Organocatalytic enantioselective decarboxylative Michael addition of β-ketoacids to α,β-unsaturated ketones

Article abstract of DOI:10.1039/c2ra21945j

Enantioselective decarboxylative Michael addition reactions promoted by chiral primary amine organocatalysts have been developed, allowing the facile synthesis of the corresponding chiral 1,5-diketones with excellent enantioselectivity (up to 97% ee). The method reported represents a valuable approach of utilizing β-ketoacids as synthetic equivalents of aryl methyl ketones.

Full text of DOI:10.1039/c2ra21945j

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Organocatalytic enantioselective decarboxylative
Michael addition of b-ketoacids to a,b-unsaturated
ketones3
Cite this: RSC Advances, 2013, 3, 1332
Received 25th August 2012,
Accepted 26th November 2012
Young Ku Kang, Hyun Joo Lee, Hyoung Wook Moon and Dae Young Kim*
DOI: 10.1039/c2ra21945j
www.rsc.org/advances
Enantioselective decarboxylative Michael addition reactions pro-
moted by chiral primary amine organocatalysts have been
developed, allowing the facile synthesis of the corresponding
chiral 1,5-diketones with excellent enantioselectivity (up to 97%
ee). The method reported represents a valuable approach of
utilizing b-ketoacids as synthetic equivalents of aryl methyl
ketones.
boxylative Michael addition of b-ketoacids to a,b-unsaturated
ketones has not been reported so far. Chiral 1,5-diketones are
among the most important synthetic feedstocks for constructing
valuable molecules including chiral cyclohexenones.11 Recently,
chiral 6-aryl-2,6-hexanediione derivatives were obtained by kinetic
resolution of racemic compounds by the Luo and Cheng group.12
To the best of our knowledge, there is no report for the catalytic
enantioselective synthesis of 6-aryl-2,6-hexanediione derivatives.
As part of a research program related to the development of
synthetic methods for the enantioselective construction of
stereogenic carbon centers,13 we recently reported the enantiose-
lective Michael additions of active methylenes and methines.14
Herein, we wish to describe the direct enantioselective decarbox-
ylative Michael addition of b-ketoacids to a,b-unsaturated ketones
catalyzed by chiral primary amine organocatalysts.
To determine suitable reaction conditions for the catalytic
enantioselective decarboxylative Michael addition reaction of
b-ketoacids, we initially investigated the reaction mechanisms
with benzoylacetic acid (1a) and (E) 4-phenylbut-3-en-2-one (2a) in
the presence of 20 mol% of catalyst in toluene at room
temperature. We first examined the impact of the structure of
catalysts I–VII (Fig. 1) on the enantioselectivities (3–83% ee,
Table 1, entries 1–7). The binaphthyl-modified primary amine
catalyst VI and thiourea organocatalyst VII were less effective (3
and 35% ee, Table 1, entries 6 and 7), whereas the cinchona-
alkaloid-derived organocatalyst I–V effectively promoted the
addition reaction in high yields and with high enantioselectivities
(61–83% ee, Table 1, entries 1–5). The best results were obtained
with catalyst III (83% ee, Table 1, entry 3). Different solvents were
then tested in the presence of 20 mol% of catalyst III together with
benzoylacetic acid (1a) and (E) 4-phenyl-3-en-2-one (2a) in order to
improve the selectivity of the reaction further. A survey of the
reaction media indicated that many common solvents such as
THF, dichloromethane, diethyl ether, ethyl acetate, acetonitrile,
and xylene (Table 1, entries 3 and 8–13) were well tolerated in this
conjugate addition without significant decreases in enantioselec-
tivity. Among the solvents probed, the best results (79% yield and
83% ee) were achieved when the reaction was conducted in
toluene (Table 1, entry 3). Based on the exploratory studies, we
The Michael addition reaction is one of the most important and
powerful methods for the formation of C–C bonds in organic
synthesis,1 and the development of an asymmetric version of this
reaction has been a subject of intense study.2 The organocatalytic
asymmetric version of this reaction can afford key chiral
intermediates from the reaction of a series of Michael donors
and acceptors.3,4 Among them, the enantioselective organocataly-
tic conjugate addition reaction of 1,3-dicarbonyl compounds to
a,b-unsaturated carbonyl compounds represents a direct and
highly appealing approach to chiral 1,5-dicarbonyl compounds
that are versatile intermediates for further transformations in
organic synthesis.5 Extensive studies have been devoted to the
development of asymmetric conjugate addition of 1,3-dicarbonyl
compounds to Michael acceptors.6 Particularly, a number of
reactions of b-keto esters as carbon nucleophiles have been
reported.7 Recently, the enantioselective decarboxylative additions
of malonic acid half-thioesters as ester enolate equivalents have
received much attention.8 Although a number of reactions of
malonic acid half-thioesters as carbon nucleophiles to various
electrophiles have been reported,9 the corresponding b-ketoacids
have received relatively little attention as carbon nucleophiles.
