Poly(4-vinylimidazolium) iodides: A highly recyclable organocatalyst precursor for benzoin condensation reaction

Article abstract of DOI:10.1039/c4ra05073h

The development of highly efficient, recyclable poly(4-vinylimidazolium) iodides (2) for the benzoin condensation reaction under mild reaction conditions is discussed: poly(4-vinyl N-heterocyclic carbene)s (3) obtained from 2 showed higher catalytic activity than monomeric 4-vinyl N-heterocyclic carbene and could be successfully recovered and reused over seven times without loss of performance. the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra05073h

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Poly(4-vinylimidazolium) iodides: a highly
recyclable organocatalyst precursor for benzoin
Cite this: RSC Adv., 2014, 4, 32371
condensation reaction†
Received 29th May 2014
Accepted 16th July 2014
Ue Ryung Seo and Young Keun Chung*
DOI: 10.1039/c4ra05073h
www.rsc.org/advances
The development of highly efficient, recyclable poly(4- in three successive benzoin condensation reactions (67%, 66%,
vinylimidazolium) iodides (2) for the benzoin condensation reaction and 64% yields, respectively).
under mild reaction conditions is discussed: poly(4-vinyl N-hetero-
Recently, poly(NHC)s iodides, being polymeric ionic liquids
cyclic carbene)s (3) obtained from 2 showed higher catalytic activity (PILs) or poly(ionic liquid)s, obtained from 4-vinylimidazolium
than monomeric 4-vinyl N-heterocyclic carbene and could be iodide have attracted our interest. Because PILs can combine
successfully recovered and reused over seven times without loss of the unique properties of ionic liquids and mechanical stability
performance.
of polymers, they would offer a great versatility in the catalyst
design. we envisioned that poly(4-vinylimidazolium) iodides
8
would be a very useful polymeric support material and function
Organocatalysis related to green chemistry has attracted much
attention and has signicantly progressed in recent years.
9,10
as a polymerized catalyst (Scheme 1). In our previous study,
1
10
we reported the preparation of poly(4-vinylimidazolium)s and
Imidazoles obtained from heterocycles are widely used as Lewis
its application in the cycloaddition of CO
diazabicyclo[5.4.0]undec-7-ene (DBU) and ZnBr
2
to epoxides using 1,8-
. Herein we
2
basic organocatalysts. One of the most studied reactions by
2
organocatalysis is the benzoin condensation reaction which
communicate the use of poly(4-vinylimidazolium)s as an
organic precatalyst for N-heterocyclic carbene-catalyzed
benzoin condensation reaction and the tandem reaction of
benzaldehyde and methyl acrylate to afford g-butyrolactone.
The in situ generated poly(NHC)s exhibits higher catalytic
activity than the corresponding monomeric analog and can be
recycled repeatedly without loss in performance in the benzoin
condensation reaction. Recently, Taton et al. reported the use of
poly(1-vinyl-3-alkylimidazolium)s as a precatalyst in benzoin
affords a-hydroxyketones (acyloins) via the self-condensation of
3
two aromatic aldehydes. a-Hydroxyketones are important
building blocks in organic synthesis and found in many bio-
4
logically active compounds.
Organocatalytic reactions are usually carried out under
homogeneous conditions. However, due to the economic and
environmental issues, many studies have been focused on
reducing waste and reusing materials. In this regard, several
studies on the heterogenization of organocatalysts using
11
condensation reactions. However, poor recyclability with a
5
6
organic polymers or mesoporous materials have been repor-
ted. However, some of them suffered from relatively low yields
and poor recyclability, probably because of poor stability or
degradation of the catalysts under the basic conditions.
Recently, a self-supported poly(NHC) (NHC ¼ N-heterocyclic
carbene), with in situ generated NHCs orthogonally positioned
lower yield was observed. As far as we are aware, our study
shows the rst successfully recyclable catalytic system for
benzoin condensation reaction.
7
along a main chain was reported. However, the catalytic activity
of this catalyst for benzoin condensation was not satisfactory.
The recyclability of the catalyst (10 mol%) was investigated only
Department of Chemistry, College of Natural Sciences, Seoul National University,
Seoul 151-747, Korea. E-mail: ykchung@snu.ac.kr
†
Electronic supplementary information (ESI) available: All experimental
1
13
procedures, characterization data, and the copies of the H NMR and C NMR
spectra are provided. See DOI: 10.1039/c4ra05073h
Scheme 1 Synthesis of 1 and 2 from histamine.
