Copper Nanocrystal Plane Effect on Stereoselectivity of Catalytic Deoxygenation of Aromatic Epoxides

Article abstract of DOI:10.1021/jacs.5b01391

Previous studies have shown that crystal planes of heterogeneous catalysts could display enhanced activity, such that higher turnover or chemoselectivity could be achieved. Here we report an example where the reaction stereoselectivity was significantly affected by the catalyst crystal planes. In copper-catalyzed deoxygenation reaction of aromatic epoxides, copper cubes, wires, and plates gave the olefin products with different cis/trans selectivities, whereas homogeneous copper catalysts showed poor selectivity. Scanning tunneling microscope and density functional theory studies revealed that the different adsorption mode and higher adsorption strength of epoxide oxygen on Cu{100} plane were responsible for the observed variation of selectivity. The copper-catalyzed deoxygenation reaction provided new practical access to cis-olefins from readily available aromatic epoxides. Our work also indicated that nanocrystal catalysts may provide useful stereochemical control in organic reactions.

Full text of DOI:10.1021/jacs.5b01391

Communication
pubs.acs.org/JACS
Copper Nanocrystal Plane Effect on Stereoselectivity of Catalytic
Deoxygenation of Aromatic Epoxides
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§,⊥
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∥
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Bin Xiao,
Zhiqiang Niu,
Yang-Gang Wang, Wei Jia, Jian Shang, Lan Zhang,
‡
,§
†
∥
‡,§
†
,†,‡,§,⊥
Dingsheng Wang, Yao Fu, Jie Zeng, Wei He, Kai Wu, Jun Li, Jinlong Yang, Lei Liu,*
and Yadong Li*
,†,‡,§
†
Center of Advanced Nanocatalysis (CAN), University of Science and Technology of China, Hefei 230026, China
Department of Chemistry, Tsinghua University, Beijing 100084, China
‡
§
Collaborative Innovation Center for Nanomaterial Science and Engineering, Tsinghua University, Beijing 100084, China
∥College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China
⊥Tsinghua-Peking Center for Life Sciences, Tsinghua University, Beijing 100084, China
*
S Supporting Information
single-crystalline nanocrystals as highly active catalysts. The
advantages manifested by the crystal plane effect, however, are
confined to reactivity enhancement. As catalysis concerns both
ABSTRACT: Previous studies have shown that crystal
planes of heterogeneous catalysts could display enhanced
activity, such that higher turnover or chemoselectivity
could be achieved. Here we report an example where the
reaction stereoselectivity was significantly affected by the
catalyst crystal planes. In copper-catalyzed deoxygenation
reaction of aromatic epoxides, copper cubes, wires, and
plates gave the olefin products with different cis/trans
selectivities, whereas homogeneous copper catalysts
showed poor selectivity. Scanning tunneling microscope
and density functional theory studies revealed that the
different adsorption mode and higher adsorption strength
of epoxide oxygen on Cu{100} plane were responsible for
the observed variation of selectivity. The copper-catalyzed
deoxygenation reaction provided new practical access to
cis-olefins from readily available aromatic epoxides. Our
work also indicated that nanocrystal catalysts may provide
useful stereochemical control in organic reactions.
9
reactivity and selectivity, it remains a desirable goal to elicit
crystal plane effect on reaction selectivity, especially stereo-
10
selectivity.
Here we report a surprising finding of crystal plane-dependent
stereoselectivity changes in a copper nanocrystal (wires, plates,
and cubes) catalyzed deoxygenation reaction of aromatic
epoxides. This reaction is not only a new catalytic transformation
useful to organic synthesis but also provides an interesting
example for the variation of reaction stereoselectivity with
differently exposed catalyst crystal planes. Our finding suggested
the potential of using well-defined nanocrystals as practical
catalysts to catalyze organic reactions with both high activity and
high stereoselectivity. Moreover, this reaction provides a new
convenient and practical access to some synthetically interesting
11
cis-olefins from readily available epoxides.
