Heteropolyacid salts of N-methyl-2-pyrrolidonium as highly efficient and reusable catalysts for Prins reactions of styrenes with formalin

Article abstract of DOI:10.1039/c0gc00729c

New organic heteropolyacid (HPA) salts prepared by exchanging counter protons of Keggin HPAs with N-methyl-2-pyrrolidonium proved to be highly efficient, conveniently recoverable, and steadily reusable heterogeneous catalysts for the organic solvent-free Prins reactions of styrenes with formalin. The Royal Society of Chemistry.

Full text of DOI:10.1039/c0gc00729c

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Heteropolyacid salts of N-methyl-2-pyrrolidonium as highly efficient and
reusable catalysts for Prins reactions of styrenes with formalin†
Weihong Zhang,a,b Yan Leng, Pingping Zhao, Jun Wang,* Dunru Zhu and Jun Huang
a
a
a
a
a
Received 25th October 2010, Accepted 7th January 2011
DOI: 10.1039/c0gc00729c
New organic heteropolyacid (HPA) salts prepared by ex-
bis(trifluoromethane)sulfonimide salt [C
8
MIm]NTf
2
was coated
changing counter protons of Keggin HPAs with N-methyl-
onto sulfonate silica, showing a better activity in Prins reactions
towards 1,3-dioxane. However, the complex preparation of the
7b
2
-pyrrolidonium proved to be highly efficient, conveniently
recoverable, and steadily reusable heterogeneous catalysts
for the organic solvent-free Prins reactions of styrenes with
formalin.
catalyst and the need for the expensive IL [C
less satisfactory.
8
MIm]NTf are still
2
It is noticeable that if the bulky and high valent het-
eropolyanion was used as the inorganic moiety for ILs, the
obtained ionic compounds always possessed such high melting
points that cannot be used as liquid media as conventional
1
,3-Dioxane derivatives, important intermediates and solvents
for organic syntheses, are mainly produced from Prins reactions
1
8
of olefins with aldehydes over acidic catalysts. As a kind of
ILs. Nevertheless, this may be a new approach to obtain
polyoxometalates (POMs), heteropolyacids (HPAs) are emerg-
ing as new environmentally benign acidic catalysts for various
organic transformations. Keggin and Wells–Dawson type HPAs
HPA-based solid hybrid catalysts with IL-like compositions.
Thus, we recently developed a new family of solid acids by
combining Keggin heteropolyanions with IL-forming organic
cations functionalized by propane sulfonate, and found that
the catalysts were highly active, conveniently recoverable, and
2
have been reported to be highly active homogeneous catalysts
for Prins reactions of styrene with paraformaldehyde in the
presence of organic solvents, however, when paraformaldehyde
was replaced by an aqueous solution of formaldehyde (formalin)
without additional solvents, a greener process, the conversion
9
steadily reusable in esterifications.
It is well-known that N-methyl-2-pyrrolidone can be readily
10
protonated to give N-methyl-2-pyrrolidonium (NMP), which
3
11
of styrene dropped drastically. In order to employ HPAs as
is occasionally used as the organic cation to produce acidic ILs.
recoverable heterogeneous catalysts, they are immobilized onto
porous carriers, nevertheless, this approach always suffers from
the leaching of active species and decrease in activity. Therefore,
the preparation of highly active, conveniently recoverable and
steadily reusable HPA-based catalysts for organic solvent-free
Prins reactions still remains as a challenging issue.
However, NMP has never been combined with a HPA anion. In
this study, we prepare a group of organic HPA salts via the
reaction of N-methyl-2-pyrrolidone with Keggin HPAs, and use
the resulting solids as acidic catalysts for Prins cyclizations of
styrene and its derivatives with formalin, as shown in Scheme 1
4
exemplified by [NMP] PW (phosphotungstic acid salt of NMP).
