Divergent synthesis of flavones and aurones via base-controlled regioselective palladium catalyzed carbonylative cyclization

Article abstract of DOI:10.1016/j.mcat.2018.03.021

A regioselective approach for construction of 5-membered and 6-membered flavonoid is established by Pd catalyzed carbonylative cyclization of 2-iodophenol with terminal alkynes using different amine bases under mild reaction condition. The catalytic experiments found that piperazine preferentially accelerate 6-endo cyclization, and triethylamine mediated Pd catalyzed 5-exo cyclization. Under optimized reaction condition, Pd catalyzed carbonylative protocol successfully applied for the diverse structures of 39 examples in good to excellent yield and regioselectivity.

Full text of DOI:10.1016/j.mcat.2018.03.021

MolecularCatalysis452(2018)264–270
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Divergent synthesis of flavones and aurones via base-controlled
regioselective palladium catalyzed carbonylative cyclization
Shan Xu, Huaming Sun, Mengyuan Zhuang, Shaohua Zheng, Yajun Jian, Weiqiang Zhang⁎,
Ziwei Gao⁎
Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, School of Chemistry & Chemical Engineering, Shaanxi Normal University, 199 South
Chang’an Road, Xi’an, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Regioselective
Flavones
Aurones
Carbonylation
Palladium
A regioselective approach for construction of 5-membered and 6-membered flavonoid is established by Pd
catalyzed carbonylative cyclization of 2-iodophenol with terminal alkynes using different amine bases under
mild reaction condition. The catalytic experiments found that piperazine preferentially accelerate 6-endo cy-
clization, and triethylamine mediated Pd catalyzed 5-exo cyclization. Under optimized reaction condition, Pd
catalyzed carbonylative protocol successfully applied for the diverse structures of 39 examples in good to ex-
cellent yield and regioselectivity.
1. Introduction
and dppp for the regioselective carbonylative annulation of iodophenol
and acetylenes, which avoided the five-membered aurones formation.
Flavones and aurones are two important branches of flavonoids
since they exhibit a wide range of biological activities. Due to the anti-
inflammatory [1], anti-cancer [2], antioxidant activities of flavonoids
[3], and their pharmaceutical applications [4], many synthetic methods
for those heterocyclic compounds were developed in recent years. For
flavones, the synthetic methods are following as carbonylation of
phenols and internal alkynes [5], base/acid mediated cyclization of o-
hydroxyketone or 2′-hydroxychalcones [6], Lewis base catalyzed
tandem acyl transfer-cyclization [7], cyclization of 1-(2-hydro-
xyphenyl)-3-aryl-1,3-propanediones [8], carbonylative coupling of aryl
bromides with 2-hydroxyacetophenones [9], annulation of salicylalde-
hydes with alkynes or benzaldehyde [10], and dehydrogenation/oxi-
dative Boron-Heck coupling of chromanone with arylboronic acid pi-
nacol esters [11]. For aurones, the methods are including the ring
closure of o-hydroxyketone [12] and oxidation/cyclization of 2-(1-hy-
droxy-3-phenylprop-2-ynyl)-phenol [13]. These methods are good
supplement to construct chromones’ skeletons of flavones and aurones,
but a straightforward synthetic procedure using simple substrates under
mild reaction condition is still highly desirable [14].
