Bifunctional phase-transfer catalysts for fixation of CO2 with epoxides under ambient pressure

Article abstract of DOI:10.1039/c9gc02058f

A series of bifunctional phase-transfer catalysts with a quaternary onium center and a hydrogen-bonding donor group were prepared for the fixation of CO₂ with commercially available epoxides under mild conditions by using a CO₂ balloon (1 atm). In the presence of 2.5 mol% of achiral bifunctional phase-transfer catalysts, cyclic carbonates were obtained in good to excellent yields (up to 95%). Additionally, optical carbonates and epoxides were obtained through the kinetic resolution of rac-epoxides by 1 mol% of chiral bifunctional phase-transfer catalysts with low enantioselectivities. These catalysts featured a simple synthetic route, good modularity and high efficiency.

Full text of DOI:10.1039/c9gc02058f

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Bifunctional phase-transfer catalysts for fixation of CO2 with
epoxides under ambient pressure
Received 00th January 20xx,
Accepted 00th January 20xx
Yue-Dan Li,a,† Dong-Xiao Cui,a,b,† Jun-Chao Zhu,a Ping Huang,a Zhuang Tian,a Yan-Yan Jia,b and Ping-
An Wanga,‡
DOI: 10.1039/x0xx00000x
A series of achiral and chiral bifunctional phase-transfer catalysts with a quaternary onium center and hydrogen-bonding
donor group were prepared for fixation of CO2 with commercially available epoxides under mild conditions by using a CO2
balloon (1 atm). In the presence of 2.5 mol% of achiral bifunctional phase-transfer catalysts, cyclic carbonates were
obtained in good to excellent yields (up to 95%). Additionally, optical carbonates and epoxides were obtained through the
kinetic resolution of rac-epoxides by 1 mol% chiral bifunctional phase-transfer catalysts with low enantioselectivities.
These catalysts featured simple synthetic route, good modularity and high efficiency.
www.rsc.org/
group of structurally variable chiral quaternary onium salts
containing multiple hydrogen-bonding donors, also called
bifunctional phase-transfer catalysts,12 have been developed
Introduction
The activation and fixation of carbon dioxide (CO2) to result in
valuable chemicals such as carbonates, carboxylic acids,
oxazolidinones as synthetic blocks play an important role both
in industries and academies.1 Novel manufactured materials
and successfully applied to many asymmetric transformations
to achieve good to excellent performances both in yields and
enantioselectivities. In recent years, Werner13 and colleagues
have developed a series of bifunctional ammonium and
phosphonium salts (A and B) for the addition of CO2 to
epoxides (Figure 1). Toda14a and Shirakawa’s groups14b have
independently reported bifunctional quaternary phosphonium
salts C and D as catalysts for CO2 fixation reaction with
including
polycarbonates,
poly(ether)carbonates
and
polyurethanes are also based on the direct utilization of CO2.2
In the past four decades, the field of catalytic CO2 conversion
has been dominated by the use of metal-based catalysts being
either heterogeneous or homogeneous in nature, even harsh
conditions such as high temperature and pressure are
necessary to most cases. Therefore, the fixation of CO2 at low
temperature (< 100 oC) and atmospheric pressure is still a
challenge.3
Previous works
O
O
O
O
CO2 (pressure)
Cat.
R
R
The advent of organocatalysis provides an alternative for
activation and fixation of CO2 by using small organic molecules
as catalysts.4 N-Heterocyclic Carbenes (NHCs),5 N-heterocyclic
bases (NHCBs),6 frustrated Lewis pairs (FLPs),7 quaternary
onium hydroxide8/quaternary onium salts (PTCs),9 polyphenols,
fluroalcohols, silanediols with co-catalysts10 and Ionic liquids
(ILs)11 are used as catalysts for catalytic CO2 activation and
fixation in recent years. Among them, phase-transfer catalysed
CO2 fixation is very attractive to chemists due to the easy
availability, simple handling and high efficiency of catalysts. A
F3C
O
O
PPh3
Br
OH
I
PPh3
I
XBu3
HO
N
H
N
H
OH
F3C
NMe2
Et4N+Br-
C
A: X = N; B: X = P
D
E
Werner13 et. al. with 2 mol% A or B, neat, 90 oC, 10 bar CO2, 2 h, 90~99% yields
Toda14a et. al. with 15 mol% C, PhCl, 120 oC, 1 atm CO2, 12 h, 72~96% yields
Shirakawa14b et. al. with 1 mol% D, neat, 60 oC, 1 atm CO2, 24 h, 72~93% yields
Kleij15 et. al. with 3 mol% E, neat, 80 oC, 10 bar CO2, 18 h, 63~90% yields
This work
Br
H
N
H
N
CF3
CF3
N
Bn
a. Department of Medicinal Chemistry, School of Pharmacy, Fourth Military Medical
University, Changle West Road 169, Xi’an, 710032, P. R. China.
