Heterogeneous Hydroxyl-Directed Hydrogenation: Control of Diastereoselectivity through Bimetallic Surface Composition

Article abstract of DOI:10.1021/acscatal.1c01434

Directed hydrogenation, in which product selectivity is dictated by the binding of an ancillary directing group on the substrate to the catalyst, is typically catalyzed by homogeneous Rh and Ir complexes. No heterogeneous catalyst has been able to achieve equivalently high directivity due to a lack of control over substrate binding orientation at the catalyst surface. In this work, we demonstrate that Pd-Cu bimetallic nanoparticles with both Pd and Cu atoms distributed across the surface are capable of high conversion and diastereoselectivity in the hydroxyl-directed hydrogenation reaction of terpinen-4-ol. We postulate that the OH directing group adsorbs to the more oxophilic Cu atom while the olefin and hydrogen bind to adjacent Pd atoms, thus enabling selective delivery of hydrogen to the olefin from the same face as the directing group with a 16:1 diastereomeric ratio.

Full text of DOI:10.1021/acscatal.1c01434

pubs.acs.org/acscatalysis
Letter
Heterogeneous Hydroxyl-Directed Hydrogenation: Control of
Diastereoselectivity through Bimetallic Surface Composition
Alexander J. Shumski, William A. Swann, Nicole J. Escorcia, and Christina W. Li*
Cite This: ACS Catal. 2021, 11, 6128−6134
Read Online
sı
*
Metrics & More
Article Recommendations
Supporting Information
ACCESS
ABSTRACT: Directed hydrogenation, in which product selectivity is dictated
by the binding of an ancillary directing group on the substrate to the catalyst, is
typically catalyzed by homogeneous Rh and Ir complexes. No heterogeneous
catalyst has been able to achieve equivalently high directivity due to a lack of
control over substrate binding orientation at the catalyst surface. In this work, we
demonstrate that Pd−Cu bimetallic nanoparticles with both Pd and Cu atoms
distributed across the surface are capable of high conversion and
diastereoselectivity in the hydroxyl-directed hydrogenation reaction of
terpinen-4-ol. We postulate that the OH directing group adsorbs to the more oxophilic Cu atom while the olefin and hydrogen
bind to adjacent Pd atoms, thus enabling selective delivery of hydrogen to the olefin from the same face as the directing group with a
16:1 diastereomeric ratio.
KEYWORDS: directed hydrogenation, substrate direction, bimetallic nanoparticle, alloy, ensemble geometry
ubstrate-directed hydrogenations are an important class of
selective organic reactions that provide access to highly
In this work, we show that a bimetallic Pd surface is capable
of achieving diastereoselective OH-directed hydrogenation
when both metal atoms are available at the catalyst surface. We
postulate that adsorption of the alcohol directing group to the
more oxophilic alloying metal and activation of the alkene and
hydrogen at adjacent Pd atoms result in diastereoselective
delivery of hydrogen on the same face as the directing group
(Scheme 1b). Previous work has shown that alloying pure Pd
increases its selectivity for a variety of hydrogenation and
condensation reactions, but these examples use the second
metal primarily to temper the reactivity of the Pd surface in
order to achieve semihydrogenation of alkynes and dienes or to
alter chemoselectivity between multiple reaction path-
ways.21−36
S
functionalized and diastereomerically pure products.1−4 High
selectivity toward directed hydrogenation has been demon-
strated using molecular catalysts based on Ir, Rh, and Co, in
which the organometallic complex simultaneously activates and
coordinates H2, the directing group, and the alkene in a well-
defined orientation at a single metal center in order to achieve
facially selective addition of H2 across the olefin (Scheme
1a).5−9 Heterogeneous systems based on supported metal
Scheme 1. Homogeneous vs Heterogeneous Directed
Hydrogenation
We began by synthesizing supported Pd-M (3:1) alloy
nanoparticles through coimpregnation of metal precursor salts
on Al2O3 followed by high temperature reduction at 800 °C in
5% H2/N2 to form the alloy. These Pd3M/Al2O3 catalysts were
screened in the hydrogenation of a model substrate, terpinen-
4-ol, in cyclohexane under balloon pressure of H2 at room
temperature (Table 1). Well-ordered bimetallic surfaces with
directing capability are expected to favor product P1, while no
significant steric preference for P2 is expected in the absence of
a directing effect.
nanoparticles tend to be more reactive, robust, and recyclable
as hydrogenation catalysts than their molecular counterparts,
but none have shown significant directing capability.10−12
A
few examples using monometallic heterogeneous catalysts such
as Raney Ni and supported Pd, Pt, and Rh nanoparticles have
shown a mild directing group effect with alcohol, ether, and
amine functionality, but the strength of the interaction
between the directing group and the surface is weak compared
to homogeneous complexes, resulting in poor diastereoinduc-
tion.13−20
Received: March 29, 2021
Revised: May 3, 2021
Published: May 6, 2021
https://doi.org/10.1021/acscatal.1c01434
© 2021 American Chemical Society
ACS Catal. 2021, 11, 6128−6134
6128

ACS Catalysis
pubs.acs.org/acscatalysis
Letter
a
Table 1. Screening of Supported Pd-M Catalysts
Scheme 2. Changes in Pd−Cu Surface Speciation As a
Function of Thermal Treatment Atmosphere, Temperature,
and Sequence
thermal annealing steps under both reducing and inert
atmospheres.
