Bifunctional silver(I) complex-catalyzed CO2 conversion at ambient conditions: Synthesis of α-methylene cyclic carbonates and derivatives

Article abstract of DOI:10.1002/cssc.201402921

Abstract The chemical conversion of CO₂ at atmospheric pressure and room temperature remains a great challenge. The triphenylphosphine complex of silver(I) carbonate was proved to be a robust bifunctional catalyst for the carboxylative cyclization of propargylic alcohols and CO₂ at ambient conditions leading to the formation of α-methylene cyclic carbonates in excellent yields. The unprecedented performance of [(PPh₃)₂Ag]₂CO₃ is presumably attributed to the simultaneous activation of CO₂ and propargylic alcohol. Moreover, the highly compatible basicity of the catalytic species allows propargylic alcohol to react with CO₂ leading to key silver alkylcarbonate intermediates: the bulkier [(Ph₃P)₂Ag^I]⁺ effectively activates the carbon-carbon triple bond and enhances O-nucleophilicity of the alkylcarbonic anion, thereby greatly promoting the intramolecular nucleophilic cyclization. Notably, this catalytic protocol also worked well for the reaction of propargylic alcohols, secondary amines, and CO₂ (at atmospheric pressure) to afford β-oxopropylcarbamates.

Full text of DOI:10.1002/cssc.201402921

DOI: 10.1002/cssc.201402921
Full Papers
Bifunctional Silver(I) Complex-Catalyzed CO2 Conversion at
Ambient Conditions: Synthesis of a-Methylene Cyclic
Carbonates and Derivatives
Qing-Wen Song,[a] Wei-Qiang Chen,[b] Ran Ma,[a] Ao Yu,[b] Qiu-Yue Li,[a] Yao Chang,[a] and
Liang-Nian He*[a]
The chemical conversion of CO2 at atmospheric pressure and
room temperature remains a great challenge. The triphenyl-
phosphine complex of silver(I) carbonate was proved to be
a robust bifunctional catalyst for the carboxylative cyclization
of propargylic alcohols and CO2 at ambient conditions leading
to the formation of a-methylene cyclic carbonates in excellent
yields. The unprecedented performance of [(PPh3)2Ag]2CO3 is
presumably attributed to the simultaneous activation of CO2
and propargylic alcohol. Moreover, the highly compatible ba-
sicity of the catalytic species allows propargylic alcohol to
react with CO2 leading to key silver alkylcarbonate intermedi-
ates: the bulkier [(Ph3P)2AgI]+ effectively activates the carbon–
carbon triple bond and enhances O-nucleophilicity of the alkyl-
carbonic anion, thereby greatly promoting the intramolecular
nucleophilic cyclization. Notably, this catalytic protocol also
worked well for the reaction of propargylic alcohols, secondary
amines, and CO2 (at atmospheric pressure) to afford b-oxopro-
pylcarbamates.
Introduction
With increasing awareness of ever-growing CO2 levels in the
atmosphere, great efforts have been made to develop strat-
egies and technologies towards the reduction of carbon emis-
sions.[1] Indeed, CO2 fixation and conversion hold great prom-
ise for recycling CO2 into value-added products due to the
great potential of CO2 as an abundant, nontoxic, easily accessi-
ble, and sustainable C1 feedstock.[2] However, only a small pro-
portion of the total abundance of CO2 is currently being con-
sumed in industry; the main reason is that establishing catalyt-
ic and economical carbon neutral processes (with a high turn-
over number of a million) is challenging due to the thermody-
namic stability and kinetic inertness of CO2. In general, a highly
reactive metal reagent or catalyst, and/or nucleophile and high
pressure/temperature are required to activate and further in-
corporate CO2 into organic compounds.[3] The chemical trans-
formation of CO2 at atmospheric pressure and room tempera-
ture remains a great challenge, though great progress has
been made. In this context, the development of efficacious
processes using CO2 as chemical feedstock under mild condi-
tions (particularly low CO2 pressure, ideally at 1 bar) could be
still highly desirable.[4]
The catalytic carboxylative cyclization of propargylic alcohols
with CO2 shows great promise for the direct incorporation of
CO2 into high-value-added chemicals; namely a-alkylidene
cyclic carbonates, which are compounds incorporating a frame-
work that is important in natural products with potential bio-
activity,[5] and have a broad range of applications as intermedi-
ates in organic synthesis.[6] To date, several metal-free catalytic
systems, such as tertiary phosphine,[7] the N-heterocyclic car-
bene (NHC)/CO2 adduct,[8] bicyclic guanidine,[9] and the N-het-
erocyclic olefin/CO2 adduct[10] have been developed for the
preparation of a-alkylidene cyclic carbonates. However, high
CO2 pressure and high reaction temperatures are usually re-
quired for an efficient reaction. In this regard, further enhanced
reactivity and selectivity could depend on transition-metal cat-
alysts (containing Pd,[11] Co,[12] Cu,[13] and Ag)[14] capable of acti-
vating the carbon–carbon triple bond of propargylic alcohols.
