Palladium(0) nanoparticle catalyzed cross-coupling of allyl acetates and aryl and vinyl siloxanes

Article abstract of DOI:10.1021/jo802214m

(Chemical Equation Presented) The cross-coupling of allyl acetates and aryl and vinyl siloxanes proceeds readily by the catalysis of in situ generated palladium(0) nanoparticles. The reactions are stereoselective, and (E)-coupling products are obtained both from cis and trans allyl acetates. The coupling with vinyl siloxanes provides a novel protocol for the synthesis of 1,4-pentadienes.

Full text of DOI:10.1021/jo802214m

SCHEME 1. Cross-Coupling Reaction of Allyl Acetate and
Organo Silane Catalyzed by Pd Nanoparticles
Palladium(0) Nanoparticle Catalyzed
Cross-Coupling of Allyl Acetates and Aryl and
Vinyl Siloxanes
Raju Dey, Kalicharan Chattopadhyay, and
Brindaban C. Ranu*
more attractive. Although the Hiyama coupling of aryl halides
and organosilanes is well-documented,4 surprisingly the Hiyama
coupling of allyl alcohol derivatives and silanes is less explored.4c,5
Besides Hiyama’s own work4c Deshong demonstrated coupling
of cyclo-allylic benzoates with hypervalent silicon complexes,5a-c
and Kabalka reported cross-coupling of activated allylic acetates
and organosilanes.5d Nevertheless, several reports of allylation
by allyl alcohol derivatives using Stille6 and Suzuki7 procedures
are well-known. In view of the versatile synthetic utility of allyl
moiety and advantages of Hiyama coupling, we report here a
very simple and efficient cross-coupling of both unactivated and
activated open-chain allylic acetates with organosiloxanes
catalyzed by palladium(0) nanoparticles (Scheme 1).
Department of Organic Chemistry, Indian Association for the
CultiVation of Science, JadaVpur, Kolkata 700 032, India
ocbcr@iacs.res.in
ReceiVed October 04, 2008
The use of palladium nanoparticles as efficient catalysts in
organic reactions has attracted considerable interest as nano-
particles provide a larger number of active sites per unit area
compared to their homogeneous counterparts.8 Our interest and
continuing program to explore the novel applications of metal
nanoparticles9 led us to investigate this Hiyama cross-coupling
of allyl acetates.
The experimental procedure is very simple. A mixture of allyl
acetate and organosiloxane in THF was stirred at 65 °C in the
presence of palladium chloride, tetrabutylammonium bromide,
and tetrabutylammonium fluoride for a required period of time
(TLC). Standard workup provided the product.
To determine the optimum reaction conditions for an efficient
coupling, the reaction was studied with variation of reactions
parameters as summarized in Table 1. The best results were
obtained using 2 mol % of PdCl2, 25 mol % of TBAB, and 1.1
equiv of TBAF in THF at 65 °C (entry 5, Table 1).
To determine the active catalytic species in this reaction, the
reaction of cinnamyl acetate and phenyltrimethoxysilane was
studied by UV (THF) spectroscopy. For comparison, the spectra
of a solution of PdCl2 in THF was recorded as reference. The
reaction mixture after 1 min, 10 min, 30 min, 1.5 h, 3 h, and
The cross-coupling of allyl acetates and aryl and vinyl
siloxanes proceeds readily by the catalysis of in situ generated
palladium(0) nanoparticles. The reactions are stereoselective,
and (E)-coupling products are obtained both from cis and
trans allyl acetates. The coupling with vinyl siloxanes
provides a novel protocol for the synthesis of 1,4-pentadienes.
The palladium-catalyzed cross-coupling reaction is a powerful
tool for the formation of carbon-carbon bonds and has wide
applications in organic synthesis.1 The most frequently used
methods to perform this operation are Stille,2 Suzuki,3 and
Hiyama4 reactions using organotin, organoborane, and orga-
nosilane coupounds, respectively. Although all three reactions
provide comparable results with regard to yields and stereose-
lectivity, the toxicity and tedious separation of tin reagents and
difficulties in the preparation and purification of Suzuki boron
reagents are the disadvantages associated with Stille and Suzuki
couplings. On the other hand, the ease of preparation and low
toxicity of organosilane reagents has made Hiyama coupling
(5) (a) Brescia, M.-R.; Deshong, P. J. Org. Chem. 1998, 63, 3156–3157. (b)
Mowery, M. E.; Deshong, P. J. Org. Chem. 1999, 64, 1684–1688. (c) Correia,
R.; Deshong, P. J. Org. Chem. 2001, 66, 7159–7165. (d) Kabalka, G. W.; Dong,
G.; Venkataiah, B.; Chen, C. J. Org. Chem. 2005, 70, 9207–9210.
(6) (a) Crawforth, C. M.; Burling, S.; Fairlamb, I. J. S.; Taylor, R. J. K.;
Whitwood, A. C. Chem. Commun. 2003, 2194–2195. (b) Castano, A. M.;
Echavarren, A. M. Tetrahedron Lett. 1996, 37, 6587–6590.
(1) (a) Tsuji, J. Palladium Reagents and Catalysts. InnoVations in Organic
Synthesis; John Wiley and Sons: New York, 1995. (b) Miyaura, N., Ed. Cross-
coupling Reactions; Springer: Berlin, Germany, 2002. (c) Tius, M. A.; Gomez-
Galeno, J.; Gu, X.; Zaidi, J. H. J. Am. Chem. Soc. 1991, 113, 5775–5783. (d)
Bochmann, M.; Kelly, K. Chem. Commun. 1989, 532–534. (e) Schmitd, U.;
Meyer, R.; Leitenberger, V.; Lieberknecht, A. Angew. Chem., Int. Ed. Engl. 1989,
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Hiyama, T. J. Organomet. Chem. 2002, 653, 58–61, references cited therein.
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10.1021/jo802214m CCC: $40.75
Published on Web 11/11/2008
2008 American Chemical Society
J. Org. Chem. 2008, 73, 9461–9464 9461

