Magnetically Recoverable N-Heterocyclic Carbene-Gold(I) Catalyst for Hydroamination of Terminal Alkynes

Article abstract of DOI:10.1055/s-0035-1562134

We prepared a magnetically recoverable gold(I) catalyst by immobilizing an N-heterocyclic carbene-gold(I) complex on magnetite and applied it to the hydroamination of alkynes. By employing 2 mol% of the magnetite-supported gold(I) catalyst, the hydroamination of terminal alkynes proceeded smoothly to provide the corresponding imine in a fair chemical yield. Moreover, after the reaction, the magnetic gold(I) catalyst was readily recovered by use of an external magnet and could be reused up to five times.

Full text of DOI:10.1055/s-0035-1562134

SYNLETT
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
©
Georg Thieme Verlag Stuttgart · New York
2016, 27, A–D
A
letter
Synlett
K. Fujita et al.
Letter
Magnetically Recoverable N-Heterocyclic Carbene–Gold(I) Cata-
lyst for Hydroamination of Terminal Alkynes
Ken-ichi Fujita*a
Akira Fujiia
Junichi Satob
Hiroyuki Yasudaa,b
OEt
O
O
Fe3O4
Si
N
N
3
AuCl
(2 mol%)
R'
R'
CF3SO3H (2 mol%)
100 °C, 24 h
N
a
National Institute of Advanced Industrial Science and
R
+
Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi,
Tsukuba, Ibaraki 305-8565, Japan
k.fujita@aist.go.jp
Graduate School of Science and Engineering, Ibaraki
University, 2-1-1 Bunkyo, Mito, Ibaraki 310-8512, Japan
R
NH2
Magnetically Recoverable
R = Ph, 4-t-BuC6H4, C6H13
R' = H, Br
b
58–77% yield
Received: 29.02.2016
Accepted after revision: 08.04.2016
Published online: 11.05.2016
Magnetic nanoparticles have emerged as smart and
promising supports for immobilization of the catalytic met-
al, because magnetic nanoparticle-supported catalysts can
be easily separated from the reaction medium by use of an
external magnet, which provides a simple separation of the
DOI: 10.1055/s-0035-1562134; Art ID: st-2016-u0143-l
Abstract We prepared a magnetically recoverable gold(I) catalyst by
immobilizing an N-heterocyclic carbene–gold(I) complex on magnetite
and applied it to the hydroamination of alkynes. By employing 2 mol%
of the magnetite-supported gold(I) catalyst, the hydroamination of ter-
minal alkynes proceeded smoothly to provide the corresponding imine
in a fair chemical yield. Moreover, after the reaction, the magnetic
gold(I) catalyst was readily recovered by use of an external magnet and
could be reused up to five times.
4
catalysts without filtration. The magnetic separation cir-
cumvents time-consuming and laborious separation steps
and allows for practical continuous catalysis. Besides the
easy separation, an interesting feature of magnetic
nanoparticles is their convenient surface modification,
which provides a wide range of magnetic-functionalized
catalysts showing identical and sometimes even higher ac-
tivity than their homogeneous counterparts in organic
transformations. Recently, magnetic nanoparticle-support-
ed and magnetically recoverable transition-metal complex-
Key words amination, N-heterocyclic carbene complexes, catalysis,
imines, supported catalysis
Gold catalysts have recently gained a great deal of atten-
tion as an emerging tool for a range of useful synthetic
transformations. In particular, gold(I) complexes have seen
increased utility as catalysts for the activation of alkynes to-
ward addition by a variety of nucleophiles. For example,
the catalytic addition of an organic N–H bond to alkynes
5
6
7
es such as palladium, ruthenium, and osmium have been
increasingly reported with a high level of catalytic activity.
