Copper-catalyzed aerobic decarboxylative coupling between cyclic α-amino acids and diverse C-H nucleophiles with low catalyst loading

Article abstract of DOI:10.1039/c8ra02340a

An aerobic decarboxylative cross-coupling of α-amino acids with diverse C-H nucleophiles has been realized using Cu₂(OH)₂CO₃ (1 mol%) as the catalyst under air. This protocol enables highly efficient formation of various C

Full text of DOI:10.1039/c8ra02340a

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Copper-catalyzed aerobic decarboxylative
coupling between cyclic a-amino acids and diverse
C–H nucleophiles with low catalyst loading†
Cite this: RSC Adv., 2018, 8, 16202
*
Jing Guo, Ying Xie, Qiao-Lei Wu, Wen-Tian Zeng, Albert S. C. Chan, Jiang Weng
*
and Gui Lu
An aerobic decarboxylative cross-coupling of a-amino acids with diverse C–H nucleophiles has been
realized using Cu2(OH)2CO3 (1 mol%) as the catalyst under air. This protocol enables highly efficient
formation of various C(sp3)–C(sp3), C(sp3)–C(sp2) and C(sp3)–C(sp) bonds under simple conditions
without the use of any ligand or extra oxidant, providing a practical approach to numerous nitrogen-
containing compounds in good to excellent yields. The efficiency and practicability were also
demonstrated by the gram-scale experiment and three-step synthesis of a Rad51 inhibitor.
Received 16th March 2018
Accepted 26th April 2018
DOI: 10.1039/c8ra02340a
rsc.li/rsc-advances
and nitromethane in the presence of peroxide and ligand
(Fig. 1a). Later they expanded the substrate scope of C–H
nucleophiles to naphthols via iron catalysis (Fig. 1b).6 Subse-
Introduction
Decarboxylative coupling has recently emerged as a powerful
method for the construction of C–C and C–heteroatom bonds in
organic synthesis.1,2 In contrast to conventional coupling reac-
tions where expensive and sensitive organometallic reagents are
usually required, decarboxylative coupling utilizes carboxylic
acids that are readily available, bench-stable and simple to
handle. Moreover, these reactions generate active carbon
species in situ along with nontoxic carbon dioxide, which
enables highly chemo- and regioselective cross-couplings with
various coupling partners.
Pyrrolidines are privileged structural motifs in a wide variety
of natural and unnatural products.3 The highly pronounced
biological activities of pyrrolidine-containing compounds have
made them attractive synthetic targets in medicinal chemistry.
In the past decade, redox-neutral C–H functionalization affords
a useful and economically efficient synthetic approach to these
active molecules.4 On the other hand, proline is among the most
abundant biomass feedstock in nature. As such, site-specic
decarboxylative functionalization of proline has received
particular attention, since this approach provides a facile and
versatile synthesis of pyrrolidine derivatives. Recently, several
research groups have developed useful synthetic methods for
these nitrogen heterocycles through decarboxylative coupling
between proline and diverse C–H nucleophiles. Li and Liang5
cooperatively developed a copper-catalyzed oxidative decarbox-
ylative C–C coupling of N-benzyl-proline with alkynes, indoles
quently, Seidel's and other groups independently realized
copper-catalyzed a-functionalization of proline under neutral
conditions, which involved the aldehyde- and ketone-induced
formation of the azomethine ylide as the key step (Fig. 1c–d).7
Chien and co-workers recently reported an intermolecular
decarbonylative Mannich reaction of N-benzyl-proline with
methyl ketones in the presence of oxalyl chloride (Fig. 1e).8 In
addition, signicant progress has been achieved in visible-light-
induced radical decarboxylative functionalization of a-amino
acids including proline derivatives (Fig. 1f).9 However, most of
the above methods suffer from one or more drawbacks, such as
the requirement of peroxide, organic ligand or transition metal
catalyst with high loading, and harsh reaction conditions. From
the viewpoint of sustainable chemistry, it is still highly desir-
able to develop simple and practical methods for decarbox-
ylative functionalization of proline and its derivatives. Herein
we wish to report a copper-catalyzed aerobic decarboxylative
coupling between cyclic a-amino acids and diverse C–H nucle-
ophiles (Fig. 1g). The major advantage of our ndings
including: (1) the use of earth-abundant copper catalyst with
1 mol% loading; (2) ligand peroxide-free conditions; (3) efficient
construction of various C(sp3)–C(sp3), C(sp3)–C(sp2) and C(sp3)–
C(sp) bonds (more than 40 examples, 54–92% yields); (4) gram-
scale synthesis; (5) application in facile synthesis of a Rad51
inhibitor.
