Gold(I)-catalyzed diketonization of alkynes and its application for the one-pot synthesis of quinoxaline derivatives

Article abstract of DOI:10.1055/s-0033-1338804

A gold(I)-catalyzed oxidative diketonization of alkynes by water in the presence of Selectfluor has been achieved. The application of the present protocol for the one-pot synthesis of quin-oxaline derivatives from alkynes and o-phenylenediamines has also been successful. Georg Thieme Verlag Stuttgart · New York.

Full text of DOI:10.1055/s-0033-1338804

LETTER
▌1371
letter
Gold(I)-Catalyzed Diketonization of Alkynes and Its Application for the One-
Pot Synthesis of Quinoxaline Derivatives
Gold(I)-Catalyzed Diketonization of Alkynes
Yunkui Liu,* Xiaoling Chen, Jian Zhang, Zhenyuan Xu
State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, Zhejiang University of Technology, Hangzhou, 310014,
P. R. of China
Fax +86(571)88320066; E-mail: ykuiliu@zjut.edu
Received: 06.03.2013; Accepted after revision: 19.04.2013
catalysis11f,11g due to the importance of fluorocompounds
Abstract: A gold(I)-catalyzed oxidative diketonization of alkynes
in synthetic, pharmaceutical, and material sciences.12 En-
by water in the presence of Selectfluor has been achieved. The ap-
couraged by our recent success in the gold(I)-catalyzed
plication of the present protocol for the one-pot synthesis of quin-
fluoroamination of alkynes in the presence of Selectflu-
or,11f we envisioned that o-(alk-1-ynyl)benzoates would
undergo the fluorolactonization to give 4-fluoro-isocou-
marin derivatives under the gold(I)/Selectfluor catalytic
system. For an initial study, ethyl 2-(phenylethynyl)ben-
zoate (1a) was treated with two equivalents of Selectfluor
and NaHCO3 in the presence of 2.5 mol% of Ph3PAuCl in
dried MeCN at 60 °C for 24 h. No 4-fluoro-isocoumarin
derivative 3a was obtained while a 1,2-diketone 2a was
isolated in 17% yield (Scheme 1; Table 1, entry 1). Our in-
terest in the novel oxidative diketonization of alkynes to
access 1,2-dicarbonyl compounds catalyzed by a gold(I)
catalyst prompted us to optimize the reaction conditions
so as to make the reaction synthetically valuable (Table
1).13,14 When the reaction was conducted in the non-dried
MeCN under the catalysis of Ph3PAuCl or Ph3PAuNTf2 in
the presence of Selectfluor at 60 °C for 24 hours, the yield
of 2a was improved to 20% and 35%, respectively (Table
1, entries 2 and 3). The performance of the reaction at 80
°C using MeCN–H2O (100:1, v/v) as a medium under oth-
erwise identical reaction conditions as in entry 3, the yield
of 2a was improved to 80% (Table 1, entry 4). It was prov-
en that the base additives were dispensable for the reac-
tion since the yield of 2a could be obtained in 82%
without an addition of NaHCO3 (Table 1, entry 5). The
employment of two equivalents of Selectfluor was indis-
pensable for the reaction otherwise the yield would be
lower or negligible (Table 1, entry 6). Solvent-screening
experiments established that a mixture of MeCN and H2O
(100:1, v/v) was the best choice of solvent (Table 1, en-
tries 5, 9–15). The catalytic performances of several other
gold(I) and gold(III) catalysts including AuCl,
oxaline derivatives from alkynes and o-phenylenediamines has also
been successful.
