On the involvement of palladium nanoparticles in the Heck and Suzuki reactions

Article abstract of DOI:10.1002/ejoc.200800852

We describe two different pathways by which palladium-catalyzed Heck and Suzuki coupling reactions take place. When sol-gel entrapped palladium acetate was used as a catalyst, the reactions proceed by the formation of metallic nanoparticles. Such particle

Full text of DOI:10.1002/ejoc.200800852

FULL PAPER
DOI: 10.1002/ejoc.200800852
On the Involvement of Palladium Nanoparticles in the Heck and Suzuki
Reactions
Dmitry Tsvelikhovsky,[a] Inna Popov,[b] Vitaly Gutkin,[b] Alina Rozin,[a]
Azariya Shvartsman,[a] and Jochanan Blum*[a]
Keywords: C–C coupling / Heterogeneous catalysis / Nanotechnology / Palladium / Sol–gel processes
We describe two different pathways by which palladium-cat-
alyzed Heck and Suzuki coupling reactions take place. When
sol–gel entrapped palladium acetate was used as a catalyst,
the reactions proceed by the formation of metallic nanopar-
longed reaction periods (48 h). The reactions in the presence
of the amino acid derivatives proceeded at a higher rate than
those by the entrapped palladium acetate. The existence of
palladium nanoparticles was investigated with the aid of
ticles. Such particles were already formed during the immo- high-resolution scanning transmission electron microscopy
bilization of the palladium salt. Upon application of palla-
dium derivatives of proline, tyrosine, or alanine, either in
their homogeneous or in their sol–gel entrapped version, no
Pd0 particles could be detected, even when the couplings
were carried out in boiling mesitylene (161 °C) or during pro-
equipped with energy dispersive X-ray spectroscopy, as well
as by X-ray photoelectron spectroscopy.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
the physical entrapment of palladium acetate within hydro-
phobicitized silica sol–gel.[4] A typical TEM image is shown
in Figure 1A. Dark contrast of heavy dense crystalline Pd
nanoparticles is seen on the image. The formation of the
sol–gel entrapped nanoparticles is also associated with the
darkening of the sol–gel matrix. The existence of most of
the metal particles as Pd0 has been confirmed by X-ray
photoelectron spectroscopy (XPS), which shows distin-
guished binding energy peaks (BE) of Pd 3d5/2 and 3d3/2 at
335.58 and 340.84 eV, respectively[5] (Figure 2A). The
shapes of these peaks may suggest the presence of small
amounts (up to 10%) of PdII species. After the use of the
catalyst in the Heck coupling reaction shown in Scheme 1
(R = H; X = Br) at 80 °C under the conditions described
in our previous paper[4] no significant changes in either the
micrograph or the XPS spectrum could be observed. The
appearance of the nanoparticles also did not change when
the aqueous solvent was replaced by an organic medium
(heptane, benzene, toluene, mesitylene). However, when the
entrapped palladium acetate (1) was replaced by sol–gel en-
trapped Z-bis(-prolinato)palladium(II) (2),[6] Z-bis(-
tyrosinato)palladium(II) (3),[7] or Z-bis(-alaninato)palladi-
um(II) (4),[8] the catalyst did not form nanoparticles in de-
tectable amounts during their entrapment within the ce-
ramic matrix or during the coupling process (see Experi-
mental Section). Unlike the reactions by Pd(OAc)2 that
could be performed in an aqueous medium under micro-
emulsion/sol–gel transport (EST) conditions,[4] the coupling
by the palladium derivatives of the amino acids had to be
conducted in benzene (or in other hydrocarbons), because
the immobilized palladium compounds leach readily into
Introduction
The wide application of palladium-catalyzed Heck and
Suzuki reactions in academic studies and in the industry[1]
piqued the interest of many researchers in the characteriza-
tion, the mechanisms, and the reaction intermediates in-
volved in these processes.[2] On the basis of experiments
from various laboratories, de Vries reported in his critical
review[2f] that “in all Heck reactions at high temperature, cer-
tainly above 120 °C, the palladium catalyst, irrespectively of
the nature of its precursor, is rapidly reduced to Pd0 which
has a strong tendency to form soluble colloids.” Although
this statement is certainly correct for many cases, we ob-
served that it is not a general rule. Some nanoparticle-free
couplings were found to take place even between 120 and
160 °C and may proceed at a higher rate than those in
which colloidal palladium is formed. Unlike the catalytic
systems that involve metallic nanoparticles,[3] the nanopart-
icle-free systems do not require the presence of any stabi-
lizer.