Mesoporous SiO2 was chosen as the support for thermal
annealing studies due to the superior uniformity and low
polydispersity of its supported nanoparticles (Table S13,
Figure S27). For the following thermal treatments, we begin
with an identical impregnated and calcined material with a
75:25 Pd/Cu ratio on SiO2. The impregnation is carried out
sequentially using metal ammonia precursors, and after
calcination, only oxidized Pd and Cu species are observed
(Figures S2 and S3). We first performed H2 reduction on the
calcined sample at temperatures ranging from room temper-
ature to 800 °C (Table 1, entries 7−9). At room temperature,
only Pd precursors can be reduced by H2, generating a catalyst
comprising reduced Pd nanoparticles interspersed with Cu
oxides (RT H2), which shows identical reactivity and
selectivity to pure Pd. Catalyst selectivity increases slightly
with increasing reduction temperature due to Pd−Cu alloy
formation (600H2). However, at best, catalysts treated with H2
alone can achieve modest directivity (3:1 dr) and full
conversion over 20 h, consistent with formation of a Pd-rich
alloy surface in the high temperature H2 environment
(800H2).
To generate a more Cu-rich surface, we annealed the
calcined sample under N2 at temperatures between 600 and
800 °C (Table 1, entries 10−12). Due to the lack of an
external reductant, higher temperatures are required to reduce
the Cu precursors and form the bimetallic alloy using only
residual ammonia in the calcined material. At 600 °C under
N2, the catalyst shows high conversion and low directivity due
to negligible Cu precursor reduction at this temperature
(600N2). As the N2 annealing temperature is raised to 700 and
800 °C, the diastereoselectivity rises dramatically to 10:1 and
17:1 dr, respectively, while the conversion drops to 66% and
30% (700N2, 800N2).
We then further reduced the catalysts annealed under N2 at
400 °C in H2 in order to more efficiently reduce and
incorporate the Cu atoms into the alloy nanoparticle (Table 1,
entries 13−15). In all cases, the reactivity increases while the
diastereoselectivity of the N2-treated catalyst is retained. Our
most selective and active catalyst, 800N2-400H2, achieves 16:1
dr and full conversion over 20 h. Raising the reduction
temperature up to 800 °C after N2 annealing (800N2-800H2)
erodes the dr back down to 6:1 due to segregation of Pd to the
surface. On the basis of these data, the optimal catalyst for
both high diastereoselectivity and high conversion in this
system requires sequential 800N2-400H2 treatment in order to
obtain a balanced distribution of Pd and Cu on the bimetallic
surface.
a
0.1 mmol substrate, 50 mg 2 wt % Pd-M catalyst, 5 mL cyclohexane,
b
H2 balloon Single run conversions and diastereomeric ratios (dr)
determined by GC with decane as an internal standard.
Using a pure Pd/Al2O3 catalyst, we observe complete
conversion of the substrate after 2 h and a diastereomeric ratio
for P1/P2 (dr) of 1:1, revealing that pure Pd nanoparticles are
incapable of binding the hydroxyl directing group, in line with
previous reports on Pd/C catalysts (Table 1, entry 1).5 Pd3Fe,
Pd3Co, and Pd3Ni catalysts show conversions similar to pure
Pd with slight increases in dr to 2−3:1 toward the directed
product (Table 1, entries 2−4). However, incomplete alloying
and phase segregation of the two metals is observed, which
results in low directivity (Figure S1).37 The late transition
metal alloys Pd3Cu and Pd3Zn show suppressed conversion
and elevated diastereoselectivity relative to monometallic Pd,
suggesting that a larger proportion of the catalyst forms the
bimetallic structure (Table 1, entries 5 and 6). In this initial
screen, Pd3Cu showed the highest diastereoselectivity for the
directed hydrogenation with a 5:1 dr at 43% conversion in 2 h.
A control sample containing only Cu showed no conversion
under these conditions (Table S12, Figure S26).
