Ligand-free hydroboration of alkynes catalyzed by heterogeneous copper powder with high efficiency

Article abstract of DOI:10.1039/c3cc48670b

Regioselective hydroboration of terminal and internal alkynes is realized by using 10 mol% copper powder (0.3-1 μm) at room temperature. 24 alkynes were efficiently converted into vinylboronates in up to 96% yield without addition of any ligand or additiv

Full text of DOI:10.1039/c3cc48670b

ChemComm
View Article Online
View Journal | View Issue
COMMUNICATION
Ligand-free hydroboration of alkynes catalyzed
by heterogeneous copper powder with high
efficiency†
Cite this: Chem. Commun., 2014,
50, 2058
Received 13th November 2013,
Accepted 18th December 2013
Jie Zhao, Zhiqiang Niu, Hua Fu* and Yadong Li*
DOI: 10.1039/c3cc48670b
www.rsc.org/chemcomm
Regioselective hydroboration of terminal and internal alkynes is
realized by using 10 mol% copper powder (0.3–1 lm) at room
temperature. 24 alkynes were efficiently converted into vinyl-
boronates in up to 96% yield without addition of any ligand or additive.
Organoboron compounds play a significant role in organic
synthesis because they are generally nontoxic, stable under atmo-
spheric conditions and highly reactive.1 Among them, vinylboronates
have drawn much attention because they are versatile intermediates
in the construction of numerous complex molecules, such as organic
semiconductors and pharmaceuticals.2 Transition metal-catalyzed
addition of boron-containing reagents to carbon–carbon multiple
bonds represents an important strategy for synthesis of organoboron
compounds.3,4 Notably, copper is preferred in organic synthesis
because of its low cost and low toxicity.5 Over the past few years,
great progress has been made in copper-catalyzed hydroboration of
either terminal alkynes (Scheme 1A)6 or internal asymmetrical alkynes
Scheme 1 Copper-catalyzed hydroboration of (A) terminal alkynes and
(B) internal alkynes. (C) Our protocol for hydroboration of terminal and
internal alkynes catalysed by heterogeneous micro copper powder.
At the outset, but-1-ynylbenzene and B2Pin2 were used as the
(Scheme 1B)7 with bis(pinacolato)diboron (B2Pin2). For example, model substrates for the screening of reaction conditions including
Yun’s group used CuCl as a catalyst for addition of B2Pin2 to catalysts, bases, solvents and atmospheres. As shown in Table 1, five
disubstituted alkynes and a,b-ethylenic carbonyl compounds.7a–c bases were tested using 10 mol% micro copper powder (0.3–1 mm,
Hoveyda’s group reported an efficient method for the synthesis of Ningbo Guangbo New Nanomaterials Stock Co., Ltd) as the catalyst,
internal (a-)vinylboronates catalyzed by an NHC–Cu complex.6b EtOH as the solvent under an Ar atmosphere at room temperature
Corma and Garcia’s group explored CuO nanoparticles together with (entries 1–5). Sodium methoxide (MeONa) provided the highest
PPh3 in the hydroboration of alkynes.6a It should be noted that most yield of 2b without detection of regioisomeric product 2b0 (entry 3).
of these protocols are exclusively performed in the presence of organic No product was observed in the absence of the base (entry 6). As
ligands and proceed via homogeneous pathways.
shown in entries 7–9, three other kinds of copper catalyst were
From the perspective of practical applications, heterogeneous tested: copper (a) (size: ꢀ100 mesh, Sinopharm Chemical Reagent
hydroboration of alkynes, which may reduce metal contamination Beijing Co. Ltd) exhibited lower catalytic activity than micro copper
of products and avoid the use of toxic ligands, is highly desired powder (cf. entries 3 and 7); copper (b) and copper (g) (Alfa Aesar),
but has not been reported thus far. Herein, we report a ligand-free the shape and size of which were not as well-distributed as micro
hydroboration of terminal and internal alkynes catalyzed by micro copper powder, also showed low activity (cf. entries 3, 8 and 9,
copper powder at room temperature (Scheme 1C).
Fig. S1, ESI†). Only 8% yield was observed in the absence of the
catalyst (entry 10). Other solvents screened (entries 11–13) were
inferior to ethanol. The yield decreased significantly when the
reaction was carried out under air as compared to that under
argon protection (entry 14).
Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China.
E-mail: ydli@mail.tsinghua.edu.cn, fuhua@mail.tsinghua.edu.cn;
Fax: +86 10-62788765; Tel: +86-10-62772315
† Electronic supplementary information (ESI) available: Details of synthesis
procedures and characterization of compounds by H-NMR, C-NMR, GC-MS (or
ESI-MS). See DOI: 10.1039/c3cc48670b
After obtaining the optimum reaction conditions (using
10 mol% micro copper powder as the catalyst, 20 mol% MeONa
2058 | Chem. Commun., 2014, 50, 2058--2060
This journal is ©The Royal Society of Chemistry 2014

View Article Online
Communication
ChemComm
Table 1 Hydroboration of but-1-ynylbenzene: optimization of conditionsa
b
Entry Cat.
Base
Solvent Yield of 2b (%) Yield of 2b0 (%)
1
2
3
4
5
6
7
8
Cu
Cu
Cu
Cu
Cu
Cu
NaOBut EtOH
LiOBut EtOH
MeONa EtOH
Cs2CO3 EtOH
24
61
90
56
66
Trace
61
79
79
8
n.d.
2
Fig. 1 TEM images of micro copper powder: (A) before reaction and (B)
after reaction.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
K2CO3
—
EtOH
EtOH
Cu(a)c MeONa EtOH
Cu(b)d MeONa EtOH
Cu(g)e MeONa EtOH
For internal alkynes, hydroboration occurred at the b-carbon
to the aryl and yielded only the (Z)-isomers (2a–d, 2t). Hydro-
boration of terminal alkynes led to E-vinylboronates via anti-
Markovnikov and syn-addition of the boron reagents. For the
aryl terminal alkynes, the electronic effect of the substrates on
the aromatic rings including neutral, electron-donating and
electron-withdrawing groups did not show obvious difference
in reactivity. The aliphatic terminal alkynes displayed higher
reactivity than the aromatic ones, and they finished the reac-
tions within 11 h. The hydroboration of alkynes under catalysis
of micro copper powder could tolerate various functional
groups including ester (2d), ether (2i), the C–Cl bond (2j), the
C–F bond (2k–m), trifluoromethyl (2n), N,S-heterocycles
(2o, 2p), and hydroxyl (2u–x) in the substrates.
9
10
11
12
13
14
—
MeONa EtOH
Cu
Cu
Cu
Cu f
MeONa CH2Cl2 Trace
MeONa THF Trace
MeONa MeOH 67
MeONa EtOH 29
a
Reaction conditions: but-1-ynylbenzene (0.5 mmol), B2Pin2
(0.75 mmol), micro copper powder (0.05 mmol) (0.3–1 mm, Ningbo
Guangbo New Nanomaterials Stock Co., Ltd), base (0.1 mmol), solvent
b
(2 mL), room temperature (B25 1C), under an Ar atmosphere. Yield
was determined by GC analysis using n-tridecane as the internal
c
standard. Copper powder (ꢀ100 mesh, Sinopharm Chemical Reagent
d
Beijing Co., Ltd). Copper powder (ꢀ170 + 270 mesh, Alfa Aesar).
e
f
Copper powder (ꢀ625 mesh, 0.5–1.5 mm, Alfa Aesar). Under air.
as the base, and 2 mL EtOH as the solvent at room temperature under
To investigate if the reaction was catalyzed by heterogeneous
an Ar atmosphere), the substrate scope of hydroboration of alkynes copper, at first we studied the structural variation of the catalyst
was investigated. As shown in Table 2, the examined substrates before and after the reaction by transmission electron micro-
provided good to excellent yields with high regioselectivity. scopy (TEM). As shown in Fig. 1, the shape and size of the
copper powder remained unchanged. Moreover, the colour of
the final reaction solution didn’t turn green or blue, which
indicated that the catalyst had not been oxidized (Fig. S2, ESI†).
The filtration test was then performed. The reaction solution was
Table 2 Hydroboration of various alkynes catalyzed by micro copper
powdera
filtered when the reaction proceeded at about 13% conversion and
the liquid phase was allowed to react under the same conditions to
the final time. It turned out that the liquid phase did not continue to
generate the product. Encouraged by these results, we subsequently
tested the metal residue in the reaction solution by using ICP-AES.
