Nickel-Catalyzed Cross-Coupling of Aryl Redoxactive Esters with Aryl Zinc Reagents

Article abstract of DOI:10.1021/acscatal.9b02913

A nickel-catalyzed aryl-aroyloxyl C(sp²)-O radical cross-coupling reaction conducted using a redox active ester with aryl zinc reagent was developed. This method demonstrates a new disconnection approach for formation of aryl aryl esters. In the one-pot sequential process, the readily available aryl carboxylic acids can be converted into functionalized aryl aryl esters and heteroaryl esters. This protocol is amenable to the gram-scale synthesis. The present method has a wide substrate scope and high functional group tolerance.

Full text of DOI:10.1021/acscatal.9b02913

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Letter
Nickel-Catalyzed Cross-Coupling of Aryl
Redoxactive Esters with Aryl Zinc Reagents
Bo-Hao Shih, R. Sidick Basha, and Chin-Fa Lee
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b02913 • Publication Date (Web): 26 Aug 2019
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ACS Catalysis
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Nickel-Catalyzed Cross-Coupling of Aryl Redoxactive Esters with
Aryl Zinc Reagents
Bo-Hao Shih,† R. Sidick Basha,† Chin Fa Lee*,†,‡,§
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†Department of Chemistry, National Chung Hsing University, Taichung, Taiwan 402, R.O.C
‡Research Center for Sustainable Energy and Nanotechnology (RCSEN), Taiwan
§Innovation and Development Center of Sustainable Agriculture (IDCSA), Taiwan
Supporting Information Placeholder
O
O
O
N
Ar/
Het
R1
O
Ni
O
O
Ar
Ar/
Het
R1
Aryl NHPI esters
+
R
new disconnection
one-pot
.
ZnCl LiCl
Aryl aryl esters
Ar
R
Arylzinc reagents
ABSTRACT: A nickel-catalyzed aryl–aroyloxyl C(sp2)–O radical cross-coupling reaction conducted using a redox active ester with
aryl zinc reagent was developed. This method demonstrates a new disconnection approach for aryl aryl esters formation. In the one-
pot sequential process, the readily available aryl carboxylic acids can be converted into functionalized aryl aryl esters and heteroaryl
esters. This protocol is amenable to the gram scale synthesis. The present method has a wide substrate scope and high functional
group tolerance.
KEYWORDS: nickel, cross-coupling, redox-active esters, EPR study, single-electron transfer
The formation of carbon-heteroatom bond serves a vital role in
cross-coupling reactions.1-2 Aryl esters are prevalent motifs
found in the building blocks of a variety of natural products3,4
and widely used in pharmacological science.5-8 The C–O
coupling to esters has been explored using limited routes
through transition metal-mediated cross-coupling reactions
with proanionic carboxylate chemistry.9-11 The use of cross-
coupling reactions with a single-electron transfer process is a
widely used and robust strategy that includes two electron
metal-mediated coupling reactions.12 Recently, the radical
cross-coupling (RCC) reaction catalyzed by transition metals
has exhibited a high in demand in chemical synthesis.13-15
However, the limited components were engineered and
flourished as coupling partners in RCC reactions. Baran and
coworkers have proposed that the unique alkyl redox active
esters are decarboxylative coupling partners in the C–C and C–
B coupling reactions.16-19 We envisioned the unmapped
challenge of the C–O coupling of aryl redox active esters by
using arylmetallic reagents (Figure 1A). In this study, a new
approach was proposed for C(sp2)–O radical cross-coupling of
aryl redoxactive esters with aryl zinc reagents to aryl aryl esters
(Figure 1B). The hypothesized mechanism is outlined in Figure
1C. The NiI–X species 1 undergoes transmetalation with aryl
zinc reagent 2 to afford a NiI–aryl species 3. The reduction of
aryl NHPI esters 4 from 3 through the single-electron transfer
process causes O–N fragmentation of 6. When the NiII–aryl
cation 5 is added to the generated benzoyloxyl radical 7 and
phthalimide anion A*, the NiIII(aryl)(benzoyloxy)(A*) species 8
is formed. The reductive elimination of 8 provides the product
9 and regenerates the NiI catalyst species 1 (ERP results, see
supproting material for the details).
At the outset, the redox active ester 4 with 2 in the
.
presence of NiCl2 6H2O with ligand L1 afforded a yield of 54%
.
