A DBFOX-Ph-based combinatorial catalyst for enantioselective fluorination of aryl acetyl and 3-butenoyl thiazolidinones

Full text of DOI:10.1002/asia.200900164

DOI: 10.1002/asia.200900164
A DBFOX-Ph-Based Combinatorial Catalyst for Enantioselective
Fluorination of Aryl Acetyl and 3-Butenoyl Thiazolidinones
Dhande Sudhakar Reddy,[a] Norio Shibata,*[a] Takao Horikawa,[a] Satoru Suzuki,[a]
Shuichi Nakamura,[a] Takeshi Toru,[a] and Motoo Shiro[b]
Organic compounds that contain one or more fluorine
atoms are ideal drug candidates. Approximately 20% of
drugs and 40% of agrochemicals on the market are estimat-
ed to contain fluorine, and it is no exaggeration to say that
fluorine-containing molecules cross the interface between
chemistry and biology.[1] Therefore, the development of effi-
cient methodologies for the preparation of fluoro-organic
compounds has a tremendous impact on our quality of life.
Especially, the research of new asymmetric processes that
permit chiral fluoro-organic compounds is of foremost im-
portance and is an extremely competitive and dynamic area
of organic chemistry.[2] Enantioselective electrophilic fluori-
nation promoted by metal complexes and organocatalysts is
lower enantioselectivities were obtained, up to 78% ee.[9]
Just recently, Lectka and co-workers reported a smart ap-
proach for the preparation of fluorinated aryl acetic acid de-
rivatives in high enantioselectivities by cinchona alkaloids
via dual metal-ketene enolates.[10] We disclose herein that
the enantioselectivity of our previous DBFOX-Ph/catalyzed
fluorination of 1 has been dramatically improved to 99% ee
by the help of a catalytic amount of hexafluoroisopropanol
(HFIP; Scheme 1). This combinatorial catalytic system is
further applicable for the first catalytic asymmetric fluorina-
tion of aryl-3-butenoyl derivative 3. The enantioselective
chlorination of 1 was also realized by the combinatorial cat-
alyst with high enantioselectivity for the first time.
À
expanding into one of the most effective strategies for C F
Our work began with the aim of further improving the
catalytic asymmetric fluorination process achieved by So-
deoka et al. (up to 88% ee)[8] and by our group (up to
78% ee).[9] In fact, the most intriguing question from both
studies is: Why are these catalysts not resulting in higher ee
values of 2, taking into account the rational three-dimen-
sional transition states proposed in their original papers?
The proton between the aromatic group, the fluorine atom,
and the amide in 2 should be highly acidic, and racemization
bond construction on chiral centers.[3] It provides, in fact, a
straightforward approach to the asymmetric synthesis of flu-
orinated b-ketoesters, b-ketophosphates, cyanoacetates, ox-
indoles, aldehydes, and malonates.[4–6] However, the enantio-
selective fluorination of aryl acetate derivatives is still a
challenge, albeit important in drug synthesis.[7] In 2007, So-
deoka and co-workers revealed an initial elegant solution on
this subject with the enantioselective fluorination of 3-(2-ar-
ylacetyl)-2-thiazolidinones 1 using binap-based catalyst, and
enantioselectivities with ee values up to 88% were ach-
ieved.[8] We also attempted a similar enantioselective fluori-
nation of 1 by DBFOX-Ph/metal complexes; however,
[a] D. S. Reddy, Prof. N. Shibata, T. Horikawa, S. Suzuki,
Dr. S. Nakamura, Prof. T. Toru
Department of Frontier Materials
Graduate School of Engineering
Nagoya Institute of Technology
Gokiso, Showa-ku, Nagoya 466-8555 (Japan)
Fax : (+81)52-735-5442
E-mail: nozshiba@nitech.ac.jp
[b] Dr. M. Shiro
Rigaku Corporation
3-9-12 Mastubara-cho, Akishima, Tokyo 196-8666 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.200900164.
Scheme 1. DBFOX-Ph/NiII in asymmetric catalysis for fluorination and
chlorination reactions of aryl acetyl and 3-butenoyl thiazolidinones.
