Catalytic asymmetric synthesis of 3,3′-diaryloxindoles as triarylmethanes with a chiral all-carbon quaternary center: Phase-transfer- catalyzed snar reaction

Article abstract of DOI:10.1002/anie.201403046

Catalytic asymmetric synthesis of unsymmetrical triarylmethanes with a chiral all-carbon quaternary center was achieved by using a chiral bifunctional quaternary phosphonium bromide catalyst in the S_NAr reaction of 3-aryloxindoles under phase-transfer conditions. The presence of a urea moiety in the chiral phase-transfer catalyst was important for obtaining high enantioselectivity in this reaction.

Full text of DOI:10.1002/anie.201403046

Angewandte
Chemie
DOI: 10.1002/anie.201403046
Asymmetric Catalysis
Catalytic Asymmetric Synthesis of 3,3’-Diaryloxindoles as
Triarylmethanes with a Chiral All-Carbon Quaternary Center:
Phase-Transfer-Catalyzed SNAr Reaction**
Seiji Shirakawa, Kenta Koga, Takashi Tokuda, Kenichiro Yamamoto, and Keiji Maruoka*
Abstract: Catalytic asymmetric synthesis of unsymmetrical
triarylmethanes with a chiral all-carbon quaternary center was
achieved by using a chiral bifunctional quaternary phospho-
nium bromide catalyst in the SNAr reaction of 3-aryloxindoles
under phase-transfer conditions. The presence of a urea moiety
in the chiral phase-transfer catalyst was important for obtain-
ing high enantioselectivity in this reaction.
T
he development of efficient synthetic methods for triaryl-
methanes is an important task for organic chemistry because
of the utility of these compounds in materials science[1] and
medicinal chemistry,[2,3] and a number of methods have been
reported to date.[4,5] In particular, research in catalytic
asymmetric methods for the synthesis of chiral, unsymmet-
rical triarylmethanes has attracted much attention in recent
years.[6,7] Although several catalytic asymmetric methods for
the synthesis of chiral triarylmethanes of type Awith a tertiary
carbon center have been reported (Scheme 1),[6] only one
general method for the synthesis of compounds of type B,
possessing a quaternary carbon center, has been developed
and involves a rhodium-catalyzed asymmetric addition of
arylboroxines to diaryl ketimines to produce chiral (triaryl)-
methylamines (Scheme 1a).[7a–c] In this context, we are
interested in the development of a new approach to the
synthesis of chiral triarylmethanes of type B. Herein we report
a valuable example of the catalytic asymmetric synthesis of
triarylmethanes, possessing a chiral all-carbon quaternary
center, by phase-transfer-catalyzed SNAr reaction of 3-aryl-
oxindoles (1),[8–10] and it results in chiral 3,3’-diaryloxindoles
(3) which are biologically interesting compounds (Sche-
me 1b).[3]
Scheme 1. Catalytic asymmetric synthesis of triarylmethanes with a qua-
ternary carbon center. Boc=tert-butoxycarbonyl, PTC=phase-transfer-
catalyst.
under the influence of the bifunctional catalyst (S)-4, which
possesses a hydroxy group, at room temperature for 24 hours
afforded the arylation product 3a in high yield with low
enantioselectivity (entry 1). Switching the catalyst to the
benzamide-substituted (S)-5 slightly improved the enantio-
selectivity (entry 2), and the benzenesulfonamide-substituted
catalyst (S)-6 gave the product 3a in low yield with low
enantioselectivity (entry 3). To further improve the enantio-
selectivity of this reaction, we next examined the newly
designed bifuctional phosphonium bromides (S)-7, which
possess a urea group.[14] It was expected that the urea moiety
of (S)-7 interacts with the nitroarene 2 through two hydrogen
bonds, thus giving rise to a well-organized transition state
which provides high stereocontrol (Figure 1).[15] Based on this
hypothesis, (S)-7a was utilized in this reaction, and to our
delight, 3a was obtained in moderate enantioselectivity
(entry 4). Further tuning of the catalyst structure of led to
We first examined the screening of chiral bifunctional
quaternary phosphonium bromides [(S)-4–6] as phase-trans-
fer catalysts,[11,12] which were developed recently by our
group,[13] for the asymmetric SNAr reaction of the 3-phenyl-
oxindole 1a (Table 1). Attempted reaction of 1a and 2,4-
dinitrofluorobenzene (2a) with solid KHCO3 in toluene
[*] Dr. S. Shirakawa, K. Koga, T. Tokuda, K. Yamamoto,
Prof. Dr. K. Maruoka
Laboratory of Synthetic Organic Chemistry and Special Laboratory
of Organocatalytic Chemistry, Department of Chemistry, Graduate
School of Science, Kyoto University
Sakyo, Kyoto 606-8502 (Japan)
E-mail: maruoka@kuchem.kyoto-u.ac.jp
[**] This work was partially supported by a Grant-in-Aid for Scientific
Research from JSPS and MEXT (Japan).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201403046.
Figure 1. Proposed working model on the asymmetric arylation of
3-aryloxindole with bifunctional catalyst (S)-7.