Unfortunately, higher CO2 pressure or additional energy is
eventually inevitable for practical syntheses, despite the fact
that, according to the literature, relatively high selectivities
have been attained with metal catalysts so far. In 2007,
Yamada et al. reported on the silver-catalyzed procedure for
the carboxylative ring-closing reaction of internal propargylic
alcohols under low CO2 pressure in the presence of an organic
base, such as 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), which
presumably acts as a promoter for the formation of alkylcar-
bonic intermediate I (Scheme 1).[14a] However, relatively large
amounts of AgOAc and stoichiometric amounts of the organic
base were still required for a smooth reaction. Notably,
Yamada et al. achieved the enantioselective incorporation of
CO2 into chiral internal propargylic alcohols catalyzed by
[a] Dr. Q.-W. Song, Dr. R. Ma, Q.-Y. Li, Y. Chang, Prof. Dr. L.-N. He
State Key Laboratory and Institute of Elemento-Organic Chemistry
Collaborative Innovation Center of Chemical Science and
Engineering (Tianjin)
Nankai University
Tianjin, 300071 (PR China)
E-mail: heln@nankai.edu.cn
[b] W.-Q. Chen, Prof. A. Yu
College of Chemistry
Nankai University
Tianjin, 300071 (PR China)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201402921.
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Table 1. Carboxylic cyclization of propargylic alcohols with CO2.[a]
Scheme 1. Dual activation mode by the two-component catalyst system
(Base: tertiary amine) in the literature (I) and well-defined robust metal com-
plex used herein (II).
Entry
Catalyst
[1 mol%]
L/Additive
[2 mol%]
t
[h]
Yield[b]
[%]
[c]
1
2
3
4
5
6
7
8
9
–
DBU
PPh3
CuCl
AgNO3
AgOAc
AgSO3CF3
Ag2WO4
Ag2O
Ag3PO4
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
AgOAc
[(PPh3)2Ag]2CO3
–
–
–
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
–
Et3N
2,2’-bipyridine
DBU
L1
12
12
12
12
12
12
12
2
–
–
–
–
–
–
–
[c]
[c]
[c]
a AgOAc/chiral Schiff base system.[14b] Recently, Mizuno et al.[4b]
reported that ammonium tungstate (able to activate CO2) al-
lowed the reaction of propargylic alcohols with CO2 to proceed
smoothly under mild conditions. Yet, higher temperatures and
longer reaction times were indispensable to reach satisfactory
yields.
[c]
[c]
[c]
41
86
81
97
2
2
2
2
2
2
2
2
2
2
1
1
1
1
2
2
10
11
[c]
Scheme 1 shows I participating in the two-component
metal–base-catalyzed reaction of propargylic alcohols and
CO2.[12,14] A tertiary amine was used as a sacrificial promoter to
abstract protons from hydroxyl groups. Higher CO2 pressure
and reaction temperatures were still unavoidable probably
because the strong interaction between base–H+ and
12
13
14
15
16[d]
17[d]
18[d]
19[e]
20[e]
21[e]
22[e]
23[f]
24[g]
–
<1
–
[c]
4
74
15
40
57
55
17
L2
L3
À
HCꢀCC(R1R2)OCO2 hindered the O-nucleophilicity of I. Fortu-
(p-MeOC6H4)3P
PPh3
(p-FC6H4)3P
(p-CF3C6H4)3P
Schiff base
–
nately, the presence of bulky cations enhanced the O-nucleo-
philicity and basicity of oxo-anions.[4b,15] In this context, we suc-
cessfully developed the two-component catalyst system com-
prising Ag2WO4 and Ph3P for this kind of carboxylative cycliza-
tion under low pressure.[16] We envisioned that the triphenyl-
phosphine complex of silver(I) carbonate, that is
[(PPh3)2Ag]2CO3, prepared from Ag2CO3 and PPh3,[17] would
serve as a well-defined single-component bifunctional catalyst
capable of activating both the CꢀC bond[18] as well as CO2, and
thus enable the reaction to efficiently proceed at low pressure
and temperature presumably going through intermediate II
(Scheme 1). Furthermore, the bulkier cation, that is
[(Ph3P)2AgI]+, could stabilize the carbonic anion and enhance
its O-nucleophilicity relative to base–H+ (I, Scheme 1), and
thus promote the subsequent intramolecular nucleophilic cycli-
zation of II resulting in the formation of a-methylene cyclic
carbonates.
