Base-Promoted Coupling of Carbon Dioxide, Amines, and Diaryliodonium Salts: A Phosgene- and Metal-Free Route to O-Aryl Carbamates

Article abstract of DOI:10.1002/chem.201502689

A phosgene- and metal-free synthesis of O-aryl carbamates is realized through a three-component coupling of carbon dioxide, amines and diaryliodonium salts. The reaction only requires a base as the promoter, providing access to a diverse array of O-aryl c

Full text of DOI:10.1002/chem.201502689

DOI: 10.1002/chem.201502689
Communication
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Synthetic Methods |Hot Paper|
Base-Promoted Coupling of Carbon Dioxide, Amines, and
Diaryliodonium Salts: A Phosgene- and Metal-Free Route to
O-Aryl Carbamates
Wenfang Xiong, Chaorong Qi,* Youbin Peng, Tianzuo Guo, Min Zhang, and
Huanfeng Jiang*[a]
In recent decades, significant progress has been achieved in
the synthesis of O-alkyl carbamates based on CO2 activation.[6]
Abstract: A phosgene- and metal-free synthesis of O-aryl
However, to our knowledge, no general approach that allows
the synthesis of O-aryl carbamates directly from CO2 by
a single-step process has been developed to date. The current
methods of obtaining this important compound class still
mainly rely on the interaction of carbamoyl chlorides[5c,7] or iso-
cyanates[8] with phenols and the aminolysis of aryl chlorofor-
mates[9] (Scheme 1a), all of which suffer from the use of highly
toxic reagents. An elegant synthesis of O-aryl carbamates was
recently developed by Reddy and co-workers through
a copper-catalyzed oxidative coupling of formamides with phe-
nols (Scheme 1b).[10] However, this protocol is only applicable
to 2-carbonyl-substituted phenols, which may restrict its syn-
thetic applications. Therefore, the development of general and
environmentally friendly methods for the synthesis of O-aryl
carbamates using CO2 as a raw material remains a tremendous
challenge in organic synthesis.
carbamates is realized through a three-component cou-
pling of carbon dioxide, amines and diaryliodonium salts.
The reaction only requires a base as the promoter, provid-
ing access to a diverse array of O-aryl carbamates in mod-
erate to high yields with excellent chemoselectivity.
The conversion of carbon dioxide (CO2) into valuable chemicals
has received much attention in recent years, not only because
CO2 is one of the major anthropogenic greenhouse gases, but
also because it has been regarded as an abundant, nontoxic,
and renewable C1 feedstock.[1] In this context, the synthesis of
organic carbamates employing CO2 as a phosgene alternative
is of great interest because of the importance of carbamates in
biologically active natural products,[2] agricultural chemicals,[3]
and pharmaceutically relevant molecules[4] (Figure 1). More-
over, carbamates can also serve as versatile intermediates and
protecting groups for various synthetic applications.[5]
Recently, diaryliodonium salts have been extensively utilized
as powerful arylating reagents[11] to construct useful and com-
plicated molecules through transition metal-catalyzed[12] or
metal-free[13] arylation reactions. However, to our knowledge,
Figure 1. Selected biologically active O-aryl carbamates.
[a] W. Xiong, Dr. C. Qi, Y. Peng, T. Guo, Prof. Dr. M. Zhang, Prof. Dr. H. Jiang
School of Chemistry and Chemical Engineering
South China University of Technology
Guangzhou 510640 (P.R. China)
E-mail: crqi@scut.edu.cn
jianghf@scut.edu.cn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201502689.
Scheme 1. Strategies for the synthesis of O-aryl carbamates. a) Common
routes; b) Reddy’s work; c) this work.
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the application of diaryliodonium salts in CO2 fixation has not
yet been explored.
trimethylbenzene was also isolated in 91% yield, which could
be reused for the preparation of the diaryliodonium salt 1a,
thus improving the atom-economy of the reaction.[13b,d,g] Exper-
imental results also confirmed that the addition of 2 equiva-
lents of DBU is essential for complete reaction (Table1, en-
tries 2 and 3). The use of other organic or inorganic bases,
such as 1,4-diazabicyclo[2.2.2]octane (DABCO), triethylamine
(Et3N), Na2CO3, or KOH, resulted in a significant decrease in the
yields (Table1, entries 4–7). Moreover, MeCN was found to be
the best reaction medium for the transformation, whereas
other solvents, such as THF, DMF, or toluene, gave inferior re-
sults (Table1, entries 8–10). Both decreasing and increasing the
reaction temperature led to dramatic decreases in the yield of
the product, which could be due to the lower reaction rate at
lower temperature and the formation of unidentified products
at higher temperature (Table1, entries 11 and 12). Further opti-
mization revealed that 10 equivalents of 2a was required to
obtain satisfactory yield; using 5 equivalents of 2a gave a low
yield (Table1, entry 13). The reaction could even proceed
smoothly under a relatively low CO2 pressure (0.75 MPa) to
give the product in 78% yield (Table1, entry 14). To evaluate
possible reaction pathways, the reaction was also performed in
the presence of 1.2 equivalents of 1,1-diphenylethene (DPE) or
2,6-di-tert-butyl-4-methylphenol (BHT) as a radical scavenger
(Table1, entries 15 and 16). The results showed that both reac-
tions proceeded with no notable decrease in yields, thus ruling
out the possibility of a free radical pathway.