Results and Discussion
Transmission electron microscopy (TEM) images clearly
show the formation of nanoparticles of palladium during
[a] Institute of Chemistry, The Hebrew University,
Jerusalem 91904, Israel
Fax: +972-2-6513832
E-mail: jblum@chem.ch.huji.ac.il
[b] Center for Nanoscience and Nanotechnology, The Hebrew Uni-
versity,
Jerusalem 91904, Israel
98
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On the Involvement of Palladium Nanoparticles in the Heck and Suzuki Reactions
Figure 1. TEM images of the sol–gel support after the reaction of bromobenzene, styrene, and triethylamine in benzene (A) in the
presence of entrapped palladium acetate (1) and (B) in the presence of entrapped palladium prolinate (2).
water. In contrast to the Pd(OAc)2-catalyzed couplings, the
color of the reaction mixtures by the amino acid derivatives
remains pale yellow throughout the entire reaction period.
Upon searching for any changes that might have occurred
during the entrapment of 2, 3, and 4 we recorded the 13C
Scheme 1.
NMR spectra before and after the encagement processes.
For example, the 125 MHz 13C NMR of palladium prolin-
ate (2) in D2O consisted of resonance peaks at δ = 24.67,
161 °C. Likewise, we carried out couplings for an extended
24.78, 29.44, 29.89, 50.14, 52.70, 63.63, 65.04, 186.50,
reaction period (48 h). The TEM images of the used cata-
188.18 ppm. This spectrum is reduced to five signals at δ =
lysts in all these experiments were free of the dark TEM
23.78, 28.75, 51.64, 64.18, 184.71 ppm when the solvent-
spots characteristic of palladium nanoparticles such as
free solid-state NMR spectrum was recorded (owing to the
those formed when Pd(OAc)2 was used as catalyst [cf. e.g.,
diminished resolution). This spectrum resembles closely the
the transmission electron micrographs of sol–gel entrapped
Pd(OAc)2 shown as Figure 1A and that of sol–gel en-
trapped 2 shown as Figure 1B]. The only significant fea-
solid-state spectrum of the sol–gel entrapped compound at
δ = 24.05, 29.54, 50.76, 63.80, 187.86 ppm.
tures in the micrographs of the palladium prolinate cata-
lyzed Heck reactions were those of the pore entrances of
ca. 2–4.5 nm. Numerous STEM and XPS analyses were car-
ried out with the attempt to locate possible clusters of Pd0,
but no such clusters were found. In contrast, the XPS stud-
ies clearly indicated the presence of PdII–O species with BE
of Pd 3d5/2 of 336.83 and Pd 3d3/2 of 342.09 eV.[9] Attempts
to find evidence for the presence of PdIV species[10] did not
give positive results.
Whereas free amino acids such as proline, tyrosine, and
alanine react readily with Pd(OAc)2 according to Schemes 2
and 3, the disubstituted N,N-dimethylglycine (DMG) can-
not form an analogous palladium compound. Thus, in the
experiments of Reetz et al.[11] the formation of the active
Figure 2. Palladium 3d5/2 and 3d3/2 profiles (A) obtained after Heck
reaction in the presence of entrapped 1; (B) obtained after the reac-
palladium nanoparticles, which presumably were stabilized
tion in the presence of entrapped 2.
by DMG, result from the existing Pd(OAc)2.
Because it has been claimed that all palladium complexes
In analogy to the Heck reactions, the sol–gel entrapped
formed during the Heck reaction decompose above 120 °C Pd(OAc)2 (1) and palladium prolinate 2 promote the Suzuki
to give Pd0 particles,[2f] we performed the coupling both un- cross couplings shown in Scheme 4. Here too, the Pd-
[4]
der homogeneous and under heterogeneous conditions (OAc)2 and Pd(OAc)2/DMG were found to generate pal-
(with sol–gel entrapped catalyst) in boiling mesitylene at ladium nanoparticles, whereas in the reactions catalyzed by
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99

D. Tsvelikhovsky, I. Popov, V. Gutkin, A. Rozin, A. Shvartsman, J. Blum
FULL PAPER
activity (before and after concentration). No palladium spe-
cies and no catalytic activity could be traced. Sol–gel en-
trapped 2, 3, and 4 proved recyclable. For example, in a
series of experiments with bromobenzene and styrene in the
presence of sol–gel entrapped 2 at 80 °C for 4 h the yields
in each of the first six runs was quantitative (Ͼ99% by GC
analysis). We recall that recycling of the catalyst was also
possible in reactions by entrapped Pd(OAc)2 that forms
metallic nanoparticles[4] (see, however, ref.[2f]).