[c]
–
1
93
[a] Unless otherwise specified, all reactions were performed with 1a
(0.211 g, 2.5 mmol), CO2 (1 bar), and catalyst (1 mol% relative to 1a) at
ambient conditions. [b] Determined by GC analysis with biphenyl as the
internal standard. [c] No reaction with full recovery of the starting materi-
al. [d] Ligand (1.0 mol%). [e] Ag2CO3 (3.5 mg, 0.5 mol%), ligand (13.1 mg,
1.0 mol%). [f] The silver system of Yamada et al. with AgOAc (2 mol%)
and a Schiff base (8 mol%). [g] Prior to use, [(PPh3)2Ag]2CO3 (34.0 mg,
1.0 mol%) was prepared according to Ref. [17].
under a CO2 balloon without any solvent at 258C for 12 h
(Table 1). Without catalyst or additive, the reaction did not
occur and the starting material was quantitatively recovered
(Table 1, entry 1). DBU and Ph3P were ineffective under the
given conditions (Table 1, entries 2 and 3). Then, CuCl and vari-
ous silver salts capable of activating carbon–carbon triple
bonds were screened in combination with Ph3P as an additive.
Unfortunately, CuCl, AgNO3, AgOAc, and AgSO3CF3 showed no
activity (Table 1, entries 4–7). Gratifyingly, a-alkylidene cyclic
carbonates (2a) were obtained in a yield of 41% by employing
Ag2WO4 (1 mol%) in conjunction with Ph3P (2 mol%) at 258C
for 2 h (Table 1, entry 8). Notably, silver(I) compounds, such as
Ag2O, Ag3PO4, and Ag2CO3 displayed higher activity than
Ag2WO4 under identical conditions (Table 1, entries 9–11 versus
8). Particularly the Ag2CO3/PPh3 (1:2 molar ratio) system gave
the best result with a quantitative yield of 2a (Table 1,
entry 11).
Herein, we describe the use of [(PPh3)2Ag]2CO3 (in situ
formed from Ag2CO3 and PPh3) as a robust and highly efficient
single-component bifunctional catalyst for the reaction of
propargylic alcohols with CO2 at room temperature and atmos-
pheric pressure; comparisons with the present state-of-the-art
two-component metal–base catalyst system were also drawn.
Notably, the catalyst worked well for the three-component re-
action of propargylic alcohols, secondary amines, and CO2
(1 bar) affording b-oxopropylcarbamates.
Results and Discussion
Several metal compounds were initially investigated for the re-
action of 2-methylbut-3-yn-2-ol (1a) as the model substrate
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However, Ag2CO3 itself was ineffective (Table 1, entry 12),
suggesting a crucial role of PPh3 in promoting catalytic per-
formance. Further screening of additives showed that Et3N,
2,2’-bipyridine, and DBU had a negative effect on the reaction
(Table 1, entries 13–15). On the other hand, bidentate phos-
phine ligands, such as Xantphos (L1) were more active than,
for example, 1,2-bis(diphenylphosphanyl)benzene (Dppbz, L2)
and 1,3-bis(diphenylphosphino)propane (Dppp, L3; Table 1,
entry 16 versus 17–18). Subsequently, several monophosphines
were investigated. The monophosphine outfitted with a me-
thoxy group performed slightly better than PPh3 (Table 1,
entry 19 versus 20), whereas fluorinated groups retarded the
reaction sharply (Table 1, entry 21). In addition, the reaction
was completely suppressed once a strong withdrawing group
was introduced (e.g., CF3; Table 1, entry 22). Especially the
silver system with the Schiff base[14b] showed almost total inac-
tivity under ambient conditions (Table 1, entry 23).
