Synthesis of tetraline derivatives through depolymerization of polyethers with aromatic compounds using a heterogeneous titanium-exchanged montmorillonite catalyst

Article abstract of DOI:10.1039/c6ra20864a

A novel depolymerization of poly(tetramethylene glycol) (PTMG) with benzene to tetralin using titanium-exchanged montmorillonite (Ti⁴⁺-mont) as a solid acid catalyst is described. This catalyst is applicable to depolymerization of PTMG with some aromatic compounds. This is the first demonstration of the potential use of PTMG as a C4 synthon for organic synthesis.

Full text of DOI:10.1039/c6ra20864a

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Synthesis of tetraline derivatives through
depolymerization of polyethers with aromatic
compounds using a heterogeneous titanium-
exchanged montmorillonite catalyst†
Cite this: RSC Adv., 2016, 6, 89231
Received 19th August 2016
Accepted 12th September 2016
Zen Maeno, Hiroyuki Torii, Shota Yamada, Takato Mitsudome, Tomoo Mizugaki
and Koichiro Jitsukawa*
DOI: 10.1039/c6ra20864a
www.rsc.org/advances
A novel depolymerization of poly(tetramethylene glycol) (PTMG) with potential to act as effective heterogeneous catalysts for the
4+
benzene to tetralin using titanium-exchanged montmorillonite (Ti
-
depolymerization of other polyethers.
mont) as a solid acid catalyst is described. This catalyst is applicable to
The aim of this work is the development of novel
depolymerization of PTMG with some aromatic compounds. This is heterogeneously-catalyzed depolymerization of polyethers
the first demonstration of the potential use of PTMG as a C4 synthon using montmorillonite catalyst. Herein, we achieved the depo-
for organic synthesis.
lymerization of poly(tetramethylene glycol) (PTMG) with
benzene to tetralin using a titanium-exchanged montmoril-
4
+
lonite catalyst (Ti -mont) (Scheme 1). This catalytic system was
applied to the depolymerization of PTMG with other aromatic
compounds. This is the rst report of the depolymerization of
polyethers through the alkylation of aromatic compounds and
of the use of PTMG as a C4 synthon.
Polymer waste is managed mainly through incineration or
deposit in landlls. However, these disposal techniques can
cause environmental damage such as carbon dioxide emissions
and soil pollution. Catalytic depolymerization is a more desir-
able management method because the waste polymers can be
1
4+
The Ti -mont was prepared according to a previously re-
2
used to synthesize low-molecular-weight chemicals. Using
homogeneous catalysts, depolymerization reactions of poly-
À1 5b
ported method (total amount of acid sites: 0.26 mmol g ).
n
Depolymerization of PTMG (M z 2000) with benzene was
3
a
ethers, such as reductive cleavage to alkanes, acylative
investigated in the presence of various solid acid catalysts,
3b–d
3e,f
cleavage to esters,
and cyclization to cyclic ethers, have
n+
n+
including cation-exchanged montmorillonite (M -mont, M
been reported. However, these homogeneous catalyst systems
oen are limited by the separation of the products and the reuse
of the catalyst. To carry out green sustainable chemistry,
a heterogeneous catalyst system for the depolymerization of
n+
denotes metal species or proton). Among the M -mont catalysts
4+
tested, Ti -mont exhibited the greatest catalytic activity to afford
3+
+
2+
tetralin in 86% yield (Table 1, entry 1). The Al -, H -, and Cu -
mont also promoted the reaction, but yields were moderate to
polyethers needs to be developed.
3+
+
low (entries 3–5). The use of La -mont or the parent Na -mont
did not provide tetralin (entries 6 and 7). The commercially
available solid acid catalyst, Naon NR50, possessed extremely
low catalytic activity (entry 8), and the solid acid catalysts,
+
Montmorillonite (Na0.66(OH) Si (Al3.34Mg0.66Fe )O , Na -
4
8
0.19 20
mont) is a clay mineral consisting of negatively charged
aluminosilicate layers and exchangeable Na cations in the
interlayer spaces. Catalytically active cation species can be
introduced into the interlayer by cation exchange. The inter-
layer space of mont expands and intercalates various molecules,
2À
+
Amberlyst 36, SiO /Al O , SO /ZrO , and H -mordenite, mini-
2
2
3
4
2
4
,5
mally promoted depolymerization to tetralin (entries 9–12).
