A general method for regioselective Heck arylation of electron-rich N-acyl-N-vinylamine with aryl halides

Article abstract of DOI:10.1016/j.tetlet.2008.02.144

A highly efficient protocol for the Pd-catalyzed regioselective Heck arylation of the electron-rich olefin N-acyl-N-vinylamine with aryl halides has been developed. In the presence of hydrogen-bond donor [H₂NiPr₂][BF₄] as

Full text of DOI:10.1016/j.tetlet.2008.02.144

Available online at www.sciencedirect.com
Tetrahedron Letters 49 (2008) 2756–2760
A general method for regioselective Heck arylation
of electron-rich N-acyl-N-vinylamine with aryl halides
a
a
a
a,
b
b,
*
*
Zhihua Liu , Dan Xu , Weijun Tang , Lijin Xu , Jun Mo , Jianliang Xiao
a Department of Chemistry, Renmin University of China, Beijing 100872, China
b Liverpool Centre for Materials and Catalysis, Department of Chemistry, University of Liverpool, Liverpool L69 7ZD, UK
Received 27 November 2007; revised 25 February 2008; accepted 26 February 2008
Available online 29 February 2008
Abstract
A highly efficient protocol for the Pd-catalyzed regioselective Heck arylation of the electron-rich olefin N-acyl-N-vinylamine with
aryl halides has been developed. In the presence of hydrogen-bond donor [H2NiPr2][BF4] as an additive, this proceeds smoothly in
isopropanol to afford exclusively the branched products in high yields.
Ó 2008 Published by Elsevier Ltd.
The Heck reaction is an important transformation that
efficiently functionalizes aromatic halides with olefins in
the presence of palladium catalysts. Considerable advances
in this reaction have been made through the development
of more active catalytic systems, but the synthetic utility
is often limited by the poor regioselectivity.1 In general,
the Heck reaction works efficiently with electron-withdraw-
ing olefins to give the terminal products (often referred to
as b-products), while the arylation of electron-rich olefins
such as enol ethers and enamides almost always generates
a mixture of internal a- and terminal b-products.2 The
Heck reaction is believed to proceed via two pathways,
one ionic leading to the branched product, and the other
neutral producing the linear variant (Scheme 1).2,3 In the
earlier studies, it was found that high a/b regioselectivity
could be obtained by employing aryl triflates or by adding
stoichiometric halide scavengers when aryl iodides and bro-
mides were used.1a,2
rich olefins could be arylated highly regioselectively by aryl
halides without recourse to triflates or halide scavengers.4,5
Similar results in ionic liquids have also been observed by
other groups.6 More recently, we have found that these
regioselective reactions could be greatly accelerated by
hydrogen-bond-donating ammonium salts, and the reac-
tions could be carried out not only in imidazolium ionic
liquids but also in common organic solvents.7 As a result,
the electron-rich olefins such as vinyl ethers and tertiary
N-vinylamides could be regioselectively arylated with aryl
halides to provide good yields in shorter reaction time.
Following our continued interest in regioselective arylation
of electron-rich olefins, herein we report that in the
presence of hydrogen-bond-donating ammonium salt, the
regioselective Heck arylation of electron-rich olefin N-
acyl-N-vinylamine lacking an N-alkylsubstituent with aryl
halides can be carried out smoothly in isopropanol, exclu-
sively generating the branched products in high yields.
These arylated enamides are important precursors in the
synthesis of optically active amines by transition-metal
catalyzed asymmetric reactions.8
In recent publications, we have reported that imidazol-
ium ionic liquids in combination with the readily available
Pd(OAc)2 and DPPP (1,3-bis(diphenylphosphino)propane)
formed an excellent catalytic system, with which electron-
Several methods to prepare internally arylated N-acyl
enamides have been reported in the literature, and they
all suffer from either low yields, or low functional group
tolerance or both.9 Recently, an alternative approach
involving Pd-catalyzed Heck reaction of aryl triflates with
*
Corresponding authors. Tel.: +86 10 62511528; fax: +86 10 62516444
(X.L.).
E-mail address: xulj@chem.ruc.edu.cn (L. Xu).
