Palladium-catalyzed C-N bond activation: The synthesis of β-amino acid derivatives from triethylamine and acrylates

Article abstract of DOI:10.1002/ejoc.200700369

In the palladium-catalyzed reaction of acrylates, C-N bond activation and acetalization occurred under different conditions. Described herein is a highly efficient palladium-catalyzed C-N bond activation reaction and subsequent new C-N bond formation to directly construct β-amino acids from triethylamine and acrylate esters in isolated yields of up to 95%. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejoc.200700369

FULL PAPER
DOI: 10.1002/ejoc.200700369
Palladium-Catalyzed C–N Bond Activation: The Synthesis of β-Amino Acid
Derivatives from Triethylamine and Acrylates
[a]
[a]
[b]
Bo Zou, Huan-Feng Jiang,* and Zhao-Yang Wang
Keywords: Palladium / C–N bond activation / Amino acids
In the palladium-catalyzed reaction of acrylates, C–N bond
activation and acetalization occurred under different condi-
tions. Described herein is a highly efficient palladium-cata-
lyzed C–N bond activation reaction and subsequent new C–
N bond formation to directly construct β-amino acids from
triethylamine and acrylate esters in isolated yields of up to
95%.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
Introduction
Palladium-catalyzed processes have proven to be a
powerful and useful tool for the synthesis of nitrogen-con-
[
1]
taining organic molecules in synthetic organic chemistry.
Palladium is able to effect an extraordinary number of very
different reactions, including C–H or C–C activation reac-
tions in the presence of a nitrogen-containing functional
groups. Recent work has focused on Pd-catalyzed C–N
(
1)
[
2]
bond formation; however, considerably less attention has
been devoted to the Pd-catalyzed C–N bond activation of
[
3]
tile building blocks in synthetic organic chemistry. In recent
years, the development of β-amino acid derivatives has pro-
vided new protocols. Shen discovered that a kind of β-
amino acid with pyrazole showed good vitro antiviral ac-
tivity against HIV-1. Wani and Tarlor isolated methyl
(2R,3S)-N-benzoyl-3-phehylisoserine, which displays sig-
nificant cytotoxicity. Several important methods have
been developed for the synthesis of β-amino acid deriva-
tives. Herein we report a new type of synthesis for β-
amino acid derivatives through Pd-catalyzed C–N bond ac-
tivation of trialkylamine and subsequent addition to acryl-
ate esters under mild conditions.
alkylamines, which might provide an alternative to tradi-
tional functional group organic chemistry. In previous
[
4]
work, triethylamine was employed as the base in the Pd-
catalyzed acetalization reaction of methyl acrylate with
methanol. If the amount of molecular oxygen is not suf-
[
6]
[
4d]
ficient,
the cleavage of the C–N bond in ethylamine oc-
[
7]
curs and a small amount of methyl 3-diethylaminoprop-
ionate, a β-amino acid ester, can be detected in the reaction
mixture [Equation (1)]. With the aim to seek novel and
highly efficient synthetic approaches, a new route to con-
struct β-amino acid esters through C–N bond cleavage of
trialkylamines and subsequent new C–N bond formation is
worthy of study.
[
8]
The synthesis of β-amino acids and their derivatives is of
great interest in industrial and academic research for medic-
inal and organic synthetic chemists, because β-amino acid
derivatives have the potential to be biologically active and
have medicinal value;[ they can also serve as highly versa-
Results and Discussion
5]
PdCl Catalysis in Supercritical Carbon Dioxide
2
It is well-known that supercritical carbon dioxide
[
a] The College of Chemistry, South China University of Technol-
ogy,
(scCO ) is an important and attractive organic reaction me-
2
Guangzhou, 510640 P. R. China
[9–10]
dia in green chemistry research.
perimental operation in scCO
According to the ex-
Fax: +86-020-87112906
E-mail: jianghf@scut.edu.cn
[
4]
,
when triethylamine (1a)
2
[b] Department of Chemistry, South China Normal University,
Guangzhou, 510006 P. R. China
and methyl acrylate (2a) were induced in the palladium
chloride catalytic system in the absence of O , methyl 3-
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
2
diethylaminopropionate (3a) was obtained as the sole prod-
4600
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Eur. J. Org. Chem. 2007, 4600–4604

Pd-Catalyzed C–N Bond Activation: Synthesis of β-Amino Acid Derivatives
uct [Equation (2)]. This result indicated that β-amino acids dropped to 26% (Table 1, Entry 7). If water was used as the
could be directly constructed from triethylamine and acryl- solute modifier, the reaction does not occur. So MeOH was
ate esters.
shown to be the best modifier to promote the reaction
Table 1, Entry 8).
