Tert-BuOK-Catalyzed condensation of ethyl diazoacetate to aldehydes and palladium-catalyzed 1,2-hydrogen migration for the synthesis of β-ketoesters under solvent-free conditions

Article abstract of DOI:10.1039/c3ra41920g

A mild and convenient method for the condensation of ethyl diazoacetate (EDA) with aldehydes catalyzed by tert-BuOK under solvent-free conditions was developed. The corresponding α-diazo-β-hydroxy esters were further converted into β-ketoesters through palladium-catalyzed 1,2-hydrogen migration under neat conditions. The two-step transformation exemplifies a simple method for the efficient and green synthesis of β-ketoesters. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ra41920g

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tert-BuOK-Catalyzed condensation of ethyl
diazoacetate to aldehydes and palladium-catalyzed
Cite this: RSC Advances, 2013, 3, 12616
1
,2-hydrogen migration for the synthesis of
Received 19th April 2013,
Accepted 7th June 2013
b-ketoesters under solvent-free conditions
DOI: 10.1039/c3ra41920g
Shufeng Chen, Fang Yuan, Haiying Zhao and Baoguo Li*
www.rsc.org/advances
A mild and convenient method for the condensation of ethyl
diazoacetate (EDA) with aldehydes catalyzed by tert-BuOK under
solvent-free conditions was developed. The corresponding
a-diazo-b-hydroxy esters were further converted into b-ketoesters
through palladium-catalyzed 1,2-hydrogen migration under neat
conditions. The two-step transformation exemplifies a simple
method for the efficient and green synthesis of b-ketoesters.
palladium as the catalyst is unknown to the best of our knowledge.
Moreover, most of these reactions were performed in organic
solvents under anhydrous conditions. According to the demands
of green chemistry, further development of more efficient and
environmentally friendly methodologies in this area continues to
be desirable. Herein, we wish to report (1) the tert-BuOK-catalyzed
condensation of ethyl diazoacetate (EDA) with aldehydes under
solvent-free conditions, and (2) the palladium-catalyzed 1,2-
hydrogen migration of corresponding a-diazo-b-hydroxy esters
under neat conditions. This two-step transformation exemplifies a
simple method for the efficient and green synthesis of b-ketoesters
(Scheme 1).
Introduction
The development of environmentally benign and cost-efficient
protocols for the purpose of green, sustainable chemistry has been
1
in demand in both academic and industrial research. A generally
Results and discussion
desirable process is the catalytic transformation of neat reactants
with the least waste under mild reaction conditions with simplicity
and cost-efficiency. Moreover, because of the increasing public
concern regarding the harmful effects of organic solvents on the
environment and human body, solvent-free organic reactions have
been receiving more attention for many years due to their clean
and environmentally friendly protocols as well as convenient
A common method for the preparation of a-diazo-b-hydroxy esters
is the nucleophilic addition of EDA to carbonyl compounds, for
which deprotonation of EDA is needed. This is usually achieved by
treating the diazo compound with excess strong base, such as
butyllithium, lithium diisopropylamide and sodium hydride
8
under absolutely anhydrous conditions. Wang and his co-workers
2
have developed a mild DBU-catalyzed condensation of acyldiazo-
product purification. It is often claimed that ‘the best solvent is
methanes to aldehydes affording b-hydroxy a-diazo carbonyl
no solvent’ in modern organic chemistry.
a-Diazo-b-hydroxy esters are a class of versatile compounds
endowed with a wealth of potential synthetic applications. For
example, a variety of b-substituted a-diazo carbonyl compounds
can be synthesized from them just through simple nucleophilic
substitution, and can then undergo various reactions through
9
10
compounds in CH CN and also in water. However, the
3
3
efficiency of this transformation in water is not satisfactory,
because a long reaction time (24–168 h) is necessary in order to
complete the reactions. Here, we describe an efficient tert-BuOK-
catalyzed condensation of EDA with aldehydes under solvent-free
conditions.
4
transition metal-catalyzed diazo decomposition. Among those,
Firstly, we employed EDA 1 and benzaldehyde 2a as substrates
to investigate this solvent-free condensation reaction. The effec-
tiveness of various bases was carefully examined for this process.
