Silver(I)-Catalyzed Synthesis of β-Oxopropylcarbamates from Propargylic Alcohols and CO2 Surrogate: A Gas-Free Process

Article abstract of DOI:10.1002/cssc.201501176

The utilization of carbon dioxide poses major challenges owing to its high thermodynamic stability and kinetic inertness. To circumvent these problems, a simple reaction system is reported comprising ammonium carbamates as carbon dioxide surrogates, propargylic alcohols, and a silver(I) catalyst, for the effective conversion of a wide range of alcohols and secondary amines into the corresponding β-oxopropylcarbamates. A key feature of this strategy includes quantitative use of a carbon resource with high product yields under gas-free and mild reaction conditions. Notably, this catalytic protocol also works well for the carboxylative cyclization of propargylic amines and carbon dioxide surrogates to afford 2-oxazolidinones.

Full text of DOI:10.1002/cssc.201501176

DOI: 10.1002/cssc.201501176
Communications
Silver(I)-Catalyzed Synthesis of b-Oxopropylcarbamates
from Propargylic Alcohols and CO2 Surrogate: A Gas-Free
Process
Qing-Wen Song,[a, b] Zhi-Hua Zhou,[a] Hong Yin,[a] and Liang-Nian He*[a]
The utilization of carbon dioxide poses major challenges
owing to its high thermodynamic stability and kinetic inert-
ness. To circumvent these problems, a simple reaction system
is reported comprising ammonium carbamates as carbon diox-
ide surrogates, propargylic alcohols, and a silver(I) catalyst, for
the effective conversion of a wide range of alcohols and secon-
dary amines into the corresponding b-oxopropylcarbamates.
A key feature of this strategy includes quantitative use of
a carbon resource with high product yields under gas-free and
mild reaction conditions. Notably, this catalytic protocol also
works well for the carboxylative cyclization of propargylic
amines and carbon dioxide surrogates to afford 2-oxazolidi-
nones.
group reported a methodology for the synthesis of oxazolidi-
none by the silver/1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)-
promoted reaction of propargylic amines and CO2 in air.[2f,3]
A large amount of DBU was initially employed to trap CO2 to
form a DBU–CO2 complex, which acted as CO2 source for the
reaction.[4] Subsequently, CO2 captured directly from exhaust
gas by aqueous ethanolamine solution was used for the car-
boxylation of alkynes as efficiently as pure CO2 gas from a com-
mercial source.[5] Newly, NH2COONH4 and (NH4)2CO3 were re-
ported as carbon source and hydrogenated to produce ammo-
nium formate with molecular hydrogen through carbon-sup-
ported palladium nanocatalysts in aqueous alcohol solutions.[6]
Our group has also developed catalytic strategies for high-effi-
ciency chemical conversion of captured CO2, that is, a CO2 cap-
ture and utilization (CCU) strategy, into value-added chemicals
such as ureas, oxazolidinones, formate, and others.[7] In these
protocols, the fixed CO2, a potential activated form, could be
stoichiometrically incorporated into value-added chemicals/
fuels under mild reaction conditions, getting rid of the desorp-
tion step. These findings pave the way for the development of
green processes and technological innovations towards low-
energy, highly effective catalytic methods for CO2 conversion.
Although CCU strategies offer much potential as effective cata-
lytic protocols for the utilization of low concentrations of CO2,
the examples are limited and more studies are required.
Carbon dioxide is as an easily available, abundant, safe, and re-
newable carbon resource. These factors make it a highly attrac-
tive C1 building block for exploitation in chemical transforma-
tions.[1] However, the utilization of CO2 poses major challenges
owing to its high thermodynamic stability and kinetic inert-
ness. There are well-established protocols with vigorous cata-
lysts including (transition) metals such as palladium, silver,
copper, ruthenium, rhodium, iridium; active molecules with
high free energy such as N-heterocyclic carbenes (NHCs); or-
ganic bases; and drastic reaction conditions such as high pres-
sures for the incorporation of CO2 into organic compounds.[2]
Although great achievements have been made, most proce-
dures require an excess of CO2; that is, high CO2 pressures.
