Chiral N,N'-dioxide-scandium(III)-catalyzed asymmetric epoxidation of 2-arylidene-1,3-diketones with hydrogen peroxide

Article abstract of DOI:10.1002/adsc.201400341

The catalytic asymmetric epoxidation of 2-arylidene-1,3-diketones using aqueous 30% H₂O₂ as oxidant has been successfully realized by a N,N'-dioxide-scandium(III) triflate [Sc(OTf)₃] complex, giving the corresponding products in moderate to good yields (up to 85%) with excellent enantioselectivities (up to 99% ee) under mild reaction conditions without extra additives.

Full text of DOI:10.1002/adsc.201400341

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DOI: 10.1002/adsc.201400341
Chiral N,N’-Dioxide–Scandium(III)-Catalyzed Asymmetric
G
Epoxidation of 2-Arylidene-1,3-diketones with Hydrogen
Peroxide
Yangyang Chu,a Xiaoyu Hao,a Lili Lin,a Weiliang Chen,a Wei Li,a Fei Tan,a
Xiaohua Liu,a and Xiaoming Fenga,*
a
Key Laboratory of Green Chemistry & Technology, Ministry of Education, College of Chemistry, Sichuan University,
Chengdu 610064, Peopleꢀs Republic of China
Fax : (+86) 28-8541-8249; e-mail: xmfeng@scu.edu.cn
Received: April 4, 2014; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201400341.
Abstract: The catalytic asymmetric epoxidation of
2-arylidene-1,3-diketones using aqueous 30% H2O2
as oxidant has been successfully realized by an
been applied in asymmetric [3+2]cycloaddition reac-
tions.[9] In 2010, Lattanzi et al. reported the first enan-
tioselective epoxidation of 2-arylidene-1,3-diketones
using a,a-diarylprolinol as organocatalyst and TBHP
as oxidant, giving the corresponding epoxides in mod-
erate to good ee values (64–85%) within 3–7 days.[10]
However, a highly efficient catalytic system for the
catalytic asymmetric epoxidation of 2-arylidene-1,3-
diketones with environmentally benign H2O2 as oxi-
dant is still desirable. The chiral N,N’-dioxide–metal
complexes developed by our group have shown pow-
erful catalytic ability in a variety of asymmetric reac-
tions owing to their tunable electronic and steric
chiral scaffolds, including the asymmetric epoxidation
of a,b-unsaturated carbonyl compounds with hydro-
gen peroxide.[12] As part of our continuing efforts in
N,N’-dioxide–scandiumACTHNUTRGENNUG(III) triflate [ScHCATUNGTREN(NUNG OTf)3] com-
plex, giving the corresponding products in moderate
to good yields (up to 85%) with excellent enantio-
selectivities (up to 99% ee) under mild reaction
conditions without extra additives.
Keywords: 2-arylidene-1,3-diketones; asymmetric
catalysis; epoxidation; hydrogen peroxide; N,N’-di-
oxide–scandiumACHTUNGTRENNUNG(III) complexes
The asymmetric epoxidation of olefins[1] is a very the epoxidation reaction of olefins, we disclose here
practical method for synthesizing chiral epoxide com- an efficient epoxidation reaction of 2-arylidene-1,3-di-
pounds, which are versatile intermediates and build- ketones with hydrogen peroxide catalyzed by an
ing blocks for the synthesis of useful natural prod- N,N’-dioxide-Sc
ACHTUNGERTN(NUNG OTf)3 complex under mild reaction
ucts[2] and drugs.[3] A milestone achievement was ob- conditions (Scheme 1).
tained by Sharpless for the asymmetric epoxidation of
Firstly, the complexes of chiral N,N’-dioxide L3 de-
allylic alcohols in the early 1980s. In recent years, rived from (S)-pipecolic acid with various metal salts
asymmetric epoxidation reactions have been accom- were investigated with 2-arylidene-1,3-diketone 1a as
plished with various oxidants using a wide range of the model substrate. Except for the complexes of lan-
catalysts, such as polypeptides,[5a] chiral ketone,[5b–f] thanide metals, the complexes of other metals could
phase-transfer catalyst,[5g] and metal complexes.[5h,i,j] not catalyze the reaction (see the Supporting Infor-
Among these oxidants, aqueous hydrogen peroxide is mation for details). For example, when the L3-Cu-
probably the most competitive for its advantages of
ACHTUNGTRENNUNG
cheapness, high atom efficiency and generating water (Table 1, entry 1). The use of the L3-LaAHCUTNGTRENNUNG
as the sole by-product. Although great progress has plex did afford the desired product in 24% yield with
been made in H2O2-efficient epoxidations,[6] the epox- 34% ee (Table 1, entry 2). Encouraged by this result,
idations of unfunctionalized[7] and trisubstituted ole- other lanthanide metals were tested and it was found
fins[8] using H2O2 as oxidant are still challenging. For that the L3-Sc
ACTHNUTRGNEN(UG OTf)3 complex could give 96% ee and
the latter, the greater steric hindrance makes the oxi- 57% yield (Table 1, entry 4 vs. entry 3). Next, the
dation process more difficult. Recently, one type of structure of the chiral N,N’-dioxide was explored.