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Starting from histamine, 4-vinylimidazolium (1) and poly- reaction, the best result was achieved (entry 12). The optimum
1
0
(
4-vinylimidazolium)s (2) were prepared. With 1 and 2 in reaction conditions were as follows: 7 mol% 2, 0.3 equiv. DBU, 1
ꢀ
hand, their abilities to catalyze the benzoin condensation were mL DMF, 40 C, and 24 h. Moreover, the catalytic activity of 2
investigated (eqn (1)).
was higher than that of 1 (entry 12 vs. 13).
Using the optimized reaction conditions, the catalytic
activity of 1 and 2 were investigated for diverse functionalized
derivatives (Table 2). Benzoins were obtained as the major
products with benzil as the byproduct in various yields (less
than 7%). Interestingly, the yields and distributions of the
products were highly dependent on the substituent on the
benzaldehydes. The yields of benzoin products in the presence
of 2 were moderate to excellent (48–96%). The electronic and
steric nature of the substituents did not affect the yield of the
benzoins. The total yields of the benzoin and benzil in the
presence of 2 were moderate to excellent (52–97%). As expected,
higher yields were observed in the presence of 2 in all the cases
than 1. Strangely, when 4-methoxybenzaldehyde was used as the
substrate, a poor yield (<10%) was obtained. However, when the
(
1)
First, the reaction conditions were investigated, including
the reaction temperature, solvent and base to optimize the yield
12
of benzoin (Table 1).
The yield of the reaction in the presence of 2 was highly
sensitive to the base, reaction solvent, and reaction tempera-
ture. In the absence of a base or in the presence of triethylamine
TEA) as the base, no reaction was observed (entries 1 and 3). A
particular range of reaction temperature (40–50 C) in dimethyl
(
ꢀ
ꢀ
reaction temperature was raised to 80 C, the expected reaction
formamide (DMF) maximized the yield of the reaction (entries
product was obtained in 77% yield and with 21% recovery of the
reactant (entry 5). Notably, pyridine-3-carboxaldehyde in the
presence of 2 afforded a benzil derivative, 1,2-di(pyridine-3-yl)
ethane-1,2-dione, as the only product in 93% yield (by H
NMR) (data not shown in Table 2). It has been reported that 2-
5–9). The amount of DBU could be reduced to 0.3 equiv. without
decreasing the yield (entries 10–11). As shown in Table 1,
benzoin was obtained as the major product with benzil in
various yields as the byproduct. The yields and product distri-
butions were also highly dependent on the trituration method
1
pyridinecarboxaldehyde is easily oxidized to a-pyridil in meth-
(
entry 11 vs. 12) because the remaining DBU functioned as a
15
anol at room temperature in air.
13
catalyst in the conversion of benzoin to benzil. The acidica-
tion of reaction mixture before exposing to air protects the
transformation of benzoin to benzil in air. Thus, when the
reaction mixture was treated with 4 M HCl in dioxane aer the
The reusability of 2 was also examined (Fig. 1). The polymer
catalyst was recovered by the addition of excess acetone into the
reaction mixture. The precipitate was ltered and washed with
acetone and dried. Because a small amount of 2 was used as the
catalyst, the effect of loss during the separation seemed to be
very signicant. Aer seven reaction cycles, 15% of 2 was lost.
As shown in Fig. 1, the catalyst was successfully reused
without loss of performance over seven cycles. Our excellent
result is in contrast to that obtained for poly(1-vinyl-
a
Table 1 Optimization of reaction conditions
11
imidazolium)s. In that study, poor recyclability with lower
yields was observed even aer the rst and subsequent recycling
Isolated yield (%)
Table 2 Benzoin reactions with various benzaldehydes
ꢀ
Entry Base
Solvent Temp ( C) Benzoin Benzil Total
1
2
3
4
5
6
7
8
9
1
1
1
1
None
t-BuOK
TEA
DMF
DMF
DMF
40
60
60
60
60
80
50
40
r.t
40
40
40
40
0
11
0
0
9
0
0
20
0
DBU (1 eq.)
DBU (1 eq.)
DBU (1 eq.)
DBU (1 eq.)
DBU (1 eq.)
DBU (1 eq.)
