Highly efficient gold nanoparticle catalyzed deoxygenation of amides, sulfoxides, and pyridine N-oxides

Article abstract of DOI:10.1002/chem.201003109

Selective deoxygenation is one of the most important reactions in the areas of total synthesis, biological chemistry, and transformation of renewable biomass resources.To date, many useful methods for the selective deoxygenation of oxygen-containing organic molecules, such as amides, nitro compounds, epoxides, sulfoxides, and those with Noxide groups, have been developed. However, these methods often include stoichiometric reactions. Some successful catalysts have been reported,but most of them are homogeneous systems and still suffer from low activities and selectivities, harsh reaction conditions, and tedious workup procedures. Therefore, further development of highly efficient heterogeneous catalysts for selective deoxygenations is highly desired.

Full text of DOI:10.1002/chem.201003109

DOI: 10.1002/chem.201003109
Highly Efficient Gold Nanoparticle Catalyzed Deoxygenation of Amides,
Sulfoxides, and Pyridine N-Oxides
Yusuke Mikami,[a] Akifumi Noujima,[a] Takato Mitsudome,[a] Tomoo Mizugaki,[a]
Koichiro Jitsukawa,[a] and Kiyotomi Kaneda*[a, b]
Selective deoxygenation is one of the most important re-
actions in the areas of total synthesis, biological chemistry,
and transformation of renewable biomass resources.[1] To
date, many useful methods for the selective deoxygenation
of oxygen-containing organic molecules, such as amides,
nitro compounds, epoxides, sulfoxides, and those with N-
oxide groups, have been developed. However, these meth-
ods often include stoichiometric reactions. Some successful
catalysts have been reported,[2–4] but most of them are ho-
mogeneous systems and still suffer from low activities and
selectivities, harsh reaction conditions, and tedious workup
procedures. Therefore, further development of highly effi-
cient heterogeneous catalysts for selective deoxygenations is
highly desired.
Gold nanoparticles (Au NPs) have received much atten-
tion in the field of catalysis because of their unique and high
oxidation ability for various reactions such as oxidation of
CO[5] and alcohols,[6] and selective epoxidation of styrene[7]
and propylene.[8] On the other hand, the reduction ability of
the Au NPs has not yet been widely studied, despite their
outstanding catalytic activity compared with other metal
NPs. Corma and co-workers have discovered an excellent
catalytic activity of TiO2-supported Au NPs for highly che-
moselective reduction of nitro compounds.[9] Cao and co-
workers have reported chemoselective reduction of carbonyl
compounds to the corresponding alcohols by TiO2 or meso-
porous CeO2-supported Au NP catalysts.[10] We have also
discovered a novel Au NP catalytic system for selective de-
oxygenation of epoxides into alkenes with an alcohol or
CO/H2O as reducing agent.[11]
From our continuing studies on Au NP catalysis, we
herein report that hydroxyapatite-supported Au NPs (here-
after called AuHAP) efficiently catalyze deoxygenation of
amides to amines with silanes as reductants. Various tertiary
and secondary amides with benzylic, allylic, aliphatic, and
heterocyclic amide substituents are selectively deoxygenated
to the corresponding amines. Moreover, this AuHAP/silane
catalytic system can be applied to deoxygenation of sulfox-
ides and pyridine N-oxides into sulfides and pyridines, re-
spectively. In these deoxygenations, the AuHAP catalyst
achieved excellent turnover numbers (TONs) up to 10000,
which is much greater than those of previously reported cat-
alytic systems.[2–4] The present AuHAP catalyst was easily
recovered and reused without any loss of activity or selectiv-
ity.
The AuHAP catalyst was synthesized according to a pro-
cedure described in our previous paper.[12] In brief, HAP
was mixed into an aqueous solution of HAuCl4 in the pres-
ence of NH3. The resulting slurry was filtered, washed with
distilled water, and dried under vacuum to give a HAP-con-
taining AuIII species. The obtained solid was treated with
KBH4 to give AuHAP as a purplish red powder with a
mean diameter of 3.0 nm.