TABLE 1. Standardization of Reaction Conditions for Hiyama
Cross-Coupling Reactiona
entry
surfactant
SDS
Pd salt additive solvent temp (°C) yield (%)
1
2
3
4
5
6
7
8
9
1
1
1
1
2
2
2
2
2
2
NaOH
TBAF
TBAF
TBAF
TBAF
TBAF
H2O
H2O
THF
THF
THF
THF
85
85
65
65
65
rt
65
100
100
100
10
10
76
74
76
20
10
20
15
15
SDS
SDS, TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
K2CO3 THF
FIGURE 3. EDX spectra of Pd nanoparticles.
TBAF
TBAF
TBAF
toluene
DMF
NMP
Several structurally varied substituted allyl acetates underwent
couplings with phenyl, tolyl,10 and vinyl siloxanes by this
procedure to produce the corresponding allylated products. The
results are reported in Table 2. Both unactivated (Table 2, entries
1-15) and activated acetates (Baylis-Hillman acetate adducts,
Table 2, entries 16-18) participated in this reaction. The
activated acetates coupled faster than unactivated ones. The
reactions were highly regioselective, providing straight chain
olefins through coupling from the less substituted carbon atom.
Only one reaction (Table 2, entry 4) furnished a mixture of
regioisomeric coupled products with a small amount (15%) of
branched olefin. The stereoselectivity achieved in these reactions
is also very good. Very interestingly, both trans (Table 2, entries
1-3, 5-8, 10) and cis (Table 2, entries 12-14) allyl acetates
produced the (E)-coupled products. No cis product was isolated
or found. The Hiyama coupling of cis allyl acetates was not
addressed earlier. The stereochemistry of the products was
10 TBAB
a Pd salt 1 ) Na2PdCl4, SDS ) sodium dodecyl sulfate. Pd Salt 2 )
PdCl2, TBAB ) tetrabutyl ammonium bromide.
1
established by comparison of H NMR chemical shifts and
coupling constants (J values) of the olefinic protons with those
reported. The use of vinylsiloxanes in the Hiyama coupling of
allylic acetates was not reported earlier, and this reaction
presents a novel protocol for the synthesis of 1,4-dienes (Table
2, entries 7-11, 13, 18). The product obtained by the coupling
of vinylsiloxane and Baylis-Hillman acetate adduct listed in
entry 18, Table 2 is of high potential, having three useful
functionalities for manipulation.
FIGURE 1. UV spectra monitoring of reaction.
It is suggested that the reaction proceeds via a pathway
outlined in Scheme 2. As Pd(0) nanoparticle is the active
catalytic species, a cycle involving Pd(0) to Pd(II) is most likely.
Oxidative addition of allyl acetate to Pd(0) generates π-allylpal-
ladium acetate complex 1, which undergoes transmetalation by
arylsiloxane in presence of TBAF to produce an intermediate
2. Possibly, this process operates straightway in case of trans
allyl acetate complex. However, for cis allyl acetates two
alternative routes may be considered. In path a the η3-complex
1 may change its heptacity from 3 to 1 to form an intermediate
complex, losing its cis stereochemistry,15 which then couples
with aryl siloxane to form 2. Path b considers the possibility of
a branched isomer as intermediate. However, in a blank
experiment it was observed that no branched isomer was formed
FIGURE 2. TEM image of Pd nanoparticles.
8 h showed the absence of the corresponding peak of PdCl2 at
350 nm; however, a new peak at 366 nm appeared after 10 min
and became maximum after 1.5 h (Figure 1). This peak remained
even after 3 h and disappeared when recorded after 8 h. The
new peak at 366 nm is possibly due to the formation of π-allyl
Pd(II) complex with the progress of the reaction, and this peak
disappeared after reductive elimination showing the formation
of Pd(0) after the completion of reaction. The TEM (transmis-
sion electron microscopy) image (Figure 2) and EDS (energy
dispersive X-ray spectroscopy) (Figure 3) confirmed the pres-
ence of Pd nanoparticles (3-5 nm). It is suggested that Pd(II)
was reduced by allyl acetate to Pd(0) and TBAB served as
stabilizer for the Pd nanoparticles.
(10) Manoso, A. S.; Ahn, C.; Soheli, A.; Handy, C. J.; Correia, R.; Seganish,
W. M.; Deshong, P. J. Org. Chem. 2004, 69, 8305–8314.
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2005, 347, 1499–1503.
(12) Moreno-Manas, M.; Pijuelo, F.; Pleixats, R. J. Org. Chem. 1995, 60,
2396–2397.
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metallics 2003, 22, 4193–4197.
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9462 J. Org. Chem. Vol. 73, No. 23, 2008

