Asymmetric catalysis at the mesoscale: Gold nanoclusters embedded in chiral self-assembled monolayer as heterogeneous catalyst for asymmetric reactions

Article abstract of DOI:10.1021/ja310640b

Research to develop highly versatile, chiral, heterogeneous catalysts for asymmetric organic transformations, without quenching the catalytic reactivity, has met with limited success. While chiral supramolecular structures, connected by weak bonds, are highly active for homogeneous asymmetric catalysis, their application in heterogeneous catalysis is rare. In this work, asymmetric catalyst was prepared by encapsulating metallic nanoclusters in chiral self-assembled monolayer (SAM), immobilized on mesoporous SiO₂ support. Using olefin cyclopropanation as an example, it was demonstrated that by controlling the SAM properties, asymmetric reactions can be catalyzed by Au clusters embedded in chiral SAM. Up to 50% enantioselectivity with high diastereoselectivity were obtained while employing Au nanoclusters coated with SAM peptides as heterogeneous catalyst for the formation of cyclopropane- containing products. Spectroscopic measurements correlated the improved enantioselectivity with the formation of a hydrogen-bonding network in the chiral SAM. These results demonstrate the synergetic effect of the catalytically active metallic sites and the surrounding chiral SAM for the formation of a mesoscale enantioselective catalyst.

Full text of DOI:10.1021/ja310640b

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Article
Asymmetric catalysis at the mesoscale: Gold nanoclusters
embed-ded in chiral self-assembled-monolayer as
heterogeneous catalyst for asymmetric reactions
Elad Gross, Jack Hung-Chang Liu, Selim Alayoglu, Matthew A.
Marcus, Sirine C. Fakra, F. Dean Toste, and Gabor A. Somorjai
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja310640b • Publication Date (Web): 13 Feb 2013
Downloaded from http://pubs.acs.org on February 17, 2013
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Asymmetric catalysis at the mesoscale: Gold nanoclusters embed-
ded in chiral self-assembled-monolayer as heterogeneous catalyst
for asymmetric reactions
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Elad Gross†, Jack H. Liu†, Selim Alayoglu†, Matthew A. Marcus‡, Sirine C. Fakra‡, F. Dean Tos-
te†* and Gabor A. Somorjai†*
† Department of Chemistry, University of California, Berkeley, California 94720, and Chemical Sciences Divisions,
Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720
‡ Advanced Light Source, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720
ABSTRACT: Research to develop highly versatile, chiral, heterogeneous catalysts for asymmetric organic transformations,
without quenching the catalytic reactivity, has met with limited success. While chiral supramolecular structures, con-
nected by weak bonds, are highly active for homogeneous asymmetric catalysis, their application in heterogeneous cataly-
sis is rare. In this work, the catalyst was prepared by encapsulating metallic nanoclusters in chiral self-assembled mono-
layer (SAM), immobilized on mesoporous SiO2 support. Using olefin cyclopropanation as an example, it was demonstrat-
ed that by controlling the SAM properties, asymmetric reactions can be catalyzed by Au clusters embedded in chiral SAM.
Up to 50% enantioselectivity with high diastereoselectivity were obtained while employing Au nanoclusters coated with
SAM peptides as heterogeneous catalyst for the formation of cyclopropane-containing products. Spectroscopic measure-
ments correlated the improved enantioselectivity with the formation of hydrogen-bonding network in the chiral SAM.
These results demonstrate the synergetic effect of the catalytically active metallic sites and the surrounding chiral SAM
for the formation of a mesoscale enantioselective catalyst.
1. Introduction
quence, the reactants for asymmetric reactions are mainly
ketones and aldehydes, in which an oxygen atom, that forms
hydrogen bonds with the ligand, is next to the pro-chiral
carbon 3-4. Moreover, the high enantioselectivity was shown
to be coupled with a decrease in the reactions rate, due to
poisoning of the highly active metallic sites by the strongly-
adsorbed chiral ligands 5.
