Catalytic enantioselective arylation of glyoxylate with arylsilanes: Practical synthesis of optically active mandelic acid derivatives

Article abstract of DOI:10.1002/asia.201000522

Just a little does the trick! The catalytic enantioselective arylation using chiral dicationic palladium complexes provides a reliable and useful access to enantiomerically enriched mandelic-acid derivatives. Significantly low catalyst loading (down to 0.002 mol%) as well as easy catalyst handling are the advantage of this practical method.

Full text of DOI:10.1002/asia.201000522

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DOI: 10.1002/asia.201000522
Catalytic Enantioselective Arylation of Glyoxylate with Arylsilanes: Practical
Synthesis of Optically Active Mandelic Acid Derivatives
Kohsuke Aikawa, Yuta Hioki, and Koichi Mikami*[a]
¯
Enantiomerically enriched mandelic acid derivatives have
been broadly utilized as versatile chiral building blocks for
the synthesis of numerous natural and biologically active
compounds.[1] While several synthetic methods for these
compounds have been developed to date,[2–5] the catalytic
enantioselective Friedel–Crafts (F-C) alkylation reaction of
glyoxylate[6] with aromatic or heteroaromatic rings is one of
Scheme 1. Catalytic enantioselective arylation using chiral dicationic Pd
the most attractive and useful methods for their practical
complex as Lewis acid catalyst.
synthesis.[7,8] On the catalytic enantioselective F-C alkylation
reaction using chiral Cu[7a,d] or Ti[7b,c] catalysts, high yield and
enantioselectivity have been achieved to efficiently synthe-
size mandelic acid derivatives. However, these catalytic re-
actions proceed only with highly reactive substrates, such as
N-methylated aniline, indoline, and indole derivatives, and
hence limit their synthetic application. Additionally, it is in-
herently difficult to control regioselectivity (p- or o-orienta-
tion) of the F-C alkylation reaction by direct use of aromatic
compounds, such as nucleophiles.[6–8] Herein, we describe a
highly regiospecific and practical catalytic enantioselective
arylation of ethyl glyoxylate with various arylsilanes using
chiral dicationic Pd complexes for the efficient synthesis of
enantiomerically enriched mandelic acid derivatives
(Scheme 1). Silyl groups of arylsilanes, which have been
rarely utilized owing to a lack of reactivity without activa-
tion reagents such as fluoride, have been employed as pro-
tective groups of aromatic compounds.[7a,8c] In sharp con-
trast, we anticipated that arylsilanes, which are easily syn-
thesized, storable, air- and moisture-stable, might increase
the enantioselectivity and reactivity by the steric effect and
the stabilizing effect of b-cation to control p- or o-orienta-
tion, respectively.[9] The highly reactive dicationic Pd com-
plex as a chiral Lewis acid catalyst is easily synthesized, air-
and moisture-stable.[10] Recently, the catalytic enantioselec-
tive arylation of carbonyl compounds with arylsilanes via an
aryl-copper intermediate was reported by Shibasaki and
Kanai.[11] However, to the best of our knowledge, the cata-
lytic enantioselective direct arylation with arylsilanes by a
Lewis acid catalyst without need for any additive has never
been reported.
We have already reported chiral dicationic Pd complex-
catalyzed highly enantioselective alkenyl-, dienyl-, and trien-
ylation employing the corresponding silyl compounds as nu-
cleophiles.[12,13] As an initial experiment under a similar con-
dition, the treatment of ethyl glyoxylate 2 with arylsilane 1a
in the presence of 5 mol% of various chiral dicationic Pd
complexes, which are generated in situ by adding slight
excess of AgSbF6 (2.2 equiv) to the corresponding neutral
dichloro Pd complexes,[10b,12] in dichloromethane at room
temperature for 24 h afforded the desired adduct 3a after
the desilylation with 1n HCl/THF solution (Table 1). The
effect of chiral diphosphine ligands was critical in this reac-
tion. While BINAP and tol-BINAP bearing binaphthyl-
backbone result in good-to-excellent yield and enantioselec-
tivity (83%, 98% ee and 71%, 99% ee) (entries 1–2), SYN-
PHOS, SEGPHOS, and MeO-BIPHEP bearing a biphenyl-
backbone decreased both the yield and enantioselectivity
(90, 94, and 82% ee, respectively) (entries 3–5). BDPP and
DUPHOS with central chirality further decreased the reac-
tivity and enantioselectivity (entries 6–7), and the reaction
with BINAPHANE did not proceed (entry 8). Toluene or
diethyl ether as a solvent led to lower yield owing to the
low solubility of the Pd complex (entries 9–10). In addition,
[a] Dr. K. Aikawa, Y. Hioki, Prof. Dr. K. Mikami
Department of Applied Chemistry
Tokyo Institute of Technology
O-okayama, Meguro-ku, 152-8552 (Japan)
Fax : (+81)3-5734-2776
E-mail: mikami.k.ab@m.titech.ac.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201000522.
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Chem. Asian J. 2010, 5, 2346 – 2350

