Copper-catalysed synthesis of α-alkylidene cyclic carbonates from propargylic alcohols and CO2

Article abstract of DOI:10.1039/d0gc03990j

We report a N-heterocyclic carbene copper(i) complex-catalysed formal cycloaddition between readily available propargylic alcohols and carbon dioxide at room temperature. By using the combination of a sterically demanding^BPDPrCuCl complex (^BPDPr = 1,3-bis(2,6-diisopropylphenyl)-1,3-diazonine-2-ylidene) and CsF, as catalytic system, primary propargylic alcohols are efficiently converted to the corresponding α-alkylidene cyclic carbonates. Gram scale (up to 89% yield) and reusability experiments (74% global yield, turnover number value = 103) showcase the robustness of the catalytic system. This practically simple protocol also tolerates secondary and tertiary propargylic alcohols under CO₂at atmospheric pressure, enabling the direct synthesis of substituted and unsubstituted α-alkylidene cyclic carbonates at room temperature.

Full text of DOI:10.1039/d0gc03990j

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DOI: 10.1039/D0GC03990J
ARTICLE
Copper-Catalysed Synthesis of α-Alkylidene Cyclic Carbonates
from Propargylic Alcohols and CO2
Alejandro Cervantes-Reyes,*a Kaveh Farshadfar,b Matthias Rudolph,a Frank Rominger,a Thomas
Received 00th January 20xx,
Accepted 00th January 20xx
Schaub,c,d Alireza Ariafard*e and A. Stephen K. Hashmi*a,c,f,g
DOI: 10.1039/x0xx00000x
We report a N-heterocyclic carbene copper(I) complex-catalysed formal cycloaddition between readily available propargylic
alcohols and carbon dioxide at room temperature. By using the combination of a sterically demanding BPDPrCuCl complex
(
BPDPr = 1,3-bis(2,6-diisopropylphenyl)-1,3-diazonine-2-ylidene) and CsF, as catalytic system, primary propargylic alcohols
are efficiently converted to the corresponding -alkylidene cyclic carbonates. Gram scale (up to 89% yield) and reusability
experiments (74% global yield, turnover number value = 103) showcase the robustness of the catalytic system. This
practically simple protocol also tolerates secondary and tertiary propargylic alcohols under CO2 at atmospheric pressure,
enabling the direct synthesis of substituted and unsubstituted -alkylidene cyclic carbonates at room temperature.
systems.7-11 More recently, the successful transformation of
these substituted propargylic substrates using silver-based
catalysts, has been possible in the presence of additives or co-
catalysts (e.g. organic bases) under pressure and/or heating,
wherein the need of chlorinated or aromatic solvents, or the
generation of undesired side-products might cause
environmental concerns.12,13
Introduction
The use and transformation of abundant gaseous waste, such
as converting carbon dioxide (CO2) as a C1 source into valuable
chemical products by using cheap and easily available industrial
chemicals, fulfils the requirements of green chemistry and atom
economy.1 Carbon dioxide, as an abundant, readily available,
nontoxic, and non-flammable carbon resource, is recognised as
a powerful building block for the preparation of value-added
chemicals. Additionally, this C1 unit is inexpensive as it is being
generated as a waste gas in enormous quantities globally.2
-Alkylidene cyclic carbonates, are a class of heterocycles with
the potential to be applied as additives in Li-ion batteries,
reactive solvents, intermediates in the manufacture of fine
chemicals, or as monomers in the synthesis of polycarbonates
or polyurethanes (see (a), Figure 1).3,4 To date, numerous
approaches for the synthesis of other cyclic carbonates from
CO2 have been reported, such as cycloaddition of epoxides with
CO2, or the carboxylative cyclisation of propargylic alcohols.5,6
In the latter, secondary or tertiary substrates could be smoothly
transformed in the presence of organocatalysts, ionic liquids
(ILs) or late transition metal catalysts, including heterogeneous
catalyst such as MOF (Metal Organic Framework)-supported
a) -Alkylidene cyclic carbonates in organic synthesis: polymer precursors

O
O
O
O
HO
O
O
Y
R
OH
O
n
n
R-OH
base
n
n
O
O
O
poly (-oxo-carbonate)s
O
bis (-ACC)
O
Y = Z
R1
R-NH
Y
O
O
R1
N
HN
N
H
N
O
O
N
Z
R
Y
O
O
poly (-oxo-urethane)s
b) N-C-N angles and %Vbur values in five- and nine-membered (NHC)CuCl complexes

