Colloidal palladium nanoparticles with in situ H2: Reducing system for ,-unsaturated carbonyl compounds

Article abstract of DOI:10.1080/00397910902788117

A new reducing system comprising Pd(OAc)2 and NaBH4 in methanol to generate palladium nanoparticles has been efficiently utilized to reduce a variety of unsaturated carbonyl compounds. These were reduced to their corresponding saturated alcohols and fully saturated compounds in selected cases. This protocol presents alternative and mild reaction conditions for reduction.

Full text of DOI:10.1080/00397910902788117

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Colloidal Palladium
Nanoparticles with In Situ
H2: Reducing System for
α,β-Unsaturated Carbonyl
Compounds
Kathlia A. De Castro a , Seungchan Oh a , Jongchan
Yun a , Jin Kyu Lim a , Gwangil An b , Dong Kook Kim
a & Hakjune Rhee a
a Hanyang University, Department of Chemistry and
Applied Chemistry , Ansan, Kyunggi-Do, Korea
b Radiopharmaceuticals Laboratory, Korea Institute
of Radiological and Medical Sciences , Nowon-Gu,
Seoul, Korea
Published online: 04 Sep 2009.
To cite this article: Kathlia A. De Castro , Seungchan Oh , Jongchan Yun ,
Jin Kyu Lim , Gwangil An , Dong Kook Kim & Hakjune Rhee (2009) Colloidal
Palladium Nanoparticles with In Situ H2: Reducing System for α,β-Unsaturated
Carbonyl Compounds, Synthetic Communications: An International Journal for
Rapid Communication of Synthetic Organic Chemistry, 39:19, 3509-3520, DOI:
10.1080/00397910902788117
To link to this article: http://dx.doi.org/10.1080/00397910902788117

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Synthetic Communications1, 39: 3509–3520, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910902788117
Colloidal Palladium Nanoparticles with In Situ
H2: Reducing System for a,b-Unsaturated
Carbonyl Compounds
Kathlia A. De Castro,1 Seungchan Oh,1 Jongchan Yun,1
Jin Kyu Lim,1 Gwangil An,2 Dong Kook Kim,1 and Hakjune Rhee1
1Hanyang University, Department of Chemistry and Applied Chemistry,
Ansan, Kyunggi-Do, Korea
2Radiopharmaceuticals Laboratory, Korea Institute of Radiological and
Medical Sciences, Nowon-Gu, Seoul, Korea
Abstract: A new reducing system comprising Pd(OAc)2 and NaBH4 in methanol
to generate palladium nanoparticles has been efficiently utilized to reduce a
variety of unsaturated carbonyl compounds. These were reduced to their corre-
sponding saturated alcohols and fully saturated compounds in selected cases. This
protocol presents alternative and mild reaction conditions for reduction.
Keywords: Hydride, hydrogenation, palladium nano-sized particle, saturated
alcohol, a,b-unsaturated carbonyl compounds
INTRODUCTION
Reduction of organic compounds is one of the most widely used and
important functional group transformations in synthetic chemistry. The
conjugate reduction of a,b-unsaturated carbonyl compounds is one of
the commonly investigated, especially the selective reductions either by
metal-catalyzed and=or hydrogen-transfer hydrogenation[1] or via
hydride reduction.[2] There were also some methodologies published that
Received January 6, 2008.
Address correspondence to Hakjune Rhee, Department of Chemistry and
Applied Chemistry, Hanyang University, Ansan, Sangrok-Gu Sa-3-Dong 1271,
Kyunggi-Do 426-791, Korea. E-mail: hrhee@hanyang.ac.kr
3509

3510
K. A. De Castro et al.
lead to the complete reduction of selected a,b-unsaturated carbonyl
compounds to saturated alcohols. These include [(Ph3P)CuH]6 and H2
gas,[2b] hydrogen transfer assisted by [(PPh3)RuH(H2)]BPh4 and
[(PPh3)OsH(H2)]BPh4,[1c] and InCl3-NaBH4 in CH3CN quenched with
methanol.[3] The used of Pd=C is also common in hydrogenation reac-
tions, and use of various additives can control chemoselectivity.[4] Cur-
rently, the used of transition-metal nanoparticles[5] has received great
attention because of their exceptional catalytic effectiveness in a gamut
of media. Among these transition metals, Pd nanoparticles are the most
popular and widely used, particularly for coupling reactions.[5c,6] In
addition, interest in Pd nanoparticles for hydrogenation is now also in
place.[5e,7] Commonly, these nanoparticles are stabilized using various
polymers to control the particle size and avoid agglomeration. In this
way, the catalytic ability of the nanoparticles will be maximized.
We introduce here a new, powerful reducing system that completely
reduce a,b-unsaturated carbonyl compounds to saturated alcohols and in
some cases to fully saturated compounds.
RESULTS AND DISCUSSION
It was well established that carbonyl compounds are readily reduced by
hydride source, whereas olefins are efficiently hydrogenated using H2
with the aid of a metal catalyst such as Pd. In view of the fact that col-
loidal Pd nanoparticles are easily formed from Pd(OAc)2 as precursor
via reduction,[8] we envisaged that combining this with a hydride source
and alcohol in a closed system will readily reduce a carbonyl and an ole-
fin completely (Scheme 1). The reaction between hydride and alcohol
would generate in situ molecular hydrogen, and thus there is no need
for an external hydrogen source.
To check our hypothesis, we first conducted the reaction in both
open and closed systems using trans-cinnamaldehyde. We obtained
74% 3-phenyl-1-propanol with the open system and 98% in the closed sys-
tem. This proved the need to preserve the hydrogen generated in situ.
Next, we considered the amount of Pd(OAc)2 necessary to reduced both
Scheme 1. R1 ¼ H or alkyl; R2 ¼ Alkyl or aryl.

