NHC-Gold(I) complexes as effective catalysts for the carboxylative cyclization of propargylamines with carbon dioxide

Article abstract of DOI:10.1021/om400949m

NHC-gold(I) complexes promote carboxylative cyclization of a range of propargylic amines to afford (Z)-5-alkylidene-1,3-oxazolidin-2-ones in methanol under neutral and mild conditions. The highly active and robust catalyst permits CO₂ utilization under a mixed gas atmosphere containing CO or H ₂. As a key intermediate, a new alkenylgold(I) complex was successfully identified from a stoichiometric reaction of Au(OH)(IPr) and 1-methylamino-2-butyne in THF. The methanol solvent influences the formation of catalytically active cationic gold species and facilitates protodeauration from the alkenylgold complex to release the cyclic urethane product due to in situ generation of methylcarbonic acid in the presence of CO₂.

Full text of DOI:10.1021/om400949m

Communication
pubs.acs.org/Organometallics
NHC−Gold(I) Complexes as Effective Catalysts for the Carboxylative
Cyclization of Propargylamines with Carbon Dioxide
Shun Hase, Yoshihito Kayaki, and Takao Ikariya*
Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, O-okayama
2-12-1-E4-1, Meguro-ku, Tokyo 152-8552, Japan
S
* Supporting Information
ABSTRACT: NHC−gold(I) complexes promote carboxylative
cyclization of a range of propargylic amines to afford (Z)-5-
alkylidene-1,3-oxazolidin-2-ones in methanol under neutral and mild
conditions. The highly active and robust catalyst permits CO2
utilization under a mixed gas atmosphere containing CO or H2. As
a key intermediate, a new alkenylgold(I) complex was successfully
identified from a stoichiometric reaction of Au(OH)(IPr) and 1-methylamino-2-butyne in THF. The methanol solvent
influences the formation of catalytically active cationic gold species and facilitates protodeauration from the alkenylgold complex
to release the cyclic urethane product due to in situ generation of methylcarbonic acid in the presence of CO2.
ransformation of carbon dioxide (CO2) into value-added
Scheme 1. Carboxylative Cyclization of Propargylamines
T
compounds has attracted significant attention as a
principal approach to utilize an abundant, inexpensive, and
renewable C1 resource.1 As one of the reliable methods,
conversion of thermodynamically less stable carbamic acid
family into robust functionalized molecules has been broadly
explored.2 With regard to the facile formation of N,N-
dialkylcarbamic acids from secondary amines in supercritical
CO2, we have reported effective CO2 fixation protocols
affording urethanes and ureas.3,4 For example, carboxylative
cyclization of propargylamines (1) with CO2 has been
successfully achieved under supercritical conditions even in
the absence of catalysts to give 5-alkylidene-1,3-oxazolidin-2-
ones (2).3 Although the carboxylation proceeds regio- and
stereoselectively through intramolecular addition of carbamic
acids to C−C triple bonds, some drawbacks still remain
concerning limited substrate scopes and severe pressurization
conditions.5
We initially examined the reaction of 1-methylamino-2-
butyne (1a; R1 = R2 = CH3) with 1 atm of CO2 in the presence
of group 11 metal compounds with a substrate/catalyst (S/C)
ratio of 50 in methanol at 40 °C for 15 h (the standard
conditions in Table 1). In contrast to the unsatisfactory results
obtained with AuCl (entry 1), the NHC−gold complex
AuCl(IPr) (IPr = 1,3-bis(2,6-diisopropylphenyl)-imidazol-2-
ylidene) was found to promote the carboxylation effectively to
afford a 5-exo-dig cyclization product, 5-ethylidene-1,3-
oxazolidin-2-one (2a), in 91% yield (entry 2).8 The reaction
proceeded independently of the CO2 pressure above 1 atm. In
our previous work on catalyst-free carboxylation under