We report herein the synthesis of a magnetically recover-
able gold(I) catalyst by the immobilization of an N-hetero-
1
1a
8,9
cyclic carbene (NHC)–gold(I) complex on magnetite and
its application to the hydroamination of alkynes. Moreover,
this magnetite-supported NHC–gold(I) catalyst was clearly
collected from the reaction mixture by use of an external
magnet and could be reused repeatedly.
(
hydroamination) to give nitrogen-containing molecules is
of great interest to both the academic and industrial com-
2
munity. Despite the widespread application of these types
of reactions in organic synthesis, some obstacles stand in
the way of their large-scale application to the pharmaceuti-
cal and fine-chemical industries, including high cost and
the possibility of contamination with catalytic gold in the
final product. One of the most promising solutions to these
problems is the immobilization of the gold(I) complex as a
catalyst on an insoluble support. Heterogenization of cata-
lytic gold(I) by immobilization on various organic and inor-
ganic supports has previously been successful, making it
possible for the immobilized gold(I) catalyst to be recovered
A magnetic nanoparticle-supported NHC–gold(I) com-
plex 5 was synthesized as follows (Scheme 1). Magnetite
(Fe O ) was chosen for use as a magnetic support, since it
3
4
could be readily prepared by a conventional coprecipitation
method.10 The silane coupling agent having an NHC–gold(I)
complex 4 was prepared according to a modification of the
synthetic scheme reported by Corma.3e Imidazolium iodide
3 was synthesized by stirring of an N,N-dimethylformamide
11
(DMF) solution of (3-iodopropyl)triethoxysilane (1) and
3
12
by filtration of the reaction mixture.
1-(2,4,6-trimethylphenyl)imidazole (2) at 90 °C for 3
hours under an argon atmosphere. After the evaporation of
DMF in vacuo, 1,2-dichloroethane and silver(I) oxide were
©
Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–D

B
Synlett
K. Fujita et al.
Letter
added, and the reaction mixture was stirred at room tem-
perature for 18 hours under an argon atmosphere. AuCl(SMe2)
was subsequently added, and the reaction mixture was
stirred at room temperature for 4 hours under an argon at-
mosphere. After the Celite® filtration of the reaction mix-
ture and the evaporation of the solvent, the silane coupling
agent having an NHC–gold(I) complex 4 was obtained.13
Finally, magnetite and 4 were refluxed in ethanol for 18
hours under an argon atmosphere to afford the magnetite-
supported NHC–gold(I) complex 5, which was separated by
magnetic decantation using an external magnet. The load-
ing of 5 was 0.17 mmol/g, which was determined by ICP-
Table 1 Hydroamination of Ethynylbenzene 6a Catalyzed by 5a
Br
catalyst 5 (x mol%)
acid (y mol%)
+ H2N
Br
N
24 h
6a
7a
8a
Entry Catalyst 5
Acid (y mol%)
Temp (°C)
Yield (%)b,c
(
x mol%)
1
2
3
4
5
1
1
1
2
2
0
none (0)
80
80
37 (7)
14
AES of gold.
12WO
3
·H
3
PO
4
·nH
2
O (1)
46 (22)
62 (14)
69 (17)
77 (20)
1 (1)
CF
CF
CF
CF
3
3
3
3
SO
SO
SO
SO
3
3
3
3
H (1)
H (2)
H (2)
H (2)
80
80
(
EtO)3Si
I
N
N
+
DMF
100
80
90 ºC, 3 h
1
2
6
a
Reaction conditions: Catalyst 5 (x mol%), acid (y mol%), 6a (3 equiv), 7a
1 equiv), carried out at the indicated temperature for 24 h under an ar-
gon atmosphere.
Ag2O
AuCl(SMe2)
r.t., 4 h
(
(
EtO)3Si
N
I
N
ClCH2CH2Cl
r.t., 18 h
b
1
Determined by the integration of H NMR absorptions referring to an in-
ternal standard.
3
c
The value in parentheses is the chemical yield of acetophenone.
