Copper-catalyzed aerobic oxidative cross-dehydrogenative coupling of amine and α-carbonyl aldehyde: A practical and efficient approach to α-ketoamides with wide substrate scope

Article abstract of DOI:10.1021/ol301130u

A copper-catalyzed aerobic oxidative cross-dehydrogenative coupling (CDC) of amine with α-carbonyl aldehyde has been developed. Many types of amines are tolerant in this transformation leading to various α-ketoamides compounds. Wide substrate scope, CDC strategy and using air as oxidant make this transformation highly efficient and practical. Molecular oxygen acts not only as the oxidant, but also as an initiator to trigger this catalytic process. Furthermore, mechanism studies show that carbonyl group of α-carbonyl aldehyde plays a role as the directing group to facilitate this chemical process.

Full text of DOI:10.1021/ol301130u

ORGANIC
LETTERS
XXXX
Vol. XX, No. XX
000–000
Copper-Catalyzed Aerobic Oxidative
Cross-Dehydrogenative Coupling of
Amine and r-Carbonyl Aldehyde:
A Practical and Efficient Approach to
r-Ketoamides with Wide Substrate Scope
Chun Zhang,† Xiaolin Zong,† Liangren Zhang,† and Ning Jiao*,‡
State Key Laboratory of Natural and Biomimetic Drugs, School of Pharmaceutical
Sciences, Peking University, Xue Yuan Rd. 38, Beijing 100191, China. State Key
Laboratory of Drug Research, Shanghai Institute of Materia Medica,
Chinese Academy of Sciences, Shanghai 201203, China
jiaoning@bjmu.edu.cn
Received May 2, 2012
ABSTRACT
A copper-catalyzed aerobic oxidative cross-dehydrogenative coupling (CDC) of amine with R-carbonyl aldehyde has been developed. Many types
of amines are tolerant in this transformation leading to various R-ketoamides compounds. Wide substrate scope, CDC strategy and using air as
oxidant make this transformation highly efficient and practical. Molecular oxygen acts not only as the oxidant, but also as an initiator to trigger this
catalytic process. Furthermore, mechanism studies show that carbonyl group of R-carbonyl aldehyde plays a role as the directing group to
facilitate this chemical process.
R-Ketoamides have attracted considerable attention not
only because of their remarkable biological and pharma-
cological activity but also because of their wide applica-
tion as useful precursors for a variety of functional group
transformations.1 Consequently, numerous synthetic
methodologies for the construction of this important unit
have been developed in the past decades.2 Recently,
aerobic oxidative cross-dehydrogenative coupling (CDC)
reactions have presented one of the most attractive and
powerful strategies for CꢀC and CꢀX bond formation.3,4
In the context of CDC strategy, three kinds of oxidative
(2) For some selected examples in recent years, see: (a) Chen, J.;
Chen, X.; Bois-Choussy, M.; Zhu, J. J. Am. Chem. Soc. 2006, 128, 87. (b)
El Kaim, L.; Gamez-Montano, R.; Grimaud, L.; Ibarra-Rivera, T.
Chem. Commun. 2008, 1350. (c) Liu, J.; Zhang, R.; Wang, S.; Sun, W.;
Xia, C. Org. Lett. 2009, 11, 1321. (d) Murphy, E. R.; Martinelli, J. R.;
Zaborenko, N.; Buchwald, S. L.; Jensen, K. F. Angew. Chem., Int. Ed.
2007, 46, 1734. (e) Fukuyama, T.; Nishitani, S.; Inouye, T.; Morimoto,
K.; Ryu, I. Org. Lett. 2006, 8, 1383. (f) Iizuka, M.; Kondo, Y. Chem.
Commun. 2006, 42, 1739. (g) Grassot, J.-M.; Masson, G.; Zhu, J. Angew.
Chem., Int. Ed. 2008, 47, 947. (h) Bouma, M.; Masson, G.; Zhu, J. J. Org.
Chem. 2010, 75, 2748. (i) Coffinier, D.; Kaim, L. E.; Grimaud, L. Org.
Lett. 2009, 11, 1825. (j) Mossetti, R.; Pirali, T.; Tron, G. C.; Zhu, J. Org.
Lett. 2010, 12, 820. (k) Liu, Q.; Perreault, S.; Rovis, T. J. Am. Chem. Soc.