We attempted the deoxygenation of N,N-dimethylbenz-
AHCTUNGTREGaNNNU mide (1) into N,N-dimethylbenzylamine (2) with various
[a] Y. Mikami, A. Noujima, Dr. T. Mitsudome, Dr. T. Mizugaki,
Prof. Dr. K. Jitsukawa, Prof. Dr. K. Kaneda
Department of Materials Engineering Science
Graduate School of Engineering Science
supported Au NP catalysts. During the optimization of the
reaction conditions, we found that heating AuHAP and 1 at
1108C in toluene in the presence of dimethylphenylsilane
for 3 h afforded complete conversion of 1 to 2 in >99%
yield (Table 1, entry 1).[13,14] Other supports for Au NPs such
as TiO2 and Al2O3 also gave good yields of 2 (Table 1, en-
tries 2 and 3). Compared with other metal NPs tested, the
Au NPs exhibited the highest catalytic activities; Pd, Ag,
and Pt NPs resulted in low yields of 2 and neither Ru nor
Rh NPs showed any activity (Table 1, entries 6–10). In addi-
Osaka University, 1-3, Machikaneyama
Toyonaka, Osaka 560-8531 (Japan)
[b] Prof. Dr. K. Kaneda
Research Center for Solar Energy Chemistry Osaka University
1-3, Machikaneyama, Toyonaka, Osaka 560-8531 (Japan)
Fax : (+81)6-6850-6260
E-mail: kaneda@cheng.es.osaka-u.ac.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201003109.
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Chem. Eur. J. 2011, 17, 1768 – 1772

COMMUNICATION
Table 1. Deoxygenation of N,N-dimethylbenzamide with hydrosilanes by
Table 2. Deoxygenation of amides by AuHAP in the presence of dime-
thylphenylsilane.[a]
various catalysts.[a]
Amide
Amine
t
Yield
[%][b]
Sel.
Catalyst
Yield [%][b]
Sel. [%][b]
[h]
[%][b]
1
2
3
4
5
6
7
8
9
AuHAP
AuTiO2
AuAl2O3
AuMgO
AuSiO2
AgHAP
PdHAP
PtHAP
RuHAP
RhHAP
HAuCl4
Au2O3
>99
88
80
67
55
29
20
7
trace
trace
trace
0
>99
>99
>99
>99
>99
>99
>99
>99
–
1
3
3
3
99 (92)
99
99
>99
>99
>99
>99
2[c]
3[d]
4[e]
24
99 (96)
5
6
10
3
80
>99
>99
10
11
12
13
–
–
–
99 (93)
HAP
0
–
7
6
95
>99
[a] Reaction conditions: AuHAP (0.1 g, 1.6mol% Au), N,N-
dimethylbenzamide (0.5 mmol), dimethylphenylsilane (2.5 mmol), tolu-
ACHTUNGTRENNUNG
ene (5 mL), 1108C, Ar, 3 h. [b] Selectivity (sel.) was determined by GC
using naphthalene as an internal standard.
8
9
4
4
6
6
91 (85)
90 (84)
92
>99
>99
>99
>99
tion, the use of the gold compounds HAuCl4 and Au2O3,
bulk Au, or parent HAP in place of AuHAP did not pro-
mote the deoxygenation at all (Table 1, entries 11–13).
These results suggested that the high catalytic activity of the
AuHAP is derived from the intrinsic activity of the Au NPs.
The solid AuHAP catalyst was filtered off from the reac-
tion mixture after 50% conversion of 1. Further stirring of
the filtrate under identical reaction conditions did not afford
any products. Inductively coupled plasma (ICP) analysis of
the filtrate did not show any leaching of Au species into the
solution (detection limit: 0.007 ppm), confirming that the
present deoxygenation proceeds on the solid AuHAP cata-
lyst.
10
11
93
12
13
14
8
8
95
95
91
>99
>99
>99
12
The AuHAP catalyst could deoxygenate a wide range of
amides to amines. Typical results are shown in Table 2. Most
of the tested benzamides exhibited high reactivities (Table 2,
entries 1 and 6–9). The derivative with an electron-donating
group was more reactive than one with an electron-with-
drawing group (Table 2, entry 5 versus 6). The ester-contain-
ing amide was converted to the amine without reduction of
the ester group (Table 2, entry 7). Benzamides substituted
with piperidine and morpholine moieties were deoxygenated
efficiently (Table 2, entries 8 and 9). Furthermore, allylic,
aliphatic, and heterocyclic tertiary amides all underwent de-
oxygenation sufficiently, providing the corresponding
amines in excellent to good yields (Table 2, entries 10–15).
Interestingly, the deoxygenation of the secondary amide of
N-methylbenzamide by the AuHAP also occurred to give
N-methyl-N-benzylamine in good yield when TMDS was
used as a reducing agent instead of dimethylphenylsilane
(Table 2, entry 16).[15]
15
12
12
90
80
>99
>99
16[f]
[a] Reaction conditions: AuHAP (0.1 g, 1.6 mol% Au), amide
(0.5 mmol), dimethylphenylsilane (2.5 mmol), toluene (5 mL), 1108C, Ar.
[b] Determined by GC by an internal standard technique. Values in pa-
rentheses are the isolated yields. [c] First recycle of the catalyst.
[d] Second recycle of the catalyst. [e] AuHAP (0.015 g), amide (2 g).
