A combination of copper(0) powder and selectfluor enables generation of cationic copper species for mild 1,2-dicarbonylation of alkynes

Article abstract of DOI:10.1055/s-0033-1338983

A combination of copper powder and Selectfluor permits the generation of cationic copper species that efficiently catalyze 1,2-dicarbonylation of alkynes under mild conditions with water and dioxygen as inexpensive and environmentally benign sources of oxygen. Georg Thieme Verlag Stuttgart New York.

Full text of DOI:10.1055/s-0033-1338983

LETTER
▌2709
lAetteCr ombination of Copper(0) Powder and Selectfluor Enables Generation of
Cationic Copper Species for Mild 1,2-Dicarbonylation of Alkynes
Copper-Catalyzed 1,2-Dicarbonylation of Alkenes
Wenxia Zhang, Jian Zhang, Yunkui Liu,* 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: 08.08.2013; Accepted after revision: 08.09.2013
philic silylium salt;11 and (d) treatment of a protic acid
Abstract: A combination of copper powder and Selectfluor permits
with a metal hydroxide complex.12 Hammond and co-
the generation of cationic copper species that efficiently catalyze
workers13 recently reported that the redox reaction be-
1,2-dicarbonylation of alkynes under mild conditions with water
tween a neutral low-valence chlorogold(I) complex and a
fluoroammonium-type fluorination reagent containing
a poorly coordinating counteranion such as Selectfluor
[1-(chloromethyl)-4-fluoro-1,4-diazoniabicyclo[2.2.2]oc-
tane ditetrafluoroborate] can generate a fluoro-coordinat-
ed cationic gold(III) species that has high catalytic activity
in electrophilic activation of alkynes (Scheme 1, e). In-
spired by Hammond’s work, we surmised that the redox
reaction of copper powder with Selectfluor should gener-
ate cationic copper species that might act as catalysts for
an efficient and mild 1,2-dicarbonylation of alkynes using
water and air as environmentally benign sources of oxy-
gen (Scheme 1, f).
and dioxygen as inexpensive and environmentally benign sources
of oxygen.
Key words: alkynes, carbonylation, catalysis, copper, ketones
1,2-Dicarbonyl functionalities are important structural
moieties that occur widely in bioactive natural products
and in synthetic molecules.1 Furthermore, 1,2-dicarbonyl
compounds can serve as versatile intermediates for the
construction of complex molecules.2 Many methods for
the synthesis of 1,2-dicarbonyl compounds have been ex-
plored,3,4 among which the transition-metal-catalyzed ox-
idation of alkynes3 is an attractive strategy because of the
ready availability of alkynes5 and the high efficiency of
transition-metal catalysts in oxidative reactions.6 Despite
the many advances made in this field in recent years, most
existing methods suffer from limitations, prominent
among which are (a) the use of expensive transition-metal
catalysts,3a–m,3p (b) the need for harsh reaction condi-
tions,3b,e,3g–i,3k–n and (c) the use of expensive sources of
oxygen.3h,i,l,n There is therefore still a need to develop gen-
eral and simple methods for the synthesis of 1,2-dicarbon-
yl derivatives from alkynes that use inexpensive
transition-metal catalysts and cheap sources of oxygen
under mild reaction conditions.