DBU (0.5 eq.) DMF
DBU (0.3 eq.) DMF
DBU (0.3 eq.) DMF
DBU (0.3 eq.) DMF
H
2
O
0
1
1
DMF
DMF
DMF
DMF
DMF
60
31
69
62
21
74
82
96
62
21
20
22
33
21
22
15
1
81
51
91
95
42
96
97
97
72
a
a
Yield by 1
Yield by 2
Entry
R
Benzoin Benzil Total Benzoin Benzil Total
1
2
3
4
5
6
7
H
69
69
40
68
3
11
9
5
0
72
80
49
73
75
45
76
96
81
48
72
77
71
80
1
7
4
3
0
4
4
97
88
52
75
77
75
84
0
1
2
3
m-Br
p-Br
p-Me
b
c
10
b
p-MeO 75
a
Reaction conditions: 1.8 mmol benzaldehyde, 7 mol% catalyst,
p-CF3
p-Cl
40
68
5
8
appropriate equiv. of base were reacted in 1 mL solvent were reacted
under nitrogen atmosphere. Acidication aer reaction. 1 as a
catalyst precursor.
b
c
a
b
ꢀ
Isolated yields. Reaction temp. 80 C.
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a,b
Table 3 Reaction of aromatic aldehydes with methyl acrylate
Yield (%) by 1
Yield (%) by 2
Entry
R
Lactone
Ally alcohol
Lactone
Ally alcohol
1
2
3
4
5
H
60
26
60
34
8
6
78
44
72
42
6
2
m-Br
p-Me
p-Cl
Fig.
reaction.
1
Recycling of polymer catalyst in benzoin condensation
Trace
Trace
Trace
Trace
Trace
Trace
c
d
p-MeO
50
52
a
Reaction conditions: 1.8 mmol benzaldehyde, 7 mol% catalyst, 0.3
ꢀ
equiv. DBU, 1 mL of DMF at 40 C for 24 h under N
2
/18 mmol methyl
catalytic reactions and
a partial deactivation of poly-
b
c
acrylate for 5 h. Isolated yield. 23% of reactant was recovered.
(1-vinylNHC)s due to the trace impurities was proposed. The
d
ꢀ
Reaction temp. 80 C and 36% of reactant was recovered.
deactivation of in situ generated poly(1-vinylNHC)s is more
ꢀ
likely to occur because of relatively high temperature (80 C).
However, we expected that the signicant difference in the
reusability between poly(1-vinylNHC)s and poly(4-vinylNHC)s
might arise from the stability of poly(NHC)s under the reac-
tion and workup conditions.
Zhai et al. reported the one-step synthesis of g-butyro-
lactones from benzoins/benzaldehydes and methyl acrylate in
the presence of a catalyst generated from the reaction of 1,3-
dimethylimidazolium with a base. We also investigated the
tandem reaction of benzaldehyde and methyl acrylate catalyzed
by 2 in the presence of DBU. Aer the reaction, g-butyrolactone
was inferior to that of the catalyst with only one imidazolium
moiety, probably because of the increase in the steric hindrance
by adding of an imidazolium ring. The highly enhanced reac-
tivity of 2 may be attributed to the synergistic effect between the
16
7,19
catalytically active sites along the polymer backbone
or a
higher density of active sites. In addition, the active sites linked
to the polymer backbone in a pendant fashion may help to
increase the reactivity.
was isolated in 78% yield and with a concomitant formation of Conclusions
an allylic alcohol in 6% yield (eqn (2)).
In conclusion, we developed a polymer-based organocatalytic
system (2) that shows high catalytic activity in benzoin
condensation reaction and the tandem formation of g-butyro-
lactone from benzaldehyde and methyl acrylate. Precatalyst 2
showed higher catalytic activity than monomeric analog (1).
Moreover, 2 showed higher catalytic activity and reusability in
the benzoin condensation reaction compared to the polymer-
ized precatalysts obtained from 1-vinylimidazolium. Organo-
catalytic system 2 synthesized in three steps from commercially
available materials, has great potential for practical use and
recovery of the polymerized catalysts in benzoin condensation
reaction. Further investigations of the polymer based organo-
catalytic system to other reactions are ongoing in our laboratory.