2008, 130, 14066. (l) Al-Rashid, Z. F.; Johnson, W. L.; Hsung, R. P.;
Wei, Y.; Yao, P.-Y.; Liu, R.; Zhao, K. J. Org. Chem. 2008, 73, 8780. (m)
Song, B.; Wang, S.; Sun, C.; Deng, H.; Xu, B. Tetrahedron Lett. 2007, 48,
8982. (n) Allais, C.; Constantieux, T.; Rodriguez, J. Synthesis 2009,
2523.
† Peking University.
‡ Chinese Academy of Sciences.
(1) (a) Patel, D. V.; Gless, R. D. J.; Webb, H.; Heather, K.; Anandan,
S. K.; Aavula, B. R. PCT Int. Appl. WO 2008073623, 2008. (b) Knust, H.;
Nettekoven, M.; Pinard, E.; Roche, O.; Rogers-Evans, M. PCT Int. Appl.
WO 2009016087, 2009. (c) Crowley, C. A.; Delaet, N. G. J.; Ernst, J.;
Grove, C. G.; Hepburn, B.; King, B.; Larson, C. J.; Miller, S.; Pryor, K.;
Shuster, L. J. PCT Int. Appl. WO 2007146712, 2007. (d) Alvarez, S.;
Alvarez, R.; Khanwalkar, H.; Germain, P.; Lemaire, G.; Rodriguez-Barrios,
F.; Gronemeyer, H.; de Lera, A. R. Bioorg. Med. Chem. 2009, 17, 4345. (e)
Jesuraj, J. L.; Sivaguru, J. Chem. Commun. 2010, 46, 4791. (f) Zhang, Z.;
Zhang, Q.; Ni, Z.; Liu, Q. Chem. Commun. 2010, 46, 1269. (g) Sai,
K. K. S.; Esteves, P. M.; Penha, E. T. D.; Klumpp, D. A. J. Org. Chem.
2008, 73, 6506. (h) Tomita, D.; Yamatsugu, K.; Kanai, M.; Shibasaki,
M. J. Am. Chem. Soc. 2009, 131, 6946. (i) Yang, L.; Wang, D.-X.;
Huang, Z.-T.; Wang, M.-X. J. Am. Chem. Soc. 2009, 131, 10390.
r
10.1021/ol301130u
XXXX American Chemical Society

approaches to R-ketoamides have been developed employ-
ing molecular oxygen5 as the oxidant.6ꢀ8
Scheme 1. Methods of Synthesis of R-Ketoamides
Despite the environmentally benign character and the
efficiency of these methods, the substrate scope still re-
mains as a challenge: (1) N-Unsubstituted anilines are
required in the reaction of amidationꢀdiketonization of
terminal alkynes (method 1, Scheme 1).6 (2) Aliphatic
amines do not work in the reaction of aryl acetaldehydes
with anilines (method 2, Scheme 1).7 (3) Only aliphatic
secondary amines work in the coupling reaction of aryl
methyl ketones with amines (method 3, Scheme 1).8 Thus,
many important R-ketoamides could not be afforded by
these methods.6ꢀ8 Furthermore, pure molecular oxygen is
required in all these three methods for high efficiency.
Therefore, development of a more practical approach with
broad substrate scope compatible with various amines is
still attractive. Herein, we report a practical and efficient
Cu-catalyzed CDC of R-carbonyl aldehyde with a variety
of amines using air as the oxidant (Scheme 1).
Interestingly, when CuBr was used as the catalyst, the
cross-dehydrogenative coupling of 4-aminobenzonitrile
(1a) with phenylglyoxal monohydrate (2a) afforded the
desired N-(4-cyanophenyl)-2-oxo-2-phenylacetamide (3aa)
in 81% yield (entry 1, Table S1). Other copper catalysts
including Cu(II) salts gave low efficiencies (entries 1ꢀ3,
Table S1, and Supporting Information). This reaction
nearly did not work in the absence of Cu catalyst (entry
4, Table S1). The reactions gave low yields, respectively,
in CH3CN, DMF, or other solvents (entries 1, 5, and 6,
Table S1, and Supporting Information). Further studies
indicated that base can promote this transformation
(entries 1, 7, and 8, Table S1, and Supporting Information).
