UC Pd: A new form of PdVC for sonogashira couplings

Article abstract of DOI:10.1002/chem.200902471

Screening of different sources of Pd/C shows reagents of highly variable nanoparticle sizes and oxidation states of the metal. Typically, catalysts with higher surface area are viewed as likely to be the more reactive. In this paper a new form of Pd/C, "U

Full text of DOI:10.1002/chem.200902471

DOI: 10.1002/chem.200902471
UC Pd: A New Form of Pd/C for Sonogashira Couplings
Christophe Duplais, Arnold J. Forman, Benjamin A. Baker, and Bruce H. Lipshutz*[a]
Abstract: Screening of different sour-
ces of Pd/C shows reagents of highly
variable nanoparticle sizes and oxida-
tion states of the metal. Typically, cata-
lysts with higher surface area are
viewed as likely to be the more reac-
tive. In this paper a new form of Pd/C,
“UC Pd” is described that is shown to
contain larger nanoparticles yet it is
the most reactive catalyst of those sold
commercially for Sonogashira coupling
reactions. UC Pd functions efficiently
in the absence of a copper co-catalyst,
under very mild and “green” condi-
tions using inexpensive 95% EtOH at
508C. It is also the only form of Pd/C
that can be recycled. In side-by-side re-
actions with several commercially
available forms of Pd/C, none compete
successfully with UC Pd under standar-
dized conditions. Physical data ob-
tained from extensive surface analysis
using TEM, XRD, XPS, and CO-TPD
measurements lead to an explanation
behind the unique reactivity of this
new recyclable form of Pd/C.
Keywords: cross-coupling
· green
chemistry · heterogeneous catalysis ·
palladium · Sonogashira couplings
Introduction
reaction conditions to be optimized (e.g., ligand, solvent),
perhaps it would not be surprising if the many commercial
forms of Pd/C were found not to be equally effective. Arriv-
ing at a reliable, reproducible Pd/C catalyst capable of medi-
ating selected heterogeneous cross-couplings seems not only
potentially important to synthesis and green chemistry, but
long overdue. In this report we describe the preparation of a
new, active form of palladium-on-charcoal (“UC Pd”) that is
unlike any of the existing commercial materials. Such differ-
ences are not only manifested by comparisons of measurable
physical data (e.g., TEM, XRD, XPS, and CO-TPD), but
also by direct side-by-side experiments in catalyzed Sonoga-
shira couplings. It has been shown that UC Pd uniquely
serves as the first effective and recyclable catalyst for
copper-free Sonogashira couplings with aryl bromides as
substrates. Moreover, given the reactivity of newly fashioned
UC Pd, the conditions under which these reactions occur
are very mild and take into consideration environmental
concerns, safety issues, and economic factors.
One approach to enhancing the appeal of green chemistry[1]
is to improve the range of transition metal-catalyzed cross-
coupling reactions conducted under heterogeneous condi-
tions.[2] Palladium-on-charcoal is a well-established reagent[3]
that has found increasing popularity in several of the more
À
common C C bond-forming name reactions (e.g., Suzuki–
Miyaura,[4] Heck[5] and Sonogashira[6–8]). However, there are
issues associated with its use (e.g., levels of leaching) and,
perhaps more difficult to address, consistency among suppli-
ers of Pd/C. Indeed, as a catalyst for hydrogenation, the
level of success can be quite variable and several different
sources of Pd/C are often required for evaluation.[9] Hence,
variation might be expected as well among non-hydrogena-
tive processes, such as Pd-catalyzed cross-coupling reactions.