As part of our ongoing program to develop efficient meth-
ods for the chemical transformation of CO2 into valuable com-
pounds, we have developed several protocols for the synthesis
of cyclic or acyclic carbamates.[14] Early this year, we reported
a novel three-component coupling reaction of CO2, amines,
and N-tosylhydrazones, in which carbamate anions, generated
in situ from CO2 and amines, were proposed to function as
a nucleophile to attack the in situ-generated carbocation spe-
cies to give the corresponding alkyl carbamates.[14g] We envi-
sioned that, by using diaryliodonium salts as electrophilic ary-
lating agents to trap the in situ-generated carbamate anions,
a straightforward route to O-aryl carbamates could be realized.
Herein we report our preliminary results on phosgene- and
metal-free synthesis of O-aryl carbamates through a three-com-
ponent coupling reaction of CO2, amines and diaryliodonium
salts (Scheme 1c).
For the initial screening and optimization of the reaction
conditions, dimesityliodonium triflate (1a) and diethylamine
(2a) were chosen as the model substrates (Table 1). To our de-
light, when the reaction was carried out under 4 MPa of CO2 in
MeCN at 808C in the presence of 2 equivalents of 1,8-diazabi-
cyclo[5.4.0]-undec-7-ene (DBU), the desired product, mesityl di-
ethylcarbamate (3aa), was obtained in an excellent yield after
12 h (Table 1, entry 1). It should be stressed that 2-iodo-1,3,5-
Having established the optimal reaction conditions, we ex-
amined the scope and limitations of the transformation by
using various amines and diaryliodonium salts. Gratifyingly, var-
ious secondary and primary aliphatic amines investigated un-
derwent the coupling reactions smoothly and furnished the
corresponding products in moderate to excellent yields upon
isolation (Table 2). Notably, morpholine (2 f), methylamine (2g),
and n-propylamine (2h) could even work well under 0.75 MPa
of CO2, delivering the corresponding products in almost equal
yields to those obtained at 4 MPa of CO2. However, when the
reaction was performed using aniline as a coupling partner,
only a trace amount of desired product was observed, which
was associated with the low basicity and nucleophilicity of the
aniline.
Table 1. Optimization of the three-component reaction of diaryliodonium
salt 1a, diethyl amine (2a) and CO2.[a]
Entry
Base
Solvent
MeCN
T [8C]
Yield [%][b]
1
2
3[d]
4
5
6
7
8
DBU
–
DBU
80
80
80
80
80
80
80
80
80
80
60
100
80
80
80
80
90 (86)[c]
37
73
72
74
66
69
80
81
50
70
32
47
78
89
85
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
THF
DABCO
Et3N
K2CO3
NaOH
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
Next, we turned our attention to the application of diarylio-
donium salts (Table 3). Pleasingly, diphenyliodonium triflate
(1b1) was also a suitable substrate for the reaction, giving rise
to the corresponding product 3ba in high yield (Table3,
entry 1). However, switching the anion of 1b1 to tetrafluorobo-
rate (1b2), bromide (1b3) and tosylate (1b4) either completely
suppressed the transformation or led to a low product yield
(Table3, entries 2–4), revealing that the nature of the anion of
salts 1 has a profound influence on the formation of the de-
sired products. To our surprise, symmetric salt 1c, containing
a methyl group in the para position of the aryl moiety, gave
product 3ca in a relatively low yield (Table3, entry 5). Inspired
by Olofsson’s elegant arylations using unsymmetric diaryliodo-
nium salts with an anisyl group as a “dummy ligand”,[13f] we
prepared a variety of asymmetric triflates 1d–k and then inves-
tigated their reactivity and selectivity in our new reaction
9
DMF
10
11
12
13[e]
14[f]
15[g]
16[h]
toluene
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
[a] Reaction conditions (unless otherwise stated): 1a (0.25 mmol), 2a
(2.5 mmol), base (2 equiv), CO2 (4 MPa), solvent (3 mL), 12 h; [b] GC yield
with dodecane as internal standard; number in parentheses is the yield
of isolated product; [c] 2-iodo-1,3,5-trimethylbenzene was also isolated in
91% yield; [d] the reaction was carried out with 1.5 equivalents of DBU;
[e] with 5 equivalents of 2a; [f] under
a CO2 pressure of 0.75 MPa;
[g] with 1.2 equivalents of 1,1-diphenylethene; [h] with 1.2 equivalents of
BHT.
