Carboxylative Cyclization of Propargylic Amines with Carbon Dioxide?- Catalyzed by Poly(amidoamine)-Dendrimer-Encapsulated Gold Nanoparticles

Article abstract of DOI:10.1055/s-0039-1690162

We prepared gold nanoparticles encapsulated in poly(amidoamine) (PAMAM) dendrimers as templating agents. The resulting gold nanoparticles were used as catalysts for the carboxylative cyclization of propargylic amines with carbon dioxide to afford the corr

Full text of DOI:10.1055/s-0039-1690162

SYNLETT
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
© Georg Thieme Verlag Stuttgart · New York
2019, 30, A–E
letter
A
en
H. Matsuo et al.
Letter
Synlett
Carboxylative Cyclization of Propargylic Amines with Carbon
Dioxide Catalyzed by Poly(amidoamine)-Dendrimer-Encapsulated
Gold Nanoparticles
Hideaki Matsuo
Akira Fujii
R2
R1
R2
AuNP@PAMAM/C
CO2
R1
N
HN R3
O
R3
Jun-Chul Choi
Tadahiro Fujitani
Ken-ichi Fujita*
H2O–toluene
40 °C
O
5 examples
11–99% yield
National Institute of Advanced Industrial Science and Technology
(AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565,
Japan
k.fujita@aist.go.jp
Received: 27.06.2019
Accepted after revision: 28.07.2019
Published online: 21.08.2019
ed to the formation of alkenyl gold intermediates derived
from the triple bonds of the alkynes.10b,11 In addition,
monodisperse gold nanoparticles have been reported to be
readily prepared by the use of poly(amidoamine) (PAMAM)
dendrimers as templates.12 Here, we report a carboxylative
cyclization of propargylic amines with CO2 to provide 1,3-
oxazolidin-2-ones that is catalyzed by PAMAM-dendrimer-
encapsulated gold nanoparticles.13 In our screening of the
preparation method for the dendrimer-encapsulated gold
nanoparticles as catalysts, the addition of mesoporous car-
bon powder to the prepared gold nanoparticles mixture
was found to be effective in promoting the carboxylative
cyclization of propargylic amines with CO2.
Gold nanoparticles were prepared within the fourth
generation-PAMAM dendrimer (PAMAM 1) by a modifica-
tion of a previously reported synthetic sequence (Scheme
1).12b First, an aqueous solution of hydrogen tetrachloroau-
rate(III) was added to an aqueous solution of dendrimer 1
under argon. The resulting mixture was vigorously stirred
for one hour to load the tetrachloroaurate(III) ion into the
dendrimer. Next, the tetrachloroaurate(III) ions were re-
duced to form PAMAM-encapsulated gold nanoparticles
(AuNP@PAMAM) by the addition of aqueous sodium tetra-
hydroborate to the mixture. Immediately after the reduc-
tion, mesoporous carbon powder was added to the reaction
DOI: 10.1055/s-0039-1690162; Art ID: st-2019-u0336-l
Abstract We prepared gold nanoparticles encapsulated in poly(ami-
doamine) (PAMAM) dendrimers as templating agents. The resulting
gold nanoparticles were used as catalysts for the carboxylative cycliza-
tion of propargylic amines with carbon dioxide to afford the corre-
sponding 1,3-oxazolidin-2-ones in yields of up to 99%.
Key words oxazolidinones, propargylic amines, carbon dioxide, gold
catalysis, nanoparticles
The capture, fixation, and utilization of carbon dioxide
(CO2) have become important scientific and technological
objectives because CO2 is nontoxic, nonflammable, abun-
dant, and easily available.1 Moreover, CO2 is one of the most
attractive C1 building blocks for replacing such toxic re-
agents as phosgene and carbon monoxide.2 However, in the
case of chemical transformations of CO2, high pressures of
CO2 and high reaction temperatures are required because of
the thermodynamic stability and kinetic inertness of CO2.