Table 1. Catalytic enantioselective arylation using chiral dicationic Pd
complexes bearing diphosphine ligands.a
Entry
P-P ligand
Solvent
Yield [%][c]
ee [%][d]
1
2
3
4
5
6
7
8
BINAP
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
toluene
Et2O
83
71
68
80
42
47
24
0
46
34
75
82
98
99
90
94
82
8
33
–
98
95
98
98
tol-BINAP
SYNPHOS
SEGPHOS
MeO-BIPHEP
BDPP
DUPHOS
BINAPHANE
BINAP
9
Scheme 2. Catalytic enantioselective Friedel–Crafts alkylation reaction of
ethyl glyoxylate 2 with aromatic or heteroaromatic compounds 4–6.
10
11[a]
12[b]
BINAP
BINAP
BINAP
CH2Cl2
CH2Cl2
[a] [Pd{(S)-BINAP}ACHTUNGTRENNUNG(H2O)2]AHCUTNGTREN(NUGN SbF6)2 (aqua-complex) isolated was used
under the same conditions. [b] Reaction was examined after filtration of
AgCl salt obtained by PdCl2[(S)-BINAP] complex and 2.0 equiv of
AgSbF6. [c] Yield of isolated product. [d] Enantiomeric excess was deter-
mined by chiral HPLC analysis.
Scheme 3. Silyl effects of 1a on enantioselective arylation. [a] Reaction
temperature was 508C.
selectivity though with lower reactivity. Arylsilanes bearing
a SiACHTNURTGENN(UG OMe)3 or SiACHTUNGTRENN(UGN OEt)3 substituent resulted in decomposi-
tion under the reaction conditions without formation of the
product 3a, in sharp contrast to the high reactivity in
Hiyama cross-coupling in the presence of activating re-
agents.[15]
the use of [Pd{(S)-BINAP}ACHTNUTRGENN(UG H2O)2]ACHTUGNTRNE(NUGN SbF6)2 isolated as a
bench-stable aqua-complex[14] provided almost the same re-
sults as in entry 1 (entry 11 vs 1). Whether AgCl, which was
obtained by the reaction of PdCl2[(S)-BINAP] complex and
exactly 2.0 equivalents of AgSbF6, was filtered or not prior
to the reaction, equally high yield and enantioselectivity
were achieved (entry 12).
The scope of the Pd-catalyzed enantioselective arylation
was further extended to a variety of arylsilanes 1b–s
(Table 2). These results demonstrated wide substrate scope;
single regioisomers were produced with high enantioselec-
tivity and yield. Arylsilanes 1b–e with methoxy groups were
examined to give a high level of enantioselectivity regardless
of the substituent positions (98!99% ee) (entries 1–4). The
reaction of 1,3-dimethoxybenzene 1 f bearing a bis(trime-
thylsilyl) substituent proceeded not on the m-position but
on the o-position with almost complete enantioselectivity
(>99% ee) (entry 5). The use of arylsilanes 1g–j with benzyl-
oxy, allyloxy, silyloxy, and phenoxy groups instead of me-
thoxy groups slightly decreased the reactivity but main-
tained the high enantioselectivity (97–99% ee) (entries 6–9).
Arylsilanes with either electron-withdrawing 1k, m–n or
more sterically demanding substituents 1l, o also led to a
decrease in the reactivity but retained a high level of enan-
tioselectivity (96–99% ee) (entries 10–14). Interestingly, re-
active functional groups, such as iodide, were well-tolerated
even under the palladium catalysis (entries 7 and 10). Addi-
tionally, N,N-dimethylaniline 1p bearing a trimethylsilyl
group produced an enantiomerically enriched aminomandel-
ic acid derivative, which is a potential starting material for
various biologically active compounds,[1d,7] in a good yield
We further investigated the reactivity of anisole 4 without
a silyl group in the Friedel–Crafts alkylation reaction
(Scheme 2). Indeed, the reaction proceeded to give the de-
sired product 3a although in 35% yield together with regio-
isomer 3b in 12% yield. Both the reactivity and enantiose-
lectivity significantly decreased compared to the correspond-
ing arylsilane 1a. The reaction with either 1,3-dimethoxy-
benzene 5 or furan 6 gave only the regioisomer 3c or 3r in