N
N
R

R
five-membered NHC:
nine-membered NHC:
N
N
R
R
M
M
%Vbur = 47.6
 = 105°
%Vbur = 57.9
= 118°

c) Synthesis of -alkylidene cyclic carbonates from propargylic alcohols

Previous work: R2, R3 H
Zn, Ru, Co, Ag, ILs, bases
Zn, Ru, Co, Ag, ILs, bases:
Challenging for primary propargyl
alcohols (so far only achieved
with a silver(I) catalyst)
O
OH
R3
O
CO
+
O
2
R2
R1
This work: R2, R3 H
R1
R3
R2
Cu(I)
[Cu] cat.
a. Organisch-Chemisches Institut, Heidelberg University, Im Neuenheimer Feld 270,
69120 Heidelberg, Germany. E-mail: hashmi@hashmi.de
b. Department of Chemistry, Azad University Central Tehran Branch, Poonak,
Tehran, Iran
Cu(I) catalysis
 Inexpensive and abundant metal (<0,2 £/mol);
 Gram scale (up to 89% yield, 15 mmol) and catalyst reusability;
 Mild reaction conditions: room temperature, 0.12.0 MPa;
 Unprecedent substituted and unsubstituted -alkylidene
cyclic carbonates synthesized.
N
N
c. Catalysis Research Laboratory (CaRLa), Im Neuenheimer Feld 584, Heidelberg,
69120, Germany.
Cu
Cl
d. BASF SE, Carl-Bosch-Str. 38, Ludwigshafen 67056, Germany.
e. School of Physical Sciences, University of Tasmania, Private Bag 75, Hobart TAS
7001, Australia
Figure
1 (a) N-heterocyclic carbene copper(I) chloride
complexes with five (left) and nine (right) members featuring N-
C-N angle () and percent buried volume (%Vbur), (b) Application
of cyclic carbonates in polymer synthesis, and (c) Synthesis of
-alkylidene cyclic carbonates from primary propargylic
alcohols and CO2.
f. Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah
21589, Saudi Arabia.
g. Heidelberg Center for the Environment (HCE), Universität Heidelberg, Im
Neuenheimer Feld 229, 69120 Heidelberg, Germany
†Electronic Supplementary Information (ESI) available: Experimental details and
characterisation of products. See DOI: 10.1039/x0xx00000x
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Up to this point, only one successful silver-based catalytic the data of the CIF file.23 Complex BPDPrCuCl differs structurally
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DOI: 10.1039/D0GC03990J
system (DavePhos/AgOAc) has achieved the challenging from other comparable bulky or expanded ring NHC copper(I)
carboxylative cyclisation of primary propargylic alcohols.14 To chloride complexes, while being less linear (CNHC−Cu−Cl 174.96°)
the best of our knowledge several examples of copper(I) and having a larger Cu−Cl bond (2.127 Å).24 This parameter
catalysts for carboxylative cyclisation of substituted propargylic evince its weakness that facilitate the halide exchange, believed
alcohols are known, but no successful example using primary to be the first step in the catalytic cycle.
propargylic alcohols (as the most relevant for industrial The synthesis and use of BPDPrCuCl in catalysis are
applications) is available in literature.15 Furthermore, the unprecedented. A copper(I) bromide complex stabilized by
development of a copper-based catalyst as a more abundant (BPDPr) ligand was firstly developed in our group and proved to
and cheaper base metal is highly desirable for this be catalytically active.21 As in the case for the bromide
transformation.16
analogue, the stability of BPDPrCuCl to air and moisture enables
N-Heterocyclic carbenes (NHCs) are excellent stabilizing ligands its use under mild conditions (room temperature, wet solvents,
for a wide range of metal complexes, due to their strong - atmospheric pressure).
donating ability and steric properties.17 Among them, expanded
ring NHC (erNHC) ligands have probed to enhance the catalytic
activity of the complex enabled by the steric protection around
the metal center as a consequence of the enlarged N-C-N angle
that pushes the N-substituents closer to the carbene carbon
(see (b), Figure 1).18,19 Thereafter, the synthesis and application
of erNHC-copper(I) complexes in organic synthesis have
become a growing research field in the last decade.20
In this work, we present our findings on the synthesis and use
of an erNHC-copper(I) complex as catalyst for the carboxylative
cyclisation of primary propargylic alcohols at room
temperature, including an extensive screening of reaction
conditions, gram-scale and reusability experiments to show the
efficiency and robustness of our catalytic design (see (c), Figure