Table 2. Carboxylative cyclization of various propargylic alcohols with
CO2.[a]
Product
Reaction conditions
Reaction time
[h]
Yield
[%]
neat
neat
neat
1
3
92
99 (97)
12
89
>99
96
–
–
4
6
Furthermore, efforts were made to explore the appropriate
ratio of PPh3/[Ag+] (see the Supporting Information, Table S1).
At a fixed Ag2CO3 loading (0.5 mol%), the 2a yield increased
from 21 to 92% with an increase in the PPh3/[Ag+] molar ratio
from 0.5 to 2 (see the Supporting Information, Table S1, en-
tries 1–4), and then remained invariable with a further increase
in PPh3 concentration (see the Supporting Information,
Table S1, entries 5–6). Accordingly, the optimal PPh3/[Ag+] ratio
was found to be 2, which was identical with the PPh3/[Ag+]
molar ratio in [(PPh3)2Ag]2CO3. Indeed, [(Ph3P)2Ag]2CO3 gave re-
sults that were comparable to the Ag2CO3/PPh3 system
(Table 1, entry 24 versus 11), presumably implying that
[(PPh3)2Ag]2CO3 could be a real catalytic species. The formation
of [(Ph3P)2Ag]2CO3 from Ag2CO3/PPh3 during the reaction was
–
4
99
–
2
1
82
99
10 bar, neat
25 bar
24
18
3
94[b], >99[c]
1
also supported by H NMR, 13C NMR, and 31P NMR analyses (see
60 bar, neat
10 bar
54[b]
90[d]
99[d]
the Supporting Information, Figure S1). In addition, the precipi-
tate recovered after the reaction was reused in a subsequent
carboxylative cyclization to produce 2a in 94% yield under
CO2 (1 bar) at room temperature for 2 h (see the Supporting
Information, Figure S2).
Having established the [(PPh3)2Ag]2CO3-catalyzed carboxyla-
tive cyclization protocol, we then studied the reactivity of vari-
ous propargylic alcohols to further explore the substrate scope
(Table 2). Numerous terminal propargylic alcohols with a variety
of alkyl and aryl substituents at the propargylic position (1a–g)
underwent the reaction smoothly with a CO2 balloon at room
temperature to afford the corresponding a-alkylidene cyclic
carbonates (2a–g) in 82–99% yield. Nonetheless, relatively
high pressure (25 or 60 bar) and temperatures (808C) were re-
quired for the substrates with a small ring or bulky isopropyl
group at the propargylic position (1h–i) to react, probably due
to the steric hindrance effect on the formation of the silver(I)
alkylcarbonic anion intermediate (II, Scheme 1). Interestingly,
the electron-rich monodentate alkyl phosphine ligand, for ex-
ample PCy3 (Cy=cyclohexyl), greatly facilitated the reaction for
the inert substrate 1h under ambient conditions.
10 bar
4
neat
neat
12
12
91[e]
87[e]
neat
15
92[f]
[a] Unless otherwise specified, all reactions are run using 1 (2.5 mmol),
fresh prepared [(Ph3P)2Ag]2CO3 (34 mg, 1 mol% relative to 1) with 0.5 mL
CHCl3 under 1 bar CO2 at 258C. GC yields are based on the propargylic al-
cohols. The isolated yields are given in parentheses. [b] 808C. [c] Ag2CO3
(2 mol%), PCy3 (8 mol%), 1 bar CO2, 258C, 24 h. [d] [(Ph3P)2Ag]2CO3
(85 mg, 2.5 mol%). [e] 3l/3m were formed from secondary propargylic al-
cohols 1l and 1m through alcoholysis of the unstable cyclic carbonate
2l/2m and subsequent tautomerization (see Figure S3). [f] On
a 120 mmol scale; yield 14.14 g; Ag2CO3 (0.1 mol%), PPh3 (0.4 mol%).
