Copper-catalyzed cross-coupling of vinyliodonium salts and diboron reagents to generate alkenyl boronic esters

Article abstract of DOI:10.1039/d0ob01121e

An efficient approach for the synthesis of alkenyl boronic esters through the copper-catalyzed cross-coupling of vinyliodonium salts and diboron reagents is reported. This method is distinguished by its mild conditions and short reaction time of less than

Full text of DOI:10.1039/d0ob01121e

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DOI: 10.1039/D0OB01121E
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Copper-catalyzed cross-coupling of vinyliodonium salts and
diboron reagents to generate alkenyl boronic esters
Dan Zhu,a Shaoyan Gan,a Robert Li-Yuan Baoa and Lei Shi*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
An efficient approach for the synthesis of alkenyl boronic esters
through the copper-catalyzed cross-coupling of vinyliodonium salts
and diboron reagents is reported. This method is distinguished by
its mild condition and short reaction time within 30 min, which
should provide an additional way for the construction of alkenyl
boronic esters.
a)
Previous approaches to alkenylboronates
B2(pin)2/ROH
R
R
path a
Bpin
R1
Cu, Pd, Rh, Ru
R2/H
Bpin
B2(pin)2/R2-X or HOR
[M]
Bpin
path b
path c
R1
R1
R2/H
Organoboron compounds are among the most versatile
building blocks, which gain great interest in the pharmaceutical
industry and academic community.1 Alkenyl boronic esters, as
an important component of organoboron compounds, have
been widely employed in a variety of valuable transformations,
including Zweifel olefination,2 Suzuki cross-couplings3 and
Petasis reactions.4 Meanwhile, as a kind of important coupling
partners, alkenyl boronic esters can also be applied in different
transition-metal-catalyzed C–O, C–N, C–F, C–Br, C–I bonds
forming protocols.5 Moreover, alkenyl boronic esters can be
converted to the ate complexes followed by radical induced
rearrangements, which further highlights the utility of alkenyl
boronic esters.6 Therefore, the extension and enrichment of
methods for the synthesis of alkenyl boronic esters is really
necessary.
B2(OR)4 or HB(OR)2
Pd, Rh, Fe, Cu
R
R
Bpin
H
B
B
R1
Bpin
R2
Bdmpd
or
O
R2
path d
R1
R1
R2
H
b)
This work
BF4 or OTf
R2
R2
Cu+IX cat.
Bpin
R3
I
R3
Ar
B2(pin)2
R1
This work
R1
Scheme 1 Methods for the synthesis of alkenyl boronic esters.
developing transformations for the synthesis of alkenyl boronic
esters further demonstrate the importance of exploring new
strategies. However, among these diverse substrates, alkenyl-
(aryl)iodonium salts19 as highly active alkene counterparts have
not been applied yet to prepare such a class of valuable
compounds. Over the last few years, numerous pioneer work
has been reported on these highly reactive substrates with a
variety of different nucleophiles, which conducted by the action
of copper catalyst.20 Herein, we report the successful realization
of such a copper-catalyzed boronation strategy through the
combination of diboron reagents and alkenyl iodonium salts in
excellent yields within 30 min under moderate reaction
condition (Scheme 1, b).
Alkenyl boronic esters can be mainly synthesized through
transition-metal-catalyzed (i.e., Pd, Cu, Rh, and Ru)
hydroboration of alkynes (Scheme 1, path a) and
carbo/hydroboration of allenes (Scheme 1, path b).7−9 In
addition,
transition-metal-catalyzed
dehydrogenative
borylation of alkenes (Scheme 1, path c) by β−H elimination is
also an efficient approach to prepare alkenylboronates.10−17 It is
worth mentioning that besides above transformations, the
boron-Wittig reaction of aldehydes or ketone enolates (Scheme
1, path d) can be identified as a novel transition-metal-free
tactics to generate vinylboronates.18 These continuously
We firstly proceed to screening out the optimal reaction
condition with cyclic alkenyl iodonium salts 1a and B2(pin)2
(bis(pinacolato)diboron) 2a as the model reaction substrates in
THF to generate the target product 3aa (Table 1). Through
a. School of Science, Harbin Institute of Technology, Shenzhen, 518055, China.
E-mail: lshi@hit.edu.cn; http://homepage.hit.edu.cn/shilei.