Typical homogeneous acids, p-TsOH monohydrate and tri-
6
including organic polymers. Based on these cation-exchange
uoroacetic acid, did not catalyze this depolymerization (entries
and intercalation abilities, an efficient method for the depoly-
merization of polyethers with acid chlorides to chloroalkyl
esters using a proton-exchanged montmorillonite catalyst was
7
developed. Thus, cation-exchanged montmorillonites have the
Department of Materials Engineering Science, Graduate School of Engineering Science,
Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan. E-mail:
jitkk@cheng.es.osaka-u.ac.jp; Fax: +81-6-6850-6261
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c6ra20864a
Scheme 1 Depolymerization of PTMG with aromatic compounds into
tetralin derivatives.
1
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4+
Table 1 Depolymerization of PTMG with benzene to tetralin using Table 2 Scope of aromatic compounds in Ti -mont catalyzed
a
a
various acid catalysts
depolymerization of PTMG to tetralin derivatives
b
Entry
1
Aromatic compound
Tetralin derivatives
b
Entry
Catalyst
Yield
4
+
c
1
2
3
4
5
6
7
8
9
Ti -mont
86
84
4+
c
Ti -mont (2nd reuse)
3
+
Al -mont
52
47
10
0
+
H -mont
2
+
Cu -mont
3
+
La -mont
2
3
+
Na -mont
0
3
0
Naon NR50
Amberlyst 36
10
11
12
13
14
SiO
2
/Al
/ZrO
2
O
3
0
0
0
0
2
À
SO
4
2
+
H -mordenite
d
p-TsOH monohydrate
d
CF
3
COOH
0
a
Reaction conditions: Catalyst (0.2 g), PTMG (0.3 mmol per
c
e
4
N.D.
b
oxytetramethylene unit), benzene (8 mL). Determined by GC using an
internal standard technique and based on the amount of
oxytetramethylene units in the used PTMG. A trace amount of
c
d
c
e
1
,2,3,4,5,6,7,8-octahydrophenanthrene was also generated. 0.052 mmol.
5
N.D.
4
+
1
3 and 14). These results indicate that Ti -mont acted as a highly
d
active solid acid catalyst, different from other acid catalysts for
the depolymerization of PTMG with benzene. Aer the reaction,
Ti -mont could be separated by ltration and reused without
6
4
+
loss of activity during reuse experiments (Table 1, entry 2).
The Ti -mont-catalyzed depolymerizations of PTMG with
various aromatic compounds also were investigated. Toluene
a
4+
4
+
Reaction conditions: Ti -mont (0.05 g), PTMG (0.3 mmol per
ꢀ
oxytetramethylene unit), aromatic compound (8 mL), 160 C, 12 h.
b
Determined by GC using an internal standard technique. Based on
c
ꢀ
d
reacted with PTMG to form a mixture of 5- and 6-methyltetralin the amount of oxyethylene units in PTMG. 180 C. PTMG
(
(
0.9 mmole per oxytetramethylene unit), 1,3-dimethoxybenzene
in 65% total yield (3 : 2 ratio of regioisomers) (Table 2, entry 2).
The depolymerization of PTMG with phenol also occurred,
giving the corresponding tetralins in 68% yield (1 : 1 ratio of
regioisomers) (entry 3). However, an aromatic compound con-
9.0 mmol). Not detected.
4
+
˚
expanded the interlayer distance of Ti -mont from 2.8 A to
4
taining an electron-withdrawing Cl or CF group gave no tetralin
3
˚
.5 A [Fig. S1(a) and (b)†]. Aer depolymerization, the layered
derivative, even at increased temperatures (entries 4 and 5).
Reaction of 1,3-dimethoxybenzene provided 5,7-dimethoxy-
4
+
structure of Ti -mont was maintained and the interlayer
distance decreased to the initial distance [Fig. S1(c)†]. Thus, the
reaction might proceed through a Friedel–Cras-type alkyl-
ation, where the C–O bond of PTMG is activated on the Brønsted
acid site to generate the PTMG fragment containing a carboca-
tion, which attacks the aromatic compound to form the alky-
lated aromatic compound. Subsequently, intramolecular
alkylation of the alkylated aromatic compound occurs on the
acid sites to afford the corresponding tetralin.
