Asymmetric intramolecular oxa-Michael addition of activated α,β-unsaturated ketones catalyzed by a chiral N,N′-dioxide nickel(II) complex: Highly enantioselective synthesis of flavanones

Article abstract of DOI:10.1002/anie.200803326

(Chemical Equation Presented) The title reaction provides a promising approach for the synthesis of chiral flavanones with broad substrate scope and is tolerant to air and moisture. Good to excellent enantioselectivities and high yields were achieved for most substrates under mild conditions.

Full text of DOI:10.1002/anie.200803326

Communications
DOI: 10.1002/anie.200803326
Asymmetric Catalysis
Asymmetric Intramolecular Oxa-Michael Addition of Activated a,b-
Unsaturated Ketones Catalyzed by a Chiral N,N’-Dioxide Nickel(II)
Complex: Highly Enantioselective Synthesis of Flavanones**
Lijia Wang, Xiaohua Liu, Zhenhua Dong, Xuan Fu, and Xiaoming Feng*
The conjugate addition of oxygen nucleophiles to electron-
deficient olefins has been a significant challenge in organic
synthesis, owing to the low reactivity coupled with the
reversibility of the reaction.[1–3] In particular, enantioselective
intramolecular oxa-Michael (IOM) addition, which provides
a promising approach for synthesis of pharmaceutically and
biologically active chiral chromanone skeletons, has been
rarely explored.[4] Thus far, most reports have focused on
catalysis though hydrogen bonding by employing organo-
catalysts of quinine or cinchona chiral scaffolds.[5] In 1999,
Ishikawa and co-workers reported the first effective
(À)-quinine-catalyzed asymmetric IOM addition to synthe-
size anti-HIV-1 active calophyllum coumarin.[5b] Recently, a
remarkable strategy, employing tert-butyl ester activated a,b-
unsaturated ketones as substrates, for the catalytic synthesis
of chiral flavanones was developed by Scheidt and co-
workers.[5d] Despite these impressive contributions, more
efficient and practical catalytic systems for asymmetric intra-
molecular oxa-Michael addition are still in high demand.
Dicarbonyl compounds are promising candidates as sub-
strates as they can chelate a series of Lewis acids, such as FeII,
CoII, and NiII complexes,[6] and engage in two-point binding to
the central metal, which allows a chelate-ordered transition
state. Also, as nickel is a nonprecious metal, nickel complex
catalysts have been widely applied to catalytic organic
synthesis.[7] Moreover, chiral nickel complexes are becoming
practical and potential catalysts in enantioselective trans-
formations.[8–11] N,N’-Dioxide ligands are excellent chiral
scaffolds as they can coordinate many different metals and
have been successfully applied in many asymmetric reac-
tions.[12,13] Herein, we present a new, readily prepared,[14]
chiral N,N’-dioxide nickel(II) complex catalyst that facilitates
this intramolecular oxa-Michael addition with broad sub-
strates in 90–99% yields with up to 99% ee.
Some representative screening results for the catalytic
enantioselective IOM addition of the activated a,b-unsatu-
rated ketone 1a in the presence of an array of N,N’-dioxide
complexes as catalysts (10 mol%) are presented in Table 1.
Initially, N,N’-dioxide L1 (see Figure 1) was complexed to
several metal salts. Although [Fe(acac)2] as the central metal
showed ineffective asymmetric induction (Table 1, entry 1),
complexes of other Group VIII metals CoII and NiII showed
good inducing potential with enantioselectivities of 63% and
83% ee, respectively (Table 1, entries 2 and 3), which con-
firmed our initial expectation. Then, the influence of the
counterions of the [NiII(L1)] complex was investigated.
Although NiBr2, Ni(ClO4)2·6H2O, and Ni(OTf)2 showed
excellent ability in other chiral NiII-catalyzed reactions, they
gave only extremely poor results (Table 1, entries 4–6).
Fortunately, when Ni(Tfacac)2·2H2O was used, the desired
Table 1: Asymmetric IOM addition of a,b-unsaturated ketone 1a.[a]
[b]
Entry
M
Ligand x [mol%]Yield [%]
ee [%][c]
1
2
3
4
5
6
Fe(acac)2
Co(acac)2
Ni(acac)2
NiBr2
Ni(ClO4)2·6H2O
Ni(OTf)2
Ni(Tfacac)2·2H2O
Ni(Tfacac)2·2H2O
Ni(Tfacac)2·2H2O
Ni(Tfacac)2·2H2O
Ni(Tfacac)2·2H2O
Ni(Tfacac)2·2H2O
Ni(Tfacac)2·2H2O
Ni(Tfacac)2·2H2O
Ni(Tfacac)2·2H2O
Ni(Tfacac)2·2H2O
Ni(Tfacac)2·2H2O
L1
L1
L1
L1
L1
L1
L1
L2
L3
L4
L5
L6
L7
L8
L9
L1
L1
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
5
88
97
96
10
5
99
99
84
trace
98
95
91
50
90
n.r.