O
O
APTC-1
b. Department of Pharmacy, Xijing Hospital, Fourth Military Medical University,
Changle West Road 15, Xi’an 710032, P. R. China.
O
O
O
2.5 mol% APTC-1, CO2 (balloon)
neat, 80 oC, 24 h
R
† Co-first authors. These two authors have the same contribution to this work.
R
‡
Corresponding author: ping_an1718@outlook.com. Electronic Supplementary
65~95% yields
Information (ESI) available: [details of any supplementary information available
should be included here]. See DOI: 10.1039/x0xx00000x
Figure 1. Bifunctional phase-transfer catalysts for CO2 fixation.
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epoxides at atmospheric pressure to provide cyclic carbonates transfer catalysts (see Supplementary MVaiteewrAiratilcsle Onflioner
in high yields. Very recently, Kleij15 and colleagues have used preparation of APTCs), respectively. In the preparation of
DOI: 10.1039/C9GC02058F
the combination of TEAB(Et4N+Br-) and squaramide E as a thiourea-containing catalysts APTC-2 and APTC-5, S-
binary catalytic system for fixation of CO2 in cyclic addition of benzylation was found in the reaction of thioureas with benzyl
various epoxides under neat condition and 10~30 bar pressure bromide. In order to avoid S-benzylation, the primary amine
to give cyclic carbonates in good yields. In fact, commercially was firstly protected by Boc2O following quarterization with
available phase-transfer catalysts (such as TBAC, TBAB and benzyl bromide, after deprotection of Boc group and coupling
TBAI) are usually used as co-catalysts with other with aryl isothiocyanate to provide APTC-2 and APTC-5 in good
organocatalysts or metal catalysts for CO2 fixation. Inspired by yields.17
these above mentioned elegant works on fixation of CO2, we With these above-prepared phase-transfer catalysts in hand,
have envisioned that quaternary onium salts with a urea, the fixation of CO2 with epoxide 1a to form cyclic carbonate 2a
thiourea or squaramide motif as hydrogen-bonding donors was investigated. Under neat conditions with a CO2 balloon (1
could be served as good bifunctional phase-transfer catalysts atm), epoxide 1a was converted into cyclic carbonate 2a after
for fixation CO2 into epoxides. We have developed a class of 24 h of heating in the presence of 2.5 mol% of APTC-1~APTC-
chiral bifunctional phase-transfer catalysts for asymmetric 10, and the results are listed in Table 1. From Table 1, we
amination of nitrooleins under neutral and water-rich found that APTC-1 with a urea group is a good catalyst for this
conditions with high yields and enantioselectivities.16 In order transformation to provide cyclic carbonate 2a in 87% yield.
to test the validity of our hypothesis, herein, we have Catalyst APTC-2 with a thiourea group produced 2a in 52%
synthesized a series of bifunctional phase-transfer catalysts yield, and APTC-3 with a squaramide group demonstrated
containing one urea, thiourea or squaramide group and poor performance in this transformation (Table 1, entries 2
demonstrated their catalytic activities in fixation CO2 with and 3) to afford trace of 2a. APTC-4 and APTC-5 containing a
epoxides to afford cyclic carbonates in good to excellent yields morpholine ring and a urea group or thiourea group gave 57%
under very mild conditions (low temperature and ambient and 42% yield of 2a, respectively. Instead of 3,5-
pressure of CO2). Furthermore, the kinetic resolution of mono- bistrifluoromethylphenyl group in APTC-1 by 4-methylphenyl
substituted epoxides through CO2 fixation by using chiral or 3,5-dimethylphenyl generated APTC-6 and APTC-7, these
quaternary onium salts as bifunctional phase-transfer catalysts two catalysts produced inferior results comparing with APTC-1
is also investigated.
