A general synthesis of end-functionalized oligoanilines via palladium-catalyzed amination

Full text of DOI:10.1021/ja973407l

J. Am. Chem. Soc. 1998, 120, 213-214
213
Scheme 1a
A General Synthesis of End-Functionalized
Oligoanilines via Palladium-Catalyzed Amination
Robert A. Singer, Joseph P. Sadighi, and
Stephen L. Buchwald*
a Reagents and conditions: (a) Pd(OAc)2 (1 mol %), BINAP (1.5 mol
%), NaOtBu (4.5 equiv), toluene, 80 °C; (b) (BOC)2O (3 equiv), 4-DMAP
(0.1 equiv), THF/toluene, reflux; (c) (i) HONH2‚HCl (2.5 equiv), pyridine,
CHCl3/THF/EtOH, rt, (ii) Et3N.
Department of Chemistry
Massachusetts Institute of Technology
Cambridge, Massachusetts 02139
Scheme 2a
ReceiVed September 30, 1997
Polyaniline has attracted much attention in the field of organic
conducting polymers due to its robust nature in the doped
emeraldine state.1 Among the many industrial applications it has
found are its use as components in rechargeable batteries,2
electromagnetic interference shielding,3 and anticorrosion coatings
for steel.4
a Reagents and conditions: (a) Pd2(dba)3 (2 mol %), BINAP (5 mol
%), NaOtBu (2.5 equiv), toluene, 80 °C; (b) (BOC)2O (3 equiv), 4-DMAP
(0.1 equiv), THF, reflux.
In 1986, Wudl and co-workers demonstrated that synthetically
prepared phenyl-capped octaaniline exhibited properties similar
to bulk polyaniline (comparable UV/vis, IR, CV, and conductiv-
ity).5 Consequently, an octaaniline could be considered a good
model or substitute for applications involving polyaniline. Aside
from the modified Honzl condensation method Wudl employed
for synthesizing oligoanilines, other methods of preparation
include titanium-alkoxide-mediated couplings with aniline deriva-
tives,6 Ullmann couplings,7 and an adaptation of the Willsta¨tter-
Moore approach.8 All of these methods have yet to demonstrate
generality in the choice of substrates for carrying out oligomer-
izations and have the common drawback of lacking the ability to
functionalize end groups.
We desired to expand the repertoire of techniques available
for constructing oligoanilines and their analogues to include a
strategy based on Pd-catalyzed amination methodology. We
speculated that such a method would demonstrate efficiency in
the preparation of oligoanilines and derivatives due to the broad
scope of the Pd-catalyzed amination reaction.9 To undertake such
an objective, three issues had to be confronted. First, an
orthogonal protecting group strategy had to be developed to
differentiate internal and terminal nitrogens. Second, a means
of masking or selectively introducing terminal bromides for use
in couplings with aniline derivatives had to be implemented.
Third, and most importantly, construction would have to be carried
out in a bidirectional mode to produce materials with symmetry.
Herein, we report our first efforts toward a unified strategy for
synthesizing oligoanilines with end group functionalization. In
light of Wudl’s pioneering studies,5 we initially chose to target
functionalized octaanilines.
As a surrogate for 4-bromoaniline in the controlled construction
of oligoanilines we used N-(diphenylmethylene)-4-bromoaniline
(1) (Scheme 1).10 The imine group serves the dual purposes of
protecting the nitrogen and activating the compound to oxidative
addition to the Pd catalyst. To build symmetrical oligomers we
employed 1,4-phenylenediamine dihydrochloride (2) as a core
piece for initiating two-directional growth. We found that
coupling took place smoothly using 1 (2 equiv) and 2 (1 equiv)
in the presence of Pd(OAc)2 (1 mol %), BINAP (1.5 mol %),
and NaOtBu (4.5 equiv) in toluene at 80 °C. To avoid oxidation
of the desired product, each of the internal amines was protected,
in situ, as a tert-butyl carbamate (BOC) by addition of (BOC)2O
and a catalytic amount of 4-(dimethylamino)pyridine (DMAP).
The crude diphenyl ketimine product was cleaved with hydrox-
ylamine hydrochloride11 to afford 3 in an overall yield of 91%.
Two approaches were taken to complete the assembly of the
octamers. The first strategy (Scheme 2) entails coupling diamine
3 with bromide 4 (2 equiv)12 followed by BOC-protection of the
initially formed intermediate. The 4-methoxyphenyl-capped
octamer (5, R ) OMe) was constructed in this manner in 79%
yield.