In this work we aimed towards the preparation of catalysts
with unique architecture that will utilize the catalytically
active metallic site and its surrounding for the formation of
highly reactive and selective catalysts. The preparation of
these synergetic systems would extend the catalytically active
site from the nanoscale to the mesoscale, which implies a
principle of operating systems with functions that are be-
yond the properties of their building blocks.
In order to overcome these limitations, we envisioned the
preparation of mesoscale structure in which the enantiose-
lectivity could be achieved by the synthetic analog of the
three-dimensional enzyme’s active site. In a similar way to
the enzymatic systems, the high selectivity would be gained
by surrounding the catalytic site with a chiral environment 8-
11. Synthetic chiral supramolecular homogeneous catalysts
that are self-assembled through hydrogen bonding show
high enantioselectivity for asymmetric organic transfor-
mations 2,8, however, this concept has not been successfully
employed for heterogeneous catalysts.
The development of sophisticated structures for unique cata-
lytic reactivity is widely used in nature. For example, the
construction of the three dimensional structures which can
produce high catalytic reactivity and selectivity, including
enantioselectivity, is often observed in enzyme-catalyzed
reactions 1.
In homogeneous and heterogeneous catalysis weak bonds
between the reactant and ligands can be essential for highly
enantioselective catalysis 2-9. For example, following adsorp-
tion of chiral ligands, such as cinchonidine, on the surface of
Pt/Al2O3, up to 95% enantioselectivity was obtained in hy-
drogenation of α-ketoester 3. The catalysts surface was coated
by the chiral ligands in order to prevent the direct interac-
tion between the metal and the pro-chiral reactant. A specif-
ic orientation of the reactant, as it approaches the catalyst, is
favored by hydrogen bonding between the cinchonidine lig-
and and the α-ketoester. These two interactions (reactant-
ligand and ligand-metal) were proved to be essential for
asymmetric heterogeneous catalytic reactions. As a conse-
In this paper, we show a new route for the preparation of
enantioselective heterogeneous catalyst, in which the metal-
lic nanoclusters are synthesized within
a chiral self-
assembled-monolayer (SAM), which is grafted on mesopo-
rous SiO2 support (MCF-17) without any other stabilizing
agent (Au@SAM/MCF-17). By this preparation method, the
Au clusters are embedded in an interconnected network of
chiral molecules, enabling high catalytic reactivity and enan-
tioselectivity for cyclopropanation reactions. It was previous-
ly demonstrated that metallic nanoparticles that were em-
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bedded or coated by SAM have unique reactivity and selec-
peak was detected following the formation of Br terminated
tivity;12-14 however, chiral SAM was not previously employed
for asymmetric catalytic reactions.
SAM (Supp. Info. Fig. S2). The high density of the SAM led to
decrease of the SiO-H (850 cm-1) and free O-H (3700 cm-1)
signals in Diffusion Reflective Infrared Fourier Transform
(DRIFT) spectra (Supp. Info. Fig. S3).
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In an earlier report, we have showed that dendrimer encap-
sulated gold nanoclusters are highly active and diastereose-
lective toward the catalytic formation of cyclopropane, fol-
lowing the oxidation of nanoclusters by PhICl2 and the for-
mation of highly oxidized Au ions. The diastereoselectivity of
the catalytically formed cyclopropane was tuned by the pack-
ing density of the dendrimer matrix 15. Leaching of Au ions to
the solution phase was prevented by using toluene as a sol-
vent, due to repulsion between the hydrophobic solvent and
the hydrophilic solid catalyst 15-17. In the current study, the
dendrimer matrix was substituted with SAM of chiral mole-
cules immobilized on mesoporous silica support and Au na-
noparticles were synthesized within the chiral matrix. Sub-
stituting the amino-acid SAM with peptide SAM led to an
enhancement in the reactivity, diastereoselectivity and enan-
tioselectivity. Spectroscopic analysis indicated that highly
stabilized SAM was formed by increasing the chain length of
the peptides that construct the SAM due to the formation of
hydrogen-bonding network in the SAM. These results indi-
cate the importance of the chiral environment that sur-
rounds the metallic site and its role in asymmetric catalytic
formation of cyclopropane molecules.
