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Catalysis in Capillaries by Pd Thin Films Using Microwave-Assisted Continuous-Flow Organic Synthesis (MACOS).

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Continuous-Flow Reactions
DOI: 10.1002/ange.200503600
Catalysis in Capillaries by Pd Thin Films Using
Microwave-Assisted Continuous-Flow Organic
Synthesis (MACOS)**
Gjergji Shore, Sylvie Morin, and Michael G. Organ*
Microwave-assisted organic synthesis (MAOS) has blossomed over the last five years to become a presence in most
academic and industrial labs.[1] At the same time, significant
efforts have been made to develop flow methods for synthetic
chemistry.[2, 3] Both methodologies reduce the time required to
conduct synthesis, but in different ways. MAOS greatly
accelerates the rate of individual chemical transformations
by direct and more effective heating of the reaction solution
than that with conventional heating sources such as an oil
bath or heating mantle. Flow techniques target handling and
processing operations between transformations, often using
solid-supported reagents/catalysts and chromatographic
media to streamline reaction workup and purification by
requiring only simple filtration. An optimal arrangement
would be to combine MAOS with flow techniques to gain the
powerful benefits of both, and this is the starting point for
microwave-assisted continuous-flow organic synthesis
(MACOS).[4–6] To fuel the development of this new area,
new reagents and catalysts that are immobilized on surfaces
must be developed that can withstand pressurized flow,
microwave irradiation, and very high temperatures.
This report details the development of thin metal films
that serve as catalysts for MACOS. There are two literature
reports that describe the deposition of Pd black on the surface
of glass reaction vessels where the metal films themselves
proved suitable to catalyze Heck and Sonogashira couplings,[7] as well as C P coupling.[8] In these cases, the
reactions were run in batch mode; that is, the reactions
were not flowed through the microwave chamber. These Pdfilm-catalyzed reactions required anywhere from 3 min to 5 d
of constant irradiation to achieve good conversion. Here we
detail the methods to deposit Pd on the inner surface of
capillaries, the complete characterization of these films
including their morphology and elemental composition, and
their use in Suzuki and Heck reactions using MACOS.
[*] G. Shore, Prof. S. Morin, Prof. M. G. Organ
The Department of Chemistry, York University
4700 Keele Street, Toronto, ON M3J 1P3 (Canada)
Fax: (+ 1) 416-735-5936
[**] This work was funded by the Ontario Research and Development
Challenge Fund (ORDCF), Canada Foundation for Innovation (CFI),
the Ontario Innovation Trust (OIT), NSERC, and York University.
The authors are grateful to Biotage Inc. for the donation of a Smith
Creator Synthesizer to develop this new methodology. We
acknowledge Ross Davidson at Surface Science Western, University
of Western Ontario, London, ON, for the SEM and EDX measurements.
Angew. Chem. 2006, 118, 2827 –2832
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
The preparation of Pd films is based on methods for the
preparation of palladium particles and sols by thermal
decomposition of organometallic compounds in organic
solvents.[9] It was reported that monodispersed Pd nanoparticles could be obtained by rapid heating of the solution.[9d]
While the methodology for the preparation of Pd sols can be
optimized carefully to prevent sedimentation and flocculation
by using the appropriate organic solvent[9c–e] or by controlling
the amount of water present,[10] we optimized sedimentation/
flocculation in order to produce the metal film. Pd thin films
were prepared by flowing a 0.1 mmol mL 1 solution of
PdACHTUNGRE(OAc)2 into a 1150-mm capillary, capping the ends, and
heating the capillary at 150 8C for 30 min. During this time,
Pd0 gradually plated out on the wall of the capillary producing
a black film with a mat appearance. Closer inspection of the
film morphology revealed that the films are highly porous and
consist of nanometer-size Pd crystallites (see Figure 1).
Figure 1. a) SEM image of the Pd films prepared inside a glass
capillary and removed. (B 50.0). b) Cross-section of the edge located
on the lefthand side of sample shown in (a) (B 5000). Image taken
from the central portion of the sample in (a) at c) B 1500, d) B 30 000,
and e) B 100 000.
