Construction of C-C Bond via C-N and C-O Cleavage

Article abstract of DOI:10.1055/s-0039-1691525

The construction of a C-C bond is a center subject in synthetic organic chemistry. The cross-electrophile coupling has provided a powerful tool to forge the C-C bond. However, this process generally requires organic halides, which has severely restricted

Full text of DOI:10.1055/s-0039-1691525

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X. Pang et al.
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Construction of C–C Bond via C–N and C–O Cleavage
Xiaobo Pang
Rong-De He
Ni
Xing-Zhong Shu*
0
0
0
0-
0
0
0
2-0
9
6
1-1
5
0
8
C O
C N
C C
Mn
State Key Laboratory of Applied Organic Chemistry (SKLAOC),
College of Chemistry and Chemical Engineering,
Lanzhou University, 222 South Tianshui Road, Lanzhou,
730000, P. R. of China
OAc
OAc
+
OAc
+
NMe3
NMe3
–
Ar
–
Ar
OTf
shuxingzh@lzu.edu.cn
OTf
FG
Ar
n
n
Received: 31.10.2019
Accepted after revision: 15.11.2019
Published online: 04.12.2019
DOI: 10.1055/s-0039-1691525; Art ID: st-2019-p0590-sp
Abstract The construction of a C–C bond is a center subject in syn-
thetic organic chemistry. The cross-electrophile coupling has provided a
powerful tool to forge the C–C bond. However, this process generally
requires organic halides, which has severely restricted the design space
for new reactions. Herein, we highlight our recent work on the coupling
reaction between C–N and C–O electrophiles. This work demonstrates
the possibility of construction of C–C bond via C–N and C–O cleavage.
A number of reactions between benzyl ammoniums and vinyl acetates,
aryl ammoniums and vinyl acetates, and benzyl ammoniums and aryl
C–O electrophiles have been studied. We also disclosed that benzyl am-
monium salts can be activated by low-valent nickel to be radicals.
Xing-Zhong Shu was born and grew up in Zhejiang, China. He ob-
tained his BSc degree from Shaoxing University and PhD degree from
Lanzhou University working with Professor Yong-Min Liang. Upon com-
pletion of his graduate work, he moved to the University of Wisconsin-
Madison to carry out postdoctoral research with Professor Weiping
Tang. In the summer of 2012, he joined Professor F. Dean Toste’s group
at the University of California-Berkeley as a postdoctoral researcher. He
began his independent career at Lanzhou University (China) in 2015
and was granted by ‘1000-plan for the young talent’. His research inter-
est focuses on the cross-electrophile reactions, asymmetric catalysis,
and organosilicon chemistry.
1
2
3
Introduction
Cross-Coupling of C–N and C–O Electrophiles
Summary and Outlook
Key words cross-coupling, cross-electrophile reaction, nickel, radi-
cals, ammoniums, vinyl acetates, vinylation
1 Introduction
The transition-metal-catalyzed cross-couplings have
become one of the most powerful tools for the construction
of C–C bonds.1 Over the past half century, great success has
been achieved in this field, and diverse electrophiles and
nucleophiles have been developed as coupling partners
(Scheme 1, path a). The broad substrate scope makes the
method particularly useful for the synthesis of valuable
compounds that can be applied in a number of fields in-
cluding pharmaceuticals and material science. However, in
general, these methods rely on the use of organometallic
reagents such as Grignard reagents, organic zinc, and or-
ganic boron compounds. The limited availability and rela-
tively low stability of organometallic species have restricted
the applicability of the methods. For instance, at present,
highly functionalized or secondary/tertiary alkyl metallic
reagents are still difficult to be produced. Moreover, the re-
quirement for pre-synthesis of organometallic reagents of-
ten leads to the decrease of the functional group compati-
bility. Consequently, the development of alternative cross-
coupling strategies could have a substantial effect on organ-
ic synthesis.
[M]
[M], Mn/Zn
path b
+
+
X
C
X
X
M'
C
C
C
C C
path a
M' = Zn, Mg, Sn, Li...
X = halides, O, N...
X: mostly halides
How to expand the scope
of electrophiles ?
Diverse coupling partners
Scheme 1 The transition-metal-catalyzed cross-coupling reactions
One of the protocols that could efficaciously circumvent
the preparation and use of organometallic reagents would
be the cross-electrophile reaction (Scheme 1, path b).2 This
protocol allows the construction of C–C bonds directly from
electrophiles and has shown high step-economy, excellent
functional group compatibility, and unique selectivity or-
thogonal to conventional methods. The history of dimeriza-
tion of electrophiles can be traced back to over 100 years
© 2019. Thieme. All rights reserved. Synlett 2019, 30, A–F
Georg Thieme Verlag KG, Rüdigerstraße 14, 70469 Stuttgart, Germany

