AgOAc-catalyzed asymmetric amination of glycine Schiff bases with azodicarboxylates

Article abstract of DOI:10.1016/j.tetlet.2009.09.124

Asymmetric amination of glycine Schiff bases with azodicarboxylates has been developed with high yields and up to 98% ee using AgOAc/Taniaphos complex as the catalyst. Crown Copyright

Full text of DOI:10.1016/j.tetlet.2009.09.124

Tetrahedron Letters 50 (2009) 6866–6868
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
AgOAc-catalyzed asymmetric amination of glycine Schiff bases
with azodicarboxylates
Qing-An Chen a, Wei Zeng a, Yong-Gui Zhou a,b,
*
a State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China
b State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Lu, Shanghai 200032, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Asymmetric amination of glycine Schiff bases with azodicarboxylates has been developed with high
yields and up to 98% ee using AgOAc/Taniaphos complex as the catalyst.
Received 12 August 2009
Revised 18 September 2009
Accepted 22 September 2009
Available online 25 September 2009
Crown Copyright Ó 2009 Published by Elsevier Ltd. All rights reserved.
Keywords:
Silver acetate
Asymmetric amination
Glycine Schiff bases
Azodicarboxylates
The stereoselective construction of the C–N bonds is an impor-
tant task in organic synthesis and catalysis. The direct catalytic
enantioselective amination has attracted much attention in this
rapidly growing area due to its simple experimental procedures
and easily available starting materials.1 In 1997, the first catalytic
nation of glycine Schiff bases with azodicarboxylates achieving
high yields and excellent enantioselectivities.
In our initial investigation, we found AgOAc/L1 system could
efficiently catalyze the amination of glycine Schiff base 2a and azodi-
carboxylate 1a with high activity and moderate enantioselectivity
(50% ee) in THF (Table 1, entry 1). In order to obtain high selectivity
of the product, the effect of ligands was evaluated (entries 1–10).
Taniaphos (L6) emerged as the best ligand (92% ee, entry 6) for this
asymmetric amination compared to other ferrocenyl-derived ligands
(entries 1–5). Some commercially available ligands had also been
explored in the enantioselective amination of 2a. However, no satis-
fied results were obtained (entries 7–10). The influence of solvents
was studied using L6 as the ligand, a better enantioselectivity was
obtainedintolueneat0 °C(entry13).Decreasingthereactiontemper-
ature dramatically deteriorated the catalytic activity, but slightly
improved the enantioselectivity (entry 14). Thus, the optimal condi-
tions for this asymmetric amination reaction are AgOAc/L6/toluene/
ꢀ25 °C.
asymmetric a-amination of carbonyl compounds using azodicarb-
oxylates as the nitrogen source was developed by Evans, who em-
ployed a chiral magnesium bis(sulfonamide) complex as the
catalyst.2 Subsequently, much progress has been achieved in the
asymmetric
b-keto esters,6
little attention has been paid to the enantioselective
of glycine Schiff bases to provide optically active -diamino car-
a a
-amination of aldehydes,3 ketones,4 -keto esters,5
a
-cyano esters,7 and other compounds.8 However,
-amination
a
a,a
bonyl compounds and their derivatives, which are of great syn-
thetic potential in the preparation of pharmaceuticals and
agrochemicals.
Recently, we have developed efficient bifunctional AgOAc-cata-
lyzed asymmetric cycloaddition and Mannich reaction with high
activities and excellent stereoselectivities, in which reactions the
enolization of carbonyl compound was promoted by acetate, and
extra base was not necessary for achieving high yields or good ste-
reoselectivities.9,10 In continuation of our research on bifunctional
AgOAc-catalyzed asymmetric reactions, we envisioned the possi-
bility of applying the same strategy to the electrophilic amination
of carbonyl compounds using azodicarboxylates as the Michael
acceptors. Herein, we described AgOAc-catalyzed asymmetric ami-
Having established the optimal conditions, the generality of the
AgOAc-catalyzed asymmetric amination reaction was investigated
(Table 2). This investigation revealed the enantioselectivity of this
reaction strongly depended on the steric property of both sub-
strates. The selectivity decreased (from 98% ee to 75% ee) with
increasing the steric hindrance of the substituents in glycine Schiff
bases 2 (entries 1–4). In contrast, increasing the steric bulkiness of
the substituents in azodicarboxylates 1 was good for improving the
enantioselectivity (entries 1, 5–7). The best stereoselectivities (98%
ee) were obtained in the asymmetric amination reaction of di-tert-
butyl azodicarboxylate 1a with benzophenone imine glycine
* Corresponding author. Tel./fax: +86 411 84379220.
E-mail address: ygzhou@dicp.ac.cn (Y.-G. Zhou).
0040-4039/$ - see front matter Crown Copyright Ó 2009 Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.09.124

