A Novel Synthetic Metal Catalytic System

Article abstract of DOI:10.1021/jo9617575

π-Conjugate polymers, polyanilines and polypyrroles, served as catalysts in dehydrogenative oxidation reactions of benzylamines, 2-phenylglycine, and 2,6-di-tert-butylphenol under oxygen. The catalytic activity was controlled by protonic acid doping. The reversible redox of the polyaniline catalyst under oxygen was supported by UV-visible spectroscopy. Polyaniline and transition metal such as copper(II) chloride or iron(III) chloride formed a complex, which was effective in the dehydrogenative oxidation of cinnamyl alcohol or mandelic acid. In the complex system, transition metals are considered to electronically interact through a π-conjugate polymer chain. Protonic acid doping and transition metal doping play an important role in reversible redox processes of polyanilines.

Full text of DOI:10.1021/jo9617575

1072
J . Org. Chem. 1997, 62, 1072-1078
A Novel Syn th etic Meta l Ca ta lytic System
Masayoshi Higuchi, Isao Ikeda, and Toshikazu Hirao*
Department of Applied Chemistry, Faculty of Engineering, Osaka University, Yamada-oka, Suita,
Osaka 565, J apan
Received September 12, 1996X
π-Conjugate polymers, polyanilines and polypyrroles, served as catalysts in dehydrogenative
oxidation reactions of benzylamines, 2-phenylglycine, and 2,6-di-tert-butylphenol under oxygen.
The catalytic activity was controlled by protonic acid doping. The reversible redox of the polyaniline
catalyst under oxygen was supported by UV-visible spectroscopy. Polyaniline and transition metal
such as copper(II) chloride or iron(III) chloride formed a complex, which was effective in the
dehydrogenative oxidation of cinnamyl alcohol or mandelic acid. In the complex system, transition
metals are considered to electronically interact through a π-conjugate polymer chain. Protonic
acid doping and transition metal doping play an important role in reversible redox processes of
polyanilines.
In tr od u ction
sisting of palladium(II) acetate and polyaniline or poly-
pyrrole is capable of inducing the Wacker oxidation.10
Efficient redox systems for the catalytic reactions are
achieved in both cases. We describe herein the full scope
of the novel synthetic metal catalytic systems based on
the redox properties of polyanilines and polypyrroles.
π-Conjugate polymers have received wide interest due
to their high potential in a variety of applications as
conducting polymers. Among π-conjugate polymers,
polyanilines are particularly attractive based on their
excellent electrical properties and stability.1 An exten-
sive investigation has been undertaken to hopefully
provide more useful materials such as batteries,2 light-
emitting diodes,3 antistatic packagings and coatings,4
microelectronic devices,5 electrochemical chromatogra-
phy,6 and corrosion inhibitors to protect semiconductors
or metals.7
Polyanilines in various oxidation states interconvert
each other, which permits us to construct a reversible
redox cycle for catalytic reactions. In previous papers,8
we demonstrated that polyanilines serve as synthetic
metal catalysts in dehydrogenative oxidation under
oxygen. The catalytic activity has been revealed to be
controlled by protonic acid doping. Coordination of
transition metals to the nitrogen atoms affords the
complexes, in which transition metals are considered to
interact through a π-conjugate chain. The characteristics
of π-conjugate polymers are reflected on the complexes,
which are expected to provide a novel catalytic system.
Complexation with copper(II) chloride or iron(III) chloride
affords a more efficient synthetic metal catalyst in the
dehydrogenation reaction.8a,9 The complex catalyst con-
Resu lts a n d Discu ssion
P olya n ilin e-Ca ta lyzed Deh yd r ogen a tive Oxid a -
tion of Ben zyla m in es. A catalytic oxidative dehydro-
genation of the benzylamine 1 proceeded on treatment
with a catalytic amount of polyaniline under oxygen to
give the N-benzylidenebenzylamine 2 (eq 1 and Table 1).
The imine 2a is considered to be derived by dehydroge-
nation to benzylideneamine and transamination with
benzylamine. R-Disubstituted benzylamine 1b was also
oxidized to the corresponding imine 2b, which was
detected as the hydrolyzed product, benzophenone (run
18). N,N-Dimethylformamide (DMF) or 1-methyl-2-pyr-
rolidinone (NMP) was used as a solvent although poly-
aniline employed here was only partially soluble (runs 2
and 3). N-Benzylformamide was obtained as a byproduct
in the former case via the amine exchange of DMF with
benzylamine. The almost insolubility in acetonitrile or
ethanol required a longer reaction time (runs 6 and 10).
The blue supernatant of the heterogeneous acetonitrile
mixture indicates the involvement of an oxidized form
of polyaniline in the dehydrogenation reaction. Oxygen
is essential for the catalytic reaction since only a small
amount of 2a was produced under nitrogen (run 5). A
distinct difference in the yields of 2a was not observed
when the reaction was carried out in the dark (run 4).11
The relationship between protonic acid doping and
catalytic activity was studied because protonation of
emeraldine base increases the conductivity of poly-
anilines.12 Higher activity was attained with an increase
of doping (run 9), but 2a was produced only in 12% yield
with the deprotonated polyaniline (run 11).
X Abstract published in Advance ACS Abstracts, J anuary 15, 1997.
(1) (a) Bergeron, J .-Y.; Dao, L. H. Macromolecules 1992, 25, 3332.