B
X. Pang et al.
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ago (Wurtz and Ullman reaction). However, until very re-
cently, the cross-electrophile reaction has received atten-
tion since the pioneering work of Gosmini, Weix, and Gong
using cobalt/nickel catalysis with metal as a reductant.3
Over the past few years, there have been great achieve-
ments in the development of reactions using organic ha-
lides.2 Despite the formidable advances in this line, com-
pared with the traditional procedures capable of coupling
diverse electrophiles with diverse nucleophiles, extension
of the scope to other types of coupling fragments is highly
desirable for enhancing the applicability of this methodolo-
gy, yet remains largely undeveloped. Herein, we highlight
our recent work on the cross-electrophile reaction between
C–N and C–O electrophiles.4
philes, including allylic alcohols8a and benzyl oxalates.8b We
then set out to explore the possibility of a coupling reaction
between C–N and C–O electrophiles (Scheme 2, path c). If
successful, such a strategy for C–C bond formation through
synergistic C–N and C–O bond cleavage would provide a
new platform for the cross-electrophile reaction and would
offer an unrecognized opportunity for new reaction discovery.
The Gosmini group has reported the arylation reaction
of vinyl acetates with aryl halides.9 Very recently, the re-
ductive carboxylation of benzyl ammoniums with CO2 has
been achieved by the Martin group.10 We wondered wheth-
er vinyl acetates and benzyl ammoniums could be coupled
under the reductive conditions. We started our investiga-
tion by studying the reaction of benzylic ammonium salt 1a
with alkenyl acetate 2a (Table 1). The hinge to success of
such a coupling reaction relies largely on the evaluation of
N-ligand skeletons (L1–L10). Among various ligands, the re-
actions with phenanthrolines proceeded efficiently, and the
use of 4,7-diphenyl-phenanthroline (Bphen, L9) gave the
best result with 89% isolated yield (Table 1, entry 1). Com-
parable results were obtained when various nickel sources
were employed (Table 1, entries 2–4). The use of NaOAc re-
sulted in the decrease of the formation of byproduct ArCH2–
H (Table 1, entry 5). When Zn was used as a reductant, ei-
ther ArCH2–H or vinyl–H was significantly formed (Table 1,
entry 6). The reaction without nickel, Mn, or Bphen gave no
desired product (Table 1, entry 7).
The scope of the reaction with respect to vinyl acetates
was then studied (Scheme 3, a). Various styryl acetates in-
cluding unsubstituted (3b), electron-rich (3c), electron-
poor (3d–e), and -extended compounds (3f) coupled with
1a efficiently. The heterocycle-substituted vinyl acetates
gave the desired products with moderate yields (3g–h). The
presence of conjugated diene was compatible (3i).
The reactions with a broad range of alkyl-substituted vi-
nyl acetates were further investigated (3j–p). Vinyl acetates
with functionalities such as phenyl group (3l), acetate (3m),
ether (3n), ester (3o), and amides (3p) were tolerated and
afforded the desired products with good yields. Cyclic vinyl
acetates coupled with 1a efficiently (3q–t). The reaction of
1,2-dialkylated substrate gave the desired product in mod-
erate yield (3u). However, when fully substituted or non-
substituted vinyl acetate was used, only a trace of target
product was detected by GC–MS.
2 Cross-Coupling of C–N and C–O Electrophiles
C–N and C–O electrophiles (e.g. ammonium salts, ace-
tates, and ethers) have recently become a complementary
alternative to organic halides within the cross-coupling are-
na.5 Their advantages include being easy to handle, stability
to long-term storage, ready availability from a wide range of
feedstock chemicals, and naturally occurring functional
groups. Progress on reactions with nucleophiles has led to
numerous useful transformations with a broad spectrum of
C–N and C–O electrophiles proven to be effective (Scheme
2, path a).5 However, because of the high activation barrier
for C–N/C–O cleavage and selectivity problems in the pres-
ence of multiple C–N and C–O bonds, the cross-electrophile
reaction involving these compounds remains a particular
challenge.6 There are only a few elegant studies that have
shown their couplings with organic halides and CO2
(Scheme 2, path b).7 To our knowledge, there has been no
report on the couplings between C–N and C–O electrophiles
(Scheme 2, path c).
well explored
+
N/O
M
C
C
path a
N,O = NR2, NR3X, OMe, OAc...
unknown
+
O
N
C
C
C
C
path c
few reports
path b
+
X
N/O
C
C
This work: Cross-coupling between C–N and C–O electrophiles
OTf
R
To further explore the scope of this reaction, a wide
range of benzylic ammonium salts were then evaluated
(Scheme 3, b); -or -substituted naphthyl substrates
(3v,w) were tolerated under our conditions. The secondary
-extended ammonium salts could also couple with vinyl
acetate 2b efficiently (3x). The presence of heterocyclic
compounds such as indole (3y) and furan (3z), and substi-
tution around the aromatic ring were tolerated (3aa–ac).
The reactions are highly selective for the functionalization
of the C–N bond, leaving functionalities such as ester (3aa–
ac), trifluoromethyl group (3ad), terminal styrene (3ae),
Ar'
AcO
[Ni]/Mn
n
n
n
NMe3
-
Ar'
Ar
Ar
Ar
OTf
R
[Ni]/Mn
n = 0, 1
n = 1
Scheme 2 Cross-couplings via C–N and C–O cleavage
Our group has been interested in the development of
new coupling fragments and coupling schemes for the
cross-electrophile reactions.8 We have recently reported
the reductive coupling of several challenged C–O electro-
© 2019. Thieme. All rights reserved. Synlett 2019, 30, A–F

