Nickel-catalysed Suzuki-Miyaura coupling of amides

Article abstract of DOI:10.1038/nchem.2388

The Suzuki-Miyaura coupling has become one of the most important and prevalent methods for the construction of C-C bonds. Although palladium catalysis has historically dominated the field, the use of nickel catalysis has become increasingly widespread because of its unique ability to cleave carbon-heteroatom bonds that are unreactive towards other transition metals. We report the first nickel-catalysed Suzuki-Miyaura coupling of amides, which proceeds by an uncommon cleavage of the amide C-N bond after N-tert-butoxycarbonyl activation. The methodology is mild, functional-group tolerant and can be strategically employed in sequential transition-metal-catalysed cross-coupling sequences to unite heterocyclic fragments. These studies demonstrate that amides, despite classically considered inert substrates, can be harnessed as synthons for use in reactions that form C-C bonds through cleavage of the C-N bond using non-precious metal catalysis.

Full text of DOI:10.1038/nchem.2388

ARTICLES
PUBLISHED ONLINE: 9 NOVEMBER 2015 | DOI: 10.1038/NCHEM.2388
Nickel-catalysed Suzuki–Miyaura coupling
of amides
Nicholas A. Weires, Emma L. Baker and Neil K. Garg
*
The Suzuki–Miyaura coupling has become one of the most important and prevalent methods for the construction of C–C
bonds. Although palladium catalysis has historically dominated the field, the use of nickel catalysis has become
increasingly widespread because of its unique ability to cleave carbon–heteroatom bonds that are unreactive towards other
transition metals. We report the first nickel-catalysed Suzuki–Miyaura coupling of amides, which proceeds by an
uncommon cleavage of the amide C–N bond after N-tert-butoxycarbonyl activation. The methodology is mild, functional-
group tolerant and can be strategically employed in sequential transition-metal-catalysed cross-coupling sequences to
unite heterocyclic fragments. These studies demonstrate that amides, despite classically considered inert substrates, can
be harnessed as synthons for use in reactions that form C–C bonds through cleavage of the C–N bond using non-precious
metal catalysis.
ross-coupling reactions catalysed by transition metals have pervasively in Suzuki–Miyaura cross-couplings18,19. Thus, we
become an indispensable tool for the construction of C–C examined the coupling of amides 5 with phenylboronic acid (6a)
C
bonds1. Although palladium catalysis has dominated the or boronic ester 6b with several of our key results involving
field and was the topic of the 2010 Nobel Prize in Chemistry, Ni/SIPr, K3PO4 and toluene (Table 1). The use of anilide 5a,
there has been much interest in the development of complementary which was an excellent coupling partner in our amide-to-ester
nickel-catalysed processes2–4. Much of the excitement that surrounds conversion8, only led to low yields of ketone product 7 (Table 1,
nickel-catalysed cross-couplings stems from nickel being more abun- entry 1). We also evaluated the use of N-Bn-N-Boc-derivative
dant and therefore less expensive compared with palladium2,3. 5b. Although the coupling of 5b with boronic acid 6a gave
However, the most striking feature of nickel catalysis is the opportu- higher yields of the desired ketone, yields were inconsistent
nity to uncover new reactivity. To this end, nickel-catalysed processes (Table 1, entry 2) and led us to consider the corresponding
to cleave C–O bonds have led to the cross-coupling of unconventional boronic ester. In turn, the use of pinacolatoboronate 6b gave
phenol-based electrophiles4, such as aryl pivalates and ethers, and only trace amounts of the desired product 7 (Table 1, entry 3).
reactions to break C–N bonds have allowed for the cross-coupling As we, and others, have seen similar inconsistencies in nickel-
of aniline derivatives5–7
.
