Expanded Ring NHC Silver Carboxylate Complexes as Efficient and Reusable Catalysts for the Carboxylative Cyclization of Unsubstituted Propargylic Derivatives

Article abstract of DOI:10.1002/cssc.202002822

Stabilized by a bulky N-heterocyclic carbene [^BPDPr, 1,3-bis(2,6-diisopropylphenyl)-1,3-diazonine-2-ylidene] ligand, new silver carboxylate complexes of the form ^BPDPrAgO₂C-R (R=Me, Ph) have been synthesized and fully char

Full text of DOI:10.1002/cssc.202002822

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Expanded Ring NHC Silver Carboxylate Complexes as
Efficient and Reusable Catalysts for the Carboxylative
Cyclization of Unsubstituted Propargylic Derivatives
Alejandro Cervantes-Reyes,*[a] Tobias Saxl,[a] Philipp M. Stein,[a] Matthias Rudolph,[a]
Frank Rominger+,[a] Abdullah M. Asiri,[b] and A. Stephen K. Hashmi*[a, b]
Stabilized by a bulky N-heterocyclic carbene [BPDPr, 1,3-bis(2,6-
diisopropylphenyl)-1,3-diazonine-2-ylidene] ligand, new silver
carboxylate complexes of the form BPDPrAgO2C-R (R=Me, Ph)
have been synthesized and fully characterized in solution and
in the solid state and implemented as sole catalysts (base-,
additive-, and, in some cases, solvent-free) in the challenging
fixation of carbon dioxide to unsubstituted propargylic deriva-
tives for the synthesis of oxazolidinones and α-methylene cyclic
carbonates. Derived from X-ray diffraction studies, the molec-
ular geometry and the concept of buried volume were
employed to describe the structural and steric features of these
silver complexes. Their stability and efficiency as catalysts have
been demonstrated by the synthesis of 29 carboxylation
products (72–98% yield) at low catalyst loadings (0.01–
1.5 mol%). Characteristics are high turnover numbers (up to
9400), catalyst recyclability (up to 96% yield after the 7th cycle
with no decomposition of the silver complex), and the
possibility to scale-up the reaction.
Introduction
catalyzed organic transformations proved their potential as
catalysts as well.[6,7]
N-heterocyclic carbenes (NHCs) are an important class of
ligands for transition metals.[1] The structural tunability of steric
and electronic properties from either the backbone ring or the
N-substituents enables distinct catalytic activities and
selectivities.[2] Within these manifolds, new carbene ligands
based on NHC backbones with larger ring sizes (more than five
atoms) have been designed.[3] These expanded ring NHC
(erNHC) ligands showcase enhanced σ-donor ability and a
superior steric bulkiness, which provides high stability and
protection to the metal center and consequently enlarges their
catalytic lifetime.[4] In this regard, NHC silver complexes were
initially considered as useful organometallic synthons due to
their ligand transfer ability,[5] and a growing number of silver-
Carbon dioxide (CO2) is the major greenhouse gas produced
on earth, and its emission to the atmosphere has been related
to the climate change.[8] Due to its high abundance, lack of
toxicity, and low-cost, the attained interest of CO2 as C1 building
block in both academia and industry relies on its tremendous
potential as renewable carbon source.[9] In this respect, the
fixation of CO2 to propargylic alcohols or propargyl amines,
promoted by organometallic catalysts, has received consider-
able attention during the last years.[10] This atom- and step-
economic strategy gives access to privileged heterocyclic motifs
such as α-methylene cyclic carbonates and oxazolidinones,
which can be employed as reactive solvents, additives in Li-ion
batteries, synthetic intermediates for the synthesis of polymers
and fine chemicals, or can be found as substructures of
biologically relevant molecules (e.g., pharmaceuticals).[11,12] In
particular, the fixation of CO2 to propargylic derivatives has
been investigated in seminal work of Yamada, Sekine, and co-
workers in the context of silver and gold catalysis.[7b,13]
The recent years have witnessed the growing use of
several NHC metal catalysts in environmentally benign reac-
tions due to their compatibility with eco-friendly solvents,
solubility in water or under solvent-free conditions, enabling a
straightforward recovery that eases their recyclability.[14]
Although many examples of supported or immobilized metals
on 3D frameworks [e.g., metal-organic frameworks (MOFs)],
phosphines, ionic liquids, or NHCs, are effective for the
cycloaddition of CO2 with propargylic derivatives, many of
them are restricted to the use of bases or additives, and allow
the conversion of only secondary or tertiary propargylic
substrates, hence limiting their application profile.[15–17] Indeed,
the Thorpe–Ingold effect in substrates bearing gem-dialkyl
substituents in propargylic position offers the optimal geome-
[a] A. Cervantes-Reyes, T. Saxl, P. M. Stein, Dr. M. Rudolph, Dr. F. Rominger,+
Prof. Dr. A. S. K. Hashmi
Organisch-Chemisches Institut
Heidelberg University
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
E-mail: alexcervantes803@gmail.com
hashmi@hashmi.de
[b] Prof. Dr. A. M. Asiri, Prof. Dr. A. S. K. Hashmi
Chemistry Department, Faculty of Science
King Abdulaziz University
Jeddah 21589 (Saudi Arabia)
[+] Crystallographic investigation.