Chiral SAM was prepared by alkylation of either (L)-proline
1, (L)-4 hydroxyproline 2, (L)-diproline 3 (L)-phenylalanine 4
and quinine 5 on Br terminated SAM/MCF-17 (Scheme 1). For
example, Boc-protected (L)-proline 1 was added to Br/MCF-
17 with the addition of K2CO3•1.5H2O in THF (followed by the
removal of the Boc protecting group). After this step, the
surface area and pore diameter of the SAM/MCF-17 support
decreased to 475 m2 g-1 and 18 nm, respectively (Supp. Info.
Fig. S1). The quenching of Br 3d signal on the chiral
SAM/MCF-17 indicated that all the surface bromide reacted
with the chiral molecules, up to the XPS detection limits
(Supp. Info. Fig. S2). Moreover, characteristic IR peaks of the
immobilized chiral molecules were detected in DRIFT spec-
tra (Supp. Info. Fig. S3).
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Solid-state CP-MAS NMR spectra of Br/MCF-17 indicated the
formation of SAM with a terminal halide on the mesoporous
silica support (Supp. Info S4). Following the alkylation of the
surface by (L)-diproline 3, characteristic carboxylate ester
peaks were detected (173 and 65 ppm) along with a decay of
the peak correlated to the bromide bearing carbon (34 ppm).
Both DRIFTS and CP-MAS NMR spectra verified that the
attachment of chiral molecules to the surface was through
the carboxylic terminal group.
2. Results and discussion:
2.1 Catalysts preparation
SAM-directed growth of Au nanoclusters in SAM/MCF-17
was performed (Scheme 1). The Au clusters were synthesized
without any stabilizing agent, ensuring that a high percent-
age of surface atoms would be catalytically accessible. While
HAuCl4 was mixed with the chiral SAM/MCF-17 in H2O, elec-
tronic interactions were formed between Au ions and the
electron rich amine groups in the SAM for the formation of
Au+3@SAM/MCF-17 12,20,21. The solid catalyst was washed
three times in order to remove the residual Au ions that were
not encapsulated in the SAM. The encapsulated Au ions were
reduced to nanoparticles by exposure to 1 atm of H2 at 40 °C
for 24 hr. XPS measurements verified the metallic (Au0)
properties of the gold clusters (Supp. Info. Fig. S2). HR-TEM
analysis indicated the formation of mono-dispersed Au clus-
ters with diameter of 6±1 nm with loading of 0.4 w/w % Au in
proline/MCF-17. Increasing the amount of encapsulated Au
by fivefold to 2 w/w % led to bimodal distribution of the Au
clusters, with small Au clusters ( 2±0.5 nm) and large Au
aggregates (20±4 nm) (Supp. Info. Fig. S5).
Scheme 1: Preparation of Au nanoclusters encapsulated
in chiral SAM/mesoporous MCF-17 support.
The synthesis of Au@SAM/MCF-17 is illustrated in scheme 1.
18
Mesoporous SiO2 (MCF-17) was dried under vacuum at 120
2.2 Reactivity
°C for 48 hours. Bromide terminated SAM (Br/MCF-17) was
prepared by refluxing the dry mesoporous SiO2 support with
Br(CH2)3SiCl3 in toluene for 48 hours. N2 BET measurements
indicated that following the addition of SAM, the surface
area and pore diameter decreased from 525 to 490 m2 g-1 and
28 to 23 nm, respectively (Supp. Info. Fig. S1), with SAM den-
sity of 1.8 molecule/nm2 (19). The high density of SAM
proved to be essential for the formation of highly selective
catalyst. A Br 3d X-ray Photoelectron Spectroscopy (XPS)
The catalytic reactivity and selectivity of Au clusters embed-
ded in SAM (Au@SAM/MCF-17) was studied using propargyl
pivalate 6 and styrene as reactants in cyclopropanation reac-
tion at room temperature that produces cis and trans dia-
stereomers of cyclopropane 7 15,22. Employing 0.4 w/w % Au
encapsulated in (L)-proline/MCF-17 (proline 1 was attached
to the surface through the OH terminal due to Boc protec-
tion of the amine) as catalyst with 10 mol % PhICl2 as an oxi-
dizer, led to 87% conversion after 12 hours, using toluene as a
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solvent (Table 1, entry 1). The selectivity toward the for-
using Au@diproline/MCF-17 catalyst, with cis:trans ratio of
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5
6
7
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mation of cyclopropane 7 was 84%, while 16% of 6 was con-
verted to the aldehyde (3-methyl-2-butenal) 8 byproduct. A
cis:trans ratio of 10:1 and 14% ee of cyclopropane 7 were
measured.
