Highly enantioselective epoxidation of α,β-unsaturated ketones catalyzed by primary-secondary diamines

Article abstract of DOI:10.1002/adsc.201100230

The asymmetric epoxidation of α,β-unsaturated ketones has been achieved by using functional and readily accessible primary-secondary diamines as the catalysts, giving the useful alkyl epoxy products with good yields and high enantioselectivities (up to 99% ee). Copyright

Full text of DOI:10.1002/adsc.201100230

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DOI: 10.1002/adsc.201100230
Highly Enantioselective Epoxidation of a,b-Unsaturated Ketones
Catalyzed by Primary-Secondary Diamines
Yingpeng Lu,a,b Changwu Zheng,a Yingquan Yang,a Gang Zhao,a,*
and Gang Zoub,*
a
Key Laboratory of Synthetic Chemistry of Natural Substances, Shanghai Institute of Organic Chemistry, Chinese
Academy of Sciences, 345 lingling Road, Shanghai 200032, Peopleꢀs Republic of China
Fax : (+86)-21-64166128; e-mail: zhaog@mail.sioc.ac.cn
Laboratory of Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, 130
b
Meilong Road, Shanghai 200237, Peopleꢀs Republic of China
Fax : (+86)-21-6425-3881; e-mail: zougang@ecust.edu.cn
Received: March 30, 2011; Revised: June 10, 2011; Published online: November 16, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adcs.201100230.
Abstract: The asymmetric epoxidation of a,b-unsa-
turated ketones has been achieved by using func-
tional and readily accessible primary-secondary di-
ACHTUNGTRENNUNGamines as the catalysts, giving the useful alkyl
epoxy products with good yields and high enantio-
selectivities (up to 99% ee).
Figure 1. Activation mode of catalysts.
Keywords: asymmetric catalysis; cumene hydroper-
oxide; enones; epoxidation; organocatalysis
termediates to activate the a,b-unsaturated alkyl ke-
tones with both good chemoselectivities and enantio-
selectivities, many efforts have been made to explore
novel and accessible types of this catalysts and their
Chiral epoxy groups are widely used as building application in asymmetric reactions.[6k,10]
blocks in a great number of organic syntheses.[1] De-
Recently, our group has developed the first novel
spite the great achievements have been made by primary-secondary diamine catalysts, which could be
Sharpless,[2] Jacobsen,[3] Shi,[4] and Shibasaki[5] in the synthesized easily from commercially available natu-
epoxidation of allylic alcohols, various unactivated al- ral amino acids, and which provided excellent activi-
kenes and some examples of a,b-unsaturated carbonyl ties and enantioseletivities in Michael additions.[11] As
compounds, methods focused on the non-metal-cata- part of our continuous efforts in developing efficient
lyzed enantioselective epoxidation of a,b-unsaturated organocatalysts for asymmetric epoxidation, we
ketones were still limited.[6]
postulated that these powerful and readily accessible
Among the organocatalysts employed for the epoxi- chiral primary-secondary diamines could also be used
dation of a,b-unsaturated ketones recently,[7–9] there for the epoxidation of a,b-unsaturated alkyl ketones,
are two types of catalysts with different activation which would provide an alternative way to a series of
modes that are the most used and valuable (Figure 1). a,b-epoxy carbonyl compounds with good yields and
The secondary amine catalysts derived from proline enantioselectivities.
and used for the epoxidation of a,b-unsaturated aryl
Preliminary studies were carried out using 6-phe-
ketones activate the substrate through hydrogen-bond nylhex-3-en-2-one as a model substrate with 20 mol%
interaction followed by a nucleophilic pathway (type of the primary amine catalysts (Figure 2) and using
I),[8] while the primary amine catalysts derived from tert-butyl hydroperoxide (TBHP) as an oxidant in tol-
Cinchona alkaloids activate the a,b-unsaturated al- uene (Table 1). The same as we observed in the previ-
ACHTUNGTRENNUNG
kylenones with an iminium transition state (type II).[9] ous Michael addition, this epoxidation using primary-
Due to the significant advantages of the primary secondary amine catalyst also proceeded smoothly.
amine catalysts in generating congested covalent in- All of the catalysts promoted this reaction and most
Adv. Synth. Catal. 2011, 353, 3129 – 3133
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Yingpeng Lu et al.