(Table 1, entries 6 and 7). This indicated that the modification
of electronic properties of catalyst lead to dramatic change of
catalytic activity. APTC-8 with a more electron-withdrawing
group (from 4-trifluromethylbenzyl bromide) at quaternary
ammonium center produced a similar catalytic activity to
APTC-4 (Table 1, entry 8 vs entry 4). Catalysts APTC-9 and
APTC-10 afforded cyclic carbonate 2a in moderate yields under
the same conditions (Table 1, entries 9 and 10). Under the
same conditions, 2a was obtained in 49% yield in the presence
of TBAB (Table 1, entry 11). The increase of catalyst-loading
from 2.5 mol% to 5 mol% furnished increased yield of cyclic
carbonate 2a (Table 1, entry 12 vs entry 1, 92%
Results and discussion
A. Synthesis of achiral bifunctional phase-transfer catalysts and
catalytic fixation CO2 by them
As illustrated in Figure 2, ten achiral phase-transfer catalysts
(APTC-1~APTC-10) were prepared from commercially available
1-(2-aminoethyl)pyrrolidine and 4-(2-aminoethyl)morpholine
in two or three steps with high modularity, and one urea,
thiourea or squaramide group was introduced to these phase-
Bn
Bn
H
N
H
N
H
N
H
N
Bn
N
H
N
H
N
CF3
CF3
CF3
N
N
S
Br
Br
O
Br
O
O
CF3
CF3
CF3
APTC-2
APTC-1
APTC-3
Bn
N
H
N
H
N
Bn
N
H
N
H
N
Bn
N
Bn
H
N
H
N
CF3
H
N
H
N
CF3
N
O
Br
O
O
Br
S
O
O
Br
Br
CF3
CF3
APTC-4
APTC-5
APTC-6
APTC-7
F3C
F3C
F3C
H
N
H
N
H
N
H
N
H
N
H
N
CF3
N
N
N
O
Br
Br
O
Br
O
CF3
APTC-8
APTC-9
APTC-10
Figure 2. The chemical structures of APTC-1~10.
2 | J. Name., 2012, 00, 1-3
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Table 1. The optimization of reaction conditions.
O
O
O
CO2 (balloon)
O
PhO
conditions, 24 h
PhO
2a
1a
entrya cat.(mol%)
solvent
neat
neat
neat
neat
neat
neat
neat
neat
neat
neat
T(oC)
yield(%)b
87
52
trace
57
42
50
trace
64
45
54
entrya
11
12
13
14
15
16
17
18
cat.(mol%)
TBABc (2.5)
APTC-1 (5.0)
APTC-1 (1.0)
APTC-1 (2.5)
APTC-1 (2.5)
APTC-1 (2.5)
APTC-1 (2.5)
APTC-1 (2.5)
APTC-1 (2.5)
none
solvent T(oC)
yield(%)b
49
92
64
61
32
59
40
33
1
2
3
4
5
6
7
8
APTC-1 (2.5)
APTC-2 (2.5)
APTC-3 (2.5)
APTC-4 (2.5)
APTC-5 (2.5)
APTC-6 (2.5)
APTC-7 (2.5)
APTC-8 (2.5)
APTC-9 (2.5)
APTC-10 (2.5)
80
80
80
80
80
80
80
80
80
80
neat
neat
neat
neat
neat
PhCl
80
80
80
60
25
80
dioxane 80
toluene 80
9
10
19
20
DMF
neat
80
80
82
0
aReaction was performed in a 0.5 mmol scale; bIsolated yield based on 1a; cTBAB = Tetra-n-butyl ammonium bromide.
vs 87%). 2a was obtained in 64% yield in the presence of 1.0 to give the comparative yield of 2a to neat conditions (Table 1,
mol% of APTC-1 (Table 1, entry 13). When the reaction entries 16~19 vs entry 1). No cyclic carbonate 2a was furnished
o
o
temperature was decreased from 80 C to 60 C, the yield of without catalyst (Table 1, entry 20). When the reaction was
cyclic carbonate 2a was slightly dropped to 61% (Table 1, entry performed in 5 mmol scale under optimal conditions, 2a was
14 vs entry 1). When the reaction was performed at room obtained in 95% yield. Therefore, the optimal reaction
temperature, the yield of 2a is 32% (Table 1, entry 15 vs entry conditions were established as follows: 2.5 mol% APTC-1, neat,
1). The solvent effect of this model reaction was also 1.0
investigated. It was found that only DMF was used as solvent
atm
CO2
(balloon),
80
oC
and
24
h.