(
PdCl Catalysis in Conventional Solvents
2
Various conventional solvents were investigated in this
reaction as shown in Table 2. With the use of the optimum
mole ratio of triethylamine/methyl acrylate/HOAc, the dif-
ferent organic solvents could be employed in most cases.
(
2)
It is necessary to add an appropriate dosage of HOAc to
accomplish the reaction; otherwise, the reaction exhausts
the stoichiometric dosage of PdCl . When the dosage of
HOAc is increased from 0.5 equiv. to 1.5 equiv. (based on
the amount of 2), the yield of 3a increased. However, when
the dosage of HOAc exceeded 1.5 equiv., the yield of 3a
decreased and N,N-diethylacetamide was detected. Thus,
the suitable dosage of HOAc was 1.0 equiv. on the basis of
the amount of substrate used.
DMF, THF and CH CN are better solvents than dioxane,
3
2
toluene, HMPA and NEt . In DMF, THF and CH CN, the
3
3
reaction could be carried out smoothly with 86, 89 and 90%
yields, respectively (Table 2, Entries 1–3). By using water as
the solvent, no methyl 3-diethylaminopropionate was de-
tected (Table 2, Entry 8).
The results of the PdCl -catalyzed reaction in a series
2
Table 2. PdCl -catalyzed reaction of triethylamine with methyl
2
[a]
of CO pressure is summarized in Table 1. As the reaction acrylate in conventional solvents.
2
pressure was increased from 9 to 20 MPa, the yield in-
creased from 80 to 95% (Table 1, Entries 1–4). Higher pres-
sure, such as 30 MPa, does not raise the yield any further
Entry
Solvent
Yield[b] [%]
1
2
3
4
5
6
7
8
DMF
THF
86
89
90
73
37
33
38
–
(
Table 1, Entry 5). The results indicate that: (1) scCO is a
2
CH CN
3
suitable solvent for the PdCl -catalyzed reaction of triethyl-
Dioxane
Toluene
HMPA
2
amine with methyl acrylate and (2) the pressure could affect
the chemical equilibrium, the chemical reaction rate and the
NEt
3
[
11]
catalytic activity.
2
H O
[
a] Reaction conditions: molar ratio of triethylamine (1a)/methyl
Table 1. PdCl
acrylate in scCO
2
-catalyzed reaction of triethylamine with methyl
[
a]
acrylate (2a)/HOAc, 2.0:1.0:1.0, 3 mol-% PdCl , 48 h, 70 °C. [b]
Isolated yield.
.
2
2
Entry Reaction pressure [MPa]
Additive
Yield[b] [%]
1
2
3
4
5
6
7
8
9
MeOH
MeOH
MeOH
MeOH
MeOH
none
80
80
92
95
95
72
26
–
10
15
20
30
15
15
15
Different Olefins
Different functionalized olefins were subjected to the re-
action under the optimized conditions. Representative ex-
amples are tabulated in Table 3. Substrates ethyl acrylate
EtOH
H O
2
[
a] Reaction conditions: molar ratio of triethylamine/methyl acryl-
ate/HOAc, 2.0:1.0:1.0, 3 mol-% PdCl
yield.
2
, 48 h, 70 °C. [b] Isolated
Table 3. PdCl
2
-catalyzed reaction of triethylamine with different
olefins.[
a]
Most organometallic or transition metals are difficult to Entry
Substrate
Product
Solvent
Yield[c] [%]
[
12]
dissolve in scCO2.