99
91
0
63(R)
83(R)
3(R)
5(R)
3(R)
97(R)
0
7[d]
8
9
10
11
12
13
14
15
16
17[e]
[*]L. J. Wang, Dr. X. H. Liu, Z. H. Dong, X. Fu, Prof. Dr. X. M. Feng
Key Laboratory of Green Chemistry & Technology, Ministry of
Education, College of Chemistry, Sichuan University
Chengdu 610064 (China)
n.d.
96(R)
93(R)
13(R)
20(R)
18(R)
n.d.
Fax: (+86)28-8541-8249
E-mail: xmfeng@scu.edu.cn
Prof. Dr. X. M. Feng
State Key Laboratory of Biotherapy, Sichuan University
Chengdu 610041 (China)
98(R)
92(R)
2
[**]We appreciate the National Natural Science Foundation of China
(No. 20732003) and the Ministry of Education (No. 20070610019)
for financial support, the Sichuan University Analytical & Testing
Centre for NMR analysis, and the State Key Laboratory of Biotherapy
for HRMS analysis.
[a]Unless otherwise noted, reactions were carried out with 1a
(0.1 mmol), [M(L)]complex (1:1, x mol%), and PhOMe (0.5 mL) at
308C for 12 h, then p-TsOH (0.2 mmol) was added at 808C for 2 h.
[b]Yield of isolated product; n.r. =no reaction. [c]Determined by chiral
HPLC analysis. The absolute configuration was determined by compar-
ison to literature data.[5d] n.d.=not determined. [d]Tfacac =1,1,1-
Trifluoroacetylacetonate. [e]20 h was needed.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200803326.
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Table 2: Olefin substrates for the catalytic asymmetric IOM addition.[a]
product was obtained in 99% yield and with 97% ee (Table 1,
entry 7). The disparate results were probably caused by the
different electronic and steric properties of the counterions.[15]
Next, the structures of ligands were examined (L1–L9,
Figure 1). The results showed that a linker chain of three
carbon atoms in the ligand was essential for the asymmetric
Entry
R
Product
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
Ph
2a
2b
2c
2d
2e
2 f
2g
2h
2i
99
90
95
90
98
98
96
98
97
99
90
98(R)[e]
93(R)[e]
97(R)[e]
99
2-ClC6H4
4-BrC6H4
4-CNC6H4
4-PhC6H4
1-naphthyl
2-naphthyl
3-MeC6H4
4-MeC6H4
2-MeC6H4
Et
92
96
92(R)[e]
93
9[d]
10
11
84(R)[e]
86
2j
2k
85
[a]Unless otherwise noted, reactions were carried out with 1 (0.1 mmol),
[Ni(L1)(Tfacac)2]·2H2O (5 mol%), and PhOMe (0.5 mL) at 308C for
12 h, then p-TsOH (0.2 mmol) was added at 808C for 2 h. [b]Yield of
isolated product. [c]Determined by chiral HPLC analysis; see the
Supporting Information for details. [d]15 mol% catalyst was used.
[e]The absolute configuration was determined by comparison to
literature data.[5d]
Figure 1. Ligands employed for the IOM addition.
addition. Ligand L2 with a two-carbon atom linkage gave
racemic product (Table 1, entry 8), whereas L3 with a five-
carbon atom linkage showed poor reactivity (Table 1,
entry 9). Further studies on the amide moiety demonstrated
that ligands L4 and L5 provided high yields (98% and 95%)
and excellent ee values (96% ee and 93% ee) regardless of
the electronic property of the substituents (Table 1, entries 10
and 11). Ligand L6 with bulky isopropyl groups led to a
dramatic decrease in enantioselectivity (Table 1, entry 12).
l-Ramiprol acid derivative L1 was superior to l-proline
derived L7 and l-pipecolic acid derived L8 as the chiral
backbone of the N,N’-dioxide in both reactivity and enantio-
selectivity (Table 1, entry 7 vs. entries 13 and 14). Moreover,
when amide ligand L9 was employed, the reaction did not
take place, which revealed that the N-oxide group is essential
for the reaction (Table 1, entry 15). Further optimization of
the reaction conditions identified that adduct 2a was pro-
duced in PhOMe with 5 mol% of [Ni(L1)(Tfacac)2]·2H2O
(ratio 1:1) in 99% yield and 98% ee (Table 1, entry 16). The
catalyst loading could be further reduced to 2 mol%, leading
to 91% yield with 92% ee after 20 h (Table 1, entry 17).