catalysed Suzuki–Miyaura couplings, which were attributed to
Recently, we discovered that Ni/SIPr (SIPr, 1,3-bis(2,6-di-iso- variations in water content20–23, we tested the use of boronic ester
propylphenyl)-4,5-dihydroimidazol-2-ylidine) could facilitate the
conversion of amides into esters (1→2, Fig. 1)8–10. With the hypoth-
Ni
Ni
O
esis that this transformation occurred by the metal-catalysed acti-
vation of the amide C–N bond, which typically is considered stable
and unreactive11, we postulated that the putative oxidative addition
intermediate 4 could be intercepted by carbon-based nucleophiles,
ultimately to forge C–C bonds (1→3). Direct acyl couplings of this
type have been achieved using acid chlorides, thioesters12, car-
boxylic anhydrides13, arylated mixed imides14 and geometrically
distorted cyclic imides15. A nickel-catalysed process to build
acyl C–C bonds from amides would provide a powerful new syn-
thetic tool for ketone synthesis, which would also complement
Weinreb’s widespread methodology16,17, but without the need
for strongly basic and pyrophoric organometallic reagents. In
this article, we report the first method for building C–C bonds
from N-tert-butoxycarbonyl (N-Boc)-activated secondary amides
using non-precious metal catalysis.
O
O
catalysis
catalysis
R´
N
OMe
R
MeOH
M
R
R˝
C–O bond
formation
C–C bond
formation
Esters
(2)
Amides
(1)
Ketones
(3)
?
O
4
R´
R˝
Putative oxidative addition
intermediate
Ni
N
Figure 1 | Development of Ni-catalysed coupling of amides with carbon
nucleophiles. After the discovery of nickel-catalysed conversion of amides
Results and discussion
Discovery and optimization of the nickel-catalysed Suzuki–Miyaura (1) into esters (2), this study sought to intercept putative oxidative addition
coupling of amides. In considering possible amide cross-coupling intermediate 4 with carbon-based nucleophiles to uncover a new
partners for C–C bond formation, we elected to pursue the use methodology for the synthesis of ketones (3) from amides (1). The
of boron-based partners. Countless boron-coupling partners, development of the reaction indicated by the dashed arrow supports the
including heterocycles that serve as building blocks to medicines intermediacy of 4 and provides a new and mild method for the synthesis of
and natural products, are commercially available and used ketones—complementary to the Weinreb ketone synthesis.
Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, USA. e-mail: neilgarg@chem.ucla.edu
*
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.2388
ARTICLES
system was evaluated, which gave naphthylphenyl ketone 18 in
70% yield (entry 12).
Table 1 | Reaction discovery and optimization.
Ph–B(OH)2 (6a)
The scope with regard to the boronic ester coupling partner, in
addition to the tolerance of the methodology towards heterocycles,
was also examined (entries 13–25). The use of methyl-substituted
boronic esters smoothly provided tolyl ketones 8–10 (Table 2,
entries 13–15), whereas the use of the para-methoxy-substituted
boronic ester gave 19 in 95% yield (entry 16). The fluoro- and tri-
fluoromethyl substituents were also tolerated to give 14 and 20,
respectively (entries 17 and 18). Moreover, a naphthylboronic
ester could be employed to provide 18 (entry 19). As shown by
the formation of 21–26, furan, thiophene, indole, pyrrole and pyr-
azole heterocycles were all tolerated (entries 20–25), which thus
demonstrates the potential utility of this methodology for the syn-
thesis of medicinally relevant scaffolds27. Indeed, diaryl or hetaryl
ketone frameworks are seen in several currently marketed drugs
or compounds under clinical evaluation28.
O
O
(2.5 equiv.)
Ni(cod)2, SIPr,
K3PO4 (2.0 equiv.),
R´
or
N
Ph
+
toluene, H2O,
50 °C, 24 h
R˝
Ph–B(pin) (6b)
(2.5 equiv.)
