Ag(I)-catalyzed C-H borylation of terminal alkynes

Article abstract of DOI:10.1016/j.tet.2014.06.033

An efficient Ag(I)-catalyzed borylation method of terminal alkynes is reported. The obtained borylated alkynes are shown to engage in C-Br, C-CN, C-N, and C-C bond formation with various reaction partners. Meanwhile the Ag(I) catalyst could be regenerated in the presence of PPh₃ and BF₃.

Full text of DOI:10.1016/j.tet.2014.06.033

Tetrahedron xxx (2014) 1e5
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Ag(I)-catalyzed CeH borylation of terminal alkynes
*
Jiu-Rong Hu , Lin-Hai Liu, Xin Hu, Hong-De Ye
Key Laboratory of Applied Organic Chemistry, Higher Institutions of Jiangxi Province, School of Chemistry and Chemical Engineering, Shangrao Normal
University, Jiangxi 334001, China
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient Ag(I)-catalyzed borylation method of terminal alkynes is reported. The obtained borylated
alkynes are shown to engage in CeBr, CeCN, CeN, and CeC bond formation with various reaction
partners. Meanwhile the Ag(I) catalyst could be regenerated in the presence of PPh3 and BF3.
Ó 2014 Elsevier Ltd. All rights reserved.
Received 26 April 2014
Received in revised form 3 June 2014
Accepted 10 June 2014
Available online xxx
Keywords:
Ag(I)-catalyzed borylation
Borylated alkynes
Regeneration
1. Introduction
presence of different silver salts, bases, and solvents to optimize the
reaction conditions. The catalytic system using 1 mol % of AgI as
Boronic acids and borate esters are useful intermediates in or-
ganic synthesis,1 particularly, for reactions involving carbonecarbon
bond formation through the palladium-catalyzed SuzukieMiyaura
cross-coupling reaction, the chemistry of which has been developed
extensively in the past decade. Prevalent methods for the formation
of carboneboron bonds involve the metal-catalyzed borylation of
aryl halides2,3 or the addition of aryllithium or magnesium species to
borates.1 Recently, transition metal (Ir, Rh, Pd, Cu)-catalyzed C(sp2,
sp3)eH borylation methods are reported more.4e9 However, to our
knowledge, most of borylated alkynes were synthesized with an
equivalent strong base,10 such as n-BuLi, and Ir-catalyzed C(sp)eH
borylation of terminal alkynes are currently under development.11
Due to the expensive price of Ir-species, there is a need to develop
other methods for this synthesis. Herein, a Ag(I)-catalyzed borylation
method of terminal alkynes is reported. The obtained borylated al-
kynes are shown to engage in CeBr, CeCN, CeN, and CeC bond
formation with various reaction partners. Meanwhile the Ag(I) cat-
alyst could be regenerated in the presence of PPh3 and BF3.
catalyst and Cs2CO3 as base gave an encouraging yield of 38% for 2a
(entry 1). Other Ag(I) salt could also afford some amount product
(2a), but AgCl establishing the best activity increased the yield to
51% (entries 1e5). Further reaction with other bases led no signifi-
cant improvement on the yield (entries 6e8). To our surprise, an
additive ligand helped increasing the yield to 83% (entries 9e12).
Notably, the mixed ligands of PPh3 and L1 gave the best yield to 92%
(entry 13). Increasing the amount of catalyst and ligand did not
improve the yield (entry 15). Very poor yield of 2a was detected
either without AgCl or without base (entries 16 and 17).
With the optimized reaction conditions (Table 1, entry 13), the
substrate scope was then examined (Table 2). Substrates bearing
various aryl-substituted terminal alkynes could afford the corre-
sponding borate esters in good to excellent yields (Table 2). Both
electron-withdrawing and electron-donating substitutions on the
aromatic ring were successfully borylated with B(OiPr)pin (Table 2,
2ae2i). More importantly, a variety of alkyl-substituted terminal
alkynes could also be borylated in good yield under the normal
conditions (Table 2, 2je2o).
Additionally, to demonstrate the utility of this borylation
method, we conducted the reaction on gram scale, which proved 2a
in 68% yield. Subsequently, we also showed that 2a could be con-
verted to a wide range of desirable synthons in good yields using
known conversion such as halogenation,13 cyanation,13 amina-
tion,14 and homocoupling15 with itself (Scheme 2).
