Room-temperature palladium(II)-catalyzed N-vinylation of sulfonamides and acylamides with vinyl acetate as vinyl source

Article abstract of DOI:10.1016/j.tetlet.2010.08.029

Room-temperature N-vinylation of various substituted sulfonamides and acylamides with vinyl acetate was achieved for the first time with a palladium/carbene catalyst system. This reaction provides a useful method for synthesis of enamides under mild conditions.

Full text of DOI:10.1016/j.tetlet.2010.08.029

Tetrahedron Letters 51 (2010) 5476–5479
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Tetrahedron Letters
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Room-temperature palladium(II)-catalyzed N-vinylation of sulfonamides
and acylamides with vinyl acetate as vinyl source
*
*
Jun Xu, Yao Fu , Bin Xiao, Tianjun Gong, Qingxiang Guo
Department of Chemistry, University of Science and Technology of China, Hefei 230026, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Room-temperature N-vinylation of various substituted sulfonamides and acylamides with vinyl acetate
was achieved for the first time with a palladium/carbene catalyst system. This reaction provides a useful
method for synthesis of enamides under mild conditions.
Received 29 June 2010
Revised 6 August 2010
Accepted 10 August 2010
Available online 14 August 2010
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
4-Methyl-N-p-tolylbenzenesulfonamide (1a) was chosen as a
model substrate to react with vinyl acetate (2) in the initial exper-
Enamides are important structural skeletons in various classes
of natural products1,2 and they are also valuable synthetic interme-
diates.3,4 Traditional methods5–9 for enamide synthesis include:
acid-catalyzed condensation of amides and aldehydes,5 Curtius
iments to optimize the reaction conditions, which had not been
used by Stahl20 in the coupling of vinyl ethers to give the corre-
sponding enamides (Table 1). A blank test, which was conducted
only in the presence of K3PO4, failed to produce the desired product
3a, and the starting material was recovered in 95% yield (entry 1).
When 10 mol % Pd(OAc)2 was added into the reaction system, the
desired N-vinylation product was obtained in 36% yield (entry 2).
Various neutral donor ligands were investigated for their effects
on palladium(II) center (entries 3–9). Recent results by Stahl and
co-workers on vinyl transfer to amide prompted us to test phenan-
throline ligand20 (entry 3), but it did not improve the catalytic
activity. The desired cross-coupling barely occurred in the pres-
ence of Pd(OAc)2 and bisphosphine ligands that are commonly
used for catalytic cross-coupling reactions (entries 4 and 5), which
indicated that bis-phosphine ligands may inhibit the desired reac-
tion. However, sterically hindered mono-phosphine ligands (en-
tries 6 and 7) and carbene ligands (entries 8 and 9) improved the
catalyst activity efficiently, which give the desired product in mod-
erate to good yield, especially Pd(OAc)2 and IPr catalytic system
exhibited higher activity. Considering the high efficiency and avail-
ability, 1 mol % Pd(OAc)2 and 2 mol % ligand (IPr) were used in the
reaction, which also gave 84% yield (entry 10). Besides, we found
that the basicity and solubility of base have significant influence
on the reaction efficiency (entries 10–14). A very low yield (28%)
was obtained when a strong base t-BuOK was used in the reaction
(entry 13). However, when K2CO3 was used as base in the reaction,
the catalytic system displayed the highest activity, which afforded
the N-vinylation product in 96% yield (entry 14). Nearly identical
activity is obtained when Pd(OTFA)2 was used as Pd(II) catalyst
(entry 15). Na2PdCl4, which was developed by Geckeler and Ba-
ver22 for N-vinylation of lactams, exhibited no catalytic activity
in the reaction (entry 16).
rearrangement of
a
,b-unsaturated acyl azides,6 acylation of imi-
nes,7 and Peterson reaction manifold.8 More recently, transition
metal-catalyzed reactions also provide alternative and convenient
approaches for the synthesis of enamides.10–12 Nowadays, copper-
catalyzed coupling13 of alkenyl halides with amide,14 a modern
variant of the Goldberg reaction,15 has been the most widely used
method as reported by Porco, Buchwald, Ma, and others.16–20
Although these general methods are often used for enamides for-
mation, they often have limited substrate scope and typically re-
quire elevated temperatures, rigorous exclusion of air and water,
and two or more equivalents of strong bases. Especially, these
methods are not suitable for enesulfonamide formation. There
are rare examples for the transition metal-catalyzed enesulfona-
mide formation, except that Stahl20 have developed the Pd(II)-cat-
alyzed vinyl transfer reaction from vinyl ethers to nitrogen
nucleophiles. Compared to vinyl ethers, the vinyl acetate21 is more
readily available from commercial sources, which is easier to han-
dle in air than the corresponding halides, sulfonates, and phos-
phates. Besides, an ester is also less dangerous than an ether
because the latter may generate explosive peroxides. Considering
these advantages, vinyl acetate can be used as a good terminal vi-
nyl source. Herein, we reported the first room-temperature palla-
dium(II)-catalyzed cross-coupling reaction between various
amides and vinyl acetate for the formation of terminal enamides.