Results and discussion
Institute of Medicinal Chemistry, School of Pharmaceutical Sciences, Sun Yat-sen University,
Guangzhou 510006, China. E-mail: lugui@mail.sysu.edu.cn; wengj2@mail.sysu.edu.cn;
Fax: +86-20-3994-3048
† Electronic supplementary information (ESI) available: Copies of 1H, 13C NMR
We chose N-benzyl proline (1a) and indole (2a) as model reac-
tion substrates. In the presence of CuBr2 (10 mol%) and 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU), the desired tertiary
spectra. See DOI: 10.1039/c8ra02340a
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Table 1 Optimization of the reaction conditionsa
Entry
Catalyst
Solvent
Base
Yieldb (%)
1
2
3
4
5
6
7
8
CuBr2
Cu(OAc)2
Cu(OTf)2
CuCl2
CuO
Cu(OH)2
Cu2(OH)2CO3
CuI
CuCl
CuBr
FeCl3
FeSO4
Fe(OAc)3
Cu2(OH)2CO3
Cu2(OH)2CO3
Cu2(OH)2CO3
Cu2(OH)2CO3
Cu2(OH)2CO3
Cu2(OH)2CO3
Cu2(OH)2CO3
Cu2(OH)2CO3
Cu2(OH)2CO3
Cu2(OH)2CO3
Cu2(OH)2CO3
Cu2(OH)2CO3
Cu2(OH)2CO3
—
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
DMF
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
TMEDA
Et3N
Ag2CO3
Cs2CO3
KOH
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
48
48
36
32
15
10
65
35
27
34
64
44
42
43
42
37
32
35
35
25
38
20
68
45
64
65
0
9
10
11
12
13
14
15
16
17
18
19
20
21
22i
23c
24d
25c,e
26c,f
27c
28c,f,g
29c,f,h
30c,f,j
31c,f,k
PhCl
Mesitylene
DMSO
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Cu2(OH)2CO3
Cu2(OH)2CO3
Cu2(OH)2CO3
Cu2(OH)2CO3
5
35
25
5
Fig. 1 Decarboxylative functionalization of cyclic a-amino acids.
a
Reaction conditions: 1a (0.4 mmol), 2a (0.2 mmol), catalyst (10 mol%),
base (4 equiv.), solvent (2 mL), 110 ꢀC, under air for 24 h. b Isolated yield.
c
f
d
e
DBU (3 equiv.).
DBU (2 equiv.).
Cu2(OH)2CO3 (5 mol%).
g
h
i
amino-indole product 3a was obtained in 48% isolated yield
aer 24 h (Table 1, entry 1). In order to improve the yield,
a series of Cu(I), Cu(II), Fe(II) and Fe(III) salts were examined and
Cu2(OH)2CO3 gave the best yield (entries 2–13). Comparing with
Cu(I) catalyst, Cu(II) catalyst resulted in higher yield. Changing
Cu catalyst to Fe catalyst could also generate the desired
product, and FeCl3 afforded comparable yield with Cu2(OH)2-
CO3 (64% vs. 65%). In view of the lower price of Cu2(OH)2CO3,
we chose Cu2(OH)2CO3 as optimal catalyst. Subsequently,
different solvents, including N,N-dimethylformamide (DMF),
chlorobenzene, dimethyl sulfoxide (DMSO) and mesitylene were
screened with Cu2(OH)2CO3 as catalyst (entries 14–17). It was
found that toluene was the optimal choice for this trans-
formation (entry 7). The attempts using other bases such as
N,N,N0,N0-tetramethylethylene diamine (TMEDA), triethyl-
amine, Ag2CO3 and Cs2CO3 were also investigated, but all
resulted in lower yields than DBU (entries 18–21). The amount
of base showed obvious effect on the reaction, the use of 3
equiv. of DBU resulted in the highest yield (entries 23–24). To
clarify whether DBU plays as a ligand or a base in this reaction,
Cu2(OH)2CO3 (1 mol%). Under N2. Under O2. Using 20 mol%
DBU as ligand and KOH (3 equiv.) as base. j Under 80 ꢀC. k Under 60 ꢀC.
we have also carried out a control experiment with catalytic
amount of DBU (20 mol%) and 3.0 equiv. of KOH, and obtained
low yield (20%, entry 22 in Table 1). In view of the comparable
basicity of DBU and KOH, we assumed that DBU might not be
a ligand but mainly act as a base. To our delight, 1 mol% of
Cu2(OH)2CO3 is sufficient to provide the desired product 3a in
65% yield within 24 h (entries 25–26). This cross-coupling did
not occur in the absence of Cu2(OH)2CO3 catalyst (entry 27). We
have also performed the reaction under N2 and O2 atmosphere
respectively, and the yields were 5% and 35% (entries 28–29 in
Table 1), which implied that O2 was essential for this trans-
formation. Lower temperatures as 80 ꢀC and 60 ꢀC caused poor
yields (<25%, entries 30–31), which indicated that high
temperature was necessary for the conversion.