Finally, we try to recycled the Ag-catalyst. All silver species after
the reaction could be recycled conveniently by reacting with
1 equiv PPh3 and 2 equiv BF3, then recrystallized by CH3OH and
2. Results and discussion
Inspired by Lu’s work,12 which use AgI as catalyst affording car-
boxylation of terminal alkynes, we attempt to use the similar Ag(I) to
achieve C(sp)eH borylation (Scheme 1). We began our investigation
by reacting PhC^CH 1a and B(OiPr)pin (pin¼Me4C2O2) in the
* Corresponding author. Fax: þ86 793 8150621; e-mail address: jiuronghu@163.
com (J.-R. Hu).
http://dx.doi.org/10.1016/j.tet.2014.06.033
0040-4020/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Hu, J.-R.; et al., Tetrahedron (2014), http://dx.doi.org/10.1016/j.tet.2014.06.033

2
J.-R. Hu et al. / Tetrahedron xxx (2014) 1e5
Ag(I) Catalyst
R
R
H
H
R
R
COOH
Bpin
CO2
ref 12
O
O
Ag(I) Catalyst
B(OiPr)pin
O
B
B(OiPr)pin
=
?
Scheme 1. The CeH bond functionalization of terminal alkynes.
Table 1
Table 2
Borylation of PhC^CH
Scope of substrates under standard conditions (Table 1, entry 13)
AgCl, L1, PPh3
HCl
Ag(I), Base,
HCl
B(OiPr)pin
B(OiPr)pin
N
R
Bpin
R
H
R
H
R
Bpin
Cs2CO3
DMF
Ligand, DMF
1
2
2a
1a
But
N
N
N
N
N
Bpin
F3C
Bpin
Bpin
Bpin
O2N
2a, 92%
2c, 86%
2b, 84%
L2
L1
H3CO
Entrya
Cat: Ag(I) (1 mol %)
Base
Ligand
Yieldc (%)
1
2
3
4
5
6
7
8
AgI
AgF
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
K2CO3
K3PO4
KOtBu
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
No
No
No
No
No
No
No
No
No
38
45
51
41
25
12
29
<5
60
43
83
68
92
81
90
<5
0
Bpin
Bpin
2f, 78%
2i, 70%
2e, 81%
AgCl
AgBr
AgOAc
AgCl
AgCl
AgCl
AgCl
AgCl
AgCl
AgCl
AgCl
AgCl
AgCl
AgCl
No
2d, 95%
Cl
tBu
HO
Bpin
Bpin
9
PPh3
Py
L1
Bpin
Bpin
2h, 82%
2k, 83%
2g, 79%
10
11
12
13
14
15b
16
17
Bpin
Bpin
L2
Ph
Cy
PPh3þL1
PPh3þL2
PPh3þL1
PPh3þL1
PPh3þL1
O
3
2j, 85%
2l, 73%
Bpin
Bpin
Bpin
Cs2CO3
Reaction conditions: 1a (0.5 mmol), B(OiPr)pin (0.75 mmol), base (1.1 mmol),
a
Si
ligand (1 mol %), catalyst (1 mol %), 5 mL DMF, 50 ꢁC, 24 h.
b
The same condition as (a) except for ligand (2 mol %), catalyst (2 mol %).
c
Isolated yield.
2n, 88%
2o, 75%
2m, 68%
16
H2O, giving cubane-like (PPh3AgCl)4 and the precursor of N-het-
erocyclic carbenes L01 (Fig. 1). Single crystal X-ray diffraction anal-
ysis reveals that L01 contains one imidazolium cation, two [BF4]ꢀ
anion, and one additional H2O molecule. The unique structure
feature for compound ðL01Þ is the hydrogen bonding interaction
between the imidazolium cation and the fluorine atoms of [BF4]ꢀ
anion. Mixing (PPh3AgCl)4 and L01 could also catalyze this bor-
ylation again in good yield (2a, 88%) under standard conditions.
reduced pressure. The crude product was purified by silica gel
column chromatography to give the corresponding products.
Note: The carbine ligand (L1) was prepared by mixing the cor-
responding imidazolidinium salt (1,3-di(2-picolyl)imidazolium
chloride) with n-LiBu (1 equiv) for 2 h at ꢀ78 ꢁC, then adding the
other starting materials.
4.1.2. Synthesis of 3. A mixture of 2a (0.05 mmol), CuBr2
(0.05 mmol) in H2O, and CH3OH (1 mL for each) was stirred at 80 ꢁC
under Ar atmosphere for 24 h. The combined organic layer was
washed with brine, extracted with ethyl acetate, dried over Na2SO4,
and evaporated under reduced pressure. The crude product was
purified by silica gel column chromatography to give the corre-
sponding products.
3. Conclusions
In summary, we have developed a Ag(I)-catalyzed terminal al-
kynes borylation method, which can afford the target products on
gram scale. Meanwhile, 2a as an example can be converted to
a wide range of desirable synthons in good yield. The Ag-catalyst
can be recovered conveniently after the reaction.