There are a few reported examples of the catalytic enantioselective
decarboxylative reactions of b-ketoacids such as Michael addition
to nitroalkenes in the presence of chiral Ni(II) complexes and
aldol-type reactions with trifluoromethyl ketones and isatins using
organocatalysts.10 However, the catalytic enantioselective decar-
Department of Chemistry, Soonchunhyang University, 22 Soonchunhyang-Ro, Asan,
Chungnam 336-745, Korea. E-mail: dyoung@sch.ac.kr; Fax: +82-41-530-1247;
Tel: +82-41-530-1244
Electronic supplementary information (ESI) available: Experimental details and
3
characterisation data. See DOI: 10.1039/c2ra21945j
1332 | RSC Adv., 2013, 3, 1332–1335
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Table 1 Optimization of the reaction conditionsa
Entry
Cat.
Solvent
Time (h)
Yield (%)b
ee (%)c
1
2
3
4
5
6
7
8
I
II
III
IV
V
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
THF
16
5
5
5
5
5
5
5
5
5
5
5
5
8
8
8
1
1
1
1
3
5
13
9
74
71
79
72
78
63
51
57
75
71
77
70
70
62
64
64
84
92
79
84
88
86
67
61
73 (S)
61 (S)
83 (R)
81 (R)
81 (R)
3 (R)
VI
VII
III
III
III
III
III
III
III
III
III
III
III
III
III
III
III
III
I
35 (R)
77 (R)
71 (R)
69 (R)
67 (R)
51 (R)
81 (R)
51 (R)
59 (R)
77 (R)
95 (R)
89 (R)
85 (R)
85 (R)
95 (R)
95 (R)
80 (R)
94 (S)
9
CH2Cl2
Et2O
10
11
12
13
14d
15e
16f
17g
18h
19i
20j
21k
22l
23m
24g
EtOAc
MeCN
p-xylene
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
Fig. 1 Structures of chiral organocatalysts.
examined the reactivity and selectivity with organocatalyst III in
the presence of different acids, such as trifluoroacetic acid,
p-toluenesulfonic acid, and amino acid derivatives as additives
(entries 14–20). Among the additives used, the best results (84%
yield and 95% ee) were achieved when the reaction was conducted
in L-phenylglycine (entry 17). The present catalytic system tolerates
catalyst loading down to 10 or 5 mol% without compromising the
yield or the enantioselectivity (Table 1, entries 21–23). In the
presence of catalyst I and L-phenylglycine, the Michael addition of
benzoylacetic acid (1a) to (E) 4-phenylbut-3-en-2-one (2a) afforded
the opposite enantiomer (S)-3a with moderate yield and excellent
enantioselectivity (94% ee, Table 1, entry 24).
To examine the generality of the catalytic enantioselective
decarboxylative Michael addition reaction of the benzoylacetic acid
derivatives 1 by using chiral primary amine organocatalyst III, we
studied the Michael addition of various benzoylacetic acid
derivatives 1 with (E) 4-phenylbut-3-en-2-one (2a). As seen from
the results summarized in Table 2, the corresponding chiral 1,5-
diketones 3a–f were obtained in excellent yields and with excellent
enantioselectivities. A range of electron-donating and electron-
withdrawing substitutions on the aryl ring of the benzoylacetic
acid derivatives 1b–e provided reaction products in high yields and
excellent enantioselectivities (90–97% ee, Table 2, entries 1–5). The
naphthyl-substituted b-ketoacid 1f provided the products with
high selectivity (81% ee, Table 2, entry 6).
a
Reaction conditions: benzoylacetic acid (1a, 0.24 mmol), (E)
4-phenylbut-3-en-2-one (2a, 0.2 mmol) catalyst (0.04 mmol) at room
temperature. Isolated yield. Enantiopurity was determined by
b
c
d
HPLC analysis using chiralpak IC column. 40 mol% TFA loading.
e
f
g
40 mol% p-TsOH loading. 40 mol% D-tartaric acid loading. 40
h
mol% L-phenylglycine loading. 40 mol% N-Boc- L-phenylglycine
loading. 40 mol% L-phenylalanine loading. 40 mol% L-histidine
i
j
k
loading. 10 mol% catalyst and 20 mol% L-phenylglycine loading.
l
m
5 mol% catalyst and 10 mol% L-phenylglycine loading. 2.5 mol%
catalyst and 5 mol% L-phenylglycine loading.