5
7
R´
Boron Ni(cod)2 SIPr
H2O
N
Entry
source (mol%) (mol%) (equiv.) Yield*
R˝
Me
N
5a
—
—
—
1
6a
6a
6b
10
10
10
10
10
10
12%
Ph
Bn
N
5b
5b
2
3
42–89%
7%
Boc
Synthetic applications that involve amide Suzuki–Miyaura coupling
methodology. Several synthetic applications were undertaken to
assess further the utility of the amide Suzuki–Miyaura couplings.
In the first of these, we performed the gram-scale coupling of
amide 27 with indolylboronic ester 28 (Fig. 2a). This smoothly
delivered ketone 29, a nanomolar tubulin-binding agent29, in 83%
yield. Next, the potential to carry out sequential palladium- and
nickel-catalysed Suzuki–Miyaura couplings of heterocycles was
tested (Fig. 2b). Furanylboronic acid (30) was first cross-coupled
with aryl chloride 31 using palladium catalysis30. This facilitated
the aryl–aryl coupling to furnish 32 in 71% yield, without
disturbing the amide. Next, amide 32 underwent nickel-catalysed
aryl–acyl coupling with pyrazolylboronic ester 33 to furnish 34.
The ability to unite heterocyclic fragments using such sequences
should prove useful in the synthesis of drug candidates, new
materials and natural products.
Bn
N
Boc
Bn
N
5b
5b
4
5
6b
6b
10
5
10
5
2.0
2.0
>99%
>99%
Boc
Bn
N
Boc
Conditions: Ni(cod)2 (5 or 10 mol%), SIPr (5 or 10 mol%), substrate 5a or 5b (1.0 equiv.), boron
source 6a or 6b (2.5 equiv.), K3PO4 (2.0 equiv.), toluene (1.0 M) and H2O (if applicable,
2.0 equiv.) at 50 °C for 24 hours. *Yields of 7 were determined by 1H NMR spectroscopic
analysis using 1,3,5-trimethoxybenzene as an internal standard.
6b with added water. We were delighted to find that the
The recently discovered nickel-catalysed esterification of amides,
coupling proceeded smoothly to give a quantitative yield of paired with the Suzuki–Miyaura coupling of amides, provides
ketone 7 when 2.0 equiv. water was used (Table 1, entry 4). opportunities to construct acyl C–C and acyl C–O bonds sequen-
Further optimization allowed us to lower the catalyst and ligand tially, as demonstrated in Fig. 2c. In one scenario, bis(amide) 35
loadings to 5 mol% without any loss in yield (Table 1, entry 5). was coupled with furanylboronic ester 36 to furnish furanyl
No evidence of decarbonylation was observed. This is significant ketone 37, without disturbing the secondary amide. Subsequent
as Yamamoto’s group and, more recently, Itami and co-workers Boc activation and Ni-catalysed esterification8 of 37 furnished
reported decarbonylation in processes to cleave acyl C–O bonds24–26
.
menthol ester 39. The coupling sequence can also be reversed.
The coupling required Ni(cod)2, (cod, cyclooctadiene), SIPr and Esterification of bis(amide) 35 with (–)-menthol (38)8 provided
K3PO4 to proceed, as demonstrated by control experiments. amidoester 40. Boc activation and Suzuki–Miyaura coupling of 40
Specifically, in the absence of any of these components, conversion with furanyl boronic ester 36 delivered ketoester 39. The ester of
into the desired ketone product was not observed. Control 40 withstood the base-mediated reaction conditions and was also
experiments are described in the Supplementary Information (see not disrupted by the nickel catalyst.
Supplementary Table 2).