Nevertheless, many transformations of CO2 have been stud-
ied with the aim of achieving sustainable development and
green syntheses. One of the useful transformations of CO2 is
the carboxylative cyclization of propargylic amines with
CO2 to give 1,3-oxazolidin-2-ones.3,4 Recently, various in-
vestigations of this reaction catalyzed by organometallic
complexes of various transition metals such as ruthenium,5
palladium,6 silver,7 and gold8 have been conducted.
1) HAuCl4
2) NaBH4, C
PAMAM
AuNP@PAMAM/C
H2O
1
2
Metal nanoparticles have great potential in the field of
catalysis.9 The large surface area of metal nanoparticles
promotes their catalytic activity, resulting in rapid chemi-
cal transformations with excellent product yields. Recently,
gold nanoparticles have been reported to act as efficient
catalysts for the cycloisomerization of 2-alkynylanilines to
give indole derivatives.10 This transformation was attribut-
CONH
NH2
G
G
G
G:
N
N
CONH
N
G
CONH
4NH2
PAMAM
1
Scheme 1 Preparation of the carbon-supported PAMAM-encapsulated
gold nanoparticles 2
© 2019. Thieme. All rights reserved. — Synlett 2019, 30, A–E
Georg Thieme Verlag KG, Rüdigerstraße 14, 70469 Stuttgart, Germany

B
H. Matsuo et al.
Letter
Synlett
mixture to support the AuNP@PAMAM and to prevent ag-
gregation of the gold nanoparticles.14 Thirty minutes after
the addition of the mesoporous carbon, the carbon-sup-
ported PAMAM-encapsulated gold nanoparticles 2 were ob-
tained as an aqueous dispersion.
Scanning transmission electron microscopy (STEM) ob-
servation of 2 revealed the presence of spherical particles of
1–2 nm in diameter as the major component, indicating the
formation of gold nanoparticles (Figure 1).15
Table 1 Carboxylative Cyclization of Propargylic Amine 3a with CO2
under Various Reaction Conditionsa
AuNP@PAMAM/C
2b (1 mol% [Au])
CO2 (balloon)
O
NBn
NHBn
3 h
O
3a
4a
Entry
Solvent
Temp (°C)
Yieldc (%)
1
2
3
4
H2O
40
40
r.t.
60
79
96
90
94
H2O–toluene
H2O–toluene
H2O–toluene
a Reaction conditions: 3a (1 equiv), 2 (1 mol% [Au]), H2O (0.1 M based on 3a;
entry 1) or 2:1 (v/v) H2O–toluene (0.067 M based on 3a; entries 2–4), 3 h,
CO2 (1 atm).
b 2: HAuCl4/PAMAM = 20:1 (mol/mol), C/[Au] = 100:1 (w/w).
c Determined by integration of 1H NMR absorptions with reference to an
internal standard.
spheric pressure. When 2 was prepared from HAuCl4 and
PAMAM in a ratio of 20:1, the 1,3-oxazolidin-2-one 4a was
obtained in moderate yield (62%; Table 2, entry 2).
Table 2 Optimization of the HAuCl4/PAMAM ratio for the Preparation of 2
AuNP@PAMAM/C
2a (1 mol% [Au])
CO2 (balloon)
O
NBn
Figure 1 STEM image of gold nanoparticles 2
NHBn
H2O–toluene
40 °C, 15 min
O
3a
4a
Next, the prepared gold nanoparticles 2 were used di-
rectly as a catalyst for the carboxylative cyclization of prop-
argylic amine 3a with CO2 (Table 1). CO2 at atmospheric
pressure (balloon) was used to displace the argon in the re-
action vessel then amine 3a was added to the aqueous dis-
persion of 2 and the mixture was stirred vigorously at 40 °C
for three hours. The aqueous-phase carboxylative cycliza-
tion of propargylic amine 3a with CO2 proceeded smoothly
in the presence of 1 mol% of gold nanoparticles 2 as a cata-
lyst to give 1,3-oxazolidin-2-one 4a in a 79% yield (Table 1,
entry 1). Encouraged by this result, we carried out the car-
boxylative cyclization of 3a in a water–toluene mixture, and
the yield of 4a increased to 96% (Table 1, entry 2). Then, the
carboxylative cyclizations of 3a were performed at various
temperatures in a water–toluene solvent system (Table 1,
entries 2–4), and 1,3-oxazolidin-2-one 4a were obtained in
good chemical yields (>90%), even at room temperature (Ta-
ble 1, entry 3).