Key word: gold catalysis, alkynes, 1,2-diketones, oxidation re-
actions, quinoxalines
The past decade has witnessed significant advances in ho-
mogeneous gold catalysis.1 Indeed, gold-catalyzed reac-
tions have now emerged as a powerful tool for the
construction of molecular diversity and structural com-
plexity.1 From a mechanistic point of view, homogeneous
gold-catalyzed reactions could be classified into two cat-
alogues, namely, reactions involving single Au(I) or
Au(III) catalytic cycle and reactions involving
Au(I)/Au(III) redox catalytic cycle. While the former type
of reaction has been applied in organic synthesis over de-
cades,1 it was not until 2008 that the first homogeneous
gold-catalyzed carbon–carbon coupling reaction involv-
ing Au(I)/Au(III) redox catalytic cycle was reported by
Tse2 and coworkers. Since the pioneering work of Tse and
the important works by Wegner,3 Zhang,4 Toste,5 Waser,6
Nevado,7 Hammond,8 and Gouverneur9 etc., the gold-cat-
alyzed reactions involving Au(I)/Au(III) redox catalytic
systems10 have received increasing interest in the area of
gold catalysis because this type of reaction opens up new
avenues for building up complex molecules otherwise dif-
ficult to be accomplished with conventional single Au(I)
or Au(III) catalytic systems.
As part of our continued interests in gold-catalyzed reac-
tions,11 recently our efforts have focused on the synthesis
of fluoro-containing compounds via Au(I)/Au(III) redox
O
O
EtO
O
catalyst (2.5 mol%)
Selectfluor (2 equiv)
O
OEt
O
Ph
solvents, additives
80 °C, 24 h
Ph
O
Ph
F
2a
1a
3a (no formation)
Scheme 1
SYNLETT 2013, 24, 1371–1376
Advanced online publication: 05.06.2013
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0033-1338804; Art ID: ST-2013-W0208-L
© Georg Thieme Verlag Stuttgart · New York

1372
Y. Liu et al.
LETTER
CO2R1
O
IMeSAuNTf2 [IMes = 1,3-bis(mesityl)imidazol-2-yli-
dene], and AuCl3 were investigated, all showed inferior
catalytic activities than that of Ph3PAuNTf2 (Table 1, en-
tries 16–18 vs. entry 5). Control experiments showed that
no desired product was detected in the absence of a gold
catalyst (Table 1, entry 19). Moreover, when conventional
Lewis or Brønsted acids were surveyed as a catalyst for
the reaction, the results showed a far less effectiveness
than that of Ph3PAuNTf2 (Table S1, Supporting Informa-
tion).
CO2R1
Ph3PAuNTf2 (2.5 mol%)
Selectfluor (2 equiv)
R2
MeCN–H2O (100:1, v/v)
80 °C, 24 h
R2
O
2
1
Scheme 2
It was found that 1 with electron-deficient aromatic rings
(R2) generally gave higher yields of 2 than those bearing
electron-rich aromatic rings (Table 2, entries 7–11 vs. en-
tries 2–6). When 1n bearing an aliphatic group of n-hexyl
was used as a substrate, the corresponding 1,2-diketone
2n could not be isolated by column chromatography (Ta-
ble 2, entry 14), but could be trapped by o-phenylenedi-
amine in solution to give the corresponding quinoxaline
derivative (see Table 3, entry 9). Interestingly, when 1o
To investigate the scope of this gold(I)-catalyzed oxida-
tive diketonization of alkynes, a series of o-(alk-1-
ynyl)benzoates 1 were subjected to the optimized reaction
conditions (Scheme 2, Table 2). In most cases, 1 could be
smoothly diketonized to give the corresponding 1,2-dike-
tones 2 in moderate to good yields (Table 2, entries 1–15).