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cssc.202002822
This publication is part of a Special Collection with all Chemistry Europe
journals on the “International Symposium on Homogeneous Catalysis”.
Please follow the link for more articles in the collection.
© 2021 The Authors. ChemSusChem published by Wiley-VCH GmbH. This is
an open access article under the terms of the Creative Commons Attribution
Non-Commercial NoDerivs License, which permits use and distribution in
any medium, provided the original work is properly cited, the use is non-
commercial and no modifications or adaptations are made.
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try for the cyclization step, while for unsubstituted propargylic
derivatives HC�CÀ CH2À YÀ H, the YÀ H (Y=NR, O) and π-
activation of the C�C bond are significantly more difficult.[18]
Therefore, the development of mild, operationally simple and
alternate carboxylation protocols that operate with a wide
substrate scope by employing readily available catalysts that
significantly improve the synthetic and catalytic processes, are
highly desirable.
In this work, we present the synthesis, structure evaluation,
and spectroscopic characterization of the first expanded ring
NHC silver carboxylate complexes, as well as their catalytic
properties in the cycloaddition of CO2 with primary propargylic
alcohols or propargyl amines. These reactions can be conducted
at room temperature and do not require a base or another
additive, thus representing a user-friendly protocol for obtain-
ing α-methylene cyclic carbonates or oxazolidinones from
readily available precursors. Furthermore gram-scale reactions
and catalyst recyclability experiments were conducted to shed
light upon the potential relevance of the silver complexes as
tool for environment-friendly industrial applications.
Scheme 1. General synthesis of BPDPrAgOAc (1) and BPDPrAgOBz (2).
the solid state by IR spectroscopy, elemental analysis, and
single-crystal X-ray diffraction (XRD) analysis. Both complexes
are bench-stable colorless solids with melting points above
°
220 C and can be indefinitely stored in the freezer. No
decomposition signs like “silver mirror” or colloidal black spots
were visible when stored under light at room temperature.
Complex 1 is soluble in CH2Cl2, CHCl3, CH3CN, and THF, among
other commonly used organic solvents. Complex 2 has good
solubility in THF or AcOEt, is sparingly soluble in CH2Cl2 or
CHCl3 (<12 mgmLÀ 1), and remains essentially insoluble above
5.0 mm concentration in CH3CN at room temperature. Both
complexes are insoluble in Et2O or hydrocarbon solvents (e.g.,
pentane, hexane, petroleum ether). In the mass spectra [matrix
assisted laser desorption ionization (MALDI+)] the presence of
peaks belonging to the (NHC-H)+ and (NHC-Ag)+ fragments
indicate a linear mononuclear geometry. Other detectable
[(NHC)2-AgO2C-R]+ or [(NHC)2Ag]+ fragments suggest that
intermolecular silver-hydrogen or silver-silver interactions
might exist.[23] Based on 13C NMR spectra these interactions
might be trivial in solution, since the presence of the carbene
carbon (CNHC) signal as two doublets at around δ=232 ppm
confirms that the complexes do not exhibit a fluxional
behavior of the silver-carbene carbon (Ag-CNHC) bond.[5,23b] The
observed CNHC resonances are significantly shifted to lower
fields compared to their IPr analogues (δ�184 ppm).[22] The
couplings 1J(107Ag-13C)=238–241 Hz and 1J(109Ag-13C)=275–
278 Hz, increased with respect to the NHC silver bromide
complex [13C NMR δ=230.0 (dd,1J(107Ag-13C)=228 Hz, 1J-
Results and Discussion
Synthesis and characterization of silver complexes
The initial motivations for this study stemmed from our recent
discovery that the sterically bulky BPDPr [1,3-bis(2,6-diisopro-
pylphenyl)-1,3-diazonine-2-ylidene] ligand[19] was able to effi-
ciently stabilize a catalytically active copper fluoride species
for the π-activation of the C�C bond of propargylic alcohols
that, in the presence of CO2, led to the cycloaddition products.