(b) Nakajima, T.; Kawagoe, T. Synth. Met. 1989, 28, C629. (c)
Mizumoto, M.; Namba, M.; Nishimura, S.; Miyadera, H.; Koseki, M.;
Kobayashi, Y. Synth. Met. 1989, 28, C639. (d) Genies, E. M.; Boyle,
A.; Lapkowski, M.; Tsintavis, C. Synth. Met. 1990, 36, 139.
(2) (a) J ohnstone, B. East. Econ. Rev. 1988, 17, 78. (b) Kitani, A.;
Kaya, M.; Sakaki, K. J . Electrochem. Soc. 1986, 133, 1069. (c)
MacDiarmid, A. G.; Yang, L. S.; Huang, W. S.; Humphrey, B. D. Synth.
Met. 1987, 18, 393.
(3) Gustafsson, G.; Cao, Y.; Treacy, G. M.; Klavetter, F.; Colaneri,
N.; Heeger, A. J . Nature 1992, 356, 47.
(4) DeBerry, D. J . Electrochem. Soc. 1985, 132, 1022.
(5) (a) Genies, E. M.; Lapkowski, M.; Santier, C.; Vieil, E. Synth.
Met. 1987, 18, 631. (b) Nguyen, M. T.; Dao, L. H. J . Electrochem. Soc.
1989, 136, 2131. (c) Paul, E. W.; Ricco, A. J .; Wrighton, M. S. J . Phys.
Chem. 1985, 89, 1441.
Electrochemical properties of polyanilines are known
to depend on the polymerization method. A large differ-
(6) Nagaoka, T.; Kakuno, K.; Fujimoto, M.; Nakao, H.; Yano, J .;
Ogura, K. J . Electroanal. Chem. 1994, 368, 315.
(7) Noufi, R.; Nozik, A. J .; White, J .; Warren, L. F. J . Electrochem.
Soc. 1982, 129, 2261.
(8) (a) Hirao, T.; Higuchi, M.; Ikeda, I.; Ohshiro, Y. J . Chem. Soc.,
Chem. Commun. 1993, 194. (b) Hirao, T.; Higuchi, M.; Ohshiro, Y.;
Ikeda, I. Chem. Lett. 1993, 1889.
(9) Higuchi, M.; Imoda, D.; Hirao, T. Macromolecules, 1996, 29, 8277.
(10) (a) Hirao, T.; Higuchi, M.; Hatano, B.; Ikeda, I. Tetrahedron
Lett. 1995, 36, 5925. (b) Higuchi, M.; Yamaguchi, S.; Hirao, T. Synlett,
in press.
(11) J ohann, D.; Otto, H. Electrochim. Acta 1991, 36, 361.
(12) Venugopal, G.; Quan, X.; J ohnson, G. E.; Houlihan, F. M.; Chin,
E.; Nalamasu, O. Chem. Mater. 1995, 7, 271.
S0022-3263(96)01757-4 CCC: $14.00 © 1997 American Chemical Society

A Novel Synthetic Metal Catalytic System
J . Org. Chem., Vol. 62, No. 4, 1997 1073
Ta ble 1. Oxid a tion of 1a
run
substrate
cat.
oxidant for polymzn
anal.
atm
solvent
time, h
yield, %
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1b
1a
1a
1a
1a
-
-
-
O2
O2
O2
O2
N2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
N2
O2
N2
MeCN
NMP
DMF
DMF
DMF
EtOH
MeCN
MeCN
MeCN
MeCN
MeCN
NMP
50
10
10
10
10
40
50
50
50
50
50
10
10
50
50
50
50
10
50
50
50
50
trace
77
74
69b
15
53
79
87
75
61
12
77
4
65
86
91
67
66
73
38
74
36
polyaniline
Cu(BF4)2
C6.00H4.50N1.00(BF4)0.11
C6.00H4.50N1.00(BF4)0.11
C6.00H4.50N1.00(BF4)0.11
C6.00H4.50N1.00(BF4)0.11
C6.00H4.50N1.00(BF4)0.11
C6.00H6.71N1.00Cl0.55
C6.00H4.04N0.99Cl1.28
C6.00H3.93N1.00Cl0.44
C6.00H4.50N1.00(BF4)0.11
C6.00H4.28N0.97
polyaniline
polyaniline
polyaniline
polyaniline
polyaniline
polyaniline
polyaniline
polyaniline
Cu(BF4)2
Cu(BF4)2
Cu(BF4)2
Cu(BF4)2
(NH4)2S2O8
VO(OEt)Cl2
Cu(BF4)2
Cu(BF4)2
Cu(BF4)2
(NH4)2S2O8
(NH4)2S2O8
(NH4)2S2O8
(NH4)2S2O8
(NH4)2S2O8
(NH4)2S2O8
Cu(BF4)2
(NH4)2S2O8
(NH4)2S2O8
(NH4)2S2O8
(NH4)2S2O8
polyaniline
polyanilinec
C6.00H5.18N0.98Cl0.52
C6.00H4.20N0.96Cl0.02
C7.00H6.98N0.97Cl0.38
C7.00H7.89N0.98Cl0.28
C7.00H6.70N1.02Cl0.26
C12.00H15.5N0.87Cl0.74
C6.00H4.50N1.00(BF4)0.11
C4.00H3.60N0.99Cl0.28
C4.00H3.60N0.99Cl0.28
C5.00H5.04N0.97Cl0.16
C5.00H5.04N0.97Cl0.16
polyanilinec
NMP
poly(o-anisidine)
poly(o-toluidine)
poly(N-methylaniline)
poly(3-(hexyloxy)aniline)
polyaniline
polypyrrole
polypyrrole
poly(N-methylpyrrole)
poly(N-methylpyrrole)
MeCN
MeCN
MeCN
MeCN
DMF
MeCN
MeCN
MeCN
MeCN
a
b
1, 1.0 mmol; cat., 10 mg; solvent, 1.0 mL. Reaction temperature, 80 °C. In the dark. c Highly soluble polymer.
ence was not observed when polyanilines polymerized
with (NH4)2S2O8 or Cu(BF4)2 were used in the dehydro-
genative oxidation (runs 7 and 9). Use of the polyaniline
polymerized with VO(OEt)Cl2 gave a better result, be-
cause it is confirmed by elemental analysis to be highly
protonated and oxidized (run 8).