Table 1 Optimization of Reaction Conditions for the Diketonization of Alkyne 1aa
Entry
Catalyst (2.5 mol%)
Solvent
Additives
Yield of 2a (%)b
1
2
Ph3PAuCl
Ph3PAuCl
Ph3PAuNTf2
Ph3PAuNTf2
Ph3PAuNTf2
Ph3PAuNTf2
Ph3PAuNTf2
Ph3PAuNTf2
Ph3PAuNTf2
Ph3PAuNTf2
Ph3PAuNTf2
Ph3PAuNTf2
Ph3PAuNTf2
Ph3PAuNTf2
Ph3PAuNTf2
AuCl
MeCNc
NaHCO3
17d
MeCNe
NaHCO3
20d
3
MeCNe
NaHCO3
35d
4
MeCN–H2O (100:1)
MeCN–H2O (100:1)
MeCN–H2O (100:1)
MeCN–H2O (100:1)
MeCN–H2O (100:1)
MeCN–H2O (20:1)
MeCN–H2O (50:1)
MeCN–H2O (200:1)
DMF–H2O (100:1)
THF–H2O (100:1)
CH2Cl2–H2O (100:1)
dioxane–H2O (100:1)
MeCN–H2O (100:1)
MeCN–H2O (100:1)
MeCN–H2O (100:1)
MeCN–H2O (100:1)
NaHCO3
80
5
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
82
6
36f, 15g, 0h
7
68i
10j
76
76
77
44
16
0
8
9
10
11
12
13
14
15
16
17
18
19
53
10
13
15
0
AuCl3
IMeSAuNTf2
–
a Reactions were carried out with 1a (0.2 mmol), Selectfluor (2 equiv), additive (2 equiv), and catalyst (2.5 mol%) in solvent (2 mL) at 80 °C
for 24 h unless otherwise stated.
b Isolated yields.
c Dried MeCN was used.
d The reaction was performed at 60 °C.
e Non-dried MeCN was used.
f In the presence of 1.0 equiv of Selectfluor.
g In the presence of 0.5 equiv of Selectfluor.
h In the absence of Selectfluor.
i The reaction time was 12 h.
j The reaction was performed at 25 °C.
Synlett 2013, 24, 1371–1376
© Georg Thieme Verlag Stuttgart · New York

LETTER
Gold(I)-Catalyzed Diketonization of Alkynes
1373
bearing a cyclopropyl group was used, the desired 1,2-
diketone 2o was successfully isolated by column chroma-
tography in 54% yield (Table 2, entry 15).
Ph3PAuNTf2 (2.5 mol%)
Selectfluor (2 equiv)
CO2Et
18O
CO2Et
(1)
MeCN–H218O (100:1, v/v)
80 °C, 24 h
Ph
18O
Ph
Table 2 Gold(I)-Catalyzed Diketonization of Alkynes 1 by Water in
1a
2a-18O (80% yield)
m/z = 286 [M+]
the Presence of Selectfluora
Entry
1 R1, R2
Product 2 Yield (%)b
O
Ph
Ph
1
2
1a Et, Ph
2a
2b
2c
2d
2e
2f
82
O
Ph3PAuNTf2 (2.5 mol%)
Selectfluor (2 equiv)
1b Et, 2-MeC6H4
1c Et, 4-MeC6H4
1d Et, 4-EtC6H4
1e Et, 4-(n-C5H11)C6H4
1f Et, 4-EtOC6H4
1g Et, 2-ClC6H4
1h Et, 3-ClC6H4
1i Et, 4-ClC6H3Ph
1j Et, 4-BrC6H4
1k Et, 4-FC6H4
1l Me, Ph
73
2p (31% yield)
Ph
Ph
(2)
+
3
78
MeCN–H2O (100:1, v/v)
80 °C, 24 h
1p
O
4
78
F
F
5
72
4p (45% yield)
6
75
O
O
Ph3PAuNTf2 (2.5 mol%)
Selectfluor (2 equiv)
7
2g
2h
2i
87
(3)
(4)
8
86
MeCN–H2O (100:1, v/v)
80 °C, 20 h
F
F
O
9
85
4p
2p (49% yield)
10
11
12
13
14
15
2j
87
EtO2C
Ph3PAuNTf2 (2.5 mol%)
Selectfluor (2 equiv)
EtO2C
O
2k
2l
88
MeCN–H2O (100:1, v/v)
Ph
80 °C, 24 h
72
Ph
O
2q or 2r (trace)
1q: meta-substituted
1r: para-substituted
1m n-Pr, Ph
2m
2n
2o
75
1n Et, n-Hex
not isolated
54
Scheme 3 Mechanistic studies
1o Et, c-Pr
a Reactions were carried out with 1 (0.2 mmol), Selectfluor (2 equiv),
and Ph3PAuNTf2 (2.5 mol%) in MeCN–H2O (100:1m v/v, 2 mL) at
80 °C for 24 h.