Unsaturated Carbonyl Reduction via Pd Nanoparticle
3511
Table 1. Reduction of trans-cinnamaldehyde using various amounts of Pd(OAc)2
Result=yield (%)
Pd(OAc)2
(equiv.)
Entry
1
2
3
4
0.001
0.005
0.01
86
59
60
0
10
34
35
98
0.05
the olefin and carbonyl group. We combined trans-cinnamaldehyde with
1.0 equiv. NaBH4 and various amount of Pd(OAc)2 in a flask capped
with a rubber septum that had a deflated balloon attached to it. Metha-
nol was then slowly added. Hydrogen gas evolution was evident through
the blown balloon that slowly shrinks as the reaction progresses. The
mixture formed a black colloidal suspension, which turned to a clear
solution with black precipitate after the reaction. Table 1 showed that
0.05 equiv. is necessary to obtain saturated alcohol in good yield.
We presumed that the mechanism of the reaction (Scheme 2) pro-
ceeded in the following manner. Sodium borohydride reduces the carbo-
nyl group first, while simultaneously reducing the Pd(OAc)2 to Pd(0) and
generating colloidal Pd nanoparticles. The excess hydrides react with
methanol to generate hydrogen gas, which is temporarily stored in situ
through the attached balloon. The nanoparticles formed from the reduc-
tion of Pd(OAc)2 act as a catalyst themselves or as a reservoir of Pd(0)
species that acts as the active catalyst of the reaction.[8] It is noteworthy
that the reaction proceeded very rapidly, so that stabilizing the nanopar-
ticles was unnecessary. The reaction completed just on time, with the
aggregation forming Pd black.
To verify the effect of size on the catalytic activity, we compared
the reaction of a-methyl cinnamaldehyde using Pd=C and Pd(OAc)2
Scheme 2. R1 ¼ H or alkyl; R2 ¼ alkyl or aryl.

3512
K. A. De Castro et al.
(Table 2). Pd=C is micrometers in size, whereas Pd(OAc)2 nanoparticles
are about 6 to 40 nm.[8] Indeed, the reactivity of Pd depends on the size,
such that nanoparticles are more reactive because of their high specific
surface area.
To check the validity of our protocol, we tested a variety of unsatu-
rated carbonyl compounds (Table 3). All a,b-unsaturated aldehydes
(entries 1 to 7) and ketones (entries 12–14, 16, and 18) gave the corre-
sponding saturated alcohols in excellent yield. Significantly, the reduction
of chalcones (entries 14, 16, 18) to the corresponding saturated alcohol
using this protocol with slight modification, such as dissolving the chal-
cone in 1 mL methanol and increasing the amount of NaBH4 to 2 equiv.,
effectively went to completion, unlike the previous method using indium
metal.[9] Interestingly, these chalcones were fully reduced to saturated
compounds when a cosolvent, chloroform, was used to dissolve the chal-
cone and 0.1 equiv of Pd(OAc)2 was used with a slightly longer reaction
time. This one-step full reduction of chalcone is noteworthy.
In addition, this protocol is not limited to only a,b-unsaturated
carbonyl compounds. It even reduced isolated unsaturated carbonyl com-
pound (entry 8). Conjugated and isolated double bonds and carbonyls
were also reduced using 2 equiv. of NaBH4. We are hoping to generalize
this protocol with other functional groups, which we will report in due
course.
In conclusion, we have illustrated a new reducing system capable
of reducing both carbonyl and olefin functionalities regardless of
position via in situ–generated Pd(0) nanoparticles and molecular
Table 2. Reduction of a-methyl cinnamaldehyde
Entry
Catalyst
Product=yield (%)
1
Pd=C
2
Pd(OAc)2

