Synthesis of Biaryls through Nickel-Catalyzed Suzuki-Miyaura Coupling of Amides by Carbon-Nitrogen Bond Cleavage

Article abstract of DOI:10.1002/anie.201601914

The first Ni-catalyzed Suzuki-Miyaura coupling of amides for the synthesis of widely occurring biaryl compounds through N-C amide bond activation is reported. The reaction tolerates a wide range of electron-withdrawing, electron-neutral, and electron-donating substituents on both coupling partners. The reaction constitutes the first example of the Ni-catalyzed generation of aryl electrophiles from bench-stable amides with potential applications for a broad range of organometallic reactions. Breaking and making: The first nickel-catalyzed Suzuki-Miyaura coupling of amides for the synthesis of biaryl compounds through N-C amide bond cleavage is reported. The reaction tolerates a wide range of sensitive and electronically diverse substituents on both coupling partners.

Full text of DOI:10.1002/anie.201601914

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International Edition: DOI: 10.1002/anie.201601914
German Edition: DOI: 10.1002/ange.201601914
Suzuki–Miyaura Coupling
Synthesis of Biaryls through Nickel-Catalyzed Suzuki–Miyaura
Coupling of Amides by Carbon–Nitrogen Bond Cleavage
Shicheng Shi, Guangrong Meng, and Michal Szostak*
Abstract: The first Ni-catalyzed Suzuki–Miyaura coupling of
amides for the synthesis of widely occurring biaryl compounds
À
through N C amide bond activation is reported. The reaction
tolerates a wide range of electron-withdrawing, electron-
neutral, and electron-donating substituents on both coupling
partners. The reaction constitutes the first example of the Ni-
catalyzed generation of aryl electrophiles from bench-stable
amides with potential applications for a broad range of
organometallic reactions.
T
he Suzuki–Miyaura biaryl coupling reaction has emerged as
one of the most powerful carbon–carbon bond-forming
reactions for arene functionalization.[1,2] Although cross-
coupling of aryl halides, triflates, and tosylates has been
successfully achieved with Pd catalysts,[3] in recent years
À
tremendous advances have been made in the use of new C O
À
and C N electrophiles (leaving group (LG) = O, N) as
attractive coupling partners using sustainable and more
economically viable Ni catalysts.[4] In this context, notable
progress has been reported in the cross-coupling of aryl
ethers,[5a–c] acetates,[5d] pivalates,[5d,e] carbamates,[5f–h] sulfama-
tes,[5f,g] and ammonium salts.[5i] Elegant examples of using
aroyl electrophiles in the Suzuki–Miyaura reaction under
redox neutral conditions, including anhydrides[6] and esters,[7]
with Rh and Ni catalysts have been developed. Furthermore,
oxidative cross-coupling of carboxylic acids with boronic
acids using Pd has been reported;[8a,b] however, this process
suffers from limited substrate scope and expensive oxidants.
Furthermore, carboxylic acids have been employed for direct
decarboxylative cross-coupling with aryl halides and olefins
by the groups of Gooßen[8c] and Myers.[8d] Despite these
notable advances, the Suzuki–Miyaura biaryl coupling of
significantly more challenging amides (barrier to N-C reso-
nance of 15–20 kcalmolÀ1; Figure 1),[9a] after direct oxidative
Figure 1. A) Amide bond activation concept for transition-metal catal-
ysis. B) Metal-catalyzed activation of amides. C) Classic electrophiles
in Suzuki–Miyaura reactions.