†Electronic
Supplementary
Information
(ESI)
available:
See
DOI:
10.1039/x0xx00000x
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preliminary screening, CuI seems to be preferable compared to experimental results show that the substrate 1aMes supplied 3aa
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CuBr or CuCl for this cross coupling as the isolated yield was with a slightly increased yield compared tDoO1I:a140−.F1P0h3f9r/oDm0O8B60%112t1oE
raised from 89% to 91% (Table 1, entry 1–3). However, when
Table 2 Copper-catalyzed C(sp2)−B cross-coupling reactiona,b
directly removed the CuI catalyst or the ligand PPh3, the
10 mol% CuI
reaction substrate B2(pin)2 2a almost cannot be allowed to
10 mol% PPh3
2.0 equiv t-BuOLi
1.0 equiv Diboron reagents (
R2
BF4 or OTf
conduct the corresponding transformation (Table 1, entry 4–5).
The reaction yields decreased sharply to 24% and 17%
respectively when change the base from t-BuOLi to t-BuONa
and t-BuOK (Table 1, entry 7–8), which greatly consistent with
previous work.21 Furthermore, in the absence of the base, the
whole reaction system become colloidal (Table 1, entry 6).
Spreading the above colloid on the glass plate to dry, and it
became a plastic stretch film, which may owe to the
polymerization of alkenyl iodonium salts in THF. Afterwards, the
yield decreases to 69% when the reaction temperature rise to
room temperature. It may be due to the instability of cyclic
alkenyl iodonium salts at the relatively high temperature (Table
1, entry 10). Therefore, we chose to reduce the reaction
R2
BX2
I
R3
2a-c)
R3
Ar
R1
R1
1a-m
o
rt (30 min)
THF / 0 C to
3aa-3ma
variation of diboron reagent
O
B
O
B
Bpin
O
O
3aa, 91%; 86%;c 94%d
3ab, 57%
3ac, 40%
N
O
B
N
B
O
o
temperature to -78 C (Table 1, entry 9), unfortunately, the
3ad, 0%
3ae, 0%
yield dropped slightly to 86% compared to 0 oC. Consequently,
the reaction condition described in entry 3 (Table 1) was
selected as the standard condition for investigation of the
reaction scope.
variation of cyclic alkenyl iodonium salt
Bpin
Bpin
Bpin
Table 1 Condition optimizationa
3ba, 86%
3ca, 73%; 81%d
3da, 47%
10 mol% copper salt
Bpin
Bpin
10 mol% PPh3
0.2 mmol B2(pin)2 (
BF4
t-Bu
2a
)
I+
Ph
Bpin
0.4 mmol t-BuOLi
3fa, 88%
3ea, 91%
o
rt (30 min)
THF / 0 C to
variation of terminal alkenyl iodonium salt
3ga, X = H, 78%
1a
3aa
, 0.3 mmol
Bpin
Bpin
Entry copper salt
(10 mol%)
variation
yield
3ha, X = F, 80%
3la, 81%
Bpin
(%)b
89
89
91
0
3ia, X = Cl, 83%
Cl
1
2
3
4
5
6
7
8
9
CuCl
CuBr
CuI
- -
CuI
CuI
CuI
CuI
CuI
CuI
none
none
none
none
3ja, X = Br, 82%
X
3ka, X = t-Bu, 81%
3ma, 77%
unsuccessful iodonium salt
OTf
I
OTf
I
without PPh3
without t-BuOLi
t-BuONa instead of t-BuOLi
t-BuOK instead of t-BuOLi
-78 oC instead of 0 oC
rt
0
0
24
17
86
69
aConditions: CuI (0.02 mmol, 10 mol %), PPh3 (0.02 mmol, 10
mol %), diboron reagents (0.2 mmol, 1.0 equiv), t-BuOLi (0.40
mmol, 2.0 equiv) and THF (2 mL) was added, a solution of
iodonium salts (Ar = Ph) (0.3mmol, 1.5 equiv) in THF (2 mL)
was added dropwise; All reactions were performed for 30
10
aConditions: CuI (0.02 mmol, 10 mol %), PPh3 (0.02 mmol, 10
mol %), (Bpin)2 (0.2 mmol, 1.0 equiv), base (0.40 mmol, 2.0
equiv) and THF (2 mL) was added, a solution of iodonium
salts (0.3mmol, 1.5 equiv) in THF (2 mL) was added dropwise;
All reactions were performed for 30 min at the
corresponding reaction temperature. bIsolated yield after
flash chromatography on silica gel.
b
min at the corresponding reaction temperature. Isolated
yield after flash chromatography on silica gel. Ar = 4−FPh;
c
dAr = Mesityl.