This strategy provided the first effective and diastereoselective
copper-catalyzed carboxylative cyclization of unsubstituted
propargylic alcohols.[20] To the best of our knowledge, there
exists at most one silver-based catalyst for the conversion of
these challenging primary propargylic substrates in the
current literature.[21] Building on these precedents, we won-
dered whether a silver complex stabilized by the BPDPr ligand
could provide a practical catalytic tool for activating the C�C
bond in unsubstituted propargylic derivatives. Derived from
the attractive structural and steric features of this carbene
ligand and the known catalytic utility of other silver
carboxylates,[22] we envisioned the preparation of BPDPrAgO2C-
R (R=Me, Ph) complexes to subsequently evaluate their
catalytic performance in CO2 fixation to propargylic substrates
at room temperature in the absence of any co-catalyst (e.g.,
base, additive).
(
109Ag-13C)=263 Hz], demonstrate a significant influence of the
carboxylate anion to the Ag-CNHC bond upon halide abstrac-
tion.
The values for the gyromagnetic ratio (γ=1.15) of both
NMR-active silver isotopes in 1 and 2 are still in accordance to
the spin-spin interactions of the 109Ag/107Ag with the singlet
[4c,5f,24]
CNHC
.
IR absorption frequencies, ν(CO2), for the asymmetric
Our study began with the preparation of the known silver
complex BPDPrAgBr followed by ligand transfer reactions
(Scheme 1) to silver carboxylate salts (i.e., acetate, benzoate).[5f]
(νasym) and symmetric (νsym) stretches of the carboxylate where
found at νasym =1607 and 1599 cmÀ 1, and νsym =1386 and
1371 cmÀ 1 for 1 and 2, respectively. The difference between
νasym and νsym values (Δν) in 1 (Δν=228 cmÀ 1) and 2 (Δν=
221 cmÀ 1) are significantly larger than those of the correspond-
Initial attempts to synthesize complex
1 via the direct
deprotonation of the NHC-HBF4 salt resulted in a moderate
yield under the selected experimental conditions.
Transmetalation reactions afforded the corresponding
silver complexes in good yields and their identity was
confirmed in solution by NMR spectroscopy and MS, and in
ing
non-coordinated
silver
carboxylates
(Δν=129–
134 cmÀ 1)[22,25] consistent with a monodentate O-coordination of
the carboxylate unit.[26]
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Single crystals suitable for X-ray analysis were obtained by
structures of 1 and 2 are shown in Figure 1a. With the
information from the CIF files, the buried volume values and
topographic maps (Figure 1c) were calculated using SambVca
2.1 (with the default settings: bondi radii scaled by 1.17,
sphere radius 3.5 Å, mesh spacing 0.10 Å, distance from the
center 2.0 Å, hydrogen atoms omitted).[28] Selected structural
parameters are deposited in Table 1. The X-ray crystal
structures denote a monodentate O-coordination of the
carboxylate anion to the silver atom and this was previously
deducted from the stretching frequencies in their IR spectra. It
is interesting to note, that neither the nature of the
carboxylate anion nor the N-aryls seem to alter the Ag-CNHC
bond length. Noteworthy, a significant deviation from linearity
was found in the CNHC-Ag-O angles for both complexes (by
slow evaporation of solvent mixtures (e.g., THF/hexane,
CH2Cl2/pentane) from saturated solutions of the complex at
room temperature overnight.[27] The solid-state molecular
°
°
�8 ) and the N-CNHC-N angles (118–119 ) overcome those of
other silver carboxylate complexes bearing five-membered
[22]
°
(IPr, SIPr, IPent) ligands (108–113 ).
