Selectivity enhancement of silica-supported sulfonic acid catalysts in water by coating of ionic liquid

Article abstract of DOI:10.1021/ol071356j

Coating of silica-supported sulfonic acid catalysts with hydrophobic ionic liquid leads to a significant improvement of catalyst selectivity. Many organic reactions, including Prins cyclization, cycloaddition of epoxide to aldehyde, and dehydrative etherification of secondary benzyl alcohols, proceed well in formalin or pure water. In particular, tandem dehydration/Prins cyclization reactions of tertiary and secondary alcohols with formaldehyde were developed for the first time.

Full text of DOI:10.1021/ol071356j

ORGANIC
LETTERS
2007
Vol. 9, No. 16
3145-3148
Selectivity Enhancement of
Silica-Supported Sulfonic Acid Catalysts
in Water by Coating of Ionic Liquid
Yanlong Gu, Ayman Karam, Franc¸ois Je´roˆme,* and Joe1l Barrault
Laboratoire de Catalyse en Chimie Organique, UMR 6503, CNRS-UniVersite´ de
Poitiers, ESIP, 40 AVenue du Recteur, Poitiers Cedex 86022, France
francois.jerome@uniV-poitiers.fr
Received June 8, 2007
ABSTRACT
Coating of silica-supported sulfonic acid catalysts with hydrophobic ionic liquid leads to a significant improvement of catalyst selectivity.
Many organic reactions, including Prins cyclization, cycloaddition of epoxide to aldehyde, and dehydrative etherification of secondary benzyl
alcohols, proceed well in formalin or pure water. In particular, tandem dehydration/Prins cyclization reactions of tertiary and secondary alcohols
with formaldehyde were developed for the first time.
Immobilization of molecular catalysts onto a solid support
is a promising strategy to facilitate the separation of the
catalyst from the products. However, this approach generally
decreases the catalytic performance, including the activity
and/or selectivity. To solve this problem, researchers delib-
erately modified the surface of catalysts to be hydrophobic,
hydrophilic, or amphiphilic with the hope of accelerating
approach of substrate to the active centers.1 Even if such
approaches were efficient to improve the catalytic activity,
these strategies were far less efficient to improve the
selectivity of solid catalysts, which still remains a huge
challenge for heterogeneous catalysis. For the moment, one
of the most common methods to improve the selectivity of
heterogeneous catalysts is to use a shape-selective material
as solid support or directly as catalyst.2 Unfortunately, most
of these shape-selective systems are difficult to design and
prepare. Furthermore, the substrate scope of shape-selective
catalysts was rather limited because their selectivities are
strictly dependent on the substrate hindrance.
Recently, Kobayashi and co-workers have developed
silica-supported metal catalysts with hydrophobic ionic
liquids for organic reactions in water.3 It was observed that
activities of silica-supported catalysts were significantly
improved by coating of ionic liquids. The authors suggested
that the ionic liquid creates a hydrophobic environment on
the siliceous surface resulting in better diffusion of organic
substrates to the catalytic sites. However, if it is now well-
(1) See some recent examples for hydrophobic derivatization: (a) Liu,
P. N.; Gu, P. M.; Wang, F.; Tu, Y. Q. Org. Lett. 2004, 6, 169. (b) Zhang,
H.; Zhang, Y.; Li, C. J. Catal. 2006, 238, 369. (c) Yang, Q.; Ma, S.; Li, J.;
Xiao, F.; Xiong, H. Chem. Commun. 2006, 2495. (d) Ahu, B. Y.; Seok, S.
I.; Baek, I. C.; Hong, S.-I. Chem. Commun. 2006, 189. For hydrophilic
derivatization: (e) Bass, J. D.; Anderson, S. L.; Katz, A. Angew. Chem.,
Int. Ed. 2003, 42, 5219. (f) Notestein, J. M.; Katz, A. Chem. Eur. J. 2006,
12, 3954. (g) Bass, J. D.; Solovyov, A.; Pascall, A. J.; Katz, A. J. Am.
Chem. Soc. 2006, 128, 3737. For amphiphilic derivatization: (h) Otani,
W.; Kinbara, K.; Zhang, Q.; Ariga, K.; Aida, T. Chem. Eur. J. 2007, 1731.
(2) (a) Tada, M.; Iwasawa, Y. Chem. Commun. 2006, 2833. (b) Roeffaers,
M. B. J.; Sels, B. F.; Uji-i, H.; De Schryver, F. C.; Jacobs, P. A.; De Vos,
D. E.; Hofkens, J. Nature 2006, 439, 572.
(3) (a) Gu, Y.; Ogawa, C.; Kobayashi, J.; Mori, C.; Kobayashi, S. Angew.
Chem, Eng. Ed. 2006, 45, 7217. (b) Gu, Y.; Ogawa, C.; Kobayashi, S.
Org. Lett. 2007, 9, 175.
10.1021/ol071356j CCC: $37.00
© 2007 American Chemical Society
Published on Web 07/12/2007