(
2)
DBU-catalyzed Baylis–Hillman reaction afforded allylic
alcohol. However, allylic alcohol was not obtained in the
17
t
16
presence of 1,3-dimethylimidazolium and a base (KO Bu). We
investigated the substrate scope of this tandem reaction in the
presence of 1 or 2 as the catalyst precursor (Table 3). In the
presence of 2 and DBU, the yields of lactones were reasonable to
high (44–78%). However, the yields in the presence of 1 and
DBU were poor to moderate (26–60%). Thus, precatalyst 2
afforded lactones in higher yields than 1. Allylic alcohols were
obtained in better yields for benzaldehyde and m-bromo- Acknowledgements
benzaldehyde than for other aldehydes (trace amounts). When
p-methoxybenzaldehyde was used, a considerable amount of
This work was supported by the National Research Foundation
of Korea (NRF) grant funded by the Korea Government (2007-
093864 and 2014-011165). URS thanks the Brain Korea 21 Plus
Fellowships.
the reactant was recovered.
0
7
Recently, Cowley et al. synthesized poly(NHC)s, with a
different backbone from our poly(NHC)s, and used in benzoin
condensation reaction. They also observed higher catalytic
activity of poly(NHC)s than the corresponding monomeric Notes and references
18
analog. Xie et al. investigated di- and triimidazolium salts as
the catalysts for benzoin condensation reaction. In contrast to
our results, the catalytic activity of di- and triimidazolium salts
1
(a) D. Enders, O. Niemeier and A. Henseler, Chem. Rev., 2007,
07, 5606–5655; (b) P. H.-Y. Cheong, C. Y. Legault, J. M. Um,
1
¨
N. Çelebi-Olç u¨ m and K. N. Houk, Chem. Rev., 2011, 111,
This journal is © The Royal Society of Chemistry 2014
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5
042–5137; (c) C. M. R. Volla, I. Atodiresei and M. Rueping,
6 (a) S. Shylesh, Z. Zhou, Q. Meng, A. Wagener, A. Seifert,
S. Ernst and W. R. Thiel, J. Mol. Catal. A: Chem., 2010, 332,
65–69; (b) Z. Zhou, Q. Meng, A. Seifert, A. Wagner, Y. Sun,
S. Emst and W. R. Thiel, Microporous Mesoporous Mater.,
2009, 121, 145–151.
Chem. Rev., 2014, 114, 2390–2431; (d) N. Mase and
C. F. Barbas, III, Org. Biomol. Chem., 2010, 8, 4043–4050; (e)
R. C. Wende and P. R. Schreiner, Green Chem., 2012, 14,
1
821–1849; (f) J.-F. Bri `e re, S. Oudeyer, V. Dalla and
V. Levacher, Chem. Soc. Rev., 2012, 41, 1696–1707; (g)
D. J. Liu and E. Y.-X. Chen, Green Chem., 2014, 16, 964–
7 A. B. Powell, Y. Suzuki, M. Ueda, C. W. Bielawski and
A. H. Cowley, J. Am. Chem. Soc., 2011, 133, 5218–5220.
8 (a) J. Texter, Macromol. Rapid Commun., 2012, 33, 1996–2014;
(b) J. Yuan, D. Mecerreyes and M. Antonietti, Prog. Polym.
Sci., 2013, 38, 1009–1036; (c) D. Mecerreyes, Prog. Polym.
Sci., 2011, 36, 1629–1648.
9 (a) H. G. Alt, J. Chem. Soc., Dalton Trans., 1999, 1703–1710; (b)
J. Zhang, X. Wang and G.-X. Jin, Coord. Chem. Rev., 2006, 250,
95–109 and references therein; (c) A. Zulauf, M. Mellah and
E. Schulz, J. Org. Chem., 2009, 74, 2242–2245.
9
81; (h) J. Alem ´a n and S. Cabrera, Chem. Soc. Rev., 2013,
42, 774–793; (i) X. Bugaut and F. Glorius, Chem. Soc. Rev.,
2012, 41, 3511–3522.
2
(a) A. Grossmann and D. Enders, Angew. Chem., Int. Ed.,
012, 51, 314–325; (b) K. Hirano, I. Piel and F. Glorius,
2
Chem. Lett., 2011, 40, 786–791; (c) T. Dr ¨o ge and F. Glorius,
Angew. Chem., Int. Ed., 2010, 49, 6940–6952; (d) H. Zhao,
F. W. Foss, Jr and R. Breslow, J. Am. Chem. Soc., 2008, 130,
1
2590–12591; (e) S. E. Denmark and L. B. Gregory, Angew. 10 U. R. Seo and Y. K. Chung, Adv. Synth. Catal., 2014, 356,
Chem., Int. Ed., 2008, 47, 1560–1638; (f) N. Marion, S. D ´ı ez-
1955–1961.
