An exo- and enantioselective 1,3-dipolar cycloaddition of azomethine ylides with alkylidene malonates catalyzed by a N,O-ligand/Cu(OAc)2-derived chiral complex

Article abstract of DOI:10.1002/anie.201007960

An exo-lent catalyst: An N,O-ligand/Cu(OAc)₂ derived chiral complex is an excellent catalyst for inducing asymmetry in the catalytic enantioselective 1,3-dipolar cycloadditions of azomethine ylides with various alkylidene malonates. A series of

Full text of DOI:10.1002/anie.201007960

DOI: 10.1002/anie.201007960
Copper Catalysis
An exo- and Enantioselective 1,3-Dipolar Cycloaddition of
Azomethine Ylides with Alkylidene Malonates Catalyzed by a N,O-
Ligand/Cu(OAc)2-Derived Chiral Complex**
Ming Wang, Zheng Wang, Yu-Hua Shi, Xiao-Xin Shi, John S. Fossey, and Wei-Ping Deng*
Highly functionalized pyrrolidines are of great importance
with applications in the synthesis of biologically active
compounds,[1] natural products,[2] and organocatalysts.[3,4]
Catalytic asymmetric 1,3-dipolar cycloadditions of azome-
thine ylides with dipolarophiles, in principle, should provide
efficient access to these versatile skeletons.[4,5] Zhang and co-
workers first reported a successful example of this strategy in
the reaction of azomethine ylides with dimethyl maleate
catalyzed by AgOAc/FAP (FAP = bis-ferrocenyl amide phos-
phine).[6] Inspired by this achievement, many efforts have
been made towards the development of asymmetric 1,3-
dipolar cycloadditions of azomethine ylides with a variety of
electron-deficient alkenes as dipolarophiles using chiral
complexes of silver,[7] copper,[8] zinc,[9] nickel,[10] calcium,[11]
and organocatalysts.[12] In spite of the relatively broad scope in
available dipolarophiles such as maleates,[8c,13] fumarates,[8f,h]
maleimides,[7d–e,8e–f,10] acrylates,[8a,i,11,14] nitroalkenes,[8g,15] a-
enones,[7a,8b,16] b-phenylsulfonyl enones,[17] and vinyl sulfo-
nes,[8d,18] alkylidene malonates[19] have rarely been employed
as dipolarophiles in asymmetric 1,3-dipolar cycloadditions of
azomethine ylides. Recently, Wang and co-workers reported
the first asymmetric enantioselective 1,3-dipolar cycloaddi-
tion of azomethine ylides with alkylidene malonates catalyzed
by AgOAc/TF-BiphamPhos (TF-BiphamPhos = 4,4’,6,6’-tet-
rakis(trifluoromethyl)biphenyl-2,2’-diamine).[20] In catalytic
systems, a variety of b-alkyl/aryl alkylidene malonates and
iminoesters delivered exclusively exo adducts. Sterically
hindered tert-butylalkylidene malonates were found to be
the best substrates in terms of enantioselectivities, and the
highest enantioselectivity was obtained when a cyclohexane
carbaldehyde derived iminoester was used. The development
of more versatile and atom-economical variants of 1,3-dipolar
cycloadditions of azomethine ylides to alkylidene malonates
with excellent enantioselectivity is still a great challenge.
We reported asymmetric catalytic 1,4-Michael addition
reactions of glycine derivative 2 with alkylidene malonates 1
to afford the corresponding 1,4-anti adducts 3 as the major
products in excellent yields and high enantioselectivities
catalyzed by novel chiral N,O-4/5/Cu(OAc)2·H2O complex-
es,[21a] the ligands of which bear resemblance to the nucleo-
philic catalysts we also recently reported (Scheme 1).[21b]
Scheme 1. Asymmetric Michael addition reactions of glycine derivative
2 with alkylidene malonates 1 catalyzed by a chiral N,O-ligated copper
complex.
Encouraged by this finding, we envisaged that our newly
developed chiral N,O-ligated copper complexes may also be
applicable to the asymmetric catalytic 1,3-dipolar cycloaddi-
tion of azomethine ylides with alkylidene malonates. Herein,
we report an exo-selective and enantioselective 1,3-dipolar
cycloaddition of azomethine ylides with alkylidene malonates
catalyzed by a complex derived from N,O-5 and Cu-
(OAc)2·H2O to give highly functionalized pyrrolidines in
excellent yields and with good to excellent enantioselectiv-
ities (up to 99% ee).
