Transition-metal-free hydroboration of terminal alkynes activated by base

Article abstract of DOI:10.1016/j.tetlet.2015.09.077

Transition-metal and ligand-free method for the hydroboration of terminal alkynes with B₂pin₂ was reported in LiOt-Bu/toluene/MeOH system under room temperature. Alkylalkynes and arylalkynes reacted efficiently with high regioselecti

Full text of DOI:10.1016/j.tetlet.2015.09.077

Accepted Manuscript
Transition-Metal-free Hydroboration of Terminal Alkynes Actived by Base
Shibin Hong, Wei Zhang, Mengyan Liu, Wei Deng
PII:
S0040-4039(15)30119-2
DOI:
http://dx.doi.org/10.1016/j.tetlet.2015.09.077
TETL 46749
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
28 July 2015
14 September 2015
18 September 2015
Please cite this article as: Hong, S., Zhang, W., Liu, M., Deng, W., Transition-Metal-free Hydroboration of Terminal
Alkynes Actived by Base, Tetrahedron Letters (2015), doi: http://dx.doi.org/10.1016/j.tetlet.2015.09.077
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Tetrahedron Letters
Transition-Metal-free Hydroboration of Terminal Alkynes Actived by Base
a
a
a
a,b
Shibin Hong , Wei Zhang , Mengyan Liu , Wei Deng *
a
Research Center of Nano Science and Technology, Shanghai University, Shanghai 200444, China
School of Chemical and Engineering, Shanghai Institute of Technology, Shanghai 201418, China
b
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
2 2
Transition-Metal and ligand-free method for the hydroboration of terminal alkynes with B pin
was reported in LiOt-Bu/Tuluene/MeOH system under room temperature. Alkylalkynes and
arylalkynes reacted efficiently with high regioselectivity.
2
015 Elsevier Ltd. All rights reserved.
Available online
Keywords:
Metal-free
Hydroboration
Alkynes
Organoboron
and green chemistry, leading to the limitation in widely
application in chemical industry and pharmaceuticals industry.
Organoboron compounds are important subjects in organic
chemistry, which play a significant role in organic synthesis, due
to highly reactivity, stability under atmospheric conditions and
1
Recently, transition-metal-free or ligand-free borylation
reactions occurred as interesting alterations. Several methods that
reactions were catalyzed by ligand without transtition-metal have
low toxicity. Much attention has been focused on looking for
easy and efficient ways to prepare kinds of organoboron reagents,
including vinylboronates. Vinylboronates have drawn much
attention because of their intrinsic scientific importance and
9
been reported to form C-B bonds and N-B bonds. Hoveyda also
2
presented transition-metal-free enantioselective method for boron
industrial applications. The traditional C-B formation via
9e
conjugate addition to α,β-unsaturated carbonyls. Furthermore,
addition reaction of boranes with alkenes or alkynes, which was
conducted under relatively harsh conditions. Recently the
synthesis of organboron compounds by hydroboration of alkene
ligand-free hydroboration of alkenes or alkynes successfully
10
catalyzed by transition metals, including platinum and copper.
3
Concerning the hydroboration of alkynes, Li used heterogeneous
micro copper powder without ligand as a catalyst for addition of
or alkyne derivatives had been disclosed . Several transition
metals have been used to catalyze hydroborations of internal or
11
9b
4
B
2
pin
2
to internal and terminal alkynes (Scheme 1A) . And Sun
transition-metal-free hydroboration of terminal
terminal alkynes over the past few years, including platinum ,
5
6
7
8
reported
a
copper , silver , gold , iridium . Using transition metal and ligand
as catalyst system poses challenges in environmental problem
alkynes under open atmosphere (Scheme 1B). Herein, we firstly
report a transition-metal- and ligand-free hydroboration of
terminal alkynes directly activied by base at room temperature
(Scheme 1C).
Previously, we reported copper-catalyzed borylation of primary
and secondary alkyl halides with bis(pinacolato)diboron (B
2
pin2,
1
2
2
). When 6-chloro-1-hexyne (1a) and B
attempt to prepare the alkyne boronate product in the presence of
Cu O/PPh /KOt-Bu. In the solvent of DMF and NMP, the
2 2
pin were used in an
2
3
reaction proceeded to conversion of both alkyne and halid group,
and produced a mixture of hydroborylated products (3a+4a) and
alkyne boronate (5a) product (Table 1, entry 1, 2). Change
solvent to toluene, only compounds of 3a and 4a were observed
with a low yield of 8%. With MeOH as additive, the yield
dramatically increased to 74%. Analysis of the products indicated
Scheme 1. Ligand-free (A) and transition-metal-free (B) hydroboration of
alkynes. (C) Our protocol of transition-metal and ligand-free of
hydro-boration of termin al alkynes directly was actived by base.