(Table 1, entry 1). By varying the amount of both NiCl2 6H2O
and ligand L2, a higher yield was obtained with 10 mol%
.
NiCl2 6H2O in combination with 20 mol% L2. By further
increasing the mol% of nickel, the yield was found to decrease
(Table 1, entries 2–5). The reaction with 2,2'-bipyrimidine
ligand L3 provides only 8% yield, thus demonstrating that the
two bidentate nitrogen ligand species hampered the reaction
efficiency. By using the phenanthroline ligand L4, 46% yield
was obtained (Table 1, entries 6 and 7). Moreover, we examined
the phenanthroline ligand analogs L5–L8. The results revealed
that compared with L5 and L8, the aryl substituted L6 and
sterically hindered L7 reduce the reaction efficiency and
provide lower product yield (Table 1, entries 8–11). Anhydrous
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NiCl2 with ligands L1, L2, L4, and L5 was found to be
Page 2 of 6
trisubstituted methyl 26 and isopropyl 27 were also successfully
coupled. The fused aromatic system smoothly afforded product
28 with a 70% yield.
1
2
3
4
5
6
7
8
inefficient (Table 1, entries 12–15). Similarly, Ni(acac)2 with
ligands L1 or L2 delivered lower yield. Moreover, Ni(acac)2
with ligands L4 and L5 afforded a product yield of 46% and
48%, respectively (Table 1, entries 16–19). NiF2 with L2
provided a product yield of 33% (Table 1, entry 20).
Prefabricated Ni complexes with L2 are ineffective (Table 1,
entries 21 and 22). Ni(cod)2 is also active as a Ni source to give
the produc in 58% yield (Table 1, entry 23). The Fe–L2 catalyst
system could also yield the product but presented a low yield
compared with the yield of Ni/L2 (Table 1, entry 24), starting
material 4 and biphenyl were detected by GC-MS analysis.
From the optimal studies, the Ni–L2 catalyst system is found to
be efficient for the C(sp2)–O radical cross-coupling reaction.
Table 1. Optimization of Reaction Conditionsa
O
O
O
O
4
N
Catalyst, Ligand
THF:DMF (3:2), Ar
O
O
+
10
.
12 h, 25 
C
PhZnCl LiCl
2
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
entry
Catalyst (mol%)
ligand
yield (%)b
.
1
NiCl2 6H2O (10)
20 mol% L1
4 mol% L2
54
51
57
73
50
08
46
54
05
11
47
10
16
12
14
17
19
46
48
33
35
23
58
44
a)
Challenge:
.
2
NiCl2 6H2O (2)
O
O
?
Ar1
O
.
A*
Ar1
3
NiCl2 6H2O (5)
10 mol% L2
20 mol% L2
40 mol% L2
20 mol% L3
20 mol% L4
20 mol% L5
20 mol% L6
20 mol% L7
20 mol% L8
20 mol% L1
20 mol% L2
20 mol% L4
20 mol% L5
20 mol% L1
20 mol% L2
20 mol% L4
20 mol% L5
20 mol% L2
20 mol% L2
20 mol% L2
20 mol% L2
20 mol% L2
+
Ar
M
O
Ar
A*
.
4
NiCl2 6H2O (10)
A*
Activating agent
.
b) This work: C(sp2)-O radical cross coupling
5
NiCl2 6H2O (20)
O
O
.
6
NiCl2 6H2O (10)
O
N
Ar/
Het
R1
O
.
7
NiCl2 6H2O (10)
Ni
O
O
Ar
Ar/
Het
R1
Aryl NHPI esters
+
.
R
8
NiCl2 6H2O (10)
new disconnection
one-pot
.
ZnCl LiCl
Aryl aryl esters
.
9
NiCl2 6H2O (10)
Ar
R
.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
NiCl2 6H2O (10)
Arylzinc reagents
c) Hypothesized Ni catalytic cycle:
.
