Palladium-Catalyzed Addition/Cyclization of (2-Hydroxyaryl)boronic Acids with Alkynylphosphonates: Access to Phosphacoumarins

Article abstract of DOI:10.1021/acs.orglett.0c03151

A facile palladium-catalyzed addition/cyclization of (2-hydroxyaryl)boronic acids with alkynylphosphonates has been developed, providing an effective strategy to construct a series of valuable phosphacoumarins. This methodology features excellent regioselectivity and broad substrate tolerance.

Full text of DOI:10.1021/acs.orglett.0c03151

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Letter
Palladium-Catalyzed Addition/Cyclization of (2-Hydroxyaryl)boronic
Acids with Alkynylphosphonates: Access to Phosphacoumarins
Dumei Ma, Shanshan Hu, Sirui Chen, Jiaoting Pan, Song Tu, Yingwu Yin,* Yuxing Gao,*
and Yufen Zhao
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ABSTRACT: A facile palladium-catalyzed addition/cyclization of
(2-hydroxyaryl)boronic acids with alkynylphosphonates has been
developed, providing an effective strategy to construct a series of
valuable phosphacoumarins. This methodology features excellent
regioselectivity and broad substrate tolerance.
that the intramolecular condensation of 2-methoxycarbonyl-
hosphorus-containing heterocycles, as an emerging group
Pof organophosphorus compounds, have drawn widespread benzylphosphonates delivered a diverse series of 4-O-
attention due to their wide applicability in organic synthesis,1
substituted phosphacoumarins, which suffer from the require-
medicinal chemistry,2 and material science.3 In particular,
ment of multistep reactions to prepare the precursors (Scheme
1b).7a Moreover, Mironov’s group revealed the intramolecular
phosphacoumarins, the phosphorus analogues of coumarin, are
coupling of readily hydrolyzable arylenedioxytrihalogenphos-
phoranes or hexacoordinated phosphorus derivatives with
terminal alkynes leading to phosphacoumarin derivatives.6c−g
In 2013, Lee’s group established the gold-catalyzed hydro-
arylation of aryl alkynylphosphonates for the synthesis of
phosphacoumarins (Scheme 1c),7b but its general use poses
severe limitations due to the noble gold/silver catalysts. Thus
developing a facile and efficient method for the synthesis of
phosphacoumarin derivatives from readily available and cheap
starting materials is continuously desirable and essential.
The transition-metal-catalyzed conjugate addition of organo-
boron reagents to unsaturated alkenes and alkynes is one of the
most useful synthetic strategies to form a C−C bond over the
past several decades.8 Among them, the palladium-catalyzed
addition/cyclization reaction involving organoborons and
alkynes provides one of the most straightforward pathways
for the formation of carbocycles and heterocycles.9 In 2008,
Yamamoto’s group reported the construction of arylcoumarins
via Cu-catalyzed hydroarylation with arylboronic acids.10
Inspired by these excellent contributions and in line with our
constant interest in the construction of heterocyclic com-
pounds bearing a phosphorus atom,9a,11 herein we revealed
that the Pd-catalyzed addition/cyclization reaction of arylbor-
made of an important subclass of phosphorus heterocycle
compounds. Phosphacoumarins are endowed with unique
biological activities4,7a and exhibit high flame-retarding
performance.5 However, reliable synthetic methods for
phosphacoumarins are still limited to date. Traditionally,
phosphacoumarins are mainly obtained from the intermolec-
ular reaction of suitable starting materials6 or from predesigned
complex phosphorus precursors via intramolecular functional-
ization.7 Chen and Rodios’ groups have employed salicylalde-
hydes and phosphonoacetates for the preparation of
phosphacoumarins via the Knoevenagel reaction, which
afforded only low yields (<21%) followed by the formation
of phosphonocoumarins (Scheme 1a).6a,b Fu’s group reported
Scheme 1. Synthetic Strategies toward Phosphacoumarins
Received: September 18, 2020
https://dx.doi.org/10.1021/acs.orglett.0c03151
© XXXX American Chemical Society
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onic acids bearing an ortho-hydroxyl group with various
alkynylphosphonates, which could be prepared conveniently,12
led to privileged phosphacoumarin scaffolds. Compared with
the known methods, this novel protocol has the advantages of
operational simplicity, excellent regioselectivity, and broad
substrate tolerance and could be an attractive practical method.
To the best of our knowledge, this approach is the first
example of a Pd-catalyzed addition/cyclization reaction of (2-
hydroxyaryl)boronic acid with alkynylphosphonates into
phosphacoumarins.