trisubstituted olefin, 2-arylidene-1,3-diketone, has When L1 which is derived from l-proline and L2
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Scheme 1. Catalytic asymmetric epoxidation reaction.
Table 1. Optimization of the reaction conditions.[a]
solvents were examined carefully. Gratifyingly,
CH3OH was the best solvent candidate which can
afford the product in 74% yield with 98% ee (Table 1,
entry 12). Other conditions such as temperature and
other metal salts were also surveyed, but no superior
results were obtained (see the Supporting Informa-
tion).
Under the optimized reaction conditions (Table 1,
entry 12), the substrate scope was investigated. As
summarized in Table 2, the enantioselectivity of the
reaction was influenced little by substitution at the
meta- and para-positions of the aromatic ring
(Table 2, entries 5–10, 12 and 13). Most of the sub-
strates provided the corresponding chiral epoxides in
Entry Ligand Metal
Solvent
Yield [%][b] ee [%][c]
Table 2. Substrate scope for the asymmetric epoxidation re-
action of 2-arylidene-1,3-diketones compounds with hydro-
gen peroxide.[a]
1
2
3
4
5
6
7
8
L3
L3
L3
L3
L1
L2
L4
L5
L3
L3
L3
L3
Cu
La
G
n.r.[d]
24
16
57
trace
39
38
52
n.r.
11
–
G
34
95
96
70
83
56
78
–
Y
A
Sc
Sc
Sc
Sc
Sc
Sc
Sc
Sc
Sc
N
Entry
R
Product
Yield [%][b]
ee [%][c]
9
10
11
12
92
98
98
1
2
3
4
5
6
7
8
Ph
3a
3b
3c
3d
3e
3f
3g
3h
3i
3g
3k
3l
3m
3n
3o
3p
3q
74
71
58
79
53
55
58
50
64
60
85
54
59
63
62
63
69
98 (R)
93 (R)
98 (R)
82 (R)
99 (R)
99 (R)
99 (R)
98 (R)
99 (R)[d]
99 (R)
99 (R)
98 (R)
97 (R)
99 (R)
99 (R)
98 (R)
97 (R)
4-NCC6H4
3-O2NC6H4
4-O2NC6H4
3-F3CC6H4
4-F3CC6H4
3-FC6H4
74
[a]
Unless otherwise noted, all reactions were performed
with 1a (0.10 mmol), (0.30 mmol), and catalyst
(10 mol%) in solvent (0.5 mL) at 308C for 48 h.
Isolated yield.
Determined by HPLC.
No reaction.
2
[b]
[c]
[d]
4-FC6H4
9
3-ClC6H4
4-ClC6H4
3,4-Cl2C6H3
3-BrC6H4
4-BrC6H4
3-PhOC6H4
3-MeOC6H4
3-MeC6H4
4-MeC6H4
10
11
12
13
14
15
16
17
which is derived from l-rapipril acid were tested, the
yields decreased sharply (Table 1, entries 5 and 6).
The substituent at the amide moiety of the N,N’-diox-
ide ligands also affected the reaction obviously. With
greater steric hindrance, better results were obtained
(Table 1, entries 4, 7 and 8). Then, the effect of sol-
vents was investigated. When H2O was introduced as
solvent, unfortunately, no product was detected
(Table 1, entry 9). If CH2Cl2 was used, a high enantio-
selectivity could be obtained albeit with 11% yield
(Table 1, entry 10). To our delight, when using tert-
butyl alcohol as the solvent, the yield increased to
61% while the enantioselectivity was maintained
(Table 1, entry 11). Encouraged by the result, alcohol
[a]
Unless otherwise noted, all reactions were conducted
with 1 (0.10 mmol), 2 (0.30 mmol), L3 (10 mol%) and Sc-
AHCTUNTGREN(GNUN OTf)3 (10 mol%) in CH3OH (0.5 mL) at 308C for 48 h.
Isolated yield.
Determined by HPLC.
The absolute configuration of adduct 3i was determined
to be R by X-ray crystallographic analysis, other products
were determined to be R from CD spectra in comparison
with the spectrum of 3i.