Fe3O4
H2O additive
the acid contributed to the activation of catalyst 5 by the
formation of a cationic gold(I) moiety in situ.2 We next
examined the catalytic amount and the reaction tempera-
ture when using trifluoromethanesulfonic acid as an acidic
promoter. By carrying out the reaction using 2 mol% of the
catalyst 5 at 100 °C, the imine 8a was obtained in the high-
est chemical yield (77%; Table 1, entry 5). Needless to say,
the use of an acidic promoter alone, without catalyst 5, did
not catalyze the hydroamination of 6a (Table 1, entry 6).
Encouraged by these results, we subsequently per-
formed the various hydroaminations of alkynes 6 by em-
ploying 2 mol% of the catalyst 5 and trifluoromethanesul-
fonic acid at 100 °C for 24 hours (Table 2). When hydroami-
nation of ethynylbenzene (6a) was performed using aniline
(7b) instead of 4-bromoaniline (7a), the corresponding
imine 8b was obtained (70%; Table 2, entry 2).17 But the
chemical yield of 8 in the case using aniline (7b) was lower
than that in the case using 4-bromoaniline (7a). This ten-
dency in the reactivity of 7 is similar to that previously re-
ported, which arose from the introduction of an electron-
withdrawing group to the phenyl group of aniline.2 Also in
the case of the hydroamination of 4-tert-butyl-ethynylben-
zene (6b), the difference in the reactivity between 7a and
7b was confirmed (Table 2, entries 3 and 4).18 In addition,
the hydroamination of 1-octyne (6c), which was an aliphat-
ic terminal alkyne, also proceeded smoothly to provide the
corresponding imine 8e in a 75% chemical yield (Table 2,
entry 5). However, the hydroamination of diphenylacety-
lene (6d), which was an internal alkyne, hardly proceeded
(1%; Table 2, entry 6).19
(
EtO)3Si
N
N
f,16
EtOH
reflux, 18 h
AuCl
4
O
O
Fe3O4
Si
N
N
OEt
AuCl
5
Scheme 1 Preparation of 5
We then examined the catalytic activity of 5 by per-
forming the hydroamination of alkenes (Table 1). The pro-
cedure employed here was a modification of Tanaka’s syn-
thetic sequence. By stirring a solution of ethynylbenzene
6a) and 4-bromoaniline (7a) with 1 mol% of the catalyst 5
at 80 °C for 24 hours under an argon atmosphere, the hy-
droamination of 6a proceeded. The magnetite-supported
gold(I) catalyst 5 was rapidly collected using an external
magnet, and the reaction mixture was then transferred out
of the reaction vessel. The corresponding imine 8a was
obtained in a 37% chemical yield, and acetophenone was
also obtained in a 7% chemical yield (Table 1, entry 1), prob-
ably due to decomposition of the imine 8a by the small
amount of water contained in the catalyst 5.
We then performed the catalytic hydroamination of
ethynylbenzene (6a) using an acidic promoter. The chemi-
cal yield of the imine 8a was enhanced by using 1 mol% of
an acid. In particular, by employing trifluoromethanesul-
fonic acid as an acidic promoter, the imine 8a was obtained
in a 62% chemical yield (Table 1, entry 3). It is supposed that
2f
(
1
5
f
©
Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–D

C
Synlett
K. Fujita et al.
Letter
Table 2 Various Hydroaminations of Alkynes 6a
nation of terminal alkynes proceeded smoothly. Moreover,
the magnetic gold(I) catalyst was readily recovered by use
of an external magnet and could be reused up to five times.
R3
catalyst 5 (2 mol%)
CF3SO3H (2 mol%)
N
R1
R2
2
H N
R3
+
R2
100 °C, 24 h
R1
Acknowledgment
6
7
8
This work was supported by JSPS KAKENHI Grant Number 26410200.
