Copper-catalyzed C-H oxidation/cross-coupling of α-amino carbonyl compounds

Article abstract of DOI:10.1002/anie.201109027

Keeping options open: The new and mild title reaction involving indoles selectively furnishes 1 and 2 with the aid of tert-butyl hydroperoxide (TBHP). The method represents the first example of a copper-catalyzed α arylation of α-amino carbonyl substrates leading to α-aryl α-imino and α-aryl α-oxo carbonyl compounds using a C-H oxidation strategy. Copyright

Full text of DOI:10.1002/anie.201109027

Angewandte
Chemie
DOI: 10.1002/anie.201109027
Synthetic Methods
À
Copper-Catalyzed C H Oxidation/Cross-Coupling of a-Amino
Carbonyl Compounds**
Ji-Cheng Wu, Ren-Jie Song, Zhi-Qiang Wang, Xiao-Cheng Huang, Ye-Xiang Xie, and
Jin-Heng Li*
À
Functionalization of a-C H bonds to the carbonyl group in a-
amino carbonyl compounds is a hot topic in organic synthesis
because many natural products or biomolecules contain these
modified a-amino carbonyl units in their structures.[1–4]
Among the efficient reactions for the functionalization of
[1–4]
À
À
C H bonds a to a carbonyl group,
transition-metal-
Scheme 1. Copper-catalyzed C H oxidation.
catalyzed a-arylation reactions have attracted significant
interest because of their synthetic utility.[3,4] Despite consid-
erable progress in the field, examples of transition-metal-
catalyzed a arylation of a-amino carbonyl compunds are
much less abundant and are more limited: the a-arylation
reaction is realized through deprotonation (in situ generation
of carbonyl enolate) with the aid of a base and requires the
use of both expensive aryl sources (often aryl halides or
pseudohalides) and transition-metal catalysts (often Pd).
Recently, transition-metal-catalyzed oxidative deprotonation
Our investigation began with the reaction of 1-phenyl-2-
(phenylamino)ethanone (1a) with 1H-indole (2a) and
10 mol% CuCl in CH2Cl2 at room temperature under an
argon atmosphere (entry 1; Table 1): a trace of the desired 2-
(1H-indol-3-yl)-1-phenyl-2-(phenylimino)ethanone
(3aa)
was observed. These results encouraged us to optimize the
other reaction parameters. After a series of trials, we were
pleased to find that the yield of 3aa could be enhanced in the
À
emerged as a powerful tool in numerous C H functionaliza-
tion reactions, wherein bases are not necessary.[2i,5,6] However,
to our knowledge, transition-metal-catalyzed a arylation of a-
amino carbonyls leading to a-aryl a-iminocarbonyls using
Table 1: Screening for optimal reaction conditions.[a]
À
a C H oxidation strategy has not been established.
Herein we report a novel and mild route to the selective
synthesis of 2-(1H-indol-3-yl)-2-imino-carbonyls (3) and 2-
À
(1H-indol-3-yl)-2-oxo-carbonyls (4) by copper-catalyzed C H
oxidation/cross-coupling of a-amino carbonyl compounds (1)
with indoles (2) in the presence of tert-butyl hydroperoxide
(TBHP; Scheme 1). Interestingly, the selectivity for either 3
or 2 can be tuned by a slight modification of the reaction
conditions. It is noteworthy that the products, 3-substituted
indoles, are valuable synthetic intermediates and important
structural units found in numerous natural products, pharma-
ceuticals, and functional materials.[7]
Entry
[Cu] (mol%)
[O] (equiv)
Solvent
Yield [%][b]
1
2
3
4
CuCl (10)
CuCl (10)
CuCl (10)
CuCl (10)
CuCl (10)
CuCl (10)
CuBr (10)
CuI (10)
CuCl2 (10)
Cu(OTf)2 (10)
–
CuCl (20)
CuCl (5)
CuCl (10)
CuCl (10)
CuCl (10)
CuCl (10)
CuCl (10)
CuCl (10)
–
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
–
trace
32
34
83
28
15
73
71
65
58
0
79
80
60
10
5
oxone (2)
mCPBA (2)
TBHP (2)
TBHP (3)
TBHP (1.3)
TBHP (2)
TBHP (2)
TBHP (2)
TBHP (2)
TBHP (2)
TBHP (2)
TBHP (2)
TBHP (2)
TBHP (2)
TBHP (2)
TBHP (2)
TBHP (2)
TBHP (2)
5[c]
6
7
8
9[d]
10
11
12
13[e]
14
15
16
17
18
19[f]
[*] J.-C. Wu,[+] R.-J. Song,[+] Z.-Q. Wang, X.-C. Huang, Y.-X. Xie,
Prof. Dr. J.-H. Li
State Key Laboratory of Chemo/Biosensing and Chemometrics
College of Chemistry and Chemical Engineering
Hunan University, Changsha 410082 (China)
E-mail: jhli@hnu.edu.cn
CH2ClCH2Cl
DMSO
MeCN
toluene
CH2Cl2
J.-C. Wu,[+] Z.-Q. Wang, X.-C. Huang
38
36
33
Key Laboratory of Chemical Biology & Traditional Chinese Medicine
Research (Ministry of Education)
Hunan Normal University, Changsha 410081 (China)
[a] Reaction conditions: 1a (0.3 mmol), 2a (0.36 mmol), [Cu] and
solvent (2 mL) under argon atmosphere for 12 h. TBHP (2 equiv, 70% in
water). [b] Yield of isolated product. [c] Some unidentified products were
observed by GC/MS analysis in addition to the 10% yield of 4aa.
