Copper-catalyzed decarboxylative hydroboration of phenylpropiolic acids under ligand-free or both ligand- and base-free conditions

Article abstract of DOI:10.1016/j.cclet.2016.02.012

An efficient copper-catalyzed decarboxylative hydroboration of phenylpropiolic acids with bis(pinacolato)diboron was developed, affording β-vinylboronates as the only products in high yields. Extra hydrogen sources such as methanol are not needed in this

Full text of DOI:10.1016/j.cclet.2016.02.012

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CCLET 3599 1–4
Chinese Chemical Letters xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
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Original article
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Copper-catalyzed decarboxylative hydroboration of phenylpropiolic
acids under ligand-free or both ligand- and base-free conditions
*
Q1 Qiang Feng, Ying-Wei Zhao, Qiu-Ling Song
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Institute of Next Generation Matter Transformation, College of Chemical Engineering, Huaqiao University, Xiamen 361021, China
A R T I C L E I N F O
A B S T R A C T
Article history:
An efficient copper-catalyzed decarboxylative hydroboration of phenylpropiolic acids with bis(pina-
Received 27 January 2016
Received in revised form 4 February 2016
Accepted 19 February 2016
Available online xxx
colato)diboron was developed, affording
b-vinylboronates as the only products in high yields. Extra
hydrogen sources such as methanol are not needed in this catalytic system. This reaction could be
performed successfully under ligand- and base-free conditions. It demonstrated that phenylpropiolic
acids can be employed as alkyne synthons in the hydroboration reaction and exhibited good reactivity
and higher selectivity than terminal alkynes.
Keywords:
ß 2016 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
Published by Elsevier B.V. All rights reserved.
Decarboxylation
Hydroboration
Ligand-free
Base-free
Copper-catalysis
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1. Introduction
frequently occurs in the Sonogashira reactions. As part of our 31
ongoing research into the development of highly efficient and 32
9
Alkenylboron compounds are especially versatile building
blocks that are widely employed as vinyl anionic or cationic
synthons in a myriad of coupling reactions [1], as well as they can
be readily transformed into various vinylic derivatives [2]. Over the
past years, the transition-metal-catalyzed addition of boron to
carbon–carbon triple bonds presents a convenient and important
strategy for alkenylboron synthesis [3]. Among them, copper-
catalyzed hydroboration of alkynes has attracted much attention
due to the readily availability, low cost, and low toxicity of copper
salts. Great progress has been made by Miyaura [4], Yun [5], Li [6],
Haveyda [7], and others [8]. However, some limitations still exist,
such as the requirement of ligand, base, and special hydrogen
source, thus the development of more efficient method with much
wide applicability to prepare vinylboronates via copper-catalyzed
regioselective hydroboration of aryl alkynes is still a challenge for
synthetic organic chemistry. On the other hand, alkynyl carboxylic
acids are regarded as ideal substitutions for terminal alkynes and
have been widely applied in the transition-metal-catalyzed
construction of C–C and C-heteroatoms bonds via decarboxylation
[9]. In particular, alkynyl carboxylic acids exhibit superiority to
terminal alkynes in the reactions: they are often more reactive, and
they can efficiently suppress the Glaser coupling reaction that
versatile copper-catalyzed decarboxylative reactions [10], and in 33
conjunction with our work on oxidative decarboxylative coupling 34
of arylpropiolic acids with dialkyl H-phosphonates [11], we 35
decided to expand this strategy in the copper-catalyzed hydro- 36
borations. Recently, we have achieved the synthesis of bis- 37
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30
deuterated
b-borylated a,b-styrene derivatives from the reaction 38
of alkynyl acids with bis(pinacolato)diboron under base-free 39
conditions [12]. Herein, we report our results on the general 40
hydroboration using alkynyl acids as the substrates under ligand- 41
free or both ligand- and base-free conditions.
42
2. Experimental
43
All experiments were conducted with a Schlenk tube. Flash 44
column chromatography was performed over silica gel (200–300 45
mesh). 1H NMR spectra were recorded on a Bruker AVIII-400M or 46
AVIII-500M spectrometers. Chemical shifts (in ppm) were refer- 47
enced to CDCl3
were obtained by using the same NMR spectrometers and were 49
calibrated with CDCl3 77.0). Unless otherwise noted, materials 50
(d
7.26) as an internal standard. 13C NMR spectra 48
(
d
obtained from commercial suppliers were used without further 51
purification. Anhydrous dioxane was obtained by refluxing for at 52
least 12 h over sodium and freshly distilled prior to use.
53
General procedure for the synthesis of (E)-4,4,5,5-tetramethyl- 54
2-styryl-1,3,2-dioxaborolane (3a): A Schlenk tube with a magnetic 55
stirring bar was charged with 3-phenylpropiolic acid (1a, 68 mg, 56
*
Corresponding author.
E-mail address: qsong@hqu.edu.cn (Q.-L. Song).
http://dx.doi.org/10.1016/j.cclet.2016.02.012
1001-8417/ß 2016 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: Q. Feng, et al., Copper-catalyzed decarboxylative hydroboration of phenylpropiolic acids under ligand-
free or both ligand- and base-free conditions, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.02.012

