Transient and Recyclable Halogenation Coupling (TRHC) for Isoflavonoid Synthesis with Site-Selective Arylation

Article abstract of DOI:10.1002/chem.201901025

A transient and recyclable C?H iodination has been designed for the synthesis of isoflavonoids through the domino reactions of o-hydroxyphenyl enaminones and aryl boronic acids in the presence of catalytic KI and Pd catalyst. Instead of the conventional cross-coupling strategy employing pre-halogenated substrates, this method transforms raw C?H bond by means of a transient C?H halogenation to smoothly relay the subsequent C-arylation. Consequently, such a method avoids the pre-functionalization for C?halogen bond installation as well as the generation of stoichiometric halogen-containing waste following the cross-coupled product, disclosing an intriguing new coupling protocol to forge the C?C bond in the virgin area between classical C?X (X=halogen) bond cross coupling and the C?H activation.

Full text of DOI:10.1002/chem.201901025

A Journal of
Accepted Article
Title: Transient and Recyclable Halogenation Coupling (TRHC) for
Isoflavonoid Synthesis with Site Selective Arylation
Authors: Jie-Ping Wan, Zhi Tu, and Yuyun Wang
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To be cited as: Chem. Eur. J. 10.1002/chem.201901025
Link to VoR: http://dx.doi.org/10.1002/chem.201901025
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COMMUNICATION
Transient and Recyclable Halogenation Coupling (TRHC) for
Isoflavonoid Synthesis with Site Selective Arylation
Jie-Ping Wan,*[a] Zhi Tu†[a] and Yuyun Wang†[a]
†These authors contributed equally
Abstract: The transient and recyclable C-H iodination has been
designed for the synthesis of isoflavonoids via the domino reactions
of o-hydroxyphenyl enaminones and aryl boronic acids in the
presence of catalytic KI and Pd-catalyst. Instead of the conventional
cross coupling strategy employing pre-halogenated substrates, this
method transforms raw C-H bond by means of a transient C-H
halogenation to smoothly relay the subsequent C-arylation.
Consequently, such method avoids the pre-functionalization for C-
halogen bond installation as well as the generation of stoichiometric
halogen-containing waste following the cross coupled product,
disclosing a intriguing new coupling protocol to forge C-C bond in the
virgin area between classical C-X (X = halogen) bond cross coupling
and the C-H activation.
application in treating diseases, drug discovery, food industry
and health care.5 Besides natural resources, the major route
accessing isoflavonoids is the Suzuki,6 Neghishi,7 Stille8 and
other coupling reactions9 using prior prepared 3-halochromones
(Scheme 1A). The reactions employing unfunctionalized
chromones with aryl reactant, however, do not provide
isoflavonoid. Instead, flavones were the products in such
reactions as determined by the inherent regioselectivity
(Scheme 1B).10 Therefore, the isoflavonoids are hitherto typical
targets which are not accessible by C-H bond coupling. Actually,
the synthesis of 3-vinyl,11 3-perfluoroalkyl,12 3-akynyl,13 3-alkyl,14
3-chalcogenyl15 3-acyloxyl chromones16 etc, have been
successively realized by coupling the C(sp2)-H bond of
chromones or proper precursors, but never equivalent arylation.
These known results confirm the challenge of synthesizing
isoflavonoids by C-H arylation. Therefore, it is inarguably
significant to establish alternative coupling method for
isoflavonoid synthesis by directly transform raw C-H bond and
avoids the aforementioned shortcomings of the C-X bond-based
cross coupling.
