Organized surface functional groups: Cooperative catalysis via thiol/sulfonic acid pairing

Article abstract of DOI:10.1021/ja074761e

The synthesis and characterization of heterogeneous catalysts containing surfaces functionalized with discrete pairs of sulfonic acid and thiol groups are reported. A catalyst having acid and thiol groups separated by three carbon atoms is ca. 3 times more active than a material containing randomly distributed acid and thiol groups in the condensation of acetone and phenol to bisphenol A and 14 times more active in the condensation of cyclohexanone and phenol to bisphenol Z. Increasing the acid/thiol distance in the paired materials decreases both the activity and selectivity. This work clearly reveals the importance of nanoscale organization of two disparate functional groups on the surface of heterogeneous catalysts.

Full text of DOI:10.1021/ja074761e

Published on Web 10/12/2007
Organized Surface Functional Groups: Cooperative Catalysis
via Thiol/Sulfonic Acid Pairing
Eric L. Margelefsky,† Ryan K. Zeidan,† Ve´ronique Dufaud,‡ and Mark E. Davis*,†
Contribution from the DiVision of Chemistry and Chemical Engineering, California Institute of
Technology, Pasadena California 91125, and Laboratoire de Chimie, Ecole Normale Supe´rieure
de Lyon, 46 allee d’Italie, 69364 Lyon cedex 07, France
Received June 28, 2007; E-mail: mdavis@cheme.caltech.edu
Abstract: The synthesis and characterization of heterogeneous catalysts containing surfaces functionalized
with discrete pairs of sulfonic acid and thiol groups are reported. A catalyst having acid and thiol groups
separated by three carbon atoms is ca. 3 times more active than a material containing randomly distributed
acid and thiol groups in the condensation of acetone and phenol to bisphenol A and 14 times more active
in the condensation of cyclohexanone and phenol to bisphenol Z. Increasing the acid/thiol distance in the
paired materials decreases both the activity and selectivity. This work clearly reveals the importance of
nanoscale organization of two disparate functional groups on the surface of heterogeneous catalysts.
surface of a silica catalyst,11-15 but the arrangement of two
Introduction
different functional groups is more difficult. To our knowledge,
There is growing interest in developing multifunctional
catalyst systems where each different functional group plays a
distinctive role in the overall catalysis. Immobilization of two
or more functional groups on a solid support allows for the
possibility of spatial control of the different groups. They can
be isolated from each other, which can be useful for sequential
one-pot reactions,1-4 or they can be intimately mixed such that
direct interaction between the two groups is possible. In the
latter case, the two groups can act in concert to provide activity
greater than either could achieve alone (so-called cooperative
catalysis).5-10
there has been only one report of the organization of two
disparate functional groups into pairs on a rigid support. Bass
and Katz16 synthesized a precursor containing carbamate and
xanthate groups that was grafted onto silica and subsequently
thermolyzed to generate pairs of amine and thiol groups. No
catalytic results were reported from this bifunctional solid.
One reaction where cooperative, heterogeneous catalysis has
been well established is the synthesis of bisphenol A (BPA)
from acetone and phenol. This reaction is catalyzed by strong
acids and promoted by thiols that have been shown to increase
both the yield and selectivity of p,p′-bisphenol A over the
undesired o,p′-isomer (Scheme 1). Industrial processes typically
employ sulfonated polystyrene resins promoted by thiols, such
as cysteamine, bound to the resin by acid/base pairing.17,18 These
resins have a random arrangement of acid and thiol groups,
leading to a broad distribution of acid/thiol distances.
The mesoporous silica SBA-1519 is well suited as a support
for immobilizing organized functional groups. Its large, uniform
pore diameter (∼6 nm) provides ample room for reactant and
product diffusion, and its thick walls provide hydrothermal
stability. The rigidity of the silica matrix ensures that the bound
functional groups do not change their positioning (unlike
polymeric supports that have the potential to shrink and swell
When seeking cooperative behavior between the functional
groups, control of the distance between the reactive groups is
essential in order to optimize the catalysis for a particular
reaction. This is dramatically illustrated with enzymes that have
multiple catalytic functional groups organized within a single
active site. There have been several reports of the nanoscale
arrangement of two or more identical functional groups on the
† California Institute of Technology.
‡ Ecole Normale Supe´rieure de Lyon.
(1) Phan, N. T. S.; Gill, C. S.; Nguyen, J. V.; Zhang, Z. J.; Jones, C. W. Angew.
Chem., Int. Ed. 2006, 45, 2209-2212.
(2) Motokura, K.; Fujita, N.; Mori, K.; Mizugaki, T.; Ebitani, K.; Kaneda, K.
