Layered double hydroxide supported nanoplatinum and nanopalladium catalyzed allylation of aldehydes: A mechanistic study

Article abstract of DOI:10.1002/adsc.200505179

Layered double hydroxide (LDH)-supported nanoplatinum(0) and nanopalladium(0) catalysts were prepared by a simple ion exchange technique and subsequent reduction with hydrazine hydrate and used for the allylation of aldehydes to give moderate to good yiel

Full text of DOI:10.1002/adsc.200505179

FULL PAPERS
DOI: 10.1002/adsc.200505179
Layered Double Hydroxide Supported Nanoplatinum and
Nanopalladium Catalyzed Allylation of Aldehydes:
A Mechanistic Study
a
b
b
b,
B. M. Choudary, Moumita Roy, Sarabindu Roy, M. Lakshmi Kantam, *
b
b
Karangula Jyothi, Bojja Sreedhar
Ogene Systems(I) Pvt. Ltd, Hyderabad, 500 037, India
Indian Institute of Chemical Technology, Hyderabad 500007, India
Fax: (þ91)-40-2716-0921; e-mail: mlakshmi@iict.res.in
a
b
Received: April 28, 2005; Accepted: September 9, 2005
Abstract: Layered double hydroxide (LDH)-support- tion using XPS and TGA-MS reveal that a monoall-
ed nanoplatinum(0) and nanopalladium(0) catalysts yl-palladium complex is the key intermediate for the
were prepared by a simple ion exchange technique catalytic cycle.
and subsequent reduction with hydrazine hydrate
and used for the allylation of aldehydes to give mod-
erate to good yields of homoallylic alcohols. Detailed Keywords: aldehydes; allylation; heterogeneous catal-
mechanistic studies of LDH-Pd(0)-catalyzed allyla- ysis; monoallyl-palladium complex; nanoplatinum
Introduction
Herein we report the efficacy of layered double hydrox-
ides (LDHs)-supported nanoplatinum and nanopalladi-
Homoallylic alcohols are versatile intermediates in the um catalysts in the allylation reaction of aldehydes using
preparation of materials, natural products, and bioactive allyl bromide, allyl alcohol and allyltributylstanne as al-
[
1]
[2]
compounds. Palladium and platinum catalyzed ally- lylating agents. Mechanistic studies were also undertak-
lation of aldehydes to homoallylic alcohols is a reliable en in this work. This is the first example of a heterogene-
and widely used methodology in modern organic syn- ous nanoplatinum-catalyzed allylation of aldehydes by
thesis. Allylation of aldehydes by homogeneous cataly- allyltributylstanne.
[
1,2]
sis using palladium or platinum complexes
offers
good to excellent yields of homoallylic alcohols but
the non-recoverable expensive palladium or platinum Results and Discussion
metal restricts its use in industry. It is highly desirable
to develop a new recoverable and reusable catalytic sys-
tem with similar or better activity than the homogene-
Preparationof LDH-Pt(0) a nd LDH-Pd(0)
ous system. The chemical industry has often favored het-
erogeneous catalysis, but the development of better cat- LDHs have recently received much attention in view of
alysts has been hindered by the presence of numerous their potential usefulness as materials, anion exchangers
[
5a–e]
kinds of active sites and also by the low concentration and more importantly as catalysts.
The LDH con-
xþ
of active sites. But recent developments in nanotechnol- sists of alternating cationic M(II) M(III) (OH)2
1
Àx
x
[
3]
n–
[5f]
ogy, mainly nanoparticles stabilized by solid supports,
and anionic A ·z H O layers. The positively charged
2
have widen the scope of heterogeneous catalysts. Re- layers contain edge-shared metal M(II) and M(III) hy-
[3b]
nÀ
cently Cai et al. reported the allylation of aldehydes droxide octahedra, with charges neutralized by A
catalyzed by a silica-supported palladium(0) complex, anions located in the interlayer spacing or at the edge
but to our knowledge there is no report on the allylation of the lamellae. Small hexagonal LDH crystals with
of aldehydes catalyzed by a heterogeneous platinum(0) Mg1 Al (OH) (Cl) ·z H O composition were synthe-
Àx
x
2
x
2
[
3a]
catalyst. Earlier a nanopalladium catalyst was pre- sized following the existing procedures (here x¼
[
5g]
pared and evaluated for CÀC bond forming reactions 0.25).