NiCl2 6H2O (10)
O
NiCl2 (10)
Ph
O
Ar
[Ar
X
M]
2
9
I
L2Ni
1
X
NiCl2 (10)
*
-A
Transmetalation
reductive
elimination
NiCl2 (10)
*
A
O
I
Nickel Catalytic Cycle
L2Ni Ar
O
III
NiCl2 (10)
L
2Ni
Ph
3
O
4
Ar
8
Ni(acac)2 (10)
Ni(acac)2 (10)
Ni(acac)2 (10)
Ni(acac)2 (10)
NiF2 (10)
*
OA
Ph
SET
II
L2Ni Ar
5
addition
O
O
O
O
Ph
O
6
N
*
A
Ph
*
OA
+
O
Ph
7
O
O-N fragmentation
Figure 1. (a) Challenge. (b) New disconnection to C(sp2)-O
radical cross-coupling. (c) Hypothesized mechanism of Ni-
catalysed C(sp2)-O radical cross-coupling.
NiCl2(PPh3)2 (10)
NiCl2(dppe) (10)
Ni(cod) (10)
Fe(OAc)2 (10)
After optimizing the reaction conditions, the varying substrate
scope of the arylzinc reagent was evaluated. As shown in Table
2, the reaction with the electron donating substituent at the ortho
position such as methyl and methoxy afforded 11 and 12 with a
product yield of 59% and 71%, respectively. Similarly, the aryl
moiety at the second position delivered 13 with a product yield
of 62%. The electron-rich substituents at the meta or para
position of the phenylzinc reagent provided the desired
products 14–17 with a product yield in the range of 43%–79%.
The ortho or para position of the phenyl ring contains strong
electron-withdrawing groups such as fluoro, chloro, and
trifluoromethyl. These groups were tolerated well and the
coupled products 18–22 were obtained with a product yield in
the range of 54%–84%. The disubstituents on the phenyl ring,
such as methyl 23, fluoro, and methyl or phenyl 24 and 25,
provided the products with a moderate to good yield. Moreover,
aReaction performed with 1 equiv of 4, 3 equiv of 2 in 0.16 mmol scale.
bIsolated yield.
2
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ACS Catalysis
and para positions on the aromatic ring proceeded well and
1
2
3
4
5
6
7
products 29–33 were obtained with a product yield in the range
of 49%–80%. Similarly, the electron withdrawing group at the
ortho, meta, and para positions reacted smoothly and afforded
products 34–39 with moderate to good yields. Moreover, the
five- or six-membered heteroaromatic system, such as pyrrole
40, furan 41, and thiophene 42, were compatabile under the
present reaction conditions.
R1
R2
R1
R2
N
N
N
N
N
N
N
N
R1 =R2 = H,
L4
R1 =R2 = H,
R1 =R2 = Bu,
L3
R1 =R2 = OMe,
L1
L2
L5
t
R1 =R2 = Ph,
L6
Me
Me
8
9
Me
Me
N
N
N
N
Me
Me
L7
L8
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 2. Scope of the Csp2-O coupling of arylzinc reagentsa
Table 3. One-pot scope of the in situ redox active esters to
Csp2-O couplinga
O
O
O
.
.
PhZnCl LiCl
O
10 mol% NiCl2 6H2O
O
4
N
NOH
.
10 mol% NiCl2 6H2O
O
O
O
20 mol% di-tBubipy
O
Ar1
O
Ar
20 mol% di-tBubipy
O
Ar1
Ar1
O
CO2H
O
N
+
THF:DMF (3:2), Ar
12 h, 25 C
DCC, CH2Cl2
THF:DMF (3:2), Ar
12 h, 25 
C
in situ
b
b
12 h, Ar, 25 
C
O
10-28
(
)
29-42
(
)
.
Ar ZnCl LiCl
O
O
Me
O
O
O
O
O
O
Me
O
O
O
O
tBu
29
, 59%
31
, 49%
30
O
, 77%
Me
MeO
10
O
, 73%
11
, 59%
12
O
, 71%
O
NO2 O
O
O
O
O
MeO
Me
33
, 63%
34
O
, 46%
O
O
O
Me
32
, 80%
OMe
15
, 55%
14
, 43%
O
I
O
I
Br
13
, 62%
O
O
O
37
, 54%
O
O
O
35
, 40%
36
, 59%
O
nBu
O
O
OMe
O
O
O
O
F
, 63%
17
, 79%
O
16
, 60%
O
18
NH
O2N
F3C
38
, 51%
39
, 59%
40
, 50%
O
O
O
O
F
O
O
CF3
O
O
O
O
Cl
, 54%
21
O
, 84%
20
, 74%
O
S
19
42
, 47%
41
, 60%
F
Me
O
O
aReaction conditions [In situ]: Aryl carboxylic acid (1 equiv), N-
O
Me
O
Cl
F
O
º
Hydroxyphthalimide (1 equiv), DCC (1 equiv) in DCM at 25 C for 12 h,
then NiCl2.6H2O (10 mol%), di-tBubpy (20 mol%), arylzinc reagent (3
22
, 73%
Me
, 37%
24
, 68%
equiv), THF/DMF (3:2) at 25 ºC for 12 h. bIsolated yield.