Furthermore,someligandscouldaffect thistransformation
(entries 1 and 9ꢀ11, table S1, and Supporting Information).
Among these ligands, 2,20-bipyridine is the best for improv-
ing the yield of 3aa to 91% (entries 9, Table S1).
Under these optimized conditions, the scope of substi-
tuted amines (1) was investigated (Scheme 2). Both electron-
rich and electron-deficient anilines could be smoothly
transformed intothe desiredproducts(3aaꢀqa, Scheme 2).
Furthermore, substituents at different positions of the
arene group (para-, meta-, and ortho-position) did not
affect the efficiency (3aaꢀca, Scheme2). Halo-substituted
aryl acetaldehydes survived well leading to halo-substi-
tuted products (3faꢀha and 3jaꢀla Scheme 2). It is note-
worthy that N-substituted anilines such as N-methyl-,
N-ethyl-, and N-phenylaniline could be smoothly trans-
formed into the desired products (3oaꢀqa, Scheme 2).
Furthermore, aliphatic secondary amines also worked
well to afford the desired products in good to excellent
yields (3raꢀua, Scheme 2). Using pure oxygen as oxidant,
N-substituted anilines and aliphatic secondary amines
afforded higher yields of desired products than under air
conditions (3oa, 3pa, and 3ra). Notably, the scope of
substituted amines could expand to the aliphatic primary
amine, which could not be achieved by the reported three
aerobic oxidative approaches (Scheme 1). For example, 1v
coupling with 2a could produce the desired product (3va)
in 34% yield (Scheme 2).
(3) For some reviews of aerobic oxidative CꢀH functionalization in
recent years, see: (a) Li, C.-J. Acc. Chem. Res. 2009, 42, 335. (b) Yeung,
C. S.; Dong, V. M. Chem. Rev. 2011, 111, 1215. (c) Zhang, C.; Tang, C.;
Jiao, N. Chem. Soc. Rev. 2012, 41, 3464. (d) Wendlandt, A. E.; Suess,
A. M.; Stahl, S. S. Angew. Chem., Int. Ed. 2011, 50, 11062. (e) Campbell,
A. N.; Stahl, S. S. Acc. Chem. Res. 201210.1021/ar2002045.
(4) For some selected examples of copper-catalyzed aerobic oxidative
CꢀH functionalization in recent years, see: (a) Chiba, S.; Zhang, L.; Lee,
J.-Y. J. Am. Chem. Soc. 2010, 132, 7266. (b) Wang, H.; Wang, Y.; Liang,
D.; Liu, L.; Zhang, J.; Zhu, Q. Angew. Chem., Int. Ed. 2011, 50, 5678. (c)
Chiba, S.; Zhang, L.; Ang, G. Y.; Hui, B. W.-Q. Org. Lett. 2010, 12,
2052. (d) Wang, J.; Wang, J.; Zhu, Y.; Lu, P.; Wang, Y. Chem. Commun.
2011, 47, 3275. (e) Hausser, A.; Trautmann, M.; Roduner, E. Chem.
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Commun. 2011, 47, 6954. (f) Wurtele, C.; Sander, O.; Lutz, V.; Waitz, T.;
Tuczek, F.; Schindler, S. J. Am. Chem. Soc. 2009, 131, 7544. (g) Lucas,
H. R.; Li, L.; Narducci Sarjeant, A. A.; Vance, M. A.; Salomon, E. I.;
Karlin, K. D. J. Am. Chem. Soc. 2009, 131, 3230. (h) Garcia-Bosch, I.;
Company, A.; Frisch, J. R.; Torrent-Sucarrat, M.; Cardellach, M.;
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Gamba, I.; Guell, M.; Casella, L.; Que, L., Jr.; Ribas, X.; Luis, J. M.;
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1293. (k) Liu, Q.; Wu, P.; Yang, Y.; Zeng, Z.; Liu, J.; Yi, H.; Lei, A.
The scope of the Cu-catalyzed aerobic oxidative dehy-
drogenative functionalization was further expanded to a
variety of substituted R-carbonyl aldehydes 2 (Table 1).
Both anilines (1a) and aliphatic amines (1r) reacted with
aldehyde (2), respectively, leading to good to excellent
yields of desired products (3aaꢀag and 3raꢀrf, Table 1).