The reasons for such variation among Pd/C catalysts could
lie in the source of carbon (e.g., wood, coconut, peat moss),
the temperature of treatment, and the levels of exposure to
oxygen, among other oxidants.[10] Given the multiple param-
eters associated with Pd/C formation, in addition to other
Results and Discussion
Preparation and reactivity of UC Pd: As previously de-
scribed for both Ni/C[11] and Cu/C,[12] UC Pd is prepared by
[a] Dr. C. Duplais, A. J. Forman, B. A. Baker, Prof. Dr. B. H. Lipshutz
University of California, Santa Barbara, CA 93106 (USA)
Fax : (+1)805-893-8265
simply dissolving PdACHTNUTRGNEUNG(NO3)2 in water to which is added com-
mercially available activated charcoal (Aldrich catalog
#278092; Scheme 1). Ultrasonication for 20 h is then fol-
lowed by distillation of the water, subsequent washing with
E-mail: lipshutz@chem.ucsb.edu
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200902471.
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FULL PAPER
toluene and diethyl ether, and drying overnight at 808C
under high vacuum. While metal reduction usually involves
flammable hydrogen or sodium borohydride, in this proce-
dure it is accomplished in situ by the charcoal support
throughout the course of the distillation of water. Ultrasoni-
cation presumably helps greatly in distributing the metal
particles throughout the charcoal matrix, as seen previously
for both Ni[11] and Cu.[12]
displayed considerably lower activity compared with UC Pd
under otherwise identical conditions (Table 2). UC Pd dis-
played a similar trend in activity in each of these substrate
combinations. By contrast, a fresh sample of 10% Pd/C
from Aldrich[13] led to moderate conversion of m-bromoani-
sole or p-bromobenzonitrile under optimized conditions.
Likewise, PdII/C Degussa E101[14] was found to be even less
prone to mediate these couplings. Pd/C from Alfa Aesar,[15]
as well as the Pd/C catalyst Selcat Q6,[16] were far more
active toward catalysis, but neither was competitive with
UC Pd.
Scheme 1. Preparation of UC Pd.
Table 2. Comparisons between Pd/C reagents for three cross-couplings.[a]
The catalytic activity of UC Pd versus that of commercial
sources was tested in Sonogashira coupling reactions. Pd/C,
in general, is known to promote couplings between terminal
acetylenes and either aryl iodides[7a–e] or aryl chlorides[7f] in
the absence of copper salts. Inexplicably, related couplings
with readily available aryl bromides have never been de-
scribed. Notably, existing procedures whether involving aryl
iodides or chlorides, while copper-free, are not truly hetero-
geneous in nature; that is, each describes Pd/C as providing
a catalytic amount of metal that mediates homogeneous cat-
alysis. On the other hand, UC Pd catalyzes heterogeneous
sp–sp2 cross-couplings with aryl bromides, and applies to a
broad range of substrates. UC Pd samples containing load-
ings of 6, 8, and 10% (by weight) were screened for activity
toward m-bromoanisole with a terminal alkyne (phenylace-
tylene; Table 1). Most effective was 10% metal (2 mol% vs
substrate); lower percentages gave dramatically reduced
levels of conversion after five hours at 508C. Triphenylphos-
phine (10 mol%) serves well as ligand for this coupling, but
only when assisted by microwave irradiation at higher tem-
perature (entry 1). However, use of reduced levels of both
UC Pd (2 mol%) and ligand (2 mol%) are effective by re-
placing Ph3P with XPhos. Inexpensive K2CO3 serves as the
base, and 95% ethanol is the solvent of choice in all cases
studied (0.3m).
Conversion [%]
Conversion [%]
Conversion [%]
UC Pd
100
60
55
45
25
100
45
53
38
5
100
65
70
54
22
Alfa Aesar
Selcat Q6
Aldrich
Degussa E101
[a] Pd/C, XPhos (2 mol%), K2CO3 (2 equiv), 95% EtOH, 508C; same Pd
loading: 10% by weight.
Effect of copper(I): Liebscher[6k] and Novꢁk[7f] have both
found that rates of Sonogashira couplings slow significantly
with Pd/C in the presence of a copper salt as co-catalyst. Re-
sults from our study also show this tendency. With only
5 mol% of CuI present, UC Pd loses half of its activity
under microwave assistance (conditions A), and essentially
all activity is gone at 508C (conditions B; Table 3, entry 1).
