A R T I C L E S
Garcia-Martinez et al.
MS spectra were obtained using a HP 5970 GC/MS. Because the
samples were acidic and thus not compatible with GC, they were
converted to trimethylsilyl esters prior to analysis using a standard
method.31-33 Specifically, 5-7 mg of the crude reaction mixture were
weighed in a 2 mL vial, and then 200 µL of N-methyl-N-(trimethylsilyl)-
trifluoroacetamide (MSTFA) was added, resulting in the complete
dissolution of the sample. This reaction was allowed to proceed for at
least 2 h, and then the sample was diluted in CH2Cl2 and injected to
the GC. There was one exception to this procedure: 3-iodobenzoic
acid was diluted in THF because it was insoluble in CH2Cl2.
this class of reaction being carried out both in water and at room
temperature simultaneously. There are two reports of poly-
disperse, polymer-stabilized Pd nanoparticles being used to
catalyze the Stille reaction,29,30 but in both cases the reactions
required the use of an organic solvent and a temperature of at
least 110 °C.29,30 Here we report the use of highly monodisperse
Pd nanoparticles for catalyzing the Stille reaction in high yield,
at room temperature, and in water.
Experimental Section
Transmission electron microscopy (TEM) was performed using a
JEOL 2010 transmission electron microscope (JEOL USA Inc.,
Peabody, MA). Samples were prepared by placing one drop of a DEN-
containing solution on a holey-carbon-coated Cu grid (EM science,
Gibbstown, NJ) and allowing the solvent to evaporate in air.
Chemicals and Materials. Fourth-generation, hydroxyl-terminated
PAMAM dendrimers (G4-OH) having an ethylenediamine core were
obtained as 25% aqueous solutions from Dendritech, Inc. (Midland,
MI). All aryl halides, organostannanes, deuterated chloroform, KOH,
NaBH4, and K2PdCl4 were used as received from the Aldrich Chemical
Co. (Milwaukee, WI). HCl 35%, HPLC-grade ethyl ether, and di-
chloromethane were purchased from EMD Chemicals Inc. Milli-Q water
(18 MΩ‚cm; Millipore, Bedford, MA) was used to prepare aqueous
solutions.
Results and Discussion
We previously showed that hydroxyl-terminated PAMAM
dendrimers (Gn-OH) can be used as templates for the encap-
sulation of Pd(0) and Pt(0) nanoparticles in aqueous solu-
tion.1-3,7,8,14-19,34 The size of the entrapped metal nanoparticles
can be controlled either by using different dendrimer generations
or by manipulating the dendrimer/metal-ion ratio.1-3 In the
present study we investigated the Pd DEN-catalyzed Stille
coupling reaction between aryl halides and organostannanes
using aqueous KOH as the solvent (Scheme 1). The catalyst
Preparation and Characterization of the Catalyst. We previously
reported the preparation of Pd DENs of the sort used here.15,18
Nevertheless, a short summary of the basic procedure follows. A 25.0
µL amount of an aqueous 1.00 mM solution of G4-OH was mixed
with 0.100 mL of 10.0 mM aqueous K2PdCl4 in a total of 10.0 mL of
water. This results in encapsulation of an average of 40 Pd2+ ions within
each dendrimer (G4-OH(Pd2+)40). Next, this composite was reduced
by adding a 10-fold molar excess of NaBH4 to give a G4-OH(Pd40)
solution. The spectroscopic properties, size, and size distribution of
these DENs were found to be consistent with published results.7,8,14-19
2-
was synthesized by mixing a 40:1 molar ratio of PdCl4 and
G4-OH. The product of this reaction is denoted as G4-OH-
(Pd2+
)
to indicate that an average of 40 Pd ions are present
40
-
within each dendrimer. Reduction of G4-OH(Pd2+
)40 with BH4
Catalytic Reactions. The same procedure was used for most of the
catalytic reactions, but small deviations were necessary in some cases.
These are identified in the appropriate locations in the Results and
Discussion section. The reaction conditions used for the coupling of
4-iodobenzoic acid with phenyltin trichloride are presented as an
example of the default procedure. A 124 mg (0.50 mmol) amount of
4-iodobenzoic acid and 604 mg (2.00 mmol) of phenyltin trichloride
were completely dissolved in 6.00 mL of 3.0 M aqueous KOH and
2.00 mL of deionized water. Next, 5.00 mL of the 2.50 µM G4-OH-
(Pd40) catalyst solution was added to the reactant mixture. The reaction
was carried out for 15 h with stirring, and then it was quenched with
50 mL of 5% aqueous HCl. The aqueous mixture was treated with
diethyl ether to extract the organic products, and the organic layer was
washed with brine, dried with MgSO4, and filtered. After removing
the solvent in a rotary evaporator, the product was analyzed (without
results in zerovalent Pd DENs containing an average of 40 Pd
atoms (G4-OH(Pd40)).
The catalyst was initially tested for the Stille reaction using
4-iodobenzoic acid and phenyltin trichloride because both are
soluble in basic aqueous solutions. More specifically, 124 mg
(0.500 mmol) of 4-iodobenzoic acid and 604 mg (2.00 mmol)
of phenyltin trichloride were dissolved in 6.00 mL of 3.0 M
aqueous KOH and 2.00 mL of deionized water. Next, 5.00 mL
of the G4-OH(Pd40) catalyst solution was added to the reactant
solution, which corresponds to 0.100 atom % of the Pd catalyst
with respect to 4-iodobenzoic acid. This is significantly lower
than the 5-10 atom % of Pd normally used to catalyze the Stille
reaction.4,6 The reaction was carried out for 15 h with stirring,
and then it was quenched with 50 mL of 5% aqueous HCl. The
aqueous mixture was treated with diethyl ether to extract the
organic products, and after removal of the solvent the product
was analyzed with no additional purification.
1
additional purification) by H NMR and GC-MS.
Characterization. UV-vis absorbance spectra were obtained at 23
( 2 °C using quartz cells and a Hewlett-Packard model 8453 UV-vis
spectrometer system (Hewlett-Packard, Wilmington, DE). UV-vis
spectra of aqueous 2.50 µM G4-OH(Pd40) solutions were collected using
deionized water as the reference.
1
Figure 1a shows the H NMR spectrum of a mixture of
1H NMR spectra were recorded using a 300 MHz Unity p300
spectrometer. Spectra of the crude reaction products were obtained using
deuterated chloroform as solvent in a standard 5 mm NMR tube.
4-iodobenzoic acid and phenyltin trichloride dissolved in CDCl3
prior to addition of the DEN catalyst. The peaks between 7.87
and 7.78 ppm are attributable to 4-iodobenzoic acid, and they
correspond to the ABq system with the characteristic coupling
constant of 1,4-disubstituted benzene rings (J ) 8.8 Hz). The
multiplet signal between 7.67 and 7.58 ppm is associated with
phenyltin trichloride. Figure 1b is the 1H NMR spectrum of the
crude reaction product. Only one compound is represented in
the spectrum, and it corresponds to the anticipated Stille coupling
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5098 J. AM. CHEM. SOC. VOL. 127, NO. 14, 2005