Initially, the reaction was carried out with diethyl-
(phenylethynyl)phosphonate (1a) and (2-hydroxyphenyl)-
boronic acid (2a) in the presence of 3 mol % Pd(OAc)2 and
6 mol % dppb in 1,4-dioxane at 60 °C (Table 1, entry 1). To
Subsequently, the effect of the solvent was studied. Solvents
such as CH3CN, DMF, 1,2-dichloroethane (DCE), and
toluene (entries 13−16) were explored, indicating that toluene
could improve the yield to 95% (entry 16). It is interesting that
the replacement of coordinating solvents (e.g., CH3CN) with
noncoordinating solvents (e.g., toluene) could lead to an
enhancement of the activity, probably due to the use of
noncoordinating solvents minimizing the competition for
coordination with the catalyst.13 In addition, the loading of
Pd(OAc)2 was also evaluated, showing that decreasing the
amounts of Pd(OAc)2 into 1 mol % afforded a significant loss
of yield (entry 17). Notably, ambient air proved to be
competent for constructing phosphacoumarins without a
significant loss of yield (entry 18). It is noticeable that the
addition of the aryl group took place exclusively at the β-
position of the alkynylphosphonates, which might be attributed
to the α-position carbanion stabilization by the phosphonyl
group. Finally, a 1 mmol scale reaction was further examined,
affording a 96% yield of 3a (entry 19).
a
Table 1. Optimization of the Reaction Conditions
With the optimal reaction conditions in hand (Table 1, entry
16), the scope of alkynylphosphonates was first surveyed. As
demonstrated in Scheme 2, a variety of alkynylphosphonates
readily reacted with phenylboronic acid 2a, generating the
corresponding phosphacoumarins in moderate to excellent
yields. Arylalkynylphosphonates bearing 2-methyl, 4-methyl, 4-
tert-butyl, and 4-methoxy groups on the phenyl ring were
suitable reaction partners in this transformation, leading to the
corresponding products 3b−3e in 52−84% yields. It is
noteworthy that steric hindrance significantly affected the
efficiency. For example, the diethyl (p-tolylethynyl)-
phosphonate 1b produced the target product 3b in a high
yield of 84%, but the sterically demanding meta-substituted
counterpart 1c gave the expected compound 3c in only 52%
yield. Additionally, halogen groups like fluoro (F), chloro (Cl),
and bromo (Br) smoothly underwent this reaction and
constructed the desired products 3f−3h in 60−88% yields.
The alkynylphosphonates with electron-withdrawing groups
such as trifluoromethyl (CF3), nitrile (CN), and nitro (NO2)
were also compatible with this protocol, giving the
corresponding products in moderate to good yields of 53−
80% (3i−3k). Notably, the addition of an isopropoxy group to
compound 1 is capable of serving as a leaving group in this
reaction to obtain the target products (3j−3l). Interestingly,
both polycyclic and heterocyclic aromatic-substituted alkynyl-
phosphonates (1m and 1n) were also allowed to generate the
expected phosphacoumarins in moderate yields. Alkynyl-
phosphonate containing a thiophene group afforded the
desired heterocyclic product 3o in only 30% yield, probably
due to the poisoning of the transition-metal catalyst caused by
the thiophene group. It is noteworthy that this strategy was an
effective route to tolerate ether and ketone functional groups
on the phosphacoumarin scaffold, leading to 3p with a good
yield of 72%. Moreover, both aliphatic-substituted and
cycloalkane-substituted alkynylphosphonates were also appli-
cable to offer the target products in yields of 48−62% (3q−3t).
The versatility of the reaction was further evaluated by the
use of different arylboronic acid derivatives (Table 2). Various
arylboronic acids bearing methyl, chloro, and fluoro groups
were investigated in combination with diethyl(phenylethynyl)-
phosphonate 1a or diisopropyl (phenylethynyl)phosphonate
1l, which provided the desired six-substituted phosphacoumar-
in in yields of 36−95% (3u−3y). Moreover, the halogen
substituents (3w, 3x, and 3y) could offer numerous possible
b
entry
catalyst
ligand
solvent
temp (°C) yield (%)
1
2
3
4
5
6
7
8
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
PdCl2
Pd(PPh3)4
Cu(OAc)2
Ni(OAc)2
dppb
dppb
dppb
dppb
dppb
dppb
dppb
dppb
dppb
dppp
PPh3
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
CH3CN
DMF
DCE
toluene
toluene
toluene
toluene
60
80
90
100
90
90
90
90
90
90
90
90
90
90
90
90
90
90
90
52
65
75
70
16
70
0
0
0
9
10
11
12
13
14
15
16
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
78
80
39
40
32
90
95
61
89
90
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
c
17
d
18
e
19
a
Reaction conditions: 1a (0.3 mmol), 2a (0.45 mmol), catalyst (3
mol %), ligand (6 mol %), H2O (2 equiv), and solvent (2 mL), 24 h,
under Ar. Isolated yields. 1 mol % of Pd(OAc)2 was used. Under
b
c
d
e
air. Reaction run on a 1 mmol scale.