significantly extend the pool of electrophiles available for
the cross-coupling, given that: a) amides are traditionally
derived from different precursors than halides, phenols, and
anilines;[11] b) amides are easy to prepare;[12] and c) amides
are typically inert to a variety of reaction conditions allowing
for ring prefunctionalization.[12] However, the challenge in
using amides as arylation precursors is the low reactivity of
À
À
addition into an N C bond, has yet to be reported, a notable
the N C bond for direct oxidative addition and control of the
deca13]
deficiency given the pivotal role of amides as key building
blocks in peptides and versatile bench-stable intermediates in
organic synthesis.[9b]
Herein, we report the first Suzuki–Miyaura biaryl cou-
pling reaction of amides by N C cleavage. This new Suzuki–
À
Biaryls are key structural motifs in numerous bioactive
medicinal agents, natural products, and polymers.[10] The
development of biaryl syntheses through cross-coupling of
readily accessible, bench-stable amide precursors would
Miyaura cross-coupling variant can be accomplished in high
yields with a broad range of amide and boronic acid
substrates. The reaction employs air-stable, inexpensive Ni
catalysts, which are economically advantageous over Pd. The
reaction proceeds with full selectivity for Ni insertion into the
À
N C amide bond, representing a general method for the
[*] S. Shi, G. Meng, Prof. Dr. M. Szostak
Department of Chemistry, Rutgers University
73 Warren Street, Newark, NJ 07102 (USA)
E-mail: michal.szostak@rutgers.edu
synthesis of aryl electrophiles from amides under Ni catalysis.
In light of the importance of amides and the advantages
offered by nickel catalysis, we expect that this concept will
provide a modular strategy for the application of ubiquitous
amides as unconventional electrophiles in aryl cross-coupling
manifolds.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
1002/anie.201601914.
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In 2015, our laboratory introduced a new mode of
activation of amide bonds in transition-metal catalysis by
geometric distortion[14] (Figure 1A). We have established that
twisted amides serve as robust and versatile precursors in
after very extensive optimization, that the proposed Suzuki–
Miyaura coupling of amides was indeed feasible using the
sterically distorted amide (1d) and a Ni0/PCy3 catalyst system.
Using the less distorted anilides such as (1a–1c),[14] only
a trace quantity of the cross-coupled product was formed,
À
elusive transformations of amide bonds for C C bond
[15a,b]
À
À
construction by N C activation under Pd catalysis.
consistent with metal insertion into the neutral amide N C
Independently, Garg et al.[15c] and Zou and Li[15d] reported
the use of twisted imides[14] for the synthesis of ketones under
Ni[15c,d] and Pd[15d] catalysis. Central to this strategy is ligand
coordination to nitrogen to disrupt nN!p*CO conjugation and
facilitate oxidative addition.[16] Ground-state distortion and
electronic activation contribute to the observed reactivity. In
all cases, amides are readily available from carboxylic acid
precursors by standard methods.[12] From a synthetic stand-
point, the ability to promote previously elusive transforma-
tions of amides through generic metal-catalyzed activation
modes with high functional group compatibility represents
a significant advance for implementing neutral, bench-stable,
readily accessible amides as precursors in cross-coupling
reactions.[17]
bond.[13] The use of a significantly less distorted amide (1e)[14]
resulted in a promising yield of the biaryl product, demon-
strating the high activity of the catalyst system. In all cases
examined, negligible formation of ketone products was
detected in crude reaction mixtures, consistent with the high
capability of the Ni catalyst to facilitate decarbonylation.[18]
À
The insertion occurred selectively at the N C bond, with
[17]
À
cleavage of the alternative s N C bond not observed.
Key optimization results are shown in Table 1. A Ni(cod)2
precatalyst with the addition of bulky, electron-rich phos-
phane ligands gave low yields of the biaryl product.[5–7]
Bidentate phosphanes resulted in markedly poor coupling.[18c]
Ni(PPh3)2Cl2 as the precatalyst gave lower yields than Ni-
(PCy3)2Cl2, consistent with the ease of oxidative addition.