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Q. Feng et al. / Chinese Chemical Letters xxx (2016) xxx–xxx
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0.5 mmol), bis(pinacolato)diboron (2a, 152 mg, 0.6 mmol),
Cu(TFA)2 (29 mg, 10 mol%), Na2CO3 (127 mg, 1.2 mmol), and
1,4-dioxane (2 mL) under N2. The reaction mixture was stirred at
80 8C for 18 h (monitored by TLC and GC). Upon completion of the
reaction, the reaction mixture was then cooled to ambient
temperature, diluted with ethyl acetate (20 mL), filtered through
a plug of silica gel, and washed with ethyl acetate (20 mL). The
organic layer was washed with saturated brine (20 mL Â 2) and
dried over anhydrous Na2SO4. The solvents were removed via
rotary evaporator and the residue was purified by flash chroma-
tography (silica gel, ethyl acetate: petroleum ether = 1:30) to give
89.7 mg of desired product 3a in 78% yield as a colorless oil. 1H
and copper trifluoroacetate (Cu(TFA)2) exhibited the best activity
(Table 1, entry 12). The yield of 3aa could be further enhanced to
83% when 2.2 equiv. of Na2CO3 was employed. To our surprise, in
the absence of the base Na2CO3, also 37% yield could be obtained
(Table 1, entry 14). Then we screened the copper catalyst again
under base-free condition and found out that Cu2O could lead to
satisfied result, probably because of its potential basicity (Table 1,
entry 18). Considering that actually a double amount of [Cu] was
involved for Cu2O, a loading of 5 mol% was used and a little lower
yield was obtained (Table 1, entry 20). Further increasing the
reaction temperature to 100 8C made the product being formed
nearly quantitatively (Table 1, entry 21). Finally, this reaction
performed smoothly at room temperature if a phosphorous ligand
was added (Table 1, entry 22). It is noteworthy that the solvents we
used were actually wet. When the reaction under the conditions as
in entry 19 was performed in anhydrous dioxane, only 7% of 3aa
was obtained. If extra 0.25 mmol of water (0.5 equiv.) was added to
anhydrous dioxane, the yield was 31%. Only when more than
1 equiv. of water was used, acceptable around 60% yield could be
achieved, indicating that water content in the wet dioxane we used
was higher than 0.4%.
After establishing the optimized reaction conditions of the
different catalytic systems, a variety of alkynyl carboxylic acids and
diboron reagents were subjected (Table 1, entries 13 and 21) to
evaluate the scope of the copper-catalyzed decarboxylative
regioselective hydroboration reaction. As shown in Scheme 1,
phenylpropiolic acids with both electron-rich and electron-
deficient substituents on the aromatic ring could be smoothly
converted into the desired products. The position of the substitutes
on the aromatic rings had some influence on yields (3b-3c, 3d-3e,
3m-3o, 3p-3q), with ortho-substitutions usually giving lower
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NMR (400 MHz, CDCl3):
d
7.48–7.50 (m, 2H), 7.41 (d, 1H,
J = 18.5 Hz), 7.29–7.32 (m, 3H), 6.18 (d, 1H, J = 18.4 Hz), 1.32 (s,
12H). 13C NMR (100 MHz, CDCl3):
126.0, 82.3, 23.8.
d
148.5, 136.4, 127.9, 127.5,
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3. Results and discussion
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We chose 3-phenylpropiolic acid (1a) and bis(pinacolato)di-
boron (2a) as the model substrates. Initially, the reaction was
conducted in the presence of 10 mol% of Cu(OTf)2 and 1.2 equiv. of
Na2CO3 in benzene at 80 8C under N2 atmosphere and no extra
ligand and hydrogen source were used. To our delight,
b-borylated
a,b
-styrene with E-configuration (3aa) was formed in 55% yield as
the single product (Table 1, entry 1). When the reaction
temperature was raised to 90 8C, a slight decrease on the product
yield was observed and no reaction occurred at room temperature
(Table 1, entries 2 and 3). Further screening of the solvents showed
that acetonitrile and 1,4-dioxane are good choice for this reaction
(Table 1, entries 5–9). We also employed other copper catalysts
Table 1
Optimization of the reaction conditions.a
Entry
Cu catalyst
Base
Solvent
T (8C)
Yield (%)b
1
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OAc)2
Cu(TFA)2
Cu(TFA)2
Cu(TFA)2
Cu(TFA)2
Cu(OAc)2
Cu(OTf)2
CuSO4
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
–
Benzene
80
90
R.T.
50
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
100
R.T.
55
46
0
2
Benzene
3
Benzene
4
Benzene
Trace
26
0
5
DMSO
6
DMF
7
CH3CN
63
8
8
DMA
9
1,4-dioxane
benzene
65
45
73
75
83
37
42
Trace
19
61
76
58
96
77
10
11
12
13
14
15
16
17
18
19
20d
21
22e
Benzene
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
c
–
–
–
CuO
–
Cu2O
–
Cu2O
–
Cu2O
–
Cu2O
–
a
Reaction conditions: (1) (0.5 mmol), (2) (1.2 equiv., 0.6 mmol), copper catalyst (0.05 mmol), base (0.6 mmol), solvent (2 mL), under N2 atmosphere.
b
c
Yields are based on GC analysis with n-dodecane as the internal standard.
1.1 mmol (2.2 equiv.) of Na2CO3 was used.
0.025 mmol of Cu2O was used.
d
e
0.05 mmol of Xantphos was added.
Please cite this article in press as: Q. Feng, et al., Copper-catalyzed decarboxylative hydroboration of phenylpropiolic acids under ligand-
free or both ligand- and base-free conditions, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.02.012