TABLE 2. Pd Nanoparticles Catalyzed Hiyama Cross-Coupling of
Allyl Acetates and Aryl/Vinyl Silanes
FIGURE 4. Recyclability chart.
FIGURE 5. TEM image of Pd nanoparticles after first cycle.
SCHEME 2. Possible Reaction Pathway
a
1
Yields refer to those of purified products characterized by IR and H
and 13C NMR spectroscopic data. b Associated with branch isomer
(15%).
from cis allyl acetate, and thus path b was eliminated. The
reductive elimination of aryl group from 2 through the less
hindered side (terminal position) leads to product.
In general, the reactions are clean. All of the products were
obtained in high yields and high purities. The Pd nanoparticles
were recovered after reaction and were reused for subsequent
runs. It was found that for up to three runs the catalysts are
appreciably active (Figure 4; recyclability chart); however every
time during exposure the PdNP catalysts show a tendency to
undergo agglomerization as observed by TEM image (Figure
5; particle size 10-12 nm in place of starting 3-5 nm) taken
before second cycle. Thus they suffer gradual loss of activity
with increase of particle size in subsequent runs.
In conclusion, we have developed a convenient and efficient
one-pot procedure for Hiyama coupling of allyl acetates and
aryl/vinyl siloxanes catalyzed by in situ generated palladium(0)
nanoparticles. In addition to excellent regio- and stereoselec-
tivity, good yields of products, simple operation, and general
applicability to a variety of allyl acetates and organosiloxanes,
this methodology provides an easy access to functionalized 1,4-
dienes. The formation of (E)-coupled product from trans as well
as cis allyl acetate is of much significance. To the best of our
knowledge, this is the first report of Pd(0) nanoparticles
J. Org. Chem. Vol. 73, No. 23, 2008 9463

This procedure was followed for all of the reactions listed in
Table 2. Although the representative procedure was based on a 1
mmol scale, gram quantities also provided similar results. Many
of these products are known compounds and were easily identified
by comparison of their spectroscopic data with those reported (see
references in Table 2). The unknown compounds were properly
characterized by their spectroscopic data (IR, 1H NMR, 13C NMR,
catalyzed cross-coupling of allyl acetates and organosiloxanes,
particularly vinylsiloxanes.
Experimental Section
General Experimental Procedure for Cross-Coupling
Reactions. Representative Procedure for Coupling of
Cinnamyl Acetate and Phenyl Trimethoxysilane (Table 2,
entry 1). A mixture of cinnamyl acetate (176 mg, 1 mmol), phenyl
trimethoxysilane (240 mg, 1.2 mmol), PdCl2 (4 mg, 2 mol %),
tetrabutylammonium bromide (80 mg, 25 mol %), and tetrabuty-
lammonium fluoride (1 M solution in THF, 1.2 mL, 1.2 mmol) in
THF (3 mL) was heated under reflux for 8 h (TLC). The reaction
mixture was extracted with dry Et2O (3 × 10 mL), and the ether
extract was washed with brine and water and dried (Na2SO4).
Evaporation of solvent left the crude product, which was purified
by column chromatography over silica gel (hexane/ether 98:2) to
give 1,3-diphenyl propene as a colorless oil (146 mg, 75%). The
spectroscopic data (IR, 1H NMR, and 13C NMR) are in good
agreement with the reported values.5c
1
HRMS). The purity of all compounds was also checked by H
NMR, 13C NMR, and elemental analysis.
Acknowledgment. This investigation has enjoyed the finan-
cial support of DST, New Delhi by a grant [SR/S5/GC-02/2006].
R.D. and K.C. are thankful to CSIR for their fellowships.
Supporting Information Available: Characterization data
(IR, 1H and 13C NMR, spectroscopic data and elemental analysis
report) of the products in entries 5-7 and 18 in Table 2. Copies
1
of H and 13C NMR spectra of all products listed in Table 2.
This material is available free of charge via the Internet at
http://pubs.acs.org.
After extraction with dry ether, the residual black Pd nanopar-
ticles were further washed with dry ether and dried. These was
reused in subsequent runs.
JO802214M
9464 J. Org. Chem. Vol. 73, No. 23, 2008