The copper concentration in the solution of the micro copper
powder system was below 0.05 ppm, which was much lower than
that of the common CuCl–PPh3 catalyst system (33.35 ppm). Noting
that copper easily undergoes surface oxidation in an ambient
atmosphere,8 we used Cu2O and CuO as the catalysts (Scheme 2A
and B), and the poor yields suggested that the oxide species on the
surface of micro copper powder was not essential for the catalytic
activity. These results collectively implied that the hydroboration
catalyzed by micro copper powder does not occur under traditional
homogeneous conditions.
We further treated but-1-ynylbenzene with pinacolborane
under our standard conditions, and the reaction did not work
(Scheme 2C), indicating the reaction did not involve the for-
mation of a pinacolborane intermediate. A possible mechanism
for hydroboration of alkynes is therefore proposed in Scheme 3.
First, reaction of NaOR (R = Me or Et) with B2Pin2 provides I and
II. Adsorption of alkyne (1) and II on the surface of micro
copper powder gives A, and the synergetic effect of 1 and II
Chem. Commun., 2014, 50, 2058--2060 | 2059
a
Reaction conditions: alkyne (0.5 mmol), B2Pin2 (0.75 mmol), micro
copper powder (0.05 mmol, Ningbo Guangbo New Nanomaterials Stock
Co. Ltd), NaOMe (0.1 mmol), ethanol (2 mL), room temperature (B25 1C),
reaction time (11 or 24 h), under an Ar atmosphere. Isolated yield.
b
This journal is ©The Royal Society of Chemistry 2014

View Article Online
ChemComm
Communication
under heterogeneous conditions. This protocol avoids the use of
any ligand or additive, and is applied to a wide spectrum of
alkynes, including terminal and internal ones as well as alkynols.
Moreover, compared with copper salts or copper complexes
that are mainly used in hydroboration reactions, the metal con-
tamination of the products was greatly reduced. We believe this
work will greatly benefit the synthesis of organic semiconductors
and drugs, further application of this heterogeneous hydroboration
and mechanism study are in progress in our laboratory.
This work was supported by the State Key Project of Fundamental
Research for Nanoscience and Nanotechnology (2011CB932401
and 2011CBA00500), National key Basic Research Program of
China (2012CB224802), the Ministry of Science and Technology
of China (Grant No. 2012CB722605), the National Natural
Science Foundation of China (Grant No. 21221062, 21172128,
21171105, 21322107 and 21131004) and China Postdoctoral
Science Foundation (Grant No. 2012M520241, 2012M520010
and 2013T60097). The authors thank Prof. Lei Liu and Prof. Wei
He for their helpful discussion.
Scheme 2 (A) 10 mol% Cu2O was used as a catalyst. (B) 10 mol% CuO
was used as a catalyst. (C) Reaction of but-1-ynylbenzene (1b) with
pinacolborane under our standard conditions.
Notes and references
1 (a) M. G. Davidson, K. Wade, T. B. Marder and A. K. Hughes,
Contemporary Boron Chemistry, Royal Society of Chemistry,
Cambridge, 2000; (b) I. A. I. Mkhalid, J. H. Barnard, T. B. Marder,
J. M. Murphy and J. F. Hartwig, Chem. Rev., 2010, 110, 890.
2 (a) Catalytic Asymmetric Conjugate Reactions, ed. A. Cordova, Wiley-
VCH, Weinheim, 2010; (b) Modern Aldol Reactions, ed. E. Mahrwald,
Wiley-VCH, Weinheim, 2010; (c) N. Miyaura and A. Suzuki, Chem.
Rev., 1995, 95, 2457; (d) T. Hayashi and K. Yamasaki, Chem. Rev.,
2003, 103, 2829; (e) T. R. Wu and M. J. Chong, J. Am. Chem. Soc., 2007,
129, 4908.
Scheme 3 A possible mechanism for hydroboration of alkynes on the
surface of copper powder.