94%. Afterwards, a series of diboron reagents were investigated
immediately. Unfortunately, the reaction process became less
effective (3ab and 3ac), or totally ineffective (3ad and 3ae),
which may be caused by the Lewis acidity and steric properties
of the boronate esters. Yields were consistently high either for
the substrate of penta cyclic alkenyl iodonium salts (3ba) or
para-substituted model substrates (3ca, 3ea and 3fa). However,
it’s worth mentioning that the vinyliodonium salt 1d can only
obtain the corresponding target product (3da) with the yield of
With the standard protocol established, we next investigated
the reaction scope for various vinyliodonium salts and diboronic
reagents (Table 2). Initially, three different kinds of
vinyliodonium salts which Ar = Ph, 4−FPh, mesityl were explored
with the standard condition to investigate the impact of the
leaving performance of iodobenzene in this process. The
2 | J. Name., 2012, 00, 1-3
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47%. It may be due to the instability of the substrate 1d in the aryl iodide by-product. Finally, the intermediate VBiewuAnrtdicelerOgnolienes
DOI: 10.1039/D0OB01121E
presence of the ortho methyl group. Different reductive elimination to generate the product and the active
vinyl(aryl)iodonium triflates with different substituents on the LCu(I)OtBu complex in the presence of t-BuOLi.
aromatic rings were then tested and the corresponding
boronation reactions provide the respective vinyl-substituted
products in good yields (3ga-3ka). Additionally, aliphatic
Conclusions
terminal alkenyl iodonium salts were also compatible with the
reaction conditions (3la and 3ma). Inspired by the previous
work about successful boration of open-chain diaryliodonium
salts,22 we then examined the compatibility of annulated
diaryliodonium salts under standard reaction condition. It is a
pity that annulated diaryliodonium salts did not generate the
expected borated products, apart from the only by-product of
substituted iodobenzene.
In conclusion, we have introduced a new method for the
preparation of alkenyl boronic esters, which can make the
reaction process more mild and efficient by using alkenyl(aryl)
iodonium salts as pre-activated alkene counterparts through
the copper-catalyzed cross-coupling. Not only are aromatic,
aliphatic terminal alkenyl iodonium salts accommodated, but
cyclic alkenyl iodonium salts are also compatible in forming the
corresponding alkenyl boronic esters in high yields through this
reaction system. Further investigations will be continued to
explore the reaction properties of vinyliodonium salts and the
potential of related transformations in our laboratory.
CuII
LiBF4 or LiOTf
L
t-BuOLi
R2
We are grateful for the financial support from the National
Natural Science Foundation of China (Grant No. 21871067), the
Natural Science Foundation of Guangdong Province (Grant No.
2018A030313038), the Shenzhen Fundamental Research
Projects (Grant No. JCYJ20180306171838187), the Harbin
Institute of Technology (Shenzhen) (Talent Development
Starting Fund from Shenzhen Government) and the Open
Project Program of Beijing National Laboratory for Molecular
Sciences (BNLMS201819).
Bpin
B2(pin)2
R3
[LCuIOtBu]
R1
Bpin-OtBu
t-BuOLi
Bpin CuIIIL R2
BF4 or OTf
[LCuIBpin]
R1
B
R3
A
R2
BF4 or OTf
Ar
Ar-I
I
Conflicts of interest
There are no conflicts to declare.
R3
R1
Scheme 2 Proposed catalytic cycle for copper-catalyzed
C(sp2)−B cross-coupling.
Notes and references
According to the previous reports,20 copper catalyst should
react preferentially with alkenyl-(aryl)iodonium salts to
generate copper(III)/alkenyl species. However, considering the
adding order of the experiment, alkenyl-(aryl)iodonium salts in
THF were finally introduced to the reaction system, so we
hypothesise that the oxidation addition might not occur first in
this reaction. To demonstrate, we then tried to very slowly
drop-add the alkenyl-(aryl)iodonium salts (in THF) more than 20
min at the room temperature to the reaction system, the yield
has not been affected compare to the result of Table 1, entry
10. Of course, we can't strictly rule out other possibilities so far,
but it seems the transmetalation process17b more likely to
proceed first. In addition, a radical pathway appears unlikely as
the addition of 1.5 equiv of TEMPO had no effect on the reaction
outcome.
Therefore, we propose the oxidative addition−reductive
elimination Cu(I)/Cu(III) catalytic cycle overviewed in Scheme 2.
Initially, the LCu(I)OtBu complex, which produced by the
reaction of CuI with t-BuOLi, undergoes the transmetalation
with pinacol diboron to afford the complex LCu(I)Bpin A and
pinBOtBu. An oxidative addition to intermediate A occurs
subsequently with the action of a vinyl iodonium salt to
generate the intermediate B accompanied by the generation of
1
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DOI: 10.1039/D0OB01121E
An efficient approach for the synthesis of alkenyl boronic esters through the copper-catalyzed
cross-coupling of vinyliodonium salts and diboron reagents is reported.
BF4 or OTf
R2
R2
Bpin
CuI/PPh3(10 mol %)
R3
I
R3
Ar
B2(pin)2
tBuOLi, THF
0oC to rt for 30 min
R1
R1
up to 94% yield