3
Currently, ionic liquids (ILs) are used as novel reaction
media due to their negligible volatility, high thermal stabil-
ity, alterable solubility, and structural versatility. For Prins
The new catalyst presented very high conversion and selectivity
5
cyclization of styrene with formaldehyde, the acidic IL 1-
butyl-3-methyl-imidazolium hydrogen sulfate [BMIm]HSO was
4
revealed to be a very effective catalyst that could be recycled
6
by vacuum distillation. For the purpose of more convenient
isolation of the catalyst, various efforts have been devoted to
7
the supported ILs; for example, 1-octyl-3-methyl-imidazolium
a
State Key Laboratory of Materials-Oriented Chemical Engineering,
College of Chemistry and Chemical Engineering, Nanjing University of
Technology, Nanjing, 210009, China. E-mail: junwang@njut.edu.cn;
Fax: +86-25-83172261; Tel: +86-25-83172264
b
School of Chemistry and Chemical Engineering, Changzhou University,
Changzhou, 213164, China
3 3
Scheme 1 Synthesis of the [NMP] PW catalyst and the [NMP] PW-
catalyzed Prins cyclization of styrene and its derivatives with an aqueous
solution of formaldehyde.
†
Electronic supplementary information (ESI) available: Experimental
1
details, H-NMR and FT-IR data, catalytic reusability, and reaction
mechanism. See DOI: 10.1039/c0gc00729c
8
32 | Green Chem., 2011, 13, 832–834
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and allowed convenient recovery and steady reuse, leading to an
organic solvent-free heterogeneous green Prins reaction.
Table 1 Reactivities of Prins reactions of styrene and its derivatives
a
3
with formalin over [NMP] PW and related catalysts
Fig. 1(A) compares the FT-IR spectrum of [NMP]
those of N-methyl-2-pyrrolidone and H 40. The four
characteristic bands for the Keggin structure of neat H
3
PW with
d
◦
e
f
Entry Catalyst
R
t/h T/ C C (%)
S (%)
3
PW12O
3
PW12
O
40
1
2
Without
4.5 : 1
4 : 1
4.5 : 1
4.5 : 1
4 : 1
3
3
3
3
3
3
3
3
8
3
2
3
3
3
95
90
95
95
90
90
95
95
90
95
90
90
90
95
11.7
87.3
98.7
0.0
98.9
61.7
-
1
H PW O
appeared at 1080, 978, 895 and 812 cm , assignable to nas(P–
O), nas(W O) (terminal oxygen), nas(W–O –W) (corner-sharing
oxygen) and nas(W–O –W) (edge-sharing oxygen), respectively.
For [NMP] PW, the four strong Keggin-structured peaks also
occurred, but with a blue shift of 978 cm to 981 cm and
3
12 40
3
4
5
6
[NH
4
]
3
PW
b
g
g
[NMP]
[NMP]
[NMP]
3
3
4
PW
98.7/95.3 99.6/99.8
4c
c
PMo
SiW
98.8
82.8
75.2
31.5
96.6
98.5
98.7
97.6
65.4
95.8
99.1
99.0
99.0
60.6
66.4
99.1
89.1
76.2
18.1
84.2
4 : 1
3
-
1
-1
7
8
9
10
11
12
1
1
CT-252
HZSM-5
98%H SO
4
4.5 : 1
4.5 : 1
4 : 1
4.5 : 1
4.5 : 1
-
1
-1
a red shift of 812 cm to 807 cm . Meanwhile, the band at
2
669 cm- for the stretching vibration of the C O bond in
N-methyl-2-pyrrolidone shifted to 1701 cm for [NMP]
Moreover, H NMR data of [NMP]
1
b
c
1
[NMP] PW
3
-
1
[NMP] PW
PW.
3
3
1
[NMP]
[NMP]H
[TPSPP]
2
HPW 4 : 1
PW 4 : 1
3
PW indicated the NMP
3
4
2
structure for the organic moiety (ESI†). These features allow
us to draw the conclusion that both organic and inorganic
h
3
PW
4.5 : 1
a
Reaction conditions: 0.4 mmol of catalyst (1.27 g for entries 7 and
, and 0.21 g for entry 9), 20 mmol of styrene ( 4-methylstyrene,
moieties of [NMP] PW retain their original structures. The
3
b
8
observed band shifts imply the formation of the ionic linkage
between the amide cation and the heteropolyanion. Elemental
analysis results (C: 5.52 wt%, N: 1.29 wt%, H: 0.92%) confirm
c
d
and a-methylstyrene). Molar ratio of formaldehyde to styrene.
e
Conversion of styrene (100%) = mmol converted styrene/mmol initial
f
styrene. Selectivity of 4-phenyl-1,3-dioxane (100%) = mmol 4-phenyl-
g
1
,3-dioxanes/mmol converted styrene. Conversion and selectivity
the catalyst’s chemical composition of (C
theoretical concentrations of C: 5.67 wt%, N: 1.32 wt%, and H:
.95%.