our delight, the desired product 3a was obtained in 52% yield.
Subsequently, we tried to improve the yield of 3a by increasing
the temperature. Pleasantly, the yield was significantly
improved to 75% at 90 °C (entries 1−3). However, a slight
decrease in the yield was observed at 100 °C (entry 4).
Screening other palladium complexes such as PdCl2 and
Pd(PPh3)4 gave yields of 16 and 70%, respectively (entries 5
and 6). Other metal salts such as Cu(OAc)2 and Ni(OAc)2
were tested, which both failed to generate the desired product
(entries 7 and 8). Without a catalyst, no product was observed
(entry 9), indicating that palladium salt is indispensable for this
transformation. Next, we examined the effect of the ligand on
the reaction (entries 10 and 11). It implied that PPh3 was the
best choice for the reaction, delivering the desired product 3a
in 80% yield (entry 11). Nevertheless, the yield dramatically
reduced to 39% in the absence of the ligand (entry 12).
B
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Scheme 2. Palladium-Catalyzed Addition/Cyclization of
Alkynylphosphonates with 2a
Table 2. Palladium-Catalyzed Addition/Cyclization of 1
a
a
with (2-Hydroxyaryl)boronic Acids
a
Reaction conditions: 1 (0.3 mmol), 2 (0.45 mmol), Pd(OAc)2 (3
mol %), PPh3 (6 mol %), H2O (2 equiv), and toluene (2 mL) at 90
b
°C for 24 h under Ar. Isolated yields.
Scheme 3. Application Studies
a
Reaction conditions: 1 (0.3 mmol), 2a (0.45 mmol), Pd(OAc)2 (3
mol %), PPh3 (6 mol %), H2O (2 equiv) and toluene (2 mL) at 90 °C
b
for 24 h under Ar. At 120 °C for 3q−3s.
postfunctionalizations of the phosphacoumarin scaffold. In
addition, ethyl phenyl(phenylethynyl)phosphinate 1u could
also react with (2-hydroxyphenyl)boronic acid 2a, generating
the expected product 3z in 73% yield.
To gain insight into the mechanism, several control
experiments were conducted. Initially, we performed the
reaction of 1a with 2a through decreasing the temperature to
60 °C in the presence of 3 mol % Pd(OAc)2 and 6 mol % PPh3
in toluene. We found that the desired product 3a was obtained
in a yield of 70%; simultaneously, the intermediate
phosphoalkene 3aa was isolated in 24% yield (Scheme 4a).
Next, the treatment of the intermediate phosphoalkene 3aa in
the absence of Pd(OAc)2 and PPh3 could give the expected
product 3a in an equivalent yield of 98% under heating
(Scheme 4b). These results indicated that 3aa is the key
intermediate during this process and the intramolecular
nucleophilic substitution of 3aa yielding 3a could be
performed effectively in the absence of catalyst and ligand.
Subsequently, phenylboronic acid-d2 (2a′, 99.8% D) was
employed to determine the source of the hydrogen on the
vinylic carbon. Notably, it turned out that 2a′ with 1l gave only
To demonstrate the synthetic application potential of this
method, a gram-scale experiment was carried out. As shown in
Scheme 3a, 7.29 g of 3a could be obtained in a yield of 85%.
The stability of phosphacoumarin 3a was preliminary evaluated
in the presence of the strong base KOH (Scheme 3b), with the
finding that 3a was hydrolyzed into phosphonic acid 3a′,
preserving the phosphacoumarin framework in an excellent
yield of 92%, indicating the strong stability of the
phosphacoumarin scaffold under these alkaline conditions.