The introduction of a phenyl group at the terminal position
also caused a decrease in reactivity. Up to that point, internal
propargylic alcohols with a terminal aryl substituent had been
generally unreactive with low-pressure CO2 even at higher re-
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action temperatures.[14,15] Our protocol worked well for internal
propargylic alcohols, such as 1j–k, at a CO2 pressure of 10 bar,
leading to the formation of 2j–k in 90 and 99% yield, respec-
tively. On the other hand, secondary alcohols (1l–m) did not
give the cyclization product, but subsequent alcoholysis prod-
ucts (3l–m) were isolated. That was understandable because
1l–m initially reacted with CO2 through the carboxylative cycli-
zation pathway to in situ form 2l–m, followed by alcoholysis
and tautomerization[9] to afford 3l–m (see the Supporting In-
formation, Figure S3).
Using a catalyst at lower loads, excellent yields (up to 92%)
were still attained in a scaled-up experiment (120 mmol,
Table 2). Thus, the present protocol could potentially be ap-
plied in the large-scale chemical fixation of CO2 at atmospheric
pressure through a facile operating procedure.
1
The reactions of 1a/1j with 13CO2 were monitored by H and
Figure 2. Models I and II with nature bond orbital (NBO) charge distribution,
calculated by the M06/6-311+ +G(d,p)/LANL2DZ//B3LYP/6-31G(d)/
LANL2DZ/SMD method. I: AgOAc-activated formation of propargyl carbonic
ester with simplified DBU; II: dual [(Me3P)2Ag]CO3-activated mode (presented
herein). H: white, C: gray, N: blue, O: red, P: orange. Bond lengths and dis-
tances in ꢁ.
13C NMR spectroscopy (Figure 1) to elucidate the catalytic role
of [(Ph3P)2Ag]2CO3. The OH signal became broad and shifted
1
from d=5.46 to 5.77 ppm (Figure 1a and b) in H NMR spectra,
bled the signal found in 13C NMR spectra of [WO4]2À/
CO2 adducts.[4b]
DFT calculations were performed (Figure 2; also
see the Supporting Information) to gain a deeper in-
sight into activation modes I and II in Scheme 1. As
demonstrated in Figure 2 (I and II), the higher elec-
tron charge at an oxygen atom of II (O1: À0.782, O2:
À0.775) is connected with stronger O-nucleophilicity
relative to that of I (O1: À0.688, O2: À0.732); the
lower stabilization energy (SE) of II (À67.7 kcalmolÀ1
)
than that of I (À32.4 kcalmolÀ1) suggests that II is
more active. On the other hand, the lower relative ac-
tivation energy (RAE) of II (À20.47 kcalmolÀ1) relative
to that of I (À18.17 kcalmolÀ1) corresponds to higher
activation efficiency with the complex of silver(I) car-
bonate as model II. In addition, such hypotheses
could also be in agreement with the effect of silver
concentration on the yield in 2a (see the Supporting
Information).
Figure 1. 1H NMR (a,b) and 13C NMR (c–f) investigation. (a,b) 1j (8.0 mg), Ag2CO3 (2.8 mg)
and Ph3P (10.5 mg) ([D6]DMSO 0.6 mL). (c,d) 1a (20.2 mg), AgNO3 (40.8 mg) (CDCl3
0.6 mL). (e) [(Ph3P)2Ag]2CO3 (158.6 mg) in 0.6 mL of CDCl3. (f) [(Ph3P)2Ag]2CO3 (158.6 mg)
in 0.6 mL of CDCl3 in the presence of 13CO2 (1 bar).
More importantly, the chemical utilization of CO2
was applied to the production of an array of value-
added products, such as oxazolidinones, quinazo-
suggesting
hydrogen
bonding
formation
between
lines, carbamates, isocyanates, and polyurethanes, through CÀ
N bond formation.[2e] However, the typical three-component re-
action of propargylic alcohols, secondary amines, and CO2 to
access b-oxopropylcarbamates usually requires high pressure
and temperature.[9,19] In this respect, effective methodologies
using CO2 as a feedstock to b-oxopropylcarbamates under at-
mospheric pressure and mild conditions are still challenging.