1). Finally, the protocol has been extended to secondary and
Figure 2 Solid state molecular structure of BPDPrCuCl (thermal
tertiary derivatives to conclusively obtain a broad substrate
probability at 50%).
scope.
Carboxylative Cyclisation of Propargylic Alcohols
Results and discussion
Synthesis and Structure of BPDPrCuCl
Our catalytic experiments were initially performed on the
carboxylative cyclisation of propargylic alcohol 1a as a model
substrate for the reaction with CO2. Preliminary screenings
Direct deprotonation of the (NHC)HBF4 salt (NHC = BPDPr: 1,3-
indicated that other standard phosphine or diamine ligands in
bis(2,6-diisopropylphenyl)-1,3-diazonine-2-ylidene) in the
combination with CuX salts (X = Cl, Br, I, OAc) provided low
presence of CuCl furnished BPDPrCuCl. Alternatively, this
yields or only trace amounts of product with the same catalyst
complex can be accessed via ligand transfer reaction to Cu(I)
loadings (Table S1).
from the corresponding (NHC)AgBr complex (Scheme 1).21
In a typical experiment, reactions were conducted in an
autoclave reactor with propargylic alcohol (5 mmol) and CO2
purged to 1.5 MPa in 10 mL of CH3CN at room temperature
(Table 1).25 We first checked that in the absence of catalyst no
a)
X = BF4
reaction occurred between 1a and CO2 after 24 hours (Table 1,
N
N
X
Dipp
Dipp
b)
N
N
entry 1). The complex IPrCuCl alone was ineffective for the
transformation (entry 3) but its use in combination with CsF
furnished the desired product in 35% isolated yield (entry 5).
The IMesCuCl/CsF system did not catalyse the reaction at all
under the same conditions (entry 6). The individual complex
BPDPrCuCl only showed a low performance with a yield of 10%
(entry 4). In contrast, by the addition of CsF, the yield of the
target product rose to 95% (entry 7), indicating a strong positive
synergistic effect between BPDPrCuCl and CsF. In the control
experiments, CuCl was unable to catalyse the carboxylative
cyclisation (entry 2) and CsF itself was ineffective under the
given reaction conditions (entry 15). However, CsF plays a
significant role in the catalytic system, as only a poor yield of
Cu
Cl
Dipp
Dipp
X = Br
BPDPrCuCl
a) CuCl, tBuOK, THF, -78°C to r.t.
b) i) Ag2O, NaBr, THF, reflux; ii) CuCl, DCM, 24 h, r.t
Scheme 1 Synthesis of the complex BPDPrCuCl (Dipp = 2,6-
diisopropylphenyl).
Slow diffusion of pentane into a saturated solution of BPDPrCuCl
in DCM, afforded a single crystal with good quality for X-ray
diffractometry analysis (Figure 2).22 For the quantification of
steric features, the percent buried volume value (Vbur = 57.9%)
and steric map (see Supporting Information) were derived from
2 | J. Name., 2012, 00, 1-3
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cyclisation product is observed without adding it (entry 4). In BPDPrCuCl/CsF system (Table 1, entry 7) showed a better
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the presence of Et3N·3HF a poor yield of desired product was catalytic efficiency than the IPrCuCl/CsF DsyOsI:te10m.10(3T9a/bDl0eG1C,0e39n9t0ryJ
obtained, however ring-opened side product B1 (10%) was also 5) under the same reaction conditions. These results
noticed, presumably formed due to the presence of remaining doubtlessly suggest that [BPDPrCu]+ acts as the crucial catalyst
propargylic alcohol that reacted with A1 in the presence of and CsF can further enhance the carboxylative cyclisation of
catalytic Et3N (entry 9). And the yield was not significantly propargylic alcohols with CO2. The mixture of (NHC)CuX (X = Cl,
improved in the presence of a quaternary ammonium fluoride I) and CsF has previously been implemented in the generation
which afforded 7% of side product (entry 10).
of catalytically active (NHC)CuF species under mild conditions
(e.g. open-flask conditions, non-anhydrous solvents, room
temperature), although some examples of (NHC)CuF complexes
have been isolated.27,28
Table 1 Optimization of the carboxylation conditionsa
O
O
OH
Cu cat. (10 mol%)
O
To confirm the pivotal role of CsF on the current copper
catalysis, we performed blank experiments under reaction-
+
CO2
O
+
O
O
CH3CN, r.t, 24 h.
O
1.5 MPa
relevant conditions. By mixing (NHC)CuCl with CsF in CDCl3, a 19
F
A1
B1
1a
NMR-active NHC-copper(I) fluoride species could be detected at