9:1 and 51% ee (Table 1, entry 3). The immobilization of
triproline did not lead to further enhancement in the prod-
ucts selectivity (Figure 1a, black curve). In a previous study,
we showed that the diastereoselectivity values of the product
cyclopropane are correlated to the packing density of the
matrix which coats the Au cluster 15. In a similar way, it was
observed here as well that the formation of highly packed
matrix around the Au clusters favors the cis diastereomer
Inductively Coupled Plasma Mass Spectrometry (ICP-MS)
measurements did not detect any leaching of Au ions to the
solution phase following the reaction. Leaching of the highly
reactive Au(III) ions was prevented by the repulsion forces
due to steric effect 20
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23-
between the hydrophilic catalyst and hydrophobic solvent
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24. Solution phase Au ions are expected to be highly active,
when compared to the embedded Au clusters and as a con-
Increasing the Au loading to 2 w/w % in Au@proline/MCF-17
deteriorated the catalytic reactivity and selectivity with
cis:trans ratio and ee values of 6:1 and 4%, respectively (Table
1, entry 4). HR-TEM imaging indicated that Au aggregates
with diameter of ~20 nm were formed due to the increase in
Au loading (Supp. Info. Fig. S5). These aggregates were not
fully embedded in the chiral matrix, resulting in a decrease in
the selectivity while employing 2 w/w % Au@proline/MCF-17
as catalyst.
sequence, any enantioselectivity would be diminished by
15
leaching of small quantity of Au ions to the solution
.
Therefore, the measured enantioselectivity values confirm
the stability of the highly oxidized Au ions within the SAM.
Table 1: Au@SAM/MCF-17 catalyzed intermolecular cy-
clopropane synthesis
Figure 1: The intermolecular enantioselective formation of cyclo-
propane 6 as function of the number of proline (black) and phenyl-
alanine (red) units in the chain molecule that construct the SAM.
Threefold enhancement in the enantioselectivity was obtained when
the amino acid was replaced with a dipeptide in the SAM of the
Au@SAM/MCF-17 catalyst. Error bars represent up to ±6% differ-
ences in reproducibility.
The reactions were performed at room temperature using toluene as
a solvent. The reactants and oxidizer amounts were 0.15 and 0.015
mmol, which are two and one order of magnitude higher than that
of the Au catalyst, respectively. a Loading was measured by ICP-MS. b
Conversion, selectivity and diastereoselectivity were measured by 1H-
c
NMR after 12 hours. Proline was anchored to the surface through
the OH group. d 3 mol % of AuCl3 was mixed with 10 mol % Pro-Pro-
OMe•HTFA and 10 mol % Et3N in toluene.
Low catalytic reactivity (8% yield) and selectivity (3:1
cis:trans ratio and no enantioselectivity) was obtained with
0.4 w/w % Au@proline/MCF-17 while immobilizing the (L)
proline 1 through its NH terminal (Table 1, entry 5). In this
case, an unprotected (L) proline 1 was employed for the for-
mation of SAM and the attachment through the NH terminal
was verified by DRIFTS spectra.
An increase in the enantioselectivity to 17% ee was measured
by substituting the SAM of proline 1 with that of hydroxypro-
line 2 (Table 1, entry 2). The increase in the enantioselectivity
can be attributed to better packing of the SAM around the
active Au clusters as a result of the formation of hydrogen-
bonding network in the SAM by the hydroxy group of the
hydroxyproline 2. The presence of OH and carboxylic-ester
signals in DRIFTS spectra indicated that the hydroxylproline
was attached to the surface through the carboxylic terminal.