3 (a) M. Suginome, T. Matsuda, T. Ohmura, A. Seki and M. Murakami,
in Comprehensive Organometallic Chemistry III, ed. R. Crabtree,
M. Mingos and I. Ojima, Elsevier, Oxford, 2007, vol. 10, pp. 725–787,
For reviews, see: (b) G. J. Irvine, M. J. G. Lesley, T. B. Marder,
N. C. Norman, C. R. Rice, E. G. Robins, W. R. Roper, G. R. Whittell
and L. J. Wright, Chem. Rev., 1998, 98, 2685; (c) I. Beletskaya and
C. Moberg, Chem. Rev., 2006, 106, 2320.
occurs, leading to B, which results in the formation of III. Finally,
treatment of III with ethanol affords the target product (2). The
present regioselective boron addition to alkynes is in agreement
with the Yun’s reports on homogeneous catalysis.7a–c
It is well known that vinylboronates are important inter-
mediates in organic synthesis and usually used in Suzuki
coupling reactions. Application of the synthesized products
(Table 2) is demonstrated in Scheme 4. Coupling reactions of
two synthesized compounds (2c and 2f) were attempted under
the catalysis of Pd(PPh3)4, and the corresponding products
(4a and 4b) were obtained in high yields.
4 (a) A. Pelter, K. Smith and H. C. Brown, Borane Reagents, Academic
¨
Press, New York, 1988; (b) C. Gunanathan, M. Holscher, F. Pan and
W. Leitner, J. Am. Chem. Soc., 2012, 134, 14349.
5 For recent reviews on copper-catalyzed cross couplings, see:
(a) S. V. Ley and A. W. Thomas, Angew. Chem., 2003, 115, 5558
(Angew. Chem., Int. Ed., 2003, 42, 5400); (b) K. Kunz, U. Scholz and
D. Ganzer, Synlett, 2003, 2428; (c) I. P. Beletskaya and
A. V. Cheprakov, Coord. Chem. Rev., 2004, 248, 2337; (d) G. Evano,
N. Blanchard and M. Toumi, Chem. Rev., 2008, 108, 3054; (e) D. Ma
and Q. Cai, Acc. Chem. Res., 2008, 41, 1450; ( f ) F. Monnier and
M. Taillefer, Angew. Chem., Int. Ed., 2009, 48, 6954; (g) D. S. Surry and
S. L. Buchwald, Chem. Sci., 2010, 1, 13; (h) H. Rao and H. Fu, Synlett,
2011, 745; (i) T. Liu and H. Fu, Synthesis, 2012, 2805 and references
cited therein.
In summary, we discovered that micro copper powder was a
robust and efficient catalyst for the preparation of vinylboronates
6 For hydroboration of terminal alkynes, see: (a) A. Grirrane, A. Corma
and H. Garcia, Chem.–Eur. J., 2011, 17, 2467; (b) H. Jang,
A. R. Zhugralin, Y. Lee and A. H. Hoveyda, J. Am. Chem. Soc., 2011,
´
´
133, 7859; (c) A. L. Moure, P. Mauleon, R. G. Arrayas and
J. C. Carretero, Org. Lett., 2013, 15, 2054.
7 For hydroboration of internal alkynes, see: (a) J.-E. Lee, J. Kwon and
J. Yun, Chem. Commun., 2008, 733; (b) H. R. Kim, I. G. Jung, K. Yoo,
K. Jang, E. S. Lee, J. Yun and S. U. Son, Chem. Commun., 2010, 46, 758;
(c) H. R. Kim and J. Yun, Chem. Commun., 2011, 47, 2943;
´
´
(d) A. L. Moure, R. G. Arrayas, D. J. Cardenas, I. Alonso and
J. C. Carretero, J. Am. Chem. Soc., 2012, 134, 7219; (e) K. Semba,
T. Fujuhara, J. Terao and Y. Tsuji, Chem.–Eur. J., 2012, 18, 10491.
8 L. I. Hung, C. K. Tsung, W. Y. Huang and P. D. Yang, Adv. Mater.,
2010, 22, 1910.
Scheme 4 Application of the synthesized products: (A) coupling reaction
of 2c and 1-iodo-4-methoxybenzene. (B) Coupling reaction of 2f and
1-iodo-4-methoxybenzene.
2060 | Chem. Commun., 2014, 50, 2058--2060
This journal is ©The Royal Society of Chemistry 2014