Chiral dinuclear phthalazine bridged bisoxazoline ligands: Synthesis and application in enantioselective Cu-catalyzed conjugate addition of ZnEt 2 to enones

Article abstract of DOI:10.1016/j.tetlet.2011.02.098

A new class of chiral dinuclear ligands with phthalazine bridged bisoxazoline scaffold was designed and prepared in convenient synthetic routes. ¹H NMR analysis showed that this class of ligands could coordinate with two metal ions, either same

Full text of DOI:10.1016/j.tetlet.2011.02.098

Tetrahedron Letters 52 (2011) 2375–2378
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Tetrahedron Letters
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Chiral dinuclear phthalazine bridged bisoxazoline ligands: synthesis and
application in enantioselective Cu-catalyzed conjugate addition of ZnEt2
to enones
⇑
Liang Zhang, Guoqiang Yang, Chaoren Shen, Sabah Arghib, Wanbin Zhang
School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A new class of chiral dinuclear ligands with phthalazine bridged bisoxazoline scaffold was designed and
prepared in convenient synthetic routes. 1H NMR analysis showed that this class of ligands could coor-
dinate with two metal ions, either same or different. These ligands afforded good to excellent yields
and enantioselectivities in Cu-catalyzed conjugate addition of ZnEt2 to enones.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 8 January 2011
Revised 15 February 2011
Accepted 22 February 2011
Available online 26 February 2011
Keywords:
Dinuclear ligand
Enantioselective catalysis
Copper
Conjugate addition
Enone
1. Introduction
element of the coordination environment and the metal–metal dis-
tance. In most of the reported chiral multi-nuclear catalysts, the
Enantioselective catalyses based on chiral metal-complexes
have been well studied by chemists over the past several decades
and have drawn much attention of synthesis chemists.1 Although
some of these transformations have been applied to industrial pro-
duction, most are still far from practical application. So, there are
strong incentives to achieve environmental friendly, high-economy
processes and much research is performed to develop better and
more selective catalysts. Until now, the most selective catalysts
are enzymes found in nature. It has been shown that a number
of active sites of enzymes contain two or more metal ions, that
operate cooperatively.2 Inspired by the study of multi-nuclear en-
zymes, chemists have designed and synthesized various multi-nu-
clear complexes to mimic the enzymes.3 In the area of
enantioselective catalysis, several research groups have paid their
attention to multi-nuclear chiral catalysts.4–8
bridge of ligands often contains the oxygen atom which coordi-
nates to two metal ions.4–6 Comparing with the oxygen-containing
bridge, N@N unit may be more suitable for coordinating with soft-
er Lewis acids, which have been used for a wide variety of catalytic
organometallic transformations. Therefore, N@N unit has been
chosen as the bridge in designing dinuclear catalysts in recent
years.9 However, there is no report of chiral dinuclear catalysts
containing N@N unit so far. Chiral dinuclear ligands with N@N unit
as the bridge are expected to form bimetal complexes with proper
metal–metal distance and offer good results in enantioselective
catalysis.
As part of our continued interest in designing novel dinuclear
chiral ligands,10 we herein present a new class of chiral dinuclear
ligands 1 (Fig. 1), in which the phthalazine is used as a bridge to
It has been concluded that the choice of the coordination envi-
ronment is very important for designing multi-nuclear catalysts
because it determines the nature of the metal ions (such as type
of metals, oxidation states, homo- or hetero-dinuclear) that can
be bound. Meanwhile, the metal–metal distance plays a crucial
role in the activity of the designed catalysts.3b The bridge which
links the metal ions in multi-nuclear catalysts is an important
Me
Me
Me
Me
O
O
N N
N N
O
O
N
N
N
N
R
R
Bisoxazoline
R
R
1
⇑
Corresponding author. Tel./fax: +86 21 54743265.
E-mail address: wanbin@sjtu.edu.cn (W. Zhang).
Figure 1. Ligand design concept.
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.02.098