1
,2-migration to an electrophilic carbon center is a common
5,6
5
transformation. It has been demonstrated that hydride, alkyl,
aryl, vinyl, acetylenyl and even thiol groups can act as 1,2-
migration groups under specific reaction conditions. Although
6
2 4 2 2 2 3
various Lewis acids including Rh (OAc) , ZnCl , ZnBr , SnCl , BF ,
GeCl , AlCl and SnCl4 have been reported to effect the 1,2-
2
3
3,7
migration transformation, the corresponding reaction using
College of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot
0
10021, China. E-mail: baoguol@sohu.com
Scheme 1 The synthesis of b-ketoesters under solvent-free conditions.
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Table 1 The effect of the base on the condensation reaction of EDA 1 with
Table 2 tert-BuOK-Catalyzed condensation reactions of EDA 1 with various
a
a
benzaldehyde 2a under solvent-free conditions
aldehydes 2a–q under solvent-free conditions
b
a
b
Entry
2, R
Reaction time (h)
3, Yield (%)
Entry
Base (mol%)
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
9
2a, Ph
2b, p-ClC
2
2
3a, 87
3b, 80
3c, 72
3d, 82
3e, 92
3f, 94
3g, 90
3h, 77
3i, 88
3j, 67
3k, 39
3l, 73
3m, 75
3n, 56
3o, 69
3p, 70
3q, 87
1
2
3
4
5
6
7
8
9
tert-BuOK (20)
KOH (20)
2
6
18
6
18
18
2
87
53
50
75
0
6
H
4
2c, p-BrC
2d, p-FC
2e, o-ClC
2f, o-FC
2g, m,p-Cl
2h, o-NO
2i, m-NO
6
H
4
0.5
0.5
0.5
0.5
0.5
2
K
2
CO
Cs CO
KOAc (20)
Et N (20)
3
(20)
(20)
6
H
4
H
4
H
4
2 6 3
C H
2
3
6
0
6
3
DBU (20)
tert-BuOK (10)
tert-BuOK (30)
60
70
84
2
C
C
6
H
H
4
2
2
2
6
H
4
0.5
2
10
2j, p-CH
2k, p-CH
3
C
6
4
H
a
Reactions were carried out with 0.6 mmol of 1, 0.5 mmol of 2a at
11
12
3
OC
6
4
1.5
0.5
0.5
0.5
0.5
0.5
0.5
b
room temperature. Yield of isolated pure products.
2l, 2-Pyridyl
2m, 2-Thienyl
2n, CH CH
2o, CH CH
2p, (CH CH
2q, PhCH CH
13
14
15
16
17
3
2
3
2
CH
2
3 2
)
2
2
The results are summarized in Table 1. tert-BuOK was found to be
the most suitable base for this transformation, giving the
corresponding a-diazo-b-hydroxy ester 3a in 87% yield after
reaction at room temperature for 2 h (Table 1, entry 1). Other
bases, such as KOH, K CO and Cs CO , could also afford the
a
Reactions were carried out with 0.6 mmol of 1 and 0.5 mmol of
b
aldehydes 2a–q at room temperature under neat conditions. Yield
of isolated pure products.
2
3
2
3
condensation product 3a, albeit with slightly lower yields (Table 1,
entries 2–4). However, when KOAc and Et N were used in this
3
1,2-hydrogen migration of a-diazo-b-hydroxy esters has been
reaction, no product was obtained (Table 1, entries 5 and 6). It was
noted that DBU was also suitable for this solvent-free reaction,
giving product 3a in 60% yield (Table 1, entry 7). Encouraged by
these results, we carefully examined the effect of the catalyst
loading on this solvent-free reaction (Table 1, entries 8 and 9). The
results indicate that 20 mol% of tert-BuOK gave the best yield of
the product under solvent-free conditions at ambient temperature.
To evaluate the scope of this new method, the optimized
conditions were then applied to the synthesis of a-diazo-b-hydroxy
esters by condensation of various aldehydes with EDA and without
any solvent, as shown in Table 2. Most aromatic aldehydes reacted
efficiently to afford the corresponding a-diazo-b-hydroxy esters in
moderate to good yields (Table 2, entries 1–13). As anticipated, the
reaction of an aromatic aldehyde bearing a strong electron-
withdrawing substituent, such as a m-nitro group, gave the
product 3i in 88% yield (Table 2, entry 9), while an aromatic
aldehyde with a strong electron-donating substituent, such as a
p-methoxy group, could only provide the product 3k in 39% yield
studied in detail to afford synthetically useful b-keto esters, there
are only two reports of this transformation using water as a green
11
medium. Moreover, to the best of our knowledge, metal-
catalyzed diazo decomposition of a-diazo-b-hydroxy esters under
solvent-free conditions has not been reported. Encouraged by
these findings and as a continuation of our research interest in
green chemistry, we herein report a palladium-catalyzed reaction
of a-diazo-b-hydroxy esters under neat conditions.
