Asymmetric henry reaction catalyzed by a chiral dinuclear nickel complex

Article abstract of DOI:10.1055/s-0033-1339112

Several chiral polyfunctional ligands were conveniently synthesized from l-amino acids and used to prepare the dinuclear complex in situ. A novel bimetallic catalyst containing dinuclear nickel was developed and applied to the asymmetric Henry reaction. With the assistance of N-methylmorpholine, good enantio selectivities (up to 91% ee) and moderate yields (up to 72%) were obtained for aryl, heteroaryl, and aliphatic aldehydes. The pathway was air tolerant and easily manipulated, and the reagents were readily available. Georg Thieme Verlag Stuttgart. New York.

Full text of DOI:10.1055/s-0033-1339112

LETTER
▌1735
lAettserymmetric Henry Reaction Catalyzed by a Chiral Dinuclear Nickel Complex
Asymmetric Henry Reaction Catalyzed by a Chiral Dinuclear Ni Complex
Yajun Liu,a Ping Deng,a Xiangyang Li,b Yan Xiong,c Hui Zhou*a
a
School of Pharmaceutical Science, Chongqing Key Laboratory of Biochemistry and Molecular Pharmacology, Chongqing Medical University,
Chongqing 400016, P. R. of China
Fax +86(23)68485161; E-mail: hzhou@cqmu.edu.cn
b
School of Pediatrics, Chongqing Medical University, Chongqing 400016, P. R. of China
c
School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, P. R. of China
Received: 22.03.2014; Accepted after revision: 18.04.2014
Ni(OAc)2·4H2O was used to prepare the dinuclear catalyst
Abstract: Several chiral polyfunctional ligands were conveniently
(Table 1, entries 1 and 2).
synthesized from L-amino acids and used to prepare the dinuclear
complex in situ. A novel bimetallic catalyst containing dinuclear
nickel was developed and applied to the asymmetric Henry reac-
tion. With the assistance of N-methylmorpholine, good enantio-
selectivities (up to 91% ee) and moderate yields (up to 72%) were
obtained for aryl, heteroaryl, and aliphatic aldehydes. The pathway
was air tolerant and easily manipulated, and the reagents were read-
ily available.
NH2
R1
NH OH HN
R1
NaBH4
EtOH
+
R1
CONHR2
R2
R2
N
H
O
O
N
H
O
OH
O
Key words: asymmetric catalysis, bimetallic catalyst, nickel, Hen-
ry reaction, nitroaldol reaction
L1 R1 = Ph, R2 = Ph
L2 R1 = i-Pr, R2 = Ph
L3 R1 = Ph, R2 = 2,6-diisopropylphenyl
L4 R1 = Ph, R2 = cyclohexyl
The catalytic asymmetric nitroaldol (Henry) reaction is a
useful carbon–carbon bond-forming reaction.[1] It affords
enantiomerically enriched β-hydroxynitroalkanes which
are key intermediates and building blocks.[2] Due to its
synthetic utility, increasing efforts have been directed to-
wards developing an efficient catalytic asymmetric Henry
reaction. Since the pioneering application of Shibasaki’s
heterometallic catalyst system[3] in the Henry reaction,
various types of catalyst systems have been studied con-
taining metal complexes[4–9] and organocatalysts.[10]
Among these systems, the multimetallic systems (such as
Shibasaki’s multimetallic complex,[11] Trost’s dinuclear
Scheme 1 Synthesis of ligands
Subsequently, the effect of ligands was examined. The
studies on the effect of the amide moiety (R2 substituent,
Scheme 1) demonstrated that the phenyl group was supe-
rior to the cyclohexyl group and the 2,6-diisopropylphe-
nyl group (Table 1, entries 1, 10, and 11). When the R1
substituent of the ligands (Scheme 1) varied from aryl
group to aliphatic group, the enantiomeric excess slightly
decreased.
In order to enhance the reactivity, a series of additives
were examined to optimize the catalyst system with the
results shown in Table 2. tert-Amine, pyridine, and p-tert-
butylphenol could obviously increase the reactivity but re-
duce the enantioselectivity (Table 2, entries 1–5). When a
more acidic one, p-nitrophenol, was added to the system,
the reactivity decreased (Table 2, entry 6). Based on the
reactivity and enantioselectivity, N-methylmorpholine
was selected as the additive for the next optimization.
zinc complex,[5a,12] Savoia’s dinuclear copper complex[13]
)
are very attractive for the high efficiency and easy modi-
fication. Herein, we report our preliminary results on the
development of a dinuclear nickel complex for the asym-
metric Henry reaction.
Chiral polyfunctional ligands L1–L4 were easily pre-
pared from L-amino acids (Scheme 1). We began our stud-
ies by examining the Henry reaction of benzaldehyde and
nitromethane in the presence of L1 and various metal salts
(Table 1, entries 1–8). The commonly used Zn(II), Cu(I),
and Cu(II) did not work well in this reaction (Table 1, en-
tries 5, 7, and 8). High activity was obtained when Mn(II)
was used as the central metal with low chiral induction
(Table 1, entries 3 and 4), while it was difficult for Mn(III)
to catalyze this reaction (Table 1, entry 6). Fortunately,
Ni(II) showed good inductive potential in this reaction
and 48% enantiomeric excess was achieved when
We continued to carry out the optimization of the reaction
conditions with the results given in Table 3. When the re-
action temperature was lowered to 0 °C, the enantioselec-
tivity was increased from 41% to 71% enantiomeric
excess (Table 3, entry 1). When the amount of nitrometh-
ane was decreased from 0.2 mL to 107 μL (10 equiv) and
the amount of N-methylmorpholine was increased from
ten mol% to ten equivalents, both of the reactivity and the
enantioselectivity were improved (Table 3, entry 2). Fur-
ther reducing the amount of nitromethane could increase
the enantioselectivity, while the activity decreased too
(Table 3, entries 3 and 4). We tried to further reduce the
reaction temperature to –10 °C, and 92% enantiomeric ex-
SYNLETT 2014, 25, 1735–1738
Advanced online publication: 05.06.2014
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0033-1339112; Art ID: st-2014-w0251-l
© Georg Thieme Verlag Stuttgart · New York