The transition metal-catalyzed cross coupling reactions act
as the predominant method in constructing C-C and C-
heteroatom bonds. Their applications are irreplaceable in both
the laboratory investigation and many industries associated with
organic synthesis. Interestingly, most of the classical cross
coupling reactions such as Ullmann, Suzuki, Heck, Negishi,
Goldberg, Buckwald-Hartwig, Sonogashira, Hiyama, Cadiot-
Chodkiewicz, Stille and Kumada coupling, the carbonylation
coupling as well as cyanation coupling etc share a common
feature: employing C(sp2)-X or C(sp)-X bond as the key
transformation group.1 While benefiting the efficiency and
synthetic power associated with the reactive C-X bond, people
also suffers from the additional labors and chemical
consumption resulting from the pre-functionalization generating
C-X bond. Moreover, the generation of stoichiometric halogen-
waste from the C-X substrate is another major challenge. Over
the past several decades, new coupling modes such as the
In light of the known transformation of o-hydroxyphenyl
enaminone to 3-halochromone,9,17 we envisage that it can be
utilized to design a catalytic and selective formal C-H arylation
for isoflavonoids synthesis by catalytically recycling the
halogenation. Herein, we wish to report for the first time the
selective arylation of 2-hydroxyphenyl enaminones for the
synthesis of isoflavonoids with relayed KI and Pd-catalysis
wherein the transient and recyclable halogenation takes place in
the presence of a terminal oxidant (Scheme 1C), thus disclosing
a new coupling mode for C-C bond formation alongside the
classical C-X bond-based cross coupling and C-H activation.
A) Metal-reagent cross coupling for isoflavonoid synthesis: classical cross coupling
transition-metal-catalyzed
C-H
activation,2
dehydrative
O
O
coupling,3 dehydrogenative and/or oxidative coupling4 have
been developed as more environmentally benign tactics. These
couplings transform directly natural C-H bond, and are thus free
of the aforementioned substrate pre-functionalization and
halogen waste production. However, due to the intrinsic stability
of most C-H bonds, the scope of efficient coupling on C-H bond
is yet far insufficient. In this context, developing new coupling
strategy beyond these known modes is highly urgent task of
modern organic synthesis.
X
Ar
Ar
M
"X"-waste
precursor
+
Pd-catalyst
X = I, Br, Cl
O
O
stoichiometric
B) C-H bond cross coupling for flavonoid synthesis: C-H activation
O
O
O
Ar
H
H
H
Pd-catalyst
Ar LG
O
O
Ar
O
LG = leaving group
unfavorable selectivity
C) Our work hypothesis: transient and recyclable halogentaion coupling (TRHC)
O
O
Ar
Pd-cat., KI
N
ArB(OH)2
Isoflavonoids are important natural products with widespread
O
OH
[O]
I-
Transient halogenation
Site selective arylation
No stoichiometric X-waste
Step efficent cross couping
I2
[a]
Prof. Dr. J.-P. Wan, Z. Tu, Y. Wang
College of Chemistry and Chemical Engineering
Jiangxi Normal University
O
O
X
Ar-B(OH)2
Pd-cat.
Nanchang 330022, P.R. China
E-mail: wanjieping@jxnu.edu.cn
Scheme 1. Coupling reactions providing isoflavonoids and flavonoids
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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To start the exploration, enaminone 1a and phenyl boronic
acid 2a were selected for reaction. To optimize the conditions,
comprehensive experiments on screening the iodo-source, Pd-
catalyst, temperature, base additive, reaction medium as well as
reagent loading were conducted (see SI). According to the
typical results (Table 1), KI was found as the hitherto best iodo-
source in 40 mol% loading (entries 1-2 and 5-6, Table 1).18 No
product 3a was formed in the absence of iodo-reagent (entry 7,
Table 1), confirming the indispensable role of an iodo-reagent in
leading this arylation. As a Pd(0) catalyst, Ph(PPh3)4 displayed
evidently better activity than Pd(OAc)2 and Pd(PPh3)2Cl2 (entries
2-4, Table 1). Increasing the loading of the catalyst (entry 8,
Table 1) and varying the species of base additive (entries 9-10,
Table 1), however, did not further improve the yield of 3a.
efficient synthesis of products containing halogen (3n-3r,
Scheme 2), alkoxyl (3s, Scheme 2), dialkyl (3t, Scheme 2) and
fused aryl (3u, Scheme 2) substructure in the chromone core.
The positive effect of the electron donating substituent in the
phenyl ring of 1 to the product yield was also observed.