With an optimized catalytic system in hand, we explored the
scope of the three-component reaction by employing a variety
of terminal propargylic alcohols and secondary amines under
CO2 at atmospheric pressure (Table 3). Generally, reactions took
place via the formation of a-methylene cyclic carbonates and
a subsequent aminolysis sequence.[9] To our delight, a series of
[(Ph3P)2Ag]2CO3 and 1j, and presumably leading to activation
of propargylic alcohol by the silver complex and also enhance-
ment of O-nucleophilicity. As seen in Figure 1c–d, changes in
signals arising from the C1, C2, and C3 sites could be attribut-
ed to the interaction between soluble silver salts and the
carbon–carbon triple bond, that is, resulting in activation of
propargylic alcohol. The interaction of [(Ph3P)2Ag]2CO3 with
13CO2 was examined by 13C NMR spectroscopy (Figure 1e–f). In
the case of 13CO2 (1 bar), analyses were performed using a solu-
tion of [(Ph3P)2Ag]2CO3 in CDCl3; a new signal centered at d=
163.02 ppm was deemed as evidence of the formation of the
carbonate species (and thus 13CO2 activation) because it resem-
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to excellent yields (76–98%) under optimal conditions. The
secondary aliphatic amines also gave 4k–n in high yields (82–
96%).
Table 3. Three-component reaction of propargylic alcohols, secondary
amines, and CO2.[a]
To further confirm the catalytic conversion of CO2 to cyclic
carbonates we performed the reaction in the presence of
13
13CO2. As shown in Scheme 2, the presence of
C
carbonyl
-labelled
Product
Yield[b]
Product
Yield[b]
98
2a and 4a verified successful conversion (see the Supporting
Information, Figure S4–S7).
98 (86[c])
To get more information about the [(Ph3P)2Ag]2CO3-catalyzed
three-component reaction of propargylic alcohols, secondary
amines, and CO2, we also performed additional control experi-
ments (Scheme 3). As shown in Table 3, the reaction of 1a, pi-
peridine, and CO2 gave 4a; whereas the reaction with a bulkier
amine (e.g., diisopropylamine) stopped at the formation of 2a.
In addition, 2a reacted with piperidine to afford 4a smoothly
in the absence of any silver catalyst. Accordingly, this kind of
three-component reaction is assumed to go through the cyclic
carbonate pathway.
96
87
96
76
98
83
93
82
On the basis of experimental results, NMR analyses, and DFT
calculations, a tentative mechanism for the [(Ph3P)2Ag]2CO3-
promoted fixation of CO2 at atmospheric pressure with propar-
gylic alcohols is illustrated in Scheme 4. Propargylic alcohol ini-
tially activated by [(Ph3P)2Ag]2CO3, which can be in situ formed
from Ag2CO3 and Ph3P, reacts with activated CO2 to generate
the silver propargylic carbonate intermediate (A) along with
(Ph3P)2AgHCO3 (Scheme 4a); (Ph3P)2AgHCO3 was inactive in
carboxylative cyclizations (see the Supporting Information).
Subsequently, an intramolecular nucleophilic cyclization occurs
at the silver(I)-activated CꢀC bond. Finally, the corresponding
cyclic carbonate is formed via intermediates B and C, followed
by proto-demetallation (with the aid of HCO3À) and by the re-
generation of [(Ph3P)2Ag]2CO3.
90
68
91
96
[a] 1 (2 mmol), amines (2 mmol, 1 equiv.), Ag2CO3 (8.3 mg, 1.5 mol%),
Ph3P (31.5 mg, 6 mol%), CO2 balloon, CH3CN (1 mL). [b] Isolated yield.
[c] On a 10 mmol scale under neat conditions for 24 h.
tertiary terminal propargylic al-
cohols (1a–g) reacted with CO2
and piperidine leading to the
formation of b-oxopropylcarba-
mates (4a–g) in 68–98% yields.
Notably, the present protocol
was also fruitful on a 10 mmol
scale (4a, Table 3).
In addition, our protocol was
also suitable for secondary prop-
argylic alcohols combined with
a variety of secondary amines to
Scheme 3. Control experiments.
afford the corresponding b-oxo-
propylcarbamates (4h–j) in high
In the case of the three-component reaction, a-alkylidene
cyclic carbonates (2), formed according to Scheme 4a, further
undergo a nucleophilic ring-opening with a secondary amine
to generate the carbamate species (Scheme 4b). Finally, the
corresponding b-oxopropylcarbamates (4) are obtained
through tautomerization.
