Preparation of novel functionalized ammonium salts that effectively catalyze depolymerization of Nylon-6 in ionic liquids

Article abstract of DOI:10.1246/cl.2009.1016

A new type of ammonium salt containing DMAP in the cation was prepared and applied for the depolymerization of nylon6 giving monomelic caprolactam in high yield through direct distillation. The ionic liquids could be used at least five times. Copyright

Full text of DOI:10.1246/cl.2009.1016

1016
Chemistry Letters Vol.38, No.11 (2009)
Preparation of Novel Functionalized Ammonium Salts that Effectively
Catalyze Depolymerization of Nylon-6 in Ionic Liquids
Shigehiro Yamamoto and Akio KamimuraÃ
Department of Applied Molecular Bioscience, Graduate School of Medicine, Yamaguchi University, Ube 755-8611
(Received July 24, 2009; CL-090695; E-mail: ak10@yamaguchi-u.ac.jp)
A new type of ammonium salt containing DMAP in the cat-
ion was prepared and applied for the depolymerization of nylon-
6 giving monomeric caprolactam in high yield through direct
distillation. The ionic liquids could be used at least five times.
Pd(OAc)2
Xphos
R1
R2
MeI
R1
R2
N
Br
+
N
N
N
H
1
CH2Cl2
NaOt-Bu
DME
•
HCl
1a; R1, R2 = -CH2CH2N(Me)CH2CH2-
1b; R1= Me, R2 = CH2CH2NMe2
2a 97%
2b 70%
Development of a new method for plastic recycling is one of
an urgent problem to be solved.1 To achieve efficient recycling
of plastics, a convenient and effective depolymerization reaction
which selectively gives monomeric materials suitable for further
recycling is necessary. During such reaction, monomeric materi-
als should be isolated in high yields as well as in sufficient purity
through simple manipulation. On the other hand, depolymeriza-
tion however usually requires high temperatures which compli-
cate the process. Nylon-6 is a commonly used polymer and de-
polymerization has been explored for over 30 years, however,
efficient isolation of the resulting monomer remains problem.2
Recently, we developed a new method for depolymerization of
nylon-6 in ionic liquids.3 This is the first example employing
ionic liquids for the depolymerization and showed a new meth-
odology for plastic chemical recycling as well as a new use of
ionic liquids. Our methodology enabled depolymerization of ny-
lon-6 into caprolactam in a selective manner, and the monomeric
material was isolated in pure form by direct distillation from
the reaction pot. For efficient progress of the reaction, N,N-di-
methylaminopyridine, (DMAP),4 was needed as a catalyst,
which promoted cleavage of amide linkages in the polymer. Al-
though the ionic liquids could be used several times, addition of
fresh DMAP was necessary for each reaction. If a DMAP unit
were to be appended to the cation of an ionic liquid, the resulting
functionalized solvent could be expected to be widely employed
in not only depolymerization but also in organic synthesis.5 In
this paper we report the preparation of new type of ammonium
salts 3 and 4 in which DMAP was installed. Prepared salts were
examined for the depolymerization reaction of nylon-6.
DMAP-containing ammonium salts 3 and 4 were prepared
in the following procedure (Scheme 1). The first step is installa-
tion of a 4-amino group by using the Buchwald–Hartwig reac-
tion.6 Commercially available 4-bromopyridinium was treated
with piperazine 1a in the presence of palladium catalyst resulting
in the formation of 4-aminopyridine derivatives 2a. The results
are summarized in Table 1. We examined various reaction con-
ditions for the conversion. Use of BINAP as a ligand only gave
2a in 27% yield (Entry 1). Prolonged reaction time did not im-
prove the yield (Entry 2). Use of dppp, dppf, and Xphos also pro-
vided similar results (Entries 3, 4, and 6). Finally we found that
use of DME as the solvent and Xphos for the additional ligand
led to successful introduction of amine at the C4 position in pyr-
idine and desired 2a was isolated in 97% yield (Entry 7).
Another 4-aminopyridine 2b was also prepared in 70% yield
under the optimized conditions. The next stage was N-methyl-
LiNTf2
4
3
N
N
N
H2O
3a; X = I, 4a; X = NTf2
3a 18%
3b 34%
4a 39%
4b 90%
N
N
N
3b; X = I, 4b; X = NTf2
Scheme 1. Preparation of DMAP-containing ammonium salts 3
and 4.
Table 1. Preparation of 4-aminopyridine 2a
Temp. Time Yielda
/ꢀC
Entry
Catalyst
Solvent
/h
/%
Pd(OAc)2/BINAP
4 mol %
Pd(OAc)2/BINAP
4 mol %
Pd(OAc)2/dppp
4 mol %
Pd(OAc)2/dppf
4 mol %
Pd(OAc)2/BINAP
4 mol %
Pd(OAc)2/Xphos
4 mol %
Pd(OAc)2/Xphos
4 mol %
1
2
3
4
5
6
7
toluene
toluene
toluene
toluene
CPME
toluene
DME
70
2
27
70
70
70
70
90
90
20
3
18
22
1
Trace
30
7
20
44
26
97
aIsolated yield.
ation of 2. Treatment of 2a with MeI in CH2Cl2 resulted in the
formation of precipitation, the filtration of which readily gave
pure N-metyhylated product 3a in 18% yield. Although the yield
was poor, this preparation was quite simple because the precip-
itate only included 3a and further methylated compounds were
not observed in the precipitate. We proceeded to optimize the re-
action conditions, however, use of large excess of MeI provided
dimethylated product as a side product. Compound 2b was also
methylated under the same conditions, and compound 3b was
isolated in pure form. It should be noted that use of other alkyl
iodide such as propyl iodide did not provide the desired product
3c but led to N-alkylation of another position as in 2a.
Copyright Ó 2009 The Chemical Society of Japan

