Iron-catalyzed Michael reactions revisited: A synthetically useful process for the preparation of tri-carbonyl compounds and chiral warfarin

Article abstract of DOI:10.1016/j.tetlet.2010.05.074

The LBAs (Lewis acid-assisted Br?nsted acid catalysis) is proposed as possible mechanistic process in the simple FeCl₃-catalyzed Michael reactions of chalcones with active methylene compounds in organic solvents. And iron salts were found to be effective promoters in the asymmetric Michael addition of 4-hydroxycoumarin to α,β-unsaturated ketone, which resulted in excellent yield and high level of enantioselectivity (up to 91% ee) in the presence of low catalytic amount of iron and simple chiral primary amine.

Full text of DOI:10.1016/j.tetlet.2010.05.074

Tetrahedron Letters 51 (2010) 3836–3839
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Iron-catalyzed Michael reactions revisited: a synthetically useful process for
the preparation of tri-carbonyl compounds and chiral warfarin
b
a,b,
Hua-Meng Yang a, Yue-Hua Gao b, Li Li a, Zhen-Yu Jiang a, Guo-Qiao Lai a, , Chun-Gu Xia , Li-Wen Xu
*
*
a Key Laboratory of Organosilicon Chemistry and Material Technology of Ministry of Education, Hangzhou Normal University, Hangzhou 310012, PR China
b State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The LBAs (Lewis acid-assisted Brønsted acid catalysis) is proposed as possible mechanistic process in the
simple FeCl3-catalyzed Michael reactions of chalcones with active methylene compounds in organic sol-
vents. And iron salts were found to be effective promoters in the asymmetric Michael addition of
4-hydroxycoumarin to a,b-unsaturated ketone, which resulted in excellent yield and high level of enanti-
oselectivity (up to 91% ee) in the presence of low catalytic amount of iron and simple chiral primary
Received 19 March 2010
Revised 14 May 2010
Accepted 18 May 2010
Available online 24 May 2010
amine.
Keywords:
Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved.
Iron catalysis
Michael reaction
Warfarin
Asymmetric catalysis
Primary amine
The Michael reaction of appropriate carboanionic reagents to
,b-unsaturated carbonyl compounds is the most direct approach
well documented and successfully used in the synthesis of multi-
carbonyl compounds from aliphatic enones or nitroolefins and
dicarbonyl compounds under solvent-free conditions.6 While the
mechanistic proposal of one-center template reaction is in accor-
dance with experimental observations, ESI, and DFT calculations,9
more information and evidence are desired to supplement or
understand the mechanism of Michael reaction. In addition, de-
spite the potential of iron salts as privileged catalysts in organic
synthesis being realized, the development of highly enantioselec-
tive iron-catalyzed organic transformation is highly desired.10
Herein, we focused on the iron-catalyzed Michael reactions by
using chalcones and active methylene as model substrates to eval-
uate the possible mechanistic process of LBAs (Lewis acid-assisted
Brønsted acid catalysis). Another aim of the present work is to
establish a novel and efficient LBAs-based chiral catalyst system
for asymmetric Michael reaction of 4-hydroxycoumarin to trans-
4-phenyl-3-buten-2-one to make chiral warfarin.
To investigate the solvent effect and catalytic efficiency of dif-
ferent iron salts in organic media, the Michael addition of active
methylenes to aromatic chalcones was considered as the ideal
model reaction. Initially, we carried out the optimization of a vari-
ety of reaction conditions, employing different iron salts for the
model Michael reaction of chalcone and acetylacetone. As shown
in Table 1, acceptable yield (39–79%) of the desired product was
obtained with FeCl3Á6H2O or Fe(ClO4)3ÁxH2O as catalyst in toluene
at room temperature after 12 h (entries 1 and 2), however, the
Michael reaction did not proceed at all with several iron salts, such
as FeSO4Á7H2O, FeCl2Á4H2O, Fe(NO3)3Á9H2O, and Fe(OAc)2 (entries
a
for the preparation of polysubstituted tri-carbonyl compounds,
which also is an efficient method for carbon–carbon bond forma-
tion and has extensive applications to remote functionalization in
organic synthesis.1 In the past decades, the reaction has been tra-
ditionally performed using various basic catalysts or reagents, such
as NaOH,2 Ba(OH)2,3 and Mg-Al-O-t-Bu hydrotalcite.4 However,
there are some disadvantages under the strong basic conditions.
For example, in the presence of strong bases, side reactions, such
as aldol or self-aldol addition, polymerizations, retrogressions,
and rearrangements, are frequently encountered. Recently, various
Michael reactions promoted by organic base/acid catalysts or un-
der microwave irradiation have been developed to avoid the usual
drawbacks of the basic process.5 Among the various metal salts
used to catalyze this Michael reaction, iron salts, such as iron(III)
chloride hexahydrate (FeCl3Á6H2O), were clearly the catalysts of
choice for the Michael reaction of alkyl enones with b-keto esters.6
Interestingly, in their report,6a the FeCl3Á6H2O showed poor cata-
lytic activity at room temperature in organic media.
In most cases, iron and its compounds are low-toxic and envi-
ronmentally friendly, therefore, it has been attracted much atten-
tion in a great deal of useful organic transformations in recent
years.7 Since 1982,8 the iron-catalyzed Michael reaction has been
* Corresponding authors. Tel.: +86 0571 28867756; fax: +86 0571 28865135
(L.-W.X.).
E-mail addresses: licpxulw@yahoo.com, liwenxu@hznu.edu.cn (L.-W. Xu).
0040-4039/$ - see front matter Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.05.074