Capreeta et al. [14g] employed PA-Ph (1,3,5,7-tetramethyl-2,4,8-
trioxa-6-phenyl-6-phosphaadamantane) as coligand for microwave-as-
sisted Pd catalyzed one-pot Sonogashira-carbonylation-annulation. In
the ionic liquids of phosphonium salt, Alper et al. [14f] established li-
gand-free palladium catalyzed carbonylative annulation reaction for
selective synthesis of flavones. Li et al. [14e] demonstrated the poten-
tial of NHCs as highly efficient ligands by which Pd(II) benzimidazolin-
2-ylidene were identified as efficient complexes for catalytic cyclo-
carbonylation of ortho-functionalized aryl iodides. Wu et al. utilized Pd/
C as heterogeneous catalyst for the cyclocarbonylation of 2-iodophenol
with terminal acetylenes to synthesis of flavones in presence of CO
(15 bar) [14a], while Pd(PPh3)4 catalyzed carbonylative synthesis of
aurones were achieved by in situ CO generation from formic acid and
Ac2O [14b]. Pd catalyzed carbonylative annulation reactions showed
interesting solvent effects on the regioselectivity. PdCl2(PPh3)2 and
benzimidazole-triazole catalyzed regioselective carbonylative annula-
tion of 2-bromophenols and terminal alkynes afforded flavones and
aurones in i-Pr2NH and n-Pr2NH, respectively [14d]. The solvent
switchable regioselective carbonylative coupling reaction of 2-iodo-
phenols and terminal alkynes using Pd(0)APTES@K10 (APTES = (3-
aminopropyl)-triethoxysilane) afforded flavones and aurones in DME
and DMF, respectively [14c]. These precedents clearly demonstrated
that varying Pd catalyst system or solvents are able to control the
outcome of Pd catalyzed cyclization. To develop simple but efficient Pd
Palladium catalyzed carbonylation reaction has drawn much in-
terest for developing new cyclization reactions [15]. Chiusoli et al.
[14m] firstly utilized Pd-catalyzed three component annulation of 2-
halophenols, terminal alkynes and CO for the synthesis of aurones.
Yang et al. [14i] designed the catalyst system of PdCl2(Ph3P)2, thiourea
⁎
Corresponding authors.
E-mail addresses: zwq@snnu.edu.cn (W. Zhang), zwgao@snnu.edu.cn (Z. Gao).
https://doi.org/10.1016/j.mcat.2018.03.021
Received 7 February 2018; Received in revised form 25 March 2018; Accepted 28 March 2018
2468-8231/©2018ElsevierB.V.Allrightsreserved.

S. Xu et al.
MolecularCatalysis452(2018)264–270
Scheme 1. Recent progress on Pd-catalyzed carbonylative cyclization for flavonoid synthesis.
Table 1
Piperazine accelerated Pd catalyzed three-component carbonylative cyclization[a].
Entry
Pd/L
Solvent
Yield[b]
Selectivity(3:4)
1
2
3
4
5
6
7
8
Pd(OAc)2/-
PdCl2/-
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CH2OH
DMF
DMSO
PhMe
dioxane
THF
41%
34%
47%
72%
80%
69%
62%
44%
66%
70%
60%
80%
86%
63:37
59:41
82:18
92:8
Pd(OAc)2/PPh3
Pd(OAc)2/Xantphos
Pd(OAc)2/dppf
Pd(OAc)2/dppp
Pd(OAc)2/dppb
Pd(OAc)2/dppf
Pd(OAc)2/dppf
Pd(OAc)2/dppf
Pd(OAc)2/dppf
Pd(OAc)2/dppf
Pd(OAc)2/dppf
94:6
88:12
85:15
73:27
91:9
90:10
75:25
94:6
9
10
11
12
13
> 99:1
[a] Reaction conditions: 2-iodophenols (0.5 mmol), alkynyl reagent (0.6 mmol), Pd source (5 mol%), ligand (10 mol%), piperazine (2.0 eq) and solvent (2 mL) was
stirred at 50 °C for 24 h. [b] The yield and selectivity was determined by 1H NMR.
catalyzed protocols continue to be of great interest for selective pre-
paration these chromones (Scheme 1). In addition, to reduce CO pres-
sure and reaction temperature, and avoiding large excess of strong
bases are still challenge to establish safe and mild Pd catalyzed carbo-
nylative annulation methodology. With this goal and our recent pro-
gress on Pd catalyzed carbonylative C-C coupling reactions [16], herein
we wish to report an efficient Pd catalyzed divergent synthesis of fla-
vones and aurones by tuning bases under mild reaction condition. By
using piperazine and triethylamine as base, respectively, Pd(OAc)2-dppf
catalyzed carbonylative annulation of functionalized 2-iodophenols and
terminal alkynes afforded 5-membered aurones and 6-membered fla-
vones with satisfactory yield and selectivity. The control experiments
unveiled that piperazine accelerated 6-endo cyclization of the inter-
mediate of alkynyl ketone whilst triethylamine mediated Pd catalyzed
5-exo cyclization.
