A copper-catalyzed oxidative coupling reaction of arylboronic acids, amines and carbon dioxide using molecular oxygen as the oxidant

Article abstract of DOI:10.1039/c6gc03465a

A Cu-catalyzed oxidative coupling reaction of arylboronic acids, amines and carbon dioxide is described for the first time in this paper. The reaction tolerates a wide range of functional groups, providing a convenient protocol for the synthesis of various O-aryl carbamates. The successful development of the transformation was enabled by the use of BF₃·OEt₂ as the promoter and molecular oxygen as the oxidant. Mechanistic studies suggested that the Cu^II carbamato complex is involved in the catalytic transformation.

Full text of DOI:10.1039/c6gc03465a

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DOI: 10.1039/C6GC03465A
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Copper-catalyzed oxidative coupling reaction of arylboronic acids,
amines and carbon dioxide using molecular oxygen as the oxidant
Wenfang Xiong,a Chaorong Qi,*a,b Tianzuo Guo,a Min Zhang,a,b Kai Chen,a and Huanfeng Jiang*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A Cu-catalyzed oxidative coupling reaction of arylboronic acids, Only recently, we demonstrated a straightforward access to O-
amines and carbon dioxide is descried for the first time. The aryl carbamates via base-promoted three-component
a
reaction tolerates a wide range of functional groups, providing a reaction of CO2, amines and diaryliodonium salts (Scheme 1,
convenient protocol for the synthesis of various O-aryl carbamates. a).7a However, this transformation requires the preformation
The successful development of the transformation was enabled by of the diaryliodonium salts and generates a stoichiometric
the use of BF3·OEt2 as the promoter and molecular oxygen as the amount of aryl iodides as the byproducts. Our continuous
oxidant. Mechanistic studies suggested that CuII carbamato interest in converting CO2 into organic carbamates7 urged us
complex is involved in the catalytic transformation.
to develop a more convenient approach to O-aryl carbamates
via the use of easily available aryl donors while with minimal
waste production.
The global warming and climate challenge has motivated
synthetic chemists to develop new and efficient
methodologies for converting carbon dioxide (CO2) into
valuable chemicals, not only because CO2 is the major
anthropogenic greenhouse gas but also because CO2 is a
nontoxic, abundantly available and renewable C1 resource.1
One of the promising examples in this field is the conversion
of CO2 into organic carbamates, which are ubiquitous moieties
Arylboronic acids are commercially available, comparatively
stable and nontoxic compounds. Over the past decades, they
have been widely used as coupling partners for the C-C bond
and C-heteroatom bond forming reactions including the Pd-
catalyzed Suzuki-Miyaura coupling reaction and the Cu-
mediated Chan-Evans-Lam cross-coupling reaction.8 However,
to the best of our knowledge, they have not yet been applied
for the assembly of O-aryl carbamates to date. We envisioned
that a more convenient route to O-aryl carbamates might be
achieved by a copper-catalyzed oxidative coupling between
CO2, amines and arylboronic acids (Scheme 1, b). The reaction
mechanism was hypothesized in Scheme 2: the arylboronic
in a large number of functionalized molecules with many
important applications in chemistry related fields.
2-4
While a
wide variety of CO2-based methods has been developed to
access different classes of carbamates,5 the direct synthesis of
O-aryl carbamates from CO2 remains outstanding challenges.
The highly toxic phosgene (COCl2) and its derivatives are still
the basic raw materials for the production of such compounds
in both laboratory preparation and industrial applications.4d-f, 6
acid initially undergoes transmetalation with complex
A to
Scheme 1. Strategies for the synthesis of O-aryl carbamates.