Pd−Cu alloys are known to show dynamic surface
reconstruction during thermal annealing depending on the
gas atmosphere and temperature regime.38−40 Pd atoms
preferentially migrate to the surface in the presence of strongly
adsorbing gases such as H2 and CO while Cu segregates to the
surface under high-temperature inert gas or vacuum conditions
(Scheme 2).41−44 To better control the surface composition of
the Pd−Cu alloy nanoparticles and to improve selectivity
toward the directed hydrogenation, we carried out a variety of
6129
https://doi.org/10.1021/acscatal.1c01434
ACS Catal. 2021, 11, 6128−6134

ACS Catalysis
pubs.acs.org/acscatalysis
Letter
To understand the structural requirements for efficient
substrate-directed hydrogenation, we characterized three
Pd3Cu/SiO2 samples that show distinct selectivity and
conversion behavior: 800H2, 800N2-400H2, and 800N2. All
catalysts show similar nanoparticle morphology and Pd−Cu
average elemental composition based on scanning-transmission
electron microscopy (STEM), energy-dispersive X-ray spec-
troscopy (EDS), and X-ray fluorescence (XRF; Figures 1 and
the 800N2 and 800N2-400H2 samples have lattice parameters
of 3.854 Å and Pd87Cu13 composition (Table S2).
X-ray absorption fine structure (EXAFS) at the Pd K-edge
shows that all samples possess the characteristic two-peak
shape of the FCC crystal structure (Figure 2c). Fitting the
EXAFS spectrum allows us to determine the coordination
numbers (CN) and bond distances (R) for all atoms within the
first coordination sphere (Table 2, Table S3, Figure S8).
Consistent with XRD, EXAFS indicates that the largest
amount of Pd−Cu alloying is observed in the 800H2 sample
followed by the 800N2-400H2 and 800N2 samples based on the
ratio of Pd−Pd to Pd−Cu CN. The 800N2 sample also shows
residual Pd−O scattering due to incomplete reduction of Pd
precursors. At the Cu K-edge, all samples show significant
unreduced Cu−O scattering in addition to Cu−Pd scattering
(Figure 2d). The ratio of Cu−Pd to Cu−O CN in each sample
parallels the degree of alloying observed at the Pd K-edge and
in the XRD pattern (Table 2, Figure S9). On the basis of these
data, we conclude that the bulk Pd−Cu alloy structure does
not dictate catalyst diastereoselectivity. In fact, the catalyst with
the highest degree of bulk alloying, 800H2, showed the lowest
directed hydrogenation selectivity, corroborating our hypoth-
esis that the surface composition must vary based on the
thermal treatment sequence and environment.
Figure 1. STEM images and EDS maps for Pd3Cu/SiO2 treated under
(a, d) 800 °C H2, (b, e) 800 °C N2-400 °C H2, and (c, f) 800 °C N2.
The scattering amplitude in the Pd K-edge EXAFS
spectrum, which reflects the total first-shell coordination
around Pd atoms, provides indirect information about the
enrichment of Pd atoms on the surface or in the core of the
nanoparticle (Figure 2c, Table 2).34,45,46 The low directivity
800H2 catalyst has the lowest EXAFS scattering intensity and a
total Pd−M CN of 8.6, significantly lower than the expected
CN of 12 for bulk Pd atoms in an FCC structure and
characteristic of Pd enrichment at the surface of the
nanoparticle. In contrast, the strongly directing 800N2-400H2
catalyst has a similar average nanoparticle size but shows much
higher scattering intensity and a total Pd−M CN of 11.2
(Figure S6). We also characterized another low directivity
sample (800H2/Al2O3) with larger average particle size
compared to the SiO2 samples (Figure S10). The 800H2/
Al2O3 sample has a total Pd-M CN of 9.4, higher than the total
CN on 800H2/SiO2 due to the larger particles, but still in a
regime that represents significant surface Pd speciation (Table
S3). Unfortunately, total coordination number cannot be
analyzed when residual oxide remains in the sample, as is the
case for the 800N2 sample and all EXAFS data at the Cu K-
edge. In addition, we obtained STEM-EDS mapping and CO
chemisorption data on the thermally treated Pd3Cu/SiO2
samples, but neither measurement has sufficient resolution to
clearly distinguish the relative distribution of Pd and Cu atoms
on the nanoparticle surface (Figure 1d−f, Table S5). Together
with the literature on Pd−Cu surface segregation, these data
suggest that subtle changes to bimetallic surface composition
engendered by the thermal treatments have a strong impact on
directed hydrogenation behavior.