Alkyne
Amine
Entry
Product Yield (%)b,c
R1
R2
6
R3
7
Supporting Information
1
Ph
Ph
H
H
H
H
H
Ph
6a
6a
6b
6b
6c
6d
Br
H
7a
7b
7a
7b
7a
7b
8a
8b
8c
8d
8e
8f
77 (20)
70 (16)
72 (19)
58 (25)
Supporting information for this article is available online at
http://dx.doi.org/10.1055/s-0035-1562134. Included are synthetic
procedures for the preparations of 3, 4, and 5, H NMR and C NMR
2
1
13
3
4-t-BuC
6
H
4
4
Br
H
data of 3 and 4, and X-ray diffraction patterns of magnetite and 5.
S
u
p
p
o
nrtIo
i
g
f
rm oaitn
S
u
p
p
ortioIgnfmr oaitn
4
4-t-BuC
6
H
d
5
C
6
H
13
Br
H
75 (21)
References and Notes
6e
Ph
1 (1)
a
(1) (a) Dorel, R.; Echavarren, A. M. Chem. Rev. 2015, 115, 9028.
(b) Joost, M.; Amgoune, A.; Bourissou, D. Angew. Chem. Int. Ed.
The reaction conditions were the same as those indicated in Table 1.
Determined by the integration of H NMR absorptions referring to an inter-
b
1
nal standard.
2015, 54, 15022. (c) Hashmi, A. S. K. Angew. Chem. Int. Ed. 2010,
c
The value in parenthesis is the chemical yield of the corresponding ketone.
d
e
1
49, 5232. (d) Li, Z.; Brouwer, C.; He, C. Chem. Rev. 2008, 108,
3239.
E/Z = 75:25 (determined by integration of H NMR absorptions).
Conditions: 1 equiv of 6d and 3 equiv of 7b were used.
(
2) (a) Anokhin, M. V.; Murashkina, A. V.; Averin, A. D.; Beletskaya,
I. P. Mendeleev Commun. 2014, 24, 332. (b) Alvarado, E.; Badaj, A.
C.; Larocque, T. G.; Lavoie, G. G. Chem. Eur. J. 2012, 18, 12112.
(c) Zeng, X.; Frey, G. D.; Kousar, S.; Bertrand, G. Chem. Eur. J.
2009, 15, 3056. (d) Corma, A.; Gonzalez-Arellano, C.; Iglesias,
M.; Navarro, M. T.; Sanchez, F. Chem. Commun. 2008, 6218.
Finally, the reusability of the catalyst 5 was examined
using ethynylbenzene (6a) and 4-bromoaniline (7a), as
20
shown in Table 3. In this experiment, it was found that the
catalyst 5 was readily collected from the reaction mixture
by use of an external magnet, and could be reused up to five
times, and that the corresponding imine 8a could be consis-
tently obtained in a fair chemical yield. In addition, it was
revealed that the formation of acetophenone was sup-
pressed by the reaction after the second, probably due to
the decrease in the small amount of water contained in the
magnetite-supported catalyst 5.
(
(
3
e) Widenhoefer, R. A.; Han, X. Eur. J. Org. Chem. 2006, 4555.
f) Mizushima, E.; Hayashi, T.; Tanaka, M. Org. Lett. 2003, 5,
349.
(
3) (a) Zhu, Y.; Laval, S.; Tang, Y.; Lian, G.; Yu, B. Asian J. Org. Chem.
2015, 4, 1034. (b) Lothschutz, C.; Szlachetko, J.; van Bokhoven, J.
A. ChemCatChem 2014, 6, 443. (c) Egi, M.; Azechi, K.; Akai, S.
Adv. Synth. Catal. 2011, 353, 287. (d) Corma, A.; Gonzalez-
Arellano, C.; Iglesias, M.; Ferreras, S. P.; Sanchez, F. Synlett
2
007, 1771. (e) Corma, A.; Puebla, E. G.; Iglesias, M.; Monge, A.;
Ferreras, S. P.; Sanchez, F. Adv. Synth. Catal. 2006, 348, 1899.
4) (a) Wang, D.; Deraedt, C.; Ruiz, J.; Astruc, D. Acc. Chem. Res.