[d] Product 4aa was isolated in 20% yield. [e] For 20 h. [f] Anhydrous
TBHP (5m in decane). Product 4aa was isolated in 28% yield.
DMSO=dimethylsulfoxide, Tf=trifluoromethanesulfonyl.
[+] These authors contributed equally to this work.
[**] We thank the Natural Science Foundation of China (No. 21172060)
and Fundamental Research Funds for the Central Universities
(Hunan University) for financial support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201109027.
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.
Angewandte
Communications
presence of oxidants, including oxone (entry 2), mCPBA
(entry 3), and TBHP (entry 4). Screening revealed that the
amount of TBHP affected the reaction (entries 5 and 6). Both
3 equivalents and 1.3 equivalents of TBHP offered the
desired product 3aa in a low yield. Another product, 1-(1H-
indol-3-yl)-2-phenylethane-1,2-dione (4aa), was isolated in
10% yield when 3 equivalents of TBHP was used (entry 5).
Subsequently, a series of other copper catalysts, including
CuBr, CuI, CuCl2, and Cu(OTf)2, were tested (entries 7–10),
and they displayed high catalytic efficiency for the reaction in
the presence of TBHP, but were inferior to CuCl. Notably,
product 4aa was obtained when CuCl2 was used as the catalyst
(entry 9). Notably, the reaction cannot take place without
a copper catalyst (entry 11). Gratifyingly, good yields were
still achieved when using either 20 mol% or 5 mol% CuCl,
but the latter required a prolonged reaction time (entries 12
and 13). It was interesting to discover that the reaction could
be carried out under neat reaction conditions, thus furnishing
3aa in moderate yield (entry 14). Among the solvents
examined, CH2Cl2 was the most effective (entries 15–18).
When using anhydrous TBHP, however, the reaction afforded
a mixture of the products 3aa and 4aa in 33% and 28%
yields, respectively (entry 19).
With the optimal reaction conditions in hand, the scope of
both the a-amino carbonyls 1 and indoles 2 was explored
(Scheme 2). Initially, several N-aryl groups on 1 were inves-
tigated in the presence of 1H-indole (2a), CuCl, TBHP, and
argon. The reactivity of the electron-rich N-aryl group was
superior to that of the electron-withdrawing N-aryl groups,
and some halo substituents (F, Cl, or Br) were tolerated (3ba–
3 fa). However, substrates with an N-nBu group, an aliphatic
group, did not lead to the desired product 3ga, but 4aa was
isolated in 50% yield. Gratifyingly, substituents, including
aryl and alkyl groups, at the 1-position of 1 were consistent
with the optimal reaction conditions (3ha–3oa). For exam-
ples, substrates with a para-substituted aryl group, such as p-
MeC6H4, p-MeOC6H4, p-PhC6H4, p-ClC6H4, or p-FC6H4,
smoothly underwent the reaction with 2a, CuCl, and TBHP
under argon, thus affording the corresponding products 3ha–
3la in good yields. Good yield was still achieved when using
the dichloro-substituted substrate (3ma). Substrate 1-(phe-
nylamino)propan-2-one, lead to the desired product 3oa in
61% yield. Notably, ethyl 2-oxo-3-(phenylamino)propanoate,
an amino ester, was also a suitable substrate, thus leading to
the target product 3pa in 69% yield.
Subsequently, the scope of the indoles 2 was examined in
the presence of 1a, CuCl, TBHP and argon (3ab–3ak).