C
X. Pang et al.
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Table 1 The Effects of Changing the Reaction Conditionsa
NiBr2(diglyme) (10 mol%)
Bphen (22 mol%)
OAc
PMP
2a
PMP: 4-MeOC6H4
PMP
NMe3
+
Mn (4.0 equiv), DMF (0.1 M)
NaOAc (1.2 equiv), 80 °C
OTf
MeO
MeO
3a
1a
Effect of ligand:
Entry
Change in conditions
none
Yield (%) of 3a
1
2
3
4
5
6
7
97 (89)b
91
NiBr2 instead of NiBr2(diglyme)
NiCl2 instead of NiBr2(diglyme)
Ni(cod)2 instead of NiBr2(diglyme)
no NaOAc
93
93
84
Zn instead of Mn
27
no Ni, Mn, or Bphen
0
a 1a (0.1 mmol), 2a (0.2 mmol), 24 h. Yields were determined by GC analysis with dodecane as an internal standard. ND: not detected.
b Isolated yield.
and boronic ester (3af) intact. The reaction of methyl- or
MOM-protected substrates (3ag–ai) with 4-tert-butylcyclo-
hexenyl acetate performed smoothly, and the obtained
product 3ai is an important intermediate for the synthesis
of the grisan analogue.
ticals, biologically active compounds, and organic materials.
Various aryl C–O electrophiles coupled with benzylic am-
monium 1a efficiently (Scheme 4). The reactions of aryl tri-
flate (4a) or tosylate (4b) gave better results than that with
mesylate (4c) or acetate (4d). The presence of a sterically
hindered group at the ortho position was tolerated (5e).
Both electron-rich (5e–g) and electron-poor (5h) substitut-
ed aryl triflates performed well.
It is well known that ammonium salts undergo oxida-
tive addition to Ni(0).11 However, our control experiments
indicate that the activation of ammonium salts under the
current circumstances might involve a radical process. This
observation was strongly supported by racemization reac-
tion of optically pure benzyl ammonium salt (Scheme 5-
1a), radical trapping experiments (Scheme 5-1b), and radi-
cal clock experiments (Scheme 5-1c).12
Based on the results described above and previous re-
ports, we temporary suggested that one of the plausible
mechanisms might include a catalytic cycle as shown in
Scheme 5-2. Oxidative addition of vinyl acetates to Ni(0)
would give vinyl–Ni(II), which might undergo radical tap-
ping and reductive elimination processes to afford the de-
sired product 3. The benzyl radicals might be generated by
reduction of benzyl ammonium salts with either nickel(I)
or nickel(0).
Anilines are abundant and available chemical feed-
stocks, thus, the research in coupling anilines or corre-
sponding derivates would extremely promote the progress
of arylation reactions. In general, the coupling reaction
with aryl ammonium salts requires highly reactive organo-
metallic reagents. We then investigated the coupling reac-
tion of aryl ammoniums with vinyl acetates under the re-
ductive conditions. To our delight, after slightly adjustment
of the reaction conditions by replacement of NiBr2(diglyme)
with NiBr2 and the use of KOAc as additive, the reactions
proceeded efficiently (Scheme 3, c). Aryl ammoniums bear-
ing an electron-withdrawing group, such as CF3 (3ak,al),
fluoride (3am), sulfone (3an), ester (3ao), and boronic ester
(3ap), gave the desired products with moderate to good
yields. However, electron-neutral and electron-rich aryl
ammoniums were unsuccessful. At present, the reaction of
allyl ammonium or alkyl ammonium salts with vinyl ace-
tate has failed to afford the target product in this work.
This coupling strategy was further applied for the con-
struction of diarylmethane comounds, which are important
substructures found in a substantial number of pharmaceu-
© 2019. Thieme. All rights reserved. Synlett 2019, 30, A–F