The importance of amine groups as an initial nucleation
point for the formation of nanoparticles was previously
demonstrated for dendrimer-encapsulated metal nanoclus-
ters 20. As observed here as well, the accessibility of the
amine group is key parameter for the preparation of Au ion-
proline complexes prior to the clusters growth step. Alkyla-
tion of proline through the amine group hinders, by either
electronic or steric effects, the complexation step. As a con-
sequence, most of the Au ions were lost in the washing pro-
cess of the catalyst (prior to clusters reduction), resulting in
low reactivity and selectivity.
On the basis of this hypothesis, diproline 3 was synthesized
and immobilized on MCF-17. The potential advantages in
replacing the amino acid with dipeptide are the addition of
another chiral center and the construction of hydrogen-
bonding network in the SAM. High reactivity and selectivity
toward the formation of cyclopropane 7 was obtained while
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Low yield and no enantioselectivity were measured while
26% ee of cyclopropane 10 with SAM of phenylalanine and
1
2
3
4
5
6
7
8
employing diproline as an ancillary ligand with Au ions (Ta-
ble 1, entry 6). The low reactivity of the homogeneous analog
to the active heterogeneous catalyst was previously ob-
served13 and was correlated to strong interactions between
Au ions and diproline molecules which prevent the catalytic
formation of cyclopropane.
diphenylalanine, respectively (Table 2, entries 3 and 4). As in
the intermolecular cyclopropanation reaction, preparing
SAM with longer peptides did not lead to any further en-
hancement in the products selectivity.
It should be noted that the Au clusters diameter (6±1 nm) is
larger than the length of the peptide in the SAM (~1.5 nm).
However, the encapsulation of Au clusters inside the three
dimensional pores of the mesoporous SiO2 prevented leach-
ing of Au ions to the solution phase, while enabling the diffu-
sion of reactants to the catalyst. No enantioselectivity was
measured when Pd@diproline/MCF-17 was used as a catalyst
for hydrogenation reactions, since the active catalyst in these
reactions are the metallic nanoclusters which are not entirely
shielded by the chiral SAM. In contrast, in the cyclopropana-
tion reactions the active catalyst are the highly oxidized met-
al ions which are well encapsulated in the SAM due to elec-
tronic interaction with the nucleophilic groups in the SAM,
enabling the diastereo- and enantioselective cyclopropana-
tion reaction.
Mixing the racemic mixture of cyclopropane
7 with
Au@diproline/MCF-17 catalyst for 10 hours in toluene did not
lead to asymmetric separation, excluding any role of the chi-
ral SAM in preferred separation of one enantiomer over the
other. No product was detected while employing dipro-
line/MCF-17 (without the Au clusters) as catalyst.
Au@diproline/MCF-17 was not active without the addition of
an oxidizer. These measurements further support the hy-
pothesis that Au(III) ions are the catalytically active species
in this reaction.
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Diastereoselectivity and enantioselectivity values of 6:1 and
5%,
were
measured
when
0.4
w/w
%
Au@phenylalanine/MCF-17 was employed as catalyst (Table
1, entry 7). Increasing the number of phenylalanine units in
the immobilized peptide from 1 to 2 and 3, increased the en-
antiomeric excess of the cyclopropane from 7 to 42 and 44,
respectively (Figure 1a, red curve).
2.3 Spectroscopic characterization
Though quinine 5 is widely used as a chiral ligand for a varie-
ty of asymmetric heterogeneous catalytic reactions, in this
case, almost no reactivity (<1%) was obtained while using
Au@quinine/MCF-17 as catalyst (Table 1, entry 8).
Table 2: Au@SAM/MCF-17 catalyzed intramolecular cy-
clopropane synthesis
Figure 2: In-situ NEXAFS spectra of reduced (a.) and oxidized (b.)
Au@diproline/MCF-17 in toluene. The Au clusters were oxidized by
the addition of 10 mol % PhICl2 to the solution phase. Following the
addition of oxidizer, the Au(0) clusters were oxidized to Au(III).