[f] 1,1,3,3-Tetramethyldisiloxane (TMDS) (1.25 mmol) was used as a re-
ductant.
the TON and turnover frequency (TOF) reached 10000 and
416 hÀ1, respectively (Table 2, entry 4). This TON value is
two orders of magnitude larger than those of previously re-
ported catalytic systems such as [Ru3(m3,h2,h3,h5-acenaph-
AHCTUNGTRENGtNUN hylene)(CO)7]/PMHS (PMHS=polymethylhydrosiloxane)
The deoxygenation of 1 by the AuHAP could be per-
formed on a two-gram scale. Thus, compound 2 was ob-
tained as the sole product in 95% isolated yield (1.9 g) and
(TON=77, TOF=5 hÀ1),[2k] [H2PtCl6]/PMHS (TON=83,
TOF=17 hÀ1),[2d] [Fe3(CO)12]/Ph2SiH2 (TON=50, TOF=
2 hÀ1),[2c] [Os3(CO)12]/Et2NH/Et3SiH (TON=100, TOF=
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K. Kaneda et al.
6 hÀ1),[2l] and Zn
0.5 hÀ1).[2a]
To investigate the scope of the deoxygenation ability of
the present AuHAP/silane catalytic system, we next applied
this system to deoxygenation of sulfoxides.[13] As exemplified
in Table 3, various sulfoxides were selectively deoxygenated
(OAc) /
E
pered by the strong coordination of the sulfide product, sur-
prisingly, the AuHAP catalyst was active even in higher sulf-
oxide/AuHAP ratios. An excellent yield of sulfide was ob-
tained with a TOF of 200 hÀ1 and a TON of up to 10000,
which is three orders of magnitude greater than those of
previously reported catalysts (Table 3, entry 4).[16]
Furthermore, this AuHAP/silane system was effective for
the deoxygenation of pyridine N-oxides, an important step
in the synthesis of functionalized pyridine derivatives.[1f,h]
The catalytic system was revealed to be compatible with var-
ious pyridine N-oxides with reducible functional groups such
as cyano, olefin, and carbonyl, as shown in Table 4.[13]
Table 3. Deoxygenation of various sulfoxides by AuHAP.[a]
Sulfoxide
Sulfide
t [h] Yield [%][b] Sel. [%][b]
1
1
1
1
>99
>99
99
>99
>99
>99
>99
2[c]
3[d]
4[e]
Table 4. Deoxygenation of various pyridine N-oxides by AuHAP.[a]
48
94 (93)
5
6
7
3
4
3
96
>99
>99
>99
Pyridine N-oxide
Pyridine
Yield [%][b]
Sel. [%][b]
1
99
99
99
95 (92)
>99
>99
>99
>99
2[c]
3[d]
4[e]
92 (85)
96 (90)
5
6
7
99
>99
>99
>99
>99 (95)
99 (92)
8
6
12
6
99 (97)
93
>99
>99
>99
8[f]
9
89
91
>99
>99
9[f]
10
89
10
96 (90)
>99
11[f]
12
12
6
98
92
>99
>99
[a] Reaction conditions: AuHAP (0.1 g, 1.6 mol% Au), substrate
(0.5 mmol), dimethylphenylsilane (1 mmol), 1,4-dioxane (3 mL), 308C,
Ar, 1 h. [b] Determined by LC by an internal standard technique. Values
in parentheses are the isolated yields. [c] First recycle of the catalyst.
[d] Second recycle of the catalyst. [e] AuHAP (0.002 g, 0.16 mmol Au),
substrate (10 mmol), dimethylphenylsilane (20 mmol), 1,4-dioxane
(20 mL), 1308C, Ar, 60 h. [f] Dimethylphenylsilane (0.75 mmol).
[a] Reaction conditions: AuHAP (0.1 g, 1.6 mol% Au), substrate
(0.5 mmol), dimethylphenylsilane (1 mmol), 1,4-dioxane (3 mL), 308C,
Ar. [b] Determined by GC or LC by an internal standard technique.
Values in parentheses are the isolated yields. [c] First recycle of the cata-
lyst. [d] Second recycle of the catalyst. [e] AuHAP (0.01 g, 0.8 mmol Au),
substrate (10 mmol), dimethylphenylsilane (20 mmol), 1,4-dioxane
(20 mL), 1308C, Ar. [f] 608C.