high regioselectivity, and -hydroboration product (3a) was main
product with a ratio of - and -hydroboration product
approximately 13:87(entry 3, 4). The conditions of ligand-free or alkyl-alkynes with B
transition-metal-free was also tried, respectively. Similar yields
of 76% and 70% were obtained, but poor regioselectivity for
ligand-free condition (entry 5, 6). In the absence of
transition-metal and ligand, the yield slightly decreased to 65%
Table 2. Transition-metal and ligand-free hydroboration of various
a
2 2
pin .
Table 1. Optimization of conditions for hydroboration reactions of 6-chlor
Bpin
Bpin
Bpin
Bpin
Cl
c
3
b, 78%
3c,68%
3
a, 80%
OH
Bpin
HO
Bpin
Bpin
O
3
d, 41%
3e, 39%
3f, 65%
Bpin
Bpin
Bpin
3
g, 60%
3h, 62%
3i, 58%
a
b
Reaction Conditions: alkynes 1 (0.25 mmol), bis(pinacolato)diboron 2 (0.38
Entry
Base
Solvent
DMF
Additive
-
Yield of
a+4a (5a)
43 (49)
3a:4a
b
mmol), LiOt-Bu (0.13 mmol) in1 mL toluene and 200 L MeOH at room
3
b
c
temperature for 24h. Isolated yield. 3 equivalents of B2pin2.
c
1
KOt-Bu
KOt-Bu
KOt-Bu
KOt-Bu
KOt-Bu
KOt-Bu
KOt-Bu
None
65:35
90:10
87:13
88:12
55:45
80:20
88:12
-
c
2
NMP
-
48 (34)
8
with high regioselectivity of 3a:4a=88:12 (entry 7).
c
3
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
-
c
Encouraged by these results, we used 6-chloro-1-hexyne (1a)
and B pin as the model substrates in the transition-metal-free
2 2
4
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
EtOH
i-PrOH
t-BuOH
MeOH
74
d
5
76
and ligand-free reactions for the screening of reaction conditions
including bases, alcohol additives and atmospheres at room
temperature, The detailed results were listed in Table 1. In the
absence of base, no reaction was observed, which indicated that
the hydroboration required base to initiate the reaction (entry 8).
Several bases, including inorganic and organic bases, were tried
in toluene with MeOH. Comparing with inorganic bases, organic
e
6
70
7
8
9
65
Trace
67
Et
3
N
80:20
82:18
65:35
90:10
90:10
90:10
88:12
90:10
89:11
64:36
90:10
77:23
65:35
-
3
bases of Et N, DIEA and DBU gived low yields and low
1
1
1
1
1
1
1
1
1
1
2
2
2
0
1
2
3
4
5
6
7
8
9
0
1
2
DIEA
DBU
53
regioselectivityies (entry 9-11). Most of alkaline metals bases,
except NaOt-Bu, showed the excellent yields (entry 12-19).
LiOt-Bu provided the highest yield of 88% with 90%
20
Li
Na
2
CO
CO
CO
3
69
2
3
71
Table 3. Transition-Metal and ligand-free hydroboration of various
a
2 2
aryl-alkynes with B pin .
K
2
3
77
NaOH
LiOMe
78
86
Bpin
Bpin
Bpin
NaOMe
NaOt-Bu
LiOt-Bu
LiOt-Bu
LiOt-Bu
LiOt-Bu
LiOt-Bu
84
Cl
F
3
j, 85%
3k, 83% Bpin
3l, 84%
52
Bpin
Bpin
88
Br
Me
n-Bu
3
m, 80%
3n, 85%
3
o, 68%
35
Bpin
Bpin
Bpin
5
MeO
3
p, 94%
NH2
F
Trace
Trace
3
q, 56%
3r, 86%
Bpin
Bpin
Bpin
[
f]
2
a
3
-
Cl
Reaction Conditions: 6-chloro-1-hexyne 1a (0.25 mmol), 2 (0.38 mmol),
F
3
u, 79%
base (0.13 mmol) in 1 mL Toluene and 200 L MeOH for 24 h, unless
otherwise noted. Yield was determined by GCMS using pyrene as an
Me
3t, 82%
b
3s, 80%Bpin
Bpin
c
internal. Reaction Conditions: Cu2O(10 mol%), PPh3 (10 mol%).
d
e
N
Reaction Conditions: Cu2O(10 mol%). Reaction Conditions: PPh3 (10
N
f
3
w, 51%
mol%). under air.
3v, 63%
a
Reaction Conditions: alkynes 1 (0.25 mmol), bis(pinacolato)diboron 2
(
0.38 mmol), LiOt-Bu (0.13 mmol) in 1 mL toluene and 200 L MeOH at
b
room temperature for 24h. Isolated yield.