To improve the solubility in scCO2,
[b]
1
2
3
4
5
6
7
8
9
1
1
1
2a
2b
2b
2b
2c
2c
2d
2d
2e
2e
2f
3a
3b
3b
3b
3c
3c
3d
3d
3e
3e
3f
scCO
scCO
2
92
84
95
56
75
69
82
20
53
59
–
some organic or inorganic compounds, so-called “modifi-
ers”, can be added to the solution.[ Among these ex-
tremely effective modifiers, MeOH is one of the most com-
mon. Our group used MeOH to promote the partial dissol-
[b]
2
13]
DMF
THF
[
b]
scCO
THF
2
ution of PdCl in scCO and developed a series of organic
[b]
2
2
scCO2
THF
reactions in scCO2.[
4,14]
Besides MeOH, other solute modi-
[b]
scCO
THF
2
fiers could be used to promote dissolution of PdCl2 in
0
1
2
scCO . Table 1 also gathers the reaction results in the pres-
2
[b]
[b]
scCO
scCO
2
ence of different kinds of solute modifiers. In the absence
of MeOH, reactions of triethylamine with terminal olefins
gave a moderate yield of 72% (Table 1, Entry 6). The ad-
dition of MeOH raised the reaction yield to 92% (Table 1,
2g
3g
2
–
[
2
a] Reaction conditions: molar ratio of triethylamine (1a)/substrate
/HOAc, 2.0:1.0:1.0, 3 mol-% PdCl , 48 h, 70 °C. [b] The total reac-
tion pressure was 15 MPa and 2 mL of methanol was added. [c]
2
Entry 3). when methanol was replaced by ethanol, the yield Isolated yield.
Eur. J. Org. Chem. 2007, 4600–4604
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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B. Zou, H.-F. Jiang, Z.-Y. Wang
FULL PAPER
(
2b) and n-butyl acrylate (2c) gave the corresponding β- either I–Pd–H and O or peroxidopalladium(II) and HI.
2
amino acid esters 3b and 3c, respectively, in moderate-to- Then, dealkylation of NEt occurred by the oxidation of
3
excellent yields in scCO , DMF and THF (Table 3, En- the I–Pd–OOH species. The ethyl group that is removed
2
tries 2–6). Acrylonitrile (2d) and acrylamide (2e), the nitro- from NEt must be transformed into MeCOOH by Me-
3
gen-containing derivatives of acrylate, could also react with CHO. The NHEt that is formed further reacts with methyl
2
triethylamine to give 3-diethylaminopropionitrile (3d) and acrylate to give (diethylamino)propionate. It is notable that
3-diethylaminopropionamide (3e), respectively, with moder- if in the presence of molecular oxygen, the reaction gives
ate yields in different solvents (Table 3, Entries 7–10). (diethylamino)propionate. Once argon replaces molecular
Methyl but-2-enoate (2f), acrylate substituted in the 3-posi- oxygen, no (diethylamino)propionate is detected. Ziolkow-
tion, did not react with triethylamine (Table 3, Entry 11), ski proposed another way of C–N bond cleavage, which was
[
3b]
because of the steric repulsion of the methyl group on the induced by the water in the reaction system. The reaction
carbon–carbon double bond. Styrene (2g; Table 3, En- starts with the activation of the C–H bonds of the amine
try 12) did not react with triethylamine, because the elec- that are coordinated to palladium and the formation of the
tron-donating group of styrene cannot stabilize the carban- Pd–C bond. The next step consists of nucleophilic attack of
ion, and the protonolysis reaction cannot occur.[
15]
OH (from water) on the α carbon of the intermediate com-
plex followed by the rearrangement to the aldehyde and sec-
ondary amine. The proposed reaction pathway is close to
the mechanism of the Wacker process.
The Mechanistic Study
Neither Hosokawa’s mechanism nor Ziolkowski’s hy-
pothesis could totally reasonably explain the phenomena in
In our catalytic reactions, it is notable that the desired our experimental results. The existence of O or water is
2
processes proceeded in the presence of HOAc and in the unfavourable for the production of diethylaminopropionate.
absence of molecular oxygen. This revealed that HOAc and
To clarify the C–N bond-cleavage mechanism, we choose
molecular oxygen play important roles in the PdCl -cata- tri-n-octylamine (1b) as a replacement for triethylamine
2
lyzed C–N activation and then C–N bond formation se- [Equation (3)], and we hoped to capture the intermediates.
quence.