Therefore, discovering effective methodologies that use feed-
stocks that are low in CO2 content, especially stoichiometric
CO2 resources, and low energy inputs is a significant, promis-
ing, and challenging area in both catalysis and sustainable
chemistry.
Alkyl-substituted ammonium carbamates are the product of
reactions between gaseous CO2 and secondary aliphatic
amines, which react rapidly (and exothermically) via unstable
alkylcarbamic acids (Scheme 1).[8] These alkyl-substituted am-
Scheme 1. Formation of carbamic acid and salts.
The exploration of green, effective strategies for the reaction
of quantitative CO2 is a very attractive topic. Recently, Yoshida’s
monium carbamates have found widespread application. For
example, dimethylammonium dimethylcarbamate (DIMCARB),
a commercially available dialkylammonium carbamate, is well-
known to be a useful dimethylamine source for preparative
amidation of carboxylic acids or ester derivatives, and a reagent
in the Willgerodt–Kindler synthesis of N,N-dimethylthiocarbox-
amides.[9] In addition, DIMCARB has attracted attention as
a self-associated, distillable ionic medium in natural product
extractions.[8b,10] Furthermore, this ammonium salt can promote
aldol condensations, Mannich-type condensations. and Knoe-
venagel condensations for metal-free syntheses of valuable
[a] Dr. Q.-W. Song, Z.-H. Zhou, H. Yin, Prof. Dr. L.-N. He
State Key Laboratory and Institute of Elemento-Organic Chemistry
Collaborative Innovation Center of Chemical Science and Engineering
Nankai University
Tianjin, 300071 (PR China)
E-mail: heln@nankai.edu.cn
[b] Dr. Q.-W. Song
Current address: State Key Laboratory of Coal Conversion
Institute of Coal Chemistry
Chinese Academy of Sciences
Taiyuan, 030001 (PR China)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201501176.
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compounds, for example, [n]-shogaols.[11] Although dialkylam-
monium carbamates are potentially valuable as reagents, reac-
tion media, promoters, or catalysts, their use in organic synthe-
sis is limited. To the best of our knowledge, the use of alkyl-
substituted ammonium carbamates as both carbonyl and ni-
trogen source in organic synthesis under gas-free conditions
has not yet been reported. More importantly, the ammonium
carbamate could be more active in lieu of free CO2, leading to
reactions that run smoothly under extremely mild condi-
tions.[7b]
To further explore the efficient synthesis of b-oxopropylcar-
bamates directly from propargylic alcohols, secondary amines,
and CO2,[21] we became interested in developing a general
effective methodology for the quantitative conversion of
CO2. Considering the economic attractiveness, we disclose
herein the first example of utilizing ammonium carbamate
(amine-CO2), one of CO2’s derivatives, in lieu of free CO2 to per-
form carboxylative cyclization with propargylic alcohols
(Scheme 2b). This offers an alternative route to b-oxopropyl-
carbamates by utilizing the ammonium carbamate as both CO2
and amine source, and thus could offer a cost-effective and
convenient way for CO2 utilization.
On the other hand, the chemical conversion of CO2 has
great significance, particularly in catalytic CÀN bond formation
with the production of value-added products such as oxazoli-
dinones, quinazolines, carbamates, isocyanates, and polyur-
ethanes.[12] One of the most promising examples is the three-
component reaction of propargylic alcohols, secondary amines,
and CO2 to access b-oxopropylcarbamates with perfect atom
economy (Scheme 2a), These compounds represent an impor-
The initial reaction is based on the silver-catalyzed three-
component reaction of propargylic alcohols, secondary amines,
and CO2 we reported earlier.[21b] An ammonium carbamate, for
example, piperidin-1-ium piperidine-1-carboxylate 2a (PIPC,
“CO2-piperidine” adduct) as a carboxylating reagent could be
more convenient for storage, transportation and application
than gaseous CO2. We envisioned that the silver com-
plex, with well-defined structure, might be a worth-
while catalyst for the PIPC transformation reac-
tion.[21,22] Therefore, we herein expand the potential
applications of the silver(I) catalyst system to the
two-component reaction of propargylic alcohols and
PIPC as indicated in Scheme 3. In this protocol, b-ox-
opropylcarbamates were obtained in 27–60% isolat-
ed yields at ambient conditions.