Entryb
HAuCl4/PAMAM
Yieldc (%)
1
2
3
10
20
30
50
62
50
a 2: C/[Au] = 100:1 (w/w).
b Reaction conditions: 3a (1 equiv), 2 (1 mol% [Au]), 2:1 (v/v) H2O–toluene
(0.067 M based on 3a), 40 °C, 15 min, CO2 (1 atm).
c Determined by integration of 1H NMR absorptions with reference to an
internal standard.
We subsequently optimized the amount of the meso-
porous carbon powder in the preparation of the PAMAM-
encapsulated gold nanoparticles 2 (Table 3). The carboxyl-
ative cyclization of propargylic amine 3a with CO2 was car-
ried out for 15 minutes in the presence of 1 mol% of 2, pre-
pared with various C/[Au] (w/w) ratios, as a catalyst.16
When the C/[Au] ratio was 50:1 or 30:1, the 1,3-oxazolidin-
2-one 4a was obtained in fair yields (85% and 82%, respec-
tively; Table 3, entries 2 and 3).17 In addition, it is note-
worthy that the yield of 1,3-oxazolidin-2-one 4a decreased
markedly to 46% when no mesoporous carbon powder was
used (entry 4). This confirmed the importance of meso-
Next, to optimize the preparation method for the
PAMAM-encapsulated gold nanoparticles 2 as a catalyst, we
prepared 2 with various HAuCl4/PAMAM ratios, and we
used the resulting samples of 2 (1 mol%) as catalysts for the
carboxylative cyclization of propargylic amine 3a in a wa-
ter–toluene at 40 °C for 15 minutes under CO2 at atmo-
© 2019. Thieme. All rights reserved. — Synlett 2019, 30, A–E

C
H. Matsuo et al.
Letter
Synlett
porous carbon as a support. The use of mesoporous carbon
to support the prepared gold-nanoparticles-containing
PAMAM probably enhances the stability of the gold
nanoparticles by suppressing their aggregation.18
propargylic amine (3b) were used as substrates for carbox-
ylative cyclization in the presence of 1 mol% of 2 as catalyst,
the corresponding 1,3-oxazolidin-2-ones 4a and 4b were
obtained in yields of 99 and 89%, respectively (Table 4, en-
tries 1 and 2). In contrast, the presence of an -methyl
group in 3c resulted in a poor yield of 4c (32%), even when 2
mol% of 2 was employed as a catalyst (entry 3). On the oth-
er hand, the presence of a terminal phenyl group in 3d re-
sulted in a moderate yield (76%) of 4d (entry 4), whereas
Table 3 Optimization of the Amount of Mesoporous Carbon Powder
for the Preparation of 2
AuNP@PAMAM/C
2a (1 mol% [Au])
CO2 (balloon)
O
NBn
NHBn
H2O–toluene
40 °C, 15 min
Table 4 Carboxylative Cyclization of Various Propargylic Amines 3 for
the Synthesis of 4a
O
3a
4a
R1
R2
Entryb
C/[Au] (w/w)
Yieldc (%)
AuNP@PAMAM/C 2
R2
CO2 (balloon)
R1
O
NR3
1
2
3
4
100
50
30
0
62
85
82
46
NHR3
H2O–toluene
40 °C
O
3
4
Entry Substrate
Product
2b
(mol% [Au])
Time Yieldc
(h)
(%)
a 2: HAuCl4/PAMAM = 20:1 (mol/mol).
b Reaction conditions: 3a (1 equiv), 2 (1 mol% [Au]), 2:1 (v/v) H2O–toluene
(0.067 M based on 3a), 40 °C, 15 min, CO2 (1 atm).
c Determined by integration of 1H NMR absorptions with reference to an
internal standard.