Although the films formed quite uniformly around the
entire glass surface, the capillaries were rotated during
heating to promote an even film thickness and counter any
possible role played by gravity in the deposition process. To
drive off any residual organic matter and to improve Pd
adherence to the glass wall, some capillaries were calcinated.
In this procedure, the liquid solution was drained out of the
capillary and it was placed back in the oven and heated at
400 8C for 1 min; this was repeated three times. This heat
treatment resulted in a noticeable enhancement of the film
Film morphology was examined using scanning electron
microscopy (SEM) and energy-dispersive X-ray (EDX)
analysis. The images in Figure 1 a–e were obtained from a
piece of the film removed from the capillary wall. Figure 1 b–e
show the Pd film morphology with increasing magnification,
and it is clear that the porosity is both extensive and regular.
The higher magnification in Figure 1d,e reveals that the films
are made of small grains of Pd that are in the range of 60 to
140 nm in diameter. The cross-sectional view in Figure 1 a
illustrates that the Pd film is approximately 6 mm thick
(evaluated from the edge on the lefthand side).
SEM analysis was also performed on Pd films prepared on
glass slides to study the effect of high-temperature calcination
on film morphology. After the film had been deposited and
washed, it was heated three separate times (1 min each time)
at 350–400 8C, and this vastly increased the porosity of the
resulting film over that of noncalcinated films. This indicates
that residual carbonaceous material is most likely located on
surface of the Pd gains. (Note that calcination temperature is
well below the Pd annealing temperature.) We also noted that
the Pd film morphology is more compact when they are
prepared on slides rather than in the capillaries. This could be
because of the slower rate of metal deposition in the
capillaries as there is less dissolved PdACHTUNGRE(OAc)2 there than in
the larger flasks in which the glass plates are coated.
EDX analysis was performed on the film formed in the
capillary as well as on films that were prepared on glass slides.
Films on slides that were not calcinated were found to contain
an average of 28.0 wt % of carbon, which dropped to
15.0 wt % for the calcinated samples. For calcinated capillary
samples the amount of elemental carbon dropped further to
5.5 wt %. This could be the result of a slower, more controlled
film deposition leading to larger and more regular pores. This
is corroborated by the increase in porosity observed for films
prepared in capillaries, which would allow trapped carbonaceous material to be removed more efficiently. The films
prepared in capillaries consist primarily of Pd (94.0 wt %)
and, other than carbon, only oxygen (0.3 wt %) was detected,
which could be associated with the presence of a thin surface
oxide on the Pd grains. The presence of such a small amount
of carbon and oxygen indicates that the film is mostly
metallic. Based on the weight change of the capillary before
and after the film preparation, the film thickness as evaluated
by SEM and the dimension of the capillary, we calculated the
density of the porous Pd film to be around 3 g cm 3. This
corresponds to a porosity of about 75 %.
The Pd-coated capillaries were first put to use in Suzuki–
Miyara coupling reactions, and the results are summarized in
Table 1. Premixed solutions containing the aryl boronic acid,
aryl bromide, base, and solvent were flowed through the
metal-coated capillary while it was subjected to microwave
irradiation (power setting 30 W) such that the IR sensor
read a constant temperature of 200 8C (see Figure 2). In all
cases, including the coupling of both electron-rich and
electron-poor aryl halides with electron-poor boronic acids,
good to excellent conversion was obtained. Perhaps most
interestingly, the reaction with the highly hindered 2-bromo1,3,5-trimethylbenzene (Table 1, entry 6) proceeded very well
although the reaction presumably proceeds at the surface of
the film (vide infra).
Two important control experiments were then performed.