Scheme 2.
Although we did not perform systematic kinetic studies,
we compared under identical experimental conditions the
initial rates of the coupling of iodobenzene with styrene in
a DMF/benzene (2:5) mixture in the presence of one of the
entrapped catalysts 1–4 at 80 °C. The corresponding rates
are listed in Table 2. The faster coupling rates by the palla-
dium derivatives of the amino acids as compared with the
rate observed with the entrapped Pd(OAc)2 catalyst, is re-
sponsible for the fact that in all coupling experiments per-
formed, the yields obtained by 2–4 where higher than those
by 1 (see Table 2).
Scheme 3.
2 (irrespectively if the complex was in its homogeneous or
immobilized version) no Pd0 species could be detected. The
results of some Heck and Suzuki coupling reactions by one
of the amino acid derivatives are summarized in Table 1,
which indicates that in analogy to the reaction promoted
by immobilized Pd(OAc)2[4] the substitution of the aromatic
bromide with an electron-donating methyl group has an in-
hibition effect on both the Heck and the Suzuki couplings,
under the present conditions. However, also the reactions
described in entries 3 and 7 (Table 1) can be brought to
completion when the time is extended to 24 h. It is notable
that although many Heck and Suzuki reactions are often
accompanied under homogeneous conditions by side prod-
ucts,[12,13] no 1,1-diarylethylenes are formed in entries 1–4
and no biphenyl (homocoupling product) is obtained in en-
tries 6 and 7.
Table 2. Comparison between the efficiencies of entrapped palla-
dium catalysts 1–4 in the Heck coupling of iodobenzene and sty-
rene.[a]
Entry
Catalyst
105 Initial rate (molL–1s–1) Yield after 1 h (%)
1
2
3
4
1
2
3
4
17.5
48.9
20.8
23.9
52
99[b]
62
70.6
[a] Reaction conditions: iodobenzene (1.34 mmol), styrene
(1.34 mmol), triethylamine (2 mmol), entrapped palladium catalyst
(0.027 mmol), DMF (2 mL), and dry benzene (5 mL); 80 °C, 1 h.
[b] 72.5% after 30 min.
At this stage we have no sound explanation for the higher
efficiency of the nanoparticle-free systems, despite earlier
Scheme 4.
Whereas the amino acid palladium catalysts are insoluble observations that stabilized nanoparticles are usually the
in many organic solvents, they can be entrapped within sil- more active ones.[2g,14]
ica sol–gel matrices, and they then react in aromatic hydro-
In order to test whether the catalytic activity of the palla-
carbons without any detectable leaching of the catalyst. Af- dium amino acid derivatives results from undetectable “ho-
ter each reaction, the catalyst-containing ceramic material meopathic” amounts of metallic nanoparticles,[2f] we grad-
was filtered off, and the filtrate was analyzed for palladium ually reduced the molar ratio of palladium/bromobenzene
by ICP. The filtrates were also tested for residual catalytic in the Heck and Suzuki couplings of entries 1 and 5 in
Table 1. cis-Bis(-prolinato)palladium-catalyzed Heck and Suzuki coupling reactions of some bromoarenes under comparable condi-
tions.[a]
Entry
Bromoarene
Arylation reagent
Product (yield after 1.5 h, %)[b]
1
2
3
4
5
6
7
C6H5Br
4-ClC6H4Br
4-CH3C6H4Br
C6H5Br
C6H5Br
4-ClC6H4Br
4-CH3C6H4Br
C6H5CH=CH2
C6H5CH=CH2
C6H5CH=CH2
C6H5CH=CH2
C6H5B(OH)2
C6H5B(OH)2
C6H5B(OH)2
(E)-C6H5CH=CHC6H5 (71)
(E)-4-ClC6H4CH=CHC6H5 (44)
(E)-4-CH3C6H4CH=CHC6H5 (18.5)
(E)-C6H5CH=CHC6H5 (72)
(C6H5)2 (84)
4-ClC6H4C6H5 (54)
4-CH3C6H4C6H5 (27)
[a] Reaction conditions for entries 1–3: bromoarene (1.34 mmol), styrene (1.34 mmol), cis-bis(prolinato)palladium (0.0134 mmol) en-
trapped within silica sol–gel [from tetramethoxysilane (3.6 mL), MeOH (2.4 mL), and H2O (2 mL)], Et3N (1.62 mg, 1.61 mmol), and
benzene (10 mL), 78 °C; conditions for entries 5–7: the same as for entries 1–3 but instead styrene phenylboronic acid (1.34 mmol) was
used; conditions for entry 4: the reaction was performed without sol–gel material in a microemulsion that contained sodium dodecyl
sulfate (3.3 wt.-%), BuOH (6.6 wt.-%), H2O (89.3 wt.-%), and Et3N was replaced by K2CO3. [b] By extending the reaction time to 4 h,
entries 1, 2, 4, 5, and 6 afforded quantitative yields and entries 3 and 7 afforded 48.5 and 71% yield, respectively. In order to obtain
quantitative yield in entries 3 and 7 the reactions were conducted for 24 h.