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Table 2. Synthesis of O-mesityl carbamates from various amines.[a]
Table 3. Synthesis of O-aryl carbamates from a variety of diaryliodonium
salts.[a]
Entry
Substrate 1
Product 3
Yield [%][b]
1
2
3
4
80
trace
trace
27
5
6
7
35
80
43
8
54
[a] Reaction conditions: dimesityliodonium triflate 1a (0.25 mmol), amine
(2.5 mmol), DBU (0.5 mmol), CO2 (4 MPa), MeCN (3 mL), 808C, 12 h; yield
is that of the isolated product; number in parentheses is the yield ob-
tained at 0.75 MPa of CO2.
9
40
38
20
63
10
11
12
under standard conditions. To our delight, these salts all under-
went reaction to give the corresponding products 3da–ka
with complete chemoselectivity (Table3, entries 6–14), and the
anisyl group was not transferred in all cases, due to electronic
effects. Notably, a variety of electron-donating and electron-
withdrawing group at the ortho, meta, or para positions of the
other aryl ring, including alkyl (iPr, Me, tBu), halide (Cl, Br), CF3,
and CN, were tolerated in this transformation. In general, tri-
flate salts bearing strong electron-withdrawing groups at the
para position of the aryl moiety exhibited higher reactivity and
furnished the desired carbamates in higher yields than those
containing weak electron-withdrawing or electron-donating
ones, which might be due to the enhanced electrophilicity of
the iodine center (Table3, entries 9–14). To further demonstrate
the versatility of our new protocol, we also examined three un-
symmetric aryl(phenyl)iodonium salts (1l–n). Products 3la and
3ma were exclusively formed in high yields (Table3, entries 14
and 15). The selectivity in both cases can be rationalized as the
influence of electronic effect and ortho effect, respectively.[13] In
contrast, the reaction with 2,4-dimethylphenyl(phenyl) salt 1n
led to products 3na and 3ba as an inseparable mixture, with
the sterically more demanding 2,4-dimethylphenyl group
being transferred preferentially (Table3, entry 16).
13
14
75
86
15
16
90
48[c]
[a] Reaction conditions: diaryliodonium salt 1 (0.25 mmol), ethylamine
(2a, 2.5 mmol), DBU (0.5 mmol), CO2 (4 MPa), MeCN (3 mL), 808C, 12 h;
[b] yield is that of the isolated product; [c] 3na/3ba=2:1; product ratio
1
determined by H NMR spectroscopy of the isolated mixture.
On the basis of the above-mentioned results and literature
reports,[6,12,13,14g,15] we have proposed a plausible mechanism
for the transformation, as depicted in Scheme 2. At the begin-
ning, CO2 reacts with ethylamine to form the carbamic acid
ammonium salt in the presence of an organic base. Then, the
carbamate anion acts as a nucleophile to attack the iodine(III)
center of diphenyliodonium triflate, upon a ligand-exchange
step, thus displacing the triflate anion ligand to give an 10-I-3
trigonal-bipyramidal intermediate A, which exists in equilibri-
um with A’ through permutational transformation. The subse-
quent reductive elimination of the tetragonal-pyramidal spe-
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Scheme 2. Proposed mechanism for the reaction.
cies A’ would finally afford product 3ba along with liberation
of iodobenzene.
In summary, we have developed a novel three-component
coupling reaction of CO2, amines, and diaaryliondonium salts.
The reaction employs easily available feedstocks and only re-
quires an organic base as the promoter, providing a practical
and general route for the synthesis of a wide range of O-aryl
carbamates. Considering the importance of O-aryl carbamate
motifs, the demonstrated phosgene- and metal-free protocol
could be of great significance in organic synthesis. Further in-
vestigations into the mechanism and application of the
method to the synthesis of more complex molecules are un-
derway in our laboratory.
Acknowledgements
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We thank the National Natural Science Foundation of China
(21172078), the National Basic Research Program of China (973
Program; 2011CB808600), the Guangdong Natural Science
Foundation (10351064101000000), and the Fundamental Re-
search Funds for the Central Universities (2015zz038) for finan-
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Keywords: amines
·
carbamates
·
carbon dioxide
·
diaryliodonium salts · three-component coupling
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Received: July 11, 2015
Published online on August 25, 2015
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