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nine (1a–1f) with different protecting groups at the
secondary amine gave the product with similar mod-
erate yields and good enantioselectivities, while the
presence of a bulky protecting group seems to be
more advantageous for the selectivity (Table 1, en-
tries 1–6). l-Phenylglycine-derived catalyst 1g was in-
ferior in this reaction and showed very low activity
and enantioselectivity (Table 1, entry 7). Catalyst de-
rived from l-leucine did not improve the result
(Table 1, entry 8). The catalyst derived from l-trypto-
phan, which showed excellent activity in the Michael
addition of malonates to enones,[11a,e] however, afford-
ed the product with no better ee value (Table 1,
entry 9). Considering that bulky steric hindrance on
the indolyl may have a good effect on the enantiose-
lectivity, two N-protected catalysts 1j and 1k were
Figure 2. Structures of the catalysts studied.
of them provided the epoxy product with good enan- prepared and evaluated in the reaction. Unfortunate-
tioselectivity. The catalysts derived from l-phenylala- ly, rather poor enantioselectivities were obtained
Table 1. Effects of catalyst, oxidant, solvent and additive in the epoxidation reaction.[a]
Entry
Catalyst
Solvent
Acid
Oxidant
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
1g
1h
1i
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
hexane
xylene
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TfOH
–
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
H2O2
UHP
CHP
CHP
CHP
CHP
CHP
CHP
CHP
CHP
CHP
68
73
74
74
77
77
46
61
74
86
76
48
91
87
88
83
58
56
83
26
71
nd
59
75
77
81
88
91
86
90
35
86
87
81
83
80
64
96
87
95
71
72
71
nd
87
nd
86
93
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23[d]
24[e]
1j
1k
1d
1d
1d
1d
1d
1d
1d
1d
1d
1d
1d
1d
1d
CHCl3
dioxane
diethyl ether
CH3OH
toluene
toluene
toluene
toluene
TFA
TFA
CHP
CHP
[a]
Unless otherwise specified, The reaction was carried out with 2a (0.2 mmol), oxidant (2 equiv.), 1 (20 mol%), additive
(20 mol%) in solvent (1.0 mL) at 308C for 48 h.
Isolated yields of 3a after chromatography.
[b]
[c]
Determined by chiral HPLC analysis. The absolute configuration was determined by comparison of the specific optical
rotation with that of the literature reported.[9b]
[d]
[e]
TFA (40 mol%) was used.
10 mol% of the catalyst was used.
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Highly Enantioselective Epoxidation of a,b-Unsaturated Ketones Catalyzed by Primary-Secondary Diamines
Table 2. Enantioselective epoxidation of a,b-unsaturated ke-
(Table 1, entries 10 and 11). It should be noted that
5–20% of the peroxide product 4 was isolated when
TBHP was used as an oxidant, so the yields of the de-
sired product were moderate. This problem could be
solved by optimizing the reaction conditions.
tones catalyzed by 1d/TFA.[a]
With the best catalyst 1d that we obtained above,
several other oxidants such as hydrogen peroxide
(30% w/w in water) (H2O2), urea-hydrogen peroxide
(UHP) and cumene hydroperoxide (CHP) were eval-
uated (Table 1, entries 12–14). The by-product 4 was
totally suppressed by using UHP or CHP as the oxi-
dant and the yields of the desired product were im-
proved to around 90%, especially an excellent enan-
tioselectivity could also be achieved when CHP was
selected as the oxidant. This improvement may be as-
cribed to the large cumene group, which is beneficial
for both enantioselective control and leaving after nu-
cleophilic attack to form the desired product. The
H2O2 oxidant, which was active in Listꢀs epoxidation
using Cinchona alkaloid-derived primary amine cata-
lysts, however, showed low reactivity in this reaction
and some of the starting material was recovered after
48 h. Then, the solvent effect in this reaction was in-
vestigated. It seemed that a non-polar solvent such as
toluene and hexane is superior to polar solvents and
protic solvents. In non-polar solvents such as toluene
and hexane, the reaction proceeded smoothly, afford-
ing the desired products with high yields and good to
excellent enantioselectivities (Table 1, entries 14–16).
Other solvents only gave the products with moderate
yields or enantioselectivities (Table 1, entries 17–20).