5
H
10NO)
3
PW12
O40 with
h
at the sixth run for testing the catalyst reusability. Triphenyl(3-
sulfopropyl)phosphonium phosphotungstate.
0
Hammett indicators (ESI†), which is comparable to those of H-
12
form zeolites. All these properties enable the newly synthesized
organic HPA salt of NMP to act as the heterogeneous catalyst
for lots of acid-catalyzed organic reactions.
Table 1 lists the catalytic performance of various catalysts for
Prins reaction of styrene with formalin. No dioxane product was
detected without using a catalyst (entry 1). The pure H
3
PW12
O
40
showed a high selectivity with a considerable conversion,
however, it caused a homogeneous system that leads to difficulty
in catalyst isolation (entry 2). The insoluble inorganic HPA salt
of ammonium caused the catalysis to be heterogeneous, but
gave only low selectivity of 61.7% (entry 3). Very interestingly,
Fig. 1 FT-IR spectra (A): (a) N-methyl-2-pyrrolidone, (b) H
and (c) [NMP] PW; and XRD patterns (B): (a) H 40, and (b)
NMP] PW.
3 40
PW12O ,
3
3
PW12O
[
3
XRD patterns of [NMP]
after drying at 100 C for 2 h) are illustrated in Fig. 1(B).
3
PW and the pure H
3
PW12
O
40 (both
the new organic HPA salt [NMP] PW not only resulted in
3
◦
a heterogeneous reaction, but also exhibited almost exclusive
selectivity to 4-phenyl-1,3-dioxane of 99.6% coupled with a very
high conversion of 98.7% (entry 4; also see Fig. S1 (ESI†)
H
3
PW12
O40 displayed a set of well resolved sharp diffraction
peaks for the secondary structure of the HPA crystal. For
[
NMP]
3
PW, some of the above featured peaks (e.g., at 2q
for optimization of reaction conditions). When using Keggin
◦
3-
of 10.19, 20.59, 23.19, 25.41, 29.39 and 34.6 ) could still be
detected. This provides a strong proof for the formation of the
similar crystal structure of the organic HPA salt of NMP to
that of Keggin phosphotungstic acid. However, the replacement
of protons by the large organic cation NMP results in the
substantial decrease in the long-range order of the crystal lattice,
demonstrated by the drastically lowered peak intensities.
12-phosphomolybdic anion (PMo12
O
40 ) and 12-silicotungstic
4
-
3-
anion (SiW12
O40 ) to replace PW12
O
40 , the prepared organic
HPA salt catalysts [NMP]
3
PMo and [NMP] SiW also provided
4
good to excellent performance (entries 5 and 6), indicating the
generalizability of the catalyst design strategy. Furthermore, in
comparison with the usual solid acidic catalysts of HZSM-5
zeolite and sulfonated resin CT-252, and the conventionally used
liquid acidic catalyst of concentrated sulfuric acid (entries 7–9),
the new organic HPA salt catalysts are much more active and/or
selective in this Prins reaction.
In addition, [NMP]
3
PW was detected to possess a high
◦
melting point of above 250 C (ESI†), mostly due to the strong
electrostatic interaction between the organic cation and the high
valent heteropolyanion. Also, its solid nature might associate
with the extended hydrogen bonding networks between anions
and cations, implied by the 3427 cm IR band for the H–O
vibration (ESI†). In addition, it was observed to be insoluble in
not only non-polar but also many polar solvents, such as water,
methanol, ethanol, formaldehyde, styrene derivatives, and so
Two styrene derivatives, 4-methylstyrene anda-methylstyrene,
were also used as the substrates over the [NMP] PW catalyst
3
-
1
(entries 10 and 11). Again, the catalyst resulted in very high
conversion and selectivity that are much higher than the previous
efforts for the synthesis of 4-methyl-4-phenyl-1,3-dioxane from
1d
a-methylstyrene using trifluoromethanesulfonic acid and the
3a
on. Its acid strength (H
0
) was between -8.2 and -3.0 tested by
Wells–Dawson HPA H
8
P
2
Mo16
V
2
O
62 as the catalysts.