established that there exists a clear synergistic effect between
ionic liquids and organomodified silica, no example of
selectiVity improVement resulting from this strategy was
reported yet. Herein, we show that the coating of silica-
supported sulfonic acids with hydrophobic ionic liquid not
only increases the catalyst activity in water but also consider-
ably improves the process selectivity in many Brønsted acid-
catalyzed reactions. This association between ionic liquid
and organomodified silica offers new routes for selectivity
improvement of heterogeneous catalysts (Figure 1).
by Reddy et al., a very messy mixture composed of oligomers
of R-methylstyrene and their corresponding Prins adducts
was produced (Table 1, entry 1). To our great delight, when
Table 1. Prins Cyclization of R-Methylstyrene in Water Using
Formalin as Formaldehyde Sourcea
entry
catalyst
SiO2-SO3H 1
SiO2-SO3H 1-[C8MIm]NTf2
no catalyst
H2SO4
HCl
CF3SO3H
HBAILe
yield (%)
1
2b
3
4
5
6
7
23c
94
0
42d
30d
<5c
<5c
Figure 1. Silica materials and ionic liquid.
a Mole ratio of formaldehyde to R-methylstyrene is 4:1. b 25 wt % IL
(respect to weight of silica catalyst) was deposited. c Yield determined at
total conversion, in this case oligomerization mainly occurs. d GC yield.
e HBAIL ) [(n-C8H17)3N(CH2)4SO3H]NTf2.
Preparation of silica-supported sulfonic acids (SiO2-
SO3H) was performed according to the reported method (see
the Supporting Information).4 The obtained functionalized
silica was then added to an acetonitrile solution of [C8MIm]-
NTf2. After 5 min of stirring at room temperature, volatile
components were removed under reduced pressure affording
a powdery and free-flowing solid here named SiO2-SO3H-
IL.
The performance, activity, and selectivity of SiO2-SO3H-
IL were first investigated in the Prins cyclization, which is
an important carbon-carbon bond-forming reaction and
widely used for organic synthesis.5,6 Previously, Prins
cyclization reaction of styrene has been carried out in organic
solvents such as acetonitrile and 1,2-dichloroethane7 using
paraformaldehyde as the HCHO source. Quite recently, Gu
et al. have reported a novel hydrophobic Brønsted acidic
ionic liquid (HBAIL)-catalyzed Prins cyclization of styrene
derivatives using formalin as the HCHO source instead of
relatively expensive paraformaldehyde.8 However, the cata-
lytic activity of HBAIL was far from satisfactory, and the
substrate scope was rather limited. Reddy et al.9 reported a
Prins cyclization using well-organized mesoporous silica
(SBA-15) functionalized with sulfonic acid groups as
catalyst. However, this acidic mesoporous silica was unable
to catalyze the reaction in formalin.
25 wt % of [C8MIm]NTf2 was loaded on SiO2-SO3H 1, a
significant yield improvement was observed affording 2a in
94% yield (Table 1, entry 2). This clearly indicates the
efficiency of ionic liquid for this transformation. As a
comparison, Brønsted mineral or organic acids, such as H2-
SO4, HCl, and triflic acid were also examined, and no yield
over 50% was obtained under the same conditions (Table 1,
entries 4-6). Similarly, with HBAIL,8 an extensive polym-
erization of R-methylstyrene was observed highlighting the
high selectivity of SiO2-SO3H 1-[C8MIm]NTf2 system
(Table 1, entry 7).
Table 2 shows the substrate scope of SiO2-SO3H 1-[C8-
MIm]NTf2 catalyst for Prins cyclization in formalin. Many
styrene derivatives were smoothly converted to the corre-
sponding 1,3-dioxanes in high to excellent yields. Interest-
ingly, SiO2-SO3H 1-[C8MIm]NTf2 can be reused at least
four times without appreciable loss of activity and selectivity
(Supporting Information).
Taking into account that, under acidic conditions, alcohols
can be dehydrated to the corresponding olefins, we then
investigated the feasibility of using SiO2-SO3H-IL to
catalyze the tandem dehydration/Prins cyclization of alcohols
in formalin. As shown in Table 3, with SiO2-SO3H 1-[C8-
MIm]NTf2 as catalyst, 2-phenyl-2-propanol was successfully
converted to the desired 1,3-dioxane in 92% yield (Table 3,
entry 1). Two tertiary alcohols such as 1-phenyl-1-cyclo-
hexanol and 2-(4-chlorophenyl)propanol were also success-
fully converted to the desired 1,3-dioxane products in 94%
and 92% yields, respectively (Table 3, entries 2 and 3).
We set out to examine SiO2-SO3H 1-catalyzed Prins
cyclization of R-methylstyrene in formalin, and as observed
(4) Li, P. H.; Wang, L. AdV. Synth. Catal. 2006, 348, 681.
(5) (a) Prins, H. J. Chem. Week 1919, 16, 1072. (b) Arundale, E.;
Mikeska, L. A. Chem. ReV. 1952, 51, 505.
(6) Bach, T.; Lo¨bel, J. Synthesis 2002, 2521.
(7) (a) Yadav, J. S.; Reddy, B. V.; Bhaishya, G. Green Chem. 2003, 5,
264. (b) Li, G.; Gu, Y.; Ding, Y.; Zhang, H.; Wang, J.; Gao, Q.; Yan, L.;
Suo, J. J. Mol. Catal. Chem. 2004, 218, 147. (c) Swapna, B. S. V.; Sridhar,
C.; Saileela, D.; Sunacha, A. Synth. Commun. 2005, 35, 1177. (d) Delmas,
M.; Gaset, A. Tetrahedron Lett. 1981, 22, 723.
(8) Gu, Y.; Ogawa, C.; Kobayashi, S. Chem. Lett. 2006, 35, 1176.
(9) Reddy, S. S.; Raju, B. D.; Kumar, V. S.; Padmasri, A. H.; Narayanan,
S.; Rao, K. S. R. Catal. Commun. 2007, 8, 261.
Unfortunately, all attempts to use secondary alcohols, for
example 1-(p-tolyl)ethanol, in this tandem reaction failed due
to formation of ether and dimerization products (Table 3,
entry 4). Interestingly, when 25 wt % of [C8MIm]NTf2 was
3146
Org. Lett., Vol. 9, No. 16, 2007