Table 3 Catalyst Recycling in Hydroamination of Ethynylbenzene 6a
Using 4-Bromoaniline (7a)
(
a
2
015, 48, 1871. (b) Baig, R. B. N.; Nadagouda, M. N.; Varma, R. S.
Coord. Chem. Rev. 2015, 287, 137. (c) Dalpozzo, R. Green Chem.
015, 17, 3671. (d) Rossi, L. M.; Costa, N. J. S.; Silva, F. P.;
Br
2
catalyst 5 (2 mol%)
CF3SO3H (2 mol%)
Wojcieszak, R. Green Chem. 2014, 16, 2906. (e) Baig, R. B. N.;
Varma, R. S. Chem. Commun. 2013, 752. (f) Polshettiwar, V.;
Luque, R.; Fihri, A.; Zhu, H.; Bouhrara, M.; Basset, J.-M. Chem.
Rev. 2011, 111, 3036. (g) Schätz, A.; Reiser, O.; Stark, W. J. Chem.
Eur. J. 2010, 16, 8950.
N
+
H2N
Br
1
00 °C, 24 h
8
a
6a
7a
(
5) (a) Chen, S.-W.; Zhang, Z.-C.; Zhai, N.-N.; Zhong, C.-M.; Lee, S.-g.
Tetrahedron 2015, 71, 648. (b) Wang, Z.; Yu, Y.; Zhang, Y. X.; Li,
S. Z.; Qian, H.; Lin, Z. Y. Green Chem. 2015, 17, 413. (c) Karimi, B.;
Mansouri, F.; Vali, H. Green Chem. 2014, 16, 2587. (d) Li, P.;
Wang, L.; Zhang, L.; Wang, G.-W. Adv. Synth. Catal. 2012, 354,
1307. (e) Yang, H.; Wang, Y.; Qin, Y.; Chong, Y.; Yang, Q.; Li, G.;
Zhang, L.; Li, W. Green Chem. 2011, 13, 1352. (f) Zhang, X.; Li, P.;
Ji, Y.; Zhang, L.; Wang, L. Synthesis 2011, 2975. (g) Phan, N. T. S.;
Le, H. V. J. Mol. Catal. A 2011, 334, 130.
Run
Yield (%)b,c
1st
73 (22)
2nd
3rd
4th
5th
83 (14)
84 (7)
85 (5)
86 (3)
a
The reaction conditions were the same as those indicated in Table 1.
Determined by the integration of H NMR absorptions referring to an in-
b
1
ternal standard.
c
The value in parenthesis is the chemical yield of acetophenone.
In summary, we prepared a novel magnetite-supported
NHC–gold(I) complex. By employing 2 mol% of the magne-
tite-supported gold(I) complex as a catalyst, the hydroami-
(6) (a) Chen, S.-W.; Zhang, Z.-C.; Ma, M.; Zhong, C.-M.; Lee, S.-g.
Org. Lett. 2014, 16, 4969. (b) Che, C.; Li, W.; Lin, S.; Chen, J.;
Zheng, J.; Wu, J.; Zheng, Q.; Zhang, G.; Yang, Z.; Jiang, B. Chem.
Commun. 2009, 5990. (c) Hirakawa, T.; Tanaka, S.; Usuki, N.;
©
Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–D

D
Synlett
K. Fujita et al.
Letter
Kanzaki, H.; Kishimoto, M.; Kitamura, M. Eur. J. Org. Chem. 2009,
89. (d) Jacinto, M. J.; Santos, O. H. C. F.; Jardim, R. F.; Landers,
ethynylbenzene (6a), namely no organic solvents was used for
washing of the catalyst 5 because this catalytic conversion was
sequentially carried out without the evaporation of solvents in
vacuo after the magnetic decantation.