As illustrated in Figure 2, the twisting of the C2-symmetric
biphenyl moiety influences the folding of the adjacent meth-
ylene spacers giving origin to a distorted non-planar NHC
backbone, characteristic of the expanded ring carbene
ligands.[2a,19] The torsion (dihedral angle) between both N-aryl
planes and the NÀ N axis reveals a considerably more twisted
°
molecular geometry of 1 (α=35.09 ) than that of 2 (α=
26.90 ) and this could be explained by a stabilizing effect of
°
the carboxylate anions. These also affect the steric distribution
around the silver center as reflected by the buried volume
values (2, VBur =54.5%; 1, VBur =52.9%). In 1, the methyl group
of the acetate exerts a poor steric influence over the ortho iso-
propyl substituents of the N-aryls in contrast to the phenyl
group, in silver benzoate complex 2, which diminishes the
distortion of the N-aryls and consequently favors the homoge-
neous structural arrangement of the NHC ligand boosting the
overall steric distribution of the complex.
Cycloaddition of CO2 with unsubstituted propargylic alcohols
or propargyl amines
Preliminary experiments confirmed our initial hypothesis. In
the cyclization of a test substrate (3) with CO2 under
atmospheric pressure at room temperature, using 5 mol%
loading of BPDPrAgOAc (1) as the catalyst, the expected
product was formed in 15% yield (see the Supporting
Information for details). By screening both catalysts in a set of
conditions we established complex BPDPrAgOBz (2) as the best
choice for the carboxylative cyclization of propargylic alcohols
Figure 1. Solid-state molecular structure (ORTEP diagram) from a front view
°
(a), side view at 90 rotation over the C2-axis (b), and steric map (c) of
BPDPrAgOAc (1, left) and BPDPrAgOBz (2, right), respectively. Ellipsoids are set
at 40% probability level. Hydrogen atoms and one crystallized H2O molecule
(in 2) are all omitted for clarity.
Table 1. Selected geometrical parameters (bond lengths and angles) of
complexes 1 and 2.
BPDPrAgOAc (1)
BPDPrAgOBz (2)
Ag-CNHC [Å]
Ag-O [Å]
2.089(2)
2.112(2)
118.4(2)
172.07(9)
35.09
2.089(3)
2.122(2)
119.3(3)
172.88(11)
26.90
°
Figure 2. Bottom view along C2-axis (z-axis) of 1 in the solid state
accompanied by its bidimensional representation. Hydrogen atoms and
[OAc] anion are omitted for clarity.
N-CNHC-N [ ]
°
CNHC-Ag-O [ ]
°
CAr-N2···N5-CAr’ (α) [ ]
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(Table S1). We therefore conducted further optimization ex-
periments with complex 2 as the catalyst. The experiments
were carried out for 18–24 h in CH3CN as the solvent
(Table S2). From beginning to end the catalyst remained
insoluble at the examined concentrations of substrate (1.0 m).
Nevertheless, an unanticipated catalytic activity was observed,
and the product was obtained in all cases in the CH3CN phase
and the NMR analysis of the solid residue corresponded to the
intact silver complex. The reaction was promoted at lower
catalyst loadings as expected albeit at the expense of higher
CO2 pressure. Along with these experiments, we could not
detect any traces of the ring-opened side product. These
observations indicate that the limiting parameter is not the
catalyst loading, but rather the applied CO2 pressure. At
present, we believe that the bulky NHC ligand stabilizes the
transient silver-alkenyl species within the catalytic cycle, thus
preventing the ring opening pathway.[20]
[Scheme 2, Eq. (1)]. A good efficiency was maintained when
running the reaction on a 10 mmol scale under 2.3 MPa CO2
pressure to furnish 3g (2.28 g) in 89% yield. Nucelar Over-
hauser effect (NOE) experiments confirmed the stereochemis-
try of the Z-alkylidene products, consistent with previous
results using bulky silver and copper catalysts.[20,21]
Prompted by our success with unsubstituted propargylic
alcohols as substrates, we then sought to explore the trans-
formation of unsubstituted propargyl amines 5 (Figure 4).