[b]
[c]
[d]
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Chiral N,N’-Dioxide–ScandiumACTHUNRTGNEUNG(III)-Catalyzed Asymmetric Epoxidation
alkene 1i coordinated to the scandiumACTHNUGRTENUNG(III) in a biden-
tate fashion to form a stable six-membered ring. The
Si face of the alkene 1i was shielded by the nearby
isopropylphenyl group of the N,N’-dioxide L3. Thus,
the peroxide attacked the b-carbon atom of the Mi-
chael acceptor 1i preferably from the Re face, and the
carbon-oxygen bond was formed. Then, the a-carbon
attacked the peroxide molecule to form the corre-
sponding R-configured product 3i as determined by
the X-ray crystallographic analysis.
Scheme 2. Scale-up version of the reaction.
good yields (53–85%) and excellent enantioselectivi-
In conclusion, we have developed an effective
ties (93–99% ee), but use of 3d led to some loss of chiral N,N’-dioxide catalyst for the asymmetric epoxi-
enantioselectivity (Table 2, entries 2 and 3 vs. entry 4). dation reaction of 2-arylidene-1,3-diketones with hy-
The best result was obtained when a trisubstituted drogen peroxide. The corresponding chiral epoxides
olefin with two chlorine substituents on the aromatic were furnished in moderate to good yields (up to
ring was applied as the substrate (85% yield and 99% 85%) with good to excellent enantioselectivities (up
ee, Table 2, entry 11). The olefins with electron-donat- to 99% ee) under mild reaction conditions. This cata-
ing groups on the aromatic ring were also good sub- lytic system is attractive for its air tolerance and per-
strates and afforded the corresponding products in formance without extra additives. Further investiga-
97–99% ee (Table 2, entries 14–17). The relatively low tions on the application of N,N’-dioxides to other re-
yields are probably caused by the steric hindrance actions are in progress.
and the poor solubility of the 2-arylidene-1,3-dike-
tones in the solvent. The alkyl substrate (1r) was also
tested under the optimized reaction conditions, un- Experimental Section
fortunately, the desired epoxide product was not de-
tected (see the Supporting Information for details). Typical Procedure for the Asymmetric Epoxidation of
The absolute configuration of adduct 3i was deter- 2-Arylidene-1,3-diketones
mined to be R by X-ray crystallographic analysis,[13]
and the others were determined to be R by comparing
their CD spectra with the spectrum of 3i.
A mixture of N,N’-dioxide L3 (6.6 mg, 0.01 mmol), ScACHTUNGTRENNUNG(OTf)3
(5.0 mg, 0.01 mmol), and 1a (31.2 mg, 0.1 mmol), was stirred
in CH3OH (0.5 mL) at 308C for 30 min. Then, H2O2
(28.2 mL, 0.3 mmol, 30% w/w in H2O) was added. The reac-
tion mixture was stirred at 308C for 48 h. After complete
consumption of the starting materials, the mixture was puri-
fied directly by column chromatography on silica gel column
(petroleum ether:ethyl acetate:CH2Cl2 =50:1:1) to afford 3a
as a white solid; yield: 24.2 mg (74%).
To show the synthetic utility of the current method,
a gram-scale synthesis of 3a by using the L3-Sc
complex was performed. As shown in Scheme 2, in
the presence of 10 mol% of L3-Sc(OTf)3, 4 mmol of
ACHTUNGERTN(NUNG OTf)3
AHCTUNGTRENNUNG
the starting materials 1a (1.2496 g) reacted with
3 equivalents of H2O2, and the corresponding product
3a was obtained in 50% yield (0.6852 g) with 97% ee
(Scheme 2).
Based on the previous work[14] and the absolute
configuration of products, a possible catalytic activa-
tion model was proposed to elucidate the origin of
Acknowledgements
We appreciate the National Natural Science Foundation of
the stereoselectivity. As outlined in Scheme 3, the China (Nos. 21321061 and 21172151), and the Minstry of
Scheme 3. Proposed catalytic model and X-ray structure of the product (R)-3i.
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Education of The Peopleꢀs Republic of China (No. NCET-
11-0345) for financial support.
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6 Chiral N,N’-Dioxide–ScandiumACTHNUGRTNEUNG(III)-Catalyzed Asymmetric
Epoxidation of 2-Arylidene-1,3-diketones with Hydrogen
Peroxide
Adv. Synth. Catal. 2014, 356, 1 – 6
Yangyang Chu, Xiaoyu Hao, Lili Lin, Weiliang Chen,
Wei Li, Fei Tan, Xiaohua Liu, Xiaoming Feng*
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