Screening revealed that 1-alkyl indoles (1-Me or 1-Bn) were
suitable for the reaction (3ab and 3ac), but 1-acetyl-
substitued indole has no reactivity (3ad). Interestingly, the
reaction had high functional group compatibility as several
functional groups, including MeO, Cl, F, CF3, Et, and Me, on
the aromatic ring of 2 were well tolerated. 5-Methoxy-1H-
indole, for instance, underwent the reaction with substrate 1a,
CuCl, and TBHP, thus providing 3ae in 73% yield. Impor-
tantly, substituents, Cl and F, were compatible with the
optimal reaction conditions, thereby providing a means for
additional modifications at the halogenated positions (3af
and 3ag). Indole having the electron-withdrawing CF3 group
Scheme 2. CuCl-catalyzed synthesis of 2-(1H-indol-3-yl)-2-imino-
carbonyls (3) under an atmosphere of argon. Reaction conditions:
1 (0.3 mmol), 2 (0.36 mmol), CuCl (10 mol%), TBHP (2 equiv, 70% in
water) and CH2Cl2 (2 mL) at room temperature under argon atmos-
phere. [a] Product 4aa was isolated in 50% yield.
delivered a moderate product yield (3ah). It is noteworthy
that 2-methyl indole furnishes the desired product 3aj in 82%
yield, but 3-methyl indole is an unsuitable substrate (3ak).
As shown in Scheme 3, the selectivity of the reaction
shifted towards 1,2-diones or 2-keto esters when the reaction
was carried out under an atmosphere of air. For example,
treatment of 1a with 2a, CuCl, TBHP, and air for 36 hours
afforded 4aa exclusively in 52% yield. The results demon-
strated that the amount of TBHP affects the reaction, and
that 3 equivalents of TBHP gives then best results. Notably,
the reaction time was shortened to 18 hours when CuCl2 was
employed as the catalyst instead of CuCl. In light of these
results, the CuCl2/TBHP/air catalytic system was used in the
preparation of various 1,2-diones and 2-keto esters. Gratify-
ingly, the CuCl2/TBHP/air catalytic system is consistent with
a wide range of substituents, including MeO, Br, Cl, F, CF3,
Ph, Et, and Me, on the aromatic rings of 1 or 2. Screening
disclosed that in the presence of CuCl2, TBHP, and air
a variety of a-amino carbonyls were oxidatively cross-coupled
with 2a, thus leading the corresponding products 4aa and
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To elucidate the mechanism, some control experiments
were carried out.[8] The results shwon in Equation (2) showed
that CuCl2, TBHP, and air are important factors in the
conversion of 3ba into 4aa. The product 3ba was converted
into 4aa in 76% yield using the CuCl2/TBHP/air system
[Eq. (2)], however, the conversion decreased sharply in the
absence air, TBHP, and CuCl2. Additinally, 3ba could not be
transformed in the presence of either air [Eq. (2)] or THBP
alone (entry 10, Table 1). However, the conversion of 3ba
could take place using air combined with either hydrous or
anhydrous TBHP. It was found that the CuCl2/H2O/air system
only gave 22% yield of 4aa without the aid of TBHP. The
results in Equation (2) suggest that TBHP plays an important
role in the conversion of 3ba into 4aa. Notably, among these
reactions some other products, including p-toluidine (6), 1-
methyl-4-nitrobenzene (7), and 1,2-dip-tolyldiazene (8), were
observed by GC/MS analysis Equation (2). Interestingly, 1-
phenyl-2-(phenylimino)ethanone [9; see structure in Eq. (3)]
is observed by in situ GC/MS analysis, and disappears when
the reaction is completed. This process was illustrated
distinctly in the reaction profile.[10] Thus, the reaction between
2a and 9 was investigated [Eq. (3)], and in the presence of
CuCl and argon and absence of TBHP only a trace amount of
3aa was observed; however with 2 equivalents of TBHP 3aa
was exclusively isolated in 72% yield. Gratifyingly, the CuCl2/
TBHP/air system furnished product 4aa alone in 55% yield.