a
Table 3 Metal-catalyzed diazo decomposition under solvent-free conditions
b
Entry
Catalyst (mol%)
Reaction time
Yield (%)
(Table 2, entry 11). Moreover, this approach was also applicable to
heteroaromatic aldehydes to give the condensation products in
high yields (Table 2, entries 12 and 13). Importantly, aliphatic
aldehydes were also tolerated, although the yields were lower
compared to the reaction with aromatic aldehydes (Table 2, entries
1
2
3
4
5
6
7
Rh
CuSO
Pd(OAc)
PdCl (2)
2
(OAc)
4
(2)
20 min
2 h
0.5 h
2.5 h
0.5 h
0.5 h
5 h
88
65
67
79
67
81
96
4
(2)
2
(2)
2
Pd
2
(dba)
2
3
(2)
(PhCN)
(PPh
PdCl
PdCl
2
(2)
(2)
14–17).
2
3
)
2
Having prepared a variety of a-diazo-b-hydroxy esters, we
conceived of this solvent-free method being applied to metal-
catalyzed diazo decomposition reactions. Although metal-catalyzed
a
Reactions were carried out with 0.5 mmol of 3a at room
b
temperature under neat conditions. Yield of isolated pure
products.
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Ethyl 2-diazo-3-hydroxy-3-phenylpropanoate 3a was first chosen
as the substrate to investigate the diazo decomposition reaction
under solvent-free conditions (Table 3). When we treated 3a in the
2 4
presence of Rh (OAc) at room temperature without any solvent,
the reaction was completed within 20 min. As expected, 1,2-
hydrogen migration product 4a was isolated in 88% yield
accompanied with a trace of the 1,2-phenyl migration product
Scheme 2 Two-step one-pot reaction under solvent-free conditions.
(Table 3, entry 1). Next, other metal salts were carefully tested in
this solvent-free reaction to find a cheaper and more easily
available catalyst. It was found that CuSO was also suitable for
this transformation, giving the b-keto ester 4a in moderate yield
4
such as b-(2-nitrophenyl) and b-(3-nitrophenyl) substituted
a-diazo-b-hydroxy esters (Table 4, entries 8 and 9). Moreover, this
method worked well even with b-2-pyridyl and b-2-thienyl
substituted substrates, giving the desired products in high yields
2
(Table 3, entry 2). Interestingly, when Pd(OAc) was used as the
catalyst in this reaction, the 1,2-hydrogen migration product 4a
was isolated with 67% yield (Table 3, entry 3). A further screening
of palladium catalysts revealed that other rare earth metal salts,
(Table 4, entries 12 and 13). Surprisingly, when b-ethyl substituted
substrate 3n was subjected to this reaction, only a trace of product
was formed (Table 4, entry 14), while other b-aliphatic substituted
substrates reacted smoothly to produce b-keto esters in moderate
yields (Table 4, entries 15 and 16).
Finally, the transformation of the two-step reaction to a direct
one-pot conversion of aldehydes to b-keto esters was explored.
Thus, aldehydes 2a and 2k were treated with EDA in the presence
of 20 mol% tert-BuOK at room temperature. After completion of
such as PdCl
giving the desired product in 79, 67, and 81% yield, respectively
Table 3, entries 4-6). However, the most effective catalyst turned
out to be PdCl (PPh (2 mol%), furnishing the product in 96%
yield (Table 3, entry 7).
2 2 3 2 2
, Pd (dba) , and PdCl (PhCN) , behaved similarly,
(
2
3 2
)
Subsequently, we examined the scope of this palladium-
catalyzed diazo decomposition reaction of various a-diazo-
b-hydroxy esters under solvent-free conditions, as shown in
Table 4. It was found that substrates with a phenyl ring bearing
a halogen atom generally gave excellent yields of the b-keto ester
products, regardless of the position of the substituent on the
phenyl ring (Table 4, entries 1–7). However, it should be noted that
the above reaction conditions did not work for solid substrates
2 3 2
the condensation, 2 mol% PdCl (PPh ) was added directly to the
resulting mixture and stirred for several hours. After the workup
procedure, b-keto esters 4a and 4k were isolated in 60% and 37%
overall yields, respectively (Scheme 2).