23
Me
O
O
O
The gram scale reaction was performed between 4c
and 2 under standard reaction conditions. The desired product
30 was obtained with a 64% yield, thus demonstrating the
practicability and scalability of the C(sp2)–O RCC reaction
(Scheme 1).
O
Me
Me
25
, 45%
26
O
, 49%
iPr
O
iPr
O
O
O
O
30
Me
O
N
iPr
28
, 70%
O
.
10 mol% NiCl2 6H2O
27
, 67%
O
Me
4c
, 3.09 g
O
20 mol% di-tBubipy
O
Me
O
Ph
+
aReaction conditions: Phenyl NHPI ester (1 equiv), arylzinc reagent (3
equiv), NiCl2.6H2O (10 mol%), di-tBubpy (20 mol%), THF/DMF (3:2), 25
ºC, 12 h. bIsolated yield.
THF:DMF (3:2), Ar
.
ZnCl LiCl
30
, 64%
12 h, 25 
C
2
1.49 g
3.09 gram scale
[
]
(3 equiv)
Scheme 1. C(sp2)-O radical cross-coupling reaction in gram
sclae
Inspired by the above studies, we used our protocol in
the one-pot transformation from aryl carboxylic acid. The in
situ generated aryl NHPI esters are utilised directly for C(sp2)–
O radical cross-coupling to synthesize functionalised aryl aryl
esters (Table 3). The electron donating group at the ortho, meta,
The relative rate of the redox active ester and aryl zinc
reagent was investigated (Scheme 2A). The reaction was
examined using electronically varied redox active esters with
3
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Page 4 of 6
C.
Reaction profile (formation of 10) D. EPR study (at room temperature)
the phenyl zinc reagent. The transmetalation process is
1
2
3
4
5
6
7
8
identical, and the reductive elimination process is solely
influenced by electronic properties. The experimental results
clearly reveal that reductive elimination highly favors the
electron withdrawing behavior. Similarly, the reaction was
inspected using the electronically varied aryl zinc reagent
(Scheme 2B). The output of the experiment clearly suggests
that transmetalation highly favors electron donating properties.
Moreover, the electron donating and electron-withdrawing
relative rates are higher when transmetalation is conducted than
when the phenyl zinc reagent is used. Control experiments were
performed to gain insights into the radical cross-coupling
reaction. The Ni catalyst and bipyridine ligand are essential for
this transformation. There is no influence of light in the
reaction, and the reaction solely proceeds at ambient conditions.
The use of halides, the Grignard reaction, and triphenyl indium
could not successfully conduct this coupling reaction. This
result clearly demonstrates the necessity of transmetalation, and
there is a specific requirement of zinc in the transmetalation
process. The radical cross-coupling reaction profile was
investigated (Scheme 2C). The observed result demonstrates
that the reaction proceeded smoothly, and the yield of the
reaction increased consistently with respect to time. Moreover,
no deterrent or dynamic variant of the reaction was observed.
Electron paramagnetic resonance (EPR) studies were
conducted for the RCC reaction of redox active ester 4, Ni–L2
catalyst, and the 2a solution by using 5,5-dimethyl-1-pyrroline-
N-oxide (DMPO) for spin trapping. The generated benzoyloxyl
radical 7 was trapped by DMPO and the resulting experimental
spectra of the DMPO spin trapping are presented in Scheme 2D.
The benzoyloxyl radical assignment is consistent with the
Hutchings report20 (Table 4, entry 1). Similarly, the substitued
radical spin trapping assignment is represented in Table 4
(entries 2 and 3).