Hexachloroplatinate was exchanged onto
and later mechanistic studies were performed by isolat- chloride-saturated LDH to obtain a light yellow-colored
[
4]
ing surface transient organometallic intermediates.
LDH-Pt(IV) and then reduced with hydrazine hydrate
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B. M. Choudary et al.
Scheme 1. Preparation of LDH-Pt and LDH-Pd catalysts.
Figure 1. XPS survey scan of LDH-Pt(0). Inset shows the
high resolution narrow scan showing peaks of (a) Pt 4f ,
7/2
(
b) Pt 4f5/2 and (c) Al 2p.
Scheme 2. The structure of LDH-PtCl6.
Table 1. Allylation of aldehydes by allyl bromide and allyl al-
cohol.
to give the air-stable, black-colored LDH-Pt(0) catalyst.
The LDH-Pd(0) catalyst was prepared according to the
earlier report and was well characterized. Scheme 1
represents the synthesis of LDH-Pt and LDH-Pd cata-
lysts. The possible structures for the catalysts are depict-
ed in Scheme 2.
[3a]
Characterizationof the LDH-Pt(0) Catalyst
All the catalysts developed were well characterized by
XRD, XPS, SEM-EDX and TEM. XRD patterns of
the initial LDH and LDH-Pt(0) hardly differ in the
range 2q¼3–658. The observed d0 basal spacing of
03
the support that appeared at 7.8 Š remains unchanged
after the anion exchange and subsequent reduction,
2
À
which indicates that the PtCl6 and Pt(0) nanoparticles
are mainly located at the edge-on surface in the respec-
tive samples. An XPS investigation of LDH-Pt(0) at Pt
4
7
f level shows a 4f line at 72.87 eV and a 4f5/2 line at
5.19 eV (Fig. 1). The average sizes of the nanoplati-
7/2
[6]
num particles in the fresh and used LDH-Pt(0) were
measured from TEM images and found to be in the
range of 30–40 nm.
Allylationof Aldehydes with LDH-Pt a nd LDH-Pd
Catalysts
stanne to extend the scope of our catalytic system. The
addition of allyltributylstanne to benzaldehyde pro-
ceeded smoothly at room temperature in the presence
The allylation reaction of aldehydes catalyzed by LDH- of LDH-Pt(0) (5 mol %) (Table 2, entry1). Impressed
Pt(0) (2 mol %) at room temperature using allyl bro- by the result of benzaldehyde allylation, we examined
mide or allyl alcohol as the allylating agent, gave the cor- the allylation of various aldehydes with allyltributyl-
responding homoallylic alcohols in moderate to good stanne using LDH-Pt and LDH-Pd catalysts (Table 2).
yields. The results are summarized in Table 1.
As per expectation, aldehydes with electron-donating
After getting good results with allyl bromide and allyl group gave lower yields of product whereas the presence
alcohol, we turned our attention towards allyltributyl- of electron-withdrawing groups produced better yields.
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Nanoplatinum- and Nanopalladium-Catalyzed Allylation of Aldehydes
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Table 2. Allylation of aldehydes with allyltributylstanne
using LDH-Pt(0).
Table 3. Comparison of activity of different catalysts in the
[a]
allylation of benzaldehyde.
ReactionMecha ni sm
In order to clarify the mechanism of the allylation reac-
tion catalyzed by LDH-Pt(0) and LDH-Pd(0), we first
investigated the role of tin species in the allylation of al-
dehydes. We performed allylation of aldehydes with all-
yl alcohol and allyl bromide in the presence of excess
amount of stannous chloride and, as expected, the yields
were good (Table 1). But when the same reactions were
performed under identical conditions excluding the
stannous chloride, no product formation was observed.