Table 2 Variation of the benzoylacetic acids 1a
With the optimal reaction conditions in hand, we then carried
on evaluating the generality of this protocol. The results of a
representative selection of a,b-unsaturated ketones for the
conjugate addition reaction are summarized in Table 3. A range
of electron-donating and electron-withdrawing substituents on the
aryl ring of the (E) 4-arylbut-3-en-2-one 2b–d provided the reaction
products in high yields (78–90%) and with excellent enantioselec-
tivities (91% ee, Table 3, entries 1–3). The heteroaryl- and
naphthyl-substituted a,b-unsaturated ketones 2e–f provided the
products with high selectivities (Table 3, entries 4 and 5).
Furthermore, (E) 5-phenylpent-4-en-3-one (2g), (E) hex-4-en-3-one
(2h), and cyclohexenone (2i) were also effective substrates for the
process (entries 6–8). The absolute configuration of the adducts 3
Entry
1, Ar
Time (h)
Yield (%)b
ee (%)c
1
2
1a, Ph
5
5
6
15
15
6
3a, 86
3b, 79
3c, 80
3d, 84
3e, 87
3f, 86
95 (R)
93
90
97
90 (R)
81
1b, 4-MeC6H4
1c, 4-MeOC6H4
1d, 2-ClC6H4
1e, 4-BrC6H4
1f, 2-naphthyl
3
4d
5d
6
a
Reaction conditions: b-ketoacids (1, 0.24 mmol), (E) 4-phenylbut-3-
en-2-one (2a, 0.2 mmol), catalyst (III, 0.04 mmol), and L-
b
phenylglycine (0.08 mmol) at room temperature. Isolated yield.
c
Enantiopurity was determined by HPLC analysis using chiralpak IC
(for 3a and 3b), IA (for 3c, 3d, and 3f), and chiralcel OD-H (for 3e)
columns. This reaction carried out using catalyst IV at 0.025 M
solution in THF/PhMe (1 : 1).
d
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Table 3 Variation of the a,b-unsaturated ketones 2a
Entry
2
Time (h)
Yield (%)b
ee (%)c
1c
2
2b
2c
2d
2e
2f
2g
2h
2i
13
4
13
6
3
5
3g, 88
3h, 90
3i, 78
3j, 82
3k, 89
3l, 78
3m, 76
3n, 78
91 (R)
91
91 (R)
95
93
97
3c
4d
5
6
7d
8e
15
6
95
91
a
Reaction conditions: benzoylacetic acid (1a, 0.24 mmol),
a,b-unsaturated ketones (2, 0.2 mmol), catalyst (III, 0.04 mmol), and
b
L-phenylglycine (0.08 mmol) at room temperature. Isolated yield.
Scheme 1 Possible reaction mechanism.
c
Enantiopurity was determined by HPLC analysis using chiralpak IA
d
(for 3g–j), IC (for 3l–n), and AS-H (for 3k) columns. This reaction
e
carried out at 0.025 M solution in PhMe. This reaction carried out
using catalyst IV.
Acknowledgements
This research was supported by the Soonchunhyang University
Research Fund (No. 20110689).
were determined for some derivatives by comparison of their
optical and HPLC properties with literature values.12
To find some evidence for the reaction mechanism, we
evaluated the reactivity of an analogous reaction and the stability
of benzoylacetic acid 1a under reaction conditions. The Michael
addition of b-keto ester 4 to a,b-unsaturated ketone 2a was
conducted in the presence of catalyst III (20 mol%) and L-
phenylglycine (40 mol%), and the Michael adduct 5 was not found
(Scheme 1, eqn 1). When 1a was exposed to catalyst III (10 mol%)
in toluene for 4 h, acetophenone 6 was isolated quantitatively
(Scheme 1, eqn 2). Based on the above results, we propose a
plausible mechanism as described in Scheme 1. The tertiary
amine of catalyst III deprotonates benzoylacetic acid 1a, followed
by the decarboxylation to afford the enol intermediate 7. The
enantioselective Michael addition of an enol to the iminium
intermediate 8 afforded the chiral 1,5-diketone 3a.
In conclusion, we have developed a highly efficient catalytic
enantioselective decarboxylative Michael addition reaction of
benzoylacetic acids 1 to a,b-unsaturated ketones 2 using a
cinchonidine-derived chiral primary amine organocatalyst. The
desired 1,5-diketones were obtained in good to high yields, and
excellent enantioselectivities (up to 97% ee) were observed for all
the substrates examined in this work. We believe that the present
method provides the first practical entry for the preparation of
chiral 6-aryl-2,6-hexanedione derivatives. Further study of this
catalytic enantioselective decarboxylative addition reaction of
b-ketoacids with various carbon electrophiles is in progress.
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