Pharmaceutical-Oriented Methoxylation of Aryl C(sp 2)-H Bonds using Copper Catalysts

Article abstract of DOI:10.1055/s-0037-1610132

A pharmaceutical-oriented, copper(II)-catalyzed methoxylation of aryl C(sp ²)-H bonds has been developed. This simple and environmentally benign reaction system occurs efficiently using oxygen as oxidant with broad substrate scope and high functional group tolerance.

Full text of DOI:10.1055/s-0037-1610132

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© Georg Thieme Verlag Stuttgart · New York
2018, 29, A–D
letter
A
en
G. Zhang et al.
Letter
Synlett
Pharmaceutical-Oriented Methoxylation of Aryl C(sp2)–H Bonds
using Copper Catalysts
Guofu Zhanga
Jianfei Zhua
Chaolai Tongb
Chengrong Ding*a
O
O
N
N
Cu-catalyzed
MeOH
R
H
H
R
N
N
O
H
19 examples
up to 93%
R = H, Me, OMe, CN, CF3, OCF3, F, Cl, Br, I, NO2, Ph
a College of Chemical Engineering, Zhejiang University of Technology,
Hangzhou 310014, P. R. of China
dingcr@zjut.edu.cn
b Jianxing Honor College, Zhejiang University of Technology, Hangzhou
310014, P. R. of China
Received: 22.01.2018
Accepted after revision: 08.04.2018
Published online: 02.05.2018
est for the synthesis of complex natural and unnatural com-
pounds since carbon–carbon and carbon–heteroatom
bonds can be constructed directly from stable organic
molecules.3 It provides a new pathway for C–O bond
coupling to generate aromatic ethers.
Due to the intrinsic inertness of the C–H bond, the in-
troduction of directing groups is a common means for C–H
activation.4 However, it usually requires additional opera-
tions to remove the directing groups in addition to the in-
troduction of exogenous groups.5 Clearly, this imposes
some limitations in terms of atom- and step-economy.
However, if the directing group itself is a part of the phar-
maceutical molecule, the transition-metal-catalyzed C–H
bond to C–O bond will be more valuable.
DOI: 10.1055/s-0037-1610132; Art ID: st-2018-r0048-l
Abstract A pharmaceutical-oriented, copper(II)-catalyzed methoxyl-
ation of aryl C(sp2)–H bonds has been developed. This simple and envi-
ronmentally benign reaction system occurs efficiently using oxygen as
oxidant with broad substrate scope and high functional group toler-
ance.
Key words pharmaceutical development, C–H methoxylation, copper
catalysis, oxygen, DMEDA
Aromatic ethers are chemically and metabolically stable
functional groups that are omnipresent in natural products
and biologically active compounds.1 The majority of meth-
ods for the synthesis of aromatic ethers involve transition-
metal-catalyzed C–O bond coupling reactions, such as
Ullmann, Buchwald–Hartwig and Chan–Evans–Lam reac-
tions.2 In recent decades, transition-metal-catalyzed func-
tionalization of C–H bonds has attracted tremendous inter-
O
Pd(OAc)2 (10 mol%)
PhI(OAc)2 (1.5 equiv)
O
O
PIP
PIP
R1
N
R1
N
(a)
H
H
R2
MeOH/m-xylene (1:1)
90 °C, N2, 24 h
R2
O
Our exploratory work
O
Pd(OAc)2 (10 mol%)
PhI(OAc)2 (3 equiv)
N
N
(b)
N
H
N
H
CH2CF3
O
O
O
H
MeOH/p-xylene (1:9)
90 °C, N2, 30 min
N
Cl
N
O
N
N
H
55%
H
O
This work
O
H2N
O
CH2CF3
O
N
N
[Cu], base
(c)
metoclopramide
O
N
H
N
flecainide
H
oxidant, MeOH, Temp.