Furthermore, aryl R-carbonyl aldehyde with both electron-
donating and electron-withdrawing groups did not affect
the efficiency of this transformation (Table 1).
R-Ketoamides are ubiquitous structural motifs that
can be found in many drugs and bioactive componds.1
The present method, which affords R-ketoamides with
wide substrate scope, provides a green and easily practical
protocol for the construction of biologically active
compounds from simple and readily available starting
ꢀ
Angew. Chem., Int. Ed. 2012, 51, 4666. (l) Basle, O.; Li, C.-J. Green Chem.
2007, 9, 1047. (m) John, A.; Nicholas, K. M. J. Org. Chem. 2011, 76,
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Soc. 2011, 133, 13942. (o) Hewgley, J. B.; Stahl, S. S.; Kozlowski, M. C.
J. Am. Chem. Soc. 2008, 130, 12232. (p) Brasche, G.; Buchwald, S. L.
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Goodhue, C. E.; Yu, J.-Q. J. Am. Chem. Soc. 2006, 128, 6790.
(5) Dioxygen has been used as an ideal oxidant; for some reviews, see:
(a) Punniyamurthy, T.; Velusamy, S.; Iqbal, J. Chem. Rev. 2005, 105,
2329. (b) Stahl, S. S. Angew. Chem., Int. Ed. 2004, 43, 3400. (c) Sigman,
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B
Org. Lett., Vol. XX, No. XX, XXXX

Scheme 2. Cu-Catalyzed Aerobic Oxidative Cross-Dehydro-
genative Coupling of Different Amine (1) with 2aa
Scheme 3. Construction of Some Bioactive Compounds
a Standard reaction conditions: see entry 9, Table S1. bIsolated yields.
c1x (0.375 mmol), 2a (0.25 mmol), CuBr (0.025 mmol), pyridine
(0.5 mmol), toluene (2.5 mL), 60 ˚C, O2 (1 atm), 12 h.
demonstrate that the R-carbonyl group of R-carbonyl
aldehyde plays the role of a directing group to facilitate
this chemical process.
a Standard reaction conditions: see entry 9, Table S1. bIsolated yields.
cThe reactions were carried in out the absence of ligand. dThe reactions
were carried in the absence of ligand using O2 (1 atm) as the oxidant.
e1 (0.375 mmol), 2a (0.25 mmol) were employed in the absence of ligand
using O2 (1 atm) as the oxidant. f1 (0.375 mmol), 2a (0.25 mmol) were
employed in the absence of ligand using air (1 atm) as the oxidant.
materials. For example, I, which is reported as an epoxide
hydrolase inhibitor (1, Scheme. 3),1a can be easily synthe-
sized from 1w and 2d in 58% yields (1, Scheme 3).
Furthermore, II, which is reported as an orexin receptor
antagonist (2, Scheme 3),1b can be easily synthesized from
simple 1x and 2a in 61% yields (2, Scheme 3).
Aldehydes are desirable starting materials in amide
synthesis due to their ready availability and nontoxic
nature.9 In the past 10 years, catalytic systems to effect
this transformation have been substantially developed into
useful organic reactions.9 However, under the standard
reaction conditions, 4-aminobenzonitrile (1a) and benzal-
dehyde (4a) did not afford the desired product 4-cyano-
phenyl benzoate 5aa (eq 1), which illustrates that this
new method underwent different reaction process in con-
trast to previous reported CꢀN coupling of aldehydes.10
Furthermore, the reaction of 1a with 2b in the presence
of 4-methoxybenzaldehyde (4b) produced 3ab in 88%
yield with 97% of 4b recovery (eq 2). The above results
The transformation of 1a and 2a in the presence of 18O2
(1 atm) and 1.25 mmol of H216O generated 3aa in 90%
yield (18O16O-3aa:16O2-3aa = 1.0:2.6; 18O2-3aa was
not detected in the reaction system) (eq 3). When the
reaction of 1a and 2a in the presence of 16O2 (1 atm) and
Table 1. Cu-Catalyzed Aerobic Oxidative Cross-Dehydrogenative Coupling of 1a and 1r with Different Aldehydes 2a
entry
1
R3
C6H5ꢀ (2a)
yield of 3b(%)
entry
1
R3
C6H5ꢀ (2a)
yield of 3b(%)
1
2
3
4
5
6
7
1a
1a
1a
1a
1a
1a
1a
91 (3aa)
89 (3ab)
94 (3ac)
65 (3ad)
63 (3ae)
61 (3af)
98 (3ag)
8
1r
1r
1r
1r
1r
1r
94 (3ra)c
93 (3rb)c
90 (3rc)c
81 (3rd)c
98 (3re)c
97 (3rf)c
4-OMeC6H4ꢀ (2b)
3,4,5-(OMe)3C6H2ꢀ (2c)
4-CF3C6H4ꢀ (2d)
4-FC6H4ꢀ (2e)
9
4-OMeC6H4ꢀ (2b)
3,4,5-(OMe)3C6Hꢀ- (2c)
4-CF3C6H4ꢀ (2d)
4-FC6H4ꢀ (2e)
10
11
12
13
3-FC6H4ꢀ (2f)
3-FC6H4ꢀ (2f)
2,4-F2C6H3ꢀ (2g)
a Standard reaction conditions: see entry 9, Table S1. b Isolated yields. c 1 (0.375 mmol) and 2a (0.25 mmol) were employed in the absence of ligand
using O2 (1 atm) as the oxidant.