leading to the efficient conversion of 1 into 2. For alkynes
substituted with a meta- or para-ester group, only trace
amount of the desired product was obtained under the
standard conditions (Scheme 3, eq. 4). Then 7 was con-
verted into an organogold(III) species 8 in the presence of
two molecules of Selectfluor (path I).8,10 Alternatively, 8
could also be generated by a hydroxyauration reaction of
alkyne 1 with H2O and a cationic gold(III) species arising
from the oxidation of Au(I) with Selectfluor followed by
the electrophilic addition of F+ to the resulting intermedi-
ate 11 (path II).8,10,11g The intermediate 8 may undergo re-
ductive elimination to afford an intermediate 9.8,11f
Hydrolysis of the α,α-difluoro-ketone 9 finally formed
1,2-diketone 2.15,16
b Isolated yields based on 1.
Several experiments were performed to gain a mechanis-
tic insight into the present reaction (Scheme 3). An 18OH2-
labeling experiment unambiguously established that the
oxygen atoms of the carbonyl groups of 2 originated from
water (Scheme 3, eq. 1). When diphenyl acetylene (1p)
was subjected to the optimized reaction conditions, 1,2-
diketone 2p was obtained in 31% yield along with an α,α-
difluoro-ketone product 4p in 45% yield (Scheme 3, eq.
2). Since two equivalents of Selectfluor were needed for
the reaction to go to completion, it is reasonable to specu-
late that the α,α-difluoro-ketone species would be an in-
termediate for the reaction. When the preparative α,α-
difluoro-ketone 4p was subjected to the standard reaction
conditions, the corresponding 1,2-diketone 2p was indeed
obtained in 49% yield (Scheme 3, eq. 3).
Quinoxaline derivatives have shown a broad spectrum of
biological activities, which has led them to be privileged
structures in drug discovery.17 1,2-Diketones are the use-
ful substrates for the synthesis of quinoxalines by the con-
densation reaction with o-phenylenediamines generally
catalyzed by a Lewis- or Brønsted acid.18 In light of gold
complexes also being efficient catalysts for the condensa-
tion reactions,11c,19 we envisioned that the 1,2-ketones re-
sulting from the gold-catalyzed diketonization of alkynes
would further undergo condensation with o-phenylenedi-
amines in the presence of the gold catalyst, thus enabling
one-pot synthesis of quinoxalines directly from alkynes
and o-phenylenediamines.
Based on these results and the previous reports,8,10,11f,11g
a
possible mechanism concerning the gold(I)-catalyzed ox-
idative diketonization of o-(alk-1-ynyl)benzoates 1 is pro-
posed (Scheme 4). Firstly, the activation of the alkyne
moiety of 1 by gold(I) species was followed by the nu-
cleophilic addition of H2O to 1 to form an organogold(I)
intermediate 7.10,11f,11g The ortho-ester group of 1 may be
helpful for gold species to activate the triple bond of 1
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 1371–1376

1374
Y. Liu et al.
LETTER
OR1
OR1
O
A+u(I)L
R2
O
path I
R2
5
1
F
OR1
Selectfluor
path II
L
A2u+(III)
A+u(I)L
10
O
1
H2O
H+
Au(I)L
OR1
OR1
reductive
elimination
O
O
R2
6
O
+
F
Au(III)
R2
L
OH
F
F
H2O
9
OR1
O
R2
H2O
11
OR1
F
H+
+
HF
Au(III)
OR1
O
L
F
O
Au(I)L
O
R2
8
OH
O
Cl
N+
N+
R2
R2
7
O
2
–
F
2
2 BF4
Scheme 4 Proposed mechanism
To pursue this goal, the oxidative diketonization of 1 was
carried out under the optimized reaction conditions fol-
lowed by the treatment of the in situ generated 1,2-di-
ketones 1 with o-phenylenediamines 12 for a further six
hours at room temperature (Scheme 5). As expected, a va-
riety of quinoxaline derivatives 13 could be synthesized in
moderate to good yields via the one-pot procedure (Table
3, entries 1–11).