While this first strategy (Scheme 2) is highly convergent, a
second route was adopted for the purpose of rapidly building a
family of octaanilines from a common precursor as shown below
in Scheme 3. This alternative route commences with a Pd-
catalyzed coupling of 3 and 8 (see below) to produce 9 after in
situ BOC-protection (74%). Diamine 10 was obtained in 86%
by hydrogenolysis of 9 with Pd(OH)2/carbon and ammonium
formate. Capping 10 by Pd-catalyzed coupling with the appropri-
ate aryl bromide (followed by in situ BOC-protection) provided
the desired octaaniline (77-82%). Octamers (5) capped with R
) H, tert-butyl, dodecyl, and cyano were prepared in this fashion
and were found to be soluble in most common organic solvents.13
(1) (a) Huang, W.-S.; Humphrey, B. D.; MacDiarmid, A. G. J. Chem. Soc.,
Faraday Trans. 1 1986, 82, 2385-2400. (b) Chen, S.-A.; Fang, W.-G.
Macromolecules 1991, 24, 1242-1248. (c) Chiang, J.-C.; MacDiarmid, A.
G. Synth. Met. 1986, 13, 193-205.
(2) MacDiarmid, A. G.; Mu, S.-L.; Somasiri, N. L. D.; Wu, W. Mol. Cryst.
Liq. Cryst. 1985, 121, 187-190.
(3) (a) Taka, T. Synth. Met. 1991, 41, 1177-1180. (b) Colaneri, N. F.;
Shacklette, L. W. IEEE Trans. Instrum. Meas. 1992, 41, 291. (c) Joo, J.;
Epstein, A. J. Appl. Phys. Lett. 1994, 65, 2278-2280.
(4) (a) DeBerry, D. W. J. Electrochem. Soc. 1985, 132, 1022-1026. (b)
Ahmad, N.; MacDiarmid, A. G. Synth. Met. 1996, 78, 103-110. (c) Lu, W.-
K.; Elsenbaumer, R. L.; Wessling, B. Synth. Met. 1995, 71, 2163-2166.
(5) (a) Lu, F.-L.; Wudl, F.; Nowak, M.; Heeger, A. J. J. Am. Chem. Soc.
1986, 108, 8311-8313. (b) Wudl, F.; Angus, R. O., Jr.; Lu, F. L.; Allemand,
P. M.; Vachon, D. J.; Nowak, M.; Liu, Z. X.; Heeger, A. J. J. Am. Chem.
Soc. 1987, 109, 3677-3684.
(10) Taguchi, K.; Westheimer, F. H. J. Org. Chem. 1971, 36, 1570-1572.
(11) Fasth, K.-J.; Antoni, G.; Langstro¨m, B. J. Chem. Soc., Perkin Trans.
1 1988, 3081-3084.
(12) Precursor 4 was prepared in two isolated steps. Intermediate 7 was
isolated in 84% yield following a Pd coupling and in situ BOC-protection.
Hydrogenolysis of 7, followed by coupling with 1,4-dibromobenzene and in
situ BOC-protection afforded 4 in 75% yield. For experimental details, see
the Supporting Information.
(6) Ochi, M.; Furusho, H.; Tanaka, J. Bull. Chem. Soc. Jpn. 1994, 67,
1749-1752.
(7) Rebourt, E.; Joule, J. A.; Monkman, A. P. Synth. Met. 1997, 84, 65-
66.
(8) Zhang, W. J.; Feng, J.; MacDiarmid, A. G.; Epstein, A. J. Synth. Met.
1997, 84, 119-120.
(9) (a) Wolfe, J. P.; Wagaw, S.; Buchwald, S. L. J. Am. Chem. Soc. 1996,
118, 7215-7216 and references therein. (b) Driver, M. S.; Hartwig, J. F. J.
Am. Chem. Soc. 1996, 118, 7217-7218.
S0002-7863(97)03407-0 CCC: $15.00 © 1998 American Chemical Society
Published on Web 01/14/1998

214 J. Am. Chem. Soc., Vol. 120, No. 1, 1998
Communications to the Editor
Scheme 3a
Scheme 5
Scheme 6a
a Reagents and conditions: (a) Pd(OAc)2 (6 mol %), BINAP (7 mol
%), NaOtBu (2.8 equiv), toluene, Et3N, 90 °C; (b) (BOC)2O (3 equiv),
4-DMAP (0.1 equiv), THF, reflux; (c) 20% Pd(OH)2/C (0.4 equiv),
(NH4)HCO2 (20 equiv), THF/EtOH (2:1), 70 °C; (d) RC6H4Br (2.1 equiv),
Pd2(dba)3 (2 mol %), BINAP (6 mol %), NaOtBu (2.5 equiv), THF, reflux.
a Reagents and conditions: (a) (NH4)HCO2 (12 equiv), 10% Pd/C (0.1
equiv), EtOH, 60 °C; (b) Br2, NaOAc, THF, 0 °C; (c) Pd2(dba)3 (2 mol
%), BINAP (5 mol %), toluene, 80 °C; (d) (BOC)2O (1.5 equiv), 4-DMAP
(0.1 equiv), THF, reflux.
Scheme 4a
6) to build nonsymmetrical oligomers. The key component in
this strategy is 13, prepared from the Pd-catalyzed coupling of
4-(trimethylsilyl)aniline20 with 1, followed by in situ BOC
protection. One portion of 13 was subjected to hydrogenolysis,
affording amine 14, and a second portion was regioselectively
brominated to produce 8. The combination of 14 and 8 was
subjected to Pd-catalyzed amination. After in situ BOC protection
of the crude product, 15 was obtained in 89% yield. This process
may be used to construct oligomers of longer chain lengths,
doubling in length with each iteration.