Furthermore, as expected, the reactions using both substituted
boronic acids and 2-hydroxyphenyl chromones also accordingly
gave rise to the diversely functionalized isoflavonoids with
efficiency (3v-3z and 3aa, Scheme 2).
Table 1 Typical results on condition optimization[a]
entry
1[c]
2
catalyst
“I”-source
base
Yield (%)[b]
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)4
Pd(OAc)2
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
KI
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
K2CO3
53
69
80
55
74
63
0
KI
3
KI
4
KI
5
NaI
Bu4NI
-
6
7
8[d]
9
KI
78
50
37
KI
Scheme 2. Scope the isoflavonoid synthesis by recyclable halogenation
coupling. Conditions: enaminone 1 (0.25 mmol), 2 (0.3 mmol), Pd(PPh3)4 (2.5
mmol%), KI (0.1 mmol), BPO (0.5 mmol), Na2CO3 (0.5 mmol) in EtOH (2.0
mL), sealed and stirred at 80 oC for 24h.
10
KI
NaOH
[a]Reaction conditions: 1a (0.25 mmol), 2a (0.3 mmol), Pd-catalyst (2.5 mol%),
KI (0.4 equiv.), BPO (0.5 mmol), base (0.5 mmol) in EtOH (2.0mL), stirred at
80 oC in sealed tube for 24h. [b]Yield of isolated product based on 1a. [c]The
loading of KI was 0.2 equiv. [d]The loading of Pd(PPh3)4 was 5 mol%.
Moreover, besides aryl boronic acids, the reactions of vinyl
boronic acids 4 were also investigated. Delightfully, the 3-vinyl
chromones 5 could be practically synthesized by this step
economical cross coupling mode with good to excellent yields
(Scheme 3), revealing further the high potential of this coupling
method.
To illustrate the application of this new coupling protocol,
enaminones and aryl boronic acids containing diverse
substituents were utilized for the reaction, respectively.
Generally, this method displayed satisfactory tolerance to both
components. In the reactions employing different aryl boronic
acids, electron donating groups such as alkyls (3b, 3c and 3h,
Scheme 2), electron withdrawing groups such as halogen (3d-3f
and 3i, Scheme 2) and trifluoromethyl (3g, Scheme 2) were
employed smoothly for the expected reaction. In addition, the
heteroaryl (3j, Scheme 2), fused aryl (3k, Scheme 2) and biaryl
(3l and 3m, Scheme 2) boronic acids were also practical
substrates. A tendency found in these data was that the electron
enriched aryl boronic acids generally afforded the products with
higher yields than those reactions using electron deficient aryl
boronic acids. As for the reaction of different enaminones 1,
satisfactory application scope was also demonstrated by the
Scheme 3. Transient C-H halogenation coupling for 3-vinyl chromone
synthesis
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Notably, the important isoflavonoid natural product Daidzein
(3ab) could be easily accessed from 3aa with high efficiency via
a typical AlCl3 promoted demethylation(Eq 1).
reaction of chromone 7 with KI provided 3-iodochromone 6
under the standard catalytic conditions (eq 9), suggesting that 7
was the key intermediate in the formation of 6.
With the results in hand, the reaction mechanism is
proposed. As outlined in Scheme 4, the reaction starts from the
single electron transfer (SET) between the iodine anion and the
in situ generated carboxyl radical resulting from the thermo-
induced homo-cleavage of BPO16 to afford iodine radical.
Besides the reversible formation of molecular iodine, the iodine
radical can incorporate the in situ generated chromone 7 via
radical addition, which gives rise to free radical spcies 8. The
chain termination of 8 with the benzoyloxyl radical leads to the
formation of intermediate 9. The β-elimination of a benzoic acid
from 9 then provides the 3-iodochromone intermediate 6. The
subsequent Pd-catalyzed Suzuki-type coupling between the
C(sp2)-I bond and the aryl/vinyl boronic acids then yields
products 3 or 5, respectively. Most notably, the simultaneously
released iodine anion can be repeatedly oxidized to iodine
radical, thus enables the recyclable halogenation and the
successive arylation/vinylation, offering this new cross coupling
strategy in the presence of only catalytic loading of iodide salt.