Fortunately, the reaction was carried out under standard
In the PdCl -catalyzed acetalization, the presence of conditions, and dioctylamine (4b), octylene (5b) and octyl-
2
HOAc is not necessary to accomplish the reaction. How- ene acetal (6) were all detected. The results hinted that: (1)
ever, the addition of HOAc is necessary in the PdCl -cata- in the absence of molecular oxygen and water, the C–N
2
lyzed reaction of triethylamine with acrylate; if not, the sub- bond cleavage still proceeds in palladium catalysis and (2)
II
strates will remain or the stoichiometric dosage of PdCl2 β-hydrogen elimination exists in the Pd -catalyzed C–N
will be exhausted. When 0.5–1.5 equiv. of HOAc was added bond-cleavage process.
to the reaction mixture, with the use of only a catalytic
amount of PdCl (3 mol-%), the reaction could be smoothly
2
carried out, and it is notable that no reaction occurred in
the absence of PdCl . The results indicate that: (1) a cata-
2
lytic amount of PdCl led to C–N bond cleavage and (2)
2
the role of HOAc might be to regenerate the active palla-
dium species through β-hydrogen elimination or pro-
[
15]
tonolysis, which led to completion of the reaction.
The oxidant is another key factor for the PdCl -catalyzed
2
(
3)
reaction. When the oxidant, such as CuCl , benzoquinone
2
or molecular oxygen, is part of the reaction mixture, the
reaction gave acetal as the sole product.[ When the oxidant
4]
The reaction pathway depicted in Scheme 1 might ac-
is absent, methyl 3-diethylaminopropionate was obtained as commodate all the above results. The C–H bond of triethyl-
the sole product. We also found that the mixture of acetal amine that is coordinated to palladium is initially activated,
and β-amino acid ester was detected when the amount of and the Pd–C bond can form. Then, C–N bond cleavage
[4d]
molecular oxygen was not enough.
Therefore, the pres- occurs to give intermediate B. Methyl (diethylamino)-
ence of the oxidant resulted in different reaction pathways. propionate (3a) might be obtained by route 1 or route 2.
According to the explanation of previous work, the mecha- From route 1, by the action of acid, intermediate B splits
II
II
nism of acetalization proceeds by a Pd -induced intramo- into the activated Pd species and diethylamine (4a). Com-
[
4c,16]
In the absence of pound 4a can further react with 2a to give 3a.[3b,17] From
an oxidant, Pd complexes abstract a hydrogen atom from route 2, the exchange of ethylene in intermediate B with 2a
the alkyl group of the amine. forms intermediate C. The insertion of the carbon–carbon
lecular oxypalladation of the alkene.
II
The cleavage of the C–N bond of NEt is of focus in double bond to the Pd–N bond affords intermediate D, and
3
the study of the mechanism. Hosokawa believed that the subsequently the C–Pd bond can be quenched by pro-
[
15]
II
oxidative cleavage of the C–N bond occurred by a I–Pd– tonolysis to afford 3a. The activated Pd species can then
OOH species.
[
3a]
The I–Pd–OOH species was derived from be regenerated and brought into a new catalytic cycle.
4602
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Eur. J. Org. Chem. 2007, 4600–4604

Pd-Catalyzed C–N Bond Activation: Synthesis of β-Amino Acid Derivatives
tification of products, some were purified by preparative TLC on
silica gel by using light petroleum ether/ethyl acetate as the eluent
1
before H NMR and IR spectroscopy).
Supporting Information (see footnote on the first page of this arti-
cle): Analytical and spectral data for compounds 3a–e.
Acknowledgments
The authors thank the National Natural Science Foundation of
China (Nos. 20332030, 20572027 and 20625205) for financial sup-
port of this work.
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[
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2
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© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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4603

B. Zou, H.-F. Jiang, Z.-Y. Wang
FULL PAPER
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Received: April 24, 2007
Published Online: July 19, 2007
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Eur. J. Org. Chem. 2007, 4600–4604