S4−S6, Table S1). Powder X-ray diffraction (XRD) shows that
all samples possess a face-centered cubic (FCC) crystal
structure as expected for a solid-solution Pd−Cu alloy, and
all peaks are shifted to a higher 2θ relative to a pure Pd phase
(Figure 2a,b). The sample directly reduced in 5% H2 (800H2)
shows a larger peak shift compared to those annealed first
under N2. The calculated lattice parameter of 3.833 Å indicates
an approximate Pd79Cu21 structure for the 800H2 sample while
In order to confirm that the diastereoselectivity observed on
the Pd−Cu alloy catalysts is in fact due to a hydroxyl directing
effect, we prepared two analogues of terpinen-4-ol (R = OH)
with different directing groups. Terpinen-4-ol methyl ether (R
= OCH3) should have weaker directing ability because the
bulky methyl group decreases the binding affinity of the oxygen
atom to the surface while p-menthene (R = H) should exhibit
no direction whatsoever (Scheme 3). When two Pd3Cu/SiO2
catalysts (800H2, 800N2-400H2) are compared to pure Pd/
Figure 2. (a) Powder XRD, (b) close-up of XRD (111) peak, (c) Pd
K-edge EXAFS, and (d) Cu K-edge EXAFS for Pd3Cu/SiO2 catalysts.
6130
https://doi.org/10.1021/acscatal.1c01434
ACS Catal. 2021, 11, 6128−6134

ACS Catalysis
pubs.acs.org/acscatalysis
Letter
Table 2. Pd and Cu K-edge XAS Fitting Parameters for Thermally Treated Pd3Cu/SiO2 Catalysts
a
sample
edge
Pd
scattering pair
CN
R (Å)
σ2 (Å2)
E0 (eV)
Pd3Cu/SiO2 800 °C H2
Pd−Pd
Pd−Cu
Cu−O
Cu−Pd
Pd−Pd
Pd−Cu
Cu−O
Cu−Pd
Pd−O
Pd−Pd
Pd−Cu
Cu−O
Cu−Pd
7.0 0.6
1.6 0.6
1.9 0.4
4.8 0.5
9.8 0.4
1.4 0.4
3.0 0.3
3.9 0.3
1.4 0.4
8.3 0.5
0.8 0.5
4.1 0.1
1.8 0.2
2.717 0.005
0.005
−5.0 0.6
−3.4 0.7
−6.3 0.3
−2.3 0.4
−6.5 0.4
Cu
Pd
Cu
Pd
1.917 0.018
2.680 0.008
2.728 0.002
0.009
0.005
0.009
0.005
Pd3Cu/SiO2800 °C N2 + 400 °C H2
Pd3Cu/SiO2 800 °C N2
1.924 0.007
2.725 0.006
2.041 0.028
2.729 0.003
Cu
1.928 0.003
2.710 0.007
0.009
−3.0 0.3
a
σ2 values are determined based on metal foil references and fixed during the EXAFS fitting.
of ∼1:3 on all catalysts due to the inherent steric preference of
the substrate, illustrating that the geometric and electronic
changes to the catalyst surface that accompany alloy formation
do not affect diastereoselectivity in the absence of a directing
group. The rates of reaction should also be sensitive to the
strength of directing group binding to the surface, which is
observed on both Pd−Cu alloy catalysts. The nondirecting R =
H substrate shows lower reactivity by a factor of 3 and 6
relative to R = OH and OMe substrates, respectively, because
no oxygen functionality is present to facilitate substrate
adsorption onto Cu surface atoms.
Scheme 3. Steric vs Directing Selectivity Preferences for
Different Directing Groups
SiO2, we indeed observe that the directing effect is attenuated
upon methylation or removal of the hydroxyl functional group.
The methyl ether substrate has a strong steric selectivity
preference due to the bulky methoxy group in the axial
position, which is reflected in the 1:7 dr (P1/P2) on pure Pd
(Table 3). While there is an increase in dr toward the directed
product from 1:7 to 1:4 and 1:2 using Pd−Cu catalysts, the
weak direction can never overcome the steric preference.
When no directing group is present (R = H), no change in
diastereoselectivity is observed between the monometallic Pd
and Pd−Cu catalysts. The hydrogenated product exhibits a dr
We also evaluated a few additional substrates to identify the
key features that enable highly diastereoselective heteroge-
neous directed hydrogenation (Table 4). Both homoallylic
(entries 1, 2) and allylic alcohols (entries 3−8) are capable of
directing the diastereoselective hydrogen addition, provided at
least one additional substituent besides the OH group is
present on the cyclohexene ring to reduce conformational
flexibility. In particular, substrates in which the OH directing
group prefers an axial position in the half-chair conformation
result in the highest diastereoselectivities (entries 1, 2, 3, 6).
Substrates wherein the directing group prefers an equatorial
position or has no conformational preference (entries 4, 5, 7,
8) show weaker directing effects but still noticeable increases
in diastereomeric ratio relative to the pure Pd/SiO2 control.
