Highly enantioselective conjugate addition of cyclic diketones to β,γ-unsaturated α-Ketoesters catalyzed by an N,N′-dioxide-Cu(OTf)2 complex

Article abstract of DOI:10.1002/chem.201002687

Copper-catalyzed conjugate addition: A highly enantioselective conjugate addition of cyclic diketones to β,γ-unsaturated α-ketoesters was achieved by using a two-carbon-linked N,N′-dioxide-Cu(OTf)₂ complex. Excellent yields (up to 99%) and enan

Full text of DOI:10.1002/chem.201002687

DOI: 10.1002/chem.201002687
Highly Enantioselective Conjugate Addition of Cyclic Diketones to
b,g-Unsaturated a-Ketoesters Catalyzed by an
N,N’-Dioxide-CuACHTNUGRTENUNG(OTf)2 Complex**
Zhenhua Dong, Juhua Feng, Xuan Fu, Xiaohua Liu, Lili Lin, and Xiaoming Feng*[a]
As one of the most powerful tools for the formation of
carbon–carbon bonds in organic synthesis, the catalytic
asymmetric Michael reaction has been the subject of inten-
sive development.[1] Among various types of Michael donors
and acceptors, the addition of cyclic 1,3-dicarbonyl com-
pounds to a,b-unsaturated carbonyl compounds has received
increased attention, since the addition products have prom-
ising biological and pharmaceutical activities.[2,3] Potentially,
the reaction could follow a cascade acetalization to con-
struct a six-membered oxygenated heterocycle; one of the
most common natural product frameworks, which exhibit an
extensive array of biological activities.[4] Compared with a,b-
unsaturated aldehydes and ketones, reactions with b,g-unsa-
turated a-ketoesters as Michael acceptors are limited.[5]
Since the pioneering work of Jørgensen and co-workers,[6a]
there have been several reports, for example, from the
groups of Xu and Zhao, of enantioselective conjugate addi-
tion reactions of cyclic 1,3-dicarbonyl compounds to b,g-un-
saturated a-ketoesters.[6c,d] However, the application of
cyclic diketones as nucleophiles is yet to be investigated.
Recently, Calter and Wang have described a highly asym-
metric conjugate addition reaction of cyclic diketones to b,g-
unsaturated a-ketoesters using a cinchona alkaloid deriva-
tive. They obtained excellent yields and enantioselectivites
and the product was easily transformed into a hexahydro-
quinoline.[6b] Considering the high synthetic versatility of the
products, the development of more efficient approaches for
the enantioselective conjugate addition of cyclic diketones
with b,g-unsaturated a-ketoesters remains challenging and
in high demand. Herein, we report a highly enantioselective
conjugate addition of cyclic diketones to b,g-unsaturated a-
ketoesters using an N,N’-dioxide/Cu(OTf)2 catalyst complex.
With a catalyst loading of 2 mol% excellent yields (up to
99%) and enantioselectivities (up to 99% ee) were obtained
for a wide range of substrates.
Chiral N,N’-dioxide metal complexes have shown power-
ful catalytic capability in various reactions owing to their
tuneable electronic and steric chiral scaffolds.[7,8] Initially, we
investigated the N,N’-dioxide ligand L1, derived from l-pro-
line, complexed with several metal salts to evaluate their
ability to promote the asymmetric addition of cyclohexane-
1,3-dione (2a) to b,g-unsaturated a-ketoester 1a in CH2Cl2
at room temperature. As shown in Table 1, the rare-metal
AHCTUNGTRENNUNG
complexes promoted the reaction smoothly in 12 h, however
the selectivity for the (S)-enantiomer was very poor
(Table 1, entries 1–3). Other conditions were investigated
using rare-metal complexes, however, no better results were
[a] Z. H. Dong, J. H. Feng, X. Fu, Dr. X. H. Liu, Dr. L. L. Lin,
Prof. Dr. X. M. Feng
Key Laboratory of Green Chemistry & Technology
Ministry of Education, College of Chemistry
Sichuan University, Chengdu 610064 (China)
Fax : (+86)28-8541-8249
obtained. To our delight, CuACHTNUTRGENNG(U OTf)2 gave the best results and
gave 3a for the (R)-enantiomer in 85% yield with 75% ee
1
after 12 h (Table 1, entry 4). The H and 13C NMR spectra
revealed that product 3a was obtained as an equilibrating
mixture of anomers and the cyclic compound was the major
product.[9] Encouraged by this result, other reaction parame-
ters were tested. Screening of solvent revealed that the
E-mail: xmfeng@scu.edu.cn
[**] OTf=trifluoromethanesulfonate.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201002687.
1118
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Chem. Eur. J. 2011, 17, 1118 – 1121