We initially tested the chiral N,O-ligand 4 in the reaction
of iminoester 6a and alkylidene malonate 7a using 11 mol%
of 4 and 10 mol% of Cu(OAc)2·H2O in the presence of
10 mol% KOtBu in THF at room temperature (Table 1). The
reaction proceeded smoothly to afford the corresponding exo
adduct 8aa exclusively and in 79% yield with moderate
enantioselectivity (Table 1, entry 1, 62% ee). Screening metal
salts showed Cu(OAc)2·H2O to give optimal results both in
terms of yields and enantioselectivities of adduct 8aa
(Table 1, entries 2–6). Screening solvents revealed CH2Cl2 to
be optimal of those tried in terms of both yields and
enantioselectivities (Table 1, entry 11). Next, the effect of
[*] M. Wang, Z. Wang, Y.-H. Shi, Prof. X.-X. Shi, Dr. J. S. Fossey,
Prof. W.-P. Deng
School of Pharmacy, East China University of Science and
Technology
130 Meilong Road, Shanghai 200237 (China)
Fax: (+86)21-6425-2431
E-mail: weiping_deng@ecust.edu.cn
Dr. J. S. Fossey
School of Chemistry, University of Birmingham
Edgbaston, Birmingham, B15 2TT (UK)
[**] This work was supported by the Shanghai Committee of Science and
Technology (06J14023, 09JC1404500) and “111” Project (No.
B07023), and the Natural Science Foundation of China for young
foreign scientists (No. 21050110426).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201007960.
Angew. Chem. Int. Ed. 2011, 50, 4897 –4900
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4897

Communications
Table 1: Asymmetric 1,3-dipolar cycloaddition of azomethine ylide 6a with alkyli-
dene malonate 7a.[a]
aliphatic aldehydes were employed, exo/endo mix-
tures were obtained (Table 2, entries 17–19,
d.r. 85:15, 92:8, and 92:8, respectively). Interestingly,
the bulkier iBu and tBu groups gave enantiomeric
excesses of up to 97%, but the yield was compro-
mised for the latter (Table 2, entries 18 and 19).
Notably, the diastereoselectivity for 7g,h was dra-
matically increased to exclusively exo-selective by
replacing the methyl group of the iminoester (6a)
with a tBu group (6b), maintaining the same level of
enantioselectivity (Table 2, entries 20 and 21, 95%
and 96% ee, respectively). Moreover, when the
conjugated alkylidene malonate 7j was used for
this cycloaddition reaction, the enantiomeric
excesses of both exo- and endo adducts 8bj were
greater than 99%. Although moderate diastereose-
lectivity (Table 2, entry 22, exo/endo = 82:18) was
observed, the two diastereoisomers could be sepa-
rated chromatographically, demonstrating the great
benefit of this one-step process in the synthesis of
both diastereoisomers of these pyrrolidines with
excellent enantiomeric purities. Moreover, the
Entry
Metal
Base (%)
Solvent
Yield [%][d]
ee [%][e]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16[b]
17
18
19
20[c]
Cu(OAc)2
Cu(OTf)2
Zn(OTf)2
CuCl2
AgOAc
AgSbF6
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
KOtBu (10)
KOtBu (10)
KOtBu (10)
KOtBu (10)
KOtBu (10)
KOtBu (10)
KOtBu (10)
KOtBu (10)
KOtBu (10)
KOtBu (10)
KOtBu (10)
NaHMDS (10)
Cs2CO3 (20)
Et3N (20)
THF
THF
THF
THF
THF
THF
toluene
Et2O
CHCl3
CH3CN
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
79
16
trace
48
30
45
trace
87
18
84
83
85
48
62
92
89
trace
60
trace
93
62
9
–
10
4
14
–
64
16
44
67
62
67
74
75
71
–
K2CO3 (2 equiv)
K2CO3 (2 equiv)
KOAc (2 equiv)
Ag2O (2 equiv)
–
double-bond-containing adduct 8bj provides
a
potential site for further functionalization in the
chemical transformation of 8. The enantiomeric
excesses of this 1,3-dipolar cycloaddition represent,
on average, an over 10% ee enhancement over those
previously reported.[20]
The relative and absolute configurations of the
corresponding adducts were assigned as exo-
(2R,3R,5R) by comparison with literature data for
8aa (see the Supporting Information). The exclu-
sively exo selectivities and excellent enantioselectiv-
28
–
88
K2CO3 (2 equiv)
[a] The reactions were carried out with 0.2 mmol of 6a and 0.1 mmol of 7a in 1 mL
of solvent at room temperature, and Cu(OAc)2 presented in above table is
Cu(OAc)2·H2O. [b] Reaction proceeded in the absence of 4 ꢀ M.S. [c] Ligand 5 was
used instead of 4. [d] Yield of isolated product. [e] Determined by HPLC analysis on
a chiral phase.