This catalyst system is particularly suitable for enesulfonamide
formation.
With the optimal catalyst in hand, we examined the scope of
this reaction with a series of functionalized aryl amines (Table 2).
* Corresponding authors. Tel.: +86 551 360 0843; fax: +86 551 360 6689.
E-mail addresses: fuyao@ustc.edu.cn (Y. Fu), qxguo@ustc.edu.cn (Q. Guo).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.08.029

J. Xu et al. / Tetrahedron Letters 51 (2010) 5476–5479
5477
Table 1
Table 2
Reaction condition screening resultsa
Examination of the scope of the cross-coupling reactiona
Ts
N
R2
Pd(OAc)2(1%),
IPr(2%)
NHTs
NHR2
N
Pd, L
+
OAc
+
R1
R1
OAc
base, rt, air
K2CO3(1eq),
rt, air
3a
2
1a
2
1a-h
Entry R1
3a-h
Yieldb (%)
Entry
Pd (mol %)
L (mol %)
Base (1 equiv)
Yieldb (%)
R2
Ts
Product
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16j
—
—
—
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
PivOK
KOAc
t-BuOK
K2CO3
K2C03
—
0
Ts
N
Pd(OAc)2 (10%)
Pd(OAc)2 (10%)
Pd(OAc)2 (10%)
Pd(OAc)2 (10%)
Pd(OAc)2 (10%)
Pd(OAc)2 (10%)
Pd(OAc)2 (10%)
Pd(OAc)2 (10%)
Pd(OAc)2 (1%)
Pd(OAc)2 (1%)
Pd(OAc)2 (1%)
Pd(OAc)2 (1%)
Pd(OAc)2 (1%)
Pd(OTFA)2 (1%)
Na2PdCI4 (1%)
36
18
20
<5
70
56
86
80
84
60
74
28
96 (90)
93
0
Phen (10%)c
dppb (10%)d
dppp (10%)e
X-Phos (10%)f
Davephos (10%)g
IPr (10%)h
Mes (10%)i
IPr (2%)
1
2
3
4
5
4-CH3
90
3a
Ts
N
H
Ts
Ts
Ts
Ts
92
94
98
98
3b
Ts
N
2,4-CH3
3-CF3
IPr (2%)
IPr (2%)
IPr (2%)
IPr (2%)
IPr (2%)
3c
Ts
F3C
N
3d
3e
—
Ts
N
a
Reaction condition: 1a (0.5 mmol), 2 (1 mL), Pd catalyst, L, base (1 equiv), rt,
O2N
3-NO2
open to the air (entries 4–7 was conducted under argon), 6 h.
b
GC yield (isolated yield in the parentheses).
Phen = 1,10-phenanthroline.
dppb = 1,4-bis(diphenylphosphino)butane.
dppp = 1,3-bis(diphenylphosphino)propane.
c
Ts
N
d
e
6
7
8
4-CH3CO Ts
99
87
82
3f
f
X-Phos = 2-dicyclohexyl-phosphino-20,40,60-tri-i-propyl-1,10-biphenyl.
Davephos = 2-(dicyclohexyl-phosphino)-20-(N,N-dimethylamino)biphenyl.
g
O
h
IPr = 1,3-bis(2,4,6-tri-isopropyl-phenyl)imidazol-2-ylidene.
Mes = 1,3-bis(2,4,6-methylphenyl)-imidazol-2-ylidene.
i
Ts
N
J
The reaction was conducted under reflux for 12 h with exclusion of moisture.
4-MeO
4-CH3
Ts
3g
MeO
O2N
O
As can be seen, the presence of an ortho-methyl group did not
diminish the efficiency of the coupling of aniline with vinyl acetate
2 (Table 1, entry 3). Functional groups that are compatible with
this Pd-catalyzed N-vinylation protocol include triflourmethyl, ni-
tro, keto, and methoxy groups (entries 4–7). The substrates with
those electron-withdrawing groups (entries 4–6) afforded higher
yield than those with electron-donating groups (entries 1, 3, 7,
and 8). Nearly quantitative conversion was obtained from
N-(4-acetylphenyl)-4-methylbenzenesulfonamide (1f) to desired
N-vinylation product (3f). 2,4-Dinitrobenzene-1-sulfonyl chlo-
ride-protected aryl amine, which is easy to be deprotected, is also
compatible for our reaction, giving 82% yield (entry 8). The primary
nucleophiles, acrylamide, and benzamide (entries 9 and 10), can
also successfully undergo the N-vinylation reaction. The lower
reactivity of these substrates led to a longer reaction time, but
the product could be obtained in moderate yield. Use of other pri-
mary nucleophiles, acetamide, and p-toluenesulfonamide, resulted
in low conversion (<10%).