Figure 3. The substrates scope for APTC-1 catalyzed cycloaddition of CO2.
O
Br
H
N
H
N
O
O
CF3
CF3
O
N
2.5 mol% APTC-1, CO2 (balloon)
O
Bn
n
neat, 80 oC, 24 h
n
R
R
APTC-1
n = 1 or 2
65~91% yields
1a-q
2a-q
O
O
O
O
O
O
O
O
O
O
OMe
O
O
O
O
O
O
O
PhO
BzO
BnO
2d, 90%
2e, 89%
2b, 91%
2c, 90%
2a, 87%
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
Ph
Ph
Ph
F
Cl
2j, 70%
2k, 71%
2f, 81%
2g, 66%
2h, 65%
2i, 70%
78.3% ee
66.7% ee
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
PhO
BnO
BzHN
BnO
BzO
2m, 89%
> 99% ee
2l, 90%
2n, 85%
2o, n.r.
2p n.r.
2q, n.r.
> 99% ee
> 99% ee
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CF3
After then, various epoxides (or oxetanes) were used as substrates
for this type of cycloaddition under optimal conditions. The reaction
results are listed in Figure 3. It was found that glycidyl ethers 1a-f
were converted to their corresponding cyclic carbonates 2a-f in
high to excellent yields, however, styrene oxides 1g-i provided
carbonates 2g-i in moderate yields. When enantiopure styrene
oxides 1j and 1k were used as substrates, the corresponding cyclic
carbonates 2j and 2k were obtained in good yields and moderate
enantioselectivities (78.3% and 66.7% ee), the reason to the
inversion of stereochemistry of cyclic carbonates is due to the
electron-withdrawing nature of benzene ring, which makes
nucleophilic ring-opening of styrene oxide both at the methine
bond and the methylene bond. Fortunately, the enantiopure
glycidyl ethers 1l-n retained their chirality under the optimal
conditions to produce 2l-n in excellent ee values (> 99% ee, see SI
for details). 2,2-Disubstitued epoxide 1o is not a good substrate for
this transformation, and no reaction was found when oxetanes 1p
and 1q were used in this type of cycloaddition even with a
prolongation of reaction time.
O
2
O
Bn
Br
O
O
N
N
N
CF3
H
H
R
O
APTC-1
R
1
CF3
F3C
O
CF3
O
Bn
N
N
N
CF3
N
O
H
H
H
O
A
N
H
Br
O
O
R
Bn
CF3
N
R
Br
C
O
Bn
N
N
N
CF3
H
H
O
O
B
C
O
R
Br
Figure 4. Proposed catalytic cycle of fixaition CO2 by epoxides.
proposed in Figure 4. At the beginning, the epoxide 1 is activated by
APTC-1 through hydrogen-bonding interaction between N-H and O
atom to form intermediate A, which undergoes nucleophilic attack
from Br- to give intermediate B, intermediate B attacks CO2 to yield
intermediate C, the intramolecular ring-closing of intermediate C
provides cyclic carbonate 2 with a release of APTC-1 to take apart
in the next catalytic cycle.
O
O
2.5 mol% APTCs
O
PhO
O
neat, 80 oC, 24 h
CO2 (balloon)
PhO
2a
1a
R
N
H
Br
N
RO
RO
N
CF3
Bn
Br
N
O
Bn
Bn
CF3
B. Synthesis of chiral bifunctional phase-transfer catalysts and
kinetic resolution of epoxides with CO2 by them
APTC-11: R = H 90% yield
APTC-11': R = Ms 33% yield
APTC-1: R = H
APTC-1': R = Me
87% yield
84% yield
Shirakawa14b and colleagues have demonstrated binol-derived
bifunctional quaternary phosphonium salt catalysts for kinetic
resolution of epoxides to produce chiral cyclic carbonatesand
epoxides in low ee values. Very recently, Meggers14c and colleagues
have developed a facile kinetic resolution of epoxides with CO2 by 1
mol% of chiral-at-iridium complex catalyst in the presence of 1.5
mol% co-catalyst Et4NBr to give chiral cyclic carbonates in high ee
values and selectivity factors (up to 86% ee and 16.6). However, the
kinetic resolution of epoxides solely by chiral organocatalysts to
achieve good ee values and selectivity factors is still a challenge.14d,e
We have deduced that chiral bifunctional quaternary ammonium
salt catalysts may also accomplish this transformation. Recently, we
have developed a class of chiral bifunctional phase-transfer
catalysts for asymmetric amination of nitrooleins under
neutral and water-rich conditions with high yields and
enantioselectivities.16 For our continuous interest on the utility
of these bifunctional phase-transfer catalysts, the kinetic resolution
of epoxides by CO2 was performed to form chiral cyclic carbonates
Scheme 1. Comparison of APTCs catalyzed cycloaddition of CO2.