Chem. Asian J. 2009, 4, 1411 – 1415
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1411

COMMUNICATION
processes might be involved during the fluorination reac-
tion. However, this possibility was ruled out since no race-
mization was observed when 2-fluoro-2-phenyl acetyl thiazo-
lidin-2-one (2a) was stirred under the fluorination condi-
tions described in our previous paper.[9] We thus concluded
that it is presumably due to the low activity of our DBFOX-
Ph/NiII catalyst system, which requires higher reaction tem-
peratures. We anticipated that the addition of external acti-
vators might lead to higher reaction efficiency even at lower
temperature. A variety of additives was evaluated for the
DBFOX-Ph/NiII-catalyzed enantioselective fluorination of
2-phenyl acetyl thiazolidin-2-one (1a) with N-fluorobenze-
nesulfonimide (NFSI; Table 1). Under our previously re-
tioselectivity and yield with synthetically useful levels (88%
yield, 80% ee, Table 1, entry 8). Finally, the most useful im-
provement both in yield and enantioselectivity are given in
Table 1, entry 9, whereby the reaction was performed in the
presence of a catalytic amount of HFIP, giving 2a in 95%
yield with 86% ee (Table 1, entry 9). Encouraged by this
result, we optimized the reaction conditions based on the
use of HFIP. The best conditions were found to be the use
of two equivalents of 2,6-lutidine and 0.3 equivalents of
HFIP at À608C, which afforded 2a in 98% ee and 91%
yield of isolated product (Table 1, entry 13). Pentafluorophe-
nol and iPrOH also afforded 2a with high enantioselectivity,
but at a low conversion even for longer reaction times
(Table 1, entries 15,16). The absolute configuration of 2a
was determined by comparison with the optical rotation and
HPLC analysis in the literature.[8,9]
Having obtained an optimized protocol for the catalytic
asymmetric fluorination, we next evaluated the scope of the
substrate (Table 2). Substrates having electron-donating and
-withdrawing groups at the meta or para position of the ben-
zene ring were found to be workable substrates in the a-flu-
orination products, affording high yields of 2 with excellent
enantioselectivities of 94–99% ee (Table 2, entries 2–7). Aryl
groups with halogen substitution also afforded high yields
with excellent enantioselectivities (94–99% ee, Table 2, en-
tries 8–10). The reactions of the sterically demanding naph-
thyl-substituted substrates 1i and 1j also gave desired prod-
ucts 2i and 2j in excellent yields with high enantioselectivi-
ties (92–99% ee, Table 2, entries 11–13). In most cases,
99% ee could be achieved when the reaction was performed
at À808C, although a longer reaction time is generally re-
quired (Table 2, entries 5, 10, and 13).
ported conditions (NiACTHNUTRGNEUNG(ClO4)2/2,6-lutidine/08C) the enantio-
selectivity was 74% ee (Table 1, entry 1).[9] In the absence of
2,6-lutidine, there was not a significant deference in enantio-
selectivity (69% ee), but the reaction did not go to comple-
tion even after six days (Table 1, entry 2).[9] The ee value
was improved to 79% ee at a lower temperature (À208C),
but resulted in a decrease in yield (24%, Table 1, entry 3).[9]
On the basis of observations by Sodeoka et al.,[8] the addi-
tion of Me3SiOTf or Et3SiOTf was examined. While the
enantioselectivities improved slightly, the obtained chemical
yields were again low (41–57%, Table 1, entries 4 and 5).
Additives of BF3·OEt2 and benzoic acid were entirely un-
productive (no reaction, Table 1, entries 6 and 7). Interest-
ingly, trifluoroethanol gave very good results for both enan-
Table 1. Enantioselective fluorination of phenyl acetyl thiazolidinone
(1a): optimization of reaction conditions[a]
Table 2. Catalytic enantioselective fluorination of aryl acetyl thiazolidi-
nones 1.[a]
Entry 2,6-lutidine
[equiv]
Additive
[equiv]
T
t
Yield
ee
[8C] [h] [%][b]
[%][c]
1
2
3
4
1.0
0
1.0
1.0
–
–
–
0
20 90
74
69
79
82
20 144 42
À20
96 24
62 41
Me3SiOTf
(1.0)
Et3SiOTf
(0.75)
À20
À20
Entry
1
Ar
2
t [days]
Yield [%][b]
ee [%][c]
5
1.0
62 57
83
1
2
3
4
1a
1b
1c
1d
1d
1e
1 f
1g
1h
1h
1i
Ph
2a
2b
2c
2d
2d
2e
2 f
2g
2h
2h
2i
2
3
2
4
7
3
4
3
3
7
7
3
7
91
93
93
93
80
90
94
90
96
87
87
94
70
98
96
96
98
99
96
94
94
93
99
92
95
99
6
7
1.0
1.0
BF3·OEt2 (1.0) À20
48 n.r.
48 n.r.