S
O
NO2
2,4-DNs
N
3h
NH2
H
N
9c
37
36
O
O
O
O
10c
NH2
N
H
a
Reaction conditions: 0.5 mmol of 1a–h, 1 mL 2, 1 mol % of Pd(OAc)2, 2 mol % IPr,
1 equiv K2CO3, rt, 12 h, open to air.
b
Isolated yield after flash column chromatography.
K3PO4 used as base, reaction time 24 h.
c
2. Experimental
Although a detailed mechanistic investigation of the palladium-
catalyzed N-vinylation reaction awaits further experimental stud-
ies, tentative proposals are outlined in Scheme 1. In the presence of
K2CO3, compound 1 coordinates to Pd(OAc)2, forming a new Pd–N
bond. Then aminopalladation of the vinyl acetate, which undergoes
vinyl acetate insertion into the Pd–N bond, affords the intermedi-
ate B. Subsequent b-OAc elimination gives the desired amination
product.
1H and 13C NMR spectra were recorded at rt at 400 and
100 MHz, respectively, on an ARX 400 Bruker spectrometer. Chem-
ical shifts are reported in parts per million referenced to the resid-
ual proton resonances of the solvents. Coupling constants are
expressed in hertz.
2.1. General procedure for the formation of enamides
In summary, our studies have led to the development of a new
process for the synthesis of enamides via N-vinylation from vinyl
acetate under very mild condition. The Pd(OAc)2 and carbene
ligands catalyst are effective with a variety of substituted sulfona-
mides and acylamide nucleophiles, and this straightforward meth-
odology gives a new sight in palladium(II)-catalyzed C–N bond
formation reaction.
Aryl amine 1 (0.5 mmol), vinyl acetate 2 (1 mL), Pd(OAc)2
(0.005 mmol, 1.12 mg), and IPr (0.01 mmol, 3.9 mg) were com-
bined in a round-bottomed flask, equipped with a magnetic stir
bar. The reaction mixture was stirred at room-temperature, open
to air, and monitored for completion by TLC. Upon completion,
the solvents were removed via rotary evaporator. Purification of

5478
J. Xu et al. / Tetrahedron Letters 51 (2010) 5476–5479
148.89, 144.98, 137.30, 136.87, 135.30, 134.61, 130.98, 130.08,
127.48, 125.06, 124.02, 94.72, 20.63.
L
L
OAc
OAc
TsNHAr+K2CO3
Ar
Ts
Pd
N
1
3
2.1.6. N-(4-Acetylphenyl)-4-methyl-N-vinylbenzene
sulfonamide (3f)
KHCO3+KOAc
1H NMR (400 MHz, CDCl3) d 7.95 (d, J = 8.4 Hz, 2H), 7.55 (d,
J = 7.9 Hz, 2H), 7.28 (d, J = 7.9 Hz, 2H), 7.20 (dd, J = 15.6, 9.0, 1H),
7.11 (d, J = 8.4, 2H), 4.31 (d, J = 8.8 Hz, 1H), 3.84 (d, J = 15.6 Hz,
1H), 2.62 (s, 3H), 2.44 (s, 3H). 13C NMR (101 MHz, CDCl3) d
197.14, 144.33, 140.26, 137.25, 135.56, 134.39, 130.55, 129.79,
129.48, 127.46, 94.81, 26.72, 21.62.
Ts
N
AcO
L
Ar
OAc
L
L
Pd
Pd
L
Ar
N
OAc
intermediate B
Ts
A
2.1.7. N-(4-Methoxyphenyl)-4-methyl-N-vinyl-
benzenesulfonamide (3g)
1H NMR (400 MHz, CDCl3) d 7.56 (d, J = 8.0 Hz, 2H), 7.27 (d,
J = 7.3 Hz, 2H), 7.25–7.18 (dd, 1H), 6.90–6.82 (dd, 4H), 4.23 (d,
J = 8.8 Hz, 1H), 3.83 (d, J = 14.8 Hz, 1H), 3.81 (s,3H), 2.43 (s, 3H).
13C NMR (101 MHz, CDCl3) d 159.82, 143.85, 135.89, 134.96,
131.38, 129.58, 127.99, 127.50, 114.65, 93.72, 55.42, 21.59.
OAc
2
Scheme 1. Proposed mechanism for palladium(II)-catalyzed N-vinylation of aryl
amine.