To testify the reaction mechanism, a series of control
experiments were then conducted. Because the absence of
iodine reagent was proved to provide no target product, we first
explored the possible iodo-species mediating the reaction. In the
designed entries, the reaction of KI with enaminone 1a in the
presence of BPO as well as the reaction employing directly I2
without BPO both gave iodinated chromone 6 with high yield (Eq
2 and 3). On the other hand, without BPO, KI along was not able
to react with 1a to give 6 (eq 4), and the oxidation of KI to
molecular iodine by BPO was confirmed by a classical starch
test (Eq 5). Moreover, alternate KI with molecular iodine afforded
efficiently the target product 3a under the standard conditions
(Eq 6). These results indicated that the molecular iodine was the
active species or precursor during the reaction, and 6 was a key
intermediate in the reaction. Later on, the 3-iodochromone 6 and
the unsubstituted chromone 7 was employed to react with
phenyl boronic acid under the Pd-catalyzed conditions,
respectively. The former led to smooth synthesis of 3a (Eq 7).
The latter one, on the contrary, didn’t enable the formation of
this product (Eq 8). These results further supported the fact that
the in situ and recyclable formation of iodinated intermediate 6
was a key point in the isoflavonoid synthesis. Additionally, the
O
O
I
KI,1 equiv
N
(2)
(3)
standard
conditions
OH
1a
O
6, 71%
I2,1 equiv.
1a
1a
6, 79%
EtOH, 80 o
C
Scheme 4. The proposed reaction mechanism by recyclable C-H halogenation
KI,1 equiv.
6, not observed
(4)
(5)
EtOH, 80 o
C
To sum up, by means of a rationally designed transient and
recyclable halogenation coupling tactic, the synthesis of
isoflavonoids and 3-vinyl chromones have been efficiently
achieved via the Pd(0)/KI-co-catalyzed reactions of o-
hydroxyphenyl enaminones and aryl/vinyl boronic acids. The
present method employs catalytic amount of KI to enable the
arylation and vinylation with C(sp2)-H substrate, which discloses
a new coupling tactic in constructing C-C bonds. Besides free of
substrate pre-functionalization and no stoichiometric halogen
waste, this present coupling tactic directs unconventional site
selectivity to the chromone C3-postion because previously
known Pd-catalyzed arylation of chromones naturally takes
place in the C2 position, demonstrating additional value of such
a cross coupling approach in switching the reaction selectivity.
EtOH, 80 o
C
I2
KI
BPO
(1 equiv)
(2 equiv)
observed by starch test
O
Ph
1a
I2 (0.4 equiv)
standard conditions
(6)
PhB(OH)2
O
O
3a, 75%
Na2CO3, 2 equiv
Pd(PPh3)4, 2.5 mol%
I
(7)
3a
83%
PhB(OH)2
EtOH, 80 o
C
O
6
O
Na2CO3, 2 equiv
Pd(PPh3)4, 2.5 mol%
(8)
(9)
3a
0%
PhB(OH)2
EtOH, 80 o
C
O
7
O
O
O
I
1 equiv KI
standard conditions
Experimental Section
O
7
6, 71%
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was employed to flash column chromatography via the elution of
mixed EtOAc and petroleum ether (v/v = 1:1) to give pure
product 3ab.
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
COMMUNICATION
A transient and recyclable C-H bond
halogenation is designed for the
synthesis of isoflavonoids via site
Jie-Ping Wan,* Zhi Tu, Yuyun Wang
Page No. – Page No.
selective
arylation
using
o-
Transient and Recyclable
Halogenation Coupling (TRHC) for
Isoflavonoid Synthesis with Site
Selective Arylation
hydroxyphenyl enaminones and aryl
boronic acids. This work discloses a
new cross coupling protocol in step-
economical and sustainable manner.
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