Table 3 Gold(I)-Catalyzed One-Pot Synthesis of Quinoxalines 13
from Alkynes 1 and o-Phenylenediamines 12a
Entry
1 R1
12
Yield of 13 (%)b
1
2
1a Ph
12a
12a
12a
12a
12a
12a
12a
12a
12a
12b
12c
13aa 78
13da 73
13fa 72
13ga 83
13ha 83
13ia 82
13ja 84
13ka 84
13na 73
13ab 77
13ac 80
1d 4-EtC6H4
1f 4-EtOC6H4
1g 2-ClC6H4
1h 3-ClC6H4
1i 4-ClC6H4
1j 4-BrC6H4
1k 4-FC6H4
1n n-Hex
3
CO2Et
4
5
1) Ph3PAuNTf2 (2.5 mol%)
EtO2C
Selectfluor (2 equiv)
MeCN–H2O (100:1, v/v)
80 °C, 24 h
R1
6
R2
R2
N
1
+
7
2) addition of 12
r.t., 6 h
R2
R2
NH2
NH2
N
13
R1
8
9
12
10
11
1a Ph
12a R2 = H
12b R2 = Me
12c R2 = Cl
1a Ph
a Reactions were carried out with 1 (0.2 mmol), Selectfluor (2 equiv),
and Ph3PAuNTf2 (2.5 mol%) in MeCN–H2O (100:1, v/v, 2 mL) at 80
°C for 24 h, followed by the addition of 12 (0.2 mmol) and reacting
for further 6 h at r.t.
Scheme 5
In summary, we described a novel gold(I)-catalyzed oxi-
dative diketonization of alkynes using water as an inex-
pensive oxygen source in the presence of Selectfluor. The
present protocol enabled the one-pot synthesis of quin-
oxalines directly from alkynes and o-phenylenediamines.
b Isolated yields based on 1.
Typical Experimental Procedure for the 1,2-Diketonization of
Alkynes
Ph3PAuNTf2 (3.7 mg, 0.005 mmol), Selectfluor (141.7 mg, 0.4
mmol, 2 equiv), ethyl 2-(phenylethynyl)benzoate (1a; 50.1 mg, 0.2
mmol), and MeCN–H2O (100:1, v/v, 2 mL) were added to a 10 mL
Synlett 2013, 24, 1371–1376
© Georg Thieme Verlag Stuttgart · New York

LETTER
Gold(I)-Catalyzed Diketonization of Alkynes
1375
flask. Then the reaction mixture was stirred at 80 °C for 24 h. Upon
completion, the resulting mixture was diluted with CH2Cl2 (10 mL)
and filtered through Celite. After evaporation of the solvent under
vacuum, the residue was purified by column chromatography on sil-
ica gel (100–200 mesh) using PE–EtOAc (30:1) as eluent to give
pure 2a.
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Zhang, L. Angew. Chem. Int. Ed. 2011, 50, 4450.
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Toste, F. D. J. Am. Chem. Soc. 2010, 132, 8885.
Typical Experimental Procedure for the One-Pot Synthesis of 3
from Alkynes 1 and o-Phenylenediamines 12
Ph3PAuNTf2 (3.7 mg, 0.005 mmol), Selectfluor (141.7 mg, 0.4
mmol, 2 equiv), ethyl 2-(phenylethynyl)benzoate (1a; 50.1 mg, 0.2
mmol), and MeCN–H2O (100:1, v/v, 2 mL) were added to a 10 mL
flask. The reaction mixture was stirred at 80 °C for 24 h followed
by the addition of o-phenylenediamine (12a; 21.6 mg, 0.2 mmol).
The resulting mixture was stirred at r.t. for 6 h. Upon completion,
the reaction mixture was diluted with CH2Cl2 (10 mL) and filtered
through Celite. After evaporation of the solvent under vacuum, the
residue was purified by column chromatography on silica gel (100–
200 mesh) using PE–EtOAc (20:1) as eluent to give pure 13aa.
