Silver-Catalyzed Carboxylative Cyclization of Primary Propargyl Alcohols with CO2

Article abstract of DOI:10.1021/acs.orglett.9b00156

By using silver complexes with bulky ligands such as DavePhos or N-heterocyclic carbenes, propargylic alcohols are smoothly converted with CO₂ into a unique class of unexplored cyclic alkylidene carbonates. These systems, for the first time, also achieve the direct carboxylative cyclization of primary propargylic alcohols. The silver-DavePhos catalyst is further applied for the bis-carboxylative cyclization of primary propargyl derivatives, thereby providing an effective route to a series of previously inaccessible and industrially relevant α-alkylidene cyclic carbonates.

Full text of DOI:10.1021/acs.orglett.9b00156

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Silver-Catalyzed Carboxylative Cyclization of Primary Propargyl
Alcohols with CO2
Saumya Dabral,†,∥ Bilguun Bayarmagnai,†,∥ Marko Hermsen,‡ Jasmin Schießl,§ Verena Mormul,‡
A. Stephen K. Hashmi,†,§ and Thomas Schaub*,†,‡
†Catalysis Research Laboratory (CaRLa), Im Neuenheimer Feld 584, Heidelberg 69120, Germany
‡BASF SE, Carl-Bosch-Str. 38, Ludwigshafen 67056, Germany
§Institute of Organic Chemistry, Heidelberg University, Im Neuenheimer Feld 270, Heidelberg 69120, Germany
S
* Supporting Information
ABSTRACT: By using silver complexes with bulky ligands such as DavePhos or
N-heterocyclic carbenes, propargylic alcohols are smoothly converted with CO2
into a unique class of unexplored cyclic alkylidene carbonates. These systems, for
the first time, also achieve the direct carboxylative cyclization of primary
propargylic alcohols. The silver-DavePhos catalyst is further applied for the bis-
carboxylative cyclization of primary propargyl derivatives, thereby providing an
effective route to a series of previously inaccessible and industrially relevant α-
alkylidene cyclic carbonates.
xovinylene carbonates (EVCs) are an important class of
Scheme 1. Synthetic Routes to 4-Methylene-1,3-dioxolan-2-
one (2)
E
cyclic carbonates that find broad applications as biological
compounds, pharmaceuticals, and agrochemicals.1 They are
attractive chemical intermediates or precursors in organic
synthesis.2 The five-membered secondary and tertiary EVCs
also form polymerizable building blocks for the synthesis of
poly(β-hydroxyurethane)s and poly(carbonates)s.1,3
Substantial efforts have been made to add CO2 to
propargylic alcohols, to produce the corresponding cyclic
carbonates with exocyclic double bonds.4,5 Despite a broad
series of catalytic systems being reported over the recent
years,6−12 their scope is still limited to easily cyclizable,
secondary9 and tertiary6−12 propargylic alcohols. To the best of
our knowledge, only one example is known, where a primary
propargylic alcohol was cyclized with CO2 to the correspond-
ing exovinylene carbonate, but in only 8% yield.13 Further
insights into the mechanism of these carboxylative cyclizations
were reported by Inoue and Yamada.4,10
The synthesis of unsubstituted EVCs is very attractive,
owing to their desirable properties in material science. For
example 4-methylene-1,3-dioxolan-2-one (2) (Scheme 1) has
already been investigated as an additive that improves the high
temperature performance of lithium ion batteries.11 While the
synthesis of the iodomethylene derivative of 2 is reported using
stoichiometric amounts of tBuOI,14 unfortunately, due to the
reduced reactivity displayed by the unsubstituted propargylic
alcohols,9,12 until now there exist only three methods to access
2 in isolable yields (Scheme 1, right). The first synthesis
published by Trost et al. (1983) is based on the ozone
mediated elimination reaction of 4-(phenylselenenyl)-methyl-
1,3-dioxolan-2-one, (Scheme 1, right top).15 The other two
routes, reported by Gagosz et al. (2006)16 and Nishizawa et al.
(2006),17 follow a gold or an undesirable mercury catalyzed
rearrangement of propargylic tert-butyl carbonate respectively
(Scheme 1, right, middle and bottom). These approaches are
neither very atom-economic nor sustainable: First propargylic
tert-butyl carbonate is preformed in an additional step starting
from alkynyl bromides and di-tert- butyl dicarbonate, followed
by the loss of the tert-butyl group as isobutene in the successive
cyclization reaction. These relatively expensive routes have so
far prevented the industrial use of 2, despite its interesting
properties.
Received: January 14, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b00156
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Organic Letters
Letter
To develop a simpler route to the α-unsubstituted class of
exovinylene carbonates, we investigated their synthesis directly
from primary propargylic alcohols and CO2 (see Scheme 1,
left). Simultaneously, we were also interested in the synthesis
of unique and easily accessible bis-exovinylene carbonates
which could potentially serve as monomers for materials.