Initially, we selected the 1,2-dicarbonylation of diphenyl-
acetylene (1a) as a model reaction for optimization of the
reaction conditions (Scheme 2 and Table 1). When diphe-
nylacetylene (1a) was treated with two equivalents of Se-
lectfluor in the presence of 5 mol% of copper powder in
50:1 (v/v) acetonitrile–water at room temperature for four
hours, benzil (2a) was obtained in 90% yield (Table 1, en-
try 11). Among the solvent systems that we investigated,
50:1 (v/v) acetonitrile–water was found to be the best
choice for the reaction (entries 1–10). Attempts to replace
the copper powder with a variety of copper salts gave in-
ferior results to those achieved with the copper(0)–Select-
fluor system (entries 1 and 15–19), suggesting that copper
powder undergoes a unique reaction with Selectfluor to
produce the active catalytic species. Even when the cata-
lytic loading of the copper powder was reduced to 1
mol%, 2a was still obtained in 85% yield (by gas chroma-
tography) after 24 hours (entry 12). Neither an increase in
the loading of the copper powder nor an increase in the re-
action temperature enhanced the yield of 2a (entries 13
and 14). The use of two equivalent of Selectfluor was nec-
essary; otherwise the reaction gave a low or negligible
yield (entries 21 and 22). A combination of copper pow-
der with another F+ reagent, N-fluorobenzenesulfonimide
(NFSI), was far less efficient than the copper(0)–Select-
fluor system (entries 1 and 23). A control experiment
showed that none of the desired product was obtained in
the absence of copper powder (entry 20).
Transition-metal complexes that contain weakly coordi-
nating anions,7 such as tetrafluoroborate, hexafluorophos-
phate, hexafluoroantimonate, or bistriflimide, usually
exhibit more-electrophilic activities in catalytic processes
than do those with coordinating anions (such as a halides),
thereby permitting more-effective promotion of electro-
philic activation of chemical bonds.7,8 Such reactions are
exemplified by cationic gold-8a,b or copper-catalyzed8c–f
activations of multiple bonds and by cationic transition-
metal-catalyzed activations of C−H bonds.8g–i Several
strategies for preparing cationic transition metal species
have been reported (Scheme 1), including: (a) metathesis
between a metal halide and a silver salt of a weakly coor-
dinating anion;7a,8a,b,9 (b) treatment of a protic acid with a
methyl–metal complex;10 (c) halide abstraction by a halo-
Having identified the optimal reaction conditions, we ex-
amined the application of our new protocol to the 1,2-di-
carbonylation of a variety of alkynes (Scheme 3). Diaryl
SYNLETT 2013, 24, 2709–2714
Advanced online publication: 05.11.2013
DOI: 10.1055/s-0033-1338983; Art ID: ST-2013-W0765-L
© Georg Thieme Verlag Stuttgart · New York
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6

2710
W. Zhang et al.
LETTER
previous work on generation of cationic transition-metal species
metathesis
a.
b.
AgX
X = weakly coordinated anion
– CH4
LmMCln
LmMXn
+
–
Me
HX
MLn
LnMX (e.g. M = Au, X = MeSO3
)
+
X = weakly coordinated anion
– Si-Cl
[(Tol)SiEt3]+[B(C6F5)4]–
+
[LAu]+[B(C6F5)4]–
c.
d.
AuL Cl
– H2O
–
HX
OH
MLn
LnMX (e.g. M = Au, X = BF4
)
+
X = weakly coordinated anion
F
+
Cl
redox reaction
–
N
–
L
Au
BF4
2 BF4
e.
AuL Cl
+
+
N
Cl
+
F
this work
f.
Cl
redox reaction
N
–
–
+
2 BF4
Cu(0)
F
Cu
BF4
+
+
N
+
F
R1
H2O, air
R2
r.t.