J. Am. Chem. Soc. 2005, 127, 9674-9675.
(3) Helms, B.; Guillaudeu, S. J.; Xie, Y.; McMurdo, M.; Hawker, C. J.; Fre´chet,
J. M. J. Angew. Chem., Int. Ed. 2005, 44, 6384-6387.
(4) Gelman, F.; Blum, J.; Avnir, D. Angew. Chem., Int. Ed. 2001, 40, 3647-
3649.
(11) Katz, A.; Davis, M. E. Nature 2000, 403, 286.
(12) Wulff, G.; Heide, B.; Helfmeier, G. J. Am. Chem. Soc. 1986, 108, 1089-
(5) Huh, S.; Chen, H. T.; Wiench, J. W.; Pruski, M.; Lin, V. Angew. Chem.,
Int. Ed. 2005, 44, 1826-1830.
1091.
(13) Tao, Y. T.; Ho, Y. H. J. Chem. Soc., Chem. Commun. 1988, 417-418.
(14) Dufaud, V.; Davis, M. E. J. Am. Chem. Soc. 2003, 125, 9403-9413.
(15) Mbaraka, I. K.; Shanks, B. H. J. Catal. 2006, 244, 78-85.
(16) Bass, J. D.; Katz, A. Chem. Mater. 2006, 18, 1611-1620.
(17) McNutt, B. W.; Gammill, B. B. U.S. Patent 3,394,089, 1968.
(18) Wieland, S.; Panster, P. In Heterogeneous Catalysis And Fine Chemicals
IV; Blaser, H. U., Baiker, A., Prins, R., Eds.; Elsevier: Amsterdam; Studies
in Surface Science and Catalysis, Vol. 108, 1997; pp 67-74.
(19) Zhao, D. Y.; Huo, Q.; Feng, J.; Chmelka, B.; Melosh, N.; Fredrickson, G.;
Stucky, G. Science 1998, 279, 548-552.
(6) Zeidan, R. K.; Hwang, S. J.; Davis, M. E. Angew. Chem., Int. Ed. 2006,
45, 6332-6335.
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(8) Miyabe, H.; Tuchida, S.; Yamauchi, M.; Takemoto, Y. Synthesis 2006,
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9
10.1021/ja074761e CCC: $37.00 © 2007 American Chemical Society
J. AM. CHEM. SOC. 2007, 129, 13691-13697
13691

A R T I C L E S
Margelefsky et al.
Scheme 1. Condensation of Phenol with Acetone, Cyclohexanone, and Hexafluoroacetone To Form Bisphenols
Scheme 2. Synthesis of Silane 1 and Grafting onto SBA-15 To Form SBA-g1
14,21-25
26
in different solvent environments). Furthermore, SBA-15 can
be easily functionalized with a wide variety of functional groups
by grafting an appropriate organosilane onto surface silanol
groups in order to form covalent bonds to the surface.
propylthiol groups are oxidized with H2O2
or HNO3
to give propylsulfonic acid groups. Such an oxidative method
is unsuitable for the preparation of bifunctional catalysts when
the second functionality is also oxidizable. Furthermore, the
oxidation is often incomplete and leads to disulfides and other
partially oxidized species.22,25,26 To circumvent this difficulty
we utilized a different route to alkylsulfonic acid groups based
on a sultone intermediate. This intermediate serves both as a
sulfonic acid precursor and as a site for anchoring a second
functionality. Reaction of the sultone ring with a thiol-containing
nucleophile generates the acid/thiol pair.
Sultone rings have been opened by a wide variety of
nucleophiles, including halides,27-29 phosphines,30,31 thiolates,32
sulfides,32 alkoxides,27,33 and amines.33-35 In particular, if the
nucleophile contains a thiol group, then the resulting material
will contain pairs of acid and thiol groups.
In order to functionalize a silica surface with sultone moieties,
the organosilane 1 was synthesized from 1,3-propanesultone and
3-iodopropyltriethoxysilane using a modification of the sultone
alkylation method of Durst and Du Manoir.36 Grafting of this
silane onto SBA-15 in refluxing toluene resulted in the
intermediate material SBA-g1 (Scheme 2). SBA-g1 is a versatile
intermediate for generating bifunctional catalysts containing a
We have recently shown20 that SBA-15 functionalized with
randomly distributed arylsulfonic acid and alkyl thiol groups is
an effective catalyst for the synthesis of BPA. We also
demonstrated that the acid and thiol groups must be near each
other to achieve high activity and selectivity: a physical mixture
of acid-functionalized and thiol-functionalized SBA-15 materials
exhibited only moderate activity, whereas when the two
functionalities were randomly distributed on the surface of the
same SBA-15 material at high loadings (at a total organic
loading ∼0.7-1.0 mmol/g) the activity was nearly 4-fold higher,
and the isomer ratio was 6-fold higher. We also showed that in
this reaction the selectivity is essentially independent of
conversion, which allows for the comparison of selectivities at
different levels of conversion. We hypothesized that the thiol
activates the protonated acetone molecule through formation
of a propylidene sulfonium species that exhibits increased
electrophilicity in addition to sterically disfavoring the o,p′-
isomer. If the acid and thiol groups are located near each other,
the thiol can rapidly attack the protonated acetone molecule,
generating the sulfonium species faster than if the groups are
spatially isolated. Therefore we hypothesized that the catalytic
activity should increase as the acid/thiol distance decreases.