4.1.3. Synthesis of 4. A mixture of 2a(0.05 mmol), CuCN (0.05 mmol),
and K2CO3 (0.15 mmol) in DMF (1 mL) was stirred at 60 ꢁC under Ar
atmosphere for 4 h. The combined organic layer was washed with
brine, extracted withethyl acetate, driedover Na2SO4, andevaporated
under reduced pressure. The crude product was purified by silica gel
column chromatography to give the corresponding products.
4. Experiment
4.1. General
4.1.1. Synthesis of 2ae2o. A mixture of 1 (0.5 mmol), B(OiPr)pin
(0.75 mmol), PPh3þL1þAgCl (1 mol %), and Cs2CO3 (1.1 mmol) in
DMF (5 mL) was stirred at 50 ꢁC under Ar atmosphere for 24 h. The
reaction mixture was acidified by 1 M solution of hydrochloric acid
in an ice water bath, and the aqueous phase was extracted with
ethyl acetate (three times). The combined organic layer was
washed with brine, dried over Na2SO4, and evaporated under
4.1.4. Synthesis of 5. A mixture of 2a (0.05 mmol), 2-oxozolidinone
(0.1 mmol), Cu(NO3)2 (TEMEDA) (10 mol %), and K2CO3 (0.15 mmol)
in CH3OH (1 mL) was stirred in rt under Ar atmosphere for 48 h. The
combined organic layer was washed with brine, extracted with
ethyl acetate, dried over Na2SO4, and evaporated under reduced
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J.-R. Hu et al. / Tetrahedron xxx (2014) 1e5
3
O
O
5, yield: 75%
Ph
N
CuCN, K2CO3
Ph
CuBr2
H2O/CH3OH
Ph
Br
Ph
CN
Bpin
DMF
3, yield: 88%
4, yield: 81%
6, yield: 94%
Ph
Ph
Scheme 2. Versatile transformations of the borylated products.
Fig. 1. (a) Molecular structure of L01; (b) chemical structure of L10 ; (c) view of the two-dimensional network formed by CeH/F hydrogen bonds.
pressure. The crude product was purified by silica gel column
chromatography to give the corresponding products.
CDCl3):
228.0945.
d
24.5; [M]þ: HRMS for C14H17BO2 calcd 228.0946, found
4.1.5. Synthesis of 6. A mixture of 2a (0.05 mmol), CuSO4
(0.02 mmol), MgSO4 (0.1 mmol) in DMF (1 mL) was stirred at 60 ꢁC
under air atmosphere for 24 h. The combined organic layer was
washed with brine, extracted with ethyl acetate, dried over Na2SO4,
and evaporated under reduced pressure. The crude product was
purified by silica gel column chromatography to give the corre-
sponding products.
4.2.2. Compound 2b.11 Yield 84%. 1H NMR (500 MHz, CDCl3):
d 7.40
(d, J¼7.5 Hz, 2H), 7.10 (d, J¼7.5 Hz, 2H), 2.35 (s, 3H), 1.28 (s, 12H); 11
B
NMR (128 MHz, CDCl3):
242.1212, found 242.1209.
d
23.9; [M]þ: HRMS for C15H19BO2 calcd
4.2.3. Compound 2c. Yield 86%. Colorless oil, 1H NMR (500 MHz,
CDCl3): 7.48 (d, J¼8.6 Hz, 2H), 7.32 (d, J¼8.6 Hz, 2H), 1.32 (s, 9H),
d
1.24 (s,12H); 11B NMR (128 MHz, CDCl3): 23.5; 13C NMR (125 MHz,
d
CDCl3):
d 140.5, 131.3, 129.8, 119.1, 108.3 (br), 85.0, 29.8, 25.2, 19.8;
[M]þ: HRMS for C18H25BO2 calcd 284.2009, found 284.2010.
4.2. Characterization data
4.2.4. Compound 2d. Yield 95%. Colorless oil, 1H NMR (500 MHz,
4.2.1. Compound 2a.11 Yield 92%. 1H NMR (500 MHz, CDCl3):
d
7.40 (m, 2H), 6.86 (m, 3H), 1.03 (s, 12H); 11B NMR (128 MHz,
CDCl3):
d
7.51 (d, J¼8.2 Hz, 2H), 6.75 (d, J¼8.2 Hz, 2H), 3.78 (s,
Please cite this article in press as: Hu, J.-R.; et al., Tetrahedron (2014), http://dx.doi.org/10.1016/j.tet.2014.06.033

4
J.-R. Hu et al. / Tetrahedron xxx (2014) 1e5
3H), 1.18 (s, 12H); 11B NMR (128 MHz, CDCl3):
d
23.1; 13C NMR
82.8, 28.5, 26.4, 22.1, 19.3; [M]þ: HRMS for C13H21BO2 calcd
(125 MHz, CDCl3):
d
141.3, 133.6, 131.8, 123.2, 101.5 (br), 85.4,
220.1156, found 220.1155.