Propargyl alcohol (1) and its derivatives such as 1,4-butynediol
are produced industrially on a multithousand-ton scale by
reacting acetylene with formaldehyde.18
Assuming that the cyclization of a α-substituted secondary
or tertiary propargyl alcohol was accelerated as a result of the
Thrope−Ingold effect,19 we were convinced that a bulky donor
ligand in combination with silver(I) salt (vide infra) could
induce a similar angle compression effect together with a
higher population of the conformer suitable for cyclization to
initiate the successful carboxylative cyclization of the primary
propargylic alcohols.
importance of the basicity of the counterion. For these
experiments, we could also show the principal recyclability of
the Ag-DavePhos catalyst (see section 2.5 of the Supporting
Information).
The scope of this reaction was investigated using MeCN as
the common solvent for better substrate solubility. As
determined by the NOE experiments, the internal propargylic
alcohols exclusively afforded the Z-alkylidene cyclic carbonates
(Scheme 2). Furthermore, according to the DFT calculations,
the thermodynamic preference for these Z alkylidene
carbonates was found to be low (only 0.1/0.6 kJ/mol for
products 4/5; see Supporting Information), which suggests
that the Z isomers are kinetically favored products. Both the
Ag-DavePhos and [(IPr)AgOAc] catalytic systems afforded
1
EVC 4 in comparable H NMR yields of 78% (isolated yield:
65%) and 70% respectively. Additionally, a range of
monosubstituted alkyl and aryl derivatives were synthesized
by taking an excess amount of 1,4-butynediol and hence
surpassing the second −OH group protection/deprotection
step (see section 3 of the Supporting Information). As the
DavePhos ligand, owing to its commercial availability, is more
easy to access than the IPrAgOAc catalyst,21 the rest of the
substrate scope was investigated using the Ag-DavePhos
catalytic system.
Based on this assumption, we tested the so-called Buchwald-
phosphine-ligands in combination with AgOAc and obtained
the desired product 2 in satisfying yields of 60−75%. Linear
carbonate 3 was formed as the only byproduct, due to the ring
opening reaction of 2 with the unreacted alcohol 1 (Table 1,
a
Table 1. Optimization of Reaction Conditions
With the system in hand we were able to synthesize a series
of new exovinylenecarbonates, based on the bulk-chemical 1,4-
butyndiol (intermediate in the industrial production of
tetrahydrofuran) as compounds 4−9, 14, and 15. The
functional and/or protecting groups such as acetyl, carbonate,
benzyl, methacrylic, and O-carbamoyl were well tolerated and
afforded compounds 5−9 in moderate to high yields. This
methodology was also effective in converting the α-substituted
alcohols into their respective carbonate products in very good
yields (Scheme 2, compounds 10−13). To our delight,
extending the methodology to substrates having two primary
reactive sites such as 1,4-dibenzyl, toluene diisocyanate (TDI),
and triethylene glycol derivatives resulted in their smooth
transformation and afforded the corresponding bis-carboxy-
lated cyclic carbonates also in excellent yields (Scheme 2,
compounds 14−16). Surprisingly neither 3-methyl and 3-
phenyl substituted propargyl alcohols nor pent-3-yn-1-ol
showed reactivity toward the formation of 17, 18, and 19
under the given reaction conditions (see Supporting
Information, part 8 for details).
Since the exocyclic olefinic groups of the new difunctional
EVCs form potential monomers via selective ring-opening
reactions by various bifunctional nucleophiles such as diamines
or diols,3c preliminary studies were focused in this direction.
Although not optimized, pleasantly, the treatment of the
benzyl protected EVC 7 with an alcohol (EtOH in
combination with catalytic amounts of DBU) and a secondary
amine (pyrrolidine) afforded the linear products 17 and 18 in
very high yields of 95% and 89%, respectively (see Supporting
Information section 9). These results are indicative for the
future applicability of the above formed EVCs (Scheme 2),
which are initially based on the cheap bulk chemical 1,4-
butynediol.
b
entry
[M]
ligand
2 [%]
1
2
3
4
5
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
IPrAgOAc
AgF
JohnPhos
MePhos
DavePhos
DavePhos
DavePhos
−
DavePhos
DavePhos
DavePhos
DavePhos
DavePhos
60
70
75
95
c
d
95 (86)
99
e
6
7
8
9
10
11
93
0
20
18
AgBF4
AgNO3
AgTFA
Ag2CO3
72
a
Propargyl alcohol 1 (5 mmol), Ag (1) salt (10 mol %), and ligand
(10 mol %) in anhydrous DCM (2 mL) were pressurized with CO2 (8
b
bar) and stirred for 16 h. Yields were determined by 1H NMR
spectroscopy using anisole as an internal standard. Isolated yields in
parentheses. 5 mol % catalyst loading in 5 mL of DCM. 1 mol %
catalyst loading. 1 (0.35 mmol), IPrAgOAc (1 mol %), CD3CN (0.7
c
d
e
mL), 20 bar of CO2.