–
–
C6H4-m-OMe
C6H4-p-OMe
C6H4-m-Me
C6H4-m-Me
C6H4-p-Me
C6H4-p-CF3
C6H4-p-F
C6H4-p-Br
C6H4-p-Br
1-naphthyl
2-naphthyl
2-naphthyl
benzoic acid
(0.2)
À20
À20
8
1.0
CF3CH2OH
(0.2)
20 88
80
5[d]
6
9
1.0
1.0
1.0
1.0
2.0
2.0
2.0
2.0
HFIP (0.2)
HFIP (0.1)
HFIP (0.1)
HFIP (0.2)
HFIP (0.3)
HFIP (0.3)
iPrOH (0.3)
PFP (0.3)
À20
À50
À80
À80
À60
0
24 95
55 90
80 78
80 74
48 91
86
90
94
92
98
88
93
85
7
8
9
10
11
12
13
14
15
16
10[d]
11
12
13[d]
8
94
1j
1j
2j
2j
À60
96 46
96 25
À60
[a] The reaction of 1 with NFSI (1.2 equiv) was carried out in the pres-
ence of Ni(ClO4)2/6H2O (10 mol%), (R,R)-DBFOX-Ph (11 mol%), 2,6-
lutidine (2.0 equiv), HFIP (0.3 equiv), and M.S. 4ꢁ in CH2Cl2 at À608C.
[b] Yield of isolated product. [c] Determined by chiral HPLC analysis.
[d] Reaction was performed at À808C.
[a] The reaction of 1a with NFSI (1.2 equiv) was carried out in the pres-
ence of additives, Ni(ClO4)2/6H2O (10 mol%), (R,R)-DBFOX-Ph
(11 mol%), 2,6-lutidine, and M.S. 4ꢁ in CH2Cl2. [b] Yield of isolated
ACHTUNGTRENNUNG
G
product. [c] Determined by chiral HPLC analysis. n.r.=no reaction.
1412
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Chem. Asian J. 2009, 4, 1411 – 1415

A DBFOX-Ph-Based Combinatorial Catalyst for Enantioselective Fluorination
The effect of HFIP is presumably an improvement of cat-
alytic turnover by protonation of a metal–oxygen bond fol-
lowed by release of the products in the transition state,[5f]
and 2,6-lutidine might assist the enolization of the sub-
strates,[8,9] because the enantioselection observed is essen-
tially the same even in the absence of HFIP and/or 2,6-luti-
dine (Table 1, entries 1–3, 9, 15, and 16); further studies on
this are required.[11]
ray crystallographic analysis of 4g,[12a] which showed the ste-
reogenic carbon center formed to be of R stereochemistry
(Figure 1).
As a further demonstration of the utility of this catalytic
system for enantioselective fluorination, we decided to carry
out the enantioselective fluorination of 3-butenoyl deriva-
tives 3, which has no precedent in the literature, except for
chiral axially-induced diastereoselective fluorination.[7] The
results are shown in Table 3. First, phenyl methyl substituted
Figure 1. X-ray crystal structure of 4g. Thermal ellipsoids are shown at
the 50% probability level.
On the basis of the absolute configurations of the fluori-
nated products 2 and 4, and in light of the reported X-ray
structure of the (R,R)-DBFOX-Ph/NiII complex,[12b–d] we as-
sumed octahedral complexes coordinated with a water mole-
cule for DBFOX-Ph/NiII/1a and DBFOX-Ph/NiII/3b opti-
mized using PM3 (Spartan06) as shown in Figure 2a,b and
2c,d. In the complex, the Si face of 1 or 3 is shielded by one
of the phenyl groups of the DBFOX-Ph ligand so that the
NFSI molecule approaches from the Re face of the sub-
strates (Figure 2).
This combinatorial catalytic system is quite general. It was
also successfully adapted to the enantioselective fluorination
of heteroaryl substrate 5. Both non-, methoxy-, methyl-, and
bromo-substituted indole derivatives 5a–d underwent fluori-
nation smoothly to provide the corresponding products 6a–
Table 3. Catalytic enantioselective fluorination of aryl-3-butenoyl thiazo-
lidinones 3.[a]
Entry
Ar (3)
R
T [8C] t [h] Yield [%][b] ee [%][c]
1
2
3
4
5
6
7
8
Ph (3a)
Ph (3a)
Ph (3a)
Ph (3b)
Ph (3b)
Ph (3c)
Me À60
Me À70
Me À80
24
26
92
24
24
8
21
21
21
24
99
99
50
96
58
99
99
99
99
97
85
87
91
78
83
86
85
86
80
85
H
À60
À70
À60
H
Ph
C6H4-p-Me (3d) Me À60
C6H4-p-Cl (3e)
C6H4-p-Br (3 f)
2-naphthyl (3g)
Me À70
Me À60
Me À60
9
10
[a] The reaction of 3 with NFSI (1.2 equiv) was carried out in the pres-
ence of Ni(ClO4)2/6H2O (10 mol%), (R,R)-DBFOX-Ph (11 mol%), 2,6-
ACHTUNGTRENNUNG
lutidine (2.0 equiv), HFIP (0.3 equiv), and M.S. 4ꢁ in CH2Cl2. [b] Yield
of isolated product. [c] Determined by chiral HPLC analysis.