25.9, 18.3; [M]þ: HRMS for C15H19BO3 calcd 258.1206, found
258.1210.
4.2.16. Compound 3.17 Yield 88%. 1H NMR (500 MHz, CDCl3):
d 7.38
(m, 2H), 7.25 (m, 3H); [M]þ: HRMS for C8H5Br calcd 181.0293, found
181.0290.
4.2.5. Compound 2e. Yield 81%. Mp 63e65 ꢁC, 1H NMR (500 MHz,
CDCl3):
d
d
7.74 (m, 4H), 1.27 (s, 12H); 11B NMR (128 MHz, CDCl3):
25.3; 13C NMR (125 MHz, CDCl3):
d
136.2, 135.1, 134.5, 128.2, 107.9
4.2.17. Compound 4.18 Yield 81%. 1H NMR (500 MHz, CDCl3):
d 7.61
(br), 86.1, 27.4, 21.5; [M]þ: HRMS for C15H16BF3O2 calcd 296.0925,
found 296.0922.
(d, J¼7.5 Hz, 2H), 7.53 (t, J¼7.4 Hz, 1H), 7.42 (t, J¼7.4 Hz, 2H); [M]þ:
HRMS for C9H5N calcd 127.1427, found 127.1428.
4.2.6. Compound 2f. Yield 78%. Mp 68e69 ꢁC, 1H NMR (500 MHz,
4.2.18. Compound 5.19 Yield 75%. 1H NMR (500 MHz, CDCl3):
d 7.44
CDCl3):
d
8.14 (d, J¼8.6 Hz, 2H), 7.64 (d, J¼8.6 Hz, 2H), 1.23 (s, 12H);
25.8; 13C NMR (125 MHz, CDCl3):
146.2, 133.8, 131.6, 129.7, 109.3 (br), 84.8, 22.3; [M]þ: HRMS for
14H16BNO4 calcd 273.0921, found 273.0920.
(m, 2H), 7.30 (m, 3H), 4.48 (t, J¼7.8 Hz, 2H), 4.01 (t, J¼7.8 Hz, 2H);
11B NMR (128 MHz, CDCl3):
d
[M]þ: HRMS for C11H9NO2 calcd 187.1947, found 187.1948.
d
C
4.2.19. Compound 6.15 Yield 94%. 1H NMR (500 MHz, CDCl3):
d 7.38
(m, 4H), 7.29 (m, 6H); [M]þ: HRMS for C16H10 calcd 202.2506, found
202.2505.
4.2.7. Compound 2g. Yield 79%. Mp 48e50 ꢁC, 1H NMR (500 MHz,
CDCl3):
7.54 (m, 4H), 1.16 (s, 12H); 11B NMR (128 MHz, CDCl3):
d
d
22.9; 13C NMR (125 MHz, CDCl3):
d 137.5, 135.8, 133.8, 131.6, 129.7,
Acknowledgements
127.5, 109.3 (br), 85.2, 21.8; [M]þ: HRMS for C14H16BClO2 calcd
262.5396, found 262.5399.
This work was supported by the Scientific and Technological
Landing Project of Higher Education of Jiangxi Province (No.
KJLD12094), and the National Natural Science Foundation of China
(Nos. 21261020; 21361022).
4.2.8. Compound 2h. Yield 82%. Colorless oil, 1H NMR (500 MHz,
CDCl3):
CDCl3):
d
7.43 (m, 4H), 1.31 (s, 9H), 1.08 (s, 12H); 11B NMR (128 MHz,
d
22.5; 13C NMR (125 MHz, CDCl3):
d 136.8, 135.4, 132.9,
130.3, 128.5, 126.3, 103.9 (br), 84.9, 29.6, 24.3, 20.5; [M]þ: HRMS for
Supplementary data
C
18H25BO2 calcd 284.2009, found 284.1006.
4.2.9. Compound 2i. Yield 70%. Mp 53e55 ꢁC, 1H NMR (500 MHz,
Experimental procedures and crystallographic data and struc-
tural refinement details (SD-Table 1); CCDC-998979 ðL01Þ contain
the supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.tet.2014.06.033.