O
R
Cl
N
R
N
H
O
Scheme 1 Direct Alkoxylation of C–H Bonds
O
CF3
Cl
diclometide
HN
This approach will provide a new method for efficient
synthesis of compounds for drug activity screening. Some
pharmaceuticals such as metoclopramide and diclometide
aroused our interest because of their intrinsic N,N-biden-
tate groups (Figure 1).6 Therefore, the primary purpose is to
make it clear whether these groups or similar structures
O
NH
O
N
KIFC1 inhibitor
Figure 1 A selection of pharmaceuticals containing bidentate groups
© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–D

B
G. Zhang et al.
Letter
Synlett
Table 1 Optimization of Reaction Conditionsa
sired product could be obtained with this directing group,
albeit in low yield. Interestingly, further work established
that when N,N′-dimethylethylenediamine (DMEDA) was
used as directing group, 10 mol% Pd(OAc)2 as catalyst, 3
equivalent PhI(OAc)2 as oxidant at 90 °C under N2 atmo-
sphere, the desired product was obtained in 55% yield in 30
min (Scheme 1b).
O
O
[Cu] X mol%
base (Y equiv)
N
N
H
H
N
N
MeOH, O2
Cl
Cl
O
120 °C, 24 h
1a
2a
DMEDA proved to be an effective directing group in
view of the above. Given that palladium is rare and expen-
sive, we turned our attention to the use of alternative cata-
lysts.8 In comparison to the Pd catalyst, Cu catalysis exhibits
advantages because it is inexpensive and environmentally
benign.9 To our knowledge, only a few examples of copper-
catalyzed C–H alkoxylation with oxygen as a clean and
cheap oxidant have been reported.10 Based on these reports,
we suspect that the structure of DMEDA will also lead to
equally efficient reactions using copper as catalyst.
Entry [Cu]
Base
(equiv)
Additive
(Y mol%)
Yield
(%)b
(X mol%)
1
2
Cu2(OH)2CO3 (5)
Cu2(OH)2CO3 (5)
Cu2(OH)2CO3 (5)
Cu2(OH)2CO3 (5)
Cu2(OH)2CO3 (5)
Cu2(OH)2CO3 (5)
CuCl2 (5)
Cs2CO3 (2.0)
KHCO3 (2.0)
K2CO3 (2.0)
CsOAc (2.0)
Na2CO3 (2.0)
KOCN (2.0)
KOCN (2.0)
KOCN (2.0)
KOCN (2.0)
KOCN (2.0)
KOCN (2.0)
KOCN (2.0)
KOCN (2.0)
KOCN (1.5)
KOCN (2.5)
KOCN (2.0)
KOCN (2.0)
21
4
3
4
4
16
27
90
87
ND
86
36
15
71
3
5
6
7
To verify our hypothesis, we initiated our investigation
with 4-chloro-N-(2-(dimethylamino)ethyl)benzamide (1a)
as a model substrate (Table 1). Gratifyingly, the desired
product was obtained in 21% yield in the presence of Cs2CO3
(2.0 equiv) as a base with 5 mol% Cu2(OH)2CO3 under O2 at
120 °C (entry 1). Screening of different bases revealed that
the reaction with KOCN gave the best yield (entries 2–6).
We next screened a series of copper salts, including CuCl2,
CuO, Cu(OAc)2, CuSO4, and C18H12CuN2O2, but no better re-
sult was observed (entries 7–11). When CuO was used as
catalyst, the desired product was not detected (entry 8).
The results above revealed that suitable anions could pro-
mote the reaction and Cu(II) was the species in solution
ready to coordinate the substrate 1a. The yield was 71%
when the reaction was carried out under air atmosphere
(entry 12). The reaction was low yielding under N2 atmo-
sphere (3%; entry 13). Clearly, it was necessary to use O2 as
atmosphere. To further improve the yield, the amount of
KOCN was screened, but no significant change was ob-
served (entries 14 and 15; see the Supporting Information).
By increasing the amount of Cu2(OH)2CO3 to 20 mol%, the
yield was improved to 92% (entry 16). To our delight, when
methenamine (20 mol%) was added to the reaction system,
a better result was observed (90% isolated yield, entry 17).
