Dioxygen-promoted Pd-catalyzed aminocarbonylation of organoboronic acids with amines and CO: A direct approach to tertiary amides

Article abstract of DOI:10.1021/acs.orglett.6b02913

A direct approach from organoboronic acids and amines to tertiary amides via Pd-catalyzed aerobic aminocarbonylation has been developed. The presence of O₂ significantly promotes the efficiency of this transformation. This method uses commercially available organoboronic acids and cheap CO and O₂ (1 atm), which renders amides an easy synthesis with broad substrate scope and high functional group tolerance.

Full text of DOI:10.1021/acs.orglett.6b02913

Letter
pubs.acs.org/OrgLett
Dioxygen-Promoted Pd-Catalyzed Aminocarbonylation of
Organoboronic Acids with Amines and CO: A Direct Approach to
Tertiary Amides
Long Ren,† Xinwei Li,† and Ning Jiao*,†,‡
†State Key Laboratory of Natural and Biomimetic Drugs, School of Pharmaceutical Sciences, Peking University, Xue Yuan Road 38,
Beijing 100191, China
‡Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University,
Shanghai 200062, China
S
* Supporting Information
ABSTRACT: A direct approach from organoboronic acids and amines to tertiary
amides via Pd-catalyzed aerobic aminocarbonylation has been developed. The presence
of O2 significantly promotes the efficiency of this transformation. This method uses
commercially available organoboronic acids and cheap CO and O2 (1 atm), which
renders amides an easy synthesis with broad substrate scope and high functional group
tolerance.
Scheme 1. Reported Syntheses of Tertiary Benzamides
mide is one of the most important structural motifs found
ubiquitously in pharmaceuticals, natural products, insect
A
repellents, polymers, and synthetic intermediates.1 Tradition-
ally, amide synthesis is the condensation of amines with
carboxylic acids or amine acylation with acid derivatives, such as
acyl chlorides, anhydrides, or active esters.2 These methods,
however, are limited in their applications by the need for harsh
conditions and poor atom-economy. In the last few decades,
many attractive approaches for the direct synthesis of tertiary
amides have been developed from various starting materials,
including methylarenes,3 alcohols,4 aldehydes,5 carboxylic
acids,6 aryl esters,7 carboxamides,8 carbamoyl chlorides,9 and
aryl halides,10 through different mechanistic processes (Scheme
1).11 In particular, the aminocarbonylation of aryl halides with
carbon monoxide (CO) and amines has been widely studied,
since the three-component coupling reaction was first reported
by Heck in 1974.12 This strategy has become a powerful tool to
synthesize amide, owing to the unique ability of CO serving as
an excellent carbonyl group source.13
In light of our continuous interest in oxidative carbonylative
reactions,14 we envisioned that the aerobic oxidative amino-
carbonylation of organoboronic acids would be of great
interest15 since the nontoxic organoboronic acids are air-/
moisture-stable and widely commercially available.16 Amide
synthesis from organoboronates was commonly documented
with carbamoyl chlorides (Scheme 2a).9b,d,17 More recently, the
approach with N-chloroamines generated in situ with NCS was
recently achieved by Wu’s group under high CO pressure
(Scheme 2b).18 In contrast, the direct aminocarbonylation of
organoboronates with amines has rarely been studied. Herein,
we report a simple approach to tertiary amides directly from
organoboronic acids and amines with CO and O2 (balloon)
(Scheme 2c).
catalyzed by [Pd(PPh3)2Cl2] was investigated under a CO/O2
mixture. After screening of additives and cocatalysts, the desired
product N-benzyl-4-methoxy-N-methylbenzamide (3a) was
obtained in 51% when 10 mol % of CuCl was loaded (entry
1, Table 1). We then conducted several control experiments.