1736
Y. Liu et al.
LETTER
Table 1 The Screening of Ligands and Metal Saltsa
Table 3 Further Optimization of Reaction Conditionsa
OH
OH
ligand (10 mol%)
metal salt (20 mol%)
L1 (10 mol%)
Ni(OAc)2 (20 mol%)
NO2
NO2
MeNO2
PhCHO
+
MeNO2
PhCHO
+
r.t. THF, 43–48 h
THF
Entry
Ligand
Metal salts
Yield (%)b ee (%)c
Entry Temp The amount of Additive
(°C) MeNO2 (equiv) (equiv)
Reaction Yield ee
time (h) (%)b (%)c
1
2
L1
L1
L1
L1
L1
L1
L1
L1
L2
L3
L4
Ni(OAc)2·4H2O
Ni(acac)2
35
48
15
3
1
2
3
4
5
0
0
200 μL
NMMd (1)
89
45
66
36
71
85
89
40
107 μL (10)
54 μL (5)
21 μL (2)
107 μL (10)
NMMd (10) 96
NMMd (10) 92
NMMd (10) 92
NMMd (10) 92
3
Mn(acac)2
77
0
4
Mn(OAc)2·4H2O
Zn(OAc)2·2H2O
Mn(acac)3
26
3d
0
trace 88
32 92
5
20
9d
n.d.e
–
–10
6
trace
n.r.f
n.r.f
40
a Reactions were carried out on a 0.2 mmol scale (benzaldehyde) with
nitromethane in THF (0.8 mL for entry 1 and 1.0 mL for entries 2–5)
in the presence of L1 (10 mol%), Ni(OAc)2·4H2O (20 mol%), and the
specified additive.
7
CuOTf·0.5C6H6
Cu(OTf)2
8
–
b Isolated yield.
9
Ni(OAc)2·4H2O
Ni(OAc)2·4H2O
Ni(OAc)2·4H2O
43
21
6
c Enantiomeric excesses were determined by HPLC analysis on a
Chiralcel OD-H column.
10
11
50
d N-Methylmorpholine.
27
a Reactions were carried out on a 0.2 mmol scale (benzaldehyde) with
nitromethane (0.2 mL) in THF (0.8 mL) in the presence of a ligand
(10 mol%) and a metal salt (20 mol%) at r.t. for 43–48 h.
b Isolated yield.
cess was achieved, but only 32% isolated yield was ob-
tained (Table 3, entry 5). Currently, the optimized
reaction conditions were established: 10 mol% L1–
[Ni(OAc)2]2 complex, ten equivalents N-methylmorpho-
line, ten equivalents nitromethane, and 0.2 mmol alde-
hyde in 1.0 mL THF at 0 °C (Table 3, entry 2). It should
be noted that the process is air- and moisture-tolerant.
c Enantiomeric excesses were determined by HPLC analysis on a Chi-
ralcel OD-H column; the absolute configuration was established as R
by comparison with literature data.
d The absolute configuration of the major product was inverse com-
pared with the others by the analysis of HPLC on a Chiralcel OD-H
column.
With the optimized reaction conditions in hand, the sub-
strate scope was explored. The results were summarized
in Table 4. No matter aryl aldehydes with a meta substit-
uent or a para substituent or an electron-withdrawing sub-
stituent as well as heteroaryl aldehyde afforded the
corresponding product in moderate yields with good en-
antioselectivities (Table 4, entries 3–6). The bulkier alde-
hyde, such as 2-naphthaldehyde, also gave good yield and
enantioselectivity (Table 4, entry 2). It is worth mention-
ing that the aliphatic aldehyde (Table 4, entry 7) was suit-
able for this catalyst system, providing high
enantioselectivity (91% ee) which was higher than the
corresponding product of benzaldehyde.
e Not determined.
f No reaction.
Table 2 The Screening of Additivesa
OH
L1 (10 mol%)
Ni(OAc)2 (20 mol%)
NO2
MeNO2
PhCHO
+
THF, r.t. 43–48 h
Entry Additive
Yield (%)b
ee (%)c
1
2
3
4
5
6
Et3N (10 mol%)
pyridine (10 mol%)
64
57
53
48
47
20
34
38
40
41
29
24
In summary, several chiral polyfunctional ligands were
conveniently synthesized from L-amino acids and they
were used to prepare the dinuclear complex in situ. Final-
ly, a novel bimetallic catalyst containing dinuclear nickel
was developed and applied to the asymmetric Henry reac-
tion with good enantioselectivities (up to 91% ee) and
moderate yield (up to 72%). The pathway was air-tolerant
and easily manipulated, and the reagents were readily
available. Further investigations are under way in our lab-
oratory including the exploration of the dinuclear catalyst,
the detailed mechanism, and the extending of the substrate
scope.
i-Pr2NEt (10 mol%)
N-methylmorpholine (10 mol%)
p-tert-butylphenol (10 mol%)
p-nitrophenol (10 mol%)
a Reactions were carried out on a 0.2 mmol scale (benzaldehyde) with
nitromethane (0.2 mL) in THF (0.8 mL) in the presence of L1 (10
mol%), Ni(OAc)2·4H2O (20 mol%), and the specified additive at r.t.
for 43–48 h.
b Isolated yield.
c Enantiomeric excesses were determined by HPLC analysis on a Chi-
ralcel OD-H column.
Synlett 2014, 25, 1735–1738
© Georg Thieme Verlag Stuttgart · New York