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3 Summary and Outlook
range of benzyl ammonium salts, aryl ammonium salts, vi-
nyl acetates, and aryl C–O electrophiles. A wide range of
functional groups were tolerated. Preliminary mechanistic
studies revealed that the ammonium salts were activated
by nickel to be carbon radicals.
The construction of C–C bonds is a center subject in
synthetic organic chemistry. We demonstrate for the first
time the possibility of construction of C–C bond via C–N
and C–O cleavage. The reaction proceeded with a broad
NiBr2(diglyme) (10mol%)
BPhen (22 mol%)
–
+
R
n
n
NMe3 OTf
R
Ar
+
AcO
Ar
Mn (4.0 equiv), NaOAc (1.2 equiv)
DMF (0.1 M), 80 °C, 24 h
n = 0, 1
3
2
1
(a) Scope of vinyl acetates (reactions with 1a)a
R
3b, R = H, 90%
3c, R = NMe2, 86%
3d, R = CO2Me, 65%
3e, R = F, 75%
Ph
Ar
O
S
Ar
Ar
Ar
Ar
Ar
3i, 42%
3h, 60%
3g, 51%
3f, 77%
AcO
dienyl acetate
furyl
thiophenyl
naphthyl
substituted aryls
α-Arylvinyl acetate
functional group compatibility
effect of chain length
n
O
R
O
R
OAc
OMe
n
R =
Ar
Ar
N
H
AcO
OMe
3j, n = 0, 73%
3k, n = 4, 80%
3l, 96%
3m, 70%
3n, 68%
3o, 65%
3p, 73%
α-Alkylvinyl acetate
Me
Me
Ar
AcO
Ar
Ar
Ar
Ar
3u, 44%c
n
3t, 87%
3q, 57%
3r, 75%
3s, 97%
1,2-disubstituted
alkenyl acetate
Cyclovinyl acetate
(b) Scope of benzylic ammonium saltsa
Me
NMe
PMP
Ph
PMP
PMP
PMP
O
3z, 66%
3y, 58%
3x, 64%
3w, 80%
3v, 67%
Various functional groups on different sites of the aromatic ring
+
NMe3
Ar
–
OTf
CO2Me
PMP
PMP
PMP
MeO2C
PMP
PMP
Benzylic ammonium salts
PMP: 4-MeOC6H4
MeO2C
F3C
3aa, 80%
3af, 42%
3ab, 65%
3ag, 75%
3ac, 50%
3ae, 60%
3ad, 76%
OMOM
OMe
PMP
PinB
MeO
tBu
tBu
tBu
3ai, 70%
3ah, 70%
(c) Scope of aryl ammonium saltsb
NMe3
–
F3C
OTf
Ar
MeO2S
3an, 65%
Ar =
EtO2C
3ao, 75%d
PinB
3ap, 41%
Ar
F
F
3ak, 55% (meta)
3al, 73% (para)
3aj, 85%
3am, 80%
Aryl ammonium salts
Scheme 3 Reaction scope of vinyl acetates and ammonium salts. a Reagents and conditions: 1 (0.2 mmol), 2 (0.4 mmol), NiBr2(diglyme) (10 mol%),
Bphen (22 mol%), NaOAc (1.2 equiv), and Mn (4.0 equiv) in DMF at 60–100 °C for 24 h. b Aryl ammonium salts (0.2 mmol), vinyl acetates (0.6 mmol),
and NiBr2(10 mol%) were used. c 2u (E/Z = 1:1.9), product 3u (E/Z = 1:1). d KOAc (1 equiv) was used.
© 2019. Thieme. All rights reserved. Synlett 2019, 30, A–F