Chemistry Letters Vol.38, No.11 (2009)
1017
decomposition of the ammonium portion of 3a and 4a (Entries
1 and 2). On the other hand, compound 4b endowed with both
DMAP and [TMPA] structures, showed the best results for the
repeated depolymerization. The yields over 5 reaction cycles
constantly exceeded 70% except for the 1st time (Entry 4). Com-
paring the counter anion, TFSA seemed to provide better results
than iodide (Entries 3 and 4). These results suggest that the am-
monium salts 3b and 4b were stable and survived through the
treatment at 300 ꢀC for several times. Thus, the installation of
DMAP led to effectictively catalyzed depolymerization of ny-
lon-6 in ionic liquid.
In conclusion we have successfully prepared a new type of
ammonium salt containing DMAP in the counter cation. This
salt well catalyzed the depolymerization of nylon-6, and was
useful for several reaction cycles. With the expectation that
the novel ammonium salts could catalyze not only high-temper-
ature reactions but also other organic transformations, we are
now investigating the use of the ammonium salt in organic syn-
thesis.
PrI
Pr
N
N
N
N
N
N
I
CH2Cl2
2a
3c; 0%
Scheme 2. N-propylation of 2a.
O
O
H
[PP13][NTf2]
N
NH
5wt% DMAP-salt
n
3 or 4
300°C
caprolactam
Scheme 3. Depolymerization of nylon-6 in ionic liquids con-
taining 3 and 4.
Table 2. Depolymerization of nylon-6
Yield %a
Entry
DMAP salt
1st
2nd
3rd
4th
5th
1
2
3
4
3a
IÀ
70
64
61
67
67
74
66
80
79
38
77
75
61
67
72
78
42
42
76
79
This work was partially supported by Grant-in-Aid for Sci-
entific Research on Priority Areas (Science of Ionic Liquids,
2005–2009) from The Ministry of Education, Culture, Sports,
Science and Technology, Japan.
4a
3b
4b
Tf2NÀ
IÀ
Tf2NÀ
aIsolated yields.
References and Notes
The last step of the preparation of 4 was anion exchange of
3, which was performed by simple extraction method to give 4a
and 4b in good yields. Unfortunately, all of these ammonium
salts 3 and 4 were solid at room temperature.
With desired DMAP-containing ammonium salts 3 and 4 in
hand, we examined the depolymerization of nylon-6 (Scheme 3).
The depolymerization was carried out in [PP13][TFSA] in the
presence of catalytic amounts of 3 and 4. The results are summa-
rized in Table 2.
The ionic liquids containing ammnonium salts 3 and 4
smoothly depolymerized nylon-6 in good yield. A mixture of ny-
lon-6 chips were added to a mixed solvent of [PP13][TFSA] with
4a and heated at 300 ꢀC. After 1 h, the mixture became homoge-
neous oil. Depolymerized caprolactam was obtained in 64%
yield (Entry 2, 1st) by distillation of the oil using Kugelrohr ap-
paratus for 6 h. All of the ammonium salts prepared here were
effective for the depolymerization reaction and caprolactam