It has been proposed that when catalytic reactions, such as the
Suzuki–Miyama coupling, are performed with heated metal
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. 2006, 118, 2827 –2832
Figure 2. Capillary reactor system for MACOS. a) Schematic diagram and photograph of a
four-capillary, parallel reactor system. During irradiation the reactor is lowered into the
microwave chamber such that the mixing chamber is seated on top of the visible ring
entering the chamber. b) A 10-cm-long capillary with a Pd film on the inner surface placed
next to a ruler.
films that metal nanoclusters are liberated
from the surface and in fact catalysis takes
place with these clusters in solution. To
probe this possibility, a Pd-coated capillary
through which DMF was flowed was
microwaved under identical conditions as
those used during the typical coupling
reactions (see Table 1, entry 4) until 2 mL
of DMF was collected. Half of the capillary
effluent was placed in a standard conical
microwave vessel. The remaining reaction
components were added (adjusted to provide the same concentrations) in analogy
to the coupling detailed in entry 3
(Table 1). The mixture was then heated
under batch-mode conditions for 20 min at
the same temperature as that recorded
during under flow conditions ( 200 8C).
Whereas the reaction under flow conditions gave quantitative conversion after
being microwaved for literally a few seconds, the batch reaction scarcely pro-
Table 1: Suzuki–Miyara coupling of aryl boronic acids and bromides using MACOS with Pd-coated capillaries.
T [8C][b]
1 a[10]
1 b[10]
1 c[2]
1 d[10]
1 e[2]
8[f ]
1 f[2]
9[f ]
1 g[2]
1 h[2]
81 (74)[g]
11[f ]
1 i[10]
84 (76)[g]
Conv. [%][c]
ACHTUNGRE(yield [%])
[a] General reaction conditions: Reaction solutions were flowed through a single inlet into the Pd-coated 1150-mm (i.d.) capillary while being irradiated
(see Figure 2) unless otherwise noted. [b] Temperature on the outer surface of the Pd-coated capillary as measured by the IR sensor of the Smith
Creator microwave synthesizer. [c] Conversion was determined by withdrawing a crude sample directly from the effluent from the capillary and
analyzing it by 1H NMR spectroscopy. The ratio of starting material to product determined the percent conversion (there were no visible by-products
present, only starting material and product in all cases). [d] DMF was flowed through a Pd-coated capillary while being irradiated as above. The eluent
was pooled and split into two portions; half was analyzed by atomic emission spectrometry for Pd (see text), and to the other half was added the
bromide, boronic acid, and base. This sample was then microwaved for 20 min at 180 8C. [e] The reaction conditions were identical to those in entry 3
except that the capillary was heated at 200 8C using an oil bath instead of using microwave heating. [f] Yield was determined by capturing a known
volume of effluent from the capillary and purifying the product by silica gel chromatography. From the volume, the actual amount (mmol) of starting
material could be calculated, and from this the theoretical yield was calculated. [g] In this case, 2.0 equiv aryl boronic acid was used.
Angew. Chem. 2006, 118, 2827 –2832
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Table 2: Heck coupling of aryl iodides with acrolein derivatives using MACOS through Pd-coated
ceeded at all. Additionally, the
other half of the effluent collected
was analyzed for Pd content by
atomic emission spectrometry
(ICP-AES) to determine if Pd
T [8C][b]
Conv. [%][c]
was liberated from the film during
ACHTUNGRE(Yield [%])
prolonged heating; no Pd could be
detected (< 2 ppm). This strongly
2 a[10]
suggests that reaction at the metal
surface is necessary to achieve the
2 b[1]
tremendous conversions observed
under flow conditions and that
little conversion results from solu3
2 c[1]
bilized Pd nanoclusters. Pd analysis
of the crude cross-coupling product
2 d[10]
mixture did show slightly elevated
levels of Pd (19.2 ppm). It is possible that oxidative addition, consid5
2 e[10]
ered by some to be the rate-determining step of cross coupling, must
2 f[10]
take place at the surface and in
doing so liberates Pd atoms and
[a] Reaction solutions containing aryl iodide (1.0 equiv), acrylate (1.3 equiv), and base (3.0 equiv) in
transmetalation and reductive
DMA were premixed and flowed through a single inlet into the Pd-coated capillary while being irradiated.
elimination occur in solution away
[b] Temperature on the outer surface of the Pd-coated capillary as measured by the IR sensor.
from the surface. In any case,
[c] Conversion was determined by 1H NMR spectroscopy relative to the residual starting material.