supercritical conditions,3 such an alkyl-substituted internal
propargylic amine could not be transformed into the urethane
product. The 1H NMR spectrum of the reaction mixture clearly
showed a quartet of triplets signal at 4.59 ppm (3JHH = 7.0 Hz,
4JHH = 2.2 Hz) due to the olefinic proton, indicating that no
other isomers were formed in the cyclization. The C−C double
bond of 2a was found to be in the Z configuration by NMR
spectroscopy and X-ray crystallographic analysis (Supporting
Information), indicating that addition of the carbamate moiety
to the alkyne proceeded predominantly in an anti fashion, as
seen in our previous report for the catalyst-free system.3 The
reaction even with the catalyst loading reduced to 0.5 mol %
From the intense research efforts in the field of catalysis with
coinage metals, various addition reactions of protic functional
groups to C−C multiple bonds have been developed.6 Indeed,
gold(I) catalysts effect intramolecular cyclization of alkynoic
acids to give lactones.7 The related carbamic acids derived from
aminoalkynes and CO2 have not been subjected to gold-
catalyzed cyclizations, whereas the other triad metals, copper
and silver, were employed for the urethane formation. Dimroth
and Pasedach originally found, nearly a half century ago, that
some copper compounds can catalyze the carboxylative
cyclization of terminal propargylamines.5a Recently, Yamada
and co-workers reported the effectiveness of silver acetate for
synthesis of cyclic urethanes from aminoalkynes under mild
conditions.5f−h Herein we disclose the carboxylative cyclization
of propargylamines catalyzed by gold(I) complexes bearing an
N-heterocyclic carbene (NHC) ligand under atmospheric
pressure of CO2 (Scheme 1) and identification of alkenylgold
species relevant to the catalytic intermediates.
Received: September 24, 2013
Published: October 2, 2013
© 2013 American Chemical Society
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Organometallics
Communication
a
a
Table 1. Carboxylative Cyclization of 1a and CO2
Table 2. Carboxylative Cyclization Catalyzed by AuCl(IPr)
b
b
entry
catalyst
solvent
yield, %
entry
substrate
R1
C2H5
R2
CH3
t, h
yield, %
1
AuCl
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
2-propanol
CH3CN
THF
0
91
85
89
83
41
61
70
84
71
52
19
0
1
1b
1c
1d
1e
1f
15
15
15
15
48
48
15
66
48
48
83
87
81
16
85
86
83
27
76
47
2
AuCl(IPr)
AuCl(IPr)
2
(CH3)2CH
(CH3)3C
H
CH3
c
3
3
CH3
d
4
AuCl(IMes)
AuCl(ItBu)
4
CH3
e
5
5
CH3
C2H5
C3H7
CH2C6H5
(CH3)2CH
CH3
6
AuCl[P(C6H5)3]
AuCl[P(OC2H5)3]
AuCl[P(C2H5)3]
AuBr(IPr)
6
1g
1h
1i
CH3
7
7
CH3
8
8
CH3
9
9
1j
C6H5
C6H5
10
11
12
13
14
15
16
17
18
AuI(IPr)
10
1k
H
a
AgCl(IPr)
Reaction conditions: the reaction was carried out with 1a (2.0 mmol)
and catalyst (0.04 mmol) in CH3OH (2.0 mL) under CO2 (1 atm) at
CuCl(IPr)
b
c
f
1
ItBu−CO2
40 °C. Determined by H NMR. Isolated yield in parentheses.
AuCl(IPr)
AuCl(IPr)
AuCl(IPr)
AuCl(IPr)
AuCl(IPr)
55
15
0
(entry 4). The carboxylative cyclization of other 1-amino-2-
butynes (1f−h) with alkyl and benzyl substituents on the
nitrogen atom also afforded (Z)-5-ethylidene-1,3-oxazolidin-2-
ones (2f−h) in 83−86% yields (entries 5−7), although
sterically congested amino groups required a longer reaction
time of 48 h. A sharp drop in the catalytic activity was observed
for the reaction of a bulky N-isopropyl variant (1i), furnishing
the desired product only in 27% yield after 66 h (entry 8). The
amine conversion was obviously reduced by replacement of the
R1 group from alkyls to phenyl, even at elongated reaction
times (entry 9). Moreover, the primary amine 1k, which was
hardly carboxylated in the preceding reports, was applicable to
the present Au system, resulting in a comparable yield (entry
10).