Org. Lett., Vol. XX, No. XX, XXXX
C

carried out under Ar, even employing 2.5 equiv of CuBr2
(eq 5). This result shows that molecular oxygen plays
a role not only as the oxidant but also as an initiator
to trigger this catalytic process. Encouraged by the
growing amount of information about Cu(I)ꢀdioxygen
reactivity,11 we hypothesized that a (μꢀη2:η2-peroxo)-
dicopper(II) complex A (Scheme 4) might be the active
catalytic species, which was produced in situ through the
reaction of LnꢀCu(I) complex with O2 on the basis of
the previous reports.11
Scheme 4. Proposed Mechanism for Direct Transformation
On the basis of the above results, a plausible mechan-
ism for the copper-catalyzed aerobic oxidative cross-
dehydrogenative coupling is illustrated in Scheme 3.
Phenylglyoxal 2a reacts with H2O to form phenylglyoxal
monohydrate 6.12 Facilitated by the carbonyl group
of R-carbonyl aldehyde, the addition of amine to aryl
glyoxal gave hemiaminal intermediate 7.10 At the same
time, Cu(I) salt was initially chelated with ligand and
oxidized by dioxygen to form the more active (μꢀη2:
η2-peroxo)dicopper(II) complex A.11 Subsequently, inter-
mediate 7 was oxidized by copper complex to produce
R-ketoamide 3aa.13
In conclusion, we have demonstrated a novel copper-
catalyzed aerobic oxidative cross-dehydrogenative coupling
(CDC) of amine with R-carbonyl aldehyde. This method
provides an efficient approach to R-ketoamides compounds,
which are ubiquitous structural units in a number of biolo-
gically active compounds. Wide substrate scope, CDC
strategy, and use of air as oxidant are the advantages of this
highly efficient and practical method. Molecular oxygen acts
not only as the oxidant but also as an initiator to trigger this
catalytic process. Furthermore, mechanism studies show
that the carbonyl group of R-carbonyl aldehyde plays a role
as a directing group to facilitate this chemical process.
1.25 mmol H218O, 3aa was obtained in 90% yield
(18O2-3aa:18O16O-3aa:16O2-3aa = 1.2:1.9:1.0) (eq 4). It
should be noted that the R-ketone at ketoamide 3 is very
active and undergoes oxygen exchange via hemiketal with
water. In contrast, the carbonyl of amide hardly exchanges
with water. Therefore, the above results indicate that the
oxygen atom of the R-ketoamide originated from sub-
strate 2 and/or H2O (for details, see the Supporting
Information). Furthermore, the desired product 3aa
was not detected when the reaction of 1a and 2a was
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Acknowledgment. Financial support from the Na-
tional Basic Research Program of China (973 Program
2009CB825300), the National Science Foundation of Chi-
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Research is greatly appreciated. We thank Yang Ou in this
group for reproducing the results of 3ra and 3rb.
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material is available free of charge via the Internet at
http://pubs.acs.org.
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The authors declare no competing financial interest.
D
Org. Lett., Vol. XX, No. XX, XXXX