Our study began with the finding of an unprecedented
deoxygenation reaction of trans-stilbene oxide (1a) with a Si−B
1
2
reagent Me PhSi-Bpin ((dimethylphenylsilyl)boronic acid
2
he discovery of catalyst single crystal plane effects on solid
13
pinacol ester). The yields and the ratio of cis/trans-stilbene
2a/3a) were determined by H NMR using 1,2,4,5-
T
surface-catalyzed reactions, marked by the observation that
Fe{111} planes exhibit higher activity than Fe{100} and Fe{110}
1
(
tetramethylbenzene as an internal standard. Without any catalyst,
the reaction could take place at 60 °C in dioxane generating cis-
and trans-stilbenes in a ratio of 1.0:2.1 (individual yield = 22%
and 45%) after 1 h. The low yield and poor selectivity called for
the finding of catalysts to improve the performance. Many
1
,2
in catalytic ammonia synthesis, has greatly promoted the
advance of surface science and catalysis. With aspiration to
develop practical catalysts that could fully capitalize the
advantage of such crystal plane effects, recent research on
single-crystalline nanocrystal catalysts with large “effective”
3
homogeneous or heterogeneous catalysts (e.g., CuCl
2
, CuO,
surface planes has witnessed remarkable advances. For instance,
CuI, Fe O , Au) failed to induce stereoselectivity to favor the
2
3
single-crystalline CeO nanorods were shown to have a higher
2
production of cis-olefin. Nonetheless, copper in charcoal (3 wt
%) as a commercially available catalyst (Aldrich 709107) was
found to favor the formation of cis-stilbene, but the cis/trans
selectivity varied from 4:1 to 1:1 among different experiments.
Hypothesizing that the variation of stereoselectivity may arise
from the heterogeneous nature of the exposed copper crystal
planes in copper on charcoal, we conducted morphology-
CO oxidation activity with CeO {100} and {110} than with
2
4
,5
CeO {111}. Somorjai et al. discovered that in the catalytic
2
hydrogenation of benzene, the exposed {100} surface of Pt cubic
nanoparticles was more active than the {100} and {111} mixture
6
surfaces of Pt cuboctahedral nanoparticles. For the methane
combustion, it was shown that the catalytic activity of Co O
3
4
7
nanocrystals followed the order of {112} > {011} ≫ {001}. The
oxidation of CO was achieved at temperatures as low as −77 °C
on Co O {110} planes that remained stable in a moist stream of
Received: February 8, 2015
Published: March 17, 2015
3
4
8
normal feed gas. These examples illustrated the potential of
©
2015 American Chemical Society
3791
DOI: 10.1021/jacs.5b01391
J. Am. Chem. Soc. 2015, 137, 3791−3794

Journal of the American Chemical Society
Communication
controlled synthesis of different nanocrystal copper catalysts and
tested their activities.
To elucidate the origin of the crystal plane effect on
stereoselectivity variation, we employed the scanning tunneling
16
Three general types of copper nanocrystals with well-defined
shapes including cubes, wires, and plates were prepared by
microscope (STM) to investigate the adsorption of trans-
stilbene oxide on the Cu{100} crystal plane (Figure 2). Our data
14,15
colloidal chemical synthesis.
The nanocrystals structure and
phase purity were determined by transmission electron
microscopy (TEM), high-resolution transmission electron
microscopy (HRTEM), and electron diffraction. The obtained
data established the exposed planes in the cubes, wires, and plates
to be {100}, {100}/{111}, and {111} planes (Figure 1). The
Figure 2. STM study of the adsorption (A) on Cu {111} surface and (B)
on Cu {100} surface.
showed that the adsorption of trans-stilbene oxide on Cu{111}
and Cu{100} generated distinct surface features. On the terraces
of Cu{111} surface, trans-stilbene oxide exhibited a “cuneus”
shape protrusion in topographic STM images, suggesting that
one of the phenyl groups lies close (maybe flatly) on the surface.
In contrast, the “dumbbell” shape image obtained on the
Cu{100} surface substantiated an up-standing adsorption mode.
In this case, the two lobes of each trans-stilbene-oxide molecule,
as visualized by the STM images, appeared as two round
protrusions with identical shape and height (each protrusion
represents a phenyl ring), indicating that the molecules are
anchored onto the surface without tilt.
To further compare the different adsorption strength of trans-
stilbene oxide on Cu{111} and Cu{100} facets, we took the
critical gap condition that was required to induce a lateral
manipulation of a single trans-stilbene oxide molecule on either
substrate by STM tip as a semiquantitative indicator. On
Cu{111}, by approaching the tip over a single molecule and
dragging it away, lateral movement of the target molecule could
be successfully realized and repeated at the gap condition of 10
mV/14 nA, but it failed on Cu{100} under the same conditions.