a
Key Lab of Functional Molecular Engineering of Guangdong Province, School of
Chemistry and Chemical Engineering, South China University of Technology,
Guangzhou 510640, P. R. China, E-mail: crqi@scut.edu.cn or jianghf@scut.edu.cn
b State Key Lab of Luminescent Materials and Devices, South China University of
Technology, Guangzhou 510640, P. R. China, E-mail: crqi@scut.edu.cn
† Electronic Supplementary Information (ESI) available: [experimental details and
characterization of all compounds, copies of 1H and 13C NMR spectra for selected
compounds]. See DOI: 10.1039/x0xx00000x
Scheme 2. Hypothetical new pathway for the synthesis of O-aryl carbamates
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replacement of BF3
·
OEt2 with HBF4 or CsF compleVteielwyAsrtuicplepOrnelisnes
generate intermediate
B
. Then, coordination exchange of
B
DOI: 10.1039/C6GC03465A
the reaction (entries 11 and 12). Replacing pyridine with DBU,
TEA or DMAP led to very poor product yields (entries 13-15),
presumably because the pyridine serve as both base and
ligand in the reaction. Further screenings showed that DCM
was the most effective solvent (see Supporting Information,
Table S1). The reaction temperature and time were also
optimized and it was found that 80 oC and 24 h are optimal for
the reaction (see Supporting Information, Table S2 and S3). In
with in situ generated carbamate anion gives species
C, which
would be oxidized to CuIII species
D
via disproportionation.
affords the desired
Finally, the reductive elimination of
product.
D
With the above hypothesis in mind, we set out to examine
the reaction of phenylboronic acid (1a), diethylamine (5a) and
CO2 under various conditions. Initially, the reaction was
conducted in the presence of Cu(OAc)2 as the catalyst and
pyridine as the base under a molecular oxygen atmosphere,
but only trace amount of desired product 6aa was detected,
and the amination prouduct,8f N,N-diethylaniline was formed
as the major product (Table 1, entry 1). To our surprise,
switching the catalyst to Cu2O led to formation of 6aa in 10%
yield (entry 2). After a thorough investigation we were pleased
to find that, upon addition of 3 equivalents of BF3·OEt2 with
respect to 1a, the reaction became faster and furnished 6aa in
an excellent yield (entry 3). Control experiments showed that,
besides BF3·OEt2, Cu2O, pyridine and molecular oxygen are all
essential in affording the product (entries 4-6). Among various
copper sources tested, Cu2O was uniquely efficient in
promoting the reaction (entries 3, 4, 7 and 8). In addition,
parallel to 1a, other boronic acid derivatives (2a, 3a and 4a)
were proven to be less effective coupling partners (entries 16-
18).
With the optimal reaction conditions in hand, we then tried
to explore the generality and limitations of the protocol. First,
we focused on the variation of arylboronic acids in
combination with diethyl amine and CO2. As shown in Scheme
3, all the tested arylboronic acids underwent smooth oxidative
coupling reactions to furnish the O-aryl carbamates in
moderate to excellent yields upon isolation. Both electron-
donating and electron-withdrawing substituents, such as
Table 1. Optimization of Reaction Conditions.a
Entry Ph-
B
Catalyst
Cu(OAc)2
Cu2O
Cu2O
-
Cu2O
Cu2O
Cu(OAc)2
CuCl2
CuI
Base
Additive
Yield [%]b
tracec
10
91 (85)
0
trace
trace
trace
trace
trace
trace
0
1
2
1a
pyridine
pyridine
pyridine
pyridine
-
pyridine
pyridine
pyridine
pyridine
pyridine
pyridine
pyridine
DBU
-
-
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
2a
3a
4a
3
4
BF3
BF3
BF3
BF3
BF3
BF3
BF3
BF3
·
·
·
·
·
·
·
·
OEt2
OEt2
OEt2
OEt2
OEt2
OEt2
OEt2
OEt2
5
6d
7
8
9
10
11
12
13
14
15
16
17
18
CuO
Cu2O
Cu2O
Cu2O
Cu2O
Cu2O
Cu2O
Cu2O
Cu2O
HBF4
CsF
0
BF3
·
·
·
·
·
·
OEt2
trace
trace
33
37
19
TEA
DMAP
pyridine
pyridine
pyridine
BF3
BF3
BF3
BF3
BF3
OEt2
OEt2
OEt2
OEt2
OEt2
34
a
Reaction conditions: organoboron reagent (0.5 mmol), 5a (2.5 mmol), catalyst
(10 mol%), base (3 equiv), additive (3 equiv), CH2Cl2 (3 mL), O2 (0.4 MPa), total
b
pressure (4.0 MPa), 80 oC, 24 h. GC yield with dodecane as internal standard;
Scheme 3. Substrate scope with respect to boronic acids. Reaction conditions: 1 (0.5
mmol), 5a (2.5 mmol), Cu2O (10 mol%), pyridine (3 equiv), BF3·OEt2 (3 equiv), DCM (3
mL), O2 (0.4 MPa), total pressure (4.0 MPa), 80 oC, 24 h. Yields of the isolated products
are shown.