Ta ble 2. Oxid a tion of 4a
While various dopants such as dodecylbenzenesulfonic
acid (DBSA) vary the solubility of polyaniline,13 highly
soluble polyaniline with HCl as a dopant has been
reported to be prepared by chemical oxidation with an
equimolar of (NH4)2S2O8.14 Use of thus-obtained poly-
aniline in NMP, however, resulted in the similar yield
of 2a as observed above (run 12). Dedoping of the
polyaniline again decreased the catalytic activity (run 13).
Substituent effects of polyaniline derivatives were
studied. Poly(o-toluidine) and poly(N-methylaniline)
showed a little higher activity (runs 15 and 16). High
solubility of poly(3-(hexyloxy)aniline) did not affect the
oxidation reaction much under the conditions employed
here (run 17).
run
polyaniline, anal.
C6.00H6.34N0.98Cl0.57
yield, %
1
2
3
40
62
55
C
C
6.00H4.50N1.00(BF4)0.11
6.00H4.39N0.98
a
4, 1.0 mmol; polyaniline, 10 mg; DMF, 1.0 mL.
system like polyanilines, so they are expected to serve
as synthetic metal catalysts. Indeed, polypyrrole and
poly(N-methylpyrrole) catalyzed the dehydrogenation of
1a to 2a under oxygen (runs 19 and 21, and eq 3). The
reaction, however, proceeded to some extent under
nitrogen (runs 20 and 22), which is presumably accounted
for by the over reduction of the pyrrole ring.15
The reaction path of dehydrogenation has not been
elucidated. Butylamine did not undergo the present
dehydrogenation reaction. Catalytic oxidation of benzyl-
amine in the presence of an equimolar amount of butyl-
amine led to the imines 2a and 3a in 37% and 40% yields,
respectively (eq 2). The unsymmetrical imine 3a is
considered to be derived by transamination with butyl-
amine. Use of 3 mol equiv of butylamine raised the ratio
of 3a .
P olya n ilin e-Ca ta lyzed Deh yd r ogen a tive Oxid a -
tion of 2-P h en ylglycin e. Polyaniline catalyst was
effective in the oxidation of 2-phenylglycine (4) to give
2a (eq 4 and Table 2). It should be noted that the
dehydrogenation reaction of the amino moiety accompa-
nies decarboxylation. The catalytic activity is controlled
by protonic acid doping. Use of the less doped polyaniline
led to the higher yield of 2a , being in contrast to the
finding that the doped polyaniline gave the better result
in the oxidation of 1a . Protonic acid doping is essential
for smooth redox of polyaniline as described later, but
(13) Cao, Y.; Smith, P.; Heeger, A. J . Synth. Met. 1992, 48, 91.
(14) Abe, M.; Ohtani, A.; Higuchi, H.; Ezoe, M.; Akizuki, S.;
Nakamoto, K.; Mochizuki, K. J P 91-28229.
(15) (a) Yamamoto, T. Prog. Polym. Sci. 1992, 17, 1153. (b) Kim,
D. Y.; Lee, J . Y.; Moon, D. K.; Kim, C. Y. Synth. Met. 1995, 69, 471.
P olyp yr r ole-Ca ta lyzed Deh yd r ogen a tive Oxid a -
tion of Ben zyla m in e. Polypyrroles constitute a redox

1074 J . Org. Chem., Vol. 62, No. 4, 1997
Higuchi et al.
Ta ble 3. Oxid a tion of 5a
yield, %
run
polyaniline, anal.
6
7
recovery of 5, %
1
2
3
4
-
trace
33
61
0
99
66
27
3
C6.00H6.34N0.98Cl0.57
0
12
14
C
C
6.00H4.49N0.99Cl0.01
6.00H4.39N0.98
81
a
5, 1.0 mmol; polyaniline, 10 mg; DMF, 1.0 mL.
the decarboxylation step in oxidation of 2a is assumed
to require the appropriate basicity of the catalyst.
P olya n ilin e-Ca ta lyzed Deh yd r ogen a tive Oxid a -
tion of 2,6-Di-ter t-bu tylp h en ol. The synthetic metal
catalytic system is applied to the oxidative coupling of
2,6-di-tert-butylphenol (5) (eq 5 and Table 3). The phenol
5 was dimerized to the diphenoquinone 6. One-electron
oxidation of the phenolate seems to be involved as a
reaction path. Higher conversion and yield were attained
with the undoped polyaniline, although the p-quinone
was formed via oxygenation. The results shown in Table
3 indicate that the similar reactivity of the catalyst is
necessary as observed in the case of 2-phenylglycine.
Red ox Beh a vior of P olya n ilin e. Catalysis appears
to depend on the reversible redox cycle of polyanilines
under oxygen. The redox process was monitored by UV-
visible spectra of the partially soluble polyaniline as
shown in Figure 1. The blue NMP solution showed a
broad absorption around 640 nm assignable to charge
transfer from benzenoid of polyaniline to quinoid.12 The
absorption disappeared on treatment with benzylamine
under nitrogen at 80 °C (Figure 1b). The spectral change
indicates that the charge transfer absorption decreases
with reduction of a quinoid moiety to a benzenoid one.