3a was used for the fluorination reaction using DBFOX-Ph/
NiII-2,6-lutidine-HFIP systems at À60 to À708C, affording
the product 4a in excellent yields with 85–87% ee (Table 3,
entries 1,2). Lowering the temperature to À808C resulted in
higher enantioselectivity with 91% ee, but the reaction did
not go to completion (Table 3, entry 3). Under these reac-
tion conditions, we next examined the scope and limitations
of this reaction using a variety of compounds 3 bearing dif-
ferent substituents on the benzene ring and olefin moiety.
The results of these experiments are summarized in Table 3,
entries 4–10. Both phenyl-substituted and nonsubstituted
olefins 3b,c are suitable substrates in this reaction, provid-
ing the corresponding fluorinated products 4b,c in high
yields with high enantioselectivities (78–86% ee, entries 4–
6). The substrates with methyl, chloro, and bromo substitu-
ents at the benzene ring (3d–f) and the bulkier naphthyl-
substituted substrate 3g were also successfully transformed
into the corresponding fluorinated products 4d–g with high
enantioselectivities (80–86% ee, entries 7–10). The absolute
configuration of the a-fluorinated adduct 4 was solved by X-
Figure 2. a) A CPK model of DBFOX-Ph/NiII/1a optimized using PM3
(Spartan06) b) A proposed transition-state structure for the DBFOX-Ph/
NiII-catalyzed enantioselective fluorination of 1a to (R)-2a. c) A CPK
model of DBFOX-Ph/NiII/3b optimized using PM3. d) A proposed transi-
tion-state structure for the DBFOX-Ph/NiII-catalyzed enantioselective
fluorination of 3b to (R)-4b.
Chem. Asian J. 2009, 4, 1411 – 1415
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www.chemasianj.org
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N. Shibata et al.
COMMUNICATION
tions of Carbon Resources” from the Ministry of Education, Culture,
Sports, Science and Technology Japan. We thank Kishida Chemical Co.,
Ltd., Japan for the generous gift of (R,R)-DBFOX-Ph.
d with good to high yields and excellent enantioselectivities
of 97–99% ee (Scheme 2).
Finally, the versatility of the combinatorial catalyst was
provided by enantioselective chlorination of aryl acetyl thia-
Keywords: asymmetric
catalysis
·
chlorination
·
enantioselectivity · fluorination · nickel
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Scheme 2. DBFOX-Ph/NiII-catalyzed enantioselective fluorination of
indole acetyl thiazolidinones 5.
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as the chlorination reagent (Scheme 3).
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Scheme 3. DBFOX-Ph/NiII-catalyzed enantioselective chlorination of
phenyl acetyl thiazolidinone (1a).
In conclusion, we have developed a catalytic asymmetric
fluorination reaction for aryl acetyl and 3-butenoyl thiazoli-
dinones, achieved with extremely high enantioselectivity by
the use of the DBFOX-Ph/NiII/HFIP/2,6-lutidine combina-
tion, which provides an effective entry to chiral a-fluorinat-
ed compounds. The products 2 and 4 should be useful for
drug development,[13] and some of these products have al-
ready been transformed into chiral monofluorinated acids,
esters, and ketones.[7,8] The catalyst system is also applicable
to enantioselective a-chlorination of aryl acetyl thiazolidi-
nones. The a-chlorinated compounds with an aryl acetyl ox-
azolidinone-type skeleton have so far been synthesized by
classical diastereoselective reactions via chiral auxiliaries.[14]
Our combinatorial catalyst would provide an efficient alter-
nate route to chiral a-fluoro and chloro ester equivalents
and is expected to find more applications in asymmetric re-
actions in general, such as amination and sulfenylation reac-
tions.
Acknowledgements
This work was supported by KAKENHI and by a Grant-in-Aid for Sci-
entific Research on Priority Areas “Advanced Molecular Transforma-
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A DBFOX-Ph-Based Combinatorial Catalyst for Enantioselective Fluorination
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Received: May 6, 2009
Published online: June 25, 2009
Chem. Asian J. 2009, 4, 1411 – 1415
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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