O
NBn
1
1
10
9 9
NHBn
(99)d
O
O
3a
3b
4a
The time-course curve of the carboxylative cyclization
of a propargylic amine 3a under the optimal conditions (Ta-
ble 3, entry 2) is shown in Figure 2. The reaction was so fast
that the chemical yield of the 1,3-oxazolidin-2-one 4a
reached 99% within half an hour.
O
NMe
2
3
4
5
1
2
2
2
24
24
24
24
8 9
NHMe
(87)d
4b
Me
Me
O
NBn
32
NHBn
O
3c
4c
Ph
Ph
3d
Me
3e
O
NMe
NBn
7 6
NHMe
(75)d
O
O
4d
Me
O
11
NHBn
4e
6
2
2 4
0
O
NH
NH2
Figure 2 Time-course curve of the carboxylative cyclization of a prop-
argylic amine 3a
O
3f
4f
a Reaction conditions: 2:1 (v/v) H2O–toluene [0.067 M based on 3a and 3b
(entries 1 and 2), 0.033 M based on 3c–f (entries 3–6)], 40 °C, CO2 (1 atm).
b 2: HAuCl4/PAMAM = 20:1 (mol/mol), C/[Au] = 50:1 (w/w).
c Determined by integration of 1H NMR absorptions with reference to an
internal standard.
Finally, we performed the carboxylative cyclization of
various propargylic amines 3 at 40 °C under CO2 at atmo-
spheric pressure in the presence of 2 as a catalyst (Table
4).19 When N-benzylpropargylic amine (3a) and N-methyl-
d Isolated yield.
© 2019. Thieme. All rights reserved. — Synlett 2019, 30, A–E

D
H. Matsuo et al.
Letter
Synlett
the presence of a terminal methyl group in 3e resulted in a
low chemical yield (11%) of the corresponding 1,3-oxazoli-
din-2-one 4e (entry 5).20 Finally, when the primary amine
3f was used as a substrate, the corresponding 1,3-oxazoli-
din-2-one 4f was not obtained (entry 6).
Figure 3 shows our proposed mechanism for the carbox-
ylative cyclization of propargylic amine 3 catalyzed by the
PAMAM-encapsulated gold nanoparticles 2. First, 3 reacts
with CO2 to form the corresponding carbamic acid 5 in si-
tu.8g Carbamic acid 5 is activated by gold nanoparticle-π in-
teractions of the triple bond and also by interaction of the
hydrogen atom of 5 with a tertiary amine moiety of the
PAMAM, as shown in structure 6.4c The corresponding 1,3-
oxazolidin-2-one 4 is then formed, with the regeneration of 2.
References and Notes
(1) Cuéllar-Franca, R. M.; Azapagic, A. J. CO2 Util. 2015, 9, 82.
(2) (a) Bhanja, P.; Modak, A.; Bhaumik, A. Chem. Eur. J. 2018, 24,
7278. (b) Xu, Y.; Lin, L.; Xiao, M.; Wang, S.; Smith, A. T.; Sun, L.;
Meng, Y. Prog. Polym. Sci. 2018, 80, 163. (c) Klankermayer, J.;
Wesselbaum, S.; Beydoun, K.; Leitner, W. Angew. Chem. Int. Ed.
2016, 55, 7296. (d) Liu, Q.; Wu, L.; Jackstell, R.; Beller, M. Nat.
Commun. 2015, 6, 5933. (e) Sakakura, T.; Choi, J.-C.; Yasuda, H.
Chem. Rev. 2007, 107, 2365.
(3) (a) Zhang, Z.; Ye, J.-H.; Wu, D.-S.; Zhou, Y.-Q.; Yu, D.-G. Chem.