With the optimal reaction conditions in hand, the reac-
tion scope with respect to aromatic substrates was estab-
lished (Scheme 2). Various kinds of benzamide derivatives
either both electron-donating or electron-withdrawing
groups on the phenyl rings (2b–s), were also tolerated un-
der the optimized conditions, and the reaction proceeded
with moderate to good yields. The reaction protocol was
sensitive to steric hindrance, with almost no methoxylated
product being produced from ortho-substituted substrates.
para-Substituted substrates bearing Br (2b) or CF3(2c)
groups were well tolerated, and those with I (2e), NO2 (2f),
F (2g), CN (2h), CH3 (2i), and OMe (2j) gave the methoxyl-
ation product in moderate to good yields. Methoxylation of
meta-substituted substrates proceeded exclusively at the
8
CuO (5)
9
Cu(OAc)2 (5)
10
11
12d
13e
14
15
16
17
CuSO4 (5)
C18H12CuN2O2 (5)
Cu2(OH)2CO3 (5)
Cu2(OH)2CO3 (5)
Cu2(OH)2CO3 (5)
Cu2(OH)2CO3 (5)
85
87
92
Cu2(OH)2CO3 (20)
Cu2(OH)2CO3 (20)
C6H12N4 (20)f
95 (90)c
a Reaction condition: 1a (0.2 mmol), [Cu] X mol%, base (Y equiv), O2
(1 atm), MeOH (2 mL).
b Yield based on 1H NMR analysis using CH2Br2 as internal standard.
c Isolated yield.
d The reaction was carried out under air atmosphere.
e The reaction was carried out under N2 atmosphere.
f Methenamine.
can be used as efficient directing groups for C–H alkoxyl-
ation. To date, a number of research groups have made con-
siderable progress in palladium-catalyzed alkoxylation of
arenes.7 In 2013, Shi and co-workers7a reported the alkoxyl-
ation of unactivated C(sp3)–H bonds with Pd(OAc)2 and
PhI(OAc)2 using a 2-(pyridine-2-yl)propan-2-amine (PIP)
directing group (Scheme 1a). In 2014, Zhang and Sun7b re-
ported a C(sp2)–H bond alkoxylation with Pd(OAc)2 and
PhI(OAc)2. Recently, a novel example of C(sp2)–H bond
alkoxylation has been reported by Liu et al.7c with Pd(OAc)2
and PhI(OAc)2 using amino acids as directing groups. It is
noteworthy that the combination of Pd(OAc)2 and PhI(OAc)2
is very efficient for promoting C–H alkoxylation.
Based on the above observations, the use of ortho-
methylbenzoic acid as a model substrate, N,N′-diethyl-
ethylenediamine (DEEDA) as directing group, Pd(OAc)2 as
catalyst, PhI(OAc)2 as oxidant and methanol as the alkoxyl-
ation reagent were used in some exploratory work. The de-
© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–D

C
G. Zhang et al.
Letter
Synlett
less hindered C–H bonds, irrespective of the electronic
nature of the substituents (2k–q). Substrates with more
complex structure were also tolerated under the optimized
conditions; 3,5-dichloro-N-(2-(dimethylamino)ethyl)ben-
zamide gave the methoxylated product 2r in 55% yield, N-
(2-(dimethylamino)ethyl)-[1,1′-biphenyl]-4-carboxamide
(2s) was also tolerated under the reaction conditions.
by disproportionative C–H activation affords the Cu(III) in-
termediate B. The latter undergoes reductive elimination to
give intermediate C, which is followed by protonation to
generate the desired product 2a and Cu(I). The copper(I)
species that was generated by disproportionative C–H acti-
vation and reductive elimination could be oxidized to Cu(II)
by oxygen, thus completing the catalytic redox cycle.
O
O
Cu(OH)2CO3 (20 mol%)
KOCN (2.0 equiv)
C6H12N4 (20 mol%)
KOCN
1a
Cu(II)
N
H
N
O2
R
H
R
L
N
N
MeOH (2 mL) , O2
120 °C, 24 h
HOCN
O
2a
1
2
Cu(I)
L
O
O
O
HOCN
O
O2
KOCN
NH
NH
NH
N
CuII
Cu(I)
O
Br
O
F3C
O
O
N
HOCN
Cl
N
N
N
NCO
L
N
2b: 90%
2c: 93%
2d: 50%
CuI
OMe
N
A
Cl
O
O
O
L
C
NH
NH
NH
O
I
O
O2N
O
F
O
N
N
N
N
CuIII
MeO
N
Cu(II)
MeOH
Cl
2e: 82%
2f: 52%
2g: 73%
L
O
O
O
B
NH
NH
NH
Scheme 3 Plausible reaction mechanism
NC
O
O
O
O
N
N
N
N
N
In summary, pharmaceutical oriented methoxylation of
aryl C(sp2)–H bonds using copper catalysts has been devel-
oped.12 This simple and environmentally benign reaction
occurs efficiently using oxygen as an oxidant with broad
substrate scope and high functional group tolerance. In
addition, it will provide an efficient method for the synthe-
sis of new active drug molecules according to the relevance
between drug structure and efficiency. Further research to
extend the applications of the DMEDA directing group in
the C–H activation field is under way.