moderate yield but with low enantioselectivity, respectively
(79%, 22% ee and 62%, 30% ee). These results indicate
that the trimethylsilyl group could not only increase the re-
activity and enantioselectivity but also completely control
the regioselectivity of the desired arylation.
The influence of the silyl substituent of arylsilane 1a on
the reaction was then investigated (Scheme 3). Although ar-
ylsilane with a SiiPr3 group did not undergo the reaction
owing to steric hinderance, the reaction of arylsilanes with
SiEt3 or SiMe2Ph groups preserved the high level of enantio-
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K. Mikami et al.
COMMUNICATION
Table 2. Catalytic enantioselective arylation of ethyl gyloxylate 2 with a
variety of arylsilanes 1b–s.
and high enatioselectivity (85%, 97% ee) (entry 15). Since
[16]
the remaining AgSbF6 was confirmed to catalyze the desi-
lylation of some arylsilanes,[17] we attempted to use the iso-
lated [Pd{(S)-BINAP}ACTHNUTRGENN(UG H2O)2]ACHUTNTGREN(NNGU SbF6)2 complex instead of the
in situ-activated catalyst to give good yields and high enan-
tioselectivies (entries 6–9, and 15). Heteroaromatic sub-
strates 1q–s possessed higher reactivity than aromatic ones;
the reaction with 2-trimethylsilyl substituted thiophene 1q,
furan 1r, and pyrrole 1 s could be facilitated to give excel-
lent enantioselectivity at lower temperature (98–99% ee)
(entries 16–18).
Entry
1
Ar or Het
Conditions
Yield [%][b]
82
ee [%][c]
>99
CH2Cl2,
RT, 24 h
The efficiency of this reaction is highlighted by the fact
that a high yield and enantioselectivity can be maintained
even with a lower catalyst loading (Scheme 4). The use of
CH2Cl2,
08C, 12 h
2
3
4
5
93
86
90
90
>99
>99
98
CH2Cl2,
08C, 24 h
CH2Cl2,
08C, 12 h
CH2Cl2,
ꢀ208C, 24 h
>99
CH2Cl2,
408C, 24 h
6[a]
7[a]
8[a]
71
82
79
97
99
97
CH2Cl2,
RT, 24 h
Scheme 4. Low catalyst loading on the catalyst system.
CH2Cl2,
RT, 48 h
0.1 to 0.005 mol% Pd complex under high concentration of
1r in toluene (5 or 10m) was sufficient to promote the reac-
tion with good results although a longer reaction time was
required (92%, 99% ee and 75%, 98% ee). Furthermore,
the catalyst loading could be reduced to 0.002 mol% (S/C=
50000), the desired product 3r[18] maintaining a high enan-
tioselectivity was obtained in 46% yield (TON=23000)
with 98% ee. Under a similar condition (0.1 or 0.01 mol%
in CH2Cl2), N-Boc protected pyrrole 1s also gave the corre-
sponding product 3s in a good yield with high enantioselec-
tivity (80%, 99% ee and 74%, 98% ee). In view of the in-
creasing demand for environmentally friendly catalyst sys-
tems, this arylation with significantly low catalyst loading
would be highly useful not only in laboratories but also in
industries.
CH2Cl2,
408C, 48 h
9[a]
86
97
AHCTUNGRTEG(NNUN CH2Cl)2,
408C, 24 h
10
11
12
13
86
81
69
62
99
96
97
97
CH2Cl2,
408C, 24 h
AHCTUNGTRENNUNG
AHCTUNGRTEG(NNUN CH2Cl)2,
608C, 48 h
CH2Cl2,
408C, 24 h
14
92
85
99
97
The combination of 2,5-bis(trimethysilyl)furan 7 with 3.0
equivalents of ethyl glyoxylate 2 in the presence of the Pd
catalyst (0.5 mol%) led to the two-directional adduct 8 in
84% yield with 99% ee without formation of the meso prod-
uct (Scheme 5).
CH2Cl2,
08C, 12 h
15[a]
CH2Cl2,
08C, 12 h
16
17
18
85
95
92
98
99
99
CH2Cl2,
ꢀ408C, 6 h
CH2Cl2,
ꢀ788C, 5 h
[a] [Pd{(S)-BINAP}ACHTUNGTRENNUNG(H2O)2]ACHUTGNTREN(NUGN SbF6)2 (aqua-complex) was used instead of in
situ catalyst. [b] Yield of isolated product. [c] Enantiomeric excess was
determined by chiral HPLC analyses.
Scheme 5. Two-directional reaction with 2,5-bis(trimethysilyl)furan 7.
2348
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Chem. Asian J. 2010, 5, 2346 – 2350