  -245 ppm. This chemical shift is within the range of
previously reported (NHC)CuF complexes (-240 to -253 ppm).27
In addition, by mass spectrometry experiments (MALDI+),
N
N
Ar
Ar
iPr
iPr
Cu
Cl
N
N
Cu
Cl
iPr
iPr
+
[LCu]+ and [LCu(-F)]2 species could be analysed in solution
IMesCuCl
IPrCuCl
Ar = Mes
Ar = Dipp
BPDPrCuCl
(Eqn. 1). Arguably, these observations suggest that CsF acts
merely as halide scavenger and LCuF is the ultimate copper
species present in solution during the catalytic process. On the
other hand, in the absence of (NHC)CuCl complex, CsF (1 equiv)
sparingly promoted the transformation of 1a into no desired
product B1 (17% NMR yield) under standard carboxylation
conditions (Eqn. 2). This indicates that a putative caesium
alcoholate intermediate (1a’) could react with A1 conducting to
the ring-opened carbonate. Conclusively, CsF could activate to
the substrate when acting alone but an excess (i.e. an
equivalent quantity) would lead to undesired ring-opened
product.
Entry
Cu cat.
--
CuCl
IPrCuCl
BPDPrCuCl
IPrCuCl
Additive
A1 (%)b
B1 (%)b
1
2
3
4
5
6
7
8
9
--
--
--
0
0
2
0
0
0
0
13
0
1
18
10
7
53
--
10
49 (35)
0
98 (94)
71 (60)
32
38
11
CsF
CsF
CsF
KF
IMesCuCl
BPDPrCuCl
BPDPrCuCl
BPDPrCuCl
Et3N·3HF
Me4N+F-
Cs2CO3
10
11
12
13
14
15
BPDPrCuCl
Eq 1.
Determination of LCuF in solution
BPDPrCuCl
BPDPrCuCl
BPDPrCuCl
CuCl
tBuOK
AgOAc
CsF
2
67
0
0
[LCuF]
LCuCl
+
CsF
iPr
iPr
CDCl3, r.t, 30 min.
N
N
(1 equiv)
99 (95)
[LCu]+
m/z = 605.29640
m/z = 1247.5768
19F NMR = -245.2 ppm
iPr
iPr
0
0
(MALDI+)
+
[LCu(-F)]2
L
--
CsF
2
a The reaction of 1a (5 mmol) was carried out in the presence of
Eq 2.
A
O
Cu cat. (10 mol%) and additive (10 mol%) in CH3CN (10 mL) at
CsF-promoted ring opening of carboxylation product
1
1
room temperature for 24 h. b Determined by H NMR analysis
H
Cs
F
O
O
O
(in parentheses isolated yields after Kugelrohr distillation).
OH
CsF (1 equiv)
CO
1a'
O
O
O
2 (2 MPa)
CH3CN, r.t, 24 h.
O
An increased quantity of B1 was observed when employing
strong bases CsCO3 and tBuOK confirming the role of a base in
the ring-opening side reaction (entries 11 and 12). KF was also
able to act as good additive, resulting in a moderate isolated
yield (entry 8). Noteworthy, the use of AgOAc as the additive
furnished high yield of A1 (entry 13). This would suggest a ligand
transfer from copper to silver given that CuOAc delivered only
a moderated yield of product when used as the additive (Table
S2, entry 7).26 As expected, a simple mixture of CuCl and CsF was
not catalytically active (entry 14). Subsequently, other NHC-
copper(I) halide complexes were also investigated. Low to
moderate yields were obtained for BPDPrCuBr in combination
with other additives (11–57%, Table S2), revealing BPDPrCuCl as
the most efficient pre-catalyst for this reaction. Besides, the
B1
1a
A1
17%
(NMR yield)
Our subsequent studies focused on the direct carboxylative
cyclisation of unsubstituted propargylic alcohols that
incorporate a diverse array of functional groups. We first
examined our reaction scope on a set of primary alkynols (1b–
1q) derived from cheap and industrially relevant 2-butyn-1,4-
diol (Figure S6).3b Generally, these substrates could be
effectively converted into -alkylidene cyclic carbonates A1–17
(Table 2). Substrates bearing acrylate (1b), acetate (1c) and
carbonate (1d) units reacted to deliver the corresponding cyclic
carbonates (A2–4) in up to 88% yield. Product A5, bearing a
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benzyl substituent was also obtained in comparatively good reached a full conversion to give A16 in excellent isolated yield.
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yield (89%).
Noteworthy, the substrate with a diDcaOrI:b1a0m.10a3t9e/Du0GnCit03(919q0)J,
afforded the expected bis(-alkylidene cyclic carbonate)
Table 2 Scope of the NHC-copper(I)-catalysed carboxylative product A17 in 85% yield. For this series of primary substrates,
cyclisation of propargylic alcohols with CO2
the pressure played a crucial role and by-product B could be
isolated in up to 43% yield when diminishing the CO2 pressure
to 1 MPa (see Supporting Information). Unexpectedly, 2-butyn-
1,4-diol itself did not yield the expected product and
O
O
O
OH
O
BPDPrCuCl / CsF
O
R1
O
or
CO2
+
R2
R1
R
CH3CN, r.t, 24 h.
R2
C1
R
1
2