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Table 1: Optimization of the reaction conditions.[a]
Entry
Catalyst
Solvent
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
(S)-4
(S)-5
(S)-6
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
CCl4
98
89
36
46
42
49
30
50
85
80
88
90
97
99
À17
35
3
(S)-7a
(S)-7b
(S)-7c
(S)-7d
(S)-7e
(S)-7 f
(S)-7e
(S)-7e
(S)-7e
(S)-7 f
(S)-7e
64
79
76
76
87
84
87
88
88
85
91
9
10
11
12
13
14[d]
Et2O
iPr2O
iPr2O
iPr2O
[a] Reaction conditions: 1a (0.050 mmol), 2a (0.10 mmol), and KHCO3
(0.075 mmol) in the presence of the chiral phase-transfer catalyst
(5 mol%) in solvent (1.0 mL) at room temperature (258C) for 24 h.
[b] Enantiomeric excess (ee) was determined by HPLC analysis using
a chiral stationary phase. [c] Yield of isolated product. [d] The reaction
was performed at À208C with K2CO3 (0.038 mmol) as a base.
Bz=benzoyl.
Scheme 2. Asymmetric arylation of the 3-aryloxindoles 1 with 2,4-
dinitrofluorobenzene (2a). [a] Run with K2CO3 (0.75 equiv) at À208C
for 24 h. [b] Run with K2CO3 (0.75 equiv) at À208C for 48 h. [c] Run
with KHCO3 (1.5 equiv) at RT for 24 h. [d] Run with KHCO3 (1.5 equiv)
at 0 8C for 48 h.
aryloxindoles (1) with 2a (Scheme 2). The introduction of
electron-donating and electron-withdrawing substituents on
both the oxindole core and the 3-aryl group uniformly gave
the products (3a–k) in good to high enantioselectivities (83–
94% ee). 3-Heteroaryl-substituted oxindole could also be
used in this reaction, thus providing the product 3l with good
enantioselectivity (82% ee).[16]
Other electron-deficient aryl fluorides could also be
employed in the SNAr reaction (Scheme 3). The asymmetric
arylation of 1a with 1-bromo-5-fluoro-2,4-dinitrobenzene and
1,5-difluoro-2,4-dinitrobenzene in the presence of (S)-7e gave
the corresponding arylation products 3m and 3n, respectively,
with high enantioselectivities. Pentafluoronitrobenzene was
also used in the reaction, thus giving the product 3o as a single
regioisomer with good enantioselectivity. The reaction with
2,4,6-trifluoropyrimidine as a heteroaryl fluoride was also
promoted by (S)-7e to afford the product 3p as a single
regioisomer with good enantioselectivity.[17] The absolute
configuration of the arylation products 3 was determined by
X-ray diffraction analysis of 3m (Figure 2).[18]
improved enantioselectivity (entries 5–9), and the arylation
product 3a was obtained with good enantioselectivity by
using the catalysts (S)-7e and (S)-7 f (entries 8 and 9). The
screening of solvents led to improved yields (entries 10–13),
and the highest yield and enantioselectivity were obtained at
a lower temperature (À208C) with K2CO3 in diisopropylether
in the presence of (S)-7e (entry 14). Notably, the reaction
with the triarylphosphine catalyst (S)-8 gave the product 3a
with low enantioselectivity (< 5% ee), that is, the quaternary
phosphonium bromide moiety of (S)-7e was essential to
obtaining high enantioselectivity.
With the effective chiral bifunctional quaternary phos-
phonium bromides (S)-7e and (S)-7 f in hand, we studied the
substrate generality of the asymmetric arylation of various 3-
2
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In summary, we have successfully developed an efficient
method for the synthesis of chiral triarylmethanes possessing
an all-carbon quaternary center by SNAr reaction of 3-
aryloxindoles under phase-transfer conditions. The reactions
were efficiently promoted by newly-designed chiral bifunc-
tional quaternary phosphonium bromides possessing a urea
moiety to give chiral triarylmethanes with good to high
enantioselectivities. Further studies will be directed toward
expansion of the reaction scope.
Received: March 6, 2014
Revised: March 28, 2014
Published online: && &&, &&&&
Keywords: arylation · asymmetric catalysis · organocatalysis ·
.
phase-transfer catalysis · synthetic methods
Scheme 3. Asymmetric arylation of 3-phenyloxindole (1a). [a] Run with
K2CO3 (0.75 equiv) at À208C for 24 h. [b] Run with KHCO3 (1.5 equiv)
at RT for 24 h. [c] Run with KHCO3 (1.5 equiv) at RT for 48 h.
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[18] CCDC 985318 (3m) contains the supplementary crystallo-
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Asymmetric Catalysis
S. Shirakawa, K. Koga, T. Tokuda,
K. Yamamoto,
K. Maruoka*
&&&& — &&&&
Catalytic Asymmetric Synthesis of 3,3’-
Diaryloxindoles as Triarylmethanes with
a Chiral All-Carbon Quaternary Center:
Phase-Transfer-Catalyzed SNAr Reaction
Going through a phase: Catalytic asym-
metric synthesis of unsymmetrical tri-
arylmethanes, with a chiral all-carbon
quaternary center, was achieved by using
a chiral bifunctional quaternary phos-
phonium bromide catalyst in the SNAr
reaction of 3-aryloxindoles. The reactions
proceeded under phase-transfer condi-
tions. Boc=tert-butoxycarbonyl.
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