In order to know the importance of the hydrogen-bonding
interaction between catalyst and substrate in this cycloaddition, we
have prepared the mono-N-Me substituted catalyst APTC-1’ and its
catalytic activity was investigated under the optimal conditions. To
our surprise, the yield of cyclic carbonate was only decreased
slightly from 87% to 84%, which is much more higher than n-
Bu4N+Br-(TBAB) as catalyst under the same conditions (Table 1,
entry 11, 87% vs 49%). We have also prepared APTC-11 which
containing two hydroxyl groups to catalyze the cycloaddition of CO2
with epoxide 1a, cyclic carbonate 2a was obtained in 90% yield,
however, when these two hydroxyl groups were blocked by Ms
(APTC-11’), the yield of 2a was decreased dramatically to 33%
under the same conditions (Scheme 1). All above-mentioned facts
indicated that APTCs with H-bonding donor group have shown
more effective catalytic activities than without H-bonding group in
the cycloaddition of CO2 to epoxides.
Based on experimental facts, the mechanism of the bifunctional and epoxides. Fourteen CPTCs (Figure 5) based on various chiral
phase-transfer catalyst APTC-1 for fixation of CO2 by epoxides was
scaffolds including 3-aminopyrrolidine, amino acids, ferrocene and
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urea-onium salts
CF3
View Article Online
DOI: 10.1039/C9GC02058F
H
H
H
H
N
N
N
N
CF3
O
O
O
N
Br
Bn
CF3
Br
N
Bn
N
N
F3C
N
H
Bn
Bn
H
Bn Br
CF3
CPTC-3
CPTC-1
CPTC-2
CF3
O
Br
N
H
N
O
O
H
N
N
Bn Br
PPh2
Bn
N
H
Br
N
Fe
N
N
H
H
H
CPTC-4
CPTC-5
CPTC-6
CF3
others
thiourea-onium salts
CF3
Bn
N
Cl
H
H
HO
S
OH
Bn
N
N
CF3
N
Br
Bn
N
N
CF3
S
HO
H
H
N
Br
Bn
PPh2
Bn
Bn
Br
Fe
CPTC-9
CF3
N
CPTC-8
CPTC-10
CPTC-7
squamide-ammonium salts
H
H
CF3
CF3
N
N
CF3
O
O
O
O
N
N
N
O
O
N
F3C
N
H
N
H
F3C
N
H
Bn
Bn
Br
H
Bn Br
CF3
Bn Br
CPTC-11
CPTC-12
CPTC-13
CF3
O
O
O
N
N
F3C
N
H
H
Bn Br
CPTC-14
Figure 5. The chemical structures of CPTC-1~14.
O
CO2 (balloon)
O
CPTC-2
neat, 60 oC, 24 h
cinchona alkaloids were investigated for kinetic resolution of
racemic epoxides.
O
O
Ph
Ph
At first, (+/-)-styrene oxide 1g was chosen as model substrate for
kinetic resolution by these above-mentioned CPTCs. The
preliminary results are shown in Scheme 2. Among them, only
CPTC-2 with a urea group has provided chiral cyclic carbonate 2g’ in
26% isolated yield and 21.7% ee. The increase of catalyst-loading of
CPTC-2 from 1 mol% to 10 mol%, the yield of the cyclic carbonate
2g’ was increased to 57% but with the decrease of
enantioselectivity (13% ee vs 21.7% ee). CPTCs with a urea group
based on L-valine scaffold including CPTC-3~5 furnished rac- cyclic
carbonate respectively. Ferrocene-based chiral bifunctional phase-
transfer catalysts CPTC-6 and CPTC-7 have also demonstrated
dissatisfactory results. Switched hydrogen bonding donor (urea or
thiourea) group to squaramide (such as CPTC-11~14), there is no
significant improvement for kinetic resolution of (+/-)-styrene oxide
1g by CO2, all of them afforded rac-cyclic carbonate. When glycidyl
phenylether 1a and glycidyl benzylether 1b were used as substrates,
all catalysts failed to give chiral cyclic carbonates under the same
conditions (see Supplementary Information for details).