Employing stronger tip−molecule interaction, namely, further
increasing the tunneling current to 20 nA or even 25 nA (more
intense condition would lead to damage of the molecule), still
could not overcome the energy barrier to move the molecule
adsorbed on Cu{100}. Such a remarkable difference in our tip-
induced manipulation experiment indirectly evidenced that the
trans-stilbene-oxide bound much more firmly to Cu{100} than to
Cu{111}.
Next we performed density functional theory (DFT)
calculations to help in understanding the reaction. The molecular
adsorption configuration of trans-stilbene oxide on Cu{111} and
{100} surfaces is shown in Figure 2. The adsorption energies on
Cu{111} and {100} surfaces were −0.17 and −0.28 eV,
indicating a relatively weak chemo-adsorption between trans-
stilbene oxide and Cu surfaces. We further considered the ring-
opening dissociative adsorption configuration (Figure 3) where
Figure 1. TEM and HRTEM images of Cu cubes (C,F), wires (D,G),
and plates (E,H). Insets in (F−H) show the corresponding diffraction
patterns.
three types of nanocrystals were then loaded on charcoal and
used as the catalysts after the capping agent was removed by
extensive washing (for detailed procedures, please see Support-
ing Information).
We observed remarkable change of cis/trans selectivities when
the three catalysts were subjected to the reaction. When Cu cube
was used as the catalyst under very low loading (0.01 mol %), the
previously minor product (i.e., cis-stilbene) observed in the
uncatalyzed reaction became the prevailing product in 95% yield
and >20:1 cis/trans selectivity. A similar cis/trans selectivity
(
20:1) was observed when Cu wires were used, albeit in a slightly
lower yield (90%). In contrast, both the yield of cis-stilbene
35%) and cis/trans selectivity (1:1) decreased drastically with
(
Cu plates. The strikingly high stereoselectivity elicited by the Cu
cubes and wires under a very low Cu catalyst loading (0.01 mol
%
), compared to those of Cu plates and the uncatalyzed reaction,
was an interesting finding. As the Cu cubes and wires
predominantly exposed the {100} crystal plane whereas the Cu
plates mainly exposed the {111} surface, we concluded that the
crystal plane effect caused the variation of stereochemistry.
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Communication
The above mechanism is consistent with the experiment on
deoxygenation of cis-stilbene oxide (Scheme 1). In this system
Scheme 1. Deoxygenation of cis-Stilbene Oxide
Figure 3. Dissociative adsorption based on DFT calculations (A) on
Cu{111} surface, Eads = −0.10 eV and (D) on Cu{100} surface, Eads
−0.60 eV.
=
the epoxide ring is broken when oxygen is bonded to the Cu
surface, which means that this intermediate tends to be captured
by a silicon anion and lead to irreversible C−O bond cleavage in
actual reaction solution (Figure 4). The calculated dissociative
trans- and cis-stilbenes were produced in a selectivity of 4.6:1
under uncatalytic conditions. When Cu plate was used, the
selectivity increased only slightly to 5.8:1. However, when Cu
cube was used, the selectivity significantly improved to >30:1.
Again, the inversion reaction mode was more effectively
promoted by Cu{100} as compared to the Cu{111}.
Furthermore, as both STM experiment and DFT calculation
showed a close contact between the phenyl ring and the Cu
surface, we were not surprised to find that an aliphatic epoxide
was not a good substrate in the Cu-catalyzed deoxygenation
reaction presumably to poor surface adsorption (Scheme 2).
Scheme 2. Use of Aliphatic Epoxide as the Substrate
As cis-olefins are found in many bioactive molecules and
synthetic intermediates, the present Cu nanocrystal-catalyzed
epoxide deoxygenation reaction provides a new and interesting
catalytic transformation in synthetic organic chemistry. The very
low catalyst loading (0.01 mol %) also renders the reaction of
practical utility. To demonstrate this we first examined the
recyclability of the Cu{100} crystal plane catalyst (namely, 0.1 wt
% of Cu nanocube loaded on charcoal). After filtration, washing
and retreatment with the standard conditions in Figure 1A, a
consistent yield (92−97% yield for each cycle) and selectivity
(>20:1) could be obtained for over at least six cycles.