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HPA salts of NMP are solids; however, they became an
insoluble liquid-like phase under working conditions of Prins
reactions. Thus, the catalysts actually brought about a liquid–
liquid heterogeneous catalysis system. At the end of the reaction,
the liquid-like catalyst was at the bottom of the mixture and
could be recovered simply by decantation. Afterwards, the
recovered catalyst was dried to resume its solid state that
was then reused for the next run. During the six-run test of
the catalyst reusability without adding any fresh catalyst, no
appreciable loss of conversion and selectivity was observed
Conclusions
In summary, we have designed and prepared a group of new or-
ganic HPA salts by combining NMP (N-methyl-2-pyrrolidium)
with Keggin type heteropolyanions. They are characterized
as high-melting point ionic solids, and proved to be novel
heterogeneous green catalysts for Prins reactions of styrene and
its derivatives with an aqueous solution of formaldehyde (for-
malin). The new catalyst [NMP] PW exhibited a very high yield
3
of 98.3% and very high selectivity of 99.6% toward the target
product 4-phenyl-1,3-dioxane in Prins cyclization of styrene with
formalin, and gave quite stable reusability demonstrated by a
six-run cycling test. The pseudoliquid behavior of the HPA salt
catalyst and the stabilization effect of the carbonyl in the amide
on the protonated formaldehyde of the reaction intermediate,
together with its solid nature and insolubility, are proposed to
account for the excellent catalytic performance.
(
entry 4; also see Fig. S2 in ESI† for details). IR spectrum for
the reused catalyst showed a very similar profile to the fresh
one (Fig. S3 in ESI†), suggesting a very durable structure of the
catalyst.
It is known from the classical mechanism for Prins cyclization
1c
of the alkene with formaldehyde (Scheme S1 in ESI†), the
formation of protonated formaldehyde by the chemisorption
of formaldehyde on the acidic site of the catalyst is the key
to the overall reaction. For [NMP]
3
PW of this study, the
Acknowledgements
carbonyl of the amide is suggested to be able to stabilize
the protonated formaldehyde by sharing its electron with
the electron-deficient center of the protonated formaldehyde
The authors are grateful for the financial support from the
National Natural Science Foundation of China (No. 20976084,
20771059, and 20476046).
13
(
Scheme 2), through which the whole reaction could be
facilitated effectively. Another interpretation of the excellent
performance of [NMP] PW may be relative to the pseudoliquid
Notes and references
3
nature of HPAs, i.e., the secondary structure of HPAs allows
the permeating of polar substrates into the bulk of HPAs,
through which the mass transfer is accelerated, just as in the
case of a homogeneous medium. It is thus proposed that
the reactant formaldehyde could penetrate into the bulk of
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[
NMP] PW, and then be chemisorbed onto the acid site of
3
NMP to generate the protonated formaldehyde that was further
stabilized therein according to Scheme 2. It is thus clear that
NMP plays a crucial role in accounting for the excellent catalytic
performance.
3
4
5
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3
Scheme 2 Stabilization effect of the amide group in [NMP] PW on the
reaction intermediate of the protonated formaldehyde.
2
7
8
9
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The partially NMP-substituted [NMP]
2
HPW and
[
NMP]H PW catalysts (entries 12 and 13) gave lower
2
conversions with the decrease of NMP content, corresponding
to the above proposal. Moreover, the very low selectivity is
suggested to relate to the two different types of acid sites:
one from the remaining protons and the other from the
10 A. Greenberg and C. A. Venanzi, J. Am. Chem. Soc., 1993, 115, 6951.
1
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NMP moiety. In addition, the control catalyst [TPSPP]
3
PW
(
entry 14), a typical sample in our earlier findings whose
9
acidity comes from sulfonated groups, presented a lower
yield of 4-phenyl-1,3-dioxane in comparison with [NMP] PW,
3
1
2, 661.
mostly due to the absence of the stabilization effect on
the protonated formaldehyde arising from NMP shown in
Scheme 2.
1
1
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8
34 | Green Chem., 2011, 13, 832–834
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