Amidoearbonylation of aldehydes utilizing cobalt oxide-supported gold nanoparticles as a heterogeneous catalyst

Article abstract of DOI:10.1246/cl.2008.1292

Cobalt oxide-supported gold-nanoparticles-catalyzed transformation of aldehydes and their equivalents to N-acyl-α-arnino acids was achieved. The desired products were obtained in moderate to excellent yields under milder reaction conditions than previous reports employing octacarbonyldicobalt as a catalyst. Copyright

Full text of DOI:10.1246/cl.2008.1292

1292
Chemistry Letters Vol.37, No.12 (2008)
Amidocarbonylation of Aldehydes Utilizing Cobalt Oxide-supported Gold Nanoparticles
as a Heterogeneous Catalyst
Akiyuki Hamasaki,1 Xiaohao Liu,1;2 and Makoto Tokunagaꢀ1;2
1Department of Chemistry, Graduate School of Sciences, Kyushu University,
6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581
2JST (Japan Science and Technology Cooperation) CREST
(Received September 8, 2008; CL-080855; E-mail: mtok@chem.kyushu-univ.jp)
Cobalt oxide-supported gold-nanoparticles-catalyzed trans-
Table 1. Optimization of reaction conditions
formation of aldehydes and their equivalents to N-acyl-ꢀ-amino
acids was achieved. The desired products were obtained in mod-
erate to excellent yields under milder reaction conditions than
previous reports employing octacarbonyldicobalt as a catalyst.
Au/Co3O4
CO/H2 (1:1, 4 MPa)
AcNH2 (1.5 equiv)
CO2H
CHO
EtOAc
(hexanal: 0.2 M)
3
NHAc
3
Entrya
Temp/ꢁC
Time/h
Yieldb/%
1
2
3
4
5
100
100
100
100
60
2
5
31
52
68
65
Amidocarbonylation reaction (Wakamatsu reaction) is an
efficient method to obtain N-acyl-ꢀ-amino acid derivatives from
aldehydes. It was first reported by Wakamatsu et al.1 using octa-
carbonyldicobalt [Co2(CO)8] as a homogeneous catalyst. In con-
trast to classical methods of ꢀ-amino acid synthesis such as the
Strecker reaction, amidocarbonylation can avoid the use of poi-
sonous cyano materials and give the product in a single operation
without subsequent hydrolysis. Additionally, from the point of
atom-efficiency, this reaction has a superb advantage because
H2O is the only waste formed through the reaction. However, re-
quirement of harsh reaction conditions, especially high pressure
of more than 15 MPa, could be a troublesome aspect of
Co2(CO)8-catalyzed amidocarbonylation.2 One solution of this
20
40
20
20
20
20
20
20
20
20
20
20
13
6
7
80
86 (64)
17
44
88
84
39f
5
120
100
100
80
100
100
100
100
8c
9d
10d
11e
12g
13h
14i
0
58
problem is to use stibine ligands for Co2(CO)8-catalyzed amido-
3
´
carbonylation of olefins reported by Gomez et al. Homogeneous
aAll reactions were carried out in the presence of Au/
Co3O4 [5 atom % Au/(Au+Co) and 5.7 atom % Co/sub-
strate] except Entry 14. bGC yield. Isolated yield in paren-
palladium4a,4b and platinum5 were found to catalyze the reaction
under milder conditions than Co(CO)8. Heterogeneous version
of palladium catalyst has also been developed.4c,4d However,
there are no heterogeneous cobalt-based catalyst for the reaction.