After mixing the oxidized catalyst (b.) with the reactants for 12
hours, another NEXAFS spectrum was measured (c.) which indicated
the reduction of Au back to its metallic state. Reference NEXAFS
spectra of Au(0) (Au foil), Au(III) (AuCl3) and Au(I) ((Ph3P)AuCl) are
shown in the inset.
a
Reactions performed at room temperature, using toluene as a sol-
vent and 0.4 w/w % loading of Au. The reactant 8 and PhICl2
amounts were 0.15 and 0.015 mmol, which are two and one order of
magnitude higher than that of the Au catalyst, respectively. The
yield and ee values were measured by 1H NMR and chiral HPLC after
12 hours.
X-ray absorption spectroscopy (XAS) measurements were
conducted at beam line 10.3.2 of the Advanced Light Source
(ALS) at Lawrence Berkeley National Laboratory in order to
characterize the active species in the Au@diproline/MCF-17
catalyst. The data was collected in fluorescence mode at the
Au L3 edge (11.918 keV). In-situ NEXAFS (Near Edge X-ray
Absorption Fine Structure) spectroscopy of the reduced cata-
lyst (1 w/w % Au@diproline/MCF-17) dissolved in toluene
(Figure 2, curve a) indicated the presence of metallic Au(0)
clusters. The addition of 10 mol % PhICl2 to the solution oxi-
dized the gold clusters to Au(III) (Figure 2, curve b). The
catalyst was then mixed with the reactants for 12 hr, after
which 90% yield of cyclopropane with cis:trans ratio of 10:1
The Au@SAM/MCF-17 catalyst was also tested for intramo-
lecular cyclopropanation reaction25. Using 0.4 w/w
%
Au@proline/MCF-17 as catalyst, 74% conversion of 9 and 10%
ee of cyclopropane product 10 were measured (Table 2, entry
1). Replacing the amino acid with a dipeptide increased the
reactivity and enantioselectivity to 95% and 30%, respectively
(Table 2, entry 2). Lower ee values were obtained while using
phenylalanine instead of proline for chiral SAM, with 7 and
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were measured. NEXAFS data (Figure 2, curve c) indicated
high-temperature combustion peak at 510°C. By increasing
1
2
3
4
5
6
7
8
that following the catalytic reaction, the Au ions were re-
duced to their metallic state, resulting in the deactivation of
the catalyst.
the number of proline units above one the SAM was further
stabilized, presumably due to interactions between the pep-
tides. Encapsulation of Au clusters in the SAM
(Au@diproline/MCF-17) did not lead to any modifications in
the TGA spectra.
In-situ NEXAFS measurements under reaction conditions
indicated that the formation of highly oxidized Au(III) ions is
directly correlated to the products formation. It was also
obtained that the catalytic rate decreased along with the
reduction of Au(III) ions (Supp. Info. Fig. S6). These results
are consistent with our previous observations that the active
catalyst for this reaction are the highly oxidized Au(III) ions.
TGA-MS analysis of 30 m/z (correlated to the formation of
NO molecules) indicated that the high temperature mass
loss peak (510°C) detected in the TGA spectra, is due to com-
bustion of the amino acid (Supp. Info. Figure S7). The inter-
actions within the molecules in the SAM, which are in high
proximity to the catalytically active Au ions, lead to better
packing of chiral molecules which favor specific orientation
of the reactants. These interactions ultimately result in the
enhancement in the diastereoselectivity and enantioselectivi-
ty values of products 7 and 10.
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Following the deactivation, the catalytic activity was re-
gained by re-oxidizing the metallic clusters with the addition
of 10 mol % PhICl2 to the solution phase. Only minor differ-
ences in clusters properties and their catalytic reactivity were
obtained when comparing the original and the recycled cata-
lysts. Similar results, regarding the reactivation of the cata-
lyst by the addition of fresh oxidizer, were obtained with
dendrimer-encapsulated metal nanoclusters as catalyst for
The nature of the interactions between the peptides in the
SAM was studied by DRIFT spectroscopy. As the number of
proline units in the peptide that construct the SAM was in-
creased, a broad peak between 2800 and 3700 cm-1 was de-
tected, which is correlated to inter- and intramolecular hy-
drogen bonding (Fig. 4, curves b-d) 27-28. The increase in the
IR abosprtion peak in this region indicates that the stabiliza-
tion of the diproline/MCF-17 and triproline/MCF-17, detected
in the TGA-MS spectra, can be correlated to the formation of
hydrogen bonding network in the SAM.
electrophilic reactions 15,23,24
.