So we thought that the stannous chloride is necessary
to facilitate the allylation of aldehyde to give the corre-
sponding homoallylic alcohol. The plausible mechanism
is shown in Scheme 3.
Aromatic a,b-unsaturated and aliphatic aldehydes also
To validate this mechanistic cycle we tried to trap the
underwent the allylation reaction in the presence of intermediate 2 or 3, but our attempts have failed. Then,
LDH-Pt(0) (10 mol %) at room temperature and with instead of using stannous chloride, we have taken an or-
5
mol % catalysts at reflux temperature in THF. THF ganotin reagent, namely allyltributylstanne, and treated
is found to be the best solvent for this reaction among it with LDH-Pd(0). Here we chose LDH-Pd(0) instead
[
7]
the solvents studied.
The use of LDH-Pd(0) of LDH-Pt(0) because in XPS the Pt signal was overlap-
(
5 mol %) gave an almost similar chemical yield (Table2, ped by an Al signal.
entries 1,2,5) but LDH-PtCl , LDH-PdCl , K PtCl and
Na PdCl gave lower yields (Table 3).
A mixture of LDH-Pd(0) and allyltributylstanne was
refluxed in THF under argon. Then the solid material
6
4
2
6
2
4
Here we observed that LDH-Pt(0) or LDH-Pd(0) was filtered off, washed with THF and vacuum dried to
gave better yields than the Pt(IV) or Pd(II) precursor, get the surface transient organometallic (STO-1) inter-
which is completely different in the case of homogene- mediate. This STO-1 was then subjected to XPS analysis
[2b]
ous catalysis. Pd(dba), dppe or Pd (PPh ) gave poor (Fig. 2). A high resolution narrow scan XPS spectra of
3
4
yields when compared to PdCl (PPh ) or the Pd on deconvolution shows a set of two lines each at
2
3 2
PdCl (dppf) complex. These results suggest that here 337.02 eV (3d ) 342.35 eV (3d ) and 335.10 eV (3d )
2
5/2
3/2
5/2
in heterogeneous catalysis some different kind of mech- 340.31 eV (3d ) (Fig.3). The latter set of lines is attrib-
3
/2
anism is operative. Among the catalysts screened, LDH- uted to unreacted Pd(0). The high resolution narrow
Pt(0) is found to be the best.
scan of STO-1 for Sn shows the 3d5/2 line at 486.35 eV
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B. M. Choudary et al.
Figure 3. TGA-DTA-MS thermogram of STO-1.
and C H , respectively (Scheme 4). The m/z value corre-
3
7
sponding to allyltributylstanne was not detected, which
suggests that the mass fragments observed are the pyrol-
ysis products of surface transient organometallic inter-
mediate (allyl)PdSnBu3.
Heating of the STO-1 with 4-nitrobenzaldehyde in
THF yielded the corresponding homoallylic alcohol.
By XPS and TGA-MS analysis of the surface transient
organometallic intermediate, we postulate that in this
catalytic reaction, oxidative addition of allyltributyl-
stanne, has taken place in LDH-Pd(0) to produce
Scheme 3. Plausible mechanism for the allylation.
and 3d3/2 lines at 494.88 eV, which indicate the presence STO-1, followed by insertion of the aldehyde into the
[4]
of a PdÀSn bond (Fig. 2). The deconvoluted C 1 s XPS PdÀC bond to form 4 and finally reductive elimination
spectrum of STO-1 displays four lines at 283.70, 284.60, of 5 from the intermediate 4 completes the catalytic cy-
2
85.95 and 288.01 eV. Although specific assignment is cle. Unfortunately we were not able to isolate 4.
difficult but we could say that there are four types of car-
bon bonds most likely to be PdÀC, SnÀC, CÀC and a car-
bon impurity in the LDH-Cl precursor of the LDH-Pd Reusability of the Catalyst
[4]
catalyst.