Asian J. 2018, 13, 2292. (b) Arshadi, S.; Vessally, E.; Sobati, M.;
Hosseinian, A.; Bekhradnia, A. J. CO2 Util. 2017, 19, 120.
(4) (a) Fujii, A.; Matsuo, H.; Choi, J.-C.; Fujitani, T.; Fujita, K. Tetra-
hedron 2018, 74, 2914. (b) Sadeghzadeh, S. M.; Zhiani, R.;
Emrani, S. Appl. Organomet. Chem. 2018, 32, 3941. (c) Fujii, A.;
Choi, J.-C.; Fujita, K. Tetrahedron Lett. 2017, 58, 4483. (d) Yu, B.;
Kim, D.; Kim, S.; Hong, S. H. ChemSusChem 2017, 10, 1080.
(e) Zhao, D.; Liu, X.-H.; Zhu, C.; Kang, Y.-S.; Wang, P.; Shi, Z.; Lu,
Y.; Sun, W.-Y. ChemCatChem 2017, 9, 4598. (f) Liu, X.; Wang, M.-
Y.; Wang, S.-Y.; Wang, Q.; He, L.-N. ChemSusChem 2017, 10,
1210. (g) Zhao, Y.; Qiu, J.; Li, Z.; Wang, H.; Fan, M.; Wang, J.
ChemSusChem 2017, 10, 2001. (h) Zhao, Y.; Qiu, J.; Tian, L.; Li, Z.;
Fan, M.; Wang, J. ACS Sustainable Chem. Eng. 2016, 4, 5553.
(i) Hu, J.; Ma, J.; Zhu, Q.; Zhang, Z.; Wu, C.; Han, B. Angew. Chem.
Int. Ed. 2015, 54, 5399. (j) Maggi, R.; Bertolotti, C.; Orlandini, E.;
Oro, C.; Sartori, G.; Selva, M. Tetrahedron Lett. 2007, 48, 2131.
(k) Kayaki, Y.; Yamamoto, M.; Suzuki, T.; Ikariya, T. Green Chem.
2006, 8, 1019.
R2
R1
AuNP@PAMAM/C
NHR3
2
3
R1
R2
CO2
R2
O
NR3
AuNP in 2
O
R1
R2
R1
4
NR3
O
NR3
HO
O
H
5
O
NR3 (PAMAM) in 2
(5) Mitsudo, T.; Hori, Y.; Yamakawa, Y.; Watanabe, Y. Tetrahedron
Lett. 1987, 28, 4417.
6
(6) (a) Shi, M.; Shen, Y.-M. J. Org. Chem. 2002, 67, 16. (b) Bacchi, A.;
Chiusoli, G. P.; Costa, M.; Gabriele, B.; Righi, C.; Salerno, G. Chem.
Commun. 1997, 1209.
Figure 3 Proposed mechanism for the carboxylative cyclization of a
propargylic amine 3 catalyzed by the PAMAM-encapsulated gold
nanoparticles 2
(7) (a) Yoshida, M.; Mizuguchi, T.; Shishido, K. Chem. Eur. J. 2012,
18, 15578. (b) Kikuchi, S.; Yoshida, S.; Sugawara, Y.; Yamada, W.;
Cheng, H.-M.; Fukui, K.; Sekine, K.; Iwakura, I.; Ikeno, T.;
Yamada, T. Bull. Chem. Soc. Jpn. 2011, 84, 698. (c) Yoshida, S.;
Fukui, K.; Kikuchi, S.; Yamada, T. Chem. Lett. 2009, 38, 786.
(8) (a) Fujita, K.; Inoue, K.; Sato, J.; Tsuchimoto, T.; Yasuda, H. Tetra-
hedron 2016, 72, 1205. (b) Sadeghzadeh, S. M. Appl. Organomet.
Chem. 2016, 30, 835. (c) Sadeghzadeh, S. M. J. Mol. Catal. A:
Chem. 2016, 423, 216. (d) Hase, S.; Kayaki, Y.; Ikariya, T. ACS
Catal. 2015, 5, 5135. (e) Yuan, R.; Lin, Z. ACS Catal. 2015, 5, 2866.