Additional ligands resulted in inferior results, which may
Under appropriate conditions, the acylmetal intermediate
À
resulting from metal insertion into the inert amide N C bond
might undergo transmetallation/decarbonylation, generating
an organometal electrophile. Ni catalysts have been success-
fully utilized in Suzuki–Miyaura reactions of unconventional
electrophiles (LG = O, N).[4,5] Seminal studies by Yamamoto
Table 1: Optimization of Ni-catalyzed Suzuki biaryl synthesis through
coupling of amides with boronic acids.[a]
on C(O) Ni O decarbonylation[18a] and more recent progress
in related cross-couplings provide precedent that decarbon-
ylation of acylnickel complexes could proceed with high
selectivity.[18b,c] However, at the outset it was unclear whether
such a pathway using significantly more challenging amides
À
À
Entry
Catalyst
L
Base
Yield[b]
[%]
À
would be feasible given the inert nature of amide N C
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Ni(cod)2
Ni(cod)2
Ni(cod)2
Ni(cod)2
Ni(cod)2
Ni(cod)2
Ni(cod)2
Ni(cod)2
PCy3
n-Bu3P
Pt-Bu3
dppf
dppe
dppp
dppb
IMes
SPhos
PPh3
n-Bu3P
n-Bu3P
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
K3PO4
11
22
28
18
<2
24
16
<2
14
20
20
29
40
42
68
59
21
76
83[c]
<5
77
bonds,[12] the reversibility of insertion/decarbonylation,[18] and
the lack of precedent for aryl–aryl bond formation through
amide bond cleavage.[13]
We began our investigations by evaluating the coupling of
amides (1) with 2-naphthyl boronic acid (2) as a standard
nucleophile (Figure 2). We focused on the challenging
neutral, aryl amide electrophiles with the aim of developing
a generally applicable method with a wide substrate scope.
Ni-catalyzed coupling of unconventional electrophiles is
facilitated using conjugated aromatics, limiting the prepara-
tive scope of the chemistry.[5b,c] Although biaryl products were
not detected using Pd catalysts, we were delighted to find,
Ni(cod)2
Ni(cod)2
Ni(OAc)2
Ni(PCy3)2Cl2
Ni(PCy3)2Cl2
Ni(PCy3)2Cl2
Ni(PCy3)2Cl2
Ni(PPh3)2Cl2
Ni(dppf)2Cl2
Ni(PCy3)2Cl2
Ni(PCy3)2Cl2
Ni(PCy3)2Cl2
Ni(PCy3)2Cl2
PCy3
-
-
-
-
-
-
-
-
15[d]
16[d]
17[d]
18[d,e]
19[d–f]
20[d–f]
21[d–g]
Na2CO3
See the Supporting Information for full experimental details. [a] Amide
(0.1 mmol), R-B(OH)2 (1.5 equiv), catalyst (10 mol%), base (2.0 equiv),
ligand (40 mol%), toluene (0.25m), 1508C, 12 h. [b] GC/1H NMR yields.
[c] Yield of isolated product. [d] Dioxane. [e] Ni(PCy3)2Cl2 (5 mol%).
[f] Na2CO3 (4.5 equiv). [g] H2O (5 equiv). cod=1,5-cyclooctadiene;
dppf=1,1’-bis(diphenylphosphanyl)ferrocene; dppe=1,2-bis(diphenyl-
phosphino)ethane; dppp=1,3-bis(diphenylphosphino)propane;
dppb=1,4-bis(diphenylphosphino)butane; IMes=1,3-di(2,4,6-trime-
thylphenyl)imidazolin-2-ylidene; SPhos=2-dicyclohexylphosphino-2’,6’-
dimethoxybiphenyl.
Figure 2. Nickel-catalyzed Suzuki biaryl synthesis of amides: the effect
of different N substituents. Conditions: 1 (1.0 equiv), 2-Np-B(OH)2
(1.5 equiv), Ni(PCy3)2Cl2 (5 mol%), Na2CO3 (4.5 equiv), dioxane
(0.25m), 1508C. See the Supporting Information for details.
Np=naphthyl.
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indicate saturation of the coordination sphere of the Ni
Overall, the optimized process is highly efficient and
catalyst. Previous studies suggested that a single phosphane
operationally simple; the reaction employs an air-stable
Ni(PCy3)2Cl2 complex, does not require the presence of
additional ligands or hygroscopic additives, and utilizes
commercially available boronic acids. This transformation
represents the first Suzuki–Miyaura biaryl coupling of amides.
With the optimized conditions in hand, we explored the
preparative scope of the biaryl coupling of amides (Table 2).