entries 1−3). Moreover, decreasing the catalyst loading from
10 mol % to 5 mol % and increasing the solvent amounts from
2 to 5 mL resulted in an overall increase in the yield of 2 to
95% (Table 1, entry 4). Moderate to high yields of EVC 2
were also achieved with other DavePhos-like phosphine and
bulky N-heterocylic carbene ligands.20 The amount of silver
(Ag) and DavePhos ligand could be further decreased to 1 mol
% without a loss in the yield (Table 1, entry 5). Excellent
selectivity for 2 was also achieved with the isolated NHC−
Ag(I) complex, [(IPr)AgOAc], at 1 mol % catalyst loading21
(Table 1, entry 6). Among the Ag(I) sources tested (Table 1,
entries 7−11), only AgF (pKa of the conjugate acid = 3.2) gave
yields comparable to those with AgOAc (pKa of the conjugate
acid = 4.7) (Table 1, entry 7), thereby highlighting the
To gain a deeper mechanistic understanding, the reaction
was followed by 31P NMR. A mixture of propargyl alcohol 1,
AgOAc, DavePhos, and CO2 in CDCl3 was found to contain
the [(DavePhos)Ag(OAc)] complex (peaks 16 and 25 ppm).
When the preformed [(DavePhos)Ag(OAc)] complex22 was
treated with propargyl alcohol 1 no change could be observed
B
DOI: 10.1021/acs.orglett.9b00156
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 2. Scope of the Cyclizative Carboxylation of Propargyl Alcohols
a
AgOAc (2 mol %) and DavePhos (2 mol %). Yields reported after column chromatography.
by 31P NMR: This indicates that no acetate−propargyl
alcoholate ligand exchange took place. The reaction was
further studied by in situ IR spectroscopy. New peaks at 1836
cm−1 (ν(CO) stretching mode) and at 1695 cm−1 (ν(C
C) stretching mode) were observed which are in line with the
formation of 2 (see section 7 of the Supporting Information).
Within 3 h the intensity of the peaks no longer increased, and
indeed the H NMR confirmed the full conversion of the
reaction.
We further studied the mechanism of the carboxylative
cyclization of 1 by DFT (see Scheme 3). The silver(I)
replaces the acetate on complex A to form intermediate B and
acetic acid. The intermediate B is lower-lying compared to the
hemiester of the propargyl alcohol and is relatively more stable
by 22.9 kJ/mol. As mentioned earlier (see Table 1), this step is
only possible for certain counterions. For silver salts of acids
with a lower pKa, the deprotonation of the hydrogen carbonate
is hindered and lower yields are observed. Furthermore, B can
rearrange to the alkyne coordinating complex C which has
side-on coordination of the carbonate and is endergonic by
36.0 kJ/mol with respect to A and CO2/propargyl alcohol.
Intermediate C is the structural precursor which undergoes
cyclization, via TS-D or TS-E, leading to either the E or Z vinyl
silver isomer. The latter is preferred by 1.2 kJ/mol, which is in
agreement with our experimental results, where only the Z
isomer is obtained when using internal alkynes (see Scheme 3).
Note that the correct diastereoselectivity is only obtained when
including solvent effects of acetonitrile within the COSMO
solvation model (see Supporting Information). Lastly,
complexes F/G irreversibly release the vinylene carbonate
upon proto-demetalation through acetic acid to regenerate the
initial complex A. This step is exergonic by 66.1 kJ/mol.
In conclusion, we established a system that allows, for the
first time, the straightforward synthesis of a wide range of
unsubstituted mono- and di-α-alkylidene cyclic carbonates
from their corresponding alkynols. The reaction conditions are
mild and, at room temperature, afford the desired products in
65−95% yield. The broad substrate scope can also be extended
to the unsubstituted α-alkylidene cyclic carbonates. The low
catalyst loading and recyclability of the catalyst system add to
the key features of this new methodology and are further
supported by DFT calculations. The development for ring-
opening polymerization reaction of the newly formed α-
alkylidene cyclic carbonates by various nucleophiles is
currently underway in our laboratories.
1
Scheme 3. Catalytic Pathway of Carboxylative Cyclization of
a
Propargyl Alcohol with CO2 Using Ag(I)-DavePhos
a
All energies indicated are free enthalpies in kJ/mol.
DavePhos acetate complex A was taken as the starting point for
the calculations. A variant of this reaction with ZnI2/NEt3 has
been extensively investigated by Ma et al.23 The selectivity
between the five- and six-membered carbonates and the effect
of solvents as explained by them was also researched by
Yamada et al. for the silver-catalyzed reactions with DBU as the
base.5 According to our calculations, the formation of
propargyl hydrogen carbonate from CO2/propargyl alcohol is
endergonic by 53.6 kJ/mol. This is generally expected, as the
organic hydrogen carbonate esters are not stable. The
intermediary formed propargyl hydrogen carbonate, however,
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b00156.
Experimental, characterization, and computational de-
tails (PDF)
C
DOI: 10.1021/acs.orglett.9b00156
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: thomas.schaub@basf.com.
ORCID
A. Stephen K. Hashmi: 0000-0002-6720-8602
Thomas Schaub: 0000-0003-2332-0376
Author Contributions
∥S.D. and B.B. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
CaRLa (Catalysis Research Laboratory) is cofinanced by the
Heidelberg University and BASF SE.
■
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D
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