Here, we report the synthesis of SBA-15 with alkylsulfonic
acid and thiol groups organized into discrete pairs on the silica
surface with varying distances between the acid and thiol groups.
The distances are fixed by changing the nature of spacing groups
between the acid and thiol functionalities, and the different
spacers also provide the ability to tune both the acid/thiol
distance and the electronic properties of the thiol. The resulting
bifunctional catalysts give high activity and selectivity in the
synthesis of bisphenol A. Pairing of the acid and thiol groups
increases catalytic activity substantially compared to a randomly
bifunctionalized catalyst.
(20) Zeidan, R. K.; Dufaud, V.; Davis, M. E. J. Catal. 2006, 239, 299-306.
(21) Margolese, D.; Melero, J. A.; Christiansen, S. C.; Chmelka, B. F.; Stucky,
G. D. Chem. Mater. 2000, 12, 2448-2459.
(22) Das, D.; Lee, J. F.; Cheng, S. F. J. Catal. 2004, 223, 152-160.
(23) Feng, Y. F.; Yang, X. Y.; Di, Y.; Du, Y. C.; Zhang, Y. L.; Xiao, F. S. J.
Phys. Chem. B 2006, 110, 14142-14147.
(24) Mbaraka, I. K.; Radu, D. R.; Lin, V. S. Y.; Shanks, B. H. J. Catal. 2003,
219, 329-336.
(25) Van Rhijn, W. M.; De Vos, D. E.; Sels, B. F.; Bossaert, W. D.; Jacobs, P.
A. J. Chem. Soc., Chem. Commun. 1998, 317-318.
(26) Lim, M. H.; Blanford, C. F.; Stein, A. Chem. Mater. 1998, 10, 467-470.
(27) Helberger, J. H.; Manecke, G.; Heyden, R. Liebigs Ann. Chem. 1949, 565,
22-35.
(28) Preston, A. J.; Gallucci, J. C.; Paquette, L. A. J. Org. Chem. 2006, 71,
6573-6578.
(29) King, J. F.; Khemani, K. C. Can. J. Chem. 1989, 67, 2162-2172.
(30) Cole, A. C.; Jensen, J. L.; Ntai, I.; Tran, K. L. T.; Weaver, K. J.; Forbes,
D. C.; Davis, J. H. J. Am. Chem. Soc. 2002, 124, 5962-5963.
(31) Paetzold, E.; Kinting, A.; Oehme, G. J. Prakt. Chem. 1987, 329, 725-
731.
(32) King, J. F.; Skonieczny, S.; Poole, G. A. Can. J. Chem. 1983, 61, 235-
243.
(33) Suga, K.; Miyashige, T.; Takada, K.; Watanabe, S.; Moriyama, M. Aust.
J. Chem. 1968, 21, 2333-2339.
(34) Erman, W. F.; Kretschmar, H. C. J. Org. Chem. 1961, 26, 4841-4850.
(35) Zeid, I.; Ismail, I. Liebigs Ann. Chem. 1974, 667-670.
(36) Durst, T.; Du Manoir, J. Can. J. Chem. 1969, 47, 1230-1233.
Results and Discussion
Alkylsulfonic acid-functionalized mesoporous silica has been
prepared through an oxidative method where surface-bound
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13692 J. AM. CHEM. SOC. VOL. 129, NO. 44, 2007

Enhanced Catalysis by Thiol/Sulfonic Acid Pairing
A R T I C L E S
Figure 1. 13C NMR (DMSO-d6) of silane 1 (top) and 13C CP/MAS NMR of SBA-g1 (bottom). * denotes solvent peak.
