Rhodium/chiral diene-catalyzed asymmetric 1,4-addition of arylboronic acids to chromones: A highly enantioselective pathway for accessing chiral flavanones

Article abstract of DOI:10.1002/asia.201403290

Chromone has been noted to be one of the most challenging substrates in the asymmetric 1,4-addi-tion of α,β-unsaturated carbonyl compounds. By employing the rhodium complex associated with a chiral diene ligand, (R,R)-Ph-bod, the 1,4-addition of a variety of aryl-boronic acids was realized to give high yields of the corresponding flavanones with excellent enantioselectivities (≥97% ee, 99% ee for most substrates). Ring-opening side products, which would lead to erosion of product enantioselectivity, were not observed under the stated reaction conditions.

Full text of DOI:10.1002/asia.201403290

DOI: 10.1002/asia.201403290
Communication
Addition Reactions
Rhodium/Chiral Diene-Catalyzed Asymmetric 1,4-Addition of
Arylboronic Acids to Chromones: A Highly Enantioselective
Pathway for Accessing Chiral Flavanones
Qijie He,[a] Chau Ming So,[b] Zhaoxiang Bian,[c] Tamio Hayashi,[b, d] and Jun Wang*[a]
In addition to organocatalysis, Feng disclosed a chiral N-oxide/
NiII complex that allowed the synthesis of enantio-enriched fla-
Abstract: Chromone has been noted to be one of the
most challenging substrates in the asymmetric 1,4-addi-
vanones from activated a,b-unsaturated ketones.[5] Although
tion of a,b-unsaturated carbonyl compounds. By employ-
these methods are developed, multi-step preparation of sub-
ing the rhodium complex associated with a chiral diene
strates is usually required. In fact, it would be beneficial if the
ligand, (R,R)-Ph-bod*, the 1,4-addition of a variety of aryl-
readily available chromone substrate could be directly em-
boronic acids was realized to give high yields of the corre-
ployed for this transformation. Thus, the asymmetric 1,4-addi-
sponding flavanones with excellent enantioselectivities
tion of organoboron reagents to a,b-unsaturated carbonyl
(ꢀ97% ee, 99% ee for most substrates). Ring-opening
compound is one of the most straightforward pathways for
targeting these enantio-enriched products.[6]
side products, which would lead to erosion of product
enantioselectivity, were not observed under the stated re-
Although a,b-unsaturated carbonyl substrates having alkyl/
action conditions.
aryl group at the b-position have been extensively studied, car-
bonyl compounds bearing an electronegative element, for ex-
ample, nitrogen or oxygen, at the b-position remain less ex-
plored,[7] especially for the chromone scaffolds. The major chal-
Organic molecules composed of flavanone scaffolds have been
versatile in pharmaceutical synthesis and applications. For in-
stance, both natural and synthetic flavanones show unique
biological activities such as antitumor, antioxidant, and anti-in-
flammatory properties.[1] Nevertheless, facile access of optically
active flavanone derivatives remains limited. Scheidt recently
reported the enantioselective synthesis of favanone via asym-
metric oxo-conjugate addition to a b-ketoester alkylidene pro-
moted by a chiral bifunctional thiourea organocatalyst.[2] The
desired products were obtained in 80–94% ee after a subse-
quent decarboxylation process. A similar approach was report-
ed by Hintermann where quinine was used as a catalyst (up to
64% ee).[3] Zhao also showed that trifluoromethyl group-con-
taining chiral cinchona alkaloids facilitate this transformation.[4]
lenge is that the competitive b-elimination of the oxa-p-allyl-
rhodium occurs during the catalysis, and thus gives the unde-
sirable substitution product instead[8] (or the ring-opening
product for chromone substrate, this work). Therefore, the se-
lection of the catalyst system for facilitating the desired prod-