Table 2. Prins Cyclization of Styrenes Using
SiO2-SO3H-[C8MIm]NTf2 as Catalyst in Formalina
Table 3. Tandem Dehydration/Prins Cyclization Reactions of
Tertiary and Secondary Alcohols Catalyzed by SiO2-SO3H with
or without Ionic Liquid in Formalina
a Mole ratio of formaldehyde to alcohol is 4:1. b Yield determined by
GC at total conversion of reactants, ether, and dimerization products were
mainly produced in this case.
a Mole ratio of formaldehyde to R-methylstyrene is 4:1. b 6 h. c 80 °C.
tions, an aqueous solution of chloroacetaldehyde (45%, w/w)
readily reacted with 1,2-dodecane oxide to give the corre-
sponding 1,3-dioxolane adduct in 92% yield (Table 4, entry
2). In pure water, analogous cycloaddition of 3-phenylpro-
pionaldehyde to 1,2-dodecane oxide also proceeded well
(93% yield) (Table 4, entry 3). Other epoxides, such as 1,2-
epoxy-9-decene and epichlorohydrin, were also applicable
in this system (Table 4, entries 4-7). Interestingly, the double
bond of 1,2-epoxy-9-decene, known to be unstable under
acidic conditions, was not affected during the catalytic
process.
Finally, the dehydrative etherifications of secondary benzyl
alcohols were investigated in water using our catalytic
system. These reactions are generally catalyzed by transition
metals such as palladium chloride,11a zinc chloride,11b bis-
(dithiobenzyl)nickel(II),11c and MeAl(NTf2)211d,e or iodine.11f
Brønsted acid catalysts are rarely used in this reaction due
to the easy dehydration of benzyl alcohols to the correspond-
ing styrene derivatives.11g,h As a perfect illustration, when
Brønsted acid SiO2-SO3H 3 was used as catalyst for
d 12 h. e Reused four times.
loaded over mesoporous silica HMS-SO3H10 instead of
SiO2-SO3H 1, a remarkable selectivity improvement was
observed and 1-(p-tolyl)ethanol was converted to 4-p-tolyl-
1,3-dioxane in 74% yield (Table 3, entry 5). It is interesting
to note that without coating of HMS-SO3H with [C8MIm]-
NTf2 only a trace amount of product was detected by GC
(Table 3, entry 6). It is difficult to offer an explanation for
these results, which suggest the existence of a complex
synergistic effect between ionic liquid and the siliceous
mesoporous structure. This point deserves further investiga-
tion.
In order to show the versatility of the SiO2-SO3H-IL
system, we then moved on to the selective transformation
of epoxides. It is well-known that epoxides are unstable under
aqueous acidic conditions and tend to form the corresponding
diols or oligomers. Remarkably, in formalin, a cycloaddition
of formaldehyde to 1,2-dodecane oxide exclusively occurred
in the presence of SiO2-SO3H 2-[C8MIm]NTf2, and the
corresponding 1,3-dioxolane derivative was obtained in 90%
yield (Table 4, entry 1). Sustrate scope of SiO2-SO3H
2-[C8MIm]NTf2 was then investigated. Under our condi-
(11) (a) Miller, K. J.; Abu-Omar, M. M. Eur. J. Org. Chem. 2003, 7,
1294. (b) Kim, S.; Chung, K. N.; Yang, S. J. Org. Chem. 1987, 52, 3917.
(c) Kesling, H. S., Jr. US 4518806, 1985. (d) Ooi, T.; Ichikawa, H.; Itagaki,
Y.; Maruoka, K. Heterocycles 2000, 52, 575. (e) Wang, C.-H. J. Org. Chem.
1963, 28, 2914. (f) Castro, B. A. J. J. Am. Chem. Soc. 1950, 72, 5311. (g)
Kalendkowska, M.; Kalendkowski, B.; Nowakowski, L.; Olkowski, H.
Neftekhimiya 1980, 20, 436. (h) Luzader, G. B. Hall, M. E. US 2927064,
1960.
(10) Karam, A.; Gu, Y.; Je´roˆme, F.; Douliez, J.-P.; Barrault, J. Chem.
Commun. 2007, 2222.
Org. Lett., Vol. 9, No. 16, 2007
3147