was isolated in more than 60% yield (Entries 1–4, 1st). The
counter anion of the salts did not cause a significant difference
in the reaction.
1
2
Technology for Feedstock Recycling of Plastic Wastes, ed.
by Research Association of Feedstock Recycling of Plastics,
Japan, CMC Books, Tokyo, Japan, 2005.
a) A. Kamimura, Y. Oishi, K. Kaiso, T. Sugimoto, K.
Kashiwagi, ChemSusChem 2008, 1, 82. b) S. R. Shukla,
A. M. Harad, D. Mahato, J. Appl. Polym. Sci. 2006, 100,
186. c) M. Goto, M. Sasaki, T. Hirose, J. Mater. Sci. 2006,
41, 1509. d) T. Iwaya, M. Sasaki, M. Goto, Polym. Degrad.
Stab. 2006, 91, 1989. e) L. Meng, Y. Zhang, Y. Huang, M.
Shibata, R. Yosomiya, Polym. Degrad. Stab. 2004, 83,
389. f) O. Sato, Y. Ikushima, Kobunshi Ronbunshu 2001,
58, 533. g) M. Goto, M. Umeda, A. Kodama, T. Hirose, S.
Nagaoka, Kobunshi Ronbunshu 2001, 58, 548. h) H.
Bockhorn, A. Hornung, U. Hornung, J. Weichmann, Ther-
mochim. Acta 1999, 337, 97. i) S. Czernik, C. C. Elam,
R. J. Evans, R. R. Meglen, L. Moens, K. Tatsumoto, J. Anal.
Appl. Pyrolysis 1998, 46, 51. j) A. Ballistreri, D. Garozzo, M.
Giuffrida, G. Impallomeni, G. Montaudo, Polym. Degrad.
Stab. 1989, 23, 25. k) H. Ohtani, T. Nagaya, Y. Sugimura,
S. Tsuge, J. Anal. Appl. Pyrolysis 1982, 4, 117. l) I.
Repeated use of the ionic liquid system was also examined.
Nylon-6 chips were added to the recovered mixed ionic liquid
and the resulting mixture was heated at 300 ꢀC under reduced
pressure for 6 h. Caprolactam was collected in a similar manner.
The yield of caprolactam was usually about 70–80%, so effec-
tive depolymerization was accomplished by using these ionic
liquids several times. It should be mentioned that no addition
of DMAP was needed for the effective depolymerization with
the present mixed ionic liquids system. Thus, DMAP installed
in the cationic part of ammonium salt survived under the high
temperature conditions and catalysed the reaction several times
effectively. Use of 3a and 4a tended to lower the yield of the de-
polymerization the 5th time; this is probably due to the partial
Luderwald, F. Merz, M. Rothe, Angew. Makromol. Chem.
1978, 67, 193. m) N. Ogata, Bull. Chem. Soc. Jpn. 1961,
34, 1201.
a) A. Kamimura, S. Yamamoto, Org. Lett. 2007, 9, 2533. b)
A. Kamimura, S. Yamamoto, Polym. Adv. Technol. 2008, 19,
1391.
Recent review: A. C. Spivey, S. Arseniyadis, Angew. Chem.,
Int. Ed. 2004, 43, 5436.
A. R. Katritzky, B. L. Duell, R. P. Seiders, H. D. Durst,
Langmuir 1987, 3, 976.
a) S. Wagaw, S. L. Buchwald, J. Org. Chem. 1996, 61, 7240.
b) Q. Shen, T. Ogata, J. F. Hartwig, J. Am. Chem. Soc. 2008,
130, 6586.
¨
3
4
5
6
Published on the web (Advance View) October 3, 2009; doi:10.1246/cl.2009.1016