In summary, we have developed the first nickel-catalysed method
for the construction of C–C bonds from amides. Our approach
Scope of methodology. Having identified suitable reaction relies on the use of nickel catalysis to activate the amide C–N
conditions, we assessed the scope of the methodology. Table 2 bond. In situ coupling of the putative oxidative addition inter-
showcases isolated yields for a variety of substrates and boronic mediate 4 with boronic esters furnishes the products of Suzuki–
esters. Beyond the parent coupling to give 7 in 96% isolated yield Miyaura cross-coupling. The methodology is mild and tolerant
(Table 2, entry 1), para-, meta- and ortho-substitutions were of a variety of functional groups, which include ketones and
tolerated, as shown by the formation of ketones 8–10, respectively amines, in addition to several heterocycles and functional groups
(entries 2–4). Additionally, electron-rich substrates that possess that bear acidic protons. With the future design and testing of
methoxy or amino substituents underwent the desired coupling to new catalysts, we expect that the scope of nickel-catalysed amide
give products 11–13 (entries 5–7). Substrates with electron- couplings will be broadened further. Nonetheless, we show here
withdrawing substituents, such as –F, –CF3, –CO2Me and –C(O) that the present methodology can be used to prepare bioactive
Me, were also suitable and gave adducts 14–17 in synthetically molecules (Fig. 2a), and can be employed strategically in sequential
useful yields (entries 8–11). It is notable that ketones and esters cross-couplings that involve palladium or nickel catalysis to unite
are compatible with this methodology (as are tertiary alcohols, heterocyclic fragments in controlled ways (Fig. 2b,c). Taken
nitriles, secondary amides, carboxylic acids, epoxides and the free together, these studies demonstrate that amides, despite classically
NHs of indoles; see Supplementary Table 4 for the robustness being considered inert substrates, can actually be harnessed as syn-
screen results), as these functional groups often pose challenges thons for use in reactions that form C–C bonds with non-precious
for Weinreb amide chemistry13. Finally, an extended aromatic metal catalysis.
2
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.2388
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Table 2 | Scope of nickel-catalysed Suzuki–Miyaura coupling of amides.
O
Ni(cod)2 (5 mol%),
O
iPr
iPr
N
R
SIPr (5 mol%),
(Het)Ar
N
+
N
(pin)B—(Het)Ar
SIPr
K3PO4, H2O,
toluene, 50 °C, 24 h
(Het)Ar
(Het)Ar
Yield*
85%
Boc
R = Me or Bn
iPr
iPr
Entry
Ketone product
Entry
Ketone product
Entry
Ketone product
Yield*
Yield*
O
O
O
1
96%
9
17
88%
F3C
OMe
F
15
7
14
O
O
O
2
3
92%
77%‡
10
18
19
81%
Me
Me
MeO
8
CF3
16
O
20
O
O
O
72%†
91%
11
94%
Me
9
O
17
18
Me
O
O
O
51%†,‡
4
5
12
70%
X
OMe
10
64%†
66%†
20
21
21 (X = O)
22 (X = S)
18
O
O
O
78%
86%†
80%
96%
13
14
73%
22
23
24
MeO
OMe
O
Me
Me
11
23
8
O
O
O
80%†
6
7
59%
Me
N
N
OMe
Me
9
Me
12
24
O
O
O
Me
81%
15
16
66%†
N
N
Me
13
O
25
10
O
O
O
82%†
N
8
90%
95%
25
N
Me
F
OMe
14
19
26
Conditions unless otherwise stated: Ni(cod)2 (5 mol%), SIPr (5 mol%), substrate (1.0 equiv.; R = Me or Bn as specified in the Supplementary Information), boron source (1.2–2.5 equiv.), K3PO4 (2.0 equiv.),
toluene (1.0 M) and H2O (2.0 equiv.) at 50 °C for 24 hours; The yields shown reflect the average of two isolation experiments. *Yields determined by 1H NMR analysis using 1,3,5-trimethoxybenzene as an
internal standard. †Ni(cod)2 (10 mol%) and SIPr (10 mol%) were used.