2376
L. Zhang et al. / Tetrahedron Letters 52 (2011) 2375–2378
OH
R
O
O
O
a
b
c
R
Me
Me
Me
Me
Me
Me
N
Me
Cl
Cl
N
H
Me
Me
Me
N
N
O
O
2a~b
3a~b
N
N
R
R
1
e
d
Me
Me
CN
Me
Me
NC
a: R = Ph b: R = Pri
Cl
c: R = But d: R = Bn
N
N
N
N
1,4-DCPZ
4
Scheme 1. Preparation of chiral dinuclear ligands 1a–d. Reagents and conditions: (a) chiral amino alcohol, NEt3, CH2Cl2, rt; (b) MsCl, DMAP (cat.), NEt3, CH2Cl2, rt; (c) LDA,
THF, À78 °C, then 1,4-DCPZ, À78 °C to rt; (d) lithium 2-cyanopropan-2-ide, THF, À78 °C to rt; (e) chiral amino alcohol, ZnCl2 (cat.), PhCl, reflux.
connect two oxazoline units. When 1 was used as a ligand for the
enantioselective Cu-catalyzed conjugate addition of ZnEt2 to enon-
es, good to excellent yields and enantioselectivities were achieved.
The chiral dinuclear ligand 1 can be readily prepared in two
routes (Scheme 1). In one route, the oxazoline units were synthe-
sized firstly. The oxazoline units 3a and b could be obtained with
up to 74% yield in two steps by stirring isobutyryl chloride with
enantiomerically pure 2-amino alcohols and triethylamine in
dichloromethane, followed by reaction with methanesulfonyl chlo-
ride catalyzed by DMAP. The desired ligands 1a and b were ob-
tained with up to 62% yield by adding the lithiated oxazoline 3a
and b to 1,4-dichlorophthalazine(1,4-DCPZ). Although the prepara-
tion of ligand 1d in this route failed at the last step, the other route
is viable for ligands 1b–d by reacting 2-amino alcohols with dinitril
4 using ZnCl2 as catalyst. The dinitril 4 was prepared by treating
the lithiated isobutyronitrile with 1,4-DCPZ in 87% yield.
With the chiral dinuclear ligand 1 in hand, we examined its cat-
alytic ability in the Cu-catalyzed conjugate addition of dialkylzinc
to enones.11 Firstly, different copper salts were screened for the
monocopper or dicopper catalytic systems of ligand 1a with ben-
zylideneacetone (5a) as the substrate (Table 1). Among the exam-
ined copper salts, the catalytic activities of CuI salts were generally
better than those of CuII salts (entries 1–7 vs 8–12). It can be seen
that Cu(CH3CN)4BF4 showed the best catalytic activity and enanti-
oselectivity when the ratio of copper to 1a was 1:1 (entry 3). At the
same time, the catalysts generated from copper salts and 1a in a
ratio of 1.9:1 or 2.0:1 showed slightly less activity and enantiose-
lectivity than the corresponding monocopper catalysts (entry 3
vs 4 and 5, entry 8 vs 9, entry 10 vs 11). Less Cu(CH3CN)4BF4 than
one equivalent of ligand 1a gave slightly lower enantioselectivity,
while more Cu(CH3CN)4BF4 than 2 equiv of ligand 1a gave poor re-
sults (entries 6 and 7).