1
,2,3,4-tetrahydronaphthalene in 64% yield (entry 6).‡ Using
tetralin as a reactant, 1,2,3,4,5,6,7,8-octahydroanthracene and
octahydrophenanthrene were obtained in a 2 : 7 ratio (Scheme
), indicating that successive alkylation occurred during reac-
tion of PTMG with benzene. In this case, a trace amount of
,2,3,4,5,6,7,8-octahydrophenanthrene also was generated
Table 1, entry 1). Interestingly, use of aniline resulted in a 36%
-
2
1
(
8
yield of N-phenylpyrrolidine (Scheme 3). The low yield might be
due to deactivation of the acid site of Ti -mont by the N-phe-
4
+
nylpyrrolidine generated.
4
+
The addition of 2,6-lutidine to the Ti -mont-catalyzed
depolymerization of PTMG with benzene suppressed the reac-
tion, suggesting that the Brønsted acid site within the interlayer
4
+
9
space of Ti -mont is the active species. The access of PTMG
4
+
4+
into the interlayer space of Ti -mont was conrmed by XRD
measurements, which showed that treatment with PTMG tetralin.
Scheme
2
Ti -mont-catalyzed depolymerization of PTMG with
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(
c) S. Enthaler and M. Weidauer, ChemSusChem, 2012, 5,
1
195; (d) S. Enthaler, Catal. Lett., 2014, 144, 850; (e)
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(
4+
B. Neumann, H.-G. Stammler and P. Jutzi, Angew. Chem.,
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Scheme
aniline.
3 Ti -mont-catalyzed depolymerization of PTMG with
4
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In conclusion, heterogeneously-catalyzed depolymerization of
PTMG with aromatic compounds to produce tetralin derivatives
4
+
1
68, 450; (f) H. Sato, K. Hirose, K. Nagai, H. Yoshioka and
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4
+
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reused while maintaining its efficiency. The present catalytic
system demonstrates the potential of PTMG as a C4 synthon for
the synthesis of cyclic compounds, which can promote the
effective utilization of waste polymers.
5
Acknowledgements
This work was supported by JSPS KAKENHI Grant Number
24246129, 26820352, 26105003, 16K14480.
2014, 2, 574.
6
7
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Notes and references
‡
Unfortunately, the reactions of other methoxy-substituted aromatic derivatives,
such as anisole and 1,4-dimethoxybenzene, afforded the trace amount of desired
product. In these cases, hydrolysis of methoxy moieties occurred during the
reaction even although the details were unknown. At lower reaction temperature
ꢀ
8 N-Phenylpyrrolidine was previously obtained from the
reaction between aniline with 1,4-butandiol or THF. This is
the rst example of the cyclic amine formation from
polyethers using solid acid catalysts. For synthesis of N-
phenylpyrrolidine from aniline and 1,4-butandiol, (a)
Q. Zou, C. Wang, J. Smith, D. Xue and J. Xiao, Chem.–Eur.
J., 2015, 21, 9656, and references therein; for synthesis from
aniline and THF, (b) Z. Zhang, C. Miao, C. Xia and W. Sun,
Org. Lett., 2016, 18, 1522, and references therein.
(
130 C), the reaction of PTMG and anisole afforded 5- and 6-methoxy-1,2,3,4-
tetrahydronaphthalene (25% yield, 2 : 3 ratio of regioisomers), but complicated
side products, including hydrolysis product, were still observed.
1
2
J. Hopewell, R. Dvorak and E. Kosior, Philos. Trans. R. Soc., B,
009, 364, 2115.
(a) Recycling and Recovery of Plastics, ed. J. Brandrup, Hanser/
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and B. V. Beke, ChemSusChem, 2014, 7, 1579.
2
3
Recent reports on depolymerization of polyethers to low-
molecular-weight chemicals; see (a) J. Yang, P. S. White and
M. Brookhart, J. Am. Chem. Soc., 2008, 130, 17509; (b)
S. Enthaler and M. Weidauer, Chem.–Eur. J., 2012, 18, 1910;
9
It is known that 2,6-lutidine coordinates to only Brønsted acid
site. See the following paper, A. Corma, Chem. Rev., 1995, 95,
559.
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RSC Adv., 2016, 6, 89231–89233 | 89233