CH2Cl2
toluene
0
0
a
Standard conditions: the reaction was carried out with 1a (2.0 mmol)
and catalyst (0.04 mmol) in solvent (2.0 mL) under a CO2
b
atmosphere at 40 °C for 15 h. Determined by the 1H NMR method,
c
using durene as an internal standard. The reaction was conducted
with catalyst (0.01 mmol) for 48 h under otherwise identical
d
conditions. IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene.
e
f
ItBu = 1,3-di-tert-butylimidazol-2-ylidene. ItBu-CO2 = 1,3-di-tert-
butylimidazolium-2-carboxylate.
from 2 mol % proceeded smoothly to give 2a in 85% yield in 48
h, indicating the IPr−Au catalyst is stable enough for the
carboxylation (entry 3).
Notably, the reaction of 1a under mixed gas conditions
proceeded smoothly to give the urethane product in a
reasonable yield. As shown in Table 3, the high efficiency
Further screening tests using a series of Au catalysts under
the same conditions revealed that similar levels of catalytic
activity (89% and 83% yields) were attained with the related
NHC complexes (entries 4 and 5), whereas Au complexes
bearing P(C6H5)3, P(OC2H5)3, and P(C2H5)3 ligands were
found to have lower catalyst activities (41−70% yield; entries
6−8). The halide ligand also delicately influenced the product
yield (84% and 71% in entries 9 and 10). Changing the central
metal to Ag and Cu caused a decrease in the yield to 52% and
19%, respectively (entries 11 and 12), even though several
NHC−Cu complexes have been realized as efficient catalysts
for CO2 fixations.9 We have also developed CO2-fixation
reactions catalyzed by isolated NHC molecules or these CO2
adducts;10 however, the carboxylation with a catalytic amount
of 1,3-di-tert-butylimidazolium-2-carboxylate (ItBu-CO2) re-
sulted in the recovery of 1a (entry 13).
Table 3. Carboxylative Cyclization of 1a under a Mixed CO2
a
Atmosphere
b
entry
mixed gas
v/v, %
t, h
yield, %
1
2
3
4
5
6
CO2
15
15
15
48
15
15
91
85
24
75
80
75
CO2/Ar
CO2/Ar
CO2/air
CO2/CO
CO2/H2
50/50
10/90
50/50
50/50
50/50
a
Reaction conditions: the reaction was carried out with 1a (2.0 mmol)
and catalyst (0.04 mmol) in CH3OH (2.0 mL) under CO2 (1 atm) at
b
1
40 °C. Determined by H NMR.
The outcome of the carboxylation was strongly influenced by
the solvent used. The use of methanol gave beneficial results in
comparison with 2-propanol or acetonitrile, and 1a remained
intact after the reaction in CH2Cl2, toluene, or THF (entries 2
and 14−18).
The NHC−Au(I) catalyst paved the way to a variety of five-
membered cyclic urethanes from propargylic amines, as
summarized in Table 2. With the optimized reaction conditions
using 2 mol % of AuCl(IPr) in methanol, aliphatic N-
methylaminoalkyne substrates (1b−d; R1 = alkyl) were
smoothly converted into the analogous Z products (2b−d) in
yields of 85−91% (entries 1−3). The reaction of a substrate
having a terminal alkyne unit, N-methylpropargylamine (1e),
gave the product 2e in 16% yield, possibly due to in situ
formation of a gold acetylide as a less catalytically active species
was maintained in the case of a 1:1 mixture of CO2 with argon,
whereas further dilution resulted in retardation of the process
(entries 1−3). A mixture with air was also usable, although a
prolonged reaction time was needed to provide a similar level
of productivity (entry 4). Unexpectedly, the gold catalyst
tolerated the presence of CO or H2 gas as well (entries 5 and 6)
and hence offers potential for CO2 utilization directly from
exhaust combustion gas.