Finally, we performed kinetics and reusability studies on our
optimized Pd3Cu/SiO2 catalyst to understand surface struc-
tural evolution and catalyst stability over time. We first
measured conversion and selectivity for terpinen-4-ol hydro-
genation over time using a freshly prepared catalyst.
Interestingly, the diastereoselectivity of the catalyst increases
significantly over the first 6 h of the reaction, likely due to
bimetallic surface reconstruction that occurs upon exposure to
the reaction medium (Figure 3). The diastereoselectivity at the
end of 20 h of reaction reaches the expected 14:1 dr and 90%
conversion. To avoid exposing the catalyst to air, we then
injected a second aliquot of the substrate directly into the flask.
The reaction continues at a slightly slower rate, but the
diastereomeric ratio of the new product formed is high from
the outset, corroborating the fact that the catalyst surface
reaches a stable state after an initial reconstruction. If we
instead filter off the Pd3Cu/SiO2 powder and dry it in the air,
we find that the reactivity of the catalyst drops significantly
upon reuse though the diastereoselectivity remains high,
indicating that the alloy surface deactivates significantly upon
Table 3. Diastereoselectivity and Conversion for Three
Directing Groups over Pd/SiO2 and Pd3Cu/SiO2 Catalysts
a
dr (P1:P2) at high conversion
catalyst
Pd/SiO2
Pd3Cu 800H2
R = OH
R = OMe
R = h
1:3
3:1
16:1
1:7
1:4
1:2
1:3
1:3
1:3
Pd3Cu 800N2−400H2
conversion (%) at fixed time
catalyst
R = OH
R = OMe
R = H
b
Pd/SiO2
99
31
21
96
64
49
74
11
8
c
Pd3Cu 800 H2
Pd3Cu 800N2-400H2
c
a
Diastereomeric ratios averaged over three runs; standard deviations
b
c
provided in Table S4. Conversions obtained at 2 h. Conversions
obtained at 4 h.
6131
https://doi.org/10.1021/acscatal.1c01434
ACS Catal. 2021, 11, 6128−6134

ACS Catalysis
pubs.acs.org/acscatalysis
Letter
high diastereoselectivity in the hydroxyl-directed hydrogena-
tion reaction of terpinen-4-ol and related substrates. We
postulate that selective binding of the directing group to Cu
surface atoms and activation of H2 and the olefin on
neighboring Pd surface atoms enable facially selective hydro-
gen addition to the olefin with a 16:1 diastereomeric ratio.
Future studies will probe the ensemble geometry and
adsorption properties of the Pd−Cu surface in greater detail
in order to more clearly elucidate the origin of catalyst
diastereoselectivity. We anticipate that multimetallic surfaces
with well-defined ensemble geometry will enable heteroge-
neous substrate-directed catalysis that retains the robustness of
materials while achieving the stereoselectivity of molecular
complexes.
Table 4. Substrate Scope for Pd3Cu/SiO2-Catalyzed
Directed Hydrogenation
a
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acscatal.1c01434.
Materials, catalyst synthesis methods, substrate synthesis
methods, catalytic hydrogenation methods, physical
characterization methods, and supplementary figures
and tables (PDF)
AUTHOR INFORMATION
■
Corresponding Author
Christina W. Li − Department of Chemistry, Purdue
University, West Lafayette, Indiana 47907, United States;
orcid.org/0000-0002-3538-9955; Email: christinawli@
purdue.edu
a
Authors
(*) Conversion, diastereomeric ratio (dr), and percent isomerization
Alexander J. Shumski − Department of Chemistry, Purdue
University, West Lafayette, Indiana 47907, United States
William A. Swann − Department of Chemistry, Purdue
University, West Lafayette, Indiana 47907, United States
Nicole J. Escorcia − Department of Chemistry, Purdue
University, West Lafayette, Indiana 47907, United States
determined by GC with decane as an internal standard except for
entry 7, where dr is determined by NMR. (^) Entries 3 and 4 and
entries 5 and 6 run as a mixture of diastereomers.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acscatal.1c01434
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by Purdue University and the
National Science Foundation (CHE-2045013). A.J.S. and
C.W.L. acknowledge support from the Purdue Research
Foundation. N.J.E. acknowledges support from the NSF
under Cooperative Agreement EEC-1647722 and the En-
gineering Research Center for the Innovative and Strategic
Transformation of Alkane Resources (CISTAR). We acknowl-
edge Dr. Joshua Wright, Dr. Mark Warren, Dr. Kamil Kucuk,
and Dr. Yujia Ding for assistance with XAS experiments. Use of
the Advanced Photon Source is supported by the U.S.
Department of Energy, Office of Science, Office of Basic
Energy Sciences, under contract DE-AC02-06CH11357.