The reaction did not work in the absence of Pd catalyst (entry
2). CuCl cocatalyst could improve the efficiency of this
transformation (cf. entries 1 and 3). As we screened the solvent,
the yield increased to 61% when the reaction proceeded in
DMSO (entry 4). When the reaction was exposed under pure
CO atmosphere without O2, 3aa was still obtained in 48% yield
Initially, the aminocarbonylation of (4-methoxyphenyl)
Received: September 27, 2016
boronic acid (1a) with N-methyl-1-phenylmethanamine (2a)
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b02913
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Organic Letters
Letter
Scheme 2. Amide Syntheses from Organoboronic Reagents
Scheme 3. Pd-Catalyzed Aminocarbonylation of
Organoboronic Acids
a
Table 1. Condition Optimization for Aminocarbonylation of
a
Organoboronic Acid 1a
b
entry
cocatalyst
solvent
yield (%)
1
CuCl (10 mol %)
CuCl (10 mol %)
MeCN
MeCN
MeCN
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
51
0
c
2
3
4
5
6
7
8
37
61
48
80
79
84
22
74
65
CuCl (10 mol %)
CuCl (10 mol %)
CuCl (5 mol %)
Cu (5 mol %)
Cu (5 mol %)
Cu (5 mol %)
Cu (5 mol %)
d
a
Standard conditions: 1 (0.4 mmol), 2a (1.3 equiv), Pd(PPh3)Cl2
(0.02 mmol), Cu (0.02 mmol), CO/O2 (2:1, balloon), DMSO (4.0
mL), 80 °C, 36 h. Isolated yields.
e,f
e g
9 −
e
e
,
,
f
f
,h
transformation, giving product 3ba in 80% yield. Both aryl
boronic acids containing electron-donating groups and
electron-withdrawing groups at the aryl rings produced the
desired amides in good yields (3ca−ma). Generally, the
efficiencies of the electron-deficient substrates were slightly
lower than those of the electron-rich substrates (3ia, 3ma). The
ortho-substituted aryl boronic acids also underwent the reaction
smoothly (3na, 3pa−ra). Additionally, a high yield was
obtained when 1,1′-biphenyl-2-ylboronic acid (1o) was applied
to this system. Some heteroaromatic substrates also performed
successfully in this reaction. The yield of product from furan-2-
ylboronic acid (1v) was found to be lower than that from
thiophene-3-ylboronic acid (1u). To our delight, substrates
with functional groups such as the nitro, cyano, bromo, formyl,
vinyl, and exposed hydroxyl group were well tolerated in this
protocol (3ia−ka, 3pa, 3sa, 3ta), affording the corresponding
amide products in moderate to high yields.
To further study the scope of this protocol, other
organoboronic acids, such as vinylboronic acid and alkylboronic
acid, were also investigated. Satisfactorily, the products α,β-
unsaturated acylamide (3xa, Scheme 4) and alkyl acylamide
(3ya) were also successfully obtained under the standard
conditions.
Next, we evaluated the methodology with other secondary
amines (Scheme 5). Moderate yields were obtained when
dibenzylamine (2b), alkyl secondary amine dibutylamine (2c),
and cyclic secondary amine (2g) were employed as the
substrates. The reactions of dipropylamine (2d), diisopropyl-
amine (2e), dicyclohexylamine (2f), and even di-n-octylamine
10
11
a
Reaction conditions: 1a (0.4 mmol), 2a (0.44 mmol), PdCl2(PPh3)2
b
(0.02 mol), CO/O2 (2:1, balloon), 80 °C, 24 h. Isolated yields.
c
d
e
Without PdCl2(PPh3)2. Under pure CO without O2. 1.3 equiv of 2a
f
g
was employed. Stirred for 36 h. CO/Ar (2:1, balloon) was used
instead of CO/O2 (2:1, balloon). CO/air (2:1, balloon) was used
instead of CO/O2 (2:1, balloon).
h
(entry 5). It is interesting to note that 80% of 3aa was produced
when the loading of CuCl was decreased to 5 mol % (entry 6).
