl2 solution with a catalytic amount of CF3CO2H
and then MeOH, ether 3 (the precursor of dications 4a,b)
was obtained as white crystals in 80% yield from 1 by
filtration. Neither workup nor purification was needed to
isolate 3. Moreover, this procedure for cyclic ether 3 was
applied to the preparation of dications 4c and 8 (Table 1).
(13) Gerstner, P.; Rohde, D.; Hartmann, H. Synthesis 2002, 2487 and
references therein.
(14) (a) Periasamy, M.; Jayakumar, K. N.; Bharathi, P. J. Org. Chem.
2000, 65, 3548 and references therein. (b) Lo´pez-Corte´s, J. G.; Penieres-
Carrillo, G.; Ortega-Alfaro, M. C.; Gutie´rrez-Pe´rez, R.; Toscano, R. A.;
Alvarez-Toledano, C. Can. J. Chem. 2000, 78, 1299.
(15) Although acenaphthene 5 regenerated dication 4a on treatment with
HClO4 in HFIP, dication catalysis has not been successful so far and is
now under investigation. See also: Wang, H.; Gabba¨ı, F. P. Angew. Chem.,
Int. Ed. 2004, 43, 184.
(16) Electron-rich benzenes other than anilines such as anisole and 1,4-
dimethoxy- and 1,3,5-trimethoxybenzenes gave no dimeric products on
treatment with dication 4a.
Cyclic ether 3 was treated with TMSClO4, prepared in
situ from AgClO4 and TMSCl in toluene,12 in order to
(9) Schadt, F. L.; Schleyer, P. V. R. Tetrahedron Lett. 1974, 27, 2335.
(10) For recent reports on the cationic reactions conducted in HFIP,
see: Ichikawa, J.; Jyono, H.; Kudo, T.; Fujiwara, M.; Yokota, M. Synthesis,
in press, and references therein.
(11) Letsinger, L. R.; Gilpin, A. J.; Vulio, J. W. J. Org. Chem. 1962,
27, 673.
(12) Jona, H.; Mandai, H.; Chavasiri, W.; Takeuchi, K.; Mukaiyama, T.
Bull. Chem. Soc. Jpn. 2002, 75, 291.
4564
Org. Lett., Vol. 6, No. 24, 2004

Dication 4c, bearing methoxy groups on the four benzene
rings, was less effective in this reaction, giving 7a in a lower
yield. Dication 8, bearing a 2,2′-biphenylene backbone, gave
reactions regioselectively occurred to afford the para-coupled
dimer of dialkylanilines without formation of ortho- and
meta-coupled products.
A plausible mechanism for the oxidative coupling is shown
in Scheme 3. Dication 4 promotes oxidation of dialkylanilines
-
a complex mixture, and Ph3C+BF4 afforded no coupling
products. These facts suggest that two methylium cations in
close proximity are essential. Treatment of 6a with DDQ or
-
a triarylaminium radical cation, (4-BrC6H4)3N+•SbCl6 , both
of which are widely used in organic chemistry as a one-
electron oxidant, gave no bisaniline or many side products
other than 7a, respectively. These results (Table 1) clearly
show that dication 4a is the oxidant of choice for the self-
coupling reaction of aniline derivatives.14
Scheme 3. Plausible Reaction Mechanism for the Oxidative
Coupling of 6
To expand the scope of the present oxidative coupling,
several other dialkylanilines were treated with 4a, and the
results are summarized in Table 2. The desired dimers 7 were
Table 2. Synthesis of Benzidines 7 from Anilines 6
entry
R
R1
R2
temp.
% yield
6 to generate cation radicals 10, which in turn react with 6
leading to the coupled intermediates 11. The produced 11 is
further oxidized by 4 or 4+• to give diiminium ions 12, which
provide dimers 7 after deprotonation during the workup
procedure. Dication 4 is transformed to acenaphthene 515 via
two-electron reduction, effected by two molar amounts of
dialkylanilines 6.
1
2
3
4
5
6
Me
Et
Et
Et
Allyl
Bn
Me
Me
Cl
Br
Me
Me
Me
Me
Cl
H
Me
Me
-78 °C
-78 °C
rt
rt
rt
95
98
78
86
81
77
-78 °C
In conclusion, the oxidative coupling reaction of aniline
derivatives has been successfully achieved by the use of 1,8-
bis(diphenylmethylium)naphthalenediyl dication 4a, which
was easily prepared from the corresponding cyclic ether 3
by deoxygenation using a silylating reagent in HFIP.
obtained in high to excellent yield from 3,5-disubstituted and
3-halogenated N,N-dialkylanilines. The self-coupling pro-
ceeded smoothly even when the amine nitrogen was protected
by an allyl or benzyl group, while the reactions of aniline or
a monoalkylated aniline led to complex mixtures. N,N,N′,N′-
Tetramethyl-1,8-diaminonaphthalene, Proton Sponge, also
underwent reaction to give the product of coupling at the
4-position 9 in 77% yield (Scheme 2). All of the coupling
Acknowledgment. We thank Prof. T. Kawashima and
Dr. N. Kanoh (the University of Tokyo) for the X-ray crystal
structure analysis. We are grateful to Central Glass Co., Ltd.,
for a generous gift of HFIP. This work was supported by a
Grant-in-Aid for Scientific Research from the Japan Society
for the Promotion of Science.
Scheme 2. Coupling of Proton Sponge with 4A
Supporting Information Available: Spectroscopic data,
X-ray crystallographic data (CIF), and experimental details.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OL048058W
Org. Lett., Vol. 6, No. 24, 2004
4565