The reaction conditions survey revealed some interesting ef-
fects on enantioselectivity (Table 2). When the catalysis proceeded
in a sole solvent, CH2Cl2 was the optimal solvent for this transfor-
mation, affording the product in high yield and high enantioselec-
tivity (entries 1–5). Interestingly, the addition of Et2O improved
the enantioselectivity and the best ratio of Et2O to CH2Cl2 was
1:1 (entries 6–9). The ligand survey revealed that 1c containing
tert-butyl group at oxazoline ring with larger steric hindrance gave
better enantioselectivity but lower yield than 1a (entry 11). The
enantioselectivity decreased dramatically when 1b or 1d was used
(entries 10 and 12). When the conjugate addition took place at 0 to
À30 °C, the reaction temperature had a large effect on reaction rate
but no obvious effect on enantioselectivity (entries 7, 13, and 14).
Table 2
Effects of reaction conditions on the asymmetric 1,4-addition reaction of benzyli-
a
deneacetone with ZnEt2
O
O
L*
Cu(MeCN)4BF4,
ZnEt2
solvent, tempreture
Ph
Me
(2.0 eq.)
Ph
Me
5a
6a
Entry
Solvent
L⁄
T (°C)
t (h)
Yieldb (%)
eec (%)
1
2
3
4
5
6
7
8
9
CH2Cl2
ClCH2CH2Cl
DME
THF
Toluene
CH2Cl2:Et2O (1:0.5)
CH2Cl2:Et2O (1:1)
CH2Cl2:Et2O (1:1.5)
CH2Cl2:Et2O (1:2)
1a
1a
1a
1a
1a
1a
1a
1a
1a
0
0
0
0
0
0
0
0
0
8
8
8
8
8
8
8
8
8
>99
>99
43
79
72
70
62
77
81
83
82
81
Table 1
Effects of copper salts on the enantioselective 1,4-addition reaction of benzylidene-
acetone with ZnEt2
11
>99
>99
>99
>99
>99
O
O
1a
C
Cu salt,
ZnEt2
CH2Cl2, 0 o
Ph
Me
(2.0 eq.)
Ph
Me
5a
6a
a
10
11
12
CH2Cl2:Et2O (1:1)
CH2Cl2:Et2O (1:1)
CH2Cl2:Et2O (1:1)
1b
1c
1d
0
0
0
12
16
16
>99
86
92
60
86
46
Entry
Cu salts
mCu:m1a
t (h)
Yieldb (%)
eec (%)
1
2
3
4
5
6
7
8
Cu(MeCN)4ClO4
Cu(MeCN)4PF6
Cu(MeCN)4BF4
Cu(MeCN)4BF4
Cu(MeCN)4BF4
Cu(MeCN)4BF4
Cu(MeCN)4BF4
Cu(OTf)2
1.0: 1.0
1.0: 1.0
1.0: 1.0
1.9: 1.0
2.0: 1.0
0.8: 1.0
2.5: 1.0
1.0: 1.0
1.9: 1.0
1.0: 1.0
1.9: 1.0
1.0: 1.0
20
20
8
8
8
>99
>99
>99
>99
>99
>99
22
41
6
72
67
77
79
79
77
74
72
40
50
12
73
66
60
13
14
15
16
17d
18e
19f
CH2Cl2:Et2O (1:1)
CH2Cl2:Et2O (1:1)
CH2Cl2:Et2O (1:1)
CH2Cl2:Et2O (1:1)
1a
1a
1a
1a
À20
À30
À35
À40
À30
À30
À30
10
18
22
22
>99
>99
>99
85
84
85
82
77
8
8
CH2Cl2:Et2O (1:1)
CH2Cl2:Et2O (1:1)
CH2Cl2:Et2O (1:1)
1a
1a
1a
22
13
9
98
>99
>99
84
84
80
20
20
20
20
20
9
Cu(OTf)2
10
11
12
Cu(ClO4)2Á6H2O
Cu(ClO4)2Á6H2O
Cu(OAc)2Á2H2O
a
b
c
d
e
f
Catalyst loading is 2.0 mol %.
Determined by GC using standard curve method.
Determined by chiral GC.
Catalyst loading is 1.0 mol %.
Catalyst loading is 4.0 mol %.
25
a
b
c
Using 2.0 mol % 1a.
Determined by GC using standard curve method.
Determined by chiral GC.
Catalyst loading is 10 mol %.