2. Experimental
2.1. General materials and methods
All manipulations were performed in the autoclave under CO at-
mosphere. Commercially available reagents were used without further
purification. NMR spectroscopic data were recorded in CDCl3 or [d6]
DMSO with a 400 or 600 MHz instrument, and TMS was used as the
internal standard; data are reported as chemical shift (ppm), multi-
plicity [s (singlet), d (doublet), t (triplet), q (quartet), m (multiplet)],
coupling constants in Hz, and integration. HRMS data were recorded in
the ESI mode with a TOF mass analyzer. Flash column chromatography
was performed on silica gel (200–300 mesh).
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MolecularCatalysis452(2018)264–270
Table 2
Scope for the carbonylative synthesis of flavones.[a,b]
[a] Reaction conditions: 2-iodophenols (0.5 mmol), Pd(OAc)2 (5 mol%), dppf (5 mol%), alkynyl reagent
(0.6 mmol), piperazine (1 mmol) and THF (2 mL) was stirred at 50 °C for 24 h. [b] Isolated yield.
266

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MolecularCatalysis452(2018)264–270
Scheme 2. The base effect in the Pd catalyzed carbonylative annulation.
2.2. General procedure for the synthesis of products 3aa-3ea
regioselectivity of Pd catalyzed carbonylative annulation for 6-mem-
bered rings. The reaction of 2-iodophenol and phenylacetylene was
chosen as a model reaction using piperazine as base to explore the
three-component cascade carbonylative reaction in the presence of
50 psi CO. The effects of different reaction parameters such as catalytic
precursor, ligand and solvent were investigated, respectively, and the
results are summarized in Table 1. Under ligand-free condition, neither
Pd(OAc)2 and PdCl2 catalyzed the annulation efficiently, nor gave 3a
and 4a selectively (Table 1, entries 1–2). Although PPh3 as coligands
did not increase the activity of Pd(OAc)2, adding P ligand indeed im-
proved the yield of benzopyranones (Table 1. entry 3). To our delight,
bidentated phosphine ligands, such as xantphos, dppf, dppp and dppb
accelerated the Pd catalyzed carbonylative cross-coupling of 2-iodo-
phenol and phenylacetylene, but also enhanced the regioselectivity of
sequential annulation of (1-(2-hydroxyphenyl-3- phenylprop-2-yn-1-
one)) (Table 1, entries 4–7). Among all the phosphine ligands in-
vestigated, dppf gave the best result with 80% yield and 94:6 selective
ratio (Table 1, entry 5). Finally, the solvent effect of the reaction was
optimized (Table 1, entries 8–13). In all the test system, the reaction in
THF gave the best result (86%), while the other tested solvents pro-
duced a mixture of the six-membered flavone 3a and the five-mem-
bered aurone 4a.
With the optimized reaction conditions in hand, we began to in-
vestigate the scope and limitation of the reaction using various sub-
strates of substituted o-iodophenols and terminal acetylenes. A series of
aliphatic and aromatic terminal alkynes with 1a and CO were suc-
cessfully applied to this reaction, and they delivered the corresponding
flavones in good to excellent yields with high regioselectivity (Table 2).
The phenylacetylene that bears electron-donating groups such as
−CH3, −C2H5, −C4H9, −C5H11 and −OMe substituent at the para
position provided good yields and regioselectivities of the corre-
sponding products (3ab-3af). We also observed that the phenylacety-
lene bears −F substituent at the para position and gave 84% yield
(3ah). Then we found that electron-donating groups (3ac,3ak) have
less distinction at para- and meta- position, while the yield of ortho-
product (3ao) decreased. Next, the aliphatic terminal alkyne such as 1-
1a-1e (0.5 mmol) and alkynyl reagent 2a-2o (0.60 mmol), Pd(OAc)2
(0.025 mmol, 0.0056 g) and dppf (0.025 mmol, 0.0138 g), piperazine
(1 mmol, 0.0861 g) in THF (2 mL) was added to a 20 mL silica tubes.
The autoclave was closed, flushed three times with CO, pressurized
with 50 psi of CO, and heated at 50 °C for 24 h. After the completion of
the reaction, the reactor was cooled down to room temperature, the
crude products were extracted with ethyl acetate and water, and the
organic layer were purified by column chromatography on silica gel
using dichloromethane or ethyl acetate/petroleum ether as eluent. All
products 3aa-3ea were identified by comparing their spectral data with
those of authentic samples.