A similar trend was observed with others sources of Pd0/C
(e.g., entry 2). Surprisingly, added CuI to PdII/C Degus-
sa E101 enhanced the isolated yield of product alkyne
(Table 3, entry 3). These results, while not readily explained,
clearly establish how the source and oxidation state of such
a fundamental reagent can play dramatic roles in catalysis
by “Pd/C”.
Comparison with commercial sources of Pd/C: Remarkably,
each of the commercially available sources of Pd/C tested
Reclaimed catalyst upon filtration retains most of its reac-
tivity, which was determined using successive reactions on
the aryl bromide, m-bromoani-
sole (Table 4). Good isolated
yields were observed through
Table 1. Optimization of conditions for Sonogashira couplings catalyzed by UC Pd.
four cycles, results that could
only be achieved with UC Pd
notwithstanding the extended
time frame (12 h vs 3 h; see
Entry
Pd/C loading [%][a]
Pd [mol%]
L
Ratio Pd/L
T [8C]
130[c]
50
50
50
Conversion [%][b]
Table 2). All vendor-supplied
catalysts, however, gave low
yields after initial use. Such
losses of reactivity associated
with commercially available
sources of Pd/C have also been
noted previously in Heck,[5g,j,k]
1
2
3
4
5
6
6
6
8
5
5
2
2
2
PPh3
PPh3
XPhos
XPhos
XPhos
1:3
1:3
1:1
1:1
1:1
95
<5
20
45
10
50
100 (95[d]
)
[a] Measured by ICP. [b] Decane was used as internal standard. [c] The reaction was performed in a microwave
(1308C, 30 min). [d] Isolated yield on 2 mmol scale.
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3367

B. H. Lipshutz et al.
Table 3. Influence of copper iodide as co-catalyst.[a]
trasts with related samples of
Pd/C, which, likewise using
TEM analyses, tend to form
much larger, less reactive, and
Conditions A[b]
no CuI
Conditions B[c]
no CuI
non-recyclable
aggregates
Entry
Catalyst
5 mol% CuI
5 mol% CuI
during the reaction.[5l,17] These
observations correlate well with
the X-ray diffraction (XRD)
pattern that indicates Pd0 crys-
tallites of 60 nm average diame-
ter (Figure 2). By contrast, all
commercially available catalysts
1
2
3
UC Pd
Aldrich
Degussa E101
95%
90%
45%
55%
40%
95%
90%
–
26%
traces
–
68%
[a] Isolated yield on 2 mmol scale. [b] 5 mol% Pd/C, 10 mol% PPh3, 95% ethanol, 1208C (microwaves),
15 min. [c] 2 mol% Pd/C, 2 mol% XPhos, 95% ethanol, 508C, 5 h.
Suzuki–Miyaura[17] couplings, as well as in Sonogashira reac-
tions.[7e]
examined by XRD consist of smaller nanoparticles (6–
15 nm on average).
Carbon monoxide temperature programmed desorption
(CO-TPD) was used to quantify the Pd surface sites due to
the specificity of the Pd-CO bonding ratio of 1:1. It is ob-
served (Figure 3) that the relative accessible palladium sur-
face areas (as integrated area under each desorption curve)
are dependent on the source. Additionally, there are small
differences in the effective binding/desorption energies
(peak positions) suggesting slightly different chemical envi-
ronments for Pd in each catalyst. Both samples of Pd/C
either sold by Alfa Aesar, or prepared as UC Pd, show the
greatest surface area of Pd (Table S1, entries 5, 6), even
though the size of the nanoparticles for UC Pd is larger than
that of all other commercial samples (Figure 2).
Table 4. Recycling of catalysts (isolated yields).