Gonz ´a lez and S. P. Nolan, Angew. Chem., Int. Ed., 2007, 46, 11 (a) J. Pinaud, J. Vignolle, Y. Gananou and D. Taton,
2
988–3000; (g) B. Bantu, G. M. Pawar, K. Wurst, U. Decker,
A. M. Schmidt and M. R. Buchmeiser, Eur. J. Inorg. Chem.,
009, 1970–1976; (h) B. Bantu, G. M. Pawar, U. Decker,
K. Wurst, A. M. Schmidt and M. R. Buchmeiser, Chem.–
Macromolecules, 2011, 44, 1900–1908; (b) P. Coupillaud,
J. Pinaud, N. Guidolin, J. Vignolle, M. F `e vre,
E. Veaudecrenne, D. Mecerreyes and D. Taton, J. Polym.
Sci., Part A: Polym. Chem., 2013, 51, 4530–4540.
2
Eur. J., 2009, 15, 3103–3109; (i) K. Thiel, R. Zehbe, 12 Aer the reaction was carried out, excess acetone was poured
J. Roeser, P. Strauch, S. Enthalerd and A. Thomas, Polym.
Chem., 2013, 4, 1848–1856; (j) K. Zeitlera and I. Magera,
Adv. Synth. Catal., 2007, 349, 1851–1857; (k) A. B. Powell,
C. W. Bielawski and A. H. Cowley, Comments Inorg. Chem.,
into the reaction ask to precipitate 2. The precipitates were
ltered off and the ltrate was added to a solution of 4 M HCl
in dioxane. Aer the solution was ltered off, the ltrate was
concentrated and chromatographed on a silica gel column.
13 Y. Shimakawa, T. Morikawa and S. Sakaguchi, Tetrahedron
Lett., 2010, 51, 1786–1789.
2010, 31, 75–82.
3
(a) Z.-J. Jia, K. Jiang, Q.-Q. Zhou, L. Dong and Y.-C. Chen,
Chem. Commun., 2013, 49, 5892–5894; (b) P. Dominguez de 14 Aer the reaction was carried out, a solution of 4 M HCl in
Maria and S. Shanmuganathan, Curr. Org. Chem., 2011, 15,
083–2097; (c) K. Thai, S. M. Langdon, F. Bilodeau and
M. Gravel, Org. Lett., 2013, 15, 2214–2217; (d) T. Ema,
K. Akihara, R. Obayashi and T. Sakai, Adv. Synth. Catal.,
dioxane was added to the reaction mixture. The resulting
solution was stirred for 1 h. Excess acetone was poured
into the reaction ask. Aer precipitates were ltered off,
the ltrate was concentrated and chromatographed on a
silica gel column.
2
2012, 354, 3283–3290.
4
5
C. Palomo, M. Oiarbide and J. M. Garc ´ı a, Chem. Soc. Rev., 15 V. Hada, A. Tungler and L. Szepesy, J. Catal., 2002, 209, 472–
012, 41, 4150–4164 and references therein. 479.
(a) J. D. Storey and C. Williamson, Tetrahedron Lett., 2005, 46, 16 W. Ye, G. Cai, Z. Zhuang, X. Jia and H. Zhai, Org. Lett., 2005,
337–7339; (b) K. Thiel, R. Zehbe, J. Roeser, P. Strauch, 7, 3769–3771.
S. Enthaler and A. Thomas, Polym. Chem., 2013, 4, 1848– 17 V. K. Aggarwal and A. Mereu, Chem. Commun., 1999, 2311–
2
7
1
3
856; (c) K. Zeitler and I. Mager, Adv. Synth. Catal., 2007,
49, 1851–1857; (d) M. Tan, Y. Zhang and J. Y. Ying, Adv. 18 G. Gao, R. Xiao, Y. Yuan, C.-H. Zhou, J. You and R.-G. Xie, J.
Chem. Res., Synop., 2002, 262–263.
M. R. Buchmeiser, Adv. Synth. Catal., 2010, 352, 917–928; 19 P. K. Dahl, B. B. De and S. Sivaram, J. Mol. Catal. A: Chem.,
f) O. Bortolini, A. Cavazzini, P. Dambruoso, 2001, 177, 71–87.
2312.
Synth. Catal., 2009, 351, 1390–1394; (e) G. M. Pawar and
(
P. P. Giovannini, L. Caciolli, A. Massi, S. Pacico and
D. Ragno, Green Chem., 2013, 15, 2981–2992.
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