MRCAT operations are supported by the Department of
Energy and the MRCAT member institutions.
Figure 3. Starting material (terpinen-4-ol) concentration and product
diastereomeric ratio vs time over a fresh Pd3Cu/SiO2 catalyst and
after reinjection of a second aliquot of starting material.
oxidation (Table S6). Catalysts that have been exposed to air
can be regenerated through a 200 °C H2 reduction, which then
results in 72% conversion and a 12:1 dr over 20 h.
In conclusion, we show that control over the composition of
a bimetallic Pd−Cu surface through thermal annealing enables
6132
https://doi.org/10.1021/acscatal.1c01434
ACS Catal. 2021, 11, 6128−6134

ACS Catalysis
REFERENCES
pubs.acs.org/acscatalysis
Letter
(24) Edwards, J. K.; Freakley, S. J.; Carley, A. F.; Kiely, C. J.;
Hutchings, G. J. Strategies for Designing Supported Gold-Palladium
Bimetallic Catalysts for the Direct Synthesis of Hydrogen Peroxide.
Acc. Chem. Res. 2014, 47, 845−854.
■
(1) Hoveyda, A. H.; Evans, D. A.; Fu, G. C. Substrate-Directable
Chemical Reactions. Chem. Rev. 1993, 93, 1307−1370.
(2) Crabtree, R. H.; Felkin, H.; Fillebeenkhan, T.; Morris, G. E.
Dihydridoiridium Diolefin Complexes as Intermediates in Homoge-
neous Hydrogenation. J. Organomet. Chem. 1979, 168, 183−195.
(3) Brown, J. M. Directed Homogeneous Hydrogenation. Angew.
Chem., Int. Ed. Engl. 1987, 26, 190−203.
(4) Stork, G.; Kahne, D. E. Stereocontrol in Homogeneous Catalytic
Hydrogenation via Hydroxyl Group Coordination. J. Am. Chem. Soc.
1983, 105, 1072−1073.
(5) Crabtree, R. H.; Davis, M. W. Directing Effects in Homogeneous
Hydrogenation with [Ir(cod)(PCy3)(py)]PF6. J. Org. Chem. 1986,
51, 2655−2661.
(6) Crabtree, R. H.; Davis, M. W. Occurrence and Origin of a
Pronounced Directing Effect of a Hydroxyl Group in Hydrogenation
with [Ir(cod)P-C-Hx3(Py)]PF6. Organometallics 1983, 2, 681−682.
(7) Evans, D. A.; Morrissey, M. M. Rhodium(I)-Catalyzed
Hydrogenation of Olefins. The Documentation of Hydroxyl-Directed
Stereochemical Control in Cyclic and Acyclic Systems. J. Am. Chem.
Soc. 1984, 106, 3866−3868.
(8) Friedfeld, M. R.; Margulieux, G. W.; Schaefer, B. A.; Chirik, P. J.
Bis(phosphine)cobalt Dialkyl Complexes for Directed Catalytic
Alkene Hydrogenation. J. Am. Chem. Soc. 2014, 136, 13178−13181.
(9) Morello, G. R.; Zhong, H. Y.; Chirik, P. J.; Hopmann, K. H.
Cobalt-catalysed alkene hydrogenation: a metallacycle can explain the
hydroxyl activating effect and the diastereoselectivity. Chem. Sci. 2018,
9, 4977−4982.
(10) Blaser, H. U.; Malan, C.; Pugin, B.; Spindler, F.; Steiner, H.;
Studer, M. Selective hydrogenation for fine chemicals: Recent trends
and new developments. Adv. Synth. Catal. 2003, 345, 103−151.
(11) Augustine, R. L. Selective heterogeneously catalyzed hydro-
genations. Catal. Today 1997, 37, 419−440.
(12) Heitbaum, M.; Glorius, F.; Escher, I. Asymmetric heteroge-
neous catalysis. Angew. Chem., Int. Ed. 2006, 45, 4732−4762.
(13) Gualandi, A.; Savoia, D. Substrate induced diastereoselective
hydrogenation/reduction of arenes and heteroarenes. RSC Adv. 2016,
6, 18419−18451.
(25) Nguyen, L.; Zhang, S. R.; Wang, L.; Li, Y. Y.; Yoshida, H.;
Patlolla, A.; Takeda, S.; Frenkel, A. I.; Tao, F. Reduction of Nitric
Oxide with Hydrogen on Catalysts of Singly Dispersed Bimetallic
Sites Pt1Com and Pd1Con. ACS Catal. 2016, 6, 840−850.
(26) Mori, K.; Sano, T.; Kobayashi, H.; Yamashita, H. Surface
Engineering of a Supported PdAg Catalyst for Hydrogenation of CO2
to Formic Acid: Elucidating the Active Pd Atoms in Alloy
Nanoparticles. J. Am. Chem. Soc. 2018, 140, 8902−8909.