H.-M. Yang et al. / Tetrahedron Letters 51 (2010) 3836–3839
3837
R3 R4
Table 1
R3 R4
Michael addition reaction of acetylacetone with chalcone promoted by different
metal salt
O
O
O
O
O
R2
FeCl3 (15 mol%)
Toluene, rt, 12h
R2
R1
O
O
66-97% yield
R1
O
O
COCH3
H2C
Catalyst
COCH3
COCH3
O
O
O
O
O
COCH3
OEt
1a
3a
2a
Entrya
Metal salt
FeCl3Á6H2O
Solvent
Yieldb (%)
O
O
O
OMe
1
2
3
4
5
6
7
8
9
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
CH2Cl2
39
79
Fe(ClO4)3ÁxH2O
FeCl2Á4H2O
Fe(NO3)3Á9H2O
Fe(OAc)2
FeSO4Á7H2O
FeBr3
3b: 89%
10:1 dr
NRc
NRc
NRc
NRc
91
3a: 97%
3c: 90%
O
O
O
O
O
O
Ph
Ph
FeCl3
FeCl3
97
88
10d
11e
12f
13
14
15
16
FeCl3
FeCl3
FeCl3
Fe(acac)3
HCl
THF
10
7
O
Cl
O
CH3OH
Ethanol
Toluene
Toluene
Toluene
Toluene
O
Trace
NRc
20g
>95
>95
3d: 71%
3e: 86%
3f: 87%
O
O
Fe(acac)3 + HCl
Fe(acac)3 + TsOH
O
O
O
O
Ph
Ph
a
Cl
Reaction conditions: 15 mol % of metal salt, 1.5 mmol/1.0 mmol, of acetylace-
tone/chalcone; and 1.5 mL of solvent at room temperature for 12 h unless otherwise
noted.
O
3g: 66%
b
Isolated yields.
No reaction.
24 h.
48 h.
O
O
Cl
c
d
3h: 89%
3i: 70%
e
f
Scheme 1. FeCl3-catalyzed Michael reactions of active methylene compounds with
chalcones: 15 mol % of FeCl3, 1.5 mmol/1.0 mmol, of active methylene compounds/
chalcones; and 1.5 mL of toluene at room temperature for 12 h.
70 h.
g
There are many by-products in this reaction.
3–6), which showed the anion of iron salt has a pronounced effect
in activation of carbonyl substrates. We then carried out the reac-
tion in the presence of anhydrous FeCl3 and FeBr3 in toluene at
room temperature, the reaction is clean and the yield was in-
creased to higher level (entries 7 and 8: 97% and 91%, respectively).
The screening of the solvent revealed that poor results were dis-
played in other solvents, such as THF and CH3OH (entries 10 and
11), and the reaction did not proceed at all when using ethanol
as the solvent (entry 12). Very interestingly, the reaction per-
formed with Fe(acac)3 failed in providing the desired Michael ad-
duct. The observed acceleration of the iron-mediated Michael
addition showed that the anion of iron salt played a crucial role
when counterion was changed from acetate to chloride.
We then investigated the reaction scope of this catalytic system
and its tolerance of functional groups in the case of other chalcones
1 and active methylene compounds 2. As shown in Scheme 1, the
optimized reaction conditions were applicable to various sub-
strates. Whether using aromatic or aliphatic diketones, iron effi-
ciently promoted the reaction with good to excellent yields.
Ethyl acetoacetate was also found to be a suitable substrate. Unfor-
tunately, the reaction of diethyl malonate with chalcone afforded
the desired product in only 25% yield with trace byproduct. On
the basis of previous results, we felt that the use of an appropriate
additive to form iron enolate of dimethyl malonate would allow us
to realize a mild and general procedure for the synthesis of 3k. As
we expected, using acetylacetone as an additive in the Michael
reaction of dimethyl malonate with chalcone, the reaction was pro-
ceeded successfully with good yield (70% isolated yield). To the
best of our knowledge, this is the first example of the iron-cata-
lyzed Michael reactions of chalcones and unactive malonate com-
pounds promoted by acetylacetone, which maybe due to the
activation of iron enolate (Scheme 2). In this case, the replacement
of FeCl3/acac system with Fe(acac)3/HCl resulted in low yield (10%),
which is due to low nucleophilic activity of ethyl malonate.
The mechanism of iron-catalyzed Michael reaction of 1,3-dicar-
bonyl compounds has been reported by Christoffers and co-work-
ers, in which a one-center template mode was established.9 It is
reasonable that Fe(acac)3 is significantly less active than the
FeCl3Á6H2O or FeCl3 because it does not readily offer a vacant coor-
dination site for the enone. However, it is difficult to explain why
anhydrous FeCl3 is significantly more active than FeCl3Á6H2O in tol-
uene. In addition, Fe(acac)3 is also an excellent catalyst in the pres-
ence of catalytic amount of brønsted acid, such as TsOH and HCl.
Based on our experimental results and macroscopic opinion, a sug-
gested and more practical mechanism with Lewis acid-assisted
Brønsted acid catalysis (LBA)11 is proposed in Scheme 3. The rea-
sons for this proposal of LBA are as follows: (a) The Fe(acac)3-cat-
alyzed Michael addition reactions of acetylacetone with chalcone
gave no desired product, indicating that the Michael addition pro-
ceeded with Brønsted acidic HCl as catalyst. (b) Without the iron
catalyst, the reaction proceeded with low yield and poor chemose-
lectivity. Many products of side reactions with mixtures formed in
the presence of only HCl. Therefore, the major role of the iron
O
EtO
H2C
EtO
O
O
O
O
15 mol% FeCl3
Toluene
OEt
OEt
O
3k
no acac:
with acac (15mol%):
25% yield
70% yield
Scheme 2. FeCl3-catalyzed Michael reaction of diethyl malonate with chalcone.