2h: 40%
2i: 50%
2j: 49%
O
O
O
F3C
I
O
NH
NH
NH
O
O
O
N
N
2k: 72%
2l: 54%
2m: 51%
O
O
O
Cl
F
Br
NH
NH
NH
O
O
O
N
N
2n: 44%
2o: 52%
2p: 51%
Funding Information
CF3
O
O
O
O
Cl
We acknowledge financial support from the National Natural Science
Foundation of China (no. 20702051), the Natural Science Foundation
of Zhejiang Province (LY13B020017), and the Key Innovation Team of
NH
NH
NH
O
O
Ph
O
N
Cl
N
N
Science and Technology in Zhejiang Province (no. 2010R50018).
N
oaitn
a
l
N
a
utarl
Secince
F
o
u
n
d
o
i
n
o
f
C
h
n
i
a
2(
0
7
0
2
0
5
1
N)
a
utra
l
Secince
F
o
u
n
d
o
i
n
o
f
Z
haei
n
j
g
P
orvn
i
c
e
L
1
3
B
0
2
0
0
1
7)
2q: 61%
2r: 55%
2s: 48%
Scheme 2 Substrate of aromatic compounds. Reagents and conditions:
1 (0.2 mmol), Cu(OH)2CO3 (20 mol%), KOCN (2 equiv), methenamine
(20 mol%), MeOH (2 mL), under O2, 120 °C, 24 h.
Supporting Information
Supporting information for this article is available online at
https://doi.org/10.1055/s-0037-1610132.
S
u
p
p
ortiInfogrmoaitn
S
u
p
p
o
nrtogI
f
rmoaitn
On the basis of the above observations and on other re-
ports, we proposed a plausible mechanism for the copper-
catalyzed C–H alkoxylation of benzamide derivatives
(Scheme 3).11 Complexation of benzamide 1a and methen-
amine (L) to the copper catalyst (intermediate A) followed
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(12) General Procedure for Cu(II)-Catalyzed Alkoxylation:
Cu2(OH)2CO3 (0.04 mmol), KOCN (0.40 mmol), methenamine
(0.04 mmol), 4-bromo-N-(2-(dimethylamino)ethyl)benzamide
(0.20 mmol), CH3OH (2 mL) were introduced into a 15 mL sealed
tube equipped with a magnetic stirrer in O2. The mixture was
vigorously stirred at 120 °C for 24 h. After cooling to room tem-
perature, the solvent was evaporated under vacuum. The
residue was dissolved in mixed solvent of dichloromethane
(30 mL) and edetate tetrasodium (EDTA) saturated aqueous
solution (30 mL). After separation, the aqueous phase was
extracted twice with dichloromethane (15 mL) and the organic
layers were combined and evaporated under vacuum. The crude
product was purified by column chromatography using silica
gel to afford 4-bromo-N-(2-(dimethylamino)ethyl)-2-methoxy-
benzamide. Isolated yield: 0.0542 g (90%). 1H NMR (500 MHz,
CDCl3): δ = 8.33 (s, 1 H), 8.01 (d, J = 8.4 Hz, 1 H), 6.97 (dd, J = 8.4,
1.8 Hz, 1 H), 6.89 (d, J = 1.8 Hz, 1 H), 3.92 (s, 3 H), 3.66 (q, J =
5.9 Hz, 2 H), 2.87–2.80 (m, 2 H), 2.50 (s, 6 H). 13C NMR (126
MHz, CHCl3): δ = 164.84, 158.08, 138.54, 133.11, 121.34, 119.82,
112.04, 57.77, 56.29, 44.68, 36.56. HRMS (ESI): m/z [M]+ calcd
for C12H18ClN2O2: 257.1051; found: 257.1062.
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© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–D