different bases was probed; among the bases tested, two
equivalents of K2CO3 was found to be optimal giving the
corresponding adduct in 92% yield and 75% ee (Table 1,
entry 15). There were only trace amounts of desired product
formed in absence of base or when KOAc was used (Table 1,
entries 17 and 19). The ee value of the exo adduct 8aa
dramatically increased to 88% when ligand 5, bearing two
stereogenic centers on the imidazole ring, was employed.
The above optimization led to 11 mol% of N,O-ligand 5/
10 mol% of Cu(OAc)2·H2O/K2CO3 (2.0 equiv)/CH2Cl2/4 ꢀ
molecular sieve (M.S.)/room temperature as the optimal
reaction conditions for this 1,3-dipolar cycloaddition. The
generality and substrate scope were then probed. The effect
of variation in the ester moiety of alkylidene malonates
showed that the smallest group (methyl) gave a relatively high
enantioselectivity (Table 2, entry 3, 92% ee), which is not
consistent with previously published results, probably because
of the asymmetric environment derived from novel chiral
N,O-ligands.[20] Therefore, the methyl ester of alkylidene
malonates was chosen for further exploration of substrates.
As shown in Table 2, a wide array of alkylidene malonates
7a–f, derived from various aromatic aldehydes, reacted
smoothly with iminoesters 6a–h to afford the corresponding
highly substituted pyrrolidines 8 in excellent yields (80–99%)
and enantioselectivities (91–95% ee) with exclusive exo-
selectivity. When alkylidene malonates 7g–i derived from
ities observed in this novel 1,3-dipolar cycloaddition can be
rationalized from the proposed transition state I shown in
Scheme 2. That 1,3-dipolar cycloaddition favors an exo
Scheme 2. Proposed transition state leading to the major product exo-
8aa.
product is potentially due to possible steric repulsion between
the H1 atom of ligand 5 and the phenyl group of alkylidene
malonates, which would disfavor an endo mode. When the
phenyl group was replaced by an alkyl group, exo/endo
mixtures were formed as a result of reduction of the
aforementioned steric repulsion. Two phenyl groups adjacent
to the hydroxy group might block the dipolarophileꢁs
approach from the “bottom” face and form exo-(2R,3R,5R)-
8aa through approach from the “top” face.
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Angew. Chem. Int. Ed. 2011, 50, 4897 –4900

Table 2: Asymmetric 1,3-dipolar cycloaddition of azomethine ylides 6 with alkyli-
dene malonates 7 using 5/Cu(OAc)2·H2O.[a]
as a bifunctional catalytic system similar to bifunc-
tional AgOAc systems.[22]
To obtain direct evidence to support the pro-
posed catalytic cycle, various attempts were made to
grow crystals for X-ray crystal structure determina-
tion of N,O-ligand/Cu complexes. We obtained a
crystal of protonated ligand 4 (see the Supporting
Information), suitable for determining the X-ray
crystal structure, from an equimolar mixture of
ligand 4 and Cu(OTf)2 in CH2Cl2. Notably, treatment
of protonated ligand 4 with tBuOK regenerated
ligand 4. This reversible protonation process of 4 is
similar to the equilibrium that exists between
intermediates B and C, which provides ancillary
support for our proposed catalytic cycle (C!D!
E!C).