Representative Data
(b) Brenzovitch, W. E.; Benitez, D.; Lackner, A. D.;
Shunatona, H. P.; Tkatchouk, E.; Goddard, W. A.; Toste, F.
D. Angew. Chem. Int. Ed. 2010, 49, 5519. (c) Brenzovitch,
W. E.; Brazeau, J. F.; Toste, F. D. Org. Lett. 2010, 12, 4728.
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A.; Toste, F. D. J. Am. Chem. Soc. 2011, 133, 14293.
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Ed. 2009, 48, 9346. (b) Brand, J. P.; Waser, J. Angew. Chem.
Int. Ed. 2010, 49, 7304.
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1512. (b) de Haro, T.; Nevado, C. Adv. Synth. Catal. 2010,
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2011, 47, 248. (d) de Haro, T.; Nevado, C. Angew. Chem.
Int. Ed. 2011, 50, 906.
Compound 2a
Pale yellow liquid. Rf = 0.37 (PE–EtOAc, 30:1). IR (neat): ν = 2979,
1691, 1591, 1451, 1373, 1290, 1202, 1090, 853, 797, 730, 642 cm–
1. 1H NMR (500 MHz, CDCl3): δ = 8.22 (d, J = 8.5 Hz, 2 H), 8.03(d,
J = 8.0 Hz, 1 H), 7.72–7.63 (m, 4 H), 7.55 (t, J = 8.0 Hz, 2 H), 4.19
(q, 2 H), 1.25(t, J = 7.0 Hz, 3 H). 13C NMR (125 MHz, CDCl3): δ =
193.7, 189.1, 166.6, 138.9, 133.8, 133.2, 132.9, 131.5, 130.8, 130.2,
130.0, 129.5, 128.4, 62.0, 14.1. MS (EI, 70eV): m/z (%) = 282(60)
[M+].
Compound 13aa
Pale yellow solid; Rf = 0.5 (PE–EtOAc, 20:1); mp 50–55 °C. IR
(KBr): ν = 3061, 2924, 1718, 1465, 1344, 1273, 1130, 1093, 767,
703, 605 cm–1. 1H NMR (500 MHz, CDCl3): δ = 8.23–8.15 (m, 2 H),
7.91 (dd, J = 8.0, 1.0 Hz, 1 H), 7.80–7.77 (m, 2 H), 7.61–7.25 (m, 8
H), 3.97 (q, J = 7.0 Hz, 2 H), 0.88 (t, J = 7.0 Hz, 3 H). 13C NMR
(125 MHz, CDCl3): δ = 166.5, 154.6, 153.7, 141.4, 140.8, 138.3,
132.1, 131.0, 130.9, 130.3, 129.9, 129.8, 129.7, 129.3, 129.1, 128.8,
128.7, 128.0, 61.0, 29.7, 13.7. MS (EI, 70eV): m/z (%) = 266 (100)
[M+].
(8) Wang, W.; Jasinski, J.; Hammond, G. B.; Xu, B. Angew.
Chem. Int. Ed. 2010, 49, 7247.
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12, 4904. (d) Hopkinson, M. N.; Giuffredi, G. T.; Gee, A.
D.; Gouverneur, V. Synlett 2010, 2737.
Acknowledgment
(10) For review articles on the gold catalysis involving
Au(I)/Au(III) redox catalytic cycle, see: (a) Hopkinson, M.
N.; Gee, A. D.; Gouverneur, V. Chem. Eur. J. 2011, 17,
8248. (b) Wegner, H. A.; Auzias, M. Angew. Chem. Int. Ed.
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Financial support from the Natural Science Foundation of China
(No. 21172197) and Zhejiang Province (No. Y4100201), the Foun-
dation of Science and Technology Department of Zhejiang Provin-
ce (2011R09002-09), and the opening Foundation of Zhejiang
Provincial Top Key Discipline is gratefully acknowledged.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.SnoIufproig
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