Unsaturated Carbonyl Reduction via Pd Nanoparticle
3513
Table 3. Complete reduction of unsaturated carbonyl compounds
Time
(min)
Yielda
(%)
Entry
Substrate
Product
1
2
5
97
98
30
3
4
30
98
60
60
95
96
5
6
30
60
98
96
7
8
30
60
98
89
95
95
9b
10b
11b
12
10
240
60
98
95
13
60
60
14c
87
(Continued )

3514
K. A. De Castro et al.
Table 3. Continued
Time
(min)
Yielda
(%)
Entry
15d
Substrate
Product
240
60
89
85
98
87
16e
17d
18e
120
60
19d
120
98
aIsolated yield.
bReacted with 2 equiv. NaBH4 and 0.05 equiv. Pd(OAc)2.
cThe substrate was dissolved first in 1 mL methanol before adding 1 equiv.
NaBH4 and 0.05 equiv. Pd(OAc)2.
dThe substrate was dissolved first in 1 mL chloroform before adding 1 equiv.
NaBH4 and 0.10 equiv. Pd(OAc)2.
eThe substrate was dissolved first in 1 mL methanol before adding 2 equiv.
NaBH4 and 0.05 equiv. Pd(OAc)2.
hydrogen. This protocol offers mild reaction conditions for reduction
within a shorter period.
EXPERIMENTAL
1H NMR spectra were obtained using a 300-MR spectrometer (300 MHz)
in CDCl3 solvents. The chemical shifts were reported in d parts per
million relative to tetramethylsilane (TMS). Spectral data of all compounds
were compared with those from commercially available compounds or with
literature data as specified. Flash-column chromatography was performed
using silica gel 60 (230–400 mesh). Analytical thin-layer chromatography
(TLC) was conducted on aluminum-backed silica-gel plates. Developed
plates were visualized with ultraviolet (UV) light or with a 2.0% phosphomo-
lybdic acid staining solution.

Unsaturated Carbonyl Reduction via Pd Nanoparticle
General Procedure for the Reduction
3515
The substrate (1.0 equiv.), NaBH4 (1.0 equiv.), and Pd(OAc)2 (0.05
equiv.) were combined in a round-bottom flask with magnetic stirrer
capped with a rubber septum that had a deflated balloon attached to
it. Methanol (5 mL) was slowly added into the flask through a syringe.
The mixture was stirred at room temperature until the reaction was
completed, based on TLC monitoring. The resulting mixture was fil-
tered, and methanol was evaporated using rotary evaporator. Brine
solution was added, and the product was extracted with CHCl3. The
organic layer was dried using anhydrous MgSO4, filtered, and concen-
trated using a rotary evaporator. The product was purified by silica-gel
column chromatography with an appropriate eluent system whenever
necessary.
Selected Data
3-Phenyl-1-propanol (Table 3, Entry 1)
1H NMR (CDCl3): d 7.27 (m, 2 H), 7.19 (m, 3 H), 3.68 (t, J ¼ 6.3 Hz,
2 H), 2.71 (t, J ¼ 7.5 Hz, 2 H), 1.90 (quintet, J ¼ 7.5 Hz, 2 H), 1.61
(br s, 1 H).
2-Methyl-3-phenyl-1-propanol (Table 3, Entry 2)[10]
1H NMR (CDCl3): d 7.27 (m, 2 H), 7.17 (m, 3 H), 3.46 (m, 2 H), 2.74 (dd,
J ¼ 13.5, 6.3 Hz, 1 H), 2.38 (dd, J ¼ 13.2, 8.1 Hz, 1 H), 2.01 (br s, 1 H),
1.93 (m, 1 H), 0.89 (d, J ¼ 7.2 Hz, 3 H).
2-Benzyl-1-heptanol (Table 3, Entry 3)[11]
1H NMR (CDCl3): d 7.27 (m, 2 H), 7.18 (m, 3 H), 3.50 (d, J ¼ 5.7 Hz,
2 H), 2.62 (d, J ¼ 7.2 Hz, 2 H), 1.78 (m, 1 H), 1.31 (m, 8 H), 1.27 (br s,
1 H), 0.87 (t, J ¼ 6.6 Hz, 3 H).
2-Benzyl-1-octanol (Table 3, Entry 4)[12]
1H NMR (CDCl3): d 7.27 (m, 2 H), 7.18 (m, 3 H), 3.51 (d, J ¼ 5.1 Hz,
2 H), 2.62 (d, J ¼ 6.9 Hz, 2 H), 1.78 (m, 1 H), 1.30 (m, 10 H), 1.26 (br s,
1 H), 0.87 (t, J ¼ 7.2 Hz, 3 H).