LETTER
Asymmetric Henry Reaction Catalyzed by a Chiral Dinuclear Ni Complex
1737
References
Table 4 Enantioselective Nitroaldol Reactions between Aldehydes
and Nitromethanea
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+
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R
THF, 0 °C
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(h)
ee
(%)b
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1
2
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PhCHO
96
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65
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58
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63
85
84
82
72
84
85
91
2-naphthaldehyde
4-MeC6H4CHO
3-MeC6H4CHO
4-ClC6H4CHO
2-furaldehyde
hexanal
96
120
120
96
96
96
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The mixture of ligand L1 (11.7 mg, 0.02 mmol) and
Ni(OAc)2·4H2O (10.0 mg, 0.04 mmol) was stirred in THF (0.5 mL)
at 35 °C under air atmosphere for 1 h to generate the catalyst. The
mixture was analyzed by ESI-MS (pos.): m/z calcd for
C41H41N4Ni2O7 [ML1 – H + Ni2(OAc)2]+: 817.17; found: 817.26.
Typical Experimental Procedure for the Henry Reaction
The mixture of ligand L1 (11.7 mg, 0.02 mmol) and
Ni(OAc)2·4H2O (10.0 mg, 0.04 mmol) was stirred in THF (0.5 mL)
at 35 °C under air atmosphere for 1 h to generate the catalyst.
Ni(OAc)2 was dissolved in THF, and a green solution was obtained.
Then the nitromethane (107 μL) was added to the solution, and the
stirring was continued for 0.5 h. After that, the reaction mixture was
cooled to 0 °C followed by the addition of the benzaldehyde (0.2
mmol), N-methylmorpholine, and THF (0.5 mL). The stirring was
continued for 96 h at 0 °C. The products were isolated by using col-
umn chromatography on silica gel (EtOAc–PE = 1:8, v/v) as a col-
1
orless oil, 66% yield, 85% ee. H NMR (500 MHz, CDCl3): δ =
7.34–7.37 (m, 5 H), 5.38 (dd, 1 H, J = 9.60 Hz, 2.85), 4.52–4.57 (m,
20
1 H), 4.46–4.43 (m, 1 H), 3.24 (br s, 1 H). [α]D –44.8 (c 0.40,
CH2Cl2). HPLC (Chiralcel OD-H column, hexane–2-
propanol = 90:10, flow 1.0 mL/min, detection at 215 nm): tR (ma-
jor) = 14.4 min and tR (minor) = 17.9 min.
Acknowledgment
We appreciate the financial support from the National Natural Sci-
ence Foundation of China (No. 20902114 and 21372265), Funda-
mental and Advanced Research Projects of Chongqing City (No.
cstc2013jcyjA10144), and Scientific and Technological Research
Program of Chongqing Municipal Education Commission
(KJ120307).
Supporting Information for this article is available online
at
http://www.thieme-connect.com/products/ejournals/journal/
10.1055/s-00000083.SunpfgIpi
o
o
nr
i
© Georg Thieme Verlag Stuttgart · New York
Synlett 2014, 25, 1735–1738

1738
Y. Liu et al.
LETTER
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