c
number in parentheses is the yield of isolated product. N,N-diethylaniline was
isolated in 18% yield as the major product. The reaction was carried out in the
d
absence of molecular oxygen.
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1a 8f
as the major product via the direct amination of View. ArtAicnleiOlinnlienes
methyl, methoxy, ethylthio, ester, morpholino and
trifluoromethyl, are well tolerated. Notably, products, arising
DOI: 10.1039/C6GC03465A
did not participate in the reaction, leading to the recovery of
from arylboronic acids bearing halogen functionalities (1f-h),
the substrates.
would offer the potentials for elaboration of complex
molecules via further transformations. Ortho-substituted
substrates (1n and 1o) gave the corresponding products (6na
and 6oa) in slightly lower yields than their meta- or para-
substituted analogues, which are associated with the influence
of steric hindrance. In addition to mono-substituted boronic
acids, polysubstituted substrate 1p and fused one 1q also
served as effective coupling partners to yield the desired
products (6pa and 6qa) in high yields. Moreover, heteroaryl
To demonstrate the synthetic utility of the new protocol,
product 6oa were employed for further transformations to
prepare a range of functionalized products. As shown in
scheme 5, 6oa could efficiently undergo Fries rearrangement9
and decarboxylative acylation10 to afford
7
and
8 in 75% and
81% yields, respectively. The Pd-catalyzed o-arylation of 6oa
with o-difluorobenzene also gave product
9 in a satisfactory
yield.4d Moreover, the use of 10 equiv of NaOH in refluxing
ethanol led to a phenol derivative upon hydrolysis.11
boronic acids
(1r and 1s) were compatible with the
transformation, albeit the yields were somewhat low.
Gratifyingly, vinylboronic acids (1t and 1u) were able to couple
with amine 5a and CO2, resulting in vinyl carbamates (6ta and
6ua) with retention of the stereoconfiguration of C=C bond
(see 6ta), and these products are difficult to prepare or
inaccessible with the conventional methods.
Subsequently, we turned our attention to examine the
amines (Scheme 4). Various acyclic secondary amines were
able to give rise to the O-aryl carbamates (6ab-6ah) in
moderate to high yields. Both the sterically hindered
diisopropylamine and the unsaturated diallylamine could give
the desired products (6ae and 6af) albeit in lower yields.
Pleasingly, the methodology was found to be suitable for the
transformation of different cyclic amines, affording the
Scheme 5. Versatile transformations of the O-aryl Carbamate products. Reaction
conditions: (a) 6oa (0.5 mmol), LDA (1.2 equiv), THF, -78 oC to room temperature,
8 h. (b) 6oa (0.3 mmol), PdOAc)2 (5 mol%), (NH4)2S2O8 (0.45 mmol), TfOH (20
mol%), DCE (2 mL) in pressure tubes. (c) 6oa (0.2 mmol), benzene (1 mL),
Pd(OAc)2 (10 mol%), Na2S2O8 (10 mol %), TFA (5 equiv), 48 h. (d) 6oa (0.5 mmol),
NaOH (10 equiv), EtOH (50 ml), reflux, 8 h.
products in 51-76% yields (6ai-6ao). However, when primary
amine such as n-butylamine was employed, only trace of the
desired product was detected, and N-butylaniline was formed
Preliminary mechanistic studies were also conducted.