Q.-A. Chen et al. / Tetrahedron Letters 50 (2009) 6866–6868
6867
Table 1
H
O
R1O2C
N
L
L
Optimization of the reaction conditions for AgOAc-catalyzed asymmetric amination
of glycine Schiff bases with azodicarboxylates
N
CO2R1
N
Ph2C
Ph
CO2R2
Ag
N
CO2R2
OAc
Ph
2
3
t-BuO2C
Ph
t-BuO2C
N
N
+
CO t-Bu
2
1a
2a
NHCO2t-Bu
CO2Me
AgOAc / L*
N
solvent
Ph
N
Ph2C NCH2CO2Me
3a
L
L
Ag
N
O
HOAc
L
L
Entrya
Ligand
Solvent
Time (h)
Yieldb (%)
eec (%)
Ag
Ph2C
O
CO2R2
CO2R1
1
2
3
4
5
6
7
8
9
10
11
12
13
14d
L1
L2
L3
L4
L5
L6
L7
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
Et2O
CH2Cl2
Toluene
Toluene
1.5
1.5
1.5
1.5
1.5
4
1.5
1.5
1.5
1.5
1.5
45
95
99
99
98
95
98
98
95
98
94
95
97
99
95
50
58
14
69
7
92
41
12
13
20
86
65
94
98
Ph2C N
R1O2C
N
CO2R2
N
A
B
R1O2C
N
CO2R1
N
(R)-SynPhos
(R,R)-Me-DuPhos
(S)-Mop
L6
L6
L6
L6
1
Scheme 1. Proposed mechanism for AgOAc-catalyzed asymmetric amination.
3.5
22
methyl ester or benzophenone imine glycine ethyl ester (entries 1
and 2).
A possible mechanism on AgOAc-catalyzed asymmetric amina-
tion is proposed. The first step in the asymmetric amination is the
deprotonation of glycine Schiff bases 2 promoted by acetate to
form reactive metal-bound azomethine ylide dipole A and acetic
acid (Scheme 1). Subsequently A undergoes enantioselective addi-
tion to azodicarboxylates 1 to produce intermediate B, which re-
acts with the above acetate acid to obtain the amination
products 3 and to regenerate the catalyst.
PPh2
PPh2
PPh2
O
N
PPh2
SPh
NMe2
NH2
Fe
Fe
Fe
Fe
L1
(S,Rp)-L2
(S,Rp)-L3
(S,Rp)-L4
(S,Sp)-
In conclusion, we have demonstrated an effective AgOAc-cata-
lyzed asymmetric amination of glycine Schiff bases with azodi-
carboxylates with high yields and with up to 98% ee, which
Me2N
O
O
Bn
P NEt2
N
PPh2
Ph2P
O
Fe PPh2
PPh2
(S,Sp)-L5
Fe
provided an easy access to optically active
a,a-diamino carbonyl
compounds from simple and easily available starting materials.
Further investigations on the assignment of the absolute config-
uration of the products and AgOAc-catalyzed other asymmetric
amination reactions are currently in progress, and related results
will be reported in due course.
L7
(S)-
Taniaphos-L6
a
Conditions: 2a (0.23 mmol), 1a (1.2 equiv), AgOAc (3 mol %), ligand (3.3 mol %),
concentration (0.12 M), 0 °C.
b
Isolated yields based on 2a.
Determined by HPLC.
At ꢀ25 °C.
c
d
Acknowledgments
Financial support from National Natural Science Foundation of
China (20672112 and 20621063) and Dalian Institute of Chemical
Physics (K2007F1), Chinese Academy of Sciences is acknowledged.
Table 2
AgOAc-catalyzed asymmetric amination of glycine Schiff bases with
Supplementary data
azodicarboxylates11,12
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2009.09.124.
AgOAc (3.0 mol%)
L6
H
N
R1O2C
N
N
CO2R1
1
2
R1O2C
Ph
(3.3 mol%)
N
CO2R1
+
-25 oC, toluene
References and notes
Ph2C NCH2CO2R2
Ph
N
CO2R2
3
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eec (%)
1
2
3
4
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7
t-Bu/Me
t-Bu/Et
t-Bu/4-BrC6H4CH2
t-Bu/t-Bu
i-Pr/Me
22
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51
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95 (3a)
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98 (3f)
98 (3g)
98
98
96
75
92
76
75
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in a dried Schlenk tube under a nitrogen atmosphere and toluene (2.0 mL) was
added. The mixture was stirred at room temperature for about 0.5 h. After it was
cooled to the indicated temperature, benzophenone imine glycine methyl ester
(0.23 mmol) was added followed by diethyl azodicarboxylate (0.276 mmol).
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analysis. Upon consumption of the limiting reagent, the pure adducts were
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12. Selected physical and spectral data for 3a: Colorless oil, 98% ee, ½a D28
ꢀ68.7 (c 1.92,
ꢁ
CH2Cl2); Rf 0.32 (hexane/EtOAc, 5/1); HPLC (Chiralpak AD-H column, hexane/i-
PrOH 90/10, 1.0 mL/min, detector: 254 nm, 30 °C) tR = 10.1 (major) and 11.0 min
(minor); 1H NMR (400 MHz, DMSO-d6, 60 °C): d 8.89 (br s, 1H), 7.62–7.60 (m, 2H),
7.52–7.44 (m, 4H), 7.39–7.35 (m, 2H), 7.17 (br s, 2H), 5.71 (br s, 1H), 3.61 (s, 3H),
1.38 (s, 9H), 1.33 (s, 9H); 13C NMR (100 MHz, DMSO-d6, 60 °C): d 170.9, 167.9,
154.8, 153.5, 138.7, 135.3, 130.8, 129.0, 128.7, 128.6, 127.9, 127.3, 80.7, 79.1, 74.2,
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