We thus are motivated to examine the utilization
of CO2 surrogates in chemical transformations,
aiming at extending an alternative CO2 recycling
strategy into the existing synthetic routes. We started
by exploring the silver-catalyzed b-oxopropylcarba-
mates synthesis from 2-methylbut-3-yn-2-ol (1a) with
PIPC (Table 1). A variety of phosphine ligands includ-
ing monophosphine and bidentate phosphine ligands were
screened in the presence of AgOAc (entries 1–7). AgOAc/PPh3
gave the most promising result, with a 52% yield of 3a
(entry 7). Among a set of representative silver compounds
such as Ag2O, Ag2CO3, AgNO3, AgBF4, and AgCl (entries 7–12),
the most efficient catalysis was accomplished with Ag2O in
conjunction with PPh3 (entry 8). With further detailed explora-
tion (entries 13–16), an excellent result was obtained
(entry 16).
Scheme 2. The synthesis of various b-oxopropylcarbamates.
tant class of carbamate compounds in agriculture and pharma-
cology, are useful intermediates in organic synthesis, and serve
as protective groups of an amine function in peptide chemis-
try.[13] In this context, several systems have already been devel-
oped, including catalytic metal complexes of ruthenium,[14]
iron,[15] copper,[16] silver,[17] lanthanum,[18] catalytic nonmetal sys-
tems such as bicyclic guanidines,[19] and noncatalytic sys-
tems.[20] In general, these protocols have severe limitations: the
inevitable use of high pressure (ꢀ2 MPa) with large amount of
additional energy to get the satisfied yields. Therefore, effec-
tive methodologies using CO2 as a feedstock under atmospher-
ic pressure (preferable with quantitative CO2) with low energy
input are highly desired.
With a good catalytic system in hand, we next explored the
scope of the two-component reaction as shown in Scheme 4.
In the beginning, a wide range of terminal tertiary propargylic
alcohols (1a–g) was employed, affording the corresponding b-
Scheme 3. Synthesis of b-oxopropylcarbamates from propargylic alcohols and PIPC.
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oxopropylcarbamates (3a–g) in good yields. The protocol is
also suitable for the secondary propargylic alcohols 1h, 1i in
58% and 68% yields, respectively. We also examined the scope
of the carbon and nitrogen donors from different ammonium
carbamates. Secondary aliphatic amines can also be used in
this cascade reaction, to give the desired products (3a, 3j–l).
To confirm the catalytic conversion of CO2 surrogate as
Table 1. Optimization of the AgI-catalyzed reaction of propargylic alco-
hols and PIPC.[a]
carbon resource, we performed the reaction with 13CO2-labeled
13
PIPC. As shown in Scheme 5a, the production of
C -la-
carbonyl
13
Entry
Catalyst
Ligand
Yield [%][b]
beled 3a’ verifies successful conversion.
C
-labeled a-al-
carbonyl
kylidene cyclic carbonate reacted with PIPC to produce the pi-
peridine incorporated 3a’ in 75% isolated yield (Scheme 5b).
The result indicated that the reaction went through the a-alky-
lidene cyclic carbonate pathway. In addition, the equilibrium
shift between PIPC and CO2 (and piperidine) leads to a change
of the linear conformation and charge distribution of the CO2
molecule (Scheme 5b), which improves the reactivity of gas-
eous CO2.[23] As a result, PIPC displayed excellent properties as
a valuable and favorable feedstock in place of gaseous CO2.