(f) Fujita, K.; Sato, J.; Inoue, K.; Tsuchimoto, T.; Yasuda, H. Tetra-
hedron Lett. 2014, 55, 3013. (g) Hase, S.; Kayaki, Y.; Ikariya, T.
Organometallics 2013, 32, 5285.
(9) (a) Rodrigues, T. S.; da Silva, A. G. M.; Camargo, P. H. C. J. Mater.
Chem. A 2019, 7, 5857. (b) Liu, L.; Corma, A. Chem. Rev. 2018,
118, 4981. (c) Kaur, R.; Bariwal, J.; Voskressensky, L. G.; Van der
Eycken, E. V. Chem. Heterocycl. Compd. (Engl. Transl.) 2018, 54,
241. (d) Campelo, J. M.; Luna, D.; Luque, R.; Marinas, J. M.;
Romero, A. A. ChemSusChem 2009, 2, 18. (e) Daniel, M.-C.;
Astruc, D. Chem. Rev. 2004, 104, 293.
In summary, by employing the PAMAM-dendrimer-en-
capsulated gold nanoparticles as a catalyst, the carboxyl-
ative cyclization of various propargylic amines proceeded
under CO2 at atmospheric pressure to afford the corre-
sponding 1,3-oxazolidin-2-ones. Gold nanoparticles of di-
ameter 1–2 nm were successfully prepared within the
PAMAM dendrimers. In addition, we found that the catalyt-
ic activity of the PAMAM-dendrimer-encapsulated gold
nanoparticles increased markedly by addition of meso-
porous carbon powder. Studies on further applications of
this catalyst to other reactions are ongoing.
Acknowledgment
This work was based on results obtained from a project (P16010)
commissioned by the New Energy and Industrial Technology Devel-
opment Organization (NEDO).
(10) (a) Savva, I.; Kalogirou, A. S.; Achilleos, M.; Vasile, E.; Koutentis,
P. A.; Krasia-Christoforou, T. Molecules 2016, 21, 1218.
(b) Perea-Buceta, J. E.; Wirtanen, T.; Laukkanen, O.-V.; Mäkelä,
M. K.; Nieger, M.; Melchionna, M.; Huittinen, N.; Lopez-San-
chez, J. A.; Helaja, J. Angew. Chem. Int. Ed. 2013, 52, 11835.
(11) Schröder, F.; Erdmann, N.; Noël, T.; Luque, R.; Van der Eycken, E.
V. Adv. Synth. Catal. 2015, 357, 3141.
Supporting Information
Supporting information for this article is available online at
https://doi.org/10.1055/s-0039-1690162.
S
u
p
p
orit
n
gInformati
o
n
S
u
p
p
orti
n
gInformati
o
n
© 2019. Thieme. All rights reserved. — Synlett 2019, 30, A–E

E
H. Matsuo et al.
Letter
Synlett
(12) (a) Kim, Y.-G.; Oh, S.-K.; Crooks, R. M. Chem. Mater. 2004, 16,
167. (b) Gröhn, F.; Bauer, B. J.; Akpalu, Y. A.; Jackson, C. L.; Amis,
E. J. Macromolecules 2000, 33, 6042. (c) Esumi, K.; Suzuki, A.;
Yamahira, A.; Torigoe, K. Langmuir 2000, 16, 2604. (d) Garcia, M.
E.; Baker, L. A.; Crooks, R. M. Anal. Chem. 1999, 71, 256.
(e) Esumi, K.; Suzuki, A.; Aihara, N.; Usui, K.; Torigoe, K. Lang-
muir 1998, 14, 3157.
(13) For previously reported transformations catalyzed by the
PAMAM-dendrimer-encapsulated gold nanoparticles, see:
(a) Nemanashi, M.; Meijboom, R. Langmuir 2015, 31, 9041.