Scheme 1. Etherification of Secondary Benzyl Alcohols
Table 4. Cycloaddition of Epoxides to Aldehydes in Water
Using SiO2-SO3H-IL as Catalyst
a Isolated yield. b GC yield.
yield. The same tendency was also observed in the case of
1-phenyl-1-propanol (Scheme 1). These results thus show
the versatility of this strategy and clearly illustrate that the
coating of siliceous materials with ionic liquid can advan-
tageously increase their catalytic performances leading to
highly active and selective catalysts in water.
In conclusion, we show here that the coating of Brønsted
acid silica materials with hydrophobic ionic liquid [C8MIm]-
NTf2 affords heterogeneous catalysts which are able to
perform highly selective organic transformations in formalin
or pure water. This synergistic effect between organomodi-
fied silica and ionic liquids is key to render the catalytic
system selective. In particular, tandem dehydration/Prins
cyclization reactions of tertiary and secondary alcohols in
formalin were developed for the first time. This association
between ionic liquid and organomodified silica offers new
routes for selectivity improvement of heterogeneous catalysts.
Acknowledgment. We thank the French Ministry of
Research and CNRS for their financial support and acknowl-
edge the postdoctoral grant for Y.G. A.K. is also greateful
to the Syrian government for his Ph.D. grant.
a 80 °C, 3.5 h. b Epichlorohydrin was used in excess (1.5 equiv relative
to aldehyde), 5 h.
Supporting Information Available: Preparation of cata-
lyst, experimental details, and characterization of products.
This material is available free of charge via the Internet at
http://pubs.acs.org.
etherification of 1-tolyl-1-ethanol, 30% of 4-methylstyrene
was produced to the detriment of the desired ether, which
was formed in only 61% yield. The coating of SiO2-SO3H
3 with 25 wt % of [C8MIm]NTf2 led again to a significant
selectivity improvement, and the ether was obtained in 96%
OL071356J
3148
Org. Lett., Vol. 9, No. 16, 2007