The absorption reappeared with coloring to blue only by
bubbling oxygen into the mixture for 20 min. The
oxidative interconversion of the leucoemeraldine base has
been reported to be accomplished by oxygen leading to
the emeraldine form.16
Protonic acid doping plays an important role in the
redox behavior of polyaniline as mentioned above. To
obtain further information of the redox behavior, in situ
UV-visible spectra at a given redox potential were
measured.17 The broad absorption around 640 nm based
on charge transfer from benzenoid to quinoid and 340
nm peak based on π-π* transition of the phenyl ring
were observed in NMP solution of the undoped polya-
niline.12 The broad 640 nm absorption successively
increased and blue-shifted at a potential from 0.2 V to
0.5 V vs SCE (Figure 2a). The absorption around 340
nm inversely decreased at a positive potential. These
spectral changes are explained by the partial electro-
F igu r e 1. (a) UV-visible spectra of polyaniline (anal. C6.00
-
H4.50N1.00(BF4)0.11) in NMP. (b) (1) Treatment with benzyl-
amine under nitrogen at 80 °C for 2 h. (2) O2 was bubbled
into the solution at room temperature for 20 min.
chemical oxidation of the emeraldine form to the perni-
graniline one bearing a quinoid structure. On the other
hand, at a negative potential between 0.0 and -0.6 V,
no hypochromic change was observed except for the slight
red shift (Figure 2b). These results indicate that the
undoped polyaniline is reduced with difficulty at a
negative potential.
In contrast to the above-mentioned behavior, a hypo-
chromic change occurred from 0.0 V to -0.6 V in the case
of the protonic acid-doped polyaniline (Figure 3). The
broad 620 nm absorption based on charge transfer
increased at a positive potential, being accompanied by
a blue-shift above 0.3 V. The 340 nm absorption based
on π-π* transition accordingly decreased. These spectral
changes imply that the protonic acid-doped emeraldine
salt of polyaniline is oxidized to the pernigraniline form
as observed in the undoped polyaniline. On the other
hand, the broad 620 nm absorption decreased and the
340 nm one increased at a negative potential between
0.0 V and -0.6 V, indicating that the protonic acid-doped
polyaniline is reduced to the leucoemeraldine form. This
spectral redox behavior of the protonic acid-doped polya-
niline was reversible between 0.5 and -0.6 V.
The above-mentioned results indicate that protonic
acid doping is involved in the redox of polyaniline. The
presence of a proton source permits the reduction of
thequinoid moiety of polyaniline into the benzenoid one.
In the undoped polyaniline solution, however, there is
not enough a proton source to perform the reduction.
Deh ydr ogen ative Oxidation with th e P olyan ilin e-
Tr a n sition Meta l Com p lex. Doping of polyaniline with
(16) Moon, D.-K.; Maruyama, T.; Osakada, K.; Yamamoto, T. Chem.
Lett. 1991, 1633.
(17) In situ UV-visible spectra were measured according to the
reported method: Yoneyama, H.; Ii, Y.; Kuwabata, S. J . Electrochem.
Soc. 1992, 139, 28.

A Novel Synthetic Metal Catalytic System
J . Org. Chem., Vol. 62, No. 4, 1997 1075
Ta ble 4. Oxid a tion of 8a
polyaniline, metal salt,
time, yield,
run
substrate
mgb
equiv
atm
O2
h
%
1
2
3
4
5
6
7
PhCHdCHCH2OH
10
-
10
20
10
10
10
-
10 trace
10
CuCl2 0.1 O2
CuCl2 0.1 O2
CuCl2 0.2 O2
CuCl2 0.2 N2
5
10 44
10 68
10
9
FeCl3 0.1
O2
20 39
20 trace
PhCH2OH
CuCl2 0.1 O2
a
b
8, 1.0 mmol; DMF, 1.0 mL. Anal. C6.00H4.50N1.00(BF4)0.11
.
Ta ble 5. Oxid a tion of 10a
run
polyaniline, anal.
CuCl2, equiv
yield, %
1
2
3
4
-
0.1
-
0.1
0.1
6
13
32
55
C6.00H4.50N1.00(BF4)0.11
C
C
6.00H4.50N1.00(BF4)0.11
6.00H4.49N0.99Cl0.01
a
10, 1.0 mmol; polyaniline, 10 mg; DMF, 1.0 mL.
F igu r e 2. UV-visible spectra of the undoped polyaniline (anal.
C6.00H4.04N0.97Cl0.03) in NMP. (a) Electrochemical oxidation. (b)
Electrochemical reduction.
not proceed catalytically under nitrogen (run 5). A poor
result was obtained with each individual component
(runs 1 and 2), indicating that a combination of synthetic
metal catalyst with copper(II) chloride is required for the
oxidation. Increase in the catalytic amount of both
components raised the yield of 9 (run 4). Iron(III)
chloride was similarly used instead (run 6). Failure in
the dehydrogenation of benzyl alcohol might be explained
by the difference in their redox potentials (run 7).
The catalytic system was applicable to the decarboxyl-
ative dehydrogenation of mandelic acid (10) to give
benzaldehyde (11, eq 7). The results shown in Table 5
indicate that the cooperative catalysis of polyaniline and
copper(II) chloride is essential. The effect of protonic acid
doping of polyaniline was observed as mentioned above.
Use of the less doped polyaniline led to the better yield
(runs 3 and 4).