cationic copper species for 1,2-dicarbonylation of alkynes:
• directly using inexpensive Cu(0) powder as catalyst precursor
• inexpensive and environmentally benign H2O and air as oxygen
sources
O
R2
R1
• mild reaction conditions
O
Scheme 1 Strategies for the generation of cationic transition-metal species
alkynes bearing various electron-donating and/or elec-
tron-withdrawing groups generally reacted smoothly and
gave the desired products 2a–h and 2j–p in 64–93% yield
(Scheme 3); a range of functional groups, including halo,
methyl, ethoxy, and ethoxycarbonyl groups, were well
tolerated under the reaction conditions. An alkyne bearing
a 2-formylphenyl ring also gave the desired product 2i,
catalyst
O
Selectfluor (2 equiv)
Ph
Ph
Ph
Ph
air, r.t., solvent, time
1a
O
2a
Scheme 2
O
Cu(0) (5 mol%)
Selectfluor (2 equiv)
R2
R1
R2
R1
MeCN–H2O (50:1)
air, r.t., 4 h
1
O
2
O
O
O
R
O
O
Cl
O
CHO
EtO
2a: R = H, 90%
2k (88%)
2i (30%)
2b: R = 2-Me, 64%
2c: R = 3-Me, 78%
2d: R = 4-Me, 72%
2e: R = 4-F, 93%
2f: R = 4-Cl, 90%
2g: R = 4-Br, 90%
Cl
O
O
Cl
O
2l (89%)
O
O
2h: R = 4-CO2Et, 79%
EtO
2j (87%)
O
Cl
O
R2
R1
O
O
COOEt
R
2q: R1 = Ph, R2 = Me, 69%
2r: R1 = R2 = n-Pr, 0%
2s: R1 = R2 = CO2Et, 0%
O
2n: R = F, 81%
2o: R = Cl, 83%
2p: R = Br, 65%
Cl
2m (80%)
Scheme 3 Cu(0)–Selectfluor-mediated 1,2-dicarbonylation of alkynes 1. Reaction conditions: 1 (0.2 mmol), Cu(0) powder (0.64 mg, 5 mol%),
Selectfluor (2 equiv), 50:1 MeCN–H2O (2 mL), r.t., 4 h. All yields are isolated yields.
Synlett 2013, 24, 2709–2714
© Georg Thieme Verlag Stuttgart · New York

LETTER
Copper-Catalyzed 1,2-Dicarbonylation of Alkenes
2711
albeit in a lower yield. Alkyne 1q with a methyl group on electron-deficient aryl rings generally reacted smoothly
one sp carbon also readily participated in the reaction to and gave the desired α-keto imides 4a–j in 53–92% yield
give the desired dione 2q in 69% yield. However, alkynes (Scheme 4). The 1,2-dicarbonylation of ynamides 3 was
1r or 1s substituted, respectively, with propyl and ethoxy- generally slower than that of diaryl alkynes 1. Ynamides
carbonyl groups on both sp carbon atoms failed to give the bearing electron-rich aryl rings on the amine group (3i and
desired products.
3j) were more readily dicarbonylated than an ynamide
bearing an electron-deficient aryl ring (3k).
Table 1 Copper-Catalyzed 1,2-Dicarbonylation of Diphenyl-
acetylene (1a) in the Presence of Selectfluor
Several experiments were conducted to gain insight into
the mechanism of the reaction (Scheme 5). When the dik-
etonization of alkyne 1a was conducted in the presence of
H218O, oxygen-18 was incorporated into only one carbon-
yl group of benzyl (2a) (Scheme 5, equation 1). When the
reaction was performed in degassed solvent under argon,
only a trace of 2a was obtained, but the reaction proceed-
ed well upon subsequent exposure to air, giving an 89%
yield of 2a (Scheme 5, equation 2). These experiments
suggest that both water and dioxygen participate in the re-
action. In the course of the reaction, no α,α-difluoro ke-
tone 5 was formed.3m When α,α-difluoro ketone 5 was
subjected to the standard conditions, no formation of ben-
zyl (2a) was detected and 5 was recovered quantitatively