Notably, 1-phenyl-2-(phenylamino)propan-1-one (1q) was
not a suitable substrate for the reaction [Eq. (4)]. The 18O-
labeling experiments disclosed that the oxygen atom in the
newly formed carbonyl group of product 4aa is derived from
water [Eq. (5)],[8] and implies that 4aa is formed from the
hydrolysis of an imino compound.[9] Notably, a stoichiometric
amount of two radical inhibitors, 2,2,6,6-tetramethylpiperi-
dine oxide (TEMPO) and 2,6-di-tert-butylphenol, were also
used in the reaction of 1a with 2a using either the CuCl/
TBHP/Ar catalytic system or CuCl2/TBHP/O2 catalytic
system. A significant drop in yield was observed in the
presence of these radical inhibitors, thus suggesting that the
present reaction includes a radical process.[8]
Scheme 3. CuCl2-catalyzed synthesis of 2-(1H-indol-3-yl)-2-oxo-carbon-
yls (4) under an atmosphere of air. Reaction conditions: 1 (0.3 mmol),
2 (0.36 mmol), CuCl2 (10 mol%), TBHP (3 equiv, 70% in water) and
CH2Cl2 (2 mL) at room temperature under air (1 atm) conditions.
[a] CuCl (10 mol%) and TBHP (2 equiv). [b] CuCl (10 mol%) and
TBHP (3 equiv) in either air or O2 (1 atm). [c] CuCl (10 mol%) and
TBHP (4 equiv).
To our surprise, 4aa could not be obtained in the presence
of CuCl, 70% TBHP in water, and argon. However 4aa was
isolated in 28% yield when using 5m TBHP in decane instead
of 70% TBHP in water (entries 4 and 19; Table 1). Moreover,
4aa was observed when using either CuCl2 or as much as
3 equivalents of TBHP (entries 5 and 9; Table 1). These
results simply show that the stronger oxidizing conditions
favor the hydrolysis reaction. On the basis of the above results
and the results in Eqs. (2)–(5), the CuII species is the real
catalyst for hydrolyzing imine 3, and two roles of TBHP are
proposed: 1) as an oxidizing agent to regenerate the active
CuII species, and 2) as an oxygen atom (H2O) source through
decomposition with O2.
4ha–4pa in moderate to good yields. Moreover, a number of
indoles were also successfully reacted with 1a under the same
reaction conditions. However, attempts at reacting 3-methyl-
1H-indole failed.
The hydrogenation of 3ba is summarized in Equation (1).
In the presence of NaBH4 and MeOH, 3ba was converted into
Consequently,
a
possible mechanism is proposed
(Scheme 4).[5,6,10,11] Initially oxidative deprotonation of sub-
strate 1a by TBHP takes place to yield intermediate 9,[10,11]
which is supported by the reaction profile.[10] Consequently,
the in situ Friedel–Crafts alkylation of 2a with intermediate 9
affords product 3aa.[5,6] Product 3aa is hydrolyzed by H2O to
2-(1H-indol-3-yl)-1-phenyl-2-(p-tolylamino)ethanol (5ba), an
amino alcohol, in 86% yield.
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Experimental Section
Typical experimental procedure for the copper-catalyzed synthesis of
2-(1H-Indol-3-yl)-2-imino-carbonyls
(3):
a-Amino
carbonyl
1
(0.3 mmol), indole (0.36 mmol), CuCl (10 mol%), TBHP
2
(2 equiv), and CH2Cl2 (2 mL) were added to a Schlenk tube. Then
the tube was charged with argon, and the reaction mixture stirred at
room temperature for the indicated time until complete consumption
of starting material as determined by TLC and GC/MS analyses.
After the reaction was finished, the reaction mixture was washed with
brine. The aqueous phase was re-extracted with ethyl acetate. The
combined organic extracts were dried over Na2SO4, concentrated in
vacuum, and the resulting residue was purified by silica gel column
chromatography (n-hexane/ethyl acetate = 5:1) to afford the desired
product 3.
Typical experimental procedure for the copper-catalyzed syn-
thesis of 2-(1H-indol-3-yl)-2-oxo-carbonyls (4): a-Amino carbonyl
1
(0.3 mmol), indole 2 (0.36 mmol), CuCl2 (10 mol%), TBHP
(3 equiv), and CH2Cl2 (2 mL) were added to a Schlenk tube. Then
the tube was charged with air, and the reaction mixture was stirred at
room temperature for the indicated time until complete consumption
of starting material as determined by TLC and GC/MS analysis. After
the reaction was finished, the reaction mixture was washed with brine.
The aqueous phase was re-extracted with ethyl acetate. The combined
organic extracts were dried over Na2SO4, concentrated in vacuum,
and the resulting residue was purified by silica gel column chroma-
tography (n-hexane/ethyl acetate = 3:1) to afford the desired product
4.
Received: December 21, 2011
Revised: January 15, 2012
Published online: February 29, 2012
Keywords: amines · copper · heterocycles · oxidation ·
.
synthetic methods
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