Entry
R1/R2
R3/R4
Yield [%]
ee [%][f]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
Ph/Me (6a)
Ph/Me (6a)
Ph/Me (6a)
Ph/tBu (6b)
p-ClPh/Me (6c)
p-ClPh/tBu (6d)
Ph/Me (6a)
Ph/Me (6a)
Ph/tBu (6b)
p-ClPh/Me (6c)
p-ClPh/tBu (6d)
p-ClPh/Me (6c)
m-ClPh/Me (6e)
p-MePh/Me (6 f)
m-MePh/Me (6g)
p-OMePh/Me (6h)
Ph/Me (6a)
Ph/Me (6a)
Ph/Me (6a)
Ph/tBu (6b)
Ph/tBu (6b)
Ph/Et (7a)
Ph/iPr (7b)
Ph/Me (7c)
Ph/Me (7c)
Ph/Me (7c)
Ph/Me (7c)
p-OMeC6H4/Me (7d)
2-furyl/Me (7e)
2-furyl/Me (7e)
2-furyl/Me (7e)
2-furyl/Me (7e)
p-BrC6H4/Me (7 f)
Ph/Me (7c)
Ph/Me (7c)
Ph/Me (7c)
Ph/Me (7c)
Et/Me (7g)
93
87
85
95
81
83
92
89
97
99
98
80
89
83
87
87
88
88
92
95
95(99)
94
95
94
93
94
92
93(>99)
91
94
92
94
96
97
In conclusion, highly efficient catalytic enantio-
selective 1,3-dipolar cycloadditions of azomethine
ylides 6 with various alkylidene malonates 7 were
developed. The new N,O-5/Cu(OAc)2-derived chiral
complex was demonstrated as an excellent catalyst
for inducing asymmetry in the synthesis of highly
functionalized pyrrolidines exo-8 as major products
(exo adducts for most of substrates) in excellent
yields (80–99%) and enantioselectivities (91–
99% ee). We believe that the structural novelty of
N,O-5, its potential behavior as bifunctional catalyst
upon coordination with Cu(OAc)2 and its excellent
potential to induce asymmetry in both Michael
additions and 1,3-dipolar cycloadditions will be of
interest not only for the field of asymmetric catalysis,
82[b]
85[c]
28[d]
81
iBu/Me (7h)
tBu/Me (7i)
iBu/Me (7h)
Et/Me (7g)
97
95
96
>99
92
84[e]
=
Ph/tBu (6b)
C(Me) CHEt/Me [(Z)-7j)
[a] The reactions were carried out with 0.2 mmol of 6 and 0.1 mmol of 7 in 1 mL of
CH2Cl2 at room temperature. [b] exo/endo ratio 85:15, by HPLC, ee value was
determined for the major product. [c] exo/endo ratio 92:8 by HPLC, ee value was
determined for the major product. [d] exo/endo ratio 92:8 by 1H NMR, ee value was
determined for the major product. [e] exo/endo ratio 82:18, by HPLC, ee value was
determined for both diastereoisomers of 8bj. [f] Data in parentheses was
determined after simple recrystallization.
It should be noted that a key feature for this
chiral complex is the proposed equilibrium between
intermediates B and C through electron transfer
from the quinoline N atom to Cu (Scheme 3). Thus,
coordination of iminoester 6a to the partly exposed
copper atom of intermediate C may be possible.
Through this coordination, transition state D could
be formed and then undergo a subsequent cyclo-
addition step with alkylidene malonates in the
presence of in situ formed KOAc. This assumption
was experimentally supported by the fact that only
trace amounts of the desired product were obtained
in the reaction of 6a and 7a in the absence of base
(Table 1, entry 17), which implies that complex A is
not the active form. Furthermore, the fact that the
cycloaddition was inactive when KOAc was used as
base (Table 1, entry 19) suggests that KOAc is not
basic enough to remove the proton of iminoester 6a
and that the OAc anion derived from the complex-
ation of copper complex B or C to iminoester 6a
may act as base for deprotonation to form transition
state D. Therefore, intermediates B or C can behave Scheme 3. The proposed catalytic cycle for the 1,3-dipolar cycloaddition.
Angew. Chem. Int. Ed. 2011, 50, 4897 –4900
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Communications
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Further investigations are ongoing in our laboratory.
Experimental Section
General procedure for an asymmetric 1,3-dipolar cycloaddition of
azomethine ylides with alkylidene malonates catalyzed by a N,O-
ligand/Cu(OAc)2 complex: Under N2 atmosphere, ligand 5(4.9 mg,
0.011 mmol), K2CO3 (27.6 mg, 0.2 mmol), Cu(OAc)2·H2O (2.0 mg,
0.01 mmol), and activated 4 ꢀ M.S. were dissolved in 1 mL dichloro-
methane, and stirred at room temperature for about 1 h. Then,
iminoesters 6 (0.2 mmol) and alkylidene malonates 7 (0.1 mmol) were
added sequentially. Once starting material was consumed (monitored
by TLC), the residue was purified by column chromatography to give
the corresponding cycloaddition products.
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Keywords: copper · cycloaddition · nitrogen heterocycles ·
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