Com p lexa t ion of P olya n ilin e w it h Cop p er (II)
Ch lor id e. The insoluble species recovered in the oxida-
tion reaction of 8 in the presence of copper(II) chloride
in DMF involved one copper species to two aniline units
of polyaniline (C6.00H5.82N1.01(BF4)0.11(CuCl2)0.53). It sug-
gests that the complexation of copper(II) chloride with
polyaniline affords the real catalyst that induces the
dehydrogenative oxidation reaction.
Complexation is supported by UV-visible spectra of the
undoped polyaniline with copper(II) chloride. Addition
of copper(II) chloride to the NMP solution of the poly-
aniline resulted in a blue shift of the absorption around
640 nm as shown in Figure 4a. This shift (∆λ) depended
on the amount of copper(II) chloride giving the saturation
curve for complexation (Figure 4b).
The cyclic voltammetry is also indicative of complex-
ation. The half-wave potential of the undoped poly-
aniline-CuCl2 complex in NMP was 0.46 V vs SCE, being
in contrast to 0.29 V for that of the undoped polyaniline
(Figure 5). The wave shape of the complex was broader.
F igu r e 3. UV-visible spectra of the doped polyaniline (anal.
C6.00H6.34N0.98Cl0.57) in NMP. (a) Electrochemical oxidation. (b)
Electrochemical reduction.
copper(II) chloride constitutes an efficient catalyst for the
dehydrogenation of cinnamyl alcohol (8) to cinnamalde-
hyde (9) in DMF (eq 6 and Table 4). The oxidation did

1076 J . Org. Chem., Vol. 62, No. 4, 1997
Higuchi et al.
F igu r e 6. UV-visible spectra of the undoped polyaniline (anal.
C6.00H4.04N0.97Cl0.03)-CuCl2 complex in NMP. (a) Electrochemi-
cal oxidation. (b) Electrochemical reduction.
Sch em e 1
F igu r e 4. (a) UV-visible spectra of the undoped polyaniline
(anal. C6.00H4.50N1.00(BF4)0.11) in NMP on successive addition
of CuCl2 as shown in (b): plots of ∆λ vs the amount of CuCl2.
on π-π* transition was reduced at the same potential.
Similar spectral changes are obtained in the case of
undoped and doped polyanilines. Such electrochemical
behavior was observed between 0.2 and 0.5 V as il-
lustrated in Figure 6a. A new absorption appeared
around 780 nm at 0.6 V. At a negative potential below
0.0 V, the decrease of the broad absorption around 600
nm was accompanied by an increase in the one around
320 nm with isosbestic points (Figure 6b). These obser-
vations indicate that the polyaniline of the complex is
partially reduced electrochemically to the leucoemeral-
dine form. The redox transformation was almost revers-
ible between 0.6 and -0.6 V. Such a spectral change is
similar to the one in protonic acid-doped polyaniline. In
the complex catalytic system, a copper dopant is consid-
ered to contribute to the formation of a reversible redox
cycle.
F igu r e 5. Cyclic voltammetry of the undoped polyaniline
Con clu sion
(anal. C6.00H4.04N0.97Cl0.03) and its complex with CuCl2 in NMP.
Scan rate is 50 mV s-1
.
Polyanilines or polypyrroles serve as catalysts for the
dehydrogenative oxidation of benzylamines, phenylgly-
cine, and 2,6-di-tert-butylphenol under oxygen. The
catalysis is considered to be based on the reversible redox
of π-conjugate polymers (Scheme 1). The activity of the
synthetic metal catalytic system is controlled by protonic
acid doping. Complexes of polyaniline with transition
metals such as copper(II) chloride or iron(III) chloride
serve as synthetic metal complex catalysts in the
These results appear to be accounted for by the complex-
ation, which is considered to raise the catalytic activity.
Further studies on the redox behavior of the undoped
polyaniline-CuCl2 complex in NMP were done by in situ
UV-visible spectra at a given redox potential.17 At 0.3 V
vs SCE, the broad absorption around 600 nm based on
charge transfer increased together with the blue shift.
On the contrary, the absorption around 320 nm based

A Novel Synthetic Metal Catalytic System
J . Org. Chem., Vol. 62, No. 4, 1997 1077
was placed in an ice/water bath and cooled. A solution of
ammonium peroxodisulfate (2.60 g, 11.4 mmol) in 1 M HCl
(50 mL) cooled in an ice/water bath was dropwise added to
the solution of 3-(hexyloxy)aniline over about 10 min with
vigorous stirring. After the addition of ammonium peroxo-
disulfate was completed, the mixture was stirred in an ice/
water bath for 90 min and at room temperature for 2 days. As
the polymerization proceeded, the solution turned brown and
the salt of poly(3-(hexyloxy)aniline) began to precipitate. The
obtained polymer was then filtered using a Bu¨chner funnel
and washed with about 1 L of deionized water. The polymer
was dissolved in DMF, reprecipitated in acetone, washed with
ether (400 mL), and dried in vacuum at room temperature for
24 h. Poly(3-(hexyloxy)aniline) (1.00 g) was obtained as a black
powder. The elemental analysis is shown in Table 1.