(Scheme 5, equation 3). These results suggested that α,α-
difluoro ketone 5 is unlikely to be an intermediate in the
reaction.3m A stoichiometric reaction of copper powder
with Selectfluor in 50:1 acetonitrile-d3–water at room
temperature gradually gave a light-blue solution (sample
I) and a white precipitate (sample II) (Scheme 5, equation
4). X-ray diffraction analysis revealed that the precipitate
II is 1-(chloromethyl)-4-hydrido-1,4-diazoniabicyclo-
[2.2.2]octane ditetrafluoroborate (6; Supplementary In-
formation, Figure S1).14 Addition of 1.25 equivalents of
1,10-phenanthroline to the solution of I resulted in a deep-
blue solution (sample III) (Scheme 5, equation 5). Elec-
trospray ionization mass spectroscopic analysis of sample
III showed a strong signal at m/z = 260 that was assigned
Entrya Catalyst
(mol%)
Solvent
Time Yieldb
(h)
(%)
1
2
Cu(0) (5)
Cu(0) (5)
Cu(0) (5)
Cu(0) (5)
Cu(0) (5)
Cu(0) (5)
Cu(0) (5)
Cu(0) (5)
Cu(0) (5)
Cu(0) (5)
Cu(0) (3)
Cu(0) (1)
Cu(0) (10)
Cu(0) (5)
CuCl (5)
CuBr (30)
CuBr (5)
Cu2O (5)
CuSO4 (3)
–
MeCN–H2O (50:1)
MeCN–H2O (20:1)
MeCN–H2O (100:1)
MeCN–H2O (200:1)
acetone–H2O (50:1)
DMF–H2O (50:1)
4
12
12
12
12
12
99 (90)c
90
88
80
0
3
4
5
6
0
7
1,4-dioxane–H2O (50:1) 12
0
8
CH2Cl2–H2O (50:1)
EtOAc–H2O (50:1)
MeOH–H2O (50:1)
MeCN–H2O (50:1)
MeCN–H2O (50:1)
MeCN–H2O (50:1)
MeCN–H2O (50:1)
MeCN–H2O (50:1)
MeCN–H2O (50:1)
MeCN–H2O (50:1)
MeCN–H2O (50:1)
MeCN–H2O (50:1)
MeCN–H2O (50:1)
MeCN–H2O (50:1)
MeCN–H2O (50:1)
MeCN–H2O (50:1)
12
12
12
7
0
9
0
10
11
12
13
14d
15
16
17
18
19
20
21e
22f
23g
0
99
85
94
93
<10
<10
<10
10
0
24
4
4
24
24
24
12
12
24
12
24
28
to the species [LCuOH]+ (L = 1,10-phenanthroline).15 19
F
NMR spectroscopic analysis of sample I showed only a
single resonance peak at δ = –151.06 ppm, which is close
to the magnetic resonance signal for [M]–F (M = alkali
metal) (see Supplementary Information).16 These results,
in conjunction with the material-balancing relationship,
suggest that the original cationic FCuIIBF4 species formed
by the redox reaction of copper(0) with Selecfluor might
eventually be transformed into an FCuIIOH17 species un-
der basic conditions. When sample I was used directly as
a catalyst under otherwise identical conditions to those
used for copper powder, benzil (2a) was obtained in 91%
yield (Scheme 5, equation 6).
0
Cu(0) (5)
Cu(0) (5)
Cu(0) (5)
38
0
8
a Reaction conditions: 1a (0.2 mmol), catalyst (0–10 mol%), Select-
fluor (2 equiv), solvent (2 mL), r.t., unless otherwise noted.
b Yield by GC with decane as internal standard.
c Isolated yield.
Therefore, a possible mechanism for the diketonization of
alkyne 1a was proposed (Scheme 6). First, the redox reac-
tion of copper powder with Selectfluor produces a cation-
ic copper species FCuIIBF4 (7) and releases the base 8.13,18
Subsequent activation of the alkyne moiety of 1a by cop-
per complex 7, followed by nucleophilic addition of water
to the triple bond and abstraction of a proton by base 8 re-
sults in formation of intermediates 9 and 6.13,18a Interme-
diate 9 isomerizes into 10, whereas complex 7 is
converted into copper species 11 under the basic condi-
d At 60 °C.
e With 1.0 equiv Selectfluor.
f In the absence of Selectfluor.
g In the presence of (PhSO2)2NF (2.0 equiv).