1g
2g'
with 1 mol% CPTC-2, 24 h
with 10 mol% CPTC-2, 24 h
with 1 mol% other CPTCs, 24 h various yieldc, rac-
26%, 21.7% eea, s = 1.7b
57%, 13% ee, s = 1.4
aIsolated yield. bDetermined by HPLC on chiral stationary phase. Calculated as
(ln[1 - c(1 + ee)])/(ln[1 - c(1- ee)]) where c is the conversion and ee is the
enantiomeric excess of the cyclic carbonate product.
c
Scheme 2. CPTCs catalyzed kinetic resolution of rac-styrene oxide
by CO2.
Experimental
Typical procedure for cycloaddition of epoxide with CO2
A mixture of 0.5 mmol of epoxide 1a and 2.5 mol% of APTC-1
(6.8 mg) was heated at 80 °C for 24 h under CO2 atmosphere
(1atm, using a balloon). After cooled to room temperature, a
small amount of CH2Cl2 was added to the mixture and it was
purified by a flash column chromatography (petroether/ethyl
acetate = 10/1 to 3/1, v/v) to give cyclic carbonate 2a as off-
white solid.
Typical procedure for kinetic resolution of epoxide by CO2
A mixture of 0.25 mmol of epoxide 1g and 1.0 mol% of CPTC-2
(1.5 mg) was heated at 60 °C for 24 h under CO2 atmosphere
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Commun., 2015, 6, 5933; (d) X.-Y. Zhang, M. Fevre, G. O.
(1atm, using a balloon). After cooled to room temperature, a
small amount of CH2Cl2 was added to the mixture and it was
purified by a flash column chromatography (petroether/ethyl
acetate = 10/1 to 3/1, v/v) to give cyclic carbonate and epoxide
respectively. The ee of products were determined by HPLC
using a chiral column (Daicel Chiralpak AD-H or OD-H), and the
absolute configurations of products were assigned according
to literature report.14
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Conclusions
In summary, we have demonstrated a series of bifunctional
quaternary ammonium salts as phase-transfer catalysts for
fixation of CO2 with epoxides to afford cyclic carbonates in
good to excellent yields under mild conditions. These
bifunctional catalysts are structure-tuneable and easily
prepared, they also demonstrated high efficiency for fixation
of CO2 under ambient pressure. Additionally, a number of
chiral bifunctional quaternary onium salts based on privileged
scaffolds were synthesized for kinetic resolution of rac-
epoxides by CO2 to produce chiral cyclic carbonate up to 21.7%
ee. The endeavour to improve the enantioselectivity of kinetic
resolution of epoxide by CO2 with chiral quaternary onium
salts was underway in our laboratory.
4
Conflicts of interest
There are no conflicts to declare.
5
Acknowledgements
We thank the National Science Foundation of China (no.
21372259 and 81503028) for financial support of this work.
Notes and references
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DOI: 10.1039/C9GC02058F
Bifunctional phase-transfer catalysts for fixation of CO2 with epoxides under
ambient pressure
Yue-Dan Li,a,† Dong-Xiao Cui,a,b,† Jun-Chao Zhu,a Ping Huang,a Zhuang Tian,a Yan-Yan Jia,b and Ping-An Wanga,‡
A series of achiral and chiral bifunctional phase-transfer catalysts with a quaternary onium center and hydrogen-bonding donor
group were prepared for fixation of CO2 with epoxides under mild conditions by using a CO2 balloon (1 atm) to produce cyclic
carbonates up to 95% yields.
Bifunctional PTCs
O
(BPTCs)
Br
O
H
H
O
O
CO2 balloon (1 atm)
N
N
CF3
N
2.5 mol% BPTCs
O
R
R
neat, 80 oC, 24 h
Ar
CF3
up to 95% yield
R = Ar, ArOCH2, ArCH2OCH2
Ar = Ph, 4-CF3C6H4, etc.