This new approach to constructing oligoanilines should prove
to be general in nature for building oligomers of any length by
combining the products assembled from the divergent/convergent
strategy with various symmetrical building blocks (2, 3, 10, or
others21). The current work provides octaanilines with flexibility
in modification of the end groups. In addition, protecting the
amines as tert-butyl carbamates facilitates manipulations of the
materials by impeding oxidation and solubilizing the oligoaniline.
The syntheses of the materials can be efficiently carried out on a
multigram scale since all intermediates and end products were
isolated as solids by recrystallization or precipitation.22 Electro-
chemical studies as well as the preparation of oligoanilines of
varying chain length are in progress and will be reported in due
course.
a Reagents and conditions: (a) Pd2(dba)3 (0.1 mol %), BINAP (0.24
mol %), NaOtBu (1.4 equiv), THF, reflux; (b) Bu4NBr3, CH2Cl2, rt; (c)
(BOC)2O (1.5 equiv), 4-DMAP (0.05 equiv), THF, reflux.
Essential to the efficiency of this route is the facile construction
of 8 as illustrated in Scheme 4. Coupling of 1 with aniline with
Pd2(dba)3 (0.1 mol %) (dba ) dibenzylideneacetone) and BINAP
(0.24 mol %) proceeded cleanly to give intermediate 11. Regio-
selective bromination followed by BOC-protection of the crude
coupling product (11) provided 8 in 81% yield for the 3-step
sequence.14
The BOC groups throughout the backbone of the octamers
prevent oxidation of the material and ease its handling by
improving solubility.15 The BOC groups were removed quanti-
tatively either by thermolysis16 (185 °C, 9 h), or by treatment
with TMSI17 at room temperature in CH2Cl2, followed by addition
1
of triethylamine and methanol. Analysis by H NMR, IR, and
UV/vis spectroscopies indicated that the deprotection was clean
in each case producing octaanilines in the leucoemeraldine state.
When these materials were oxidized to the emeraldine state18
under acidic conditions, we observed, by UV/visible spectroscopy,
an absorption between 700 and 1000 nm, characteristic of the
narrow bandgap in oligoanilines and polyaniline.5b
Oligoanilines other than octamers or tetramers (capping of 3)
are accessible by appending different side chains to 3. Toward
this end, we developed a divergent/convergent approach19 (Scheme
Acknowledgment. We are grateful to the Office of Naval Research
for partial support of this research. We thank Eastman Kodak for
additional funding. We are appreciative of the guidance and suggestions
provided by Prof. Timothy M. Swager.
(13) The BOC-protected oligomers (5) were quite soluble in solvents such
as CH2Cl2, EtOAc, THF, and Et2O; however, the deprotected oligoanilines
were only sparingly soluble in polar solvents such as DMF or NMP, as
observed in previous studies.5-8
(14) Berthelot, J.; Guette, C.; Desbe`ne, P.-L.; Basselier, J.-J.; Chaquin, P.;
Masure, D. Can. J. Chem. 1989, 67, 2061-2066.
Supporting Information Available: Experimental details and char-
acterization data (13 pages). See any current masthead page for ordering
and Internet access instructions.
JA973407L
(15) An additional benefit of the BOC group lies in its ability to activate
toward Pd oxidative addition substrates which contain a secondary aromatic
amine para to a bromide (such as 4 or 8). In the absence of BOC-protection,
such substrates generally fail to couple with amines under standard Pd-
catalyzed amination conditions.
(19) For a description of the divergent/convergent approach applied to the
construction of other electroactive oligomers, see: Tour, J. M. Chem. ReV.
1996, 96, 537-553.
(20) Walton, D. R. M. J. Chem. Soc. C 1966, 1706-1707.
(16) Rawal, V. H.; Jones, R. J.; Cava, M. P. J. Org. Chem. 1987, 52, 19-
(21) Odd-numbered oligoanilines should be accessible from (p-H2NC6H4)2N-
BOC (in place of 3 in Scheme 2) which may be derived from 4,4′-
dibromodiphenylamine as described in: Wolfe, J. P.; A° hman, J.; Sadighi, J.
P.; Singer, R. A.; Buchwald, S. L. Tetrahedron Lett. 1997, 38, 6367-6370.
(22) All isolated intermediates and products (3-5, 7-10, 13-15) were
28.
(17) Lott, R. S.; Chauhan, V. S.; Stammer, C. H. J. Chem. Soc., Chem.
Commun. 1979, 495-496.
(18) MacDiarmid, A. G.; Chiang, J. C.; Richter, A. F.; Somasiri, N. L. D.;
Epstein, A. J. Conduct. Polym. 1987, 105-120.
1
characterized by H and 13C NMR spectroscopy, IR, and elemental analysis.