A1
decomposition material was observed after
a typical
17
13


carboxylation experiment. The stereochemistry of the products
obtained under the current copper catalysis was determined by
NOE experiments which revealed the formation of pure Z-
alkylidene cyclic carbonates. To our delight, an X-ray quality
single crystal of product A8 could be grown and its solid state
molecular structure confirmed the Z-configuration of the -
alkylidene cyclic carbonate (Figure 3).22
Unsubstituted -Alkylidene Cyclic Carbonatesa
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
H
O
O
A
MeO
O
A
A
3 86%
A
4 78%
1 94%
2 88%
O
O
O
O
O
O
O
O
O
O
O
O
R
R
X
R
H
A5
R
R
-Me
= m
A10
=
=
=
=
=
89%
= o
X
X
Cl A12
87%
90%
=
=
85%
81%
R
R
R
R
Me A6 95% (3.13 g, 89%)b
tBu A7 90%
-OMe A
Br A
11
O
13
H
N
92% (X-ray)
96%
Ph A8
OMe A
O
9
O
O
O
O
O
O
O
HN
O
O
O
R
O
O
O
A
O
O
R
R
R
Me A15
OMe
=
=
88%
94%
A16
17 85%c
O
A
14 84%
Substituted -Alkylidene Cyclic Carbonatesd
O
O
O
O
O
O
O
R
R
R
R
R
Me
Et
nPr
nBu
C
C
C
C
=
=
=
=
=
1 81% (94%) a
2 90%a
O
C
O
O
O
O
3 92%a
R
4 88%a
Figure 3 Solid state molecular structure of -alkylidene cyclic
carbonate A8 (Ellipsoids are drawn at 50% probability level).
5 82%a
7 86% (92%)a
nPent C
C
6 78%
C
8 71%
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
In virtue of the observed outcomes, scale-up catalysis was
performed with 15 mmol 1f and 5 mol% BPDPrCuCl/CsF under
2.5 MPa CO2 at room temperature for 24 h. The substrate
underwent conversion to A6 in 89% isolated yield and the total
turnover number value for the NHC-copper(I) catalyst was 17.8.
We next sought to evaluate the scope of this transformation
9 69% (80%)a
C
C
10 78%
C
C
C
13 91%
11 81%
12 84%
a
The reactions were performed with 1 (2 mmol) and
BPDPrCuCl/CsF (5 mol%) under CO2 (2 MPa). Isolated yields after
column chromatography. b At 15 mmol scale and 2.5 MPa CO2. with respect to substituted propargylic alcohols.29 Given the
Thorpe-Ingold effect in gem-substituted substrates,30 we
envisioned that the operationally simple carboxylative
cyclisation of such compounds would easily proceed to afford
the cyclic carbonates with high efficiency. In line with this
assumption, the use of 5 mol% of catalyst under pressure
conditions, promoted the total conversion of selected
substrates and even with half of the catalyst amount, the
expected products were gained with no significant drop in yield.
In the presence of the NHC-copper(I) system, all reactions
smoothly delivered the desired carboxylative cyclisation
products C1–13 in 67–94% yield at a 2.5 mol% catalyst loading at
room temperature. For tertiary substrates, we found that the
reaction was not dependent on the applied pressure, and
reactions using a CO2-filled balloon afforded moderated to good
yields. A brief survey on literature reveals that substituted
product C7 can also be synthesized using other copper catalysts:
Cu(CH3CN)PF6/bipy (2 equiv Et3N, 100°C, 3.8 MPa, 94% yield),15a
Polymer/amide copper(I)-supported (supercritical CO2, 60°C, 12
MPa, 99% yield),15b CuBr (Bu3N, electrolytic conditions, 3 MPa,
c10 mol% BPDPrCuCl/CsF was employed. Performed with 2 (2
mmol) and BPDPrCuCl/CsF (2.5 mol%) under CO2 (balloon).
Isolated yields after column chromatography.
d
To our delight, several alkynols bearing electronically different
substituents at the aryl unit were well-tolerated, and
unsubstituted cyclic carbonates with electronically neutral p-
methyl (A6), p-tert-butyl (A7), p-phenyl (A8) and electron-rich p-
methoxy (A10) substituents were produced in good to excellent
isolated yields (90–96%). The ortho-methyl-substituted product
A10 was furnished in a slightly decreased yield compared to the
para-substituted counterpart (A6). Analogously, the meta-
methoxy-substituted product A11 was obtained in a slightly
diminished yield compared to its para-isomer. Substrates with
halogens incorporated in the aryl unit (1l and 1m) also worked
well under the reaction conditions to prepare the
corresponding products A12 and A13 in 85% and 81% yields,
respectively. Moreover, the electron-rich aryl substrate (1p)
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92% yield),7b or CuI/(Bu4P)-Levulinate (1 MPa, r.t, 97%)15g
among others, from which a Cu@MOF catalytic system (DBU,
r.t, CO2 balloon, 92%)15e stands out as a mild and low pressure
Computational study of the mechanism