Furthermore, 2a could react with diyne 1ab to generate a novel
bisphosphacoumarin-4-yl-benzene framework (3ab) in an
excellent yield of 90%, which could be of interest in the
material field (Scheme 3c).14
C
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available aromatic and aliphatic alkynylphosphonates could
smoothly deliver the desired phosphacoumarins in moderate to
excellent yields. This transformation features operational
simplicity, high regioselectivity, and a broad substrate
tolerance, showing great potential for wide application in
material science and biological research. Further studies on
mechanistic investigations as well as research applications are
currently underway in our laboratory.
Scheme 4. Control Experiments
ASSOCIATED CONTENT
* Supporting Information
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The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c03151.
General experimental procedures, characterization de-
1
tails, and copies of H, 13C, and 31P NMR spectra of
compounds (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
a trace of phosphoalkene 3aa′ under strictly anhydrous
conditions, indicating that the hydrogen on the double bond
is not from boronic acid (Scheme 4c). The addition of 100 μL
of D2O (99.9% D) to the reaction system led to the products
3aa′/[D1]-3aa′ in a total yield of 95% with a ratio of 3aa′/
Yingwu Yin − Department of Chemical and Biochemical
Engineering, College of Chemistry and Chemical Engineering,
Xiamen University, Xiamen 361005, Fujian, China;
orcid.org/0000-0002-9123-7761; Email: ywyin@
xmu.edu.cn
Yuxing Gao − Department of Chemistry and Key Laboratory for
Chemical Biology of Fujian Province, College of Chemistry and
Chemical Engineering, Xiamen University, Xiamen 361005,
Fujian, China; orcid.org/0000-0002-6420-0656;
Email: gaoxingchem@xmu.edu.cn
1
[D1]-3aa′ of 1:6.9 determined by H NMR analysis (Scheme
4d). These results clearly revealed that the hydrogen on the
vinylic carbon is derived from water.
On the basis of these experimental results and previous
reports,8c,g,15 a plausible mechanism is proposed in Scheme 5.
Scheme 5. Plausible Mechanistic Pathway
Authors
Dumei Ma − Department of Chemical and Biochemical
Engineering, College of Chemistry and Chemical Engineering,
Xiamen University, Xiamen 361005, Fujian, China
Shanshan Hu − Department of Chemistry and Key Laboratory
for Chemical Biology of Fujian Province, College of Chemistry
and Chemical Engineering, Xiamen University, Xiamen 361005,
Fujian, China
Sirui Chen − Department of Chemical and Biochemical
Engineering, College of Chemistry and Chemical Engineering,
Xiamen University, Xiamen 361005, Fujian, China
Jiaoting Pan − Department of Chemistry and Key Laboratory
for Chemical Biology of Fujian Province, College of Chemistry
and Chemical Engineering, Xiamen University, Xiamen 361005,
Fujian, China
Song Tu − Department of Chemical and Biochemical
Engineering, College of Chemistry and Chemical Engineering,
Xiamen University, Xiamen 361005, Fujian, China
Yufen Zhao − Department of Chemistry and Key Laboratory for
Chemical Biology of Fujian Province, College of Chemistry and
Chemical Engineering, Xiamen University, Xiamen 361005,
Fujian, China; orcid.org/0000-0002-8513-1354
Initially, the oxidative addition of Pd(0) species 4 with (2-
hydroxyaryl)boronic acid 2 readily took place to deliver the
intermediate 5.8d,15,16 Next, the selective insertion of
alkynylphosphonate 1 into the Pd−C in a syn fashion
generated the intermediate 6. Then, the hydrogen from
water was abstracted by 6 to form the intermediate 7. The
reductive elimination of 7 led to the intermediate alkene 8
along with the reproduction of the active Pd(0) species.
Finally, an intramolecular nucleophilic substitution of 8
resulted in the formation of the desired product 3,
accompanied by the release of one molecular alcohol.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c03151
Author Contributions
All authors have given approval to the final version of the
manuscript.
In conclusion, a convenient and efficient one-pot method for
the construction of a variety of valuable phosphacoumarins has
been successfully established via a palladium-catalyzed
addition/cyclization reaction. In this reaction, both readily
Notes
The authors declare no competing financial interest.
D
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Letter
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ACKNOWLEDGMENTS
■
We are grateful for the financial support from the Chinese
National Natural Science Foundation (21202135, 21642010),
Fundamental Research Funds for the Central Universities, P.
R. China (20720170039, 20720200035), the Science and
Technology Planning Project of Xiamen City, P. R. China (no.
3502Z20193003), and the Fujian University-Industry Research
Cooperation Project (2018N5013).
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Org. Lett. XXXX, XXX, XXX−XXX