Catalytic Enantioselective Arylation of Glyoxylate with Arylsilanes
As silicon transfer may proceed by means of an intra- or
intermolecular process, a silyl crossover experiment was per-
formed (Scheme 6). Treatment of 0.5 equivalents of each of
the arylsilanes and ethyl glyoxylate 2 with 5 mol% of
tives. Further investigations to extend the reaction scope of
this process in organic synthesis are currently in progress.
Experimental Section
Typical procedure: To
a solution of PdCl2[(S)-BINAP] (8.0 mg,
0.01 mmol) in CH2Cl2 (2.0 mL) was added AgSbF6 (7.6 mg, 0.022 mmol)
at room temperature under argon atmosphere. After stirring for 30 min,
freshly distilled ethyl glyoxylate 2 (61.3 mg, 0.6 mmol) and arylsilane 1a
(36.1 mg, 0.2 mmol) were added to the mixture at 08C. The reaction mix-
ture was stirred at room temperature for 24 h, and then the reaction mix-
ture was directly loaded onto a silica-gel column (hexane/AcOEt=1:1)
to remove the catalyst. To a solution of the crude product in THF
(2.0 mL) was added 1n HCl (1.0 mL). After stirring for 1 h at room tem-
perature, the mixture was extracted with ether. The organic layer was
washed with brine, dried over Na2SO4, and evaporated under reduced
pressure. Purification by silica-gel chromatography (hexane/AcOEt=4:1)
gave the corresponding alcohol product (R)-3a. The enantiomeric excess
was determined by chiral HPLC analysis using CHIRALPAK AD-H
column.
Scheme 6. Crossover experiment of silyl substituents.
[Pd{(S)-BINAP}ACHTUNGTRENNUNG(H2O)2]ACHTUNGTREN(NUGN SbF6)2 complex provided only three
products, to prove the exclusive intramolecular silicon trans-
fer.[19] The alcohol product obtained in 7% yield was gener-
ated after desilylation during the course of the reaction.
Acknowledgements
1
Significantly, H and 31P NMR spectroscopic analyses by
This research was supported in part by a grant program “Collaborative
Development of Innovative Seeds” from the Japan Science and Technol-
ogy Agency. We are grateful to Takasago International Co. for providing
BINAP, tol-BINAP, and SEGPHOS.
treatment of arylsilane 1a with a stoichiometric [Pd{(S)-
BINAP}]ACHTUNGTRENNUNG(SbF6)2 complex in CD2Cl2 indicated that a well-
recognized arylpalladium intermediate through transmetala-
tion did not form. These results show that this direct aryla-
tion using arylsilanes without silyl activation reagents such
as fluoride, would be facilitated by the chiral dicationic Pd
complex employed as a Lewis acid catalyst activating the
Keywords: alkylation · arylation · lewis acids · palladium ·
silanes
carbonyl electrophile rather than the CACTHNUTRGNEUNG
(sp2)-Si bond. In a
plausible reaction mechanism, the attack of arylsilane 1a to
ethyl glyoxylate 2 through the bidentate coordination of the
Pd complex (2/Pd cat*) followed by the intramolecular
transfer of the silyl group and desilylation upon acidic work-
up lead to the desired product 3a (Scheme 7).
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enantioselective arylation reaction of ethyl glyoxylate with a
variety of arylsilanes by the chiral dicationic Pd complex to
afford a high yield and enantioselectivity. This protocol pro-
vides a reliable and useful access to enantiomerically en-
riched mandelic acid derivatives. The use of significantly
low catalyst loading up to 0.002 mol% as well as easy han-
dling are the advantages of this new method for the practi-
cal synthesis of highly enantioenriched mandelic acid deriva-
Chem. Asian J. 2010, 5, 2346 – 2350
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
2349

K. Mikami et al.
COMMUNICATION
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Received: July 28, 2010
Published online: September 13, 2010
2350
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