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DOI: 10.1039/D0GC03990J
At this juncture, we employed the density functional theory
(DFT)
calculations
at
the
SMD/B3LYP-D3/def2-
protocol, making the current NHC-copper(I) complex
a
TZVP//SMD/B3LYP-D3/6-31G(d),SDD level of theory in
acetonitrile to better understand the mechanistic details of this
carboxylative cyclisation reaction. The catalytic cycle shown in
Scheme 2 summarizes our DFT results for such a catalysis.
Substrate 1a is used as a representative for propargylic alcohols.
Accordingly, the reaction is commenced by a ligand exchange
between 1a and BPDPrCuF to give the copper alkoxy complex 2
stabilized by the HF acid. This process is calculated to be
endergonic by 5.5 kcal/mol and proceeds via four-membered
transition structure TS1, which only lies 9.0 kcal/mol above the
reference point (Figure 5a).
comparatively suitable catalytic system for the transformation
of tertiary substrates (base-free, CO2 balloon, 69–92%).
A wide range of substrates was tolerated in the cycloaddition
reaction, nonetheless some phenyl- or alkyl-substituted
primary propargylic alcohols were inactive, and phenol-
containing products decomposed upon chromatographic
purification (Figure S5). It is worth noting that, for previously
reported carboxylative cyclisation protocols, the primary
propargylic alcohols failed completely to yield the expected
products by using other Cu(I)-based catalytic systems, or metal-
free mediated reactions.7-8,15,16c Our system provides yields
comparable to the silver-catalysed approach using 1 mol%
catalyst loading (i.e. A1: 90% (Ag)/94% (Cu); A2: 82% (Ag)/88%
(Cu); A3: 80% (Ag)/86% (Cu); A4: 65% (Ag)/78% (Cu); A5: 70%
(Ag)/89% (Cu); A17: 93% (Ag)/85% (Cu);).14 However, for
substituted substrates the mentioned reported systems reacted
smoothly, which highlights the distinctive feature of the present
catalytic system compared to other previously studied copper
catalysts in the carboxylative cyclisation of propargylic alcohols.
O
O
C
O
A1
HO
1a
BPDPrCuF
BPDPrCuF
BPDPrCu
HF
BPDPrCu
O
H
O
2
F
C
BPDPrCuF
BPDPrCuF
O
O
8
HF
O
O
C
O
BPDPrCu
O
Catalyst reusability
3
BPDPrCu
A preliminary recycling experiment of the catalytic system was
performed after product distillation (Kugel-rohr, 100°C, 0.6
mbar) and 45% conversion after the second cycle was feasible.
The separation of carbonate product and catalyst by distillation
seems to be not favourable because of the general instability of
(NHC)CuF species to air or moisture during the product
separation, and so the overall catalytic activity in the process
would be strongly decreased. However, to study the reusability
of BPDPrCuCl/CsF system in comparable amounts to our
previous experiments (5 mol %), we carried out successive
substrate additions (1a, 5 mmol) to the reactor instead of
working up the reaction (Figure 4).
CO2
7
more reactive
towards cyclization
BPDPrCu
O
O
O
BPDPrCu
C
O
C
O
O
4
6
less reactive
towards cyclization
Scheme 2 Reaction mechanism for carboxylative cyclisation of
1a proposed by the DFT calculations.
1
OH
O
1
O
O
24 h
r.t
A1
[LCuF]
4 additions
74% total
TON = 103
Figure 4 Successive substrate additions to count for catalyst
reusability.
The product was quantified by NMR analysis of crude samples
after each addition giving 74% overall yield after four loads of
substrate 1. Under these conditions, the catalyst amount
available in the global process was 0.71 mol% affording a
turnover number value of 103. Finally, an NMR spectrum of raw
mixture after the first addition (second run) showed traces of
intact copper catalyst, indicative of its stability in solution (see
Supporting Information for details).
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
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ARTICLE
Journal Name
(a)
The HF liberation from the hydrogen-bond complex 2 then gives
intermediate 3 in an uphill process. However, consistent with
View Article Online
DOI: 10.1039/D0GC03990J
BPDPrCu
O
1.889
BPDPrCu
HF
O
2.060
3
C
previously reported studies on (NHC)CuF complexes,31 the
released HF can gain stability through a hydrogen bond
interaction with another BPDPrCuF catalyst to give the copper–
bifluoride complex BPDPrCuF⋯HF Interestingly, the formed
intermediate 3 is computed to be extremely reactive to activate