With the optimal reaction condition in hand (entry 26 in
Table 1), we next set out to examine the scope of the new
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Table 3 Scope of cyclic a-amino acidsa,b
protocol with different C–H nucleophiles and the results were
shown in Table 2. To our delight, this reaction exhibited broad
substrate scope with respect to a wide range of sp2, sp3 and sp
C–H nucleophiles. Indoles with either electron-withdrawing
groups (5-F, 5-Cl, 5-Br, 5-NO2) or electron-donating groups (4-
Me, 5-Me, 6-Me and 7-Me) reacted smoothly with N-benzyl
proline 1a and afforded the corresponding C(sp2)–C(sp3)
coupling products 3a–3j in moderate to good yields (52–74%),
albeit indoles bearing electron-withdrawing substituents (3f, 3g,
3h) provided slightly higher yields than that with electron-
donating groups. It should be noted that increasing the cata-
lyst loading to 10 mol% could further improve the yields to 73–
81% (see the yields in parentheses). The coupling of 1a with 4-
chloro-5H-pyrrolo[3,2-d]pyrimidine also proceeded smoothly,
affording 3k in 88% yield. Moreover, a- and b-naphthols were
suitable coupling partners, the corresponding C(sp2)–C(sp3)
coupling products 3l and 3m can be obtained in 89% and 84%
yields, respectively. In addition, C(sp)–C(sp3) bond could also be
constructed via the reaction of various aromatic alkynes with N-
benzyl proline, affording the related coupling products 3n–3p in
good yields (70–75%). Furthermore, this method was also
suitable for the decarboxylative coupling of C(sp3)–C(sp3)
bonds, and the reactions proceeded smoothly for nitromethane
and acetophenone nucleophiles. Among them, products 3q and
3r are synthetic precursors for some bioactive natural prod-
ucts.10 The hindered C–H nucleophiles including propiophe-
a
Reaction conditions: 1a (0.4 mmol), nucleophile (0.2 mmol),
Cu2(OH)2CO3 (1 mol%), DBU (3 equiv.), toluene (2 mL), under air for
b
c
24 h at 110 ꢀC. Isolated yields. 10 mol% Cu2(OH)2CO3, 130 ꢀC,
d
48 h. Using 3 equiv. of nucleophile.
We next explored the scope of cyclic a-amino acids and the
none, 1,2-diphenylethan-1-one and diethyl malonate were also results were summarized in Table 3. Besides pyrrolidine,
tested in this reaction with N-benzyl proline 1a under standard piperidine is also common heterocyclic skeleton existing in
conditions. Unfortunately, these nucleophiles were found to be numerous natural products11 albeit the decarboxylative
incompatible with the developed protocol, and gave complex coupling of piperidine carboxylic acid has less been explor-
mixture with only trace products (detected by mass spectros- ed.5,6,7a We examined the decarboxylative coupling of N-benzyl
copy). Furthermore, o-cresol and phenol were also tested, but piperidine carboxylic acid with diverse C–H nucleophiles
only trace products were obtained (detected by mass including indoles, naphthols and alkynes. A range of indoles
spectroscopy).
bearing electron-donating groups (–Me) or electron-
withdrawing groups (–F and –NO2) at the C5 position reacted
smoothly with N-benzyl piperidine-2-carboxylic acid, affording
the coupling products 3x–3aa with moderate to good yields (62–
70%). Increasing the amount of copper catalyst to 10 mol%
resulted in higher yields (68–75%). This coupling reaction also
proceeded successfully with a-naphthol and b-naphthol,
leading to 3ab and 3ac in 50% and 80% yields, respectively.
Moreover, phenylacetylene readily underwent the coupling to
afford the corresponding piperidine-alkyne product 3ad with
good yield. A range of N-benzyl prolines bearing different
substituents (F-, Br- and Me-) at the para-position of the phenyl
ring were also investigated, giving various products including
indolyl pyrrolidines 3aq–3as, tertiary aminonaphthols 3ah–3aj,
propargylamines 3ak–3am and b-nitroamines 3an–3ap in
moderate to high yields (67–92%). In particular, tetrahy-
droisoquinoline acid, an important moiety in many pharma-
ceuticals and natural products, was also found to be competent
substrate under the standard decarboxylative coupling condi-
tions (3aq, 82%). Moreover, when N-benzyl L-proline was used in
this reaction, only racemic product 3a was obtained, which was
in agreement with the oxidative decarboxylation of L-proline
and subsequent racemization of product.