CDCl3):
7.11 (m, 4H), 1.07 (s, 12H); 11B NMR (128 MHz, CDCl3):
d
d
23.3; 13C NMR (125 MHz, CDCl3):
d 130.8,129.4, 127.6, 125.0,123.5,
118.9, 105.1 (br), 84.6, 20.1; [M]þ: HRMS for C14H17BO3 calcd
244.0940, found 244.0939.
4.2.10. Compound 2j. Yield 85%. Colorless oil, 1H NMR (500 MHz,
CDCl3):
d
d
7.18 (m, 5H), 1.02 (s, 12H); 11B NMR (128 MHz, CDCl3):
23.8; 13C NMR (125 MHz, CDCl3):
d 130.8, 123.5, 121.6, 118.4, 105.8
References and notes
(br), 83.9, 19.6; [M]þ: HRMS for C14H17BO3 calcd 244.0940, found
244.0942.
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Hall, D. G., Ed.; Wiley-VCH: Weinheim, Germany, 2005.
2. For selected examples of Pd-catalyzed borylation of aryl halides, see: (a) Ish-
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Murata, M.; Watanabe, S.; Masuda, Y. J. Org. Chem. 1997, 62, 6458e6459; (c)
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2010, 132, 17701e17703.
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C.; Resmerita, A. M.; Corcoran, P.; Hoang, L. M.; Rosen, B. M.; Percec, V. J. Am.
Chem. Soc. 2010, 132, 1800e1801; (b) Huang, K.; Yu, D. G.; Zheng, S. F.; Wu, Z. H.;
Shi, Z. J. Chem.dEur. J. 2011, 17, 786e791; Cu-catalyzed coupling: (c) Zhu, W.;
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4.2.11. Compound 2k. Yield 83%. Colorless oil, 1H NMR (500 MHz,
CDCl3):
(128 MHz, CDCl3):
d
2.75 (m, 1H), 2.00e1.63 (m, 10H), 0.92 (s, 12H); 11B NMR
d
22.5; 13C NMR (125 MHz, CDCl3):
d 101.2 (br),
85.5, 29.8, 25.5, 24.9, 22.6, 18.7; [M]þ: HRMS for C15H25BO2 calcd
248.1688, found 248.1685.
4.2.12. Compound 2l.11 Yield 73%. 1H NMR (500 MHz, CDCl3):
d 2.25
(t, J¼7.0 Hz, 2H), 1.48 (m, 2H), 1.39 (m, 2H), 1.25 (s, 12H), 0.85 (t,
J¼7.5 Hz, 3H); 11B NMR (128 MHz, CDCl3):
d
23.5; [M]þ: HRMS for
C
12H21BO2 calcd 208.1049, found 208.1050.
4.2.13. Compound 2m. Yield 68%. Colorless oil, 1H NMR
(500 MHz, CDCl3):
1.33 (s. 9H), 1.01 (s, 12H); 11B NMR
(128 MHz, CDCl3):
4. Activation and Functionalization of CeH Bonds InGoldberg, K. I., Goldman, A. S.,
Eds.. ACS Symposium Series; American Chemical Society: Washington, DC, 2004;
Vol. 885.
d
d
22.2; 13C NMR (125 MHz, CDCl3):
d 101.9
(br), 84.1, 28.3, 21.9, 19.8; [M]þ: HRMS for C12H21BO2 calcd
208.1049, found 208.1045.
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4.2.14. Compound 2n.11 Yield 88%. 1H NMR (500 MHz, CDCl3):
d
0.95 (s, 12H), 0.08 (s, 9H); 11B NMR (128 MHz, CDCl3):
23.5; [M]þ: HRMS for C11H21BO2Si calcd 224.1797, found
d
224.1795.
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ꢀ
Commun. 2010, 159e161; (d) Selander, N.; Willy, B.; Szabo, K. J. Angew. Chem.,
CDCl3):
(128 MHz, CDCl3):
d
2.73 (m, 1H), 1.92e1.60 (m. 8H), 0.99 (s, 12H); 11B NMR
ꢀ
Int. Ed. 2010, 49, 4051e4053; (e) Ros, A.; Estepa, B.; Lopez-Rodríguez, R.; lvarez,
d
22.1; 13C NMR (125 MHz, CDCl3):
d
100.5 (br),
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Please cite this article in press as: Hu, J.-R.; et al., Tetrahedron (2014), http://dx.doi.org/10.1016/j.tet.2014.06.033

J.-R. Hu et al. / Tetrahedron xxx (2014) 1e5
5
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Please cite this article in press as: Hu, J.-R.; et al., Tetrahedron (2014), http://dx.doi.org/10.1016/j.tet.2014.06.033