7
R.; Rossi, L. M. Appl. Catal., A 2009, 360, 177. (e) Mori, K.; Kanai,
S.; Hara, T.; Mizugaki, T.; Ebitani, K.; Jitsukawa, K.; Kaneda, K.
Chem. Mater. 2007, 19, 1249. (f) Kotani, M.; Koike, T.;
Yamaguchi, K.; Mizuno, N. Green Chem. 2006, 8, 735. (g) Hu, A.;
Yee, G. T.; Lin, W. J. Am. Chem. Soc. 2005, 127, 12486.
(16) (a) Yuan, D.; Tang, H.; Xiao, L.; Huynh, H. V. Dalton Trans. 2011,
40, 8788. (b) Duan, H.; Sengupta, S.; Petersen, J. L.; Akhmedov,
N. G.; Shi, X. J. Am. Chem. Soc. 2009, 131, 12100.
21
22
23
24
(
7) (a) Cano, R.; Perez, J. M.; Ramon, D. J. Appl. Catal., A 2014, 470,
(17) NMR spectra of 8a, 8b, 8d, and 8f are in accordance with
those reported in the literature.
177. (b) Fujita, K.; Umeki, S.; Yasuda, H. Synlett 2013, 24, 947.
(
c) Fujita, K.; Umeki, S.; Yamazaki, M.; Ainoya, T.; Tsuchimoto,
(18) Selected Data for New Compounds
T.; Yasuda, H. Tetrahedron Lett. 2011, 52, 3137.
(E)-4-Bromo-N-{1-(4-tert-butylphenyl)ethylidene}aniline (8c)
White powder; yield: 72%; mp 138.0–138.6 °C. IR (KBr): 2963,
2901, 2866, 1628, 1601, 1474, 1296, 1211, 1007, 853, 841, 586
(
8) A previously reported magnetically recoverable phosphine–
gold(I) catalyst:Yang, W.; Wei, L.; Yi, F.; Cai, M. Catal. Sci. Tech-
nol. 2016, 6, in press; DOI: 10.1039/C5CY02159F.
–1 1
cm
. H NMR (400 MHz, CDCl ): δ = 7.89 (d, J = 8.4 Hz, 2 H,
3
(
9) Previously reported magnetically recoverable gold(III) catalysts:
ArH), 7.45 (t, J = 9.2 Hz, 4 H, ArH), 6.66 (d, J = 8.4 Hz, 2 H, ArH),
2.20 (s, 3 H, CH ), 1.35 [s, 9 H, C(CH ) ]. C NMR (100 MHz,
3 3 3
13
(
a) Sadeghzadeh, S. M. RSC Adv. 2014, 4, 43315. (b) Li, B.; Gao, L.;
Bian, F.; Yu, W. Tetrahedron Lett. 2013, 54, 1063.
10) (a) Liu, X.; Ma, Z.; Xing, J.; Liu, H. J. Magn. Magn. Mater. 2004,
70, 1. (b) Massart, R. IEEE Trans. Magn. 1981, 17, 1247.
11) Lambardo, M.; Easwar, S.; Marco, A. D.; Pasi, F.; Trombini, C. Org.
Biomol. Chem. 2008, 6, 4224.
CDCl ): δ = 166.0, 154.3, 151.0, 136.5, 132.0, 127.1, 125.5, 121.4,
3
(
(
(
(
(
116.1, 35.0, 31.3, 17.4. Anal. Calcd (%) for C18H20NBr: C, 65.46; H,
6.10; N, 4.24; Br, 24.19. Found (%): C, 65.53; H, 5.98; N, 4.28; Br,
24.11.
2
4-Bromo-N-(2-octylidene)aniline (8e)
12) Zeng, X.; Yang, X.; Zhang, Y.; Qing, C.; Zhang, H. Bioorg. Med.
Chem. Lett. 2010, 20, 1844.
Colorless oil; E/Z mixture; yield: 75%. IR (KBr): 2954, 2927,
–1
2857, 1661, 1480, 1365, 1231, 1167, 1069, 1008, 842, 654 cm
.