Recently we have explored a novel function of cobalt oxide-
supported gold nanoparticles (Au/Co3O4) catalyst which effec-
tively catalyzes hydroformylation of olefins.6 The Co0 active
species is expected to be generated under syngas (CO/H2) pres-
sure (4 MPa) around 100 ꢁC. As a showcase of the applications
of these catalyst systems, amidocarbonylation utilizing Au/
Co3O4 catalyst are demonstrated in this paper.
c
d
thesis. Gas pressure was 2 MPa. CO/H2 ratio was 3:1.
eHexanal/acetamide ratio was 2:1. Based on acetamide.
f
gHexanal concentration was 1 M. In the absence of the
catalyst. In the presence of 19 atom % Co/substrate.
h
i
exclusive formation of bisamide.8 This means Au/Co3O4-cata-
lyzed amidocarbonylation can be conducted under milder condi-
tion with respect to both syngas pressure and temperature than
that of Co2(CO)8-catalyzed reaction. Although the highest yield
was achieved at 80 ꢁC, most of the reactions for optimization
were carried out at 100 ꢁC to clarify the effect of each factor.
Amidocarbonylation of hexanal with acetamide was exam-
ined under several conditions (Table 1). When the reactions were
conducted in less than 5 h (Entries 1 and 2), the yields were low
to moderate even though calculated conversions of hexanal were
77% and 87% respectively, as a result of formation of bisamide
product7 along with the desired product. It was revealed that the
by-product disappeared by elongation of reaction time to 20 h
and yield of the amino acid was increased (Entry 3). Further
longer reaction time (40 h) did not improve the yield any more
(Entry 4). Reaction temperature was also a critical factor. The
accepted temperature range seemed to be relatively narrow and
the optimal one was between 80 to 100 ꢁC (Entries 3 and 5–7).
Magnus and Slater reported CO2(CO)8-catalyzed amidocar-
bonylation of butyraldehyde requires higher temperature around
150 ꢁC and insufficient temperature (90–100 ꢁC) resulted in the
Under the same H2 partial pressure (PH ¼ 1 MPa), higher CO
2
pressure was preferable (Entries 8 and 9, PCO ¼ 1 MPa vs.
3 MPa). This result concurs with the previous report on PCO
dependence of reaction rates.9 Existence of excess aldehyde
(Entry 11) or a concentrated condition (Entry 12) lead to the for-
mation of significant amounts of aldol product and resulted in
decreasing yield. In the absence of the catalyst, no formation
of amino acid, bisamide or aldol product were observed (Entry
13). Increasing the catalyst amount did not improve reaction
efficiency (Entry 14).
Reactions employing various aldehydes and amides were
performed to reveal the scope of the reaction (Table 2). The re-
activities of aldehyde materials seemed to be quite affected by
Copyright Ó 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.12 (2008)
1293
Table 2. Amidocarbonylation reactions of various aldehydes
and amides
Au/Co3O4
CO/H2 (3:1, 4 MPa)
BzNH2 (1.5 equiv)
Au/Co3O4
OEt
OEt
CO2Et
NHBz
ð2Þ
CO/H2 (3:1, 4 MPa)
CO2H
1,4-dioxane (substrate: 0.2 M)
R2CONHR3 (1.5 equiv)
80 °C, 20 h
R1 CHO
R1
NCOR2
R3
21%
EtOAc (aldehyde: 0.2 M)
80 °C, 20 h
Recycle use of Au/Co3O4 catalyst was examined by simple
decantation. While good recyclability was observed in the case
of hydroformylation,6 the catalyst activity was completely lost
in the first recycle run. Izawa assumed the real active species
of Co2(CO)8-catalyzed hydroformylation and amidocarbonyla-
tion were HCo(CO)3 and strong acidic HCo(CO)4, respectively.9
Au/Co3O4-catalyzed amidocarbonylation in the presence of 1.5
equiv of pyridine under optimized conditions gave only 19%
yield (cf. Table 1, Entry 6). This result indicates base-sensitive
active species are also involved in the case of Au/Co3O4-cata-
lyzed amidocarbonylation, and even weak basicity of amides
could affect the recycle of Au/Co3O4.