3838
H.-M. Yang et al. / Tetrahedron Letters 51 (2010) 3836–3839
Table 2
O
O
Ph
Ph
Effect of iron and primary amines on warfarin synthesisa
O
2a
COCH3
Fe/Primary amine
(1:1, 5-10 mol%)
OH
O
O
OH
O
FeCl3
O
COCH3
3a
O
O
O
Cl
O
Ph
Ph
AcOH, THF, rt
Fe
O
O
HCl
up to 91% ee
Ph
Ph
O
(1)
Ph
H2N
Ph
O
O
Cl
Fe
O
Ph
Ph
NH2
(R,R)-DPEN
H2N HN
P
O
Fe(acac)2Cl
O
Ph
NHTs
H+
O
H2N
Ph
O
Ph
Ph
3
1
2
1a
Ph
(R,R)-PO-DPEN
OH
Ph
(R,R)-Ts-DPEN
Ph
Ph
(2)
(3)
Scheme 3. Proposed LBAs mechanism for the FeCl3-catalyzed Michael addition
reactions.
H2N
H2N
NH2
H2N
NHTs
6
5
4
catalyst lied in the formation of iron–dicarbonyl complex for the
improvement of selectivity. (c) When a catalytic amount of HCl
or TsOH was used in the Fe(acac)3-catalyzed Michael addition reac-
tions of acetylacetone with chalcone, the desired products were
also formed with excellent chemoselectivity and yields (up to
97%), which supported the proposed mechanism of Lewis acid-as-
sisted Brønsted acid catalysis (LBA).
Encouraged by previous findings, we turned our attention to the
Michael addition of a b-keto ester analogue, 4-hydroxycoumarin,
to trans-4-phenyl-3-buten-2-one (benzylideneacetone) for the
synthesis of warfarin. Wafarin is a Vitamin K antagonist, and both
racemic and chiral wafarin have been introduced for clinical use as
the most widely prescribed anticoagulant drug in North American.12
In addition, the syntheses of chiral warfarin were traditionally per-
formed via auxiliary strategy,13 asymmetric hydrogenation,14 and
hetero-Diels–Alder addition15, and recently by organocatalytic Mi-
chael addition of 4-hydroxycoumarin to trans-4-phenyl-3-buten-
2-one.16
We then investigated the asymmetric FeCl3-catalyzed Michael
addition of 4-hydroxycoumarin to trans-4-phenyl-3-buten-2-one.
In an initial screening, we evaluated several simple and commercial
available chiral amines, especially primary amino derivatives, for
their ability to coordinate with iron and promote the Michael addi-
tion through possible enamine intermediate of 4-hydroxycouma-
rin. As representative chiral amines, primary amines 1–6 are
listed here and they exhibited different levels of enantio-induction.
As shown in Table 2, we found that Ts-DPEN (2)17 could promote
the Michael addition of 4-hydroxycoumarin to trans-4-phenyl-3-
buten-2-one in the absence of acetic acid and FeCl3 (entry 2), how-
ever, the yield is low and enantioselectivity is moderate (64% ee). To
our delight, the addition of FeCl3 could improve the yield, and we
found that the decrease of catalytic amount of iron and primary
amine to 5 mol % acceptable yield and higher enantioselectivity
(81% ee) was achieved in this reaction (entry 3). The synergistic ef-
fect of iron salt and primary amine is pronounced, which proved