0040-4039/$ - see front matter Ó 2008 Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2008.02.144

Z. Liu et al. / Tetrahedron Letters 49 (2008) 2756–2760
2757
Ar
ArX
R
P
P
X
R
Ar
R
P
P
Ar
P
X
P
Pd
Pd
Pd
Ar
X
R
Path A
Base
P
P
P
P
X
H
Pd
Pd
Base.XH
Path B
+ X-
P
P
Ar
X
P
P
Ar
R
P
P
X
Pd
Pd
Pd
Ar
R
ArX
R
Ar
R
Scheme 1.
N-acyl enamides has been reported by two groups.10 With
this efficient protocol, highly regioselective a-arylation of
N-acyl enamides can be accomplished in good yields. It is
notable that this method works effectively for the prepar-
ation of aryl-N-acyl enamides bearing functionality that
would not be tolerated by the previous stoichiometric
methods. However, a drawback of this chemistry is that
it requires the use of base-sensitive, thermally labile and
costly triflates. In this context, it is very attractive to
develop an additional protocol for the efficient arylation
of N-acyl enamides with inexpensive and easily accessible
aryl halides.
Considering the successful regioselective arylation of
various electron-rich olefins in ionic liquids, we first
attempted the a-arylation of N-vinylacetamide 2a in
[bmim][BF4] (1-butyl-3-methylimidazolium tetrafluorobo-
rate) with aryl bromide 1a as the model substrate. The aryl-
ation was carried out in the ionic liquid [bmim][BF4] at
115 °C under the previously employed conditions, where
the active catalyst was generated in situ from Pd(OAc)2
and DPPP.4,5,7 However, no reaction was observed after
24 h. In our early study, it was found that the reaction of
aryl bromide with N-methyl-N-vinylacetamide or N-vinyl
pyrrolidone generated no desired product in ionic liquid
[bmim][BF4]; but the addition of DMSO as a cosolvent
led to the desired product.4a With this in mind, we then
examined the reaction in [bmim][BF4]/DMSO. However,
still no reaction was observed. Other solvents were
screened, and the results are shown in Table 1. Surpri-
singly, complete conversions were obtained in almost all
solvents. No b-substituted 4a product was detected;
however, the yields of the target compound 3a were differ-
ent. Although good results were obtained in the reaction of
aryl triflates with the enamide in dioxane,10b only 24% yield
was produced in our case (entry 3). Lower yields were
obtained in DMF or toluene (entries 4 and 5). Switching
to DMSO led to 29% yield, and the best result was achieved
in isopropanol (37% yield, entries 5 and 6). In all the
reactions tested, the main side products arose from
homo-coupling and debromination.
We have recently disclosed that the regioselective aryl-
ation of electron-rich olefins can be accelerated by the
potential hydrogen-bond donors such as [H2NiPr2][BF4].7
The effect of the additive was investigated in this study.
Introduction of 1.5 equiv of [H2NiPr2][BF4] to [bmim][BF4]
had no clearly promoting effect on the reaction rate (entry
7). When using [bmim][BF4]/DMSO mixture, an 80% con-
version was observed, but only 10% desired product was
obtained. Better results were obtained in DMSO, dioxane,
toluene and DMF (entries 10–13). And the best result was
observed in isopropanol, in which 50% yield was achieved
(entry 14). The addition of non-hydrogen-bonded donor
[NBu4][BF4] gave rise to only slightly improved results
(entry 15). Therefore, in our subsequent study, isopropanol
was the choice of solvent and [H2NiPr2][BF4] was chosen as
promoter. This study revealed that the amount of
[H2NiPr2][BF4] played a crucial role. When increasing the
amount of [H2NiPr2][BF4] in isopropanol from 1.5 to
5 equiv, the reaction time could be reduced to 15 h with
78% yield (entry 16). Remarkably increasing the additive
to 10 equiv enabled the reaction to go to completion in
8 h with 92% isolated yield (entry 17). However, further
increase of the additive had no significant effect on the rate
(entry 18). According to Jutand’s recent reports,11 the
higher reaction rate is probably due to a higher ionic
strength, which favours the formation of cationic Pd(II)
intermediate in pathway B, and so the product 3a. How-
ever, hydrogen bonding between the additive and the bro-
mide anion may also contribute to the formation of the
Pd(II) species,7,12 as [NBu4][BF4] is less effective.
We then examined various bidentate phosphines aiming
to find a more suitable ligand. The results are listed in
Table 2. The linear product was not observed regardless
of the ligand used. However, the desired product was not

2758
Z. Liu et al. / Tetrahedron Letters 49 (2008) 2756–2760
Table 1
Solvent effect on the Heck arylation of N-vinylacetamide
NHAc
Pd(OAc)2 (3 mol %)
ligand (6 mol %)
MeO
MeO
Br
MeO
MeO
MeO
MeO
NHAc
+
+
NEt3 (2 eq.)