COMMUNICATION
Table 1. Optimization of enantioselective conjugate addition of cyclohex-
99% yield and with 99% ee (Table 1, entry 16). To our de-
light, the catalyst loading could be decreased from 10 to
2 mol% without decreasing the yield or enantioselectivity
(Table 1, entry 17). Notably, this process could also tolerate
air and moisture. The experimental procedure was very
simple, since neither ligand nor metal could promote the re-
action, therefore preparing the catalyst beforehand was un-
necessary (Table 1, entries 17–19). The experimental proce-
dure was as follows: the substrate, ligand, and metal were
weighed in a dry reaction tube followed by addition of
CH2Cl2 and the reaction mixture was stirred at room tem-
perature for 12 h.
Under the optimized reaction conditions (Table 1,
entry 17), the substrate scope for asymmetric conjugate ad-
dition of cyclic diketones to various b,g-unsaturated a-ke-
toesters was examined and the results are summarized in
Table 2. Substrate 1b with an ethyl group on the ester
moiety (R2) gave a 97% yield with 98% ee (Table 2,
entry 3). Additionally, dimedone (2b) is also a good nucleo-
phile for the reaction, and gave the product in 99% yield
with 97% ee (Table 2, entry 2). The substrates with electron-
withdrawing or donating groups at the meta-, para-, or
ortho-position of the aromatic ring were well tolerated in
terms of yield and enantioselectivity (up to 99% yield and
ane-1,3-dione (2a) to 1a.[a]
Entry
Ligand
Metal
La(OTf)3
Yb(OTf)3
(OTf)3
(OTf)2
(OTf)2
(OTf)2
(OTf)2
(OTf)2
(OTf)2
(OTf)2
(OTf)2
(OTf)2
(OTf)2
(OTf)2
(OTf)2
(OTf)2
(OTf)2
Solvent
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
L1
L1
L1
L1
L1
L1
L1
L1
L2
L3
L4
L5
L6
L7
L8
L9
L9
L9
–
G
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
PhMe
CHCl3
THF
Et2O
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
94
78
95
85
41
79
40
77
99
77
80
71
57
92
90
99
11(S)[e]
E
41(S)[e]
Y
N
31(S)[e]
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
–
N
75(R)[e]
R
7
E
7
41
13
67
49
27
0
T
R
9
R
10
11
12
13
14
15
16
17[d]
18[d]
19[d]
R
N
G
R
0
E
65
69
99
99(R)[e]
–
G
G
C
99
no reaction
no reaction
Cu
G
–
[a] Unless otherwise noted, the reactions were performed with 2a
(0.10 mmol), 1a (0.12 mmol), N,N’-dioxide (0.01 mmol), metal
(0.01 mmol) in solvent (1.0 mL) at room temperature for 12 h. [b] Yield
of the isolated product. [c] Determined by HPLC analysis. [d] 2 mol%
Table 2. Substrate scope for catalytic asymmetric conjugate addition of
catalyst loading (0.004 mmol L9 and 0.004 mmol CuACTHUNRGTNEUNG(OTf)2), 1a
cyclic diketones to b,g-unsaturated a-ketoesters.[a]
(0.2 mmol), 2a (0.2 mmol) in CH2Cl2 (1.0 mL) at room temperature for
12 h. [e] Entries 1–3 S, the others R, the absolute configuration was deter-
mined by comparison to literature data.[6c]
choice of solvent had a pronounced effect on reactivity and
enantioselectivity. Other organic solvents including toluene,
CHCl3, THF, and Et2O all showed highly deleterious effects
on reactivity and enantioselectivity (less than 41% ee,
Table 1, entries 5–8).
Entry[a]
R1, R2
Product
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
Ph, Me (1a)
Ph, Me (1a)
Ph, Et (1b)
3-MeC6H4, Me (1c)
4-MeC6H4, Me (1d)
3-MeOC6H4, Me (1e)
4-MeOC6H4, Me (1 f)
3a
99
99
97
99
93
99
91
99 (R)[e]
97 (R)[e]
98 (R)[e]
99
99
98
3a’[d]
3b
3c
Next we focused on the optimization of the reaction con-
ditions to improve the efficiency of CuACTHNUTRGENUG(N OTf)2 with other li-
3d
3e
3 f
gands. Various N,N’-dioxide ligands with different chiral
backbone moieties and amines were investigated. The steric
effect of the amide moiety played a crucial role in the enan-
tioselectivity of the reaction. Decreasing the steric hindrance
of the amide moiety led to a dramatic decrease in the enan-
tioselectivity (Table 1, entries 9–11).When L5 and L6 were
used (derived from aniline and benzylamine, respectively), a
racemic mixture was obtained (Table 1, entries 12 and 13,
respectively). The chiral backbone of the N,N’-dioxides also
had significant impact on the enantioselectivity. Neither L7
(derived from l-pipecolic acid) nor L8 (derived from l-ram-
ipril acid) gave better results than the l-proline-derived L1
(Table 1, entries 14 and 15). Surprisingly, remarkable im-
provement on activity and enantioselectivity was achieved
by shortening the linkage between the two chiral backbones.
Previously, we have found that the N,N’-dioxide ligand is op-
timized by incorporating a three-carbon linkage. However,
N,N’-dioxide L9 containing a two-carbon linkage gave 3a in
99
8
3g
96
99
9
4-PhC5H4, Me (1h)
2-ClC6H4, Me (1i)
3-ClC6H4, Me (1j)
4-ClC6H4, Me (1k)
2,4-Cl2C6H3, Me (1l)
3-BrC6H4, Me (1m)
4-BrC6H4, Me (1n)