STO-1 was then subjected to TGA-MS (Fig. 3) and The catalysts were recovered by simple filtration and
the observed m/z values of the evolved gas fragments washed with acetone and oven-dried .The catalyst was
are 26, 41 and 43 a.m.u. corresponding to C H , C H
reused and consistent activity was noticed even after
2
2
3
5
Figure 2. XPS survey scan of STO-1 along with a) Pd 3d of STO-1, b) Sn 3d of STO-1, c) C 1 s of STO-1
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Nanoplatinum- and Nanopalladium-Catalyzed Allylation of Aldehydes
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were collected on a Simens/D-5000 diffractometer using
CuKa radiation. The particle size and external morphology
of the samples were observed on a JEOL JEM-100CX trans-
mission electron microscopy (TEM). SEM-EDX was per-
formed on a Hitachi SEM S-520, EDX-Oxford Link ISIS-300
instrument. Thermogravimetric (TG), differential thermal
analysis and mass spectra of the evolved gas during the thermal
decomposition of catalyst were studied on a TGA/SDTA Met-
e
tler Toledo 851 system coupled to an MS Balzers GSD 300T,
using open alumina crucibles, containing samples weighing
À1
about 8–10 mg with a linear heating rate of 108C min . All
known compounds were characterized by comparing their
physical data with those in the literature. Solvents used for ex-
periments were dried and distilled according to literature pro-
cedures. All the reactants were commercially available and
used without purification.
Preparationof LDH-PtCl 6
Scheme 4. Allylation of aldehydes using LDH-Pd(0).
LDH-Cl (Mg:Al¼3:1, 2 g) was suspended in 150 mL of an
aqueous solution of K PtCl (0.388 g, 0.8 mmol,) and stirred
2
6
at 258C for 24 h under a nitrogen atmosphere. The solid cata-
lyst was filtered, washed thoroughly with 500 mL of water
and vacuum-dried to obtain LDH-PtCl6.
the fourth cycle (Table 2, entries 1 and 5). When a fresh
reaction was conducted with the filtrate obtained at the
end of the allylation reaction, no product formation was
observed. Moreover, the absence of Pt in the filtrate was
determined by AAS. The platinum content of the cata-
lyst was found to be almost the same in the fresh catalyst
Preparationof LDH-Pt(0)
LDH-PtCl (1 g) was reduced with hydrazine hydrate (1 g,
6
and in the used catalyst after the fifth cycle by SEM- 20 mmol) in ethanol (10 mL) for 3 h under a nitrogen atmos-
EDX. This clearly rules out the possibility of leaching phere, then filtered and washed with ethanol to give an air-sta-
of platinum from the support.
ble, black powder (0.374 mmol Pt per gram).
Allylationof Aldehydes usi ng Allyl Bromide
Conclusion
In an oven-dried, 10-mL, round-bottom flask were charged
In summary, LDH-Pt(0) and LDH-Pd(0) catalysts were benzaldehyde (1 mmol), allyl bromide (2 mmol), stannous
successfully employed in the allylation of aldehydes to chloride (1.5 mmol) and LDH-Pt(0) (2 mol %) and DMF
(
2 mL) and stirred at room temperature under a nitrogen at-
give homoallylic alcohols in high yields. Moreover, the
mechanistic study revealed that a monoallyl-palladi-
um-tin complex was the key intermediate in our hetero-
geneous system as opposed to the biallylpalladium com-
plex in the homogeneous media as observed by Yama-
moto et al. Finally, the simple procedure, easy recov-
ery and reusability of the catalytic systems are expected
mosphere. After the completion of the reaction as monitored
by TLC, the catalyst was filtered and the filtrate was diluted
with 10 mL ethyl acetate and washed with 1 N HCl solution fol-
lowed by saturated aqueous NaCl solution and the organic lay-
er was concentrated to afford the crude reaction mixture. This
crude reaction mixture was subjected to column chromatogra-
[2b]
phy and the pure product was separated using 3% ethyl acetate
1
to contribute to the development of benign chemical in hexane. H NMR (300 MHz,CDCl ): d¼2.2(br, 1H), 2.42–
3
processes and products.