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X. Pang et al.
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NiBr2(diglyme) (10 mol%)
BPhen (22 mol%)
X
1a
Ar
Ar
Mn (4.0 equiv), NaOAc (1.2 equiv)
DMF (0.1 M), 80 °C, 24 h
MeO
6
7
MsO
TsO
AcO
TfO
X
Ar
4a, 85%
With aryl-OTf as electrophiles
4d, 0%
4b, 81%
4c, 37%
iPr
tBu
MeO
MeO
MeO
Me
5e, 54%
5g, 87%
5f, 86%
OMe
CN
MeO
F
MeO
MeO
Me
5h, 89%
5i, 82%
5j, 72%
Scheme 4 Reaction scope of aryl C–O electrophiles
The cross-electrophile reaction is currently restricted to
the use of organic halides. This reaction provides a new
platform for the cross-electrophile reaction and offers new
opportunity for new reaction discovery. More cross-elec-
trophile reactions based on C–N and C–O electrophiles
would be expected. Since the broad existence of N– and O–
containing compounds in the natural world and the syn-
thetic realm, such methodologies would serve as newly de-
scribed and complementary bond disconnection synthetic
strategy that is likely to find broad applications, including
molecule synthesis and late-stage modification of biologi-
cally active compounds. Moreover, for the first time, this
study has disclosed that the ammonium salts can be acti-
vated by nickel catalysts to be carbon radicals. This finding
may provide a new dimension to the functionalization of
C–N bonds, and the metal-catalyzed enantioselective func-
tionalization of alkyl ammonium salts would be possible
and expected.
(1) Mechanistic study
(a) The reaction with enantioenriched substrate
Me
Me
2a (2 equiv)
Ph
NMe3
OTf
standard conditions
6, >99% ee
7, 64% yield, 0% ee
(b) Radical trapping experiments
standard conditions
Ar
1a
Ar
H
+
Ar
+
Ar
O
additives
8
9
10
10
8
9
H
H
additives
(a) none
MeO2C
CO2Me
35%
0
46%
N
(b)
(c)
HE (1.0 equiv)
54%
44%
0
15%
H
Hantzsch ester (HE)
O
2 (10 ppm)
31%
trace
(c) Radical clock experiments
standard conditions
+
1a
Ar
1.0 equiv
11, 3.0 equiv
12, 32%
(2) Proposed Mechanism
Ni(0)
Funding Information
2
Mn
National Natural Science Foundation of China (Grant No. 21772072
Ni(II)
and 21502078).
N
ati
o
n
a
lNaturalS
c
i
e
n
c
e
F
o
u
n
d
ati
o
n
of
C
h
i
n
a
(2
1
5
0
2
0
7
8)Nati
o
n
a
lNaturalS
c
i
e
n
c
e
F
o
u
n
d
ati
o
n
of
C
h
i
n
a
(2
1
7
7
2
0
7
2)
OAc
A
Ni(I)
Ni(0 or I)
Acknowledgment
Ar
NMe3
OTf
Ar
1
We are grateful to Prof. Wei Yu and Prof. Xue-Yuan Liu for helpful dis-
cussion.
(III)
Ni
3
Ar
OAc
B
Scheme 5 Mechanism study
© 2019. Thieme. All rights reserved. Synlett 2019, 30, A–F

F
X. Pang et al.
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