2.1.8. 2,4-Dinitro-N-p-tolyl-N-vinyl-benzenesulfonamide (3h)
1H NMR (400 MHz, acetone) d 8.85 (s, 1H), 8.59 (d, J = 8.7 Hz,
1H), 7.97 (d, J = 8.7 Hz, 1H), 7.30 (d, J = 7.6 Hz, 2H), 7.19 (dd,
J = 15.3 Hz, 8.8 Hz, 1H), 7.05 (d, J = 7.2 Hz, 2H), 4.44 (d, J = 8.8 Hz,
1H), 3.89 (d, J = 15.3 Hz, 1H), 2.38 (s, 3H). 13C NMR (101 MHz, ace-
tone) d 150.81, 148.00, 140.20, 135.53, 134.53, 133.41, 131.47,
130.61, 130.27, 126.48, 120.22, 94.37, 20.31.
the residue by column chromatography (silica gel, ethyl acetate/
petroleum ether = 1:10) yielded the corresponding products.
2.1.1. 4-Methyl-N-p-tolyl-N-vinyl-benzenesulfonamide (3a)
1H NMR (400 MHz, CDCl3) d 7.56 (d, J = 8.3 Hz, 2H), 7.27 (d,
J = 7.8 Hz, 2H), 7.21 (dd, J = 15.5 Hz, 8.9 Hz, 1H), 7.14 (d,
J = 8.1 Hz, 2H), 6.84 (d, J = 8.3 Hz, 2H), 4.23 (dd, J = 8.9 Hz, 0.7,
1H), 3.82 (dd, J = 15.5 Hz, 0.7, 1H), 2.43 (s, 3H), 2.35 (s, 3H). 13C
NMR (101 MHz, CDCl3) d 143.82, 139.11, 136.02, 134.85, 132.99,
130.13, 129.98, 129.57, 127.52, 93.88, 21.59, 21.23.
2.1.9. N-Vinylacrylamide
1H NMR (400 MHz, CDCl3) d 7.89 (s, 1H), 7.14–6.99 (m, 1H), 6.38
(d, J = 16.9 Hz, 1H), 6.17 (dd, J = 17.0 Hz, 10.3, 1H), 5.73 (d, J = 10.3,
1H), 4.71 (d, J = 15.9, 1H), 4.48 (d, J = 8.8, 1H). 13C NMR (101 MHz,
CDCl3) d 162.89, 130.13, 128.75, 128.18, 96.48.
2.1.2. 4-Methyl-N-phenyl-N-vinylbenzenesulfonamide (3b)
1H NMR (400 MHz, CDCl3) d 7.56 (d, J = 8.3 Hz, 2H), 7.34 (m, 3H),
7.27 (d, J = 8.3 Hz, 2H), 7.26–7.17 (dd, 1H), 6.98 (m, 2H), 4.25 (dd,
J = 8.9 Hz, 0.7, 1H), 3.81 (d, J = 16 Hz, 1H), 2.43 (s, 3H). 13C NMR
2.1.10. N-Vinylbenzamide
1H NMR (400 MHz, CDCl3) 7.81 (d, J = 7.0 Hz, 2H), 7.75 (br s, 1H),
7.54 (t, J = 7.5 Hz), 7.46 (t, J = 7.6 Hz, 2H), 7.15–7.24 (m, 1H), 4.77
(d, J = 16.0 Hz, 1H), 4.53 (d, J = 9.0 Hz, 1H); 13C NMR (101 MHz,
CDCl3) d 164.7, 133.7, 132.2, 129.3, 128.9, 127.3, 96.5.
(101 MHz, CDCl3)
d 143.95, 135.92, 135.79, 134.78, 130.33,
129.61, 129.45, 129.06, 127.52, 94.09, 21.60.
Acknowledgments
2.1.3. N-(2,4-Dimethylphenyl)-4-methyl-N-vinyl-
benzenesulfonamide (3c)
This study was supported by the National Natural Science Foun-
dation of China (No. 20972148). We also thank the USTC Super-
computer Center.
1H NMR (400 MHz, acetone) d 7.64 (d, J = 8.3 Hz, 2H), 7.43 (d,
J = 8.0 Hz, 2H), 7.23 (dd, J = 15.4 Hz, 8.8 Hz, 1H), 7.17 (s, 1H), 6.96
(d, J = 8.0 Hz, 1H), 6.45 (d, J = 8.0 Hz, 1H), 4.21 (d, J = 8.8 Hz, 1H),
3.59 (d, J = 15.4 Hz, 1H), 2.45 (s, 3H), 2.30 (s, 3H), 2.09 (s, 3H).
13C NMR (101 MHz, acetone) d 144.22, 139.24, 138.67, 136.90,
134.15, 132.11, 129.80, 129.22, 127.42, 127.37, 92.36, 28.95,
20.59, 20.20, 16.95.
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