After screening of copper catalysts and other metal cocatalysts,
the copper powder showed a similar capacity, providing
benzamide 3aa in 79% yield (entry 7). However, increasing
the temperature or the volume ratio of CO/O2 resulted in
decline in yield (see the SI). Using powdered copper as the
cocatalyst, 84% yield of 3aa was obtained by employing 1.3
equiv of 2a and prolonging the reaction time to 36 h (entry 8).
It is noteworthy that the efficiency decreased significantly when
the reaction was carried out under CO/Ar (2:1, balloon)
instead of CO/O2 (2:1, balloon) (cf. entries 8−10), which
demonstrates that the O2 functions not only as a gas diluting
the CO concentration but also as an oxidant to promote the
reaction. Considering the effect of copper powder, we also
conducted a reaction in the absence of copper (entry 11),
which afforded 3aa in 65% yield.
In order to test the general applicability of this optimization,
the substrate scope of organoboronic acids was investigated
(Scheme 3). Phenylboronic acid (1b) proceeded well in this
B
DOI: 10.1021/acs.orglett.6b02913
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
addition of arylboronic acids to Pd(0) produces F,20 which
subsequently undergoes CO insertion and ligand exchange with
amines to form intermediate C. Finally, the desired product 3 is
produced via the reductive elimination of C and E. The control
experiments demonstrate that the presence of O2 significantly
promotes the efficiency of this transformation (entries 8−10),
which suggests an aerobic oxidative process regenerating Pd(II)
catalyst by O2 to complete the catalytic circle. However,
although it has been demonstrated by several techniques, such
as scanning electron microscopy and X-ray diffraction analysis,
that copper powder can be in situ oxidized by O2 into active
species Cu(I),21 the function of copper in this transformation is
still not completely clear.
Scheme 4. Pd-Catalyzed Aminocarbonylation of
Vinylboronic Acid and Alkylboronic Acid
Scheme 5. Pd-Catalyzed Aminocarbonylation of
a
Organoboronic Acid 1a with Different Amines
In summary, we have developed a simple Pd-catalyzed
tertiary amide synthesis from readily available organoboronic
acids, amines, and CO in the presence of O2. This method
contributes to a direct access to tertiary amides with broad
substrate scopes and high functional group tolerance. The use
of cheap common metal catalysts under ambient pressure
makes this reaction easily operated. An aerobic oxidative
process is involved in this transformation. Further studies on
the mechanistic details and reactions with other amines are
underway in our laboratory.
a
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.6b02913.
Standard conditions: 1a (0.4 mmol), 2 (1.3 equiv), Pd(PPh3)Cl2
■
S
(0.02 mmol), Cu (0.02 mmol), CO/O (2:1, balloon), DMSO (4.0
mL), 80 °C, 36 h. Isolated yields.
(2i) bearing long-chain alkyl groups also afforded products,
respectively, though with an array of lower yields between 19
and 30%. No desired product was detected from the bidentate
amine DMEDA (see the SI). Apparently, the strong
coordinative amines would retard the reaction. Product was
not obtained when a tertiary amine was employed as the N-
partner (see the SI). Five-menbered cyclic amine indoline (2h)
performed well to give the product 3ah in 54% yield.
Research details, experimental procedures, full character-
ization of products, and NMR spectra (PDF)
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: jiaoning@pku.edu.cn.
Notes
According to our previous work14b and the above observed
results, a comprehensive mechanism is proposed in Scheme 6.
The authors declare no competing financial interest.
Scheme 6. Proposed Comprehensive Mechanism
ACKNOWLEDGMENTS
■
Financial support from National Basic Research Program of
China (973 Program) (Grant No. 2015CB856600), National
Natural Science Foundation of China (Nos. 21325206 and
21632001), the National Young Top-notch Talent Support
Program, and the Peking University Health Science Center
(BMU20150505) is greatly appreciated. We thank Kai Wu in
this group for reproducing the results of 3ba and 3ah.
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