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Q. Feng et al. / Chinese Chemical Letters xxx (2016) xxx–xxx
3
Scheme 1. Substrate scope. Reaction conditions: (a) 1 (0.5 mmol), 2 (1.2 equiv., 0.6 mmol), Cu(TFA)2 (10 mol%), Na2CO3 (2.2 equiv., 1.1 mmol), 1,4-dioxane (2 mL) under N2,
80 8C, 18 h, isolated yield; (b) 1 (0.5 mmol), 2 (1.2 equiv., 0.6 mmol), Cu2O (10 mol%), 1,4-dioxane (2 mL) under N2, 100 8C, 18 h, isolated yield.
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yields of b-borylated a,b-styrene when compared to meta- and
para-substitutions, probably because of steric hindrance. It is
noteworthy that most halo-substituted aryl groups survived well,
leading to halo-substituted aromatic
b-borylated a,b-styrene in
good yields which could be used for further transformations (3m-
3r). In addition, 4-phenyl, 1-naphthyl, 4-trifluoromethyl, 4-cyano
substituted 3-phenylpropiolic acid and 3-(thiophen-2-yl)propiolic
acid were transformed into corresponding
b-borylated a,b-
styrenes smoothly as well (3h-3l).
In order to understand the reaction mechanism, some control
experiments were performed. When potassium 3-phenylpropio-
late was performed as starting material in anhydrous solvent
(Scheme 2, eq. 1), only trace of hydroboration product was formed.
This result indicates that the hydrogen of alkynyl carboxylic acids
and water in the solvent under ‘‘standard condition’’ offers the
protons as the electrophilic source. When D2O was added to the
standard reaction system, high deuterium incorporation for both
olefinic protons in 3a-D2 was obtained (Scheme 2, eq. 2). Utilizing
phenylacetylene instead of 3-phenylpropiolic acid as the substrate
to perform the reactions with D2O under the standard conditions
resulted in
incorporation than alkynyl carboxylic acids (Scheme 2, eq. 3)
139 Q2 (Scheme 3).
a slight lower reactivity and poorer deuterium
Scheme 2. Control experiments.
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Based on previous copper-catalyzed hydroboration reactions
and our own work, we suggested that the reaction may be
Scheme 3. Proposed mechanism.
Please cite this article in press as: Q. Feng, et al., Copper-catalyzed decarboxylative hydroboration of phenylpropiolic acids under ligand-
free or both ligand- and base-free conditions, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.02.012

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Q. Feng et al. / Chinese Chemical Letters xxx (2016) xxx–xxx
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4. Conclusion
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In conclusion, we have developed efficient catalytic systems to
synthesize alkenylboronates via copper-catalyzed decarboxylative
regioselective hydroboration of alkynyl carboxylic acids under
ligand-free or both ligand and base-free conditions. The applica-
tion of alkynyl carboxylic acids instead of terminal alkynes can lead
to a highly active and selective hydroboration reaction. Mechanic
investigations supported the formation of an alkenyl-bis-copper
reactive intermediate. This novel strategy has great potential in the
development of bis-functionalization of carbon–carbon triple
bond. Further studies on exploration of the reaction scope,
mechanistic elucidation, and synthetic application of this protocol
are ongoing in our laboratory.
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162
Acknowledgment
(c) A.L. Moure, R. Go´mez Arraya´s, D.J. Ca´rdenas, I. Alonso, J.C. Carretero, Regio-
controlled CuI-catalyzed borylation of propargylic-functionalized internal
alkynes, J. Am. Chem. Soc. 134 (2012) 7219–7222.
163 Q3
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Financial support from the National Science Foundation of
China (No. 21202049), the Recruitment Program of Global Experts
(1000 Talents Plan) and Fujian Hundred Talents Plan and Program
of Innovative Research Team of Huaqiao University are gratefully
acknowledged. We also thank Instrumental Analysis Center of
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168 Q4 Huaqiao University for analysis support.
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Please cite this article in press as: Q. Feng, et al., Copper-catalyzed decarboxylative hydroboration of phenylpropiolic acids under ligand-
free or both ligand- and base-free conditions, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.02.012