After the above deoxygenations, the AuHAP catalyst was
easily recovered and reused without any loss of catalytic ac-
tivity, for example, 99% yields were obtained in the two re-
cycling reactions (Table 2, entries 2 and 3; Table 3, entries 2
and 3; and Table 4, entries 2 and 3). TEM and extended X-
ray absorption fine structure (EXAFS) analysis of both
fresh and reused AuHAP revealed that the size and oxida-
tion state of the Au NPs on HAP did not change significant-
ly,[17] suggesting that the HAP support might play an impor-
tant role in dispersing and stabilizing the Au NPs.[18]
In a preliminary study by FTIR spectroscopy, interactions
between Au NPs on HAP and silane were investigated. Di-
methylphenylsilane was treated with AuHAP and an ab-
sorption band assigned to a Si H stretching vibration of the
adsorbed silane shifted to lower frequencies with respect to
under mild reaction conditions. Phenyl, benzylic, and ali-
phatic sulfoxides were converted into the corresponding sul-
fides in excellent yields with over 99% selectivity. Notably,
functionalized sulfoxides were chemoselectively reduced,
that is, only the S=O moiety was deoxygenated, whereas
olefin, ester, propargyl, cyano, and ketone groups remained
intact under the present conditions (Table 3, entries 6, 7,
and 10–12). This result is in sharp contrast to that of the re-
ported catalytic system in which hydrogenolysis of an ester
moiety and reduction of a sulfoxide occurred.[3c] Even
though the catalytic activity of the AuHAP may be ham-
À
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Gold Nanoparticle Catalyzed Deoxygenation
COMMUNICATION
that of the HAP-adsorbed silane.[19] No change in the C=O
stretching frequency of 1 was observed when AuHAP was
treated with 1. Based on these phenomena, the present
AuHAP-catalyzed deoxygenation may proceed through a
pathway initiated by the Au–silane interaction, which is sim-
ilar to that proposed by Beller and co-workers (Figure 4S in
the Supporting Information).[2a]
In conclusion, the present AuHAP system serves as an ef-
ficient heterogeneous catalyst for deoxygenation of amides,
sulfoxides, and pyridine N-oxides. The methodology de-
scribed herein is a useful protocol for removing oxygen be-
cause of the following advantages: 1) excellent catalytic ac-
tivities and selectivities; 2) the use of easy-to-handle cata-
lysts; 3) applicability to a wide range of amides, sulfoxides,
and pyridine N-oxides; and 4) reusability of the catalyst
without any loss in its efficiency.
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Experimental Section
Characterization of AuHAP: Elemental analysis revealed gold loadings
of 1.65%. X-ray absorption spectra and TEM showed that the mean di-
ameter (d) and the standard deviation (s) of the Au NPs formed on the
HAP support were d=3.0 nm and s=0.9 nm, respectively.
General protocol for deoxygenation by AuHAP: All manipulations were
conducted under Ar atmosphere. AuHAP (0.10 g, Au: 0.0083 mmol) and
solvent (3–5 mL) were introduced into a Schlenk tube followed by the
addition of amide, sulfoxide, or N-oxide (0.5 mmol). Then dimethylphe-
nylsilane (1–2.5 mmol) was added. The reaction mixture was vigorously
stirred at the selected temperature in a silicone oil bath. After the deoxy-
genation reaction, the AuHAP was filtered and the reaction mixture was
analyzed by GC or LC with naphthalene as an internal standard to deter-
mine the conversions and yields.
Acknowledgements
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Keywords: amides · deoxygenation · gold · heterogeneous
catalysis · nanoparticles
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K. Kaneda et al.
[13] See the Supporting Information for detailed results of the optimiza-
tion studies.
[17] See Figures 1S and 2S in the Supporting Information for EXAFS
and TEM analysis of AuHAP.
[14] Our previously reported catalytic system, in which Au NPs catalyzed
the deoxygenation of epoxides with an alcohol or CO as a reductant,
could not be applied to the present deoxygenation of amides, sulfox-
ides, and N-oxides.
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Kaneda, Angew. Chem. 2008, 120, 8056; Angew. Chem. Int. Ed.
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À
[15] It is reported that two proximate Si H bonds of TMDS are effective
in the deoxygenation of secondary amides. See ref. 2.
[16] The values of TON and TOF in the previously reported catalytic de-
oxygenation of diphenylsulfoxide are as follows: [MoO2Cl
2ACHTUNGTERN(NUNG dmf)2]/P-
ACHTUNGTRENNUNG
[3a]
TOF=44 hÀ1),[3b] [MoO2Cl2]/H2 (TON=20, TOF=1 hÀ1),
[ReIO2A (acac)(dppe)]/
G
E
A
[19] See Figure 3S in the Supporting Information for the FTIR study.
HBcat. (acac=acetylacetonate, dppe=1,2-bis(diphenylphosphino)-
[3d]
ethane; TON=20, TOF=20 hÀ1),
(TON=7, TOF=0.4 hÀ1).[3f]
and [ReO
(CH3)]/PPh3
Received: October 28, 2010
Published online: January 12, 2011
1772
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Chem. Eur. J. 2011, 17, 1768 – 1772