Table 1. Catalyst Characterization Data
entry
material
d (nm)a
S
BET (m2/g)b
D
p (nm)c
H+ (mmol/g)d
S (mmol/g)e
S/H+ (expected)
S/H+ (found)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
SBA-15
SBA-g1
SBA-A
11.3
11.5
11.6
810
613
603
6.1
6.1
6.5
N/A
N/A
0.17
0.38
0.18
0.18
0.22
0.18
0.21
0.17
0.20
0.16
0.20
0.21
N/A
0.26
0.26
0.81
0.40
1.04
0.54
N/A
N/A
1
2
2
N/A
N/A
1.5
2.1
2.2
SBA-AS-p
SBA-AT-p
SBA-AT-r
SBA-AT-s1
SBA-AT-s2
SBA-AT-s3
SBA-AT-s4
SBA-AT-s5
SBA-AT-s6f
SBA-AT-s7
SBA-AT-s8
11.5
676
585
6.2
6.3
N/A
3
5.8
2.5
11.1
0.50
0.56
3
3
2.5
2.7
a Unit cell parameter, from X-ray diffraction data. b Surface area, calculated from adsorption branch of N2 isotherm using the BET method. c Pore diameter
from BJH analysis of desorption branch of N2 isotherm. d From titration. e From elemental analysis (performed by QTI, Whitehouse NJ). f This material
turned pink upon acidification due to a trace amount of organic byproduct resulting from the decomposition of the monosodium salt of o-xylyl dithiol.
sulfonic acid group and another functional group without the
need for thiol oxidation.
area drops from 810 to 613 m2/g upon grafting 1 at 0.2 mmol/g
but that the pore diameter remains constant at 6.1 nm.
SBA-g1 was characterized by cross-polarization magic angle
spinning (CP/MAS) NMR, X-ray diffraction (XRD), nitrogen
adsorption/desorption porosimetry, and elemental analysis. The
13C CP/MAS NMR spectrum shows the same resonances as
the molecular precursor 1 (Figure 1); thus, the sultone ring
remains intact throughout the grafting process. The sulfur
content of the materials (from elemental analysis) was used as
a measure of the organic loading, and it was found that
approximately 30-80% of the silane used in the grafting
reaction ended up covalently bound to the surface. The XRD
pattern of the functionalized material is identical to that of the
parent SBA-15 and indicates that long-range ordering of the
material is not affected by the grafting process (see Supporting
Information, Figure S2). Nitrogen adsorption/desorption poro-
simetry (Table 1, Figure S3) shows that the SBA-15 surface
Sultone Ring-Opening. The sultone ring in SBA-g1 can be
hydrolyzed or thiolyzed to produce different surface groups
(Scheme 3). Hydrolysis of the sultone in neutral water produces
a solid that was used as a control catalyst because it contains
acid groups but no thiols (SBA-A). Ring-opening with sodium
hydrosulfide produces acid/thiol pairs separated by three carbon
atoms (SBA-AT-p). 13C CP/MAS NMR (Figure 2) shows
complete ring-opening of the sultone in both cases as evidenced
by the disappearance of the carbon resonance at 68 ppm. The
acid loadings of these materials were measured by ion exchange
with NaCl followed by filtration and titration of the resulting
HCl with NaOH.20 The sulfur content of SBA-AT-p was found
to be twice as high as the acid content measured by titration,
confirming the 1:1 ratio of acid to thiol groups. The catalyst
characterization data are summarized in Table 1.
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A R T I C L E S
Margelefsky et al.
Scheme 3. Ring-Opening of Surface-Bound Sultone To Form Sulfonic Acid-Containing Materials
Catalytic Reactions. The catalysts were used in the synthesis
of bisphenol A. Surface loadings were purposely maintained
low (∼0.2 mmol/g) in order to keep the resulting catalytic sites
sufficiently isolated to observe the effects of discrete acid/thiol
pairing. The results are given in Table 2. SBA-A gives a very
low per-site-yield (PSY ) 3.1, defined as mmol total products/
mmol H+) and selectivity (1.8, defined as p,p′/o,p′ product ratio)
due to the lack of thiol. Addition of homogeneous thiol improves
both the yield and selectivity, although both are still only modest
(10 and 8.4 respectively). SBA-AT-p is a highly active catalyst
and gives a per-site-yield of 83 and a selectivity of 14. Addition
of homogeneous thiol to this material decreases the per-site-
yield to 74, and the selectivity is slightly increased to 15. A
third catalyst was prepared via sultone thiolysis with sodium
methanesulfide. This material contains pairs of acid and methyl
sulfide groups, and is denoted SBA-AS-p. In the bisphenol A
reaction SBA-AS-p is only slightly better than SBA-A (Table
2, Entry 5). These results confirm that the thiol functionality is
necessary for high catalytic activity and also indicates that
sulfide linkages can be used to tether other groups without
substantially affecting catalysis (vide infra).