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On the Involvement of Palladium Nanoparticles in the Heck and Suzuki Reactions
Table 1 from 1:102 to 1:104 and found that the yields also
decreased gradually. Below 1:103 hardly any reaction took
place during the first 20 h.
mental composition. Initial powders were dispersed in ethanol and
dropped onto a standard 400 mesh carbon coated copper TEM
grid. The various bromo- and iodoarenes, styrene, tetramethoxysi-
lane, palladium acetate, sodium dodecyl sulfate, B-phenylboronic
acid, -alanine, -tyrosine, -proline, N,N-dimethylglycine, 1,1Ј-
(1E)-1,2-ethenediylbis(benzene), 1-methyl-4-[(1E)-2-phenylethenyl]-
benzene, 1-chloro-4-[(1E)-2-phenylethenyl]benzene, and biphenyl
were obtained from commercial sources. 4-Methyl-1,1Ј-biphenyl,[4]
4-chloro-1,1Ј-biphenyl,[4] Z-bis(-prolinato)palladium,[6] Z-bis(-
tyrosinato)palladium,[7] and Z-(-alaninato)palladium[8] were pre-
pared according to literature procedures.
Conclusions
Although our experiments indicate clearly the formation
of palladium nanoparticles during the entrapment of
Pd(OAc)2 within silica sol–gel matrices, as well as during
the application of the heterogeneous salt in the Heck and
Suzuki coupling reactions, we found no proof for the gener-
ation of metallic particles in the presence of the amino acid
derivatives 2–4. As no Pd0 nanoparticles were found even
after conducting the coupling reactions in the presence of
2–4 at 160 °C or after extending the reaction time to 48 h,
we assume that the reaction by 1 proceeds by a different
pathway than by the entrapped palladium amino acid deriv-
atives. It is possible that despite the fact that XPS analysis
was unable to detect the presence of unstable PdIV species,
Entrapment of 2, 3 and 4 within Sol–gel Matrices
Method A: A solution of tetramethoxysilane (3.6 mL) in triply dis-
tilled water (TDW, 2 mL) was stirred magnetically with a solution
of palladium acetate (30 mg, 0.134 mmol) in dichloromethane
(1 mL), and with a solution of the amino acid (0.67 mmol) in
MeOH (2.4 mL). Usually 4–5 drops of MeOH was added to obtain
a clear transparent solution. After ca. 10 h gelation was complete.
The pale-yellow gel was aged for 24 h and then dried under 0.5 Torr
at 80 °C for 24 h, washed with MeOH (20 mL), sonicated with
dichloromethane (20 mL), and dried again until constant weight
the faster nanoparticle-free processes take place through a was achieved.