has been decomposed completely to Bpin-O-t-Bu (Scheme 2A).
Replacing B pin to pinacolborane under our standard conditions,
the reaction did not work (Scheme 2B), indicating
2
2
a
pinacolborane intermediate was not involved. Finally, we
conducted our model reaction under different amounts of
LiOt-Bu. In the range of 1.5 to 0.5 equivalent bases, the yields
almost kept stable nearly 88%. Even low to 0.1 equivalent base,
4
0% yield was still obtained, but the reaction did not work
without base, indicating that base acted as catalyst to active the
pin
B
2
2
.
Li
R
Bpin
H
R
H
Scheme 2. Addition reactions for confirm the mechanism for metal- and
ligand-free hydroboration of terminal alkynes with B pin
2
2
.
MeOH
B pin
2
2
regioisomeric product 3a (entry 19). Replacement of LiOt-Bu
with NaOt-Bu, the yield decreased significantly to 52% and poor
regioselectivity (entry 18). Four alcohol additives were tested.
Using MeOH, EtOH, i-PrOH and t-BuOH as additives, the yields
dramatically decreased to 88%, 35%, 5% and trace (Entries
Li Bpin
+
LiOt-Bu
t-BuOBpin
H
R
Bpin
H
MeOBpin
LiOMe
19-22), respectively, indicating MeOH was the better ability of
B pin
2
2
hydrogen donor. The yield also decreased significantly when the
reaction was carried out under air as compared to that under
argon protection (entry 23).
Scheme 3. A possible mechanism of transition-metal- and ligand-free
hydroboration of terminal alkynes with B pin
2
2
.
A possible mechanism for hydroboration of terminal alkynes is
therefore proposed in Scheme 3. First, B pin was actived by
LiOt-Bu to form Li-Bpin as intermediate, which reacted with
alkynes via addition reaction. Then, methanol acted as hydrogen
donor reagent to provide target compounds, and LiOMe was
With optimized reaction conditions including LiOt-Bu as base,
the hydroboration of various terminal alkylalkynes was
examined. As shown in Table 2, the substrates gave good yields
with high regioselectivity. The reaction of mono-alkyne and
di-alkyne gave the desired products with good yields (3b, 3c).
Unprotected propargyl alcohols shown decreased yields about
2
2
2 2
obtained as by-product, which actived another B pin for reaction
circles.
4
3
0% (3d, 3e). The benzyl protected propargyl alcohol and
-phenylpropyne gave the desired products in 65% and 60%
Acknowledgments
yield (3f, 3g). Primary and secondary alkyl-substituted alkyne
substrates afforded the desired products in similar yields (3h, 3i),
indicating well toleration of a sterically bulky substituent for the
reaction.
We gratefully acknowledge financial support from the Eastern
Scholar, Shanghai Pujiang Program, Key Subject of Shanghai
Municipal Education Commission, Natural Science Foundation
of Shanghai (12ZR1410500), and NSFC (Nos. 21102088,
Next, the reactions for terminal arylalkynes were investigated
under same conditions (Table 3). The most substituted
arylalkynes gave better yields than alkylalkynes, possibly due to
the electron-rich aromatic rings. A wide range of substitutive
terminal arylalkynes on para-position were tolerated under these
conditions, and electron-withdrawing and electron-donating
groups on phenyl groups led to similar yields, including floro
5
1202138 and 21174081).
Supplementary Material
Supplementary data associated with this article can be found,
in the online version.
(
(
3k), chloro (3l), bromo (3m), methyl (3n), n-butyl (3o) and ether
3p). Unprotected amine alkynes shown low yield of 56% in
References and notes
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afford the products in 63% and 51% yields, respectively (3v,
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5
.
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Transition-Metal-free Hydroboration of
Terminal Alkynes Actived by Base
Leave this area blank for abstract info.
Shibin Hong, Wei Zhang, Mengyan Liu, Wei Deng*
O
O
O
LiOt-Bu (0.5 equiv)
Bpin
R
B B
+
R
Toluene/MeOH, RT,24h
O
R=alkyl, aryl
2
3 exampls, yield:94-39%