The selectivity of these catalysts was similar to that of SBA-A,
at a somewhat higher yield. 3-Mercapto-1-propanesulfonic acid
also showed higher activity compared to that of its heteroge-
neous counterpart, SBA-AT-p, but somewhat lower selectivity.
The reduced activity of the heterogeneous catalysts may be due
to mass-transfer limitations, as the reagents must diffuse into
the 1-D cylindrical pores of the SBA-15.
To investigate the possibility that the catalytic activity of
SBA-AT-p was due to surface species leaching into solution, a
hot-filtration test was performed. The solid catalyst was removed
after 8 h, and no additional conversion was detected after an
additional 9 or 16 h at 90 °C (see Figure 3). This result, along
with the observation that unfunctionalized SBA-15 exhibits no
catalytic activity toward bisphenol A, suggests that the catalytic
activity of SBA-AT-p is entirely due to its surface-bound acid/
thiol pairs, rather than to the silica support or leached organic
species.
Randomly-Distributed Acid/Thiol Catalysts. To test the
effect of site organization, a catalyst containing randomly
distributed alkylsulfonic acid and propylthiol groups was
synthesized via the simultaneous grafting of two organosilanes.
Silane 1 was again used as a source of alkylsulfonic acid groups
since it allows for a nonoxidative synthesis. Since sultone rings
can be opened by thiols at elevated temperatures, silane 2 was
used as a source of disulfide-protected thiols. 1 and 2 were
grafted onto SBA-15, followed by sultone hydrolysis and
Homogeneous analogues of SBA-A and SBA-AT-p were also
tested (Table 2, entries 6-8). 1-Propanesulfonic acid and
3-hydroxy-1-propanesulfonic acid gave nearly identical catalytic
results. This suggests that the adjacent hydroxyl in SBA-A is
not responsible for the poor catalytic activity of this material.
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13694 J. AM. CHEM. SOC. VOL. 129, NO. 44, 2007

Enhanced Catalysis by Thiol/Sulfonic Acid Pairing
A R T I C L E S
Figure 2. Comparison of 13C CP/MAS NMR spectra of SBA-g1 (top) and sulfonic acid-containing SBA-15 materials. The complete disappearance of the
sultone peak at 68 ppm indicates complete ring-opening of the sultone.
Table 2. Catalysis Data in Bisphenol A Synthesisa for
Heterogeneous and Homogeneous Catalysts
heterogeneous
catalyst
homogeneous
catalyst
entry
PSYb
isomer ratioc
1
2
3
4
5
6
7
8
SBA-A
SBA-A
SBA-AT-p
SBA-AT-p
SBA-AS-p
none
none
PrSH
none
PrSH
3.1
10
83
1.8
8.4
14
74
15
none
8.1
8.1
10
2.7
1.5
1.7
11
PrSO3H
HOPrSO3H
HSPrSO3H
none
none
113
a Reaction conditions: 0.02 mmol H+, 0 or 0.02 mmol propanethiol, 6
mmol acetone, 24 mmol phenol, 90 °C, 24 h. b Per site yield (mmol total
product/mmol H+). c p,p′/o,p′.
disulfide reduction (Scheme 4). The resulting randomly dis-
tributed acid/thiol material is denoted SBA-AT-r. Due to the
differential grafting efficiency of the two silanes, the acid/thiol
ratio could not be controlled precisely at 1:1; elemental analysis
showed that there were in fact nearly 5 times more thiol groups
than acid groups. SBA-AT-r exhibited good selectivity in the
synthesis of BPA, but with a nearly 3-fold reduction in per-
site-yield compared to that of the paired material SBA-AT-p
(Table 3, entries 2-3).
To investigate the substrate scope of the heterogeneous
catalysts, the ketone was varied to produce other bisphenol
products. Using the more sterically hindered cyclohexanone as
reactant (to produce bisphenol Z), the effect of pairing is even
more pronounced (Table 3, entries 4-6). Thiol-free SBA-A
gives low activity, the randomly bifunctionalized material gives
only a small improvement, and the acid/thiol-paired catalyst has
a much higher activity (14 times higher than for the randomly
distributed catalyst). When hexafluoroacetone is used (to
produce bisphenol AF), the effect of thiol promotion is greatly
Figure 3. Hot-filtration test results. Reaction conditions: 0.02 mmol H+,
6 mmol acetone, 24 mmol phenol, 90 °C. Catalyst was removed after 8 h,
and filtrate was heated at 90 °C for another 16 h.
reduced, probably due to the greater reactivity of the fluorinated
reactant (Table 3, entries 7-8). The increase in activity from
SBA-A to SBA-AT-p is less than 2-fold. Thus, we conclude
that the improvement in catalytic activity gained by pairing acid
and thiol groups varies for different condensation reactions and
is more pronounced for less reactive ketones.