We also examined the application of the present protocol
for the 1,2-dicarbonylation of a variety of ynamides 3 to
give the corresponding α-keto imides 4 (Scheme 4). In
most cases, ynamides 3 bearing various electron-rich or
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 2709–2714

2712
W. Zhang et al.
LETTER
Cu(0) (5 mol%)
Selectfluor (2 equiv)
R1
O
R1
R2
N
Ar
N
Ar
R2
MeCN/H2O = 50:1 (v/v)
air, r.t., 12 h
O
4
3
O
O
O
O
O
O
O
N
O
O
N
N
N
N
O
O
O
O
O
O
O
O
O
Et
Br
4a (92%)a
4b (92%)a
4c (73%)a
4d (60%)b
Cl
O
O
O
O
O
O
O
N
Cl
N
N
N
O
O
O
O
O
O
O
O
Cl
4e (80%)
4f (85%)
4g (78%)
4h (64%)
O
O
Ts
Ts
Br
SO2
Me
N
Me
Me
N
O
4i (53%)c
O
4j (67%)c
O
4k (38%)c
Scheme 4 Cu(0)–Selectfluor-mediated 1,2-dicarbonylation of ynamides 3. Reaction conditions: 3 (0.2 mmol), Cu(0) powder (0.64 mg,
5 mol%), Selectfluor (2 equiv), 50:1 MeCN–H2O (2 mL), r.t. 12 h, unless otherwise noted. All yields are isolated yields. a Reaction time 8 h.
b Reaction time 9.5 h. c Reaction time 24 h.
tions. Intermediate 10 might, alternatively be generated In summary, we have shown for the first time that a com-
by hydroxycupration of alkyne 1a by copper species 11 bination of copper(0) powder and Selectfluor gives cat-
through an inner-sphere mechanism. Oxidation of inter- ionic copper species that effect efficient and mild 1,2-
mediate 10 with dioxygen gives peroxycopper(III) inter- dicarbonylation of alkynes. This method for 1,2-dicarbon-
mediate 12, which rearranges to intermediate 13.19 ylation of alkynes has the advantages of requiring inex-
Finally, cleavage of the O–O bond in 13, followed by ox- pensive copper(0) powder as the catalyst precursor, using
idation of the resulting alcohol in the presence of copper, water and dioxygen as inexpensive and environmentally
Selectfluor, and water gives the desired product 2a and re- benign sources of oxygen, and proceeding under very
generates the copper(II) species 7 or 11.19,20
mild reaction conditions.
Cu powder (5 mol%)
Selectfluor (2 equiv)
18O
Ph
Ph
Ph
Ph
(1)
(2)
MeCN–H218O (50:1)
r.t., 4 h
1a
O
2a-18
O
yield = 88%
(m/z = 212 [M+])
Cu powder (5 mol%)
Selectfluor (2 equiv)
2a
1a
exposure to air
MeCN–H2O (50:1) (degassed)
under argon, r.t., 4 h
(trace)
89%
O
Cu powder (5 mol%)
Selectfluor (2 equiv)
(3)
(4)
2a
+
5
F
F
MeCN–H2O (50:1)
r.t., 4 h
(no formation)
(recovered)
5
Cl
+
MeCN–H2O (50:1)
N
Cu
1
sample I
+
:
Selectfluor
1
+
–
N
2 BF4
+
r.t.