Under the optimized reaction conditions described in
Table S2, entry 10, we then evaluated the generality of the
silver-catalyzed carboxylative cyclization (Figure 3). A variety
of unsubstituted propargylic alcohols (3), derived from 2-
butyn-1,4-diol, a low-cost and commercially available chem-
ical, were converted. Various substitution patterns on the aryl
ring in 3 were tested. The presence of alkyl (4c, 90%) or aryl
(4f, 84%) substituents in the para position of the aryl ring of 3
provided slightly higher yields than for an electron-with-
drawing group (p-CF3, 4p, 72%). Likewise, the presence of
para-substituted halogen gave a good yield (4k, 84%). No
significant difference was observed for ortho-, meta-, and para-
chloro-substituted aryl moieties (78–84% yield). And the
presence of an alkyl ortho substituent in 3 led to a slightly
diminished yield (e.g., 4d, 83%), compared to its para-
substituted isomer (4c, 90%) suggesting a marginal steric
effect. Encouraged by these results, we assessed a gram scale
experiment using complex 2 as catalyst under our reaction
conditions to gain more insight into its catalytic performance
Scheme 2. Scale-up experiments.
Figure 4. Substrate scope for solvent-free carboxylative cyclization of
propargyl amines 5. [a] Isolated yields are presented. [b] Performed in the
absence of solvent for 12 h.
Figure 3. Survey of unsubstituted propargylic alcohols 3 in the carboxylative
cyclization. [a] Isolated yields are presented.
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Silver complexes 1 and 2 were also used as catalysts.
Preliminary screenings (see Supporting Information, Table S3)
showed that 1 mol% catalyst could promote the transforma-
tion of 5a into 6a in 99% NMR yield after 10 min using CO2 at
atmospheric pressure. Under the studied experimental con-
ditions full and clean conversions were observed, which
indicates the resilience of the silver species in non-dried
solvents, without the need for the prevention of oxygen. In
the absence of a catalyst, no conversion of the propargyl
amine occurred, and the ligand-free AgOAc-catalyzed reaction
(1 mol%, 24 h, r.t.) required the addition of 1,8-diazabicyclo
[5.4.0]undec-7-ene (DBU; 5 mol%) to deliver a commensurable
yield. This observation, together with the results disclosed in
Figure 4, clearly evidence the intimate interplay between the
carbene ligand and substrate in our catalytic proposal. The
influence of the catalyst loading (Table S4) was therefore
investigated. The conversion of substrate 5a did not decrease
with reduced amounts of the catalyst (0.1–0.5 mol%) and 90%
yield was still observed by using only a 0.05 mol% catalyst
loading after 2 h. The stability and good solubility of the
complexes in CH2Cl2, allowed for the preparation of stock
solutions to employ even lower loadings of the catalyst. Based
on these preliminary experiments, we found that the use of a
0.01 mol% loading of complex 1 was found to be ideal for the
catalysis.
To verify the applicability of the optimized conditions, a
library of propargyl amines was subjected to the current silver
catalysis at room temperature. In a typical experiment, a
Teflon-coated screw-cap vial was charged with substrate,
solvent and catalyst, and a CO2-filled balloon (�1.5 L) was
coupled via cannula and the mixture was stirred at room
temperature until no starting material was detected [as judged
by thin layer chromatography (TLC) or GC-MS analysis]. The
results are summarized in Figure 4. Isolated yields are
presented after flash chromatography, and for selected
substrates, isolated yields of solvent-free reactions are pre-
sented (in parentheses). In all examined cases, the conversion
was higher than 98% (as revealed by NMR analysis of crude
samples). Total reaction times span from 0.5 to 16 h, although
for some substrates (5n, N-phenyl, and 5p, N-2-adamantyl) no
product or only traces were observed after 24 h. Interestingly,
upon exposure of unprotected propargyl amine or N-tosyl-
protected propargyl amine (5o) no detectable product was
obtained. This can be explained by the low N-nucleophilicity
of both the aniline-type nitrogen in 5n and the tosyl-
protected 5o, and the significant steric bulk of the N-
substituent in 5p. As predicted, substrates with additional
gem-substituents in the propargylic position, smoothly af-
forded the carboxylation products 6l and 6m in 92–98% yield
after 1.5 h reaction. The X-ray crystal structure of 6k is
presented in Figure 5.
Figure 5. X-ray crystal structure (ORTEP representation) of 6k at 45%
thermal probability.[26]
oped NHC silver complexes are a promising and more efficient
catalytic alternative.