that the concept of combining transition metal catalysis and organ-
ocatalysis is a promising strategy to improve the enantioselectvi-
ty.18 We then screened different primary amines for the iron/
primary amine-mediated Michael reaction of 4-hydroxycoumarin
with trans-4-phenyl-3-buten-2-one, only unmodified DPEN (1) re-
served the enantioselectivity and other primary amines resulted
in poor yields and enantioselectivities (entries 4–8 and 16). We ex-
plored the iron salts with different anions for this Michael reaction,
and the results are summarized in entries 9–12 of Table 2.
FeCl2Á4H2O, Fe(ClO4)3, and FeCl3 gave good enantioselectivities,
which prompted us to continue investigate the effect of AcOH.
When the amount of AcOH was increased to 10 equiv, all the three
iron salts evaluated gave good yields and excellent enantioselectiv-
Entry
FeXn
Amine
Fe/amine (mol %)
Yieldb (%)
eec (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
—
—
2
2
2
1
3
4
5
6
1
1
1
1
3
1
1
1
10
10
5
5
5
5
5
5
5
5
5
5
5
10
10
5
47
20
60
64
37
30
NRe
NRe
28
39
41
87
80
50
90
47
64
64d
81
83
75
40
—
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
Fe(ClO4)3
Fe(acac)3
—
73
53
68
83
90
91
90
84
FeSO4Á7H2O
FeCl2Á4H2O
FeCl3
Fe(ClO4)3
FeCl2Á4H2O
—
a
Experimental conditions: a mixture of iron salt and primary amine (1:1, 5–
10 mol %), 4-hydroxycoumarin (1 mmol), trans-4-phenyl-3-buten-2-one (1.5 mmol),
AcOH (entries 1–12 except entry 3: 2 equiv, entries 13–16: 10 equiv AcOH), and THF
(2 mL), at room temperature for 24 h.
b
Isolated yield.
c
Determined by HPLC using a chiral Daicel AD-H column.
d
No addition of AcOH.
e
No reaction.
ity (90–91% ee) in the presence of primary amine 1 or 3, even with
the analog of Ts-DPEN, PO-DPEN (3).19
In summary, iron-catalyzed Michael reaction of chalcone with
active methylene compounds works smoothly in organic media,
which has the advantages of mild reaction conditions, convenient
operation, high atom efficiency, and structural diversity of the
desired products. Further investigations of asymmetric iron-cata-
lyzed Michael addition of 4-hydroxycoumarin to trans-4-phenyl-
3-buten-2-one provided a new synthetically useful method for
the preparation of chiral warfarin with excellent enantioselectivity
(up to 91% ee). Present studies are directed toward the improve-
ment of enantioselectivity of the reaction by the cooperation of
metal catalysis and organocatalysis.
Acknowledgments
This work was supported by the National Natural Science Foun-
der of China (No. 20973051) and Zhejiang Provincial Natural Sci-
ence Foundation of China (Y4090139).
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2010.05.074.

H.-M. Yang et al. / Tetrahedron Letters 51 (2010) 3836–3839
3839
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