Combining the catalytic and spectroscopic data, it can be
concluded that the threefold enhancement in the enantiose-
lectivity obtained by substituting proline with diproline SAM
is due to better packing and stabilization of the SAM around
the active catalyst. The highly packed diproline SAM favors
specific symmetry of the reactant as it approach to the cata-
lytically active Au(III) species which results in the increase in
product selectivity.
Figure 3: Thermal gravimetric analysis (TGA) first derivative curve
for Br/MCF-17 (a.), proline/MCF-17 (b.), diproline/MCF-17 (c.) and
triproline/MCF-17 (d.). The minima in the TGA curve are correlated
to combustion of the SAM. The immobilization of proline on the
Br/MCF-17 stabilized the SAM. Comparison of the diproline/MCF-17
and proline/MCF-17 TGA spectra indicates that the SAM is further
stabilized due to addition of another proline to the peptide chain.
The properties of the SAM/MCF-17 were analyzed by TGA-
MS (Thermal-Gravimetric-Analysis Mass-Spectrometry)
measurements, performed under thermal treatment with
oxidative conditions and a temperature ramp of 5°C/min.
The combustion temperature, defined by the rate of mass
loss reflects the SAM properties 26. First derivative of mass
loss of Br/MCF-17 indicated that the SAM is combusted at
250°C (Fig. 3, curve a). The esterification of proline on
Br/MCF-17 stabilized the SAM and the decomposition tem-
perature of proline/MCF-17 increased by 100°C to 355°C (Fig.
3, curve b). The SAM was further stabilized by the addition of
another proline unit to the peptide chain. TGA spectra of
diproline/MCF-17 and triproline/MCF-17 (curves c and d in
Figure 3, respectively) indicated the formation of additional
Figure 4: DRIFT spectroscopy of Br/MCF-17 (a.), prolin/MCF-17 (b.)
diproline/MCF-17 (c.) and triproline/MCF-17 (d.). The samples were
annealed to 70°C under vacuum for 24 hours, in order to ensure the
removal of adsorbed water molecules. Increasing the number of
proline links in the chain is followed by an increase in the DRIFTS
peak between 2800 to 3700 cm-1, which is correlated to intermolecu-
lar and intramolecular hydrogen bonding in the SAM.
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3. Conclusions
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These results demonstrate the advantages of mesoscale cata-
lysts that assemble together two different building blocks for
the formation of an architecturally designed catalyst. In this
case, the combination of metallic catalyst and chiral SAM
matrix, enabled the preparation of reactive and enantioselec-
tive heterogeneous catalysts. Unlike other asymmetric heter-
ogeneous catalysts, in which ligand-reactants interactions are
needed for asymmetric reactions, by using the chiral SAM,
the enantioselectivity is enhanced due to the chirality of the
matrix in which the catalyst is embedded. It should be noted
that in this study only few chiral molecule were tested for the
formation of SAM as an initial proof of concept for employ-
ing this system for asymmetric reactions. Preparation of SAM
with other homopeptide or heteropeptide may lead to better
catalytic reactivity and selectivity. The use of robotic system
for the preparation of a library of peptides is planned in or-
der to gain a better scope of the SAM-based mesoscale cata-
lyst.
ASSOCIATED CONTENT
Supporting Information. Details of the experimental pro-
cedure, spectroscopic and microscopic data. This material is
available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
* F. D. Toste, fdtoste@berkeley.edu ;
G. A. Somorjai, somorjai@berkeley.edu
ACKNOWLEDGMENT
We acknowledge support from the Director, Office of
Science, Office of Basic Energy Sciences, Division of
Chemical Sciences, Geological and Biosciences of the US
DOE under contract DE-AC02-05CH11231.
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