uct formation (1,4-adduct) is particularly important. In 2010,
Liao disclosed the enantioselective synthesis of flavanones
through Rh-catalyzed 1,4-addition.[9] With the chiral bis-sulfox-
ide ligand, the desired products were obtained in moderate
yields (25–75%). Ar4BNa was found necessary to serve as the
arylating agent. The same group later reported a modified Rh/
heterodisulfoxide complex that catalyzes the addition of
ArB(OH)2 (35–70% yield and 92–95% ee).[10] In 2011, Sakai re-
ported a highly electron-deficient diphosphine ligand (S)-MeO-
F12-BIPHEP for promoting the Rh-catalyzed 1,4-addition to
chromones.[11] Although the enantioselectivity is very high, this
catalyst system has the problem that the reaction conditions
must be optimized for each substrate to obtain high yields of
the corresponding 1,4-addition products.[12] One example of
flavanone (31% yield with 99% ee) was shown using the (S)-
BICMAP diphosphine ligand.[13] In addition to Rh catalysis, Pd
catalysis can also be employed in conjugate addition of aryl-
boronic acids to chromone.[14] Stoltz recently developed a Pd/
PyOX system which catalyzes the reaction of arylboronic acids
with chromone to give the adduct with 36–91% yield and 76–
98% ee.[15]
[a] Q. He, Prof. Dr. J. Wang
Department of Chemistry
South University of Science and Technology of China
Shenzhen, 518055 (China)
Fax: (+86)755-88018304
E-mail: wang.j@sustc.edu.cn
[b] Dr. C. M. So, Prof. Dr. T. Hayashi
Institute of Materials Research and Engineering, A*STAR
3 Research Link, Singapore 117602 (Singapore)
[c] Prof. Dr. Z. Bian
School of Chinese Medicine
Hong Kong Baptist University
Kowloon Tong, Hong Kong
[d] Prof. Dr. T. Hayashi
Department of Chemistry
National University of Singapore
3 Science Drive 3, Singapore 117543 (Singapore)
Chiral dienes have emerged to be one of the most efficient
ligands for Rh-catalyzed asymmetric 1,4-addition.[16] Neverthe-
less, the efficacy of the diene ligands has not yet been shown
for chromone, which is less reactive than standard a,b-unsatu-
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/asia.201403290.
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Table 1. Optimization of reaction conditions for asymmetric 1,4-addition
of PhB(OH)2 to chromone.[a]
Table 2. Rh-catalyzed asymmetric 1,4-addition of arylboronic acids to
chromone.[a]
Entry
Ar
2
3, Yield [%][c]
ee [%][d]
Entry Ligand
Base (eq)
2a [eq] T [8C] Yield [%][b] ee [%][c]
1
2
3
4
C6H5
2a
2b
2c
2d
2e
2 f
2g
2h
2i
3aa, 94
3ab, 86
3ac, 84
3ad, 90
3ae, 84
3af, 80
3ag, 93
3ah, 72
3ai, 66
3aj, 80
3ak, 91
3al, 88
>99
99
>99
>99
>99
99
>99
98
98
1
(S)-Binap
KOH (0.5)
KOH (0.5)
5.0
5.0
5.0
5.0
5.0
3.0
2.0
90
90
90
90
90
90
90
90
90
90
90
70
50
0
6
–
4-MeO-C6H4
3-MeO-C6H4
4-Me-C6H4
3-Me-C6H4
2,4-Me-C6H3
4-tBu-C6H4
3-F-C6H4
2
(S)-P-Phos
n.d.[d]
3
4
5
6
(R,R)-Ph-bod KOH (0.5)
(R,R)-Ph-bod KOH (1.0)
(R,R)-Ph-bod KOH (2.0)
(R,R)-Ph-bod KOH (1.0)
(R,R)-Ph-bod KOH (1.0)
83
94
90
94
74
69
59
54
90
91
89
>99 (R)
>99 (R)
>99 (R)
>99 (R)
>99 (R)
87 (R)
5
6[b]
7[b]
8[b]
9
7
8
9
(R,R)-Ph-bod K2CO3 (1.0) 3.0
(R,R)-Ph-bod Cs2CO3 (1.0) 3.0
4-F-C6H4
10[b]
11[b]
12[b]
4-Et-C6H4
1-naphthyl
2-naphthyl
2j
2k
2l
97
>99
>99
68 (R)
10
11
12
13
(R,R)-Ph-bod Et3N (1.0)
(R,R)-Ph-bod KF (1.0)
(R,R)-Ph-bod KOH (1.0)
(R,R)-Ph-bod KOH (1.0)
3.0
3.0
3.0
3.0
>99 (R)
>99 (R)
>99 (R)
>99 (R)
[a] Reaction conditions: [RhCl(C2H4)2]2 (10 mol% Rh) and (R,R)-Ph-bod*
(Rh:L=1:1.1) in 1,4-dioxane (0.9 mL) was stirred at room temperature for
30 min under N2 atmosphere. Then, 1a (0.15 mmol), 2 (0.45 mmol), KOH
(0.15 mmol), and H2O (0.1 mL) were added, and the reaction mixture was
stirred in a pre-heated oil bath at 908C for 22 h. [b] ArB(OH)2 (0.75 mmol)
was used. [c] Isolated yields are reported. [d] Determined by HPLC analy-
ses.