13) 1H NMR and 13C NMR data of 3 and 4 are shown in the Support-
ing Information.
1
H NMR and C NMR resonances were presented as two signals
13
1
(indicated as major and minor). H NMR (400 MHz, THF-d ): δ =
8
14) Selected Data for Compound 5
7.38–7.35 (m, 2 H, ArH), 6.56–6.52 (m, 2 H, ArH), 2.37 (t, J = 7.5
Hz, 2 H, CH , major), 2.11 (t, J = 7.9 Hz, 2 H, CH , minor), 2.08 (s,
Black powder. IR (KBr): 3086, 2916, 1651, 1558, 1512, 1458,
2
2
–1
1
0
042, 949, 903, 856 cm . Anal.; found (%): C, 2.29; H, 0.33; N,
.22; Cl, 0.87; Au, 3.36. An X-ray diffraction pattern is shown in
3 H, CH , minor), 1.74 (s, 3 H, CH , major), 1.69–1.62 (m, 2 H,
3
3
CH , major), 1.51–1.44 (m, 2 H, CH , minor), 1.42–1.32 (m, 6 H,
2
2
the Supporting Information.
15) General Procedure
(CH ) , major), 1.26–1.17 (m, 6 H, (CH ) , minor), 0.91 (t, J = 6.7
2
3
2
3
13
(
Hz, 3 H, CH , major), 0.85 (t, J = 7.0 Hz, 3 H, CH , minor).
C
3
3
To a solution of the alkyne 6 (9 mmol) and the amine 7 (3
mmol) were successively added the indicated amounts of the
magnetite-supported gold(I) catalyst 5 and an acid under an
argon atmosphere. The reaction mixture was stirred at the indi-
cated temperature for 24 h under an argon atmosphere. The
magnetite-supported gold(I) catalyst 5 was separated by mag-
netic decantation using an external magnet, and the reaction
mixture was then transferred out of the reaction vessel, fol-
lowed by washing of the catalyst 5 with the alkyne 6 three
times under argon atmosphere. The chemical yield of the imine
NMR (100 MHz, THF-d ): δ (major) = 172.0, 152.2, 132.32,
121.85, 115.8, 41.8, 32.5, 29.7, 26.5, 23.3, 19.3, 14.3; δ (minor) =
8
172.4, 151.7, 132.27, 121.80, 115.6, 34.5, 32.2, 29.9, 27.5, 25.6,
+
23.1, 14.2. MS (EI): m/z calcd for C14H20NBr: 281.0779 [M ];
found: 281.0780.
(19) In Table 2, entry 6, the magnetically recovered gold(I) catalyst 5
was washed with aniline (7b) after the hydroamination.
(20) The magnetically recovered gold(I) catalyst 5 was reused for the
subsequent hydroamination of the alkyne 6a by the addition of
6a, 7a, and trifluoromethanesulfonic acid to the reaction vessel.
(21) Mirabdolbaghi, R.; Dudding, T. Org. Lett. 2015, 17, 1930.
(22) Weemers, J. J. M.; Sypaseuth, F. D.; Bäuerlein, P. S.; van der
Graaff, W. N. P.; Filot, I. A. W.; Lutz, M.; Müller, C. Eur. J. Org.
Chem. 2014, 350.
8
and the corresponding ketone were determined by integrating
1
H NMR absorptions referring to an internal standard [4-tert-
butyltoluene (1 mmol)], which was added to the reaction mix-
ture. Any organic solvents were not used for washing of the cat-
alyst 5 in order to avoid the vaporization of products and an
internal standard during the evaporation of organic solvents in
vacuo. Also in the case of catalyst recycling as shown in Table 3,
(23) Liu, Y.; Du, H. J. Am. Chem. Soc. 2013, 135, 6810.
(24) Anderson, L. L.; Arnold, J.; Bergman, R. G. Org. Lett. 2004, 6,
2519.
©
Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–D