3516
K. A. De Castro et al.
2-Methyl-1-pentanol (Table 3, Entry 5)
1H NMR (CDCl3): d 3.44 (m, 2 H), 1.61 (m, 1 H), 1.40–1.06 (br m, 4 H),
0.89 (t, J ¼ 6.9 Hz, 3 H).
1-Hexanol (Table 3, Entry 6)
1H NMR (CDCl3): d 3.63 (m, 2 H), 1.56 (m, 2 H), 1.37 (m, 6 H), 1.30 (br
s, 1 H), 0.89 (t, J ¼ 6.6 Hz, 3 H).
1-Heptanol (Table 3, Entry 7)
1H NMR (CDCl3): d 3.63 (t, J ¼ 6.3 Hz, 2 H), 1.56 (m, 2 H), 1.37 (m, 8 H),
1.30 (br s, 1 H), 0.88 (t, J ¼ 6.6 Hz, 3 H).
1-Undecanol (Table 3, Entry 8)
1H NMR (CDCl3): d 3.61 (m, 2 H), 1.54 (m, 2 H), 1.26 (br m, 16 H), 0.85
(t, J ¼ 6.6 Hz, 3 H).
2,4-Diethyl-1-heptanol (Table 3, Entry 10)
1H NMR (CDCl3): d 4.05 (s, 2 H), 1.41–0.97 (br m, 12 H), 0.88–0.80 (br
m, 9 H).
3,7-Dimethyl-1-octanol (Table 3, Entry 11)
1H NMR (CDCl3): d 3.67 (m, 2 H), 1.66–1.10 (br m, 10 H), 0.87 (m, 9 H).
2-Pentyl-2-cyclopentanol (Table 3, Entry 12)[13]
1H NMR (CDCl3): d 4.12 (br s, 1 H), 3.79 (q, J ¼ 5.1 Hz, 1 H), 1.96–1.09
(br m, 15 H), 0.88 (t, J ¼ 6.6 Hz, 3 H).
4-Phenyl-2-butanol (Table 3, Entry 13)
1H NMR (CDCl3): d 7.25 (m, 2 H), 7.16 (m, 3 H), 3.80 (m, 2 H), 2.70
(m, 2 H), 1.75 (m, 2 H), 1.21 (d, J ¼ 6.3 Hz, 3 H).

Unsaturated Carbonyl Reduction via Pd Nanoparticle
3517
1,3-Diphenyl-1-propanol (Table 3, Entry 14)[14]
1H NMR (CDCl3): d 7.33 (m, 4 H), 7.25 (m, 3 H), 7.16 (m, 3 H), 4.67
(m, 1 H), 2.70 (m, 2 H), 2.07 (m, 2 H), 1.83 (br s, 1 H).
1,3-Diphenylpropane (Table 3, Entry 15)[15]
1H NMR (CDCl3): d 7.28 (m, 5 H), 7.19 (m, 5 H), 2.65 (t, J ¼ 7.8 Hz, 4 H),
1.96 (quintet, J ¼ 7.8 Hz, 2 H).
3-(4-Methoxy-phenyl)-1-phenyl-1-propanol (Table 3, Entry 16)[16]
1H NMR (CDCl3): d 7.33 (d, J ¼ 4.2 Hz, 2 H), 7.25 (t, J ¼ 4.5 Hz, 3 H),
7.08 (d, J ¼ 8.4 Hz, 2 H), 6.80 (d, J ¼ 8.1 Hz, 2 H), 4.66 (m, 1 H), 3.77
(s, 3 H), 2.65 (m, 2 H), 2.03 (m, 2 H), 1.82 (d, J ¼ 3.3 Hz, 1 H).
1-Methoxy-4-(3-phenylpropyl)benzene (Table 3, Entry 17)[17]
1H NMR (CDCl3): d 7.30 (m, 2 H), 7.17 (m, 3 H), 7.10 (d, J ¼ 8.4 Hz,
2 H), 6.82 (d, J ¼ 8.7 Hz, 2 H), 3.79 (s, 3 H), 2.61 (m, 4 H), 1.92 (quintet,
J ¼ 7.5 Hz, 2 H).
1-(4-Methoxy-phenyl)-3-phenyl-1-propanol (Table 3, Entry 18)[16]
1H NMR (CDCl3): d 7.25 (m, 5 H), 7.16 (d, J ¼ 6.3 Hz, 3 H), 6.87
(d, J ¼ 8.4 Hz, 2 H), 4.62 (m, 1 H), 3.79 (s, 3 H), 2.67 (m, 2 H), 2.05
(m, 2 H), 1.75 (d, J ¼ 3.3 Hz, 1 H).
ACKNOWLEDGMENTS
K. D. C. acknowledges financial support from the Korean Ministry of
Education through the second stage of the BK21 project for the Hanyang
University graduate program. This work was supported by the research
fund of Hanynag University (HY-2004-I).
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