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.2388
ARTICLES
O
Pd(OAc)2 (1 mol%),
SPhos (2.5 mol%),
K3PO4,
O
a
O
b
Boc
B(OH)2
N
Boc
MeO
MeO
Bn
(pin)B
+
N
N
toluene, 100 °C
Bn
+
O
Bn
Boc
(71% yield)
N
Cl
Me
O
30
31
32
OMe
27
28
Pd-catalysed Suzuki–Miyaura coupling
Ni(cod)2 (5 mol%),
SIPr (5 mol%),
K3PO4, H2O,
Gram-scale
coupling
(pin)B
N
33
O
toluene, 50 °C
N
(83% yield)
Me
N
N
Ni(cod)2 (10 mol%), SIPr (10 mol%),
K3PO4, H2O, toluene, 50 °C
O
Me
MeO
MeO
O
34
(58% yield)
N
Me
Ni-catalysed Suzuki–Miyaura coupling of amide
OMe
29
Potent antiproliferative agent
(18 nM IC50
)
c
O
(pin)B
O
O
Me
Me
i. Boc2O, DMAP,
CH3CN, 23 °C
Boc
36
O
H
N
H
N
O
O
N
Bn
K3PO4,
Ni(cod)2 (10 mol%),
SIPr (10 mol%),
ii. 38 Ni(cod)2 (10 mol%),
O
Bn
Bn
O
SIPr (10 mol%),
toluene, 80 °C
O
O
H2O, toluene, 50 °C
Me
35
37
39
(57% yield, 2 steps)
(92% yield)
Ni-catalysed Suzuki–Miyaura coupling of amide
Me
Ni-catalysed esterification of amide
(–)-menthol
Me
(38)
HO
O
i. Boc2O, DMAP, CH3CN, 23 °C
Me
Me
H
O
ii. 36, K3PO4, Ni(cod)2 (10 mol%),
Ni(cod)2 (10 mol%), SIPr (10 mol%),
toluene, 80 °C
N
SIPr (10 mol%), H2O, toluene, 50 °C
Bn
Me
Me
(90% yield)
(82% yield, 2 steps)
O
40
Ni-catalysed esterification of amide
Ni-catalysed Suzuki–Miyaura coupling of amide
Figure 2 | Several applications of the Suzuki–Miyaura coupling of amides were carried out to assess the synthetic utility of the methodology.
a, The gram-scale coupling of trimethoxy amide 27 and heterocyclic fragment 28 was conducted to furnish tubulin-binding agent 29, a potent
antiproliferative diaryl ketone. b, A sequence of Suzuki–Miyaura couplings was performed to access 34, using palladium and nickel catalysis, and
thus highlight the capacity of this method to unite heterocyclic frameworks. c, Sequential Ni-catalysed esterification and Suzuki–Miyaura couplings
of amides were carried out to illustrate the ability of amides to be used in multistep synthesis through controlled activation–coupling sequences.
DMAP, 4-dimethylaminopyridine; pin, pinacolato; SPhos, 2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl.
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carbamates. J. Am. Chem. Soc. 136, 5587–5590 (2014).
Received 18 July 2015; accepted 29 September 2015;
published online 9 November 2015
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The authors are grateful to Boehringer Ingelheim, DuPont, Bristol-Myers Squibb, the
Camille and Henry Dreyfus Foundation, the A. P. Sloan Foundation, the S. T. Li Foundation
and the University of California, Los Angeles. S. Anthony (UCLA) is acknowledged for
experimental assistance. We are grateful to the National Science Foundation (NSF) (N.A.W.
and E.L.B., No. DGE-1144087) and the Foote Family (N.A.W.) for fellowship support.
These studies were also supported by shared instrumentation grants from the NSF (No.
CHE-1048804) and the National Center for Research Resources (No. S10RR025631).
Author contributions
N.A.W. and E.L.B. designed and performed the experiments and analysed the experimental
data. N.K.G. directed the investigations and prepared the manuscript with contributions
from all the authors; all the authors contributed to discussions.
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Additional information
Supplementary information and chemical compound information are available in the
online version of the paper. Reprints and permissions information is available online at
www.nature.com/reprints. Correspondence and requests for materials should be addressed
to N.K.G.
Competing financial interests
The authors declare no competing financial interests.
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