Table 2 Scope of C–H nucleophilesa,b
a
Reaction conditions: 1a (0.4 mmol), nucleophile (0.2 mmol),
Cu2(OH)2CO3 (1 mol%), DBU (3 equiv.), toluene (2 mL), under air for
b
c
24 h at 110 ꢀC. Isolated yields. 10 mol% Cu2(OH)2CO3, 130 ꢀC,
d
48 h. Using 3 equiv. of nucleophile.
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Scheme 1 Gram-scale preparation of 3l.
Scheme 4 Proposed mechanism for the copper-catalyzed decar-
boxylative C–H cross-coupling.
Scheme 2 New synthetic route of Rad51 inhibitor.
employed under the standard conditions with indole, no desired
product was observed (Scheme 3b), which implied the necessity
of benzyl group for the formation of azomethine ylide inter-
mediate.5,6,7a To test whether the azomethine ylide intermediate
5a was involved in this reaction, we have prepared the deuterium-
labeled N-benzylproline (1a-d7) and subjected it to the reaction
with indole under our optimized conditions. The 1H NMR spec-
trum of 3a-d7 indicated that all deuterium atoms were retained,
no deuterium–hydrogen exchange occurred at the benzylic
position, which excluded the possibility of azomethine ylide
intermediate 5a (Scheme 3c). To our delight, N-benzyl iminium
intermediate 5b (m/z ¼ 160) could be detected by ESI-MS from
the reaction mixture, which demonstrated that N-alkyl iminium
might be the reactive intermediate (Scheme 3d). We also tried to
prepare the possible N-benzyliminium intermediate 5d in situ
from 5c, then carried out the subsequently Mannich reaction
with indole in the presence of DBU. The desired product 3y can
be isolated in 21% yield (Scheme 3e).
On the basis of literatures5,6 and our experimental observa-
tions, a tentative mechanism has been depicted in Scheme 4.
Initially, the deprotonation of N-Bn-proline 1a by DBU gener-
ated intermediate A, then the oxidative decarboxylation of A
catalyzed by Cu2(OH)2CO3 afforded N-benzyl iminium inter-
mediate B, which underwent a Mannich reaction with carbon
nucleophile to deliver the desired product. The regeneration of
the Cu catalyst was realized via the oxidation of O2 in air. It
should be mentioned that this mechanism is different from Li's
work, where an azomethine ylide was proposed as reactive
intermediate.5
To explore the practical utility of this method, a gram-scale
experiment was performed with N-benzyl proline 1a and a-
naphthol 2l (Scheme 1). The coupling product 3l could be ob-
tained in 88% yield with only 1 mol% of copper catalyst, which
demonstrated that this protocol was scalable and practical.
The efficiency and convenience of this copper-catalyzed
decarboxylative coupling reaction was further demonstrated
in the facile synthesis of IBR2 analogue 4a, which is a Rad51
inhibitor for the treatment of triple-negative human breast
cancer (Scheme 2).12,13 The decarboxylative coupling of tetra-
hydroisoquinoline acid with indole was employed as the key
step to afford intermediate 3aq, subsequent N-deprotection and
N-sulfonylation led to the IBR2 analogue 4a (70% yield, 3 steps).
This new synthetic route is a signicant simplication when
comparing with the previous eight to thirteen steps using
conventional synthetic approaches.12,13a
In order to gain mechanistic insight into this transformation,
some control experiments were performed. The reaction pro-
ceeded well in the presence of radical scavenger TEMPO (2,2,6,6-
tetramethylpiperidinooxy), affording the desired product 3a in
64% yield (Scheme 3a), which ruled out the involvement of
a radical coupling pathway. The function of N-protective group
was also explored. When N-Boc-proline or N–H free proline was
Conclusions
In summary, we have developed a ligand-free, copper-catalyzed
aerobic decarboxylative coupling of cyclic a-amino acids and
diverse C–H nucleophiles (indoles, naphthols, alkynes, ketones
and nitroalkanes) with extremely low catalyst loading (1 mol%).
This simple catalytic system enabled the direct construction of
various C(sp3)–C(sp3), C(sp3)–C(sp2) and C(sp3)–C(sp) bonds in
numerous nitrogen-containing heterocycles. The efficiency and
practicability of this protocol was also demonstrated by gram-
scale preparation and three-step synthesis of a Rad51 inhibitor.
Scheme 3 Control experiments.
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Conflicts of interest
There are no conicts to declare.
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Acknowledgements
This work was nancially supported by the National Natural
Science Foundation of China (No. 21502240), the Guangdong
Natural Science Foundation (No. 2015A030313184), Science and
Technology Program of Guangzhou (No. 201607010118).
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