To gain further insights into the mechanism, we conducted
the two-component reaction of 3,3-dimethylbut-1-yne or pent-
4-yn-2-ol, and PIPC under the optimized reaction conditions.
No corresponding carbamate products were obtained, along
with the full recovery of the alkyne and alcohol substrates
(Scheme 6), indicating that the CꢁC bond involved in the reac-
tion should not be susceptible to attack by a nucleophilic am-
monium carbamate species, even though it was activated by
the silver complex.
1
2
3
4
5
6
7
8
9
10
11
12
13c
14d
15d,e
16e,f
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
Ag2O
Ag2CO3
AgNO3
AgBF4
AgCl
L1
L2
L3
L4
L5
L6
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
9
18
18
49
5
27
52
66
58
55
39
31
70
70
87
91
Ag2O
Ag2O
Ag2O
Ag2O
[a] Unless otherwise specified, all reactions were performed with 1a
(84.1 mg, 1.0 mmol), PIPC (0.2142 g, 1.0 mmol), Cat. (2 mol%), Ligand
(4 mol%), CH3CN (1 mL), 608C, 12 h. [b] Determined by NMR with 1,1,2,2-
tetracholoroethane as the internal standard. [c] 24 h. [d] 8 mol% PPh3.
[e] PIPC (0.3213 g, 1.5 mmol). [f] 18 h.
A plausible reaction mechanism is proposed as shown in
Scheme 7 on the basis of the above data and results published
Scheme 4. Scope of the reaction of propargylic alcohols with ammonium carbamates.
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Scheme 5. Isotope labeling experiments.
Scheme 6. Mechanistic counterevidence experiments.
Scheme 7. Plausible mechanism.
in the literature.[14,19–21] Ammonium carbamates are reversibly
formed from amine and CO2 molecule which can act as
valuable carbon and nitrogen source as well as a potentially
activated form of CO2 gas. The initial step is the formation
of a propargylic carbonate intermediate A through the reac-
tion of propargylic alcohol with ammonium carbamate with
the transfer of a proton (H+) to amine, followed by interchang-
ing with silver(I) catalyst to form the silver propargylic carbo-
nates B. Then, intramolecular 5-exo-cyclization would then pro-
ceed through the activated CꢁC bond by the active silver spe-
cies to give intermediate C via an anti addition mode, followed
by proto-demetallation to release the a-alkylidene cyclic car-
bonate (Z-isomer. For detailed experimental data and data
published in the literature, see the Supporting Informa-
tion),[24,25] which is subsequently attacked by a secondary
amine to generate the carbamate species. Finally, the corre-
sponding b-oxopropylcarbamate 3 is obtained via tautomeriza-
tion.
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Scheme 8. Reaction of propargylic amines with PIPC.
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The successful transformation of propargylic alcohols and
ammonium carbamates into b-oxopropylcarbamates under
mild conditions prompted us to expand the potential applica-
tion of this catalytic strategy to the reaction of propargylic
amines with ammonium carbamates. In addition, a-alkylidene
cyclic carbamates synthesized from propargylamines and CO2
were apt to afford products with Z-isomers on the stereoselec-
tivity with a thermodynamic controlled result.[25] To our delight,
under standard reaction conditions as listed in Scheme 8 the
corresponding 2-oxazolidinones 4a and 4b (single Z-isomer)
were afforded in excellent yields, which extends the applica-
tion scope in the quantitative fixation of CO2.
In summary, we establish the first successful protocol of
silver-catalyzed two-component reaction of propargylic alco-
hols and ammonium carbamates for expeditious quantitative
chemical fixation of CO2 to prepare carbamate motifs at mild
conditions. The method is straightforward, efficient, and offers
high atom-economy. In addition, this approach does not re-
quire a large excess of CO2 or high reaction pressures and tem-
peratures. A broad substrate and reaction application scope
under mild reaction conditions is been demonstrated. Further
studies to develop more, related transformations and elucidate
the mechanism are underway.
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This work was financially supported by the National Natural Sci-
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Received: August 31, 2015
Published online on November 6, 2015
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