High ly Solu ble P olya n ilin e P olym er ized w ith Am -
m on iu m P er oxod isu lfa te.14 A solution of aniline (5.00 g,
50.4 mmol) in 1 M HCl (150 mL) was placed in an ice/acetone
bath and cooled. A solution of ammonium peroxodisulfate
(11.5 g, 50.4 mmol) in 1 M HCl (100 mL) cooled in an
ice/acetone bath was added dropwise to the solution of aniline
over about 10 min with vigorous stirring. As the polymeri-
zation proceeded, the solution turned dark green and the salt
of polyaniline began to precipitate. After the addition of
ammonium peroxodisulfate was completed, the mixture was
stirred for additional 2 h. Polyaniline was then filtered using
a Bu¨chner funnel, washed with 1 M HCl (300 mL), acetone
(300 mL), and then ether (300 mL), and dried in vacuum at
room temperature for 24 h. Polyaniline (4.77 g) was obtained
as a black powder. The elemental analysis is shown in Table
1.
P oly(o-a n isid in e), P oly(o-tolu id in e), a n d P oly(N-m e-
t h yla n ilin e) P olym er ized w it h Am m on iu m P er oxod i-
su lfa te. A typical procedure for polymerization is as follows.
Poly(o-anisidine) was prepared by chemical oxidation of o-
anisidine with ammonium peroxodisulfate in an acidic me-
dium.5 A solution of o-anisidine (5.46 g, 44.3 mmol) in 1 M
HCl (150 mL) was placed in an ice/water bath and cooled. A
solution of ammonium peroxodisulfate (2.52 g, 11.1 mmol) in
1 M HCl (100 mL) cooled in an ice/water bath was added
dropwise to the solution of o-anisidine over about 10 min with
vigorous stirring. As the polymerization proceeded, the solu-
tion turned dark green and the salt of poly(o-anisidine) began
to precipitate. After the addition of ammonium peroxodisul-
fate was completed, the mixture was stirred for additional 2
h. Workup and isolation of the polymer were carried out
similarly as mentioned above. Poly(o-anisidine) (0.99 g) was
obtained as a black powder. The elemental analyses are
shown in Table 1.
Ded op in g of P olya n ilin e. The polyaniline obtained above
was treated with an aqueous solution of 0.1 M sodium
hydroxide at room temperature for 24 h with stirring in order
to convert it to the undoped base, which was filtered using a
Bu¨chner funnel, washed with deionized water (300 mL),
acetonitrile (300 mL), and then ether (300 mL), and dried in
vacuum at room temperature for 24 h. The elemental analyses
are shown in Table 1.
P olyp yr r ole. A solution of pyrrole (3.35 g, 50.0 mmol) in
1 M HCl (150 mL) was placed in an ice/water bath and cooled.
A solution of ammonium peroxodisulfate (2.85 g, 12.5 mmol)
in 1 M HCl (100 mL) cooled in an ice/water bath was added
dropwise to the solution of pyrrole over about 10 min with
vigorous stirring. As the polymerization proceeded, the solu-
tion turned black and the salt of polypyrrole began to precipi-
tate. After the addition of ammonium peroxodisulfate was
completed, the mixture was stirred for additional 2 h. Poly-
pyrrole was filtered using a Bu¨chner funnel, washed with 1
M HCl (300 mL), acetonitrile (300 mL), and then ether (300
mL), and dried in vacuum at room temperature for 24 h.
Polypyrrole (0.88 g) was obtained as a black powder. The
elemental analysis is shown in Table 1.
Sch em e 2
oxidation reaction (Scheme 2). In this system, transition
metals are considered to be coordinated to polyaniline
and electronically interact with each other through a
π-conjugate chain. Since π-conjugate polymers are known
to form a sequential potential field, the similar kind of
field is considered to be attained in the complex system.
The present systems are furthermore envisaged to pro-
vide combinational catalysis.
Exp er im en ta l Section
P olya n ilin e P olym er ized w ith Am m on iu m P er oxod i-
su lfa te. Polyaniline was prepared by chemical oxidation of
aniline with ammonium peroxodisulfate in an acidic medium.5
A solution of aniline (20.4 g, 219 mmol) in 1 M HCl (300 mL)
was placed in an ice/water bath and cooled. A solution of
ammonium peroxodisulfate (11.5 g, 50.4 mmol) in 1 M HCl
(200 mL) cooled in an ice/water bath was added dropwise to
the solution of aniline over about 10 min with vigorous stirring.
As the polymerization proceeded, the solution turned dark
green and the salt of polyaniline began to precipitate. After
the addition of ammonium peroxodisulfate was completed, the
mixture was stirred for additional 2 h. Polyaniline (4.49 g)
was then filtered using a Bu¨chner funnel, washed with 1 M
HCl (300 mL), acetonitrile (300 mL), and then ether (300 mL),
and dried in vacuum at room temperature for 24 h. The
elemental analyses are shown in Table 1.
P olya n ilin e P olym er ized w ith Cu (BF 4)2. To aniline
(0.93 g, 10.0 mmol) placed in an ice/water bath was added 42%
HBF4 aq (1.83 mL, 10.0 mmol) over 10 min. After the addition
of 42% HBF4 aq was completed, the mixture was stirred for
additional 30 min. Cu(BF4)2 (4.74 g, 20.0 mmol) in acetonitrile
(15 mL) was added to the solution, which was stirred in an
ice/water bath for 2 h and at room temperature for 10 h.
Workup and isolation of the polymer were carried out similarly
as mentioned above. Polyaniline (0.86 g) was obtained as a
black powder. Thus obtained polyaniline contains less than
2 ppm of copper species. The elemental analysis is shown in
Table 1.