NHAc
additive, solvent
OMe
OMe
3a
OMe
4a
115 ºC, 24h
1a
2a
Entry
Solvent
Additive (equiv)
Conv.a (%)
Yieldb (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16c
17d
18d
a
[bmim][BF4]
None
None
None
None
None
None
None
[H2NiPr2][BF4] (1.5)
[H2NiPr2][BF4] (1.5)
[H2NiPr2][BF4] (1.5)
[H2NiPr2][BF4] (1.5)
[H2NiPr2][BF4] (1.5)
[H2NiPr2][BF4] (1.5)
[H2NiPr2][BF4] (1.5)
[NBu4][BF4] (1.5)
nd
nd
[bmim][BF4]/DMSO
1,4-Dioxane
DMF
Toluene
DMSO
Isopropanol
[bmim][BF4]
[bmim][BF4]/DMSO
1,4-Dioxane
DMF
100
100
100
100
100
nd
24
18
17
29
37
80
10
37
25
28
40
50
40
76
92
92
100
100
100
100
100
100
100
100
100
Toluene
DMSO
Isopropanol
Isopropanol
Isopropanol
Isopropanol
Isopropanol
[H2NiPr2][BF4] (5)
[H2NiPr2][BF4] (10)
[H2NiPr2][BF4] (15)
Determined by 1H NMR data; >99/1 a/b ratios based on NMR.
b
c
Isolated yields.
Reaction time 15 h.
Reaction time 8 h.
d
are listed in Table 3.14 We first focused on the olefination
of aryl bromides. As shown in Table 3, bromobenzenes
bearing substituents at various positions are all effective
coupling partners, providing exclusively the branched
products with high isolated yields. For example, olefination
of 4-bromobenzonitirle 1d with enamide 2a led to the prod-
uct in 93% yield (entry 4). When the corresponding aryl tri-
flate was employed, only 75% yield was obtained.10b
However, a lower yield was found in the reaction of
ortho-substituted bromide 1c with enamide 2a (entry 3).
It appears that longer reaction time is necessary for the
olefination of bromides with electron-withdrawing substit-
uents (entries 4–7). This has been noticed before and may
result from a slower olefin insertion process, which can
be viewed as an intramolecular nucleophilic attack.3a The
regioselective reaction was also extended to heteroaromatic
bromides. For example, good result was achieved in the
reaction of 4-bromoisoquinoline (entry 9). Since the reac-
tion time was not optimized, shorter reaction time may
be possible for these cross-coupling processes.
Table 2
Ligand effect on the Heck arylation of N-vinylacetamidea
Entry
Ligand
Time (h)
Conv.b (%)
Yieldc (%)
a/b
1
2
3
4
5
6
a
None
12
12
8
10
8
20
25
100
100
100
100
nd
nd
92
41
90
40
DPPEd
DPPP
>99/1
>99/1
>99/1
>99/1
rac-BINAPd
DPPFd
DPPBd
12
Conditions: 1a (1.0 equiv), 2a (3 equiv), Pd(OAc)2 (0.03 equiv), DPPP
(0.06 equiv), NEt3 (2 equiv), [H2NiPr2][BF4] (10 equiv), isopropanol,
reflux.
b
Determined by 1H NMR data. When 4a was not detected, an a/b ratio
of >99/1 was assigned.
c
Isolated yields.
d
DPPE: 1,2-bis(diphenylphosphino)ethane; DPPB: 1,4-bis(diphenyl-
phosphino)butane; DPPF: 1,10-bis(diphenylphosphino)ferrocene; rac-
BINAP: ( )-2-20-bis(diphenylphosphino)-1,10-binaphthalene.
detected with DPPE as the ligand. The combination of
DPPP and Pd(OAc)2 led to the best result (entry 3), and
reaction with DPPF gave similar results (entry 5). Lower
yields were observed when using BINAP and DPPB.
Switching the base from NEt3 to others such as Cy2NMe
and DIPEA (N,N-diisopropylethylamine) showed no sig-
nificant promoting effect in terms of yields.