On the basis of the results presented above and the previous
understanding of this type of transition-metal-catalyzed diazo
5,6
decomposition reaction, we conceived that this Pd-catalyzed 1,2-
hydrogen migration reaction may proceed via a Pd carbene
intermediate, as shown in Scheme 3. It has been noted that the
long reaction time of the 1,2-hydrogen migration using a
palladium catalyst can be attributed to the electron-donating
Table 4 Palladium-catalyzed diazo decomposition reaction of various a-diazo-
a
b-hydroxy esters under solvent-free conditions
2 3 2
effect of triphenylphosphine in PdCl (PPh ) , which decelerates
the formation progress of the Pd carbene intermediate.
Conclusions
b
In summary, we have developed a mild and convenient method
for the condensation of EDA with aldehydes catalyzed by tert-
BuOK under solvent-free conditions. The corresponding a-diazo-
b-hydroxy esters were further converted into b-ketoesters through
palladium-catalyzed 1,2-hydrogen migration under neat condi-
Entry
3, R
Reaction time (h)
4, Yield (%)
1
2
3
4
5
6
7
3a, Ph
3b, p-ClC
5
3
5
5
5
5
11
10
10
2
2
3
2
9
8
9
4a, 96
4b, 93
4c, 95
4d, 93
4e, 93
4f, 95
4g, 95
4h, 0
6
H
4
3c, p-BrC
3d, p-FC
3e, o-ClC
3f, o-FC
3g, m,p-Cl
3h, o-NO
3i, m-NO
3j, p-CH
3k, p-CH
3l, 2-Pyridyl
3m, 2-Thienyl
3n, CH CH
3o, CH CH
3p, (CH CH
6
H
4
6
H
4
6
H
4
6
H
2
4
C
6
H
4
3
c
8
2
C
C
6
H
H
c
9
2
6
H
4
4i, 0
10
11
12
13
14
15
16
3
C
6
4
4j, 90
4k, 85
4l, 78
4m, 95
4n, trace
4o, 64
4p, 60
3
OC
6
H
4
3
2
3
2
CH
2
3
)
2
a
Reactions were carried out with 1 equiv. of a-diazo-b-hydroxy esters
a–p and 0.02 equiv. of PdCl (PPh at room temperature under
3
2
3 2
)
b
c
neat conditions. Yield of isolated pure products. The reactant and
catalyst could not be mixed effectively.
Scheme 3 Possible reaction pathway of the Pd-catalyzed 1,2-hydrogen migration
reaction.
1
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tions. The two-step transformation exemplifies a simple method
4140; (k) M. S. Singh and S. Chowdhury, RSC Adv., 2012, 2,
4547.
for the efficient and green synthesis of b-ketoesters.
3
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Experimental
General procedure for the tert-BuOK-catalyzed condensation
reaction of EDA with aldehydes
To a 10 mL flask with a stir bar were added aldehyde 2 (0.5 mmol),
EDA 1 (0.6 mmol) and tert-BuOK (0.1 mmol), and the mixture was
stirred vigorously at room temperature. The progress of the
reaction was monitored by TLC. After completion of the reaction,
the resulting mixture was separated directly by flash column
chromatography over silica gel to afford corresponding a-diazo-
b-hydroxy ester 3.
3
773; (e) Z. Zhang and J. Wang, Tetrahedron, 2008, 64, 6577.
General procedure for the palladium-catalyzed diazo
decomposition reaction of a-diazo-b-hydroxy esters under
solvent-free conditions
5
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3
2 3 2
(0.5 mmol) and PdCl (PPh ) (0.01 mmol), and the mixture was
stirred at room temperature. The progress of the reaction was
monitored by TLC. After completion of the reaction, the resulting
mixture was isolated by flash column chromatography on silica gel
to give corresponding b-keto ester 4.
1
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All of these a-diazo-b-hydroxy esters 3 and b-keto esters 4 are
known compounds and were easily identified by comparison of
1
10–12
their spectroscopic data with those previously reported.
5
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Acknowledgements
The project is generously supported by National Natural
Science Foundation of China (Nos. 20902044 and 21102068)
and the Program for Young Talents of Science and Technology
in Universities of Inner Mongolia Autonomous Region (NJYT-
12-B03).
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