It is worth noting that, our described protocols can be
carried out at room temperature under base-, additive-, or
(even) solvent-free conditions, constituting an additional
bonus from a practical and operational point of view. On the
basis of our latter results, we anticipated that our carboxylative
cyclization protocol could be amenable for a three-component
reaction. Thus, an experiment was run on a 1 mmol scale using
1-ethynyl-1-cyclohexanol and n-butylamine as the substrates,
in the presence of 0.1 mol% catalyst under our standard
reaction conditions [Scheme 2, Eq. (2)]. Nearly full conversion
of both substrates was observed and 94% isolated yield of
oxazolidinone 3 l’ was obtained. Like the activation of C�C
bonds in propargylic alcohols at the gram-scale was found to
be clean and efficient, the use of propargyl amines as
substrates under silver catalysis at a large-scale is highly
relevant. In this respect, a scale-up experiment was carried out
using propargyl amine 5g (10 mmol) with 0.1 mol% loading of
complex 1 under CO2 at atmospheric pressure producing 6g
(2.16 g) in 91% yield [Scheme 2, Eq. (3)].
With regard to the mechanism, the reaction should follow
the reaction path already computed for other ligands, with the
cyclization step being the rate-limiting step.[21]
Catalyst recycling
From the point of view of sustainable catalysis, it is interesting
that the silver complexes could also be recovered and
reutilized (Figure 6). α-Methylene cyclic carbonates are usually
soluble in polar organic solvents (e.g., CH3CN, AcOEt, etc.)
whereas the silver catalyst 2 remains insoluble above 1.0 m
concentration of the substrate/product. This case of heteroge-
neous catalysis opens up the opportunity for product separa-
tion from the catalyst by simple filtration through a membrane
or decanting the liquor containing the product, which makes
it possible to recover and recycle the silver catalyst. To validate
this hypothesis, the carboxylative cyclization experiments
were performed with 3j under our conventional protocol
Strikingly, the catalytic system did not allow the conver-
sion of primary propargyl amines bearing internal alkynes
(e.g., methyl- or phenyl-terminated alkynes); nonetheless,
when compared to other successful state-of-the-art silver-
based catalytic systems for the carboxylative cyclization of
unsubstituted propargyl amines (Figure S1), the herein devel-
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Conclusion
New silver carboxylate complexes bearing a sterically demand-
ing carbene ligand of expanded ring backbone have been
synthesized and fully characterized. A task-specific catalytic
application, the challenging carboxylative cyclization of un-
substituted propargyl derivatives, demonstrated their poten-
tial as efficient catalysts. The obtained results suggest that the
use of an expanded ring N-heterocyclic carbene (NHC)-silver
carboxylate-based architecture is a promising catalytic ap-
proach for the π-activation of C�C bonds[21] for the chemical
fixation of CO2. The molecular geometry and steric features of
the bulky NHC ligand and the presence of a stabilizing
monodentate carboxylate unit in a linear arrangement with
the silver center, provide a unique stability to the complex
enhancing its catalytic activity. The easy catalyst recovery after
the clean transformation, can make these complexes adequate
for environmentally friendly metal-catalyzed reactions because
of their recyclability property. In consequence, we have
synthesized stable and robust NHC silver complexes for the
cycloaddition of CO2 with primary propargylic alcohols or
propargyl amines at room temperature. Without any doubt,
the success of these new silver catalysts could provide new
impulses for other related silver-catalyzed reactions.
Figure 6. Catalysts recyclability. Performed with 1.5 mol% of complex 1
(yellow bars) or 2 (blue bars) under batch conditions, and by successive
additions of substrate using complex 1 (red marks) with a final 0.16 mol%
catalyst loading.
using CH3CN as the solvent. At the end of the reaction, the
suspension was kept on the bench for 0.5 h and the solution
containing the product was separated by syringe. We checked
that this procedure also allowed the residual, unconverted
substrate to be separated from the catalyst. The solid residue
was washed with cold CH3CN to leave the active silver catalyst Experimental Section
in the reaction vessel (typical loss of catalyst in each run about
General procedure for α-methylene cyclic carbonates: A 20 mL
1.4% of the initial catalyst loading; overall 14% of the silver
were lost over the 10 runs, see the Supporting Information),
and a fresh load of substrate was added. The procedure was
repeated several times to evaluate the catalyst performance.