With a highly active yet robust Cu{100} catalyst in hand, we
examined its utilization for a range of trans-stilbene oxides (Table
1). The scope of the substitution encompasses both electron-
donating (i.e., Me, 2b) and withdrawing ones (i.e., F, 2c). Aryl
chloride (2d) was tolerated in the reaction, making possible
additional cross-coupling at the C−Cl bond. A number of
synthetically relevant groups such as acetal (2e), unprotected
acid (2f), and alcohol (2e) also survived the transformation. In a
scaled-up reaction at gram quantity (2g, 5 mmol), the
nanocrystal Cu{100} catalyst provided a high isolated yield
(96%) of the cis-product. These results indicated that Cu{100}
crystal constitutes a practical catalyst for the deoxygenation of
trans-stilbene oxides to produce cis-stilbenes amenable to gram-
scale operations. As olefins can be readily epoxidized with
retention of stereochemistry, the Cu nanocrystal catalyzed
reaction offers a new route for the production of cis-olefins with
the advantage of late-stage synthesis from readily available trans-
stilbenes. Furthermore, we took advantage of the fact that an
aliphatic epoxide was not reactive to selectively deoxygenate an
aryl epoxide moiety in 1i (Scheme 3). Such a highly selective
deoxygenation process has not been achieved in the past,
Figure 4. Proposed mechanism for cis/trans-stilbene formation.
adsorption energy on Cu{100} is 0.50 eV lower than on Cu{111}
surface, indicating a significantly stronger adsorption of trans-
stilbene oxide to Cu{100} surfaces than to Cu{111} surface.
Overall, we conclude that the adsorbed trans-stilbene oxide can
move easily on Cu{111} surface, while the chemisorbed trans-
stilbene oxide is difficult to diffuse on Cu{100} surface,
consistent with the STM results.
On the basis of the above experimental and theoretical results,
we proposed that the copper-catalyzed deoxygenation reaction of
stilbene oxides should proceed through a Peterson-type
transformation (Figure 4). In this transformation the Si−B
bond was activated to generate a silicon anion presumably
through a base-promoted process as discussed in previous
1
2
studies. Two configurations of olefins were obtained (cis/trans
selectivity = 1.0:2.1) under the uncatalyzed (e.g., copper free)
conditions because there are two different modes for the attack of
the silicon group to the epoxide (pathway 1 and pathway 2).
However, once copper catalyst is added to the reaction, the
strong chemo-adsorption between trans-stilbene oxide and Cu
surfaces forces the attack of the silicon group to the epoxide from
only one possible direction. It is remarkable that when Cu cube
was under very low catalyst loading (0.01 mol %), cis-stilbene is
already obtained in 95% yield and >20:1 cis/trans selectivity. This
observation suggests a very high catalytic effect due to the strong
surface adsorption on Cu{100} surfaces. By comparison, when
Cu{111} surface with much weaker adsorption is used, the
copper plates afforded a much lower yield of cis-stilbene (35%)
and cis/trans selectivity (1:1).
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a
Table 1. Scope of Deoxygenation of trans-Stilbene Oxides
ACKNOWLEDGMENTS
■
We thank the supports by the National Basic Research Program
of China (973 program; No. 2011CB932401), the “863”
Program of the Ministry of Science and Technology (Grant
2
2
012AA02A700), NSFC (21390393, 21131004, 21302178,
1221062), and FRFCU WK2060190029.
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̈
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(
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(
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17
reaction in organic synthesis. Taken together, we conclude that
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ASSOCIATED CONTENT
Supporting Information
Experimental details. This material is available free of charge via
the Internet at http://pubs.acs.org.
■
(
17) Examples for site-selective oxidation and deoxygenation of
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*
S
(
AUTHOR INFORMATION
Corresponding Authors
lliu@mail.tsinghua.edu.cn
ydli@tsinghua.edu.cn
■
*
*
Author Contributions
#
B.X. and Z.N. contributed equally to this work.
Notes
The authors declare no competing financial interest.
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DOI: 10.1021/jacs.5b01391
J. Am. Chem. Soc. 2015, 137, 3791−3794