P olya n ilin e P olym er ized w ith VO(OEt)Cl2. To aniline
(0.93 g, 10.0 mmol) placed in an ice/water bath was added 42%
HBF4 aq (1.83 mL, 10.0 mmol) over 10 min. After the addition
of 42% HBF4 aq was completed, the mixture was stirred for
additional 30 min. VO(OEt)Cl2 (2.47 mL, 20.0 mmol) in
acetonitrile (15 mL) was added to the solution, which was
stirred in an ice/water bath for 2 h and at room temperature
for 10 h. Workup and isolation of the polymer were carried
out similarly as mentioned above. Polyaniline (0.87 g) was
obtained as a black powder. The elemental analysis is shown
in Table 1.
Syn th esis of P oly(3-(h exyloxy)a n ilin e). To a mixture
of K2CO3 (4.15 g, 30.0 mmol) and m-nitrophenol (2.78 g, 20.0
mmol) in acetonitrile (100 mL) was slowly added n-hexyl
bromide (4.20 mL, 30.0 mmol) over 10 min. The mixture was
refluxed for 30 h. The reaction mixture was filtered off and
concentrated. 3-(Hexyloxy)nitrobenzene (3.00 g, yield 67%)
was isolated by column chromatography eluting with chloro-
form. 3-(Hexyloxy)nitrobenzene (3.00 g, 13.4 mmol) in ethanol
(30 mL) was treated with 3 atm of hydrogen in the presence
of PtO2 (50 mg, 0.22 mmol) at 60 °C for 3 days. After filtration
of the reaction mixture, 3-(hexyloxy)aniline (2.20 g, yield 85%)
was obtained by Kugelrohr distillation of the filtrate (oven
temp 135 °C/0.5 mmHg); IR 3373, 2930, 1600 cm-1; H NMR
1
(600 MHz, CDCl3) δ 7.05 (t, 1H, J ) 7.8 Hz), 6.33 (d, 1H, J )
7.8 Hz), 6.28 (d, 1H, J ) 7.8 Hz), 6.25 (s, 1H), 3.92 (t, 2H, J )
6.6 Hz), 3.64 (br, 2H), 1.76 (quint, 2H, J ) 6.6 Hz), 1.45 (m,
2H), 1.34 (m, 4H), 0.91 (t, 3H, J ) 6.0 Hz). The solution of
3-(hexyloxy)aniline (2.20 g, 11.4 mmol) in 1 M HCl (50 mL)
P oly(N-m et h ylp yr r ole). A solution of N-methylpyrrole
(2.03 g, 25.0 mmol) in 1 M HCl (150 mL) was placed in an
ice/water bath and cooled. A solution of ammonium peroxo-
disulfate (5.71 g, 25.0 mmol) in 1 M HCl (100 mL) cooled in
an ice/water bath was added dropwise to the solution of

1078 J . Org. Chem., Vol. 62, No. 4, 1997
Higuchi et al.
N-methylpyrrole over about 10 min with vigorous stirring. As
the polymerization proceeded, the solution turned black and
the salt of poly(N-methylpyrrole) began to precipitate. After
the addition of ammonium peroxodisulfate was completed, the
mixture was stirred for additional 2 h. Workup and isolation
of the polymer were carried out similarly as mentioned above.
Poly(N-methylpyrrole) (1.00 g) was obtained as a black powder.
The elemental analysis is shown in Table 1.
P olya n ilin e-Ca ta lyzed Deh yd r ogen a tive Oxid a tion of
Ben zyla m in e. A typical experimental procedure for the
oxidation reaction of benzylamine is as follows. Polyaniline
(10 mg; anal. C6.00H4.50N1.00(BF4)0.11) was dried in vacuum in a
reaction vessel. Benzylamine (107 mg 1.00 mmol) in aceto-
nitrile (1.0 mL) was added under oxygen. The mixture was
stirred under oxygen at 80 °C for 50 h. Ether (2 mL) was
added to the reaction mixture at room temperature. After
stirring for 10 min, the mixture was filtered and concentrated.
Yields of 2a were determined by GLC (PEG 20M 10% 2.1 m,
200 °C).
10 min, the mixture was filtered and concentrated. Yield of 9
was determined by GLC (PEG 20M 10% 2.1 m, 200 °C).
Deh yd r ogen a tive Oxid a tion of Ma n d elic Acid Ca ta -
lyzed by th e P olya n ilin e-Cop p er Com p lex. A typical
experimental procedure for the oxidation reaction of mandelic
acid is as follows. The mixture of polyaniline (10 mg; anal.
C6.00H4.50N1.00(BF4)0.11) and copper(II) chloride (13.4 mg, 0.10
mmol) in DMF (1.0 mL) was stirred at room temperature for
30 min. Mandelic acid (152 mg, 1.00 mmol) was added to the
resulting mixture, which was stirred under oxygen at 80 °C
for 50 h. Ether (2 mL) was added to the reaction mixture at
room temperature. After stirring for 10 min, the mixture was
filtered and concentrated. Yield of 11 was determined by GLC
(PEG 20M 10% 2.1 m, 200 °C).
Ch em ica l Red ox of P olya n ilin e. The UV-visible spec-
trum of 3 mL of solution dissolved polyaniline (1.0 mg; anal.
C6.00H4.50N1.00(BF4)0.11) in NMP (20 mL) was measured under
nitrogen at 30 °C (Figure 1a). The polyaniline (10 mg) was
treated with benzylamine (107 mg, 1.00 mmol) in NMP (1.0
mL) under nitrogen at 80 °C for 2 h. A 30 µL volume of the
reaction mixture was added to NMP (3 mL) in the cell. The
UV-visible spectrum of the solution was taken under nitrogen
at 30 °C (Figure 1b-1). Oxygen was bubbled into the solution
at room temperature for 20 min. Spectrum 1 in Figure 1b
gradually changed to spectrum 2.