2.3. General procedure for the synthesis of products 4aa-4cf
1a-1c (0.5 mmol) and alkynyl reagent 2a-2k (0.60 mmol), Pd(OAc)2
(0.025 mmol, 0.0056 g) and dppf (0.05 mmol, 0.0277 g), triethylamine
(1 mmol, 139 μL) in THF (2 mL) was added to a 20 mL silica tubes. The
autoclave was closed, flushed three times with CO, pressurized with
50 psi of CO, and heated at 50 °C for 24 h. After the completion of the
reaction, the reactor was cooled down to room temperature, the crude
products were extracted with ethyl acetate and water, and the organic
layer were purified by column chromatography on silica gel using di-
chloromethane or ethyl acetate/petroleum ether as eluent. All products
4aa-4cf were identified by comparing their spectral data with those of
authentic samples.
3. Results and discussion
Piperazine contains two nitrogen donors at opposite positions in its
six-membered ring, and is considered as a weak base with two pKb
values of 5.35 and 9.73 at 25 °C. It was found that piperazine ac-
celerated 6-endo cyclization of acylated o-alkynoylphenols [16], which
was also the intermediate of the Pd catalyzed carbonylative annulation.
We envisioned piperazine might significantly enhance the
267

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MolecularCatalysis452(2018)264–270
Table 3
Scope for the carbonylative synthesis of aurones.[a,b]
[a] Reaction conditions: 2-iodophenols (0.5 mmol), Pd(OAc)2 (5 mol%), dppf (10 mol%), alkynyl reagent
(0.6 mmol), triethylamine (1 mmol) and THF (2 mL) was stirred at 50 °C for 24 h. [b] Isolated yield.
hexyne gave the corresponding flavones (3ai) along with 30% yield.
Then 2-acetylene thiophene gave moderate yield (3ag). Next, chloro,
methyl-substituted o-iodophenols reacted with several terminal acet-
ylenes. We found that 4-chloro-2-iodophenol reacted with electron-
withdrawing phenylacetylene (3bj) was better than electron-donating
(3bb), there was little difference when 4-chloro, 4-methy substituted 2-
iodophenol reacted with the same phenylacetylene (3bb,3db).
To establish a suitable protocol for synthesis of 5-membered aur-
ones, we systematically investigated the influence of base on the out-
coming of Pd catalyzed carbonylative annulation reaction. The cyclic
amines as bases controlled the regioselectivity between 6-membered
and 5-membered ring products. Piperazine preferentially accelerated
the formation of 6-membered ring flavones with 86% yield and 1-me-
thyl piperazine also afforded 6-membered flavone with 83% yield.
However, 1,4-dimethyl piperazine and 4-methylmorpholine as bases
failed to promote 6-endo or 5-exo cyclization, which only afforded trace
amount of carbonylative cross-coupling products (1-(2-hydroxyphenyl-
3-phenylprop-2-yn-1-one)). The secondary amines of morpholine and
diethylamine gave 3a with decent yield and selectivity. It was notably
that triethylamine accelerated Pd catalyzed carbonylative 5-membered
annulation, and selectively afforded 4a with 85% yield. N,N-
Diisopropylethylamine gave 4a with 70% yield and the selectivity 7:93.
Finally, we evaluated the activity of inorganic bases, such as K2CO3,
Cs2CO3 and K3PO4, which also afforded 4a but with the lower
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MolecularCatalysis452(2018)264–270
Table 4
Controlled experiment of cyclization of (1-(2-hydroxyphenyl-3-phenylprop-2-yn-1-one)).[a]
[d]
Entry
Pd/L
Base(2.0 equiv.)
Solvent
Yield (%)
3a
4a
1[b]
2
Pd(OAc)2/dppf
–
Pd(OAc)2/dppf
–
Pd(OAc)2/dppf
piperazine
piperazine
Et3N
Et3N
–
THF
THF
THF
THF
THF
97
> 99
18
9
3
–
82
15
13
3[c]
4
5
–
[a] Reaction condition: (1-(2-hydroxyphenyl-3-phenylprop-2-yn-1-one)) 0.2 mmol, Pd(OAc)2 (5 mol%), THF (2 mL), 50 °C. [b] piperazine (1 mmol), dppf (5 mol%).