2.55 (m, 2H), 4.7 (t, J¼6.78 Hz, 1H), 5.10–5.20 (m, 2H),
5
.65–5.85 (m, 1H), 7.15–7.35(m, 5H).
All products were identified by NMR and matched with au-
[3b]
thentic data as reported in the literature.
Experimental Section
General Remarks
Allylationof Aldehydes usi ng Allyl Alcohol
FT-IR spectra were recorded on a Perkin-Elmer spectrophoto-
meter. H NMR spectra were recorded on a Bruker (300 MHz)
In an oven-dried, 10-mL, round-bottom flask were charged
benzaldehyde (1 mmol), allyl alcohol (3 mmol), stannous
chloride (3 mmol), LDH-Pt(0) (2 mol %) and 1,3-dimethyl-2-
imidazolidinone (DMI) (2 mL) and stirred at room tempera-
ture under a nitrogen atmosphere. After the completion of
the reaction as monitored by TLC, the catalyst was filtered
1
spectrophotometer using CDCl as solvent and TMS as the in-
3
ternal standard. XPS spectra were recorded on a KRATOS
AXIS 165 with a dual anode (Mg and Al) apparatus using
the Mg Ka anode. X-ray powder diffraction (XRD) data
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B. M. Choudary et al.
and the filtrate was diluted with 10 mL ethyl acetate and wash- References and Notes
ed with 1 N HCl solution followed by saturated aqueous NaCl
solution and the organic layer was concentrated to afford the
crude reaction mixture. This crude reaction mixture was sub-
jected to column chromatography and the pure product was
separated using a mixture of ethyl acetate and hexane.
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Allylation of Aldehydes using Allyltributylstanne
[
2] a ) H. Nakamura, N. Asao, Y. Yamamoto, J. Chem. Soc.
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In an oven-dried, 10-mL, round-bottom flask were charged
benzaldehyde (1 mmol), allyltributylstanne (1.2 mmol) and
LDH-Pt(0) (5 mol %) and dry THF (3 ml) and stirred at
room temperature under a nitrogen atmosphere. After the
completion of the reaction as monitored by TLC, the catalyst
was filtered and the filtrate was concentrated to afford the
crude reaction mixture. This crude reaction mixture was sub-
jected to column chromatography and the pure product was
separated using a mixture of ethyl acetate and hexane.
[
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Preparationof STO-1
LDH-Pd(0) (1000 mg, Pd 1.5 mmol) and allyltributylstanne
(
1 mL) were charged in a 10-mL, round-bottom flask, 5 mL
dry THF was added and the mixture refluxed under argon
for 3 h. Then the solid material was filtered and washed with
THF twice and vacuum dried for 1 h to afford STO-1
(
1040 mg). STO-1 was analyzed for tin by SEM-EDX and the
tin content was found to be 0.11 mmol per gram. Next STO-1
was subjected to XPS analysis for Pd, Sn and C. Then STO-1
(
1000 mg) and 4-nitrobenzaldehyde (30 mg, 0.2 mmol) were
mixed with 3 mL dry THF and refluxed under N for 10 h to
get the corresponding homoallylic alcohol.
2
[6] J. F. Moulder, W. F. Stickle, P. E. Sobol, K. D. Bomden,
Handbook of X-ray Photoelectron Spectroscopy, Perkin-
Elmer Corp., 1992.
[
7] Benzaldehyde (1 mmol), allyltributylstanne (1.2 mmol),
LDH-Pt(0) and solvent (3 mL) were refluxed under nitro-
gen for 9 h. Isolated yields: dry THF (83%), THF:water¼
Acknowledgements
M. Roy thanks UGC, New Delhi for providing a research fel-
lowship. S. Roy thanks CSIR, New Delhi for providing a re-
search fellowship.
9:1 (75%), toluene (65%) CH CN (60%), CH CN:
3 3
water¼9:1 (55%), and DMF (50%).
2014
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Adv. Synth. Catal. 2005, 347, 2009 – 2014