Acetate 1 and benzoate 2 both exhibited excellent catalytic
activity in the first run, nevertheless, the yield afforded by
catalyst 1 gradually decreased although no decomposition
was observed. We assume that upon treatment of the crude
reaction, some catalyst could be solubilized in CH3CN and
withdrawn from residue, making a lower quantity available for
the next run. Under the same experimental conditions,
complex 2 displayed a good recyclability and could be
recovered and reused at least 7 times without a strong loss of
activity (90% yield after the 7th cycle) and no decomposition
of the catalyst was observed in the NMR spectra (see the
Supporting Information, Figure S2). Given the good solubility
of 1 in CH3CN, we decided to make successive additions
instead of working up the reaction mixture after each cycle.
We selected propargylic alcohol 3z (6 mmol per addition) as
the substrate and set 1 mol% of catalyst loading under CO2
(2.3 MPa) at room temperature for 24 h. Under these con-
ditions, complex 1 displayed high catalytic activity with no
decomposition signals (as judged by NMR spectroscopy)
affording 96% yield after 7 cycles with a final catalyst loading
of 0.16 mol% and a turnover number of 584.
scintillation vial provided with a Teflon-coated stirring bar was
charged with substrate 3 (1 mmol), regular CH3CN (1 mL), and
silver complex 2 (11.6 mg, 1.5 mol%) at room temperature under
open-air conditions. Vial was transferred into an autoclave and
pressurized with CO2 at approximately 2.0 MPa for 18 h under
stirring (>550 rpm). Overpressure was slowly released; the
suspension was filtered through a plug of celite to remove the
catalyst. In some cases, we observed that the presence of silver
catalyst traces in the crude product after filtration can catalyze its
decomposition and the release of benzylic alcohol derivative (as
detected by NMR spectroscopy). To avoid this, the crude product
was taken up in AcOEt (2.5 mL), stirred in the presence of
activated carbon (spatula tip) for 0.5 h and again filtering through
a plug of celite. Additional CH3CN (1 mL) was used to wash the
solid residue and the plug of celite. The filtrate was concentrated
in vacuo and crude products were purified by flash chromatog-
raphy (AcOEt/petroleum ether, 2:8) to yield α-methylene cyclic
carbonates as colorless oils or a colorless solid (4f).
(Z)-methyl (2-(2-oxo-1,3-dioxolan-4-ylidene)ethyl) carbonate (4a):
Colorless oil (163 mg, 87% yield). 1H NMR (301 MHz, CDCl3): δ=
4.98–4.91 (m, 3H), 4.71–4.67 (m, 2H), 3.70 ppm (s, 1H).13C NMR
(76 MHz, CDCl3)): δ=155.40, 151.96, 146.11, 96.83, 67.43, 60.82,
54.82 ppm.
(Z)-4-(2-(benzyloxy)ethylidene)-1,3-dioxolan-2-one (4b): Colorless
oil (196 mg, 89% yield). 1H NMR (301 MHz, CDCl3): δ=7.28–7.18 (m,
5H), 4.90–4.85 (m, 3H), 4.45 (s, 2H), 4.14–4.10 ppm (m, 2H). 13C NMR
(76 MHz, CDCl3): δ=152.28, 143.95, 137.78, 128.49, 127.88, 100.32,
72.77, 67.34, 63.36 ppm.
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Keywords: carbon dioxide · cyclic carbonates · N-heterocyclic
carbenes · oxazolidinones · silver
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Manuscript received: December 8, 2020
Revised manuscript received: February 28, 2021
Accepted manuscript online: March 9, 2021
Version of record online: ■■■, ■■■■
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FULL PAPERS
A. Cervantes-Reyes*, T. Saxl, P. M.
Stein, Dr. M. Rudolph, Dr. F.
Rominger, Prof. Dr. A. M. Asiri,
Prof. Dr. A. S. K. Hashmi*
1 – 9
Expanded Ring NHC Silver Car-
boxylate Complexes as Efficient
and Reusable Catalysts for the
Carboxylative Cyclization of Un-
substituted Propargylic Deriva-
tives
CO2 fixation: The first expanded ring
N-heterocyclic carbene silver carboxy-
late complexes are efficient and
reusable catalysts in the chemical
fixation of carbon dioxide to unsubsti-
tuted propargylic derivatives under
homogeneous or heterogeneous
reaction conditions.