[c] triethylamine (1 mmol), dppf (10 mol%). [d] The yield was determined by 1H NMR.
Scheme 3. Plausible reaction mechanism for Pd catalyzed divergent synthesis of flavones and aurones.
selectivity than triethylamine did. These results showed that the amines
controlled the product selectivity of flavone and aurone in Pd catalyzed
carbonylative annulation.
triethylamine preferentially mediated 5-endo cyclization reaction in
presence of Pd(OAc)2/dppf, and afforded aurone with 82% yield.
However, triethylamine, itself, cannot mediate 6-endo and 5-exo cy-
clization efficiently and selectively. Pd(OAc)2/dppf failed to initiate 6-
endo cyclization of (1-(2-hydroxyphenyl-3- phenylprop-2-yn-1-one)).
On the basis of present results and literature [14i,14k], a plausible
mechanism is proposed in Scheme 3. In this catalytic cycle, the Pd
catalyzed carbonylative Sonogashira cross-coupling of 2-iodophenol
and phenyl acetylene generates o-hydroxyketone I. The intermediate I
readily undergoes 6-endo or 5-exo cyclization, in which amine bases
determined the regioselectivity of whole reaction. In pathway A, I un-
derwent Michael addition by piperazine to form adduct II, which was
observed as cationic specise (calcd for m/z C19H21N2O2 ([M+H]+),
309.1598, found 309.1596 ([M+H]+)) by HRMS during the reaction
course. Cyclic enolate III convert into flavones IV after piperazine
moiety left. In pathway B, Pd(0) species oxidatively add to H-O of al-
kynyl ketone intermediate I and form V. The intramolecular insertion of
acetylene to Pd-H generates VI, which might be accelerated by the
proton shuttle of triethylamine. An alternative form of V might be a Pd-
phenoxide bearing acetate anion. The insertion of the alkyne into the
PdeO bond followed by protonolysis would produce VII.
As shown in Scheme 2, both piperazine and triethylamine ac-
celerated Pd catalyzed carbonylative cross-coupling reaction. As for the
annulation of (1-(2-hydroxyphenyl- 3-phenylprop-2-yn-1-one)), piper-
azine accelerated 6-endo cyclization for flavone product, whilst more
basic and steric hindered triethylamine preferentially 5-exo cyclization
for aurone product. Then, we explored the scope and limitation of
triethylamine controlled palladium catalyzed carbonylative protocol for
the construction of 5-membered ring aurones (Table 3). First, various
aromatic alkynes were examined, substrates with electron-donating
groups including methoxy, methyl, ethyl and pentyl gave the corre-
sponding products in very good yields (4ab-4ae). Those substrates with
methoxy group substituted at ortho-position (4ak) gave higher yield
than para-substitution (4ab). Electron-withdrawing groups such as
fluoro provide 72% yield (4af). Subsequent examination of methyl and
fluoro substituted in 4-position of 2-iodophenol, there were not obvious
difference when they reacted with same aromatic alkynes (4ba, 4bb,
4bf vs 4ca, 4cb, 4cf) and provide good yields respectively. Moreover,
heteroaryl group such thiophene moiety worked well to afford the de-
sired aurone product in good yield (4ag,75%).
To illustrate the origin of the regioselectivity in the reaction, the
annulation of the intermediate (1-(2-hydroxyphenyl-3-phenylprop-2-
yn-1-one)) I was studied in the presence of piperazine and triethyla-
mine, respectively (Table 4). The control experiments clearly showed
that piperazine significantly accelerated 6-endo cyclization of I with
over 97% yield in presence or absence of Pd(OAc)2/dppf. In contrast,
4. Conclusions
In summary, we have developed a simple and effective method for
divergent synthesis of flavones and aurones under mild reaction con-
dition. With piperazine and triethylamine as base, Pd catalyzed re-
gioselective carbonylative annulation reaction of 2-iodophenols,
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MolecularCatalysis452(2018)264–270
terminal alkynes, and CO afforded the desired flavones and aurones
good to excellent yields, respectively. Additionally, the pathway of
amines mediated 6-endo and 5-exo cyclization was illustrated by con-
trolled experiments of alkynyl ketone intermediate.
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