[a] Reaction conditions: [RhCl(C2H4)2]2 (10 mol% Rh) and (R,R)-Ph-bod*
(Rh:L=1:1.1) in 1,4-dioxane (0.9 mL) was stirred at room temperature for
30 min under N2 atmosphere. Then, 1a (0.125 mmol), 2a (equivalent with
respect to 1a, as indicated), base (as indicated), and H2O (0.1 mL) were
added, and the reaction mixture was stirred in a pre-heated oil bath at
908C for 22 h. [b] Isolated yields are reported. [c] Determined by HPLC
analyses. Absolute configuration was determined by comparison with lit-
erature data. [d] n.d., not determined.
With the optimized reaction conditions in hand, we next
probed the efficacy of the catalyst system (Table 2). The enan-
tioselectivities were very high (ꢀ97% ee) at introduction of
both electron-donating and -withdrawing arenes. Sterically hin-
dered arylboronic acid also afforded the desired product in
good yield with excellent ee (entry 6).
rated ketones.[9] Herein, we report our results obtained for Rh/
chiral diene-catalyzed asymmetric 1,4-addition of arylboronic
acids to chromones, where high yields were generally ob-
served with remarkable enantioselectivities (99% ee for most
substrates).
To further investigate the substrate scope, a series of substi-
tuted chromones 1b–1i were examined (Table 3). No signifi-
cant electronic effects were observed as most of the desired
adducts were obtained in good yields and excellent enantiose-
lectivities (97–99% ee). Chloride remained intact under the
present reaction conditions (3ea and 3ga), thus allowing fur-
ther potential functionalization using traditional cross-coupling
methods.[20] The substrate of an extended aromatic structure,
4H-benzo[h]chromen-4-one, also resulted in remarkable ee
(3ha). In order to show the synthetic utility of this protocol,
we performed a scale-up experiment (50-fold of entry 1 in
Table 2) with only 5 mol% Rh catalyst. Product 3aa was ob-
tained in excellent yield (93%) and ee (>99%) from substrate
1a (1.095 g).
The R configuration of the flavanone 3aa obtained with
(R,R)-Ph-bod* is rationalized by the coordination of chromone
to a rhodium with its a-re face. The coordination with the
other face is much less favorable due to the steric repulsions
between the aromatic moiety of chromone and one of the
phenyl groups on the diene ligand (Scheme 1). All the prod-
ucts under the present conditions with (R,R)-Ph-bod* ligand
are most likely to have the same absolute configuration (R).
In conclusion, we have succeeded in demonstrating that the
chiral diene ligand (R,R)-Ph-bod* enables the Rh-catalyzed 1,4-
addition of arylboronic acids to chromones, which are chal-
We initially started to optimize the reaction conditions using
unsubstituted chromone 1a and phenylboronic acid 2a as the
model substrates (Table 1). To our delight, commercially avail-
able chiral diene (R,R)-Ph-bod*[17] gave a high yield of 3aa with
>99% ee (R) in the presence of 0.5 equiv of KOH in 1,4-diox-
ane/H2O (9:1) at 908C for 22 h (entry 3). Under the same condi-
tions, rhodium complexes with the bisphosphine ligand (S)-
Binap[18] or (S)-P-Phos,[19] did not catalyze the reacion (entries 1
and 2). When the amount of KOH was increased from 0.5 to
1.0 equivalent, the product yield was slightly improved
(entry 4), however, a further increase in the amount of KOH did
not improve the product yield (entry 5). Notably, the presence
of KOH did not lead to erosion of product enantioselectivity
through the ring-opening side reaction. Both yields and enan-
tioselectivities remained unchanged when the loading of 2a
was decreased to 3.0 equivalents (entry 6). A survey of com-
monly available inorganic bases and salts revealed that KOH
and KF gave the best results (entries 8–11). Although organic
base (Et3N) provided excellent enantioselectivity, the product
yield was moderate (entry 10). Though the enantioselectivities
were not affected by the reaction temperature, the substrate
was not fully consumed at 50–708C as judged by GC analysis
(entries 12–13).