(27) Boucher, M. B.; Zugic, B.; Cladaras, G.; Kammert, J.;
Marcinkowski, M. D.; Lawton, T. J.; Sykes, E. C. H.; Flytzani-
Stephanopoulos, M. Single atom alloy surface analogs in Pd0.18Cu15
nanoparticles for selective hydrogenation reactions. Phys. Chem. Chem.
Phys. 2013, 15, 12187−12196.
(28) Kyriakou, G.; Boucher, M. B.; Jewell, A. D.; Lewis, E. A.;
Lawton, T. J.; Baber, A. E.; Tierney, H. L.; Flytzani-Stephanopoulos,
M.; Sykes, E. C. H. Isolated Metal Atom Geometries as a Strategy for
Selective Heterogeneous Hydrogenations. Science 2012, 335, 1209−
1212.
(29) Pei, G. X.; Liu, X. Y.; Yang, X. F.; Zhang, L. L.; Wang, A. Q.; Li,
L.; Wang, H.; Wang, X. D.; Zhang, T. Performance of Cu-Alloyed Pd
Single-Atom Catalyst for Semihydrogenation of Acetylene under
Simulated Front-End Conditions. ACS Catal. 2017, 7, 1491−1500.
(30) Armbruster, M.; Kovnir, K.; Behrens, M.; Teschner, D.; Grin,
Y.; Schlogl, R. Pd-Ga Intermetallic Compounds as Highly Selective
Semihydrogenation Catalysts. J. Am. Chem. Soc. 2010, 132, 14745−
14747.
(31) Tew, M. W.; Emerich, H.; van Bokhoven, J. A. Formation and
Characterization of PdZn Alloy: A Very Selective Catalyst for Alkyne
Semihydrogenation. J. Phys. Chem. C 2011, 115, 8457−8465.
(32) Pei, G. X.; Liu, X. Y.; Wang, A. Q.; Lee, A. F.; Isaacs, M. A.; Li,
L.; Pan, X. L.; Yang, X. F.; Wang, X. D.; Tai, Z. J.; Wilson, K.; Zhang,
T. Ag Alloyed Pd Single-Atom Catalysts for Efficient Selective
Hydrogenation of Acetylene to Ethylene in Excess Ethylene. ACS
Catal. 2015, 5, 3717−3725.
(33) Aich, P.; Wei, H. J.; Basan, B.; Kropf, A. J.; Schweitzer, N. M.;
Marshall, C. L.; Miller, J. T.; Meyer, R. Single-Atom Alloy Pd-Ag
Catalyst for Selective Hydrogenation of Acrolein. J. Phys. Chem. C
2015, 119, 18140−18148.
(34) Goulas, K. A.; Sreekumar, S.; Song, Y.; Kharidehal, P.; Gunbas,
G.; Dietrich, P. J.; Johnson, G. R.; Wang, Y. C.; Grippo, A. M.;
Grabow, L. C.; Gokhale, A. A.; Toste, F. D. Synergistic Effects in
Bimetallic Palladium-Copper Catalysts Improve Selectivity in Oxy-
genate Coupling Reactions. J. Am. Chem. Soc. 2016, 138, 6805−6812.
(35) Goulas, K. A.; Song, Y. Y.; Johnson, G. R.; Chen, J. P.; Gokhale,
A. A.; Grabow, L. C.; Toste, F. D. Selectivity tuning over
monometallic and bimetallic dehydrogenation catalysts: effects of
support and particle size. Catal. Sci. Technol. 2018, 8, 314−327.
(36) Muslehiddinoglu, J.; Li, J.; Tummala, S.; Deshpande, R. Highly
Diastereoselective Hydrogenation of Imines by a Bimetallic Pd-Cu
Heterogeneous Catalyst. Org. Process Res. Dev. 2010, 14, 890−894.
(37) Furukawa, S.; Suzuki, R.; Komatsu, T. Selective Activation of
Alcohols in the Presence of Reactive Amines over Intermetallic PdZn:
Efficient Catalysis for Alcohol-Based N-Alkylation of Various Amines.
ACS Catal. 2016, 6, 5946−5953.
(14) Huff, B. E.; Khau, V. V.; LeTourneau, M. E.; Martinelli, M. J.;
Nayyar, N. K.; Peterson, B. C. Diastereoselectivity in the
heterogeneous hydrogenation reactions of phosphorous substituted
olefins. Tetrahedron Lett. 1997, 38, 8627−8630.
(15) Ranade, V. S.; Consiglio, G.; Prins, R. Functional-group-
directed diastereoselective hydrogenation of aromatic compounds. 1.