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 2. Relative rate, control experiments, reaction profile
and EPR (performed at room temperature) studies
Table 4. EPR trapping resultsa
entry
radicala
g
aN
aH
•
1
2
3
PhCO2
2.011
12.7
9.1 (1.4)b
9.3 (1.3)b
•
4-OMePhCO2
2.010
12.6
•
4-CF3PhCO2
2.010 12.3
9.5 (1.3)b
aTrapping agent DMPO was used. bSplitting was observed.
In summary,
a nickel-catalyzed aryl–aroyloxyl
C(sp2)–O radical cross-coupling reaction conducted using a
redox active ester with aryl zinc reagent was demonstrated. This
method demonstrates a new disconnection approach for aryl
aryl esters. In the one-pot sequential process, the readily
available aryl carboxylic acids can be converted into
functionalized aryl aryl esters and heteroaryl esters. This
protocol is amenable to the gram scale synthesis. This method
is simple, practical, and convenient. Moreover, it has a wide
substrate scope, high functional group tolerance, and good
yields. To the best of our knowledge, this is the first report on
the use of intermolecular aroyloxyl radicals in the C(sp2)–O
radical cross-coupling reaction.
A.
Relative rate experiments
a)
O
O
O
O
O
Ph
.
Ph
O N
O
O
N
10 mol% NiCl2 6H2O
X
B
20 mol% di-tBubipy
4
O
O
+
(1 equiv)
+
O
THF:DMF (3:2), Ar
(1 equiv)
12 h, 25 
C
.
Ph
O
Ph ZnCl LiCl
X
2
10
(3 equiv)
AUTHOR INFORMATION
B 10
:
X = OMe:
= 1:6
B 10
:
X = CF3:
= 4:1
Corresponding Author
b)
O
O
O
O
*Email: cfalee@dragon.nchu.edu.tw
ORCID
Chin-Fa Lee: 0000-0003-0735-5691
Notes
The authors declare no competing financial interest.
.
ZnCl LiCl
Ph
Ph
O
A
X
.
Ph
O N
10 mol% NiCl2 6H2O
4
20 mol% di-tBubipy
O
+
(1 equiv)
+
THF:DMF (3:2), Ar
X
12 h, 25 
C
.
Ph ZnCl LiCl
O
(3 equiv)
2
10
(3 equiv)
A 10
:
X = OMe:
= 6:1
A 10
:
X = CF3:
= 1.02:1
B.
Control experiments
ASSOCIATED CONTENT
Supporting Information
.
O
O
10 mol% NiCl2 6H2O
O
20 mol% di-tBubipy
Ph
O N
Ph
X
+
Ph
O
The Supporting Information is available free of charge on the ACS
Publications website.
Experimental procedures, spectroscopic data and copies of NMR
THF:DMF (3:2), Ar
4
(3 equiv)
10
O
12 h, 25 
C
(1 equiv)
10
X
.
X
= ZnCl LiCl
0%
Br
.
w/o NiCl2 6H2O : 0%
0%
0%
MgBr
InPh2
ACKNOWLEDGMENT
w/o di-tBubipy : 0%
.
w/o NiCl2 6H2O & di-tBubipy : 0%
This work was financially supported by the Ministry of Science and
Technology, Taiwan (MOST 107-2113-M-005-019-MY3);
National Chung Hsing University; Research Center for Sustainable
Energy and Nanotechnology; and the "Innovation and
Development Center of Sustainable Agriculture" from The
Featured Areas Research Center Program within the framework of
in dark : 70%
4
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ACS Catalysis
by Redox-Active Esters and Alkylzinc Reagents. Science 2016,
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Graphic
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Re: Nickel-Catalyzed Cross-Coupling of Aryl Redoxactive Esters with Aryl Zinc
Reagents
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Bo-Hao Shih, R. Sidick Basha, Chin Fa Lee*
A nickel-catalyzed aryl–aroyloxy C(sp2)–O radical cross-coupling reaction conducted
using a redox active ester with aryl zinc reagent was developed. This method
demonstrates a new disconnection approach for aryl aryl esters formation. In the one-
pot sequential process, the readily available aryl carboxylic acids can be converted into
functionalized aryl aryl esters and heteroaryl esters. This protocol is amenable to the
gram scale synthesis. Improtantly, the present method is simple, practical, and
convenient. Moreover, it has a wide substrate scope, high functional group tolerance,
and good yields. To the best of our knowledge, this is the first report on the use of
intermolecular aroyloxy radicals in the C(sp2)–O radical cross-coupling reaction.
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