Initially, we treated Cu2O with dibenzylamine and CO2 in the
o
presence of molecular oxygen in CH2Cl2 at 80 C for 24 h. To
our delight,
a
blue-violet CuII carbamato complex,
Cu(O2CNBz2)2(NHBz2)2 (11a), which was previously reported by
Dell’Amico and co-workers, 12 was obtained in 84% yield (eq 1).
As expected, this complex was able to serve as a catalyst to
facilitate the reactions of a variety of aryl boronic acids with
dibenzylamine and CO2 under the standard conditions,
affording the corresponding carbamates (6ag-6lg) in moderate
to high yields (eq 2). The structure of the product 6lg was
confirmed unambiguously by X-ray crystallography.13 These
results strongly suggested that carbamato complex of
copper(II) of formula Cu(O2CNR2)2(NHR2)2, might be formed in
situ and act as the catalytically active species. It should be
pointed out that in these cases, pyridine, BF3·OEt2, and oxygen
are all required for the reaction, which is consistent with the
results obtained with Cu2O as catalyst (see Supporting
Information, Table S4).
Scheme 4. Substrate scope of amines. Reaction conditions: 1a (0.5 mmol), 5 (2.5 mmol),
Cu2O (10 mol%), pyridine (3 equiv), BF3·OEt2 (3 equiv), DCM (3 mL), O2 (0.4 MPa), total
pressure (4.0 MPa), 80 oC, 24 h. Yields of the isolated products are shown.
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another equivalent of 12, a higher oxidation stateVCieuwIIAI rctioclme Opnlleinxe
14 is then generated. Finally, the reducDtiOvIe: 1e0.l1im03i9n/aCt6iGoCn0o34f615A4
would yield the carbamate product along with liberation of CuI
species 15, which is oxidized by O2 to regenerate 12
.
Conclusions
we have developed the first copper-catalyzed oxidative
coupling of arylboronic acids, amines and CO2 by using
molecular O2 as a green oxidant. The reaction provides a
practical protocol for the synthesis of a wide range of
functionalized O-aryl carbamates. The key to the success is the
identification of a catalytic system involving the use of cost-
effective copper(I) oxide as a pre-catalyst, BF3·OEt2 as the
promoter. Mechanistic studies suggests that a CuII complex
Cu(O2CNR2)2(NHR2)2 acts as the catalytically active species for
the transformation. Further work focusing on product forming
details and applications of the method in synthetic diversity is
currently ongoing in our laboratory.
Furthermore, although it has long been regarded that
carbomato metal complexes generally exhibit no reactivity to
nucleophiles,14 we found that the stoichiometric reaction of
11a with phenylboronic acid (1a) in the presence of pyridine
and BF3·OEt2 gave the desired product 6ag in a moderate yield
(eq 2), revealing that the N,N-dialkylcarbamato group could
transfer from their metal complexes to the aryl ring of the
boronic acids under our conditions. Note that the
unprecedented reaction could proceed under a N2 atmosphere.
Thus, the molecular O2 is not involved from such a complex to
the terminal product.
Acknowledgements
We thank the Ministry of Science and Technology of the
People’s Republic of China (2016YFA0602900), the National
Natural Science Foundation of China (21572071 and
21420102003) and the Fundamental Research Funds for the
Central Universities (2015zz038) for financial support.
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plausible catalytic cycle of the newly developed three-
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presence of molecular oxygen. With the assistance of BF3·OEt2, 11
would undergo ligand exchange with pyridine to give the active CuII
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Scheme 6. Proposed catalytic cycle for the Cu-catalyzed oxidative coupling of
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Table of contents entry
A Cu-catalyzed oxidative coupling of arylboronic acids, amines and carbon dioxide is
descried, providing a route to O-aryl carbamates.