H
6 (sample II)
1,10-phenanthroline
(1.25 equiv relative to Cu)
sample III
sample I
(5)
(6)
sample I (5 mol%)
Selectfluor (2 equiv)
1a
2a
(91% yield)
MeCN–H2O (50:1)
r.t., 4 h
Scheme 5 Mechanistic studies
Synlett 2013, 24, 2709–2714
© Georg Thieme Verlag Stuttgart · New York

LETTER
Copper-Catalyzed 1,2-Dicarbonylation of Alkenes
2713
Cu(0)
Cl
+
N
–
Selectfluor
N
BF4
8
+
8 + H218O
FCuBF4
H218O
Cl
+
N
7
Ph
Ph
–
N
2 BF4
+
6
1a
H
H2O18
6
11
Ph
outer-sphere
HO18-CuF
Ph
18OH
1a
18O
Ph
Ph
Ph
inner-sphere
Ph
CuF
CuF
9
10
O2
18O
18O
Ph
18O
Ph
18O
Ph
H218O
18O
Ph
Selectfluor
Ph
Ph
Ph
Ph
Ph
Ph
F
F
hydrolysis
O18
5-18
O
Cu(III)F
O
7 or 11
O
O
2a-(18O)2
(m/z = 214 [M+])
• O
O
12
2a-18
O
Cu(II)F
13
(m/z = 212 [M+])
Scheme 6 Proposed mechanism
1,2-Dicarbonylation of Alkynes 1 or Ynamides 3; General Pro-
cedure
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.SnuIpofoiprmntgirStanoIuifpog
r
t
iornat
A 10 mL flask was charged with 1 or 3 (0.2 mmol), Cu(0) powder
(0.64 mg, 5 mol%), Selectfluor (141.7 mg, 0.4 mmol, 2 equiv), and
50:1 MeCN–H2O (2 mL). The mixture was then stirred at r.t. for the
appropriate time. When the reaction was complete, the mixture was
diluted with CH2Cl2 (10 mL) and filtered through Celite. The sol-
vent was evaporated under vacuum and the residue was purified by
column chromatography [silica gel (100–200 mesh), PE–EtOAc
(20:1 to 3:1)] to give pure product 2 or 4.
References and Notes
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Representative Products
1-(4-Chlorophenyl)-2-(4-ethoxyphenyl)ethane-1,2-dione (2j).
Purified by column chromatography [silica gel, PE–EtOAc (10:1)]
as a pale-yellow solid; yield: 50.2 mg (87%); Rf = 0.43; mp 120–121
ºC. IR (neat) = 1660 (C=O) cm–1. 1H NMR (CDCl3, 500 MHz): δ =
7.95–7.93 (m, 4 H), 7.50 (d, J = 8.5 Hz, 2 H), 6.98 (d, J = 8.0 Hz, 2
H), 4.14 (q, J = 7.0 Hz, 2 H), 1.47 (t, J = 7.0 Hz, 3 H). 13C NMR
(CDCl3, 125 MHz): δ = 193.7, 192.5, 164.3, 141.4, 132.5, 131.6,
131.3, 129.4, 125.8, 114.9, 64.6, 14.6. MS (EI, 70 eV): m/z (%) =
288 (10) [M+]. HRMS (ESI); m/z [M + H]+ calcd for C16H14ClO3:
289.0631; found: 289.0636.
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Y.; Zhao, Z.; Lindsley, C. W. Org. Lett. 2004, 6, 1453.
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3-[Oxo(phenyl)acetyl]-1,3-oxazolidin-2-one (4a)
Purified by column chromatography [silica gel, PE–EtOAc (10:1)]
as a pale-yellow solid; yield: 40.3 mg (92%): Rf = 0.50; mp 105–107
1
ºC. IR (neat) = 1785, 1696 (C=O) cm–1. H NMR (CDCl3, 500
MHz): δ = 7.89 (d, J = 7.0 Hz, 2 H), 7.66 (t, J = 7.5 Hz, 1 H), 7.52
(t, J = 8.0 Hz, 2 H), 4.60 (t, J = 8.0 Hz, 2 H), 4.17 (t, J = 8.0 Hz, 2
H). MS (EI, 70 eV): m/z (%) = 219 (2) [M+], 105 (100).
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Financial support from the Natural Science Foundation of China
(Nos. 21172197 and 21372201), Zhejiang Province (No.
Y4100201), the Foundation of Science and Technology Department
of Zhejiang Province (No. 2011R09002-09), and the Opening
Foundation of Zhejiang Provincial Top Key Discipline is gratefully
acknowledged.
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 2709–2714

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Synlett 2013, 24, 2709–2714
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