catalytic cycle that includes PdII and PdIV species [but not
Method B: A mixture of a solution of palladium acetate (400 mg,
1.78 mmol) in dichloromethane (5 mL) and the amino acid
(8.9 mmol) in MeOH (5 mL) was agitated in an open-glass vessel
Pd0] as suggested, for example, by Herrmann et al. and by
Shaw.[2a,15]
Finally, we wish to draw attention to the existence of a for 30 min at room temperature. The pale-yellow powder was fil-
tered and recrystallized from MeOH, added to a mixture of tetra-
methoxysilane (48 mL) and TDW (27 mL), awaited gelation, and
worked up as described above. The palladium content of the en-
trapped complexes was determined by ICP. The palladium-en-
trapped sol–gel material was heated at reflux for 24 h in dry ben-
zene, cooled, and filtered. The concentrated filtrate was then sub-
jected to TEM analysis, which indicated in all cases the absence of
palladium nanoparticles.
combined catalyst system composed of sol–gel entrapped
[Rh(cod)Cl]2 and Na[HRu3(CO)11] that promotes the hy-
drogenation of methylated arenes with the formation of
metallic nanoparticles at 80 °C, but that takes place without
their intermediary at 20 °C.[16]
Experimental Section
General Procedure for the Heck and Suzuki Cross-Coupling Reac-
tions by Sol–gel Entrapped 2–4: The sol–gel entrapped catalyst (2,
3, or 4, 0.0134 mmol) and the organic substrates (0.67–1.34 mmol
of each of them), triethylamine (2.6 mmol), and benzene (7 mL)
were placed in a glass pressure tube. The mixture was agitated at
80 °C for the desired length of time. After cooling the reaction mix-
ture to room temperature the ceramic material was filtered off, ex-
tracted with diethyl ether (2ϫ50 mL), dried at 80 °C under 1 Torr,
and analyzed by ICP for leached palladium. Under the above con-
ditions no leaching could be detected and the recovered immobi-
lized catalyst could be used without further workup in the next
run. The organic solutions were concentrated, and the residues
were either separated by column chromatography or analyzed by
GC, GC–MS, and NMR spectroscopy. The physical data of the
products were compared with those of authentic samples.
General: 1H and 13C NMR spectra were recorded with either a
Bruker DRX-400 or a Bruker Avance II-500 instrument in CDCl3.
Infrared spectra were obtained with a Bruker Vector 22-FRIT spec-
trometer. Mass spectral measurements were performed with a Hew-
lett–Packard model 4989 Å mass spectrometer equipped with an
HP model 5890 series II gas chromatograph. ICP-MS analyses were
performed with a Perkin–Elmer model ELAN DRC II instrument.
A Micrometrics ASAP 2020 instrument was used for BET-N2 and
BJH-N2 surface area and pore diameter measurements of the sol–
gel matrices. Gas chromatic separations were performed with a
Hewlett–Packard model Agilent, 4890D, by using either a 15 m
long capillary column packed with bonded and crosslinked (5%
phenyl)methyl polysiloxane (HP-5) or a 30 m long column packed
with Carbowax 20 -poly(ethylene glycol) in fused silica (Supelco
25301-U). XPS measurements were performed with a Kratos Axis
Ultra X-ray photoelectron spectrometer. Spectra were acquired
with monochromated Al-Kα (1486.7 eV) X-ray source with 0° take-
off angle. The pressure in the test chamber was maintained at
1.5ϫ10–9 Torr during the acquisition process. High-resolution XPS
scans were collected for Pd 3d and C 1s peaks with pass energy
20 eV. The XPS binding energy was calibrated with respect to the
peak position of C 1s as 285.0 eV. Data analysis was performed
with Vision processing data reduction software (Kratos Analytical
Ltd.) and CasaXPS (Casa Software Ltd.). Transmission electron
microscopy was done with Scanning Transmission Electron Micro-
scope (STEM) Tecnai G2 F20 (FEI Company, USA) operated at
Comparative experiments were also performed (a) in DMF/ben-
zene (2:5) without the ceramic support and (b) in a microemulsion
(without sol–gel material) prepared from sodium dodecyl sulfate
(≈3 wt.-%), n-propanol (≈7 wt.-%), the aromatic substrates
(1.34 mmol of each, ≈1 wt.-%), and water (≈89 wt.-%). Reactions
in aqueous media were carried out usually in the presence of
K2CO3 rather than in the presence of triethylamine.
Acknowledgments
We gratefully acknowledge the financial support of this study by
200 kV and equipped with EDAX-EDS for identification of ele- the Israel Science Foundation (ISF) through grant no. 269/06.
Eur. J. Org. Chem. 2009, 98–102
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Received: September 4, 2008
Published Online: November 21, 2008
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