The effect of the additive was also examined. With-
out trifluoroacetic acid (TFA) as the additive, the re-
action could not proceed (Table 1, entry 22). No posi-
tive effect was observed when trifluoromethylsulfonic
acid (TfOH) was used (Table 1, entry 21). Increasing
the loading of TFA led to a sharp decrease in yield
due to decomposition of the product (Table 1,
entry 23). Reducing the catalyst loading caused an ob-
viously decrease in reactivity and a slight decrease in
enantioselectivity (Table 1, entry 24). Thus, the reac-
tion was best performed by using a combination of
1d/TFA (1:1), CHP as the oxidant in toluene at 308C
to provide the epoxide with 87% yield and 96% ee
(Table 1, entry 14).
Entry Product
Time Yield ee
[h]
[%][b] [%][c]
1
2
3a 24
87
84
96
92
3b 24
3c 24
3
86
90
4
3d 24
3e 24
3f 24
3g 36
3h 24
3i 48
68
82
80
72
84
74
91
5
95
6
88[d]
99
7
8
99
9[e]
91
10[e]
11[e]
3j 48
3k 48
74
70
92
92
12
13
3l 24
3m 48
3n 24
79
79
70
92
83
14
66[f]
With the optimized reaction conditions in hand, the
scope of the epoxidation was investigated and the re-
sults are summarized in Table 2. In most cases, the ep-
oxide products were exclusively formed in good yields
and high to excellent ee values. a,b-Unsaturated
methyl ketones with an aryl group on the side of
double bond worked very well to give the products
with high yields and above 90% ee (Table 2, en-
tries 1–3). Long-chain alkyl-, alkoxy- and siloxy-sub-
stituted a,b-unsaturated ketones also performed well
to provide good yields of the products. The relatively
low yield of 3d was due to its volatility (Table 2, en-
[a]
Unless otherwise specified, the reaction was carried out
with 2 (0.2 mmol), CHP (2 equiv), 1 (20 mol%), TFA (20
mol%) in toluene (1.0 mL) at 308C.
[b]
[c]
[d]
[e]
[f]
Isolated yields after chromatography.
Determined by chiral HPLC/GC analysis.
Determined by chiral HPLC after being transformed.
The reaction was carried out at 408C.
Determined by comparison of the optical rotation value
with the literature.[9c]
Adv. Synth. Catal. 2011, 353, 3129 – 3133
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Yingpeng Lu et al.
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tries 4–7). A long-chain alkyl group on the side of the Experimental Section
carbonyl was also found to be favorable in this reac-
tion. Excellent enantioselectivities (99% ee) were ob- General Procedure for the Asymmetric Epoxidation
tained with these substrates (Table 2, entries 7 and 8).
a,b-Unsaturated phenethyl ketones were also reactive
in this reaction and high enantioselectivities were ob-
Catalyst 1d (8.0 mg, 0.04 mmol) was added to the solution of
trifluoroacetic acid (4.6 mg, 0.04 mmol) in toluene (1.0 mL).
Then enone 2 (0.2 mmol) was added followed by the addi-
tained although a long reaction time was needed due
to the larger aryl groups with low activities (Table 2,
entries 9–11). Substrates containing ester groups were
also synthesized and subject to the reaction. The ep-
oxidation proceeded selectively on the unsaturated
ketones with double bond connected to the ester un-
changed (Table 2, entries 12 and 13). Chalcones, and
conjugate enones with a trisubstituted or terminal
alkene moiety proved to be unreactive under this con-
dition.[12]
tion of cumene hydroperoxide (2.0 equiv., 0.4 mmol, 61 mg)
at ambient temperature. The mixture was stirred at 308C for
24–48 h. Water (3 mL) was added. The mixture was extract-
ed with ethyl acetate (2ꢂ3 mL) and the combined organic
phases were washed with brine, dried (Na2SO4), filtered and
concentrated. Crude products were purified by silica gel
column chromatography using hexane/ethyl acetate as
eluent to afford the pure epoxides 3.
A bifunctional iminium mechanism similar to those
previously proposed for the primary amine salt cata-
lysts in the iminium catalysis of enone may be in-
voked to explain the observed stereochemical results
(Figure 3).[9a] We presumed that the primary amine
Acknowledgements
Research support from National Basic Research Program of
China (973 Program, 2010CB833300), the National Natural
Science Foundation of China (No.20172064, 203900502,
21032006), Shanghai Natural Science Council, and Excellent
Young Scholars Foundation of National Natural Science
Foundation of China (20525208) is gratefully acknowledged.
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[12] Substrates 1-cyclohexenylethanone and 5-phenylpent-1-
en-3-one were examined under these conditions. How-
ever, no reaction occurred after stirring for 3 days at
408C.
Adv. Synth. Catal. 2011, 353, 3129 – 3133
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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