[d] Yield was determined by capturing a known volume of effluent from the capillary and purifying the
these trace quantities providing
product by silica gel chromatography. From the volume, the actual amount (mmol) of starting material
could be calculated, and from this the theoretical yield was calculated.
clean products.
The need for microwave irradiation with such films has also been
ditions. If the conversion is in fact an accurate reflection of
questioned. That is, if it is simply the heated film that is
individual steps in the catalytic cycle, it is informative to note
responsible for the conversions observed, then heating by any
that conversions in entries 1–4 (Table 2) follow the general
other method to reach the same temperature should result in
trend that electron-deficient aryl iodides are more reactive,
identical conversion rate and yield. To this end, the reaction in
suggesting that oxidative addition with the Pd film is rate
entry 3 (Table 1) was repeated with the exception that the
limiting. Since this is the trend one would expect with
capillary was immersed in an oil bath set at 200 8C. The
homogeneous catalysis, a similar mechanism may be operacapillary was allowed to come to temperature, and the flow
tional for the Pd film.
reaction carried out under otherwise identical conditions
It is clear that continuous-flow reactors will receive more
(Table 1, entry 5). The conversion obtained was just over half
emphasis in organic synthesis in the future. Considerable
of that obtained by heating with microwave irradiation. Thus,
advances have been made in multistep synthesis using flow
in the absence of irradiation, the same conversion rate cannot
methods through supported-reagent and scavenger colbe achieved, meaning that the microwave is essential for this
umns.[3] Greater attention will be given to improving the
process. This raises an interesting question then: How does
irradiation cause these tremendous rate enhancements, which
practicality of flow methods to increase their implementation.
an apparent Arrhenius relationship cannot explain? With
Because residence time in the flow reactor is generally shorter
simple heat transfer, the highest temperature that the film can
than the reaction time of batch synthesis, conversion rates
obtain is the same temperature as the heat source, 200 8C in
must be high. Microwave irradiation will most certainly help
this case with the oil bath. The origin of the “microwave
increase the variety of reactions that can be done in a flow
effect” in reaction-rate enhancement has been debated, and
format. In this work we have shown that metal-film-coated
many argue that it is just more effective heating and nothing
capillaries in conjunction with microwave irradiation do
to do with microwaves per se. It is possible that “hot spots”
indeed lead to tremendous rate enhancements, and therefore
are being created in the metal film that are well above the
MACOS holds great promise for the future of flow synthesis.
averaged 200 8C temperature recorded by the IR sensor of the
Capillaries as reaction vessels, while they are small, offer
microwave, and it is these areas of the film that are
several advantages over other commercially available sysresponsible for the tremendous rate enhancements observed
tems. They produce the small quantities required by modern
for these couplings.
biochemists for screening purposes very efficiently, quickly,
Next we investigated the Heck coupling (Table 2) and
and with very little waste. For example, 10 mg of product can
found once again that the Pd film was very effective at
be obtained from our flow reactor system within 1 min. Scalepromoting this transformation under continuous-flow conup is achieved by simply flowing longer, and reaction
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. 2006, 118, 2827 –2832
conditions do not have to be optimized for different batch
sizes. We have obtained gram quantities by flowing reaction
mixtures through a single capillary for about 90 min; bundling
capillaries together will produce much larger quantities in the
same time period. Unlike expension chip-type reactors (e.g.,
US$ 3280 per chip from Microlyne Inc.), several capillaries
can be purchased for US$ 0.01. Further, owing to simple,
straight design of the capillary system developed in this
report, and other systems from our group,[6] particulate matter
cannot accumulate and block the reactor as is the case with
more elaborate chip designs[11] and even large-bore spiral[4c]
reactors designed specifically for microwave applications.
In summary, the Pd0 films prepared in this report are
highly porous and composed of nanometer-size grains
(94.0 wt % Pd and 5.5 wt % carbon). The film thickness is
approximately 6 mm and the film porosity is of the order of
75 %. The films are robust and stand up well to the physical
wear and tear of flow conditions and the high temperatures
associated with microwave irradiation. The Pd film serves as
an excellent catalyst for Suzuki–Miyama and Heck reactions,
given the very short durations that the reaction mixture
actually spends in the capillary, which is far less than one
minute. In further investigations we are examining Pd and
other metal films for metal-catalyzed reactions, as well as
other reactions that are not specifically metal catalyzed.