Run UC Pd
[%]
Alfa Aesar
[%]
Degussa E101
[%]
Aldrich
[%]
Selcat Q6
[%]
1st
95
92
89
87
94
45
10
–
75
21
–
84
49
15
–
90
54
20
–
2nd
3rd
4th
–
Since the data as to particle size alone cannot be used to
explain both the greater reactivity and unique recyclability
of UC Pd compared to other sources of Pd/C, additional sur-
face analyses were conducted. X-ray photoelectron spectros-
copy (XPS) was employed to determine the oxidation state
of palladium at the surface. Given that the “escape depth”
for photoelectrons generated within any given substance is
quite shallow, XPS measures a signal from only the top 2–
4 nm of a material. This surface specificity is particularly in-
formative, in contrast to the “bulk” signal observed in XRD.
All Pd/C catalysts examined by this technique contain
roughly 30% Pd0 at their surface, with the exception of De-
gussa E101 that shows 15% (Table S2). The amounts of PdII
and PdIV present vary widely between samples, and hence,
no trend with respect to particle size or activity is observed.
However, after use, UC Pd retains its distribution of oxida-
tion states (ca. 30% Pd0; Figure 4), while the Degussa E101
and Alfa Aesar catalysts drop from 15 to 5%, and from 27
to 9% Pd0, respectively. The substantially better stability of
UC Pd may derive from its higher levels of reduced metal at
the surface relative to other catalysts due to its larger metal
nanoparticles. These are less likely to stabilize oxides on
their surface relative to smaller nanoparticles.[18] The impli-
cation is that there exists an optimum particle size which
balances surface area with surface stability. Thus, while to
some extent counterintuitive, using smaller size particles
does not translate into greater reactivity. This balance im-
pacts both catalyst activity and recyclability; only UC Pd
emerges with minimal modification to its surface following
participation in Sonogashira cross-couplings.
Several examples that lead to products 4–17 and attest to
the generality of this mild, heterogeneous cross-coupling are
illustrated in Table 5. Aryl bromides bearing either activat-
ing or deactivating substituents gave the corresponding dis-
ubstituted acetylenes in high isolated yields. Successful
cross-couplings involving an assortment of terminal acety-
lenes containing a variety of functional groups, such as ni-
trile, chloride, acetal, and alcohol groups, lend further cre-
dence to the attractiveness of this protocol. Featured cases
include aryl bromides bearing o-CF3 (entry 2), m-amino (en-
tries 3, 4), p-cyano (entries 5, 6), p-nitro (entry 8), keto (en-
tries 9, 10), and 3-pyridyl groups (entry 11), and more highly
functionalized substrates (entries 12 and 13). Acetylenes
containing an w-cyano (entries 2, 3), a-quaternary carbon
(entries 9, 11, 13), and a cholesteryl residue (entry 14) all re-
acted smoothly. Double arylation of a diyne (entry 4) is also
worth mentioning. Tests were conducted on UC Pd to assess
the extent to which it provides mainly heterogeneous cataly-
sis. Analysis of a reaction mixture (as in Table 4) premature-
ly stopped (at 60% conversion) and filtered hot to remove
Pd/C showed no further consumption of educts upon further
treatment at 508C. ICP-AES analysis on the filtrate further
confirmed that only traces of Pd are detected in solution (2–
4 ppm).
Analyses of Pd/C: Bright field transmission electron micros-
copy (TEM) imaging of UC Pd shows palladium nanoparti-
cles with a fairly broad size distribution, in the 10–300 nm
range, both before and after catalysis (Figure 1). This con-
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Sonogashira Couplings
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Table 5. UC Pd-catalyzed cross-couplings to form functionalized alkynes.
Entry Alkyne
Product
Yield
[%][a,b]
1
2
90
95
3
4
96[c]
82[c,e]
5
6
7
91
85
88
8
9
80[d]
87
Figure 1. TEM analysis a) initial reagent, UC Pd; b) UC Pd after cataly-
sis.
10
11
80[c]
92
12
13
14
55[d,f]
81[d]
90[c,g]
Figure 2. XRD profiles of Pd/C catalysts.