Entry
R1
R2
R3
Yieldb/%
1
2
3
4
5
6
7c
8d
Et
i-Pr
CH3
CH3
CH3
CH3
CH3
Ph
H
H
H
H
H
H
Et
H
53
64
Heptyl
Nonyl
c-Hexyl
Pentyl
Pentyl
Ph
81 (68)
quant. (65)
79 (51)
28 (16)
32 (18)
0e
CH3
CH3
aAll reactions were carried out in the presence of Au/
Co3O4 (5 atom % Au/(Au+Co) and 5.7 atom % Co/sub-
TEM images of the catalyst before being used for amidocar-
bonylation are shown in Supporting Information.10 It was clearly
observed gold nanoparticles (ca. 2–4 nm diameter) were depos-
ited on cobalt oxide surface.
strate. bGC yield. Isolated yield in parenthesis. 3 equiv
c
of amide was used. dCO/H2 ratio was 1:1 and temperature
was 100 ꢁC. 65% of N-benzylacetamide was found.
e
In this paper, application of Au/Co3O4 catalyst for amido-
carbonylation reactions was described. This catalyst system ef-
fectively provides the product under milder reaction conditions
with respect to pressure and temperature than that of classical
Co2(CO)8-catalyzed reactions. Studies on detailed reaction
mechanisms are underway.
alkyl chain lengths, generally higher aldehydes gave better
yields (Entries 1–4). Low reactivity of isobutyraldehyde was
not considered owing to steric hindrance of branched chain since
cyclohexane carboaldehyde gave acceptable yield (Entry 5).
Benzamide shows poor reactivity for this reaction (Entry 6). In-
creasing syngas pressure to 6 MPa did not improve the yield
(25%). Secondary amide can be used for the reaction though
the efficiency was slightly unsatisfactory (Entry 7). According
to previous reports, aldehyde substrates lacking ꢀ-hydrogen
are not suitable for Co2(CO)8-catalyzed amidocarbonylation.8
It should be noted that treatment of benzaldehyde with acet-
amide in the presence of Au/Co3O4 catalyst gave N-benzylacet-
amide in 65% yield instead of desired ꢀ-amino acid (Entry 11).
This result indicated that reductive amidation took precedence
over amidocarbonylation and resembled the case of Co2(CO)8-
catalyzed reaction.
Several compounds such as olefins and acetals are known to
give aldehyde functionality under amidocarbonylation condi-
tions and formed aldehydes are spontaneously transformed into
ꢀ-amino acids.9 Treatment of 1-pentene with 1.5 equiv of acet-
amide under Au/Co3O4-catalyzed amidocarbonylation condi-
tions (CO/H2 = 3:1, 4 MPa, 80 ꢁC, 20 h) produced 2-acetami-
doheptanoic acid in 19% yield along with a small amount of
2-acetamido-3-methylhexanoic acid (eq 1). Similarly, treatment
of acetal or phenylacetaldehyde dimethylacetal with benzamide
gave 21% N-benzoylalanine ethyl ester (eq 2) or 27% N-ben-
zoylphenylalanine methyl ester respectively. Although some
modifications of the reaction conditions are required to improve
reaction efficiency, Au/Co3O4 also catalyzed one-pot transfor-
mation of aldehydes equivalents.
This work is supported by Grant-in-Aid for Global-COE
program, ‘‘Science for Future Molecular Systems’’ from the
MEXT (Japan). We appreciate Professor Katsuki and his group
for permission to use NMR equipment.
Dedicated to Professor Ryoji Noyori on the occasion of his
70th birthday.
References and Notes
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Sharma, J. L. Arias, J. L. Velasco, J. Perez-Flores, R. M. Gomez,
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´
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C. G. Velazquez, J. Mol. Catal. A: Chem. 2007, 274, 65.
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Au/Co3O4
CO/H2 (3:1, 4 MPa)
CO2H
8
9
AcNH2 (1.5 equiv)
ð1Þ
EtOAc (substrate: 0.2 M)
NHAc
10 Supporting Information is available electronically on the CSJ-
Journal Web site, http://www.csj.jp/journals/chem-lett/index.html.
3
80 °C, 20 h
19%
Published on the web (Advance View) December 5, 2008; doi:10.1246/cl.2008.1292