CO2 via transition structure TS3 to furnish copper carbonate 4.
The ensuing carbonate complex has the potential to be
transformed to various linkage isomers as shown in Figure 5b.
The stability of the linkage isomers 4-7 are calculated to span a
relatively narrow range from 5.8 to 16.2 kcal/mol, suggesting
that these isomers should be in equilibrium during the course
of the catalytic reaction. However, among the linkage isomers,
only 6 and 7 have ability to yield heterocycle products. The
more stable isomer 6 forms a product with E-configuration
through the insertion of the alkyne into the Cu-O bond via
transition structure TS6. The less stable isomer 7 forms the
experimentally observed product with Z-configuration through
the nucleophilic anti attack of the carbonate oxygen onto the
copper(I)-activated triple bond via transition structure TS7. As
can be seen from Figure 5c, TS7 lies 4.1 kcal/mol lower in energy
than TS6. This result means that consistent with the
experimental findings, the less stable structure 7 is much more
reactive towards the cyclization process and thus the reaction
should preferentially proceed via the nucleophilic attack
pathway. In the last step, the -alkylidene cyclic carbonate
product (A1) is formed by a stereospecific protodemetallation
process concomitant with regeneration of both L-CuF units via
transition structure TS8. Based on our DFT results,
transformation 7  8 is predicted to be the rate-determining
step.
O
14.6 (14.3)
BPDPrCuF
BPDPrCuF
2.026O
BPDPrCu
2.436
TS3
O
1.094
HF
H
F 1.285
TS1
10.0 (-7.9)
9.0 (-1.5)
9.5 (-10.6)
CO2
1.959
BPDPrCu
O
7.7 (-1.5)
1.959
C
O
1.850
BPDPrCu
O
5.5 (-6.7)
O
3
HO
4
1a
1.897
BPDPrCu
O
1.339
H
2
F
0.0 (0.0)
BPDPrCuF
O
O
(b)
C
O
2.114
O
O
O
C
BPDPrCu
2.116
4
C
O
2.078
O
BPDPrCu
BPDPrCu
O
9.5 (-10.6)
1.909
6
5
7
16.2 (-7.6)
5.8 (-15.4)
10.7 (-12.7)
1.996
BPDPrCu
(c)
2.7022.664
O
2.117
O
C
TS6
O
27.3 (5.5)
1.992
BPDPrCu
2.547
2.159
O
TS7
C
O
23.2 (0.4)
O
1.172
F
H
1.392
H
As discussed above, formation of alkoxy complex 3 is a
prerequisite for the catalytic reaction to proceed. As mentioned
above, this is easily achieved by the ligand exchange between
1a and BPDPrCuF. It is worth mentioning that we were unable to
locate a transition structure for formation of alkoxy complex 3
from BPDPrCuCl. This can be attributed lower basicity of
BPDPrCuCl than BPDPrCuF in deprotonating the propargylic
alcohol as supported by the thermodynamic calculations for the
simple model reactions shown in Figure 6; the reaction between
BPDPrCuCl and 1a is about 13.6 kcal/mol more endergonic than
the reaction between BPDPrCuF and 1a. Consequently, as
discussed above, the in-situ formation of BPDPrCuF should be
the key feature for the catalysis. Our calculations based on the
simple model reaction given in Figure 6 indicates that the
formation of BPDPrCuF is thermodynamically favoured with
2.029
O
C
BP DPrCu
TS8
O
O
17.1 (-4.8)
16.2 (-7.6)
O
O
C
O
HF
DPrCuF
BP
BPDPrCuF
BPDPrCu
DPrCuF
BP
10.7 (-12.7)
7
O
BPDPrCu
C
O
O
5.7 (-17.3)
O
C
O
1.924
6
BPDPrCu
O
A1
O
8
-11.9 (-24.9)
C
O
O
Figure 5 (a) Energy profile for
ligand exchange ΔGrxn = -5.2 kcal/mol.
between BPDPrCuF and 1a followed by CO2 activation. (b)
Calculated stability for different linkages isomer (4-7). (c) Energy
profiles for the formation of A1 starting from active
intermediate 6. The relative Gibbs free energies potential
energies (in parentheses) obtained from SMD/B3LYP-D3/def2-
TZVP//SMD/B3LYP-D3/6-31G(d),SDD in acetonitrile are given in
kcal/mol and the selective bond lengths (the pink italic values)
in Å.
6 | J. Name., 2012, 00, 1-3
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catalysis. A computational study of the reaction mechanism
View Article Online
DOI: 10.1039/D0GC03990J
confirmed a -acidic activity of the copper-based catalyst and
the steering role of the bulky NHC ligand. Furthermore, this
straightforward and atom-economical approach enables access
to a wide range of substituted and unsubstituted -alkylidene
cyclic carbonates from propargylic alcohols by using CO2 at
room temperature with relatively low catalyst loadings (2.5–5
mol%).
Noteworthy, the herein implemented active catalyst could be
applied in gram-scale and reusability experiments (74% overall
yield after four successive additions) under easy, non-hazardous
reaction conditions, and waste minimization (no side products