Varying Acid/Thiol Distance. The acid/thiol distance was
varied to investigate what effects separation distance has on
activity and selectivity in catalyzing the synthesis of BPA. By
opening the sultone ring with the monosodium salt of a dithiol,
a thiol group is tethered to the acid via a sulfide linkage. In
this way it is possible to tune both the length of the acid/thiol
spacer and the electronic properties of the thiol (i.e., alkyl vs
phenyl vs benzyl; see Scheme 3). 13C CP/MAS NMR showed
quantitative conversion of the sultone moieties to sulfonic acids
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A R T I C L E S
Margelefsky et al.
Table 4. Catalysis Data in Bisphenol A Synthesisa for
Scheme 4. Simultaneous Grafting of Silanes 1 and 2 to Form
Randomly Bifunctionalized Sultone/Disulfide Material and
Subsequent Deprotection to Form Random Acid/Thiol Material
SBA-AT-r
Heterogeneous Acid/Thiol Spacer Catalysts
heterogeneous
entry
catalyst
spacer type
PSYb
isomer ratioc
1
2
3
4
5
6
7
8
9
SBA-AT-p
SBA-AT-s1
SBA-AT-s2
SBA-AT-s3
SBA-AT-s4
SBA-AT-s5
SBA-AT-s6
SBA-AT-s7
SBA-AT-s8
none
83
42
20
22
5.2
21
64
37
47
14
12
alkyl, n ) 2
alkyl, n ) 3
6.5
6.8
1.9
4.3
9.6
7.9
7.9
alkyl, n ) 6
aryl, m ) 0, ortho
aryl, m ) 0, para
aryl, m ) 1, ortho
aryl, m ) 1, meta
aryl, m ) 1, para
a Reaction conditions: 0.02 mmol H+, 6 mmol acetone, 24 mmol phenol,
90 °C, 24 h. Catalyst loading ∼0.2 mmol H+/g. b Per site yield (mmol total
product/mmol H+). b p,p′/o,p′.
no better than that of the thiol-free material SBA-A. The poor
activity of this material could be due to steric hindrance between
the adjacent sulfide and thiol groups that could prevent the thiol
from reacting with acetone. The reduced nucleophilicity of
phenyl thiols vs alkyl thiols is also likely responsible for some
of the lower activity of these two catalysts.
Catalysts SBA-AT-s6, -s7, and -s8 (derived from o-, m-, and
p-xylyl dithiols, respectively) contain benzyl thiols. These
materials generally show greater activity than those containing
alkyl and phenyl spacers (Table 4, entries 7-9). The catalyst
SBA-AT-s6, containing an o-benzyl spacer is the most active
of all the spacer materials investigated here. We believe this is
because the thiol is positioned such that it can point directly at
the acid group and shortens the acid/thiol distance.
Table 3. Catalysis Data for Heterogeneous Catalysts in
Synthesisa of Various Bisphenols
heterogeneous
catalyst
bisphenol
product
entry
PSYb
isomer ratioc
1
2
3
4
5
6
7
8
SBA-A
A
A
A
Z
Z
Z
AF
AF
3.1
83
29
0.3
14
1.0
8.4
15
1.8
14
Conclusions
SBA-AT-p
SBA-AT-r
SBA-A
SBA-AT-p
SBA-AT-r
SBA-A
22
The distance between the acid and thiol group in a series of
heterogeneous catalysts was shown to have a profound influence
on the catalytic behavior in the synthesis of bisphenols. These
data highlight the importance of the spatial positioning of
functional groups in the development of heterogeneous catalysts
and also reveal how the nanoscale organization of catalytic
surfaces can be tuned to provide a level of reactivity control
unachievable through traditional random functionalization ap-
proaches.
N/Ad
13
N/Ad
N/Ad
N/Ad
SBA-AT-p
a Reaction conditions: 0.02 mmol H+, 6 mmol ketone, 24 mmol phenol,
90 °C, 24 h. Catalyst loading ∼0.2 mmol H+/g. b Per site yield (mmol total
product/mmol H+). c p,p′/o,p′. d o,p′-isomer below detection limit.
for these products (Figure S6, S7). Elemental analysis revealed
sulfur loadings to be approximately three times the acid loading
(Table 1, Entries 7, 13, 14), corresponding to the expected 1:1
acid/thiol ratio, and the surface area, pore size, and long-range
ordering were maintained. The resulting materials with various
alkyl and aryl spacers (denoted SBA-AT-s1 through SBA-AT-
s8) were used to catalyze the BPA reaction (Table 4). Catalysts
SBA-AT-s1, -s2, and -s3 (ring-opened by 1,2-ethanedithiol, 1,3-
propanedithiol, and 1,6-hexanedithiol, respectively) showed
reduced activity and selectivity compared to SBA-AT-p, where
the acid and thiol groups are in closer proximity (Table 4, entries
1-4). The activity decrease with distance is quite dramatic, as
increasing the length of the acid/thiol spacer by three atoms
(SBA-AT-s1 vs SBA-AT-p) decreases the yield by a factor of
2. Additional increase in the spacer length by one atom (SBA-
AT-s2) reduces yield by another factor of 2, down to a level
similar to that of SBA-AT-r (Table 3, entry 3). SBA-AT-s3
exhibits activity similar to that of SBA-AT-s2.