Conclusions
An elegant protocol was established for the expeditious chem-
Scheme 2. Isotope labeling experiments.
ical fixation of CO2 at atmospheric pressure to prepare a-alkyli-
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equipped with a stir bar was charged with Ag2CO3 (8.3 mg,
1.5 mol%), Ph3P (31.5 mg, 6 mol%), 1a (168 mg, 2 mmol), piperi-
dine (170 mg, 2 mmol), and CH3CN (1 mL). Next, the Schlenk tube
was attached to a balloon filled with CO2 (99.99) and sealed; the
mixture was left to react at 308C for 16 h. Excessive CO2 was care-
fully released after the reaction. The residue was flushed with
CH2Cl2 (3ꢂ5 mL) and removed under vacuum. The residue was pu-
rified by flash column chromatography on silica gel (petroleum
ether/ethyl acetate=100:1–10:1) to give 4a as a light brown oil
(421 mg, 98% yield). 1H NMR (400 MHz, CDCl3): d=3.39 (m, 4H,
2N-CH2), 2.11 (s, 3H, COCH3), 1.59–1.49 (m, 6H, ÀCH2CH2CH2À),
1.43 ppm (s, 6H, 2CH3); 13C NMR (100.6 MHz, CDCl3): d=207.5 (C=
O), 154.0 (NÀC=O), 82.7, 44.6, 25.6, 24.0, 23.4, 23.2 ppm; IR (neat):
n˜ =1827, 1685, 1271, 1172, 1085, 1031 cmÀ1; GC-MS (EI, 70 eV): m/z
(%): 213.15 (0.33), 171.15 (1.66), 170.20 (15.03), 129.15 (1.20),
128.15 (12.49), 114.15 (1.02), 113.15 (8.01), 112.15 (100), 69.10
(45.97).
Acknowledgements
Scheme 4. Plausible mechanism for the [(Ph3P)2Ag]2CO3-catalyzed carboxylic
reaction of propargylic alcohols, secondary amines, and CO2: (a) formation of
of alkylidene cyclic carbonates and (b) formation of b-oxopropylcarbamates
through the alkylidene cyclic carbonate intermediate.
We are grateful to the National Natural Sciences Foundation of
China (grant number 21421001, 21121002, 21472103), Specialized
Research Fund for the Doctoral Program of Higher Education
(project 20130031110013), MOE Innovation Team (IRT13022) of
China for financial support.
dene cyclic carbonates in good to excellent yields at room
temperature. Such an efficient strategy relies on the ingenious
design of the relatively stable [(Ph3P)2Ag]2CO3 (which could
also be in situ generated from Ag2CO3 and PPh3) as a robust bi-
functional catalyst to spontaneously trap CO2 at atmospheric
pressure and simultaneously activate propargylic alcohol and
CO2. Notably, this interesting procedure could be easily scaled
up for the fixation of CO2 at atmospheric pressure. That cata-
lytic protocol was also applied to the three-component reac-
tion of terminal propargylic alcohols, secondary amines, and
CO2.
Keywords: carbon dioxide fixation
functional calculations · silver · synthetic methods
· catalysis · density
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Experimental Section
General procedure for the synthesis of a-alkylidene cyclic carbo-
nates: Taking the carboxylative cyclization of 2-methylbut-3-yn-2-ol
(1a) with CO2 as an example: a 10 mL Schlenk tube equipped with
a stir bar was charged with [(PPh3)2Ag]2CO3 (34.0 mg, 1.0 mol%)
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FULL PAPERS
Q.-W. Song, W.-Q. Chen, R. Ma, A. Yu,
Q.-Y. Li, Y. Chang, L.-N. He*
Catalytic fixation of CO2:
[(PPh3)2Ag]2CO3 is proven to be a robust
bifunctional catalyst for the chemical
upgrading of CO2 at atmospheric pres-
sure to produce valuable compounds at
ambient conditions. [(PPh3)2Ag]2CO3 can
activate both CO2 and propargylic alco-
hol, and promote the subsequent intra-
molecular nucleophilic cyclization.
&& – &&
Bifunctional Silver(I) Complex-
Catalyzed CO2 Conversion at Ambient
Conditions: Synthesis of a-Methylene
Cyclic Carbonates and Derivatives
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ÝÝ These are not the final page numbers!