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ture for 30 min under N2 atmosphere. Chromone
1a (21.9 mg, 0.15 mmol), phenylboronic acid 2a
(54.9 mg, 0.45 mmol), KOH (8.4 mg, 0.15 mmol)
and H2O (0.1 mL) were then added, and the reac-
tion mixture was stirred in a pre-heated oil bath
at 908C for 22 h. The resulting mixture was
cooled to room temperature, and the solvent
was removed under reduced pressure. The resi-
due was purified by chromatography on silica gel
(petroleum ether/EtOAc 60:1 to 20:1) to afford
the desired 1.4-addition product 3aa (31.6 mg,
94% yield) as a white solid. The ee was deter-
mined to be >99% ee by chiral stationary phase
HPLC analysis [Chiralcel OJ-3, hexane/iPrOH=
75:25, flow rate=1.0 mLminꢁ1, 254 nm, tR of (S)-
3aa=15.4 min (0.3%), tR of (R)-3aa=17.4 min
(99.7%)]. ½aꢂD20 = +67 (c=0.50, CHCl3). 1H NMR
(500 MHz, CDCl3): d=7.94 (dd, J=2.0, 8.5 Hz, 1H),
7.54–7.38 (m, 6H), 7.08–7.05 (m, 2H), 5.49 (dd,
J=3.0, 13.5 Hz, 1H), 3.10 (dd, J=13.5, 16.5 Hz,
1H), 2.90 ppm (dd, J=3.0, 17.0 Hz, 1H). Analytical
data match those reported in the literature.[2]
HRMS (ESI) calcd for C15H12O2 (M+H)+: 225.0916;
found: 225.0929.
Table 3. Rh-catalyzed 1,4-addition of arylboronic acids to substituted chromones.[a]
[a] Reaction conditions: [RhCl(C2H4)2]2 (10 mol% Rh) and (R,R)-Ph-bod* (Rh:L=1:1.1) in 1,4-di-
oxane (0.9 mL) was stirred at room temperature for 30 min under N2 atmosphere. Then,
1 (0.15 mmol), 2 (0.45 mmol), KOH (0.15 mmol), and H2O (0.1 mL) were added, and the reac-
tion mixture was stirred in a pre-heated oil bath at 908C for 22 h. Isolated yields are reported.
Ee values were determined by HPLC analyses. [b] ArB(OH)2 (0.75 mmol) was used. [c] (PhBO)3
(0.45 mmol) was used instead of PhB(OH)2.
Acknowledgements
We gratefully thank the startup fund from
South University of Science and Technology of
China (SUSTC) and the faculty research grant
(FRG2/12-13/004) from Hong Kong Baptist
University.
Keywords: asymmetric 1,4-addition · chiral dienes · chiral
flavanones · chromones · rhodium catalysis
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Received: November 12, 2014
Revised: December 7, 2014
Published online on && &&, 0000
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COMMUNICATION
Addition Reactions
Qijie He, Chau Ming So, Zhaoxiang Bian,
Tamio Hayashi, Jun Wang*
&& – &&
A great addition: By employing the
rhodium complex associated with
nones with excellent enantioselectivities
(ꢀ97% ee, 99% ee for most substrates).
Ring-opening side product, which
would lead to erosion of product enan-
tioselectivity, was not observed under
the stated reaction conditions.
Rhodium/Chiral Diene-Catalyzed
Asymmetric 1,4-Addition of
Arylboronic Acids to Chromones: A
Highly Enantioselective Pathway for
Accessing Chiral Flavanones
a chiral diene ligand, (R,R)-Ph-bod*, the
1,4-addition of a variety of arylboronic
acids was realized to give high yields
(up to 94%) of the corresponding flava-
Chem. Asian J. 2015, 00, 0 – 0
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