3 equiv) in DMF (2 mL) was prepared. Condition B: In reactions
with pyridineboronic acid, the stock solution contained aryl halide
(0.6 mmol, 1 equiv), pyridine boronic acid (1.2 equiv), base (K2CO3,
120 mg, 1.5 equiv), and CsF (217 mg, 2.4 equiv) all dissolved in DMA/
H2O (2:1; 3 mL). The plumbing of the MACOS microreactor system
was primed with the same solvent as the stock solution. A 1-mL
aliquot from the homogenous stock solution was taken up in a
Hamilton gas-tight syringe and connected to the microreactor by
PEEK tubing with the aid of microtight fittings. The syringe was
placed in a Harvard 22 syringe pump that was set to deliver
10 mL min 1, and the single-mode microwave was programmed to
heat constantly at the temperatures indicated in Table 1. The output
from the reactor was fed into a collection tube and then analyzed by
H NMR spectroscopy immediately after the reaction. All products
are known and their spectra were identical to those reported in the
literature (see Table 1 for references).
General procedure for the Heck coupling: A stock solution
containing the aryl halide (1.2 mmol, 1.2 equiv), acrylate (1 mmol,
1 equiv), and base (triethylamine, 3 mmol, 3 equiv) in 1.5 mL DMA
was prepared. The MACOS reaction was performed using the same
technique as that described above for the Suzuki–Miyama reaction.
All products are known and their spectra were identical to that
reported in the literature (see Table 2 for references).
Received: October 12, 2005
Revised: November 24, 2005
Published online: March 20, 2006
Keywords: cross-coupling · microwaves · palladium ·
synthetic methods · thin films
Experimental Section
Pd film coating of capillaries: Borosilicate capillaries (1150 mm i.d.)
were filled with a 0.1 mmol mL 1 solution of palladium acetate in
DMF. The capillaries were then capped at both ends with septa and
placed in a muffle furnace at room temperature, and the temperature
was gradually increased up to 150 8C. After 10 min, metallic
palladium was gradually released from the solution and deposited
on the inner wall of the capillary. The capillaries were rotated a few
times during deposition to encourage uniform film thickness, and
heating was continued at 150 8C for a total of 30 min. Capillaries were
calcinated by removing the plugs, pouring out the remaining solution,
rinsing with fresh DMF, and heating (3 I 1 min each time) at 400 8C
before use.
Preparation and analysis of Pd thin films: Capillaries were
cleaved open and pieces of the films were fixed on carbon tape for
analysis. Films were also prepared on 1-cm2 flat borosilicate glass
slides by dipping them in the same PdACHTUNGRE(OAc)2 solution described
above. Sample imaging was carried out with a Hitachi S-4500 field
emission scanning electron microscope (SEM) equipped with an
EDAX Phoenix model energy-dispersive X-ray (EDX) analyzer.
EDX analysis can detect all elements above atomic number 5 and has
a minimum detection limit of 0.5 wt % for most elements. A 5-kV
electron beam was used to obtain SEM images and EDX spectra.
Both the lower and upper SE detectors were used for imaging
Microwave irradiation experiments: All microwave experiments
were performed in a MACOS reactor designed and constructed in our
laboratory (see Figure 2), operating at a frequency of 2.45 GHz with
irradiation power from 0 to 300 W. The reactions were carried out in
Pd-film-coated glass capillaries (1150 mm i.d.) that were placed in the
cavity of the Biotage Smith Creator Synthesizer. The recorded
temperature was measured by an IR sensor that was focused on the
outer surface of the reaction capillaries.
General procedure for the Suzuki–Miyara coupling: Condition A: A stock solution containing the aryl halide (0.6 mmol,
1 equiv), arylboronic acid (1.2 equiv), base (2 m KOH, 0.9 mL,
Angew. Chem. 2006, 118, 2827 –2832
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