In order to gain mechanistic insight into the carboxylation,
stoichiometric reactions using an NHC−Au(I) complex under
aprotic and nonacidic conditions were evaluated. The treatment
of AuOH(IPr)11 with an equimolar amount of 1a under a CO2
atmosphere in dehydrated THF at 40 °C afforded the
alkenylgold complex 3a, which was isolated in 54% yield after
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Communication
recrystallization from THF/n-pentane (Scheme 2). The
colorless complex 3a was unambiguously characterized by
under the standard reaction conditions mentioned above. On
the basis of the experimental results on 3a, a mechanism
involving the alkenylgold(I) species is proposed (Scheme 3).
Scheme 2. Synthesis of the Alkenylgold(I) Complex 3a as a
Model Intermediate in Carboxylative Cyclization
Scheme 3. Proposed Mechanism of Carboxylative
Cyclization of Propargylamines Catalyzed by the NHC−Au
Complexes
NMR spectroscopy, elemental analysis, and X-ray crystallog-
raphy. The crystal structure (Figure 1) shows that the Au(I)
The chlorogold(I) precursor can form a catalytically active
cationic species via facile dissociation of the anionic ligand in a
polar methanol medium. The alkyne moiety on propargylic
carbamate formed through a spontaneous carboxylation of the
amine substrate is activated on the cationic gold(I) center. The
carbamate anion attacks nucleophilically on the triple bond in
an anti fashion to generate the corresponding neutral
alkenylgold intermediate, as typified by 3a. The presence of
CO2 in CH3OH enhances the following protodeauration in
which the cationic species is regenerated with liberation of the
urethane 2.
In summary, we have demonstrated that NHC−Au(I)
complexes serve as efficient catalysts, providing 1,3-oxazoli-
din-2-ones from propargylamines and CO2 under mild
conditions. Moreover, we have successfully identified a key
alkenylgold(I) intermediate in the catalytic carboxylative
cyclization and realized unique advantages of methanol to
drive the catalytic cycle, especially with regard to the CO2-
mediated acceleration in the product-releasing step.
Figure 1. ORTEP diagram of 3a with 50% probability ellipsoids.
Hydrogen atoms are omitted for clarity. Selected bond lengths and
angles are given in the Supporting Information.
atom is bound to the NHC carbene and alkenyl carbons in a
typical linear two-coordinate geometry. The gold−carbon bond
lengths are 2.029 and 2.046 Å (mean values), which are within
the typical range for related alkenylgold compounds.12,13 The
alkylidenecarbamate scaffold including the five-membered ring
is virtually planar, and the geometry is almost identical with that
of the liberated product 2a (see the Supporting Information).
These structural data suggest that the cyclization of the amine−
CO2 adduct will be promoted by coordination of the C−C
triple bond to the Au(I) center to form the anti addition
product exclusively.
Since the coproduct H2O does not have enough acidity to
cleave the alkenyl−Au bond, the intermediate 3a is obtainable
from the stoichiometric reaction. The isolable complex 3a was
smoothly deaurated upon equimolar addition of acetic acid in
CD3OD at room temperature to give 2a in 83% yield within 1
h. In contrast, 91% of 3a remained in a CD3OD solution under
neutral conditions even after 24 h; thus, the alkenyl−Au bond
proved to be more stable than expected. Of particular note is
that the protodeauration of 3a was significantly accelerated by
exposure with atmospheric CO2 to furnish 2a in 54% yield after
3 h. Presumably, the combination of methanol and CO2 will
lead to marginal formation of methylcarbonic acid
(CH3OCO2H),14 which facilitates the protodeauration step in
the catalytic carboxylative cyclization.
ASSOCIATED CONTENT
* Supporting Information
Text, figures, tables, and a CIF file giving experimental and
crystallographic data. This material is available free of charge via
the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail for T.I.: tikariya@apc.titech.ac.jp.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by JSPS KAKENHI Grant
Numbers 22225004 and 24350079 and partially supported by
the GCOE Program.
It is noteworthy that catalytic use of the isolated complex 3a
showed a comparable activity (90% yield of 2a) with AuCl(IPr)
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(12) Liu, L.-P.; Hammond, G. B. Chem. Soc. Rev. 2012, 41, 3129−
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