Com p lexa tion of P olya n ilin e w ith Cu Cl2. The UV-
visible spectrum of 3 mL of solution dissolved polyaniline (0.54
mg; anal. C6.00H4.50N1.00(BF4)0.11) in NMP (20 mL) was mea-
sured under nitrogen at 30 °C. The solution was treated
successively with 5 µL portion of 10 mM CuCl2 solution to give
the spectra as shown in Figure 4a. Figure 4b shows plots of
∆λ vs the amount of CuCl2.
Cyclic Volta m m etr y of P olya n ilin e a n d th e Com p lex
w ith Cu Cl2. All electrochemical measurements were carried
out under nitrogen at 25 °C. Cyclic voltammograms were
obtained in the NMP solution containing 0.1 M Bu4NClO4 as
P olya n ilin e-Ca ta lyzed Deh yd r ogen a tive Oxid a tion of
Am in od ip h en ylm eth a n e. Polyaniline (10 mg; anal. C6.00
-
H4.50N1.00(BF4)0.11) was dried in vacuum in a reaction vessel.
Aminodiphenylmethane (183 mg, 1.00 mmol) in DMF (1.0 mL)
was added under oxygen. The mixture was stirred under
oxygen at 80 °C for 10 h. Ether (2 mL) was added to the
reaction mixture at room temperature. After stirring for 10
min, the mixture was filtered and concentrated. Ethanol (2
mL), deionized water (0.2 mL), and SiO2 (1.0 g) were added to
the mixture, which was stirred for 24 h. The mixture was
filtered and concentrated. The yield of benzophenone was
determined by GLC (PEG 20M 10% 2.1 m, 200 °C).
P olya n ilin e-Ca ta lyzed Deh yd r ogen a tive Oxid a tion of
2-P h en ylglycin e. A typical experimental procedure for the
oxidation reaction of 2-phenylglycine is as follows. Polyaniline
(10 mg; anal. C6.00H4.50N1.00(BF4)0.11) was dried in vacuum in a
reaction vessel. 2-Phenylglycine (151 mg, 1.00 mmol) in DMF
(1.0 mL) was added under oxygen. The mixture was stirred
under oxygen at 80 °C for 20 h. Ether (2 mL) was added to
the reaction mixture at room temperature. After stirring for
10 min, the mixture was filtered and concentrated. The yield
of 2a was determined by GLC (PEG 20M 10% 2.1 m, 200 °C).
P olya n ilin e-Ca ta lyzed Deh yd r ogen a tive Oxid a tion of
2,6-Di-ter t-bu tylp h en ol. A typical experimental procedure
for the oxidation reaction of 2,6-di-tert-butylphenol is as
follows. Polyaniline (10 mg; anal. C6.00H4.39N0.98) was dried in
vacuum in a reaction vessel. 2,6-Di-tert-butylphenol (206 mg,
1.00 mmol) in DMF (1.0 mL) was added under oxygen. The
mixture was stirred under oxygen at 70 °C for 40 h. Ether (2
mL) was added to the reaction mixture at room temperature.
After stirring for 10 min, the mixture was filtered and
concentrated. The yields of 6 and 7 were determined by GLC
(SE-30 1.0 m, 50-250 °C, 10 °C/min).
a supporting electrolyte: polyaniline (10 mg; anal. C6.00
-
H4.04N0.97Cl0.03) in NMP (5 mL) in Figure 5; polyaniline (10 mg)
and CuCl2 (0.27 mg, 2.0 × 10-3 mmol) in NMP (5 mL).
Potentials were determined with a three-electrode system
consisting of a glassy carbon electrode, a platinum auxiliary
electrode, and a KCl-saturated calomel reference electrode at
50 mV s-1 scan rate.
In Situ UV-Visible Sp ectr a of P olya n ilin e a n d th e
Com p lex w ith Cu Cl2 a t a Given P oten tia l. In situ UV-
visible spectral measurements were carried out under nitrogen
at 25 °C.17 In situ UV-visible spectra were obtained in the
NMP solution containing 0.1 M Bu4NClO4 as a supporting
electrolyte at an electrical potential between -0.6 V and 0.6
V vs SCE with
a
reported hand-made thin layer cell:17
polyaniline (8.0 mg; anal. C6.00H4.04N0.97Cl0.03) in NMP (4 mL)
in Figure 2; polyaniline (8.0 mg; anal. C6.00H6.34N0.98Cl0.57) in
NMP (4 mL) in Figure 3; polyaniline (8.0 mg; anal.
C6.00H4.04N0.97Cl0.03) and CuCl2 (10.8 mg, 0.08 mmol) in NMP
(4 mL) in Figure 6.
Deh ydr ogen ative Oxidation of Cin n am yl Alcoh ol Cata-
lyzed by th e P olya n ilin e-Tr a n sition Meta l Com p lex. A
typical experimental procedure for the oxidation reaction of
cinnamyl alcohol is as follows. A mixture of polyaniline (10
mg; anal. C6.00H4.50N1.00(BF4)0.11) and copper(II) chloride (13.4
mg, 0.10 mmol) in DMF (1.0 mL) was stirred at room
temperature for 30 min. Cinnamyl alcohol (134 mg, 1.00
mmol) was added to the resulting mixture, which was stirred
under oxygen at 80 °C for 10 h. Ether (2 mL) was added to
the reaction mixture at room temperature. After stirring for
Ack n ow led gm en t. Thanks are due to the Analyti-
cal Center, Faculty of Engineering, Osaka University,
for the use of the NMR instrument.
J O9617575