In the case of acid and thiol groups, it was found that the
catalytic activity for bisphenol synthesis is highest when the
two groups are as close as possible. It may be that for other
pairs of groups, such as mutually destructive ones, that the
activity vs distance behavior could be qualitatively different.
The sultone-containing silica SBA-g1 was used here to
generate acid/thiol pairs. By the reaction of this intermediate
with other nucleophiles, it is possible to create a wide array of
surface-paired materials.
Experimental Section
Materials. Tetrahydrofuran (THF) and toluene were distilled over
sodium immediately before use. All other solvents were analytical grade
and used as received. 3-Iodopropyltriethoxysilane was prepared from
3-chloropropyltriethoxysilane (Gelest) and sodium iodide according to
the literature procedure.37 Organosilane 2 was synthesized from
3-mercaptopropyltriethoxysilane and 2,2′-dipyridyldisulfide according
to the literature procedure14 followed by purification by chromatography
Catalysts SBA-AT-s4 and -s5, containing ortho- and para
phenylthiol groups, respectively, are both poor catalysts (Table
4, entries 5-6). In particular, the ortho variant shows activity
(37) Matsura, V.; Guari, Y.; Larionova, J.; Guerin, C.; Caneschi, A.; Sangregorio,
C.; Lancelle-Beltran, E.; Mehdi, A.; Corriu, R. J. P. J. Mater. Chem. 2004,
14, 3026-3033.
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13696 J. AM. CHEM. SOC. VOL. 129, NO. 44, 2007

Enhanced Catalysis by Thiol/Sulfonic Acid Pairing
A R T I C L E S
on silica gel. Anhydrous sodium hydrosulfide was purchased from Alfa
Aesar. All other chemicals were purchased from Aldrich and used as
received. All reactions were performed under an argon atmosphere.
3-(3-Triethoxysilylpropyl)-[1,2]-oxathiolane 2,2-dioxide (Orga-
nosilane 1). 1,3-Propanesultone (2.7 g, 22 mmol) was dissolved in dry
THF (50 mL). The solution was cooled to -78 °C, and n-butyllithium
(1.6 M in hexanes, 14 mL, 22 mmol) was added slowly over 10 min.
After stirring for another 10 min, 3-iodopropyltriethoxysilane (3.3 g,
10 mmol) was added slowly over 5 min. After stirring for 15 min, the
temperature was raised to -60 °C. After stirring for 9 h, the temperature
was raised to 0 °C, water (50 mL) was added, and the mixture was
transferred to a separatory funnel. The aqueous layer was removed,
and the organic layer was washed with saturated NaCl solution (50
mL) and dried over anhydrous MgSO4. After the solvent was removed
in vacuo, chromatography on silica gel (3:2 hexanes/ethyl acetate, Rf
mL) and water (5 × 30 mL) and dried under high vacuum overnight.
SBA-AS-p was prepared with the same procedure, except that sodium
methanesulfide was used in place of sodium hydrosulfide.
SBA-AT-r (Random Acid/Thiol Catalyst). SBA-15 (0.5 g) was
grafted with 1 (0.16 g, 0.5 mmol) and 2 (0.17 g, 0.5 mmol) according
to the procedure above for SBA-g1. The two organosilanes were added
dropwise simultaneously. After Soxhlet extraction and drying, the pale-
beige solids were suspended in water (50 mL) and stirred at 40 °C for
2 days. The solids were filtered, washed with water (2 × 30 mL) and
0.5 N HCl (3 × 30 mL), and suspended in an aqueous solution of
tris(2-carboxyethyl)phosphine hydrochloride (0.015 M, 50 mL). This
suspension was stirred at 55 °C for 2 days, and then the solids were
filtered, washed with water and methanol (4 × 30 mL each), and dried
under high vacuum overnight to yield a nearly white powder.