As shown, the scope of the reaction is very broad and
tolerates the coupling of electron-neutral, electron-withdraw-
ing, and electron-rich substrates. The generality of the amide
component was investigated using 4-trifluoromethylphenyl
boronic acid to facilitate product isolation (yielding products
3a–3m).[5h] The generality was further demonstrated in
additional examples employing electronically varied sub-
À
ligand might be required to facilitate metal insertion into a C
X bond (X = N, O).[7b,15a] p-Deficient ligands had a negative
impact on the reaction (see the Supporting Information).[19]
Nucleophilic additives had a deleterious effect on the
reaction, rendering
a nucleophilic catalysis mechanism
unlikely (see the Supporting Information). Importantly,
efficient coupling is observed with only 5 mol% of Ni
À
catalyst, which compares favorably with the related C X
biaryl couplings (X = O, N).[5a–i] The reaction is highly
À
practical and tolerates water, in contrast to C O couplings,
in which boronic acid/ester equilibria complicate the reac-
tion.[5d–h] Finally, we note that the reaction can proceed at
temperatures as low as 808C.
Table 2: Nickel-catalyzed Suzuki biaryl synthesis through cross-coupling of amides with boronic acids.[a,b]
[a] Amide (1.0 equiv), R-B(OH)2 (1.5 equiv), base (4.5 equiv), Ni(PCy3)2Cl2 (5 mol%), dioxane (0.25m), 1508C. [b] Yield of isolated product. [c] Gram
scale. See the Supporting Information for details.
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strates (yielding 3n–3q). Electron-deficient nucleophiles are
generally less reactive in transmetallation.[1–5] Fluorine-con-
taining biaryls are of great value in medicinal and materials
chemistry.[21] Importantly, the coupling proceeds in high yields
in the presence of carbonyl derivatives, such as esters and
ketones (products 3e, 3 f). Remarkably, the reaction tolerates
even an unprotected aldehyde (product 3h), albeit in lower
[5]
À
yield. Aldehydes are not tolerated in related C O couplings,
clearly showing the advantage of amide electrophiles in
coupling manifolds. Ketone-containing substrates are yet to
[7]
À
be reported in C O ester couplings. Sterically hindered
Scheme 1. Proposed mechanism.
substrates showed good reactivity (yielding products 3i and
3j). The scope of the boronic acid component was inves-
tigated using 4-trifluoromethylphenyl amide as a standard
substrate (to form products 3o–3z). The generality was
additionally demonstrated through the coupling of electroni-
cally varied substrates with neutral boronic acids (yielding
products 3ab–3ad). The coupling with 4-trifluoromethyl
boronic acid affords the symmetrical biaryl compound 3t. In
the absence of amide, less than 9% of the biaryl is formed.
The reaction with 4-tert-butylboronic acid (forming com-
pound 3r) demonstrates a gram-scale coupling. Medicinally
relevant heterocycles, including sensitive pyridine, thiophene,
and furan rings, are well-tolerated (3u–3w). Notably, the
reaction tolerates strongly electron-withdrawing substrates
lighting the power of transition-metal-catalyzed activation of
bonds traditionally considered to be inert.
Acknowledgements
We acknowledge Rutgers University for financial support.
The Bruker 500 MHz spectrometer used in this study was
supported by the NSF-MRI grant (CHE-1229030).
À
Keywords: amide bonds · biaryls · cross-coupling · N
C activation · nickel
(forming products 3z and 3aa),[20] complementing C O
À
coupling manifolds.[5] Overall, the Suzuki–Miyaura biaryl
coupling of amides demonstrates a broad reaction scope.
How to cite: Angew. Chem. Int. Ed. 2016, 55, 6959–6963
Angew. Chem. 2016, 128, 7073–7077
À
Selected examples highlight complementarity to C O cou-
pling reactions.[5] At present, the major limitation is the
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À
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In conclusion, we have developed the first biaryl Suzuki–
À
Miyaura coupling of amides by N C bond activation. The
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Received: February 24, 2016
Published online: April 21, 2016
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