Grxn = 28.2 kcal/mol
Grxn = 14.6 kcal/mol
BPDPrCuCl
BPDPrCu
O
HCl
HF
HO
1a
3
3
BPDPrCuF
BPDPrCu
O
H2O
1a
Grxn = -5.2 kcal/mol
BPDPrCuCl
Cl
BPDPrCuF
F
Figure 6. Thermodynamic calculations for some simple model are observed). This could represent a feasible choice for
reactions.
sustainable industrial-scale syntheses of cyclic carbonates and
synthetically useful derivatives.
To determine whether the nature of the substituent R affects
the favourability of the outer-sphere mechanism over the inner-
sphere one, we calculated the key transition structures of the
carboxylative cyclisation for substrate 5a. As can be seen from
Figure 7, TS7’ is more stable than TS6’ by about 7.8 kcal/mol. This
result further confirms that the observed product with Z-
configuration is most likely produced by the outer-sphere
mechanism, occurring from an alkyne -complex analogous to
7.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
A.C.R is grateful for a Ph.D fellowship from CONACyT (México).
R
R
BPDPrCu
Notes and references
inner-sphere
2BPDPrCuF
+
O
1
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O
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parenthesis)
obtained
from
SMD/B3LYP-D3/def2-
TZVP//SMD/B3LYP-D3/6-31G(d),SDD in acetonitrile are given in
kcal/mol.
Conclusions
In summary, we have developed the first NHC-copper(I)
catalytic system for the -activation of C≡C bonds of primary
propargyl alcohols for the efficient chemical fixation of CO2
leading to -alkylidene cyclic carbonates with atom and step
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control experiments indicate that, in comparison to the other
copper complexes investigated, the (NHC)CuCl complex is
unique in its effectiveness for the carboxylative cyclisation in
combination with CsF owing to the in-situ formation of a
(NHC)CuF species which is believed to be the key feature for the
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22 CCDC 2008623 (BPDPrCuCl) and 2008624 (A8) contain the
supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge
Crystallographic
Data
Centre
via
www.ccdc.cam.ac.uk/data_request/cif.
23 Settings for the calculation: r = 3.5 Å (radius of the sphere
around the metal center), d = 2.0 Å (bond length CNHC–Cu),
Bondi radii scaled by 1.17 and a mesh spacing of 0.10 Å was
used to scan the sphere of the buried voxels. All hydrogens
were omitted for the calculation, see: (a) A. Poater, F. Ragone,
16 For other copper-catalyzed CO2 utilizations see: (a) L. Zhang
and Z. Hou, Chem. Sci., 2013, 4, 3395–3403; (b) S. Wang, G.
8 | J. Name., 2012, 00, 1-3
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25 Preliminary screening includes environmentally benign
solvents such as: γ-valerolactone, isopentyl acetate, p-
cymene etc. obtaining in up to 20% yield (See Table S3). Due
to good solubility of most substrates in CH3CN or DCM, these
available.
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were chosen as the solvents for this study. For green solvents 31 Complexes of type LCuF–HF have been previously studied,
see: (a) T. K. Olszewski, K. Skowerski, A. Tracz and J. Bialecki,
Green Chem., 2014, 16, 1125–1130; (b) C. Sambiagio, R. H.
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Green Chem., 2016, 18, 288–296.
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26 This observation led us to investigate the presence of a silver
species (L-AgOAc) from the mixture of L-CuCl and AgOAc in
solution, which acted as the catalyst in the experiment
described in Table 1 (entry 13). The generation and fate of L-
AgOAc complex will be disclosed soon.
A well-defined, sterically encumbered, nine-membered NHC-copper(I) complex has been shown effective in the chemical fixation of CO2 into
propargyl alcohols for the formation of substituted and unsubstituted -alkylidene cyclic carbonates at room temperature.
(NHC)Cu catalysis
O
OH
BPDPrCuCl, CsF
R2
O
O
R3
+
CO2
R1
iPr
iPr
R2
CH3CN, r.t, 24 h.
R1
N
N
R3
30 examples
up to 96% isolated yield
(0.1  2 MPa)
Cu
Cl
iPr
iPr


Efficient chemical fixation of CO2

BPDPrCuCl
sterically demanding (NHC)Cu catalyst
Vbur = 57.9%

Stereoselectivity (only Z products obtained)
New unsubstituted -alkylidene cyclic carbonates
Catalyst reusability (up to 74% yield, TON = 103)




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