L. Zhang et al. / Tetrahedron Letters 52 (2011) 2375–2378
2377
However, further lowering the reaction temperature would de-
H5
crease the enantioselectivity obviously (entries 15 and 16). This
behavior is similar to that of phosphoramidite ligands.11 Another
interesting behavior of our chiral dinuclear ligands for this reaction
is that higher catalyst loading more than 4.0 mol % led to slightly
lower enantioselectivity (entries 18 and 19). Based on the above
results, the optimal reaction conditions are using 2.0 mol % 1a as
a dinuclear ligand, 2.0 mol % Cu(CH3CN)4BF4 as a copper source,
the mixture of CH2Cl2 and Et2O in a ratio of 1:1 as a solvent and
taking the reaction at À30 °C.
H4
N N
O
O
H3
H2
N
H1
Ph
N
1a
+2Cu+ZnEt2
H1
Ph
H5
H2
H2
H4
H3
HEt
H3
H5
1a
H1
H4
+2Cu
H3
With the optimized reaction conditions identified, several other
H2
H4 H5
H1
a,b-unsaturated ketones were examined (Table 3). As the size of
1a
R2-group increased, the enantioselectivity increased (entries 1–
4). When the R2-group was isopropyl, the highest enantioselectiv-
ity of 95% ee was achieved. However, bulky R2-groups could slow
down the reaction (entries 4 and 5). Chalcone also worked well,
giving 87% ee (entry 6). For substrates possessing R1-groups with
different electron natures, the change of enantioselectivity was
not obvious (entries 7–11). And with the increasing electron-
donating effect of the R1-group, the yield decreased. R1-group at
different positions affected the enantioselectivity slightly (entries
11–13). For substrate 5n, of which the R1 was o-Me and R2 was
Et, the ee of product 6n was 89%, slightly higher than the result
of 5m possessing o-Me as R1 and Me as R2 (entry 14 vs 13).
Then, we studied the coordination behavior of ligand 1a with
Cu(CH3CN)4BF4 and ZnEt2 in CD2Cl2 by 1H NMR (Fig. 2). Firstly, it
can be seen that the free ligand could not coordinate with ZnEt2
(Fig. 2, 1a and 1a+ZnEt2). Secondly, when one equivalent of
Cu(CH3CN)4BF4 was added to ligand 1a, the splits of peaks H1–H5
and H at phenyl groups turned out to be broad (Fig. 2, 1a+Cu). This
behavior can be rationalized in two ways. One possibility is that
the Cu ion moved very quickly between the two coordination sites
of dinuclear ligand 1a (Fig. 3, 7a and 7b). Alternatively, oligomer or
supermolecular might formed by coordination of each CuI with two
ligands and each ligand with two CuI ions (Fig. 3 and 8). To testify
the two proposed explanations, we evaporated the solvent and
added CD2Cl2 to the complex. No H peak of MeCN was observed
in the 1H NMR determination, which supported the second expla-
nation. Thirdly, after addition of excessive ZnEt2 to the Cu-1a oligo-
mer 8, most of the single peaks split into broad double peaks (Fig. 2,
H3
H2
H1
H5
H5
H4
H4
HEt
1a+ZnEt2
H3
H1
H2
1a
+Cu
HEt
H4H5
8
H1
1a+Cu+ZnEt2
H2
H3
7
6
5
4
3
2
1
Figure 2. 1H NMR analysis of complexes of 1a/Cu/Zn.
N
N
N
N
O
O
O
O
N
N
N
N
CuLn
LnCu
7a
7b
N
N
L
N
N
L
L
Cu
Cu
Cu
Cu
L
N
N
O
O
N
N
N
N
N
N
N
N
N
N
Cu
Ln
Cu
Ln
n
8
9
Figure 3. Proposed complexes of 1a with CuI.
Table 3
Substrates scope of enantioselective 1,4-addition reaction catalyzed by optimized
chiral bimetal system
O
1a+Cu+ZnEt2, also see SI for enlarged spectrum), which might be
due to the forming of bimetal complex of 1a with Cu and Zn ions.
Fourthly, for the complex of Cu(CH3CN)4BF4 and 1a in a ratio of 2:1,
the 1H NMR results were very similar to those of Cu-1a oligomer 8
(Fig. 2, 1a+2Cu). But the broad single peaks moved to slightly lower
field comparing with that of 8. Meanwhile, obvious H peak of
MeCN was observed in the spectrum after evaporating and adding
CD2Cl2. Interestingly, after adding ZnEt2 to the above 1a+2Cu com-
plex, clear split peaks at lower field were observed (Fig. 2,
1a+2Cu+ZnEt2). All of these might show that monomer di-copper
complex of 1a formed (Fig. 3 and 9).
O
Cu(MeCN)4BF4/1a
ZnEt2
R2
(2.0mol%)
R2
R1
R1
(2.0eq.)
CH2Cl2/Et2O (1/1)
5a~5n
6a~6n
-30 °C, 18h
c
Entry
Substrate
R1
R2
Yielda (%)
eeb, (%)
1
2
3
4
5
6
7
8
5a
5b
5c
5d
5e
5f
5g
5h
5i
H
H
H
H
H
H
p-Cl
p-Br
Me
Et
>99
92
92
70
NR
98
99
99
99
49
73
95
78
65
85
91
90
95
––
87
86
85
75
80
80
77
86
89
Prn
Pri
But
Ph
Me
Me
Me
Me
Me
Me
Me
Et
Based on the reported mechanism11 and the above 1H NMR
analysis, we assume that our system undergoes a bimetallic cata-
lytic process. Our ligand has two equal coordination sites. One site
coordinated with CuI ion which activated the C@C double bond of
enone. The other site coordinated with another metal ion, ZnII or
CuI, acting as Lewis acid to catch the carbonyl group. In addition,
the reason why monocopper catalytic systems gave good results
than dicopper ones might be the stronger acidity of ZnII than that
of CuI (Table 1).
9
p-NO2
10
11
12
13
14
5j
p-OMe
p-Me
m-Me
o-Me
o-Me
5k
5l
5m
5n
Yields were determined by GC using standard curve method or 1H NMR using
MeOH as internal standard.
a
b
Enantioselectivities were determined by chiral GC or HPLC.
c
In conclusion, we have developed a new class of chiral dinuclear
ligands with phthalazine bridged bisoxazoline scaffold. The new
The absolute configurations of products were determined by comparing the
retention time in HPLC and optical rotation with data of previous reports.12

2378
L. Zhang et al. / Tetrahedron Letters 52 (2011) 2375–2378
ligands could be prepared easily in two or three steps. 1H NMR
analysis showed that this class of ligands was suitable for coordi-
nating with two metal ions, either same or different. In Cu-cata-
lyzed conjugate addition, the chiral dinuclear ligand 1a afforded
good to excellent catalytic activities and enantioselectivities.
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We thank Prof. Tsuneo Imamoto and Dr. Masashi Sugiya for
helpful discussion. This work was partially supported by the Na-
tional Natural Science Foundation of China (20772081) and Science
and Technology Commission of Shanghai Municipality
(09JC1407800), and Nippon Chemical Industrial Co. Ltd. Our thanks
also go to the Instrumental Analysis Center of Shanghai Jiao Tong
University.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.02.098. These data
include experimental procedures, characterization data of new
compounds and HPLC spectra of products of catalytic conjugate
addition.
References and notes
1. For reviews, see: (a) Jacobsen, E. N.; Pfaltz, A.; Yamamoto, H. Comprehensive
Asymmetric Catalysis; Springer: New York, 1999; (b) Ojima, I. Catalytic
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