[a] Conditions: aryl bromide (2 mmol), alkyne (4 mmol), K2CO3 (4 mmol),
2 mol% UC Pd, 2 mol% XPhos, 95% ethanol (6 mL), 508C, 5 h. [b] Isolated
yield. [c] Run over 10 h. [d] Run over 15 h. [e] Alkyne (2 mmol), aryl bromide
(8 mmol), K2CO3 (4 mmol). [f] 5 mol% of Pd/C, 5 mol% XPhos, K2CO3
(6 mmol). [g] Alkyne (1 mmol), aryl bromide (2 mmol), K2CO3 (2 mmol).
rectly with UC Pd in terms of reactivity profile, recyclability,
and nature with respect to particle size and metal oxidation
state(s): all were found to vary widely; none is as reactive,
nor can any be recycled effectively. Representative cross-
couplings, as illustrated with the Sonogashira coupling reac-
tion, support these differences and highlight the counterin-
tuitive notion in catalysis that smaller particle size is typical-
ly correlated with greater reactivity, which is not the case
with UC Pd.
Conclusion
A novel form of palladium-on-charcoal (UC Pd), easily pre-
pared from Pd
ACHTUNGTRENNUNG(NO3)2 in water, has been developed. Several
commercially available sources of Pd/C were compared di-
Chem. Eur. J. 2010, 16, 3366 – 3371
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3369

B. H. Lipshutz et al.
and then the terminal alkyne (4 mmol). The flask was submerged in a
pre-heated 508C water bath and stirred vigorously for the indicated time.
The mixture was then poured into a flask containing silica (3–4 g) and
the sides of the reaction flask were washed with diethyl ether. Solvents
were removed under vaccum and the resulting dry, crude silica was intro-
duced on top of a silica gel chromatography column to purify the prod-
uct.
Acknowledgements
We are grateful to the NIH and NSF for providing financial support. We
are indebted to Dr. Louise Weaver (University of New Brunswick) for
TEM data, Xiaoying Ouyang for CO-TPD data, and Professor S. Scott
(UCSB) for valuable discussions. A.J.F. gratefully acknowledges support
from the US EPA STAR Fellowship program and the UCSB Partnership
for International Research and Education in Electron Chemistry and
Catalysis at Interfaces (PIRE-ECCI) fellowship program (NSF grant
number OISE-0530268).
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Experimental Section
Preparation of 10% UC Pd: Darco KB activated carbon (Aldrich, cata-
log #278092, 100 mesh, ꢀ25% H2O) was dried overnight under vaccum
at 1508C. The dry activated carbon (3.6 g) was added to a 100 mL round-
bottom flask containing a stir bar. A solution of PdACTHNUTRGNE(UNG NO3)2·H2O (Strem
Chemicals, catalog #93-4608, Pd ꢀ40%, 1 g, 3.75 mmol) in deionized
H2O (50 mL) was added to the activated carbon, and additional deion-
ized H2O (30 mL) was added to wash down the sides of the flask. The
flask was stirred vigourously at RT for 1 h and then submerged in a ultra-
sonic bath for 20 h. It was then attached to a distillation setup and placed
in a pre-heated (175–1808C) sand bath while stirring. As the distillation
ended, the sand temperature began to rise and was held below 2008C.
After cooling the charcoal to RT, it was washed with toluene (50 mL)
and then ether (50 mL), and the resulting solid filtered in vacuo in a frit-
ted funnel. The fritted funnel was turned upside down under vaccum
overnight until the Pd/C fell from the frit into the collection flask placed
in a preheated 808C sand bath. The impregnated charcoal (ca. 3.94 g)
was transferred to, and stored in, separate vials. Thus, 10.2 wt.% Pd/C
(calculated by ICP analysis), or 0.93 mmol Pd/g catalyst was obtained.
General procedure for Pd/C-catalyzed cross-coupling reaction between
aryl bromides and terminal alkynes: In a 10 mL round-bottom flask
under argon containing Pd/C (42 mg, 0.93 mmolgÀ1 ca. 0.04 mmol),
XPhos (18 mg, 0.04 mmol) and potassium carbonate (552 mg, 4 mmol)
was added 95% ethanol (6 mL) followed by the aryl bromide (2 mmol)
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Sonogashira Couplings
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