(b) Nemanashi, M.; Meijboom, R. J. Colloid Interface Sci. 2013,
389, 260. (c) Kracke, P.; Haas, T.; Saltsburg, H.; Flytzani-Stepha-
nopoulos, M. J. Phys. Chem. C 2010, 114, 16401. (d) Esumi, K.;
Miyamoto, K.; Yoshimura, T. J. Colloid Interface Sci. 2002, 254,
402.
mixture was stirred for 1 h, then a 0.1 M solution of NaBH4 in
0.3 M aq NaOH (0.3 mL) was added dropwise with vigorous stir-
ring to reduce the AuCl4– to Au nanoparticles. Immediately after
the addition of the NaBH4, mesoporous carbon powder (30 mg)
was added, and the reaction mixture was vigorously stirred for
30 min to give the carbon-supported PAMAM-encapsulated
gold nanoparticles 2 as an aqueous dispersion (3 mL in total), to
which toluene (1.5 mL) was added with stirring. The atmo-
sphere in the reaction vessel was then changed from argon to
CO2 and the mixture was heated at 40 °C. The appropriate prop-
argylic amine 3 (0.3 mmol for Table 4, entries 1 and 2; 0.15
mmol for entries 3–6) was then added, and the mixture was
vigorously stirred at 40 ºC under CO2 at atmospheric pressure
for the appropriate time. Catalyst 2 was separated by filtration
under a vacuum and washed with CH2Cl2. The filtrate was col-
lected and extracted with CH2Cl2 (×4). The combined organic
layer was dried (Na2SO4) and filtered, and the yield of the1,3-
oxazolidin-2-one 4 was determined by integration of its 1H
NMR absorption with reference to an internal standard [2-(ben-
zyloxy)naphthalene]. 4a, 4b, and 4d were isolated by removal of
the CH2Cl2 under reduced pressure and purified by column
chromatography (silica gel, hexane–AcOEt).
(14) Takahashi, M.; Imaoka, T.; Yamamoto, K. RSC Adv. 2015, 5,
100693.
(15) 2: HAuCl4/PAMAM = 20:1 (mol/mol), C/[Au] = 50:1 (w/w). A
histogram showing the particle-size distribution of the gold
nanoparticles 2 is given in the Supporting Information.
(16) [Au]: weight of gold in hydrogen tetrachloroaurate(III).
(17) In all cases in which 2 was prepared without hydrogen tetra-
chloroaurate(III) and/or PAMAM 1, the carboxylative cyclization
of 3a with CO2 did not proceed.
3-Benzyl-5-methylidene-1,3-oxazolidin-2-one (4a)
yield: 57.4 mg (99%). 1H NMR (400 MHz, CDCl3):  = 7.40–7.30
(m, 3 H), 7.30–7.25 (m, 2 H), 4.74 (td, J = 2.6, 3.1 Hz, 1 H), 4.47 (s,
2 H), 4.24 (td, J = 2.2, 3.1 Hz, 1 H), 4.02 (t, J = 2.4 Hz, 2 H). 13C
NMR (100 MHz, CDCl3):  = 155.6, 149.0, 135.0, 129.0, 128.25,
128.17, 86.7, 47.8, 47.2.
(18) Takahashi, M.; Imaoka, T.; Hongo, Y.; Yamamoto, K. Angew.
Chem. Int. Ed. 2013, 52, 7419.
(19) 1,3-Oxazolidin-2-ones 4a–e : General Procedure
A reaction vessel was charged with 10% methanolic solution of
the fourth-generation PAMAM 1 (21 mg, 0.15 mol) under
argon. The MeOH was removed in vacuo, and 1 was redissolved
in H2O (2.1 mL). A 0.005 M aqueous solution of HAuCl4 (0.6 mL)
was slowly added to the aqueous solution of 1 with stirring. The
(20) In Figure 3, it is assumed that the nucleophilic ring closure from
6 to 4 is probably retarded when an electron-donating methyl
group is present at R1.
© 2019. Thieme. All rights reserved. — Synlett 2019, 30, A–E