4-FC6H4, Me (1o)
2-naphthyl, Me (1p)
2-thienyl, Me (1q)
PhCH=CH, Me (1r)
3h
3i
3j
3k
3l
3m
3n
3o
3p
3q
3r
95
96
99
90
73
93
87
99
99
92
67
99
91
99
99
93
99
98
98
99
98
95
10
11
12
13
14
15
16
17
18
19
[a] Unless otherwise noted, the reactions were performed with 2a
(0.20 mmol), (0.20 mmol), N,N’-dioxide L9 (2 mol%), Cu(OTf)2
(2 mol%) in CH2Cl2 (1.0 mL) and the reaction was stirred at room tem-
perature for 12 h. [b] Yield of the isolated product. [c] Determined by
HPLC analysis. [d] Dimedone 2b instead of 2a. [e] The absolute configu-
ration was determined by comparison to literature data.[6b,c]
1
ACHTUNGTRENNUNG
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X. Feng et al.
99% ee, Table 2, entries 4–16). Notably, the condensed-ring
and heteroaromatic alkenes also performed well, to give the
corresponding products in excellent yields and enantioselec-
tivities (Table 2, entries 17 and 18). Moreover, the substrate
with a cinnamyl group also gave a moderate yield and an ex-
cellent ee value (Table 2, entry 19). Upon scaling up to gram
quantities, the desired product 3a was still obtained with ex-
cellent results (1.673 g, 88% yield, 98% ee, Scheme 1) with
2 mol% of the L9-CuACHTNUGRTENUNG(OTf)2 complex.
Scheme 3. Proposed transition states.
complex. With a low catalyst loading of 2 mol% and mild
reaction conditions, a series of synthetically useful chiral bi-
cyclic compounds were obtained with excellent results (up
to 99% ee and near quantitative yield). Upon scale up, the
reaction maintained a good yield and excellent enantioselec-
tivity. The addition product was also easily transformed into
hexahydroquinoline. Remarkably, atmospheric oxygen and
water did not affect the reaction outcome and the procedure
was straightforward. Further application of the catalyst
system to other reactions is currently ongoing.
Scheme 1. Asymmetric Michael reaction on a gram scale.
The highly enantiomerically enriched compounds 3 ob-
tained by this method can be easily converted into hexahy-
droquinoline. Hexahydroquinoline derivatives are important
due to their wide applications in medicinal chemistry, in-
cluding calcium channel activity, bronchodilators, antiather-
osclerotics, antidiabetic, and so forth.[10] Hence, the synthesis
of hexahydroquinoline derivatives has attracted considera-
ble attention.[11] For example, compound 3a could be con-
verted to hexahydroquinoline 4a (Scheme 2) in high yield
(90%) and with no loss of enantioselectivity (99% ee).[12]
Experimental Section
General procedure: Dichloromethane (1.0 mL) was added to a mixture
of the ligand L9 (2.4 mg, 0.004 mmol), CuACTHNUTRGNENUG(OTf)2 (1.4 mg, 0.004 mmol),
and b,g-unsaturated a-ketoester 1a (38.0 mg, 0.2 mmol) and diketone 2a
(22.4 mg, 0.2 mmol), then stirred at room temperature for 12 h. After the
reaction was complete, the mixture was purified by flash chromatography
on silica gel (petroleum ether/ethyl acetate: 2:1) to afford the desired
product 3a in 99% yield with 99% ee.
Acknowledgements
We thank the National Natural Science Foundation of China (nos.
20732003 and 21021001), PCSIRT (no. IRT0846), and the National Basic
Research Program of China (973 Program: no. 2010CB833300) for finan-
cial support, and Sichuan University Analytical & Testing Centre for
NMR spectroscopic analyses.
Scheme 2. Conversion of 3a into hexahydroquinoline 4a.
To understand the reaction mechanism, a C2-symmetric
amide L10 (the precursor of the chiral N,N’-dioxide L9) was
synthesized (Scheme 3). No adduct was obtained when L10-
Keywords: asymmetric
catalysis
·
copper
·
hexahydroquinoline · ketoesters · Michael addition
CuACHTUNGTRENNUNG(OTf)2 was employed as the catalyst, which confirmed
that the N-oxide plays a key role in this reaction. A possible
transition state was postulated based on this result and the
absolute configuration of the product. We speculate that
N,N’-dioxide L9 and b,g-unsaturated a-ketoester 1a coordi-
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nate with CuACHTUNGTRENNUNG(OTf)2 to form a complex. Subsequently, the
cyclic diketone nucleophile attacks from the Si face of the
double bond to afford the corresponding product 3a with
excellent enantioselectivity.
In conclusion, we have developed a highly enantioselec-
tive conjugate addition of cyclic diketones to b,g-unsaturat-
ed a-ketoesters catalyzed by a chiral N,N’-dioxide/CuACTHNUTRGNEUNG(OTf)2
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Highly Enantioselective Conjugate Addition of Cyclic Diketones
COMMUNICATION
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Received: September 19, 2010
Published online: January 5, 2011
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