J. Org. Chem. 1999, 64, 8862−8867.
(16) Ranade, V. S.; Consiglio, G.; Prins, R. Functional-group-
directed diastereoselective hydrogenation of aromatic compounds. 2.
J. Org. Chem. 2000, 65, 1132−1138.
(17) MaGee, D. I.; Lee, M. L.; Decken, A. Construction of cis- and
trans-decahydroisoquinolines via heterogeneous catalytic hydrogena-
tion. J. Org. Chem. 1999, 64, 2549−2554.
(18) Borszeky, K.; Mallat, T.; Baiker, A. Diastereoselective
hydrogenation at indenols: Evidence for sterically and electronically
unfavorable adsorption on palladium. J. Catal. 1999, 188, 413−416.
(19) Jiang, C.; Frontier, A. J. Stereoselective synthesis of pyrrolidine
derivatives via reduction of substituted pyrroles. Org. Lett. 2007, 9,
4939−4942.
(20) Sehgal, R. K.; Koenigsberger, R. U.; Howard, T. J. Effect of
Ring Size on Hydrogenation of Cyclic Allylic Alcohols. J. Org. Chem.
1975, 40, 3073−3078.
(38) Zafeiratos, S.; Piccinin, S.; Teschner, D. Alloys in catalysis:
phase separation and surface segregation phenomena in response to
the reactive environment. Catal. Sci. Technol. 2012, 2, 1787−1801.
(39) Han, P.; Axnanda, S.; Lyubinetsky, I.; Goodman, D. W. Atomic-
scale assembly of a heterogeneous catalytic site. J. Am. Chem. Soc.
2007, 129, 14355−14361.
(40) Wang, C. Y.; Chen, D. P.; Sang, X. H.; Unocic, R. R.; Skrabalak,
S. E. Size-Dependent Disorder-Order Transformation in the Synthesis
of Monodisperse Intermetallic PdCu Nanocatalysts. ACS Nano 2016,
10, 6345−6353.
(21) Vile, G.; Albani, D.; Almora-Barrios, N.; Lopez, N.; Perez-
Ramirez, J. Advances in the Design of Nanostructured Catalysts for
Selective Hydrogenation. ChemCatChem 2016, 8, 21−33.
(22) Zhang, L. L.; Zhou, M. X.; Wang, A. Q.; Zhang, T. Selective
Hydrogenation over Supported Metal Catalysts: From Nanoparticles
to Single Atoms. Chem. Rev. 2020, 120, 683−733.
(23) Sankar, M.; Dimitratos, N.; Miedziak, P. J.; Wells, P. P.; Kiely,
C. J.; Hutchings, G. J. Designing bimetallic catalysts for a green and
sustainable future. Chem. Soc. Rev. 2012, 41, 8099−8139.
6133
https://doi.org/10.1021/acscatal.1c01434
ACS Catal. 2021, 11, 6128−6134

ACS Catalysis
pubs.acs.org/acscatalysis
Letter
(41) Miller, J. B.; Matranga, C.; Gellman, A. J. Surface segregation in
a polycrystalline Pd70Cu30 alloy hydrogen purification membrane.
Surf. Sci. 2008, 602, 375−382.
(42) Zhao, M.; Brouwer, J. C.; Sloof, W. G.; Bottger, A. J. Surface
segregation of Pd-Cu alloy in various gas atmospheres. Int. J. Hydrogen
Energy 2020, 45, 21567−21572.
(43) Bradley, J. S.; Hill, E. W.; Chaudret, B.; Duteil, A. Surface
Chemistry on Colloidal Metals. Reversible Adsorbate-Induced Surface
Composition Changes in Colloidal Palladium-Copper Alloys.
Langmuir 1995, 11, 693−695.
(44) Noakes, T. C. Q.; Bailey, P.; Laroze, S.; Bloxham, L. H.; Raval,
R.; Baddeley, C. J. Pd/Cu alloys as hydrodechlorination catalysts: a
medium-energy ion scattering study of surface composition under
exposure to chlorinated hydrocarbons. Surf. Interface Anal. 2000, 30,
81−84.
(45) Beale, A. M.; Weckhuysen, B. M. EXAFS as a tool to
interrogate the size and shape of mono and bimetallic catalyst
nanoparticles. Phys. Chem. Chem. Phys. 2010, 12, 5562−5574.
́
̌
́
(46) Marinkovic, N. S.; Sasaki, K.; Adzic, R. R. Nanoparticle size
evaluation of catalysts by EXAFS: Advantages and limitations. Zast.
Mater. 2016, 57, 101−109.
6134
https://doi.org/10.1021/acscatal.1c01434
ACS Catal. 2021, 11, 6128−6134