SBA-AT-s (Acid/Thiol Spacer Catalysts). Dithiol (15 mmol) was
dissolved in anhydrous DMF (10 mL), and sodium hydride (60 wt %
in mineral oil, 5 mmol) was added. This mixture was stirred until all
solids had dissolved and then was added slowly to a suspension of
SBA-g1 (0.5 g) in anhydrous DMF (10 mL). After stirring for 24 h at
room temperature, the solids were filtered, washed, and acidified as
described for SBA-AT-p.
1
) 0.3) afforded 1 (0.61 g, 19%) as a colorless liquid. H NMR (300
MHz, DMSO-d6) δ 4.38 (m, 2H), 3.73 (q, J ) 7 Hz, 6H), 3.48 (m,
1H), 2.58 (m, 1H), 2.13 (m, 1H), 1.70 (m, 2H), 1.46 (m, 2H), 1.13 (t,
J ) 7 Hz, 9H), 0.61 (t, J ) 8 Hz, 2H). 13C NMR (300 MHz, DMSO-
d6) δ 68.6, 58.4, 55.5, 31.6, 29.9, 20.6, 18.9, 10.3. Anal. Calcd for
C12H26O6SSi: C, 44.15; H, 8.03; S, 9.82. Found: C, 43.05; H, 7.46; S,
9.85. HRMS (FAB+): m/z (M + H+) (C12H27SiSO6) Calcd, 327.1298;
Found, 327.1302.
SBA-g1 (Sultone-Functionalized Silica). SBA-15 (1.0 g, synthe-
sized according to the literature procedure38) was dried under flowing
argon at 170 °C for 4 h. After cooling, dry toluene (50 mL) was added
via syringe, and the mixture was stirred vigorously to form a uniform
suspension. An appropriate amount of 1 (typically ∼0.7 mmol) was
added dropwise via syringe. The suspension was stirred for 45 min at
room temperature and then refluxed for 16 h. After cooling to room
temperature, the solids were filtered and washed with toluene and
dichloromethane (3 × 20 mL each). The solids were Soxhlet extracted
with dichloromethane overnight, dried under vacuum, and stored under
an argon atmosphere in a drybox until further use.
SBA-A (Thiol-Free Sulfonic Acid Catalyst). SBA-g1 (0.5 g) was
added to water (40 mL), and the resulting suspension was stirred at 40
°C for 2 days. Then the solids were filtered, washed with water (3 ×
30 mL), 0.5 N HCl (3 × 30 mL), and water (4 × 30 mL), and dried
under high vacuum overnight.
SBA-AT-p (Acid/Thiol-Paired Catalyst). SBA-g1 (0.5 g) was
suspended in anhydrous DMF (15 mL). Anhydrous sodium hydrosulfide
(86 mg, 1.5 mmol) was dissolved in anhydrous DMF (10 mL), and the
resulting blue solution was added dropwise to the stirred silica
suspension. After stirring for 24 h at room temperature, the solids were
filtered, washed with DMF (3 × 30 mL) and water (5 × 30 mL), and
then suspended in 0.5 N HCl (30 mL) to acidify. After stirring for 3 h
the white solids were filtered and washed with 0.5 N HCl (3 × 30
Acid Titration. To the silica material to be analyzed (∼30 mg) was
added aqueous NaCl (2 N, 4 mL), and the suspension was stirred for
24 h. The solids were removed by filtration and washed with water (4
× 2 mL), and the combined filtrate was titrated with 0.01 N NaOH
using phenol red as indicator.
Catalytic Reaction: Condensation of Phenol and Ketone. An
amount of catalyst corresponding to 0.02 mmol H+ (∼100 mg) was
added to a vial and dried under high vacuum at 80 °C overnight. Phenol
(2.2 g, 24 mmol) and ketone (6 mmol) were added, and the vial was
sealed under argon and stirred at 90 °C for 24 h. The catalyst was
removed by filtration and washed with acetonitrile to a total filtrate
volume of 25 mL, and the products were quantified by HPLC. Per-
site-yield was calculated on the basis of the number of acid sites present,
and selectivity was defined as the ratio of bisphenol isomers (p,p′/
o,p′).
Acknowledgment. This work was supported by a National
Science Foundation Graduate Research Fellowship and by the
Department of Energy.
Supporting Information Available: X-ray diffraction data,
1H NMR spectrum of silane 1, nitrogen adsorption/desorption
isotherms, 29Si CP/MAS spectrum of SBA-g1, 13C CP/MAS
NMR spectra of SBA-AT-r and spacer materials SBA-AT-s(1-
8). This material is available free of charge via the Internet at
http://pubs.acs.org.
(38) Zhao, D. Y.; Huo, Q.; Feng, J.; Chmelka, B.; Stucky, G. J. Am. Chem.
Soc. 1998, 120, 6024-6036.
JA074761E
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