Furthermore, this process was tolerant to air and moisture.
Under the optimal reaction conditions (Table 1, entry 16),
a series of representative olefin substrates (1a–1k) were
investigated, and the corresponding products (2a–2k) were
obtained in high yields and with up to 99% ee (Table 2). In
the case of electron-poor substituents (2b–2e), the catalyst
system [Ni(L1)(Tfacac)2]·2H2O led to excellent yields and
enantioselectivities (Table 2, entries 2–5, 90–98% yields, 92–
99% ee). Condensed ring substrates 1 f and 1g also gave the
desired products 2 f and 2g in high yields and with 96% ee and
92% ee, respectively (Table 2, entries 6 and 7). The electron-
rich substituents on 1h–1j underwent the IOM addition and
decarboxylation processes to yield the enantiomeric adducts
(2h–2j) in excellent yields and with ee values in the range 84–
93% ee (Table 2, entries 8–10). The aliphatic substrate 1k was
also found to be suitable, affording chromanone 2k in 90%
yield and with 85% ee (Table 2, entry 11).
To extend the substrate scope, our further examination
focused on the IOM addition of several representative phenol
substrates (3a–3e). The results in Table 3 show that different
phenol moieties were tolerated in the reaction and good to
excellent results (up to 96% ee) were achieved (Table 3,
entries 1–4). The methyl-substituted substrates 3a and 3b
provided the corresponding products 4a and 4b in high yields
and with 96% ee and 95% ee, respectively (Table 3, entries 1
and 2). The reaction of 4-methoxyphenyl-substituted 3c gave
the product 4c in 95% yield and with 80% ee (Table 3,
entry 3). Moreover, naphthyl-substituted 3d was also found to
be an excellent substrate for the reaction and afforded the
Table 3: Phenol substrates for the catalytic asymmetric IOM addition.[a]
Entry
R1
R2
Product
Yield [%][b]
ee [%][c]
1
H
Me
H
Me
H
MeO
4a
4b
4c
4d
4e
97
90
95
90
96
96
2[d]
3
95(R)[e]
80
4
5
-(CH)4-
Cl
90(R)[e]
40
H
[a]Unless otherwise noted, reactions were carried out with 3 (0.1 mmol),
[Ni(L1)(Tfacac)2]·2H2O (5 mol%), and PhOMe (0.5 mL) at 308C for
12 h, then p-TsOH (0.2 mmol) was added at 808C for 2 h. [b]Yield of
isolated product. [c]Determined by chiral HPLC analysis; see the
Supporting Information for details. [d]10 mol% catalyst was used.
[e]The absolute configuration was determined by comparison to
literature data.[5d]
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Communications
desired product 4d in 90% yield and with 90% ee (Table 3,
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entry 4). However, electron-poor substituents on the phenol
moiety dramatically decreased the enantioselectivity
(Table 3, entry 5).
In addition, when the reaction with 1a was scaled up
tenfold with 5 mol% nickel complex at ambient temperature
in open vessel, good results (99% yield and 96% ee) were still
obtained (Scheme 1). Significantly, flavanones can be easily
transformed into versatile building blocks for pharmaceutical
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Scheme 1. Asymmetric IOM addition of 1a on a tenfold scale (left)
and example for its application (right).
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In summary, we have developed a highly efficient catalytic
enantioselective intramolecular oxa-Michael addition using a
new chiral N,N’-dioxide nickel(II) complex. This process
provided a promising approach for the synthesis of chiral
flavanones with broad substrate scope and which was tolerant
to air and moisture. In the presence of 5 mol% [NiII(L1)]
complex, good to excellent enantioselectivities (up to
99% ee) and high yields were achieved for most of the
substrates under mild conditions. Further investigations of the
mechanism of this catalytic system are still in progress.
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Experimental Section
General experimental procedure:
A mixture of L1 (2.7 mg,
0.005 mmol) and Ni(Tfacac)2·2H2O (2.0 mg, 0.005 mmol) in PhOMe
(0.5 mL) was stirred at ambient temperature in an open vessel.
Substrate 1 or 3 (0.1 mmol) was added, and the reaction mixture was
stirred at 308C for 12 h. After completion of cyclization, pTsOH
(34 mg, 0.2 mmol) was added, and the solution was heated to 808C for
2 h (monitored by TLC). The solution was then cooled, and the
reaction mixture was purified by flash chromatography on silica gel to
obtain the final product 2 or 4.
Received: July 9, 2008
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Published online: October 7, 2008
Keywords: asymmetric catalysis · flavanones · nickel ·
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nitrogen oxide · oxa-Michael addition
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