Having established the reliable conditions for the aryl-
ation of enamide, we then examined the scope of this reac-
tion with a variety of aryl halides and 2a, and the results
This reaction is not limited to aryl bromides, and aryl
iodides participated equally well. For example, the reac-
tions of aryl iodides 1j and 1k also afforded good results
in shorter reaction times (entries 10 and 11). The utility
of this new protocol was further demonstrated by the ary-
lation of enamide with aryl chlorides (entries 12 and 13).
However, the reactions were sluggish, furnishing the corre-
sponding products in less than 50% yield after 45 h. It is

Z. Liu et al. / Tetrahedron Letters 49 (2008) 2756–2760
2759
Table 3
In conclusion, we have developed an efficient method for
the highly regioselective Heck arylation of N-vinylacet-
amide with aryl halides, avoiding the use of aryl triflates
or halide scavengers. The key to the success of this method
lies in the use of an ammonium salt, [H2NiPr2][BF4], as the
additive, which is believed to facilitate the formation of
charged organopalladium intermediate, thereby promoting
the ionic pathway of the Heck reaction.
Heck arylation of N-vinylacetamide with aryl halides in isopropanola
Entry Aryl bromide
Time Product
(h)
Yield
(%)
MeO
MeO
1
1a
1b
8
3a 92
MeO
Br
MeO
NHAc
MeO
MeO
Br
NHAc
2
8
3b 85
3c 72
Acknowledgements
Me
Me
Br
We thank the Renmin University of China and the
National Natural Science Foundation of China (No.
20542009 and 20672139) for financial support.
NHAc
Me
3
1c 18
Me
Br
References and notes
NHAc
4
1d 19
1e 15
1f 12
3d 93
3e 86
3f 89
NC
NC
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Br
NHAc
5
MeOC
MeOC
EtOOC
Br
NHAc
NHAc
6
EtOOC
MeOOC
Br
MeOOC
7
1g 12
3g 82
3h 87
NHAc
Br
8
1h
8
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Xu, L.; Chen, W.; Ross, J.; Xiao, J. Org. Lett. 2001, 3, 295–297.
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Chem. Soc. 1996, 118, 5142–5143; (c) Zhu, G.; Casalnuovo, A. L.;
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Boice, G. N.; Murry, J. A.; Corley, E.; DiMichele, L.; Hughes, D.
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NHAc
N
Br
9
1i 18
3i 71
N
I
NHAc
NHAc
10
1j
5
5
3f 83
3g 81
EtOOC
EtOOC
MeOOC
I
MeOOC
11
1k
Cl
NHAc
12
1l 45
1m 45
3d 32
3f 41
NC
NC
10. (a) Harrison, P.; Meek, G. Tetrahedron Lett. 2004, 45, 9277–9280; (b)
Lindhardt, A.; Skrydstrup, T. J. Org. Chem. 2005, 70, 5997–6003.
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2007, 26, 1757–1761; (b) Amatore, C.; Godin, B.; Jutand, A.;
Lemaitre, F. Chem. Eur. J. 2007, 13, 2002–2011.
Cl
NHAc
13
EtOOC
EtOOC
a
Conditions: aryl halide (1.0 equiv), olefin (3 equiv), Pd(OAc)2
(0.03 equiv), DPPP (0.06 equiv), NEt3 (2 equiv), [H2NiPr2][BF4]
(10 equiv), isopropanol, reflux.
12. Arvela, R. K.; Pasquini, S.; Larhed, M. J. Org. Chem. 2007, 72, 6390–
6396.
13. Yin, J.; Buchwald, S. L. Org. Lett. 2000, 2, 1101–1104.
14. General procedure for the arylation of 2a: An oven-dried, two-necked
round-bottom flask containing a stir bar was charged with aryl halide
1 (1.0 mmol), 2a (170 mg, 2.0 mmol), Pd(OAc)2 (7 mg, 0.03 mmol),
DPPP (25 mg, 0.06 mmol), [H2NiPr2][BF4] (1.89 g, 10 mmol), and dry
also notable that no competing amidation reaction was
observed in these reactions.13

2760
Z. Liu et al. / Tetrahedron Letters 49 (2008) 2756–2760
isopropanol (2 mL) under nitrogen at room temperature. After
acetylphenyl)-ethenamine (3e). 1H NMR (300 MHz, CDCl3): d 2.12 (s,
3H), 2.53 (s, 3H), 5.05 (s, 1H), 5.80 (s, 1H), 7.03 (br s, 1H), 7.45 (d,
J = 8.2 Hz, 2H), 7.84 (d, J = 8.2 Hz, 2H). 13C NMR (75 MHz,
CDCl3): d 24.3, 28.7, 106.7, 126.2, 128.3, 136.5, 140.8, 141.6, 169.5,
197.6. HRMS C12H13NO2 calcd: 203.0946; found, 203.0949. Ethyl 4-
(1-acetamidovinyl) benzoate (3f). 1H NMR (300 MHz, CDCl3): d 1.39
(t, J = 7.2 Hz, 3H), 2.12 (s, 3H), 4.36 (q, J = 7.2 Hz, 2H), 5.18 (s, 1H),
5.85 (s, 1H), 7.01 (br s, 1H), 7.46 (d, J = 8.1 Hz, 2H), 7.06 (d,
J = 8.1 Hz, 2H). 13C NMR (75 MHz, CDCl3): d 14.3, 24.4, 61.1,
104.8, 125.9, 129.9, 130.5, 142.4, 152.6, 166.1, 169.2. HRMS
degassing three times, NEt3 (202 mg, 2 mmol) was injected. The flask
was placed in an oil bath, and the mixture was stirred and heated
under reflux. The reaction was monitored by TLC. After the reaction
went to completion, saturated aqueous NaHCO3 was added, and the
aqueous phase was extracted with dichloromethane three times. The
combined organic layer was dried over Na2SO4, filtered, and
concentrated in vacuo. The crude product was purified via flash
chromatography on silica gel using a mixture of ethyl acetate and
hexane (1/1 to 1/5 in volume). N-Acetyl-1-(30,40,50-trimetoxyphenyl)-
ethenamine (3a). 1H NMR (300 MHz, CDCl3): d 2.13 (s, 3H), 3.80 (s,
3H), 3.84 (s, 6H), 5.02 (s, 1H), 5.83 (s, 1H), 6.60 (s, 2H), 7.08 (br s,
1H); 13C NMR (75 MHz, CDCl3): d 24.1, 55.8, 60.6, 102.2, 103.7,
128.6, 132.1, 140.7, 153.3, 169.3. HRMS C13H17NO4 calcd: 251.1158;
found, 251.1158. N-Acetyl-1-(40-methylphenyl)-ethenamine (3b). 1H
NMR (300 MHz, CDCl3): d 2.10 (s, 3H), 2.35 (s, 3H), 5.08 (s, 1H),
5.88 (s, 1H), 6.77 (br s, 1H), 7.30 (d, J = 7.9 Hz, 2H), 7.45(d,
J = 7.9 Hz, 2H); 13C NMR (75 MHz, CDCl3): d 20.5, 23.9, 101.2,
125.4, 128.8, 134.9, 137.9, 140.0, 168.5. HRMS C11H13NO calcd:
175.0997; found, 175.0999. N-Acetyl-1-(20-methylphenyl)-ethenamine
(3c). 1H NMR (300 MHz, CDCl3): d 2.03 (s, 3H), 2.35 (s, 3H), 4.70 (s,
1H), 6.06 (s, 1H), 6.65 (br s, 1H), 7.09–7.68 (m, 4H); 13C NMR
(75 MHz, CDCl3): d 19.5, 23.8, 102.2, 126.0, 128.6, 129.2, 130.5,
135.8, 138.9, 140.8, 169.5. HRMS C11H13NO calcd: 175.0997; found,
175.0996. N-Acetyl-1-(40-cyanophenyl)-ethenamine (3d). 1H NMR
(300 MHz, CDCl3): d 1.97 (s, 3H), 5.06 (s, 1H), 5.75 (s, 1H), 6.84
(br s, 1H), 7.45–7.47 (m, 2H), 7.55–7.61 (m, 2H). 13C NMR (75 MHz,
CD3CN): d 23.9, 106.2, 112.4, 119.6, 127.9, 133.2, 141.7, 143.4, 170.4.
HRMS C11H10N2O calcd: 186.0793; found, 186.0796. N-Acetyl-1-(40-
C
13H15NO3 calcd: 233.1052; found, 233.1055. Methyl 3-(1-acetami-
dovinyl) benzoate (3g). 1H NMR (300 MHz, CDCl3): d 2.08 (s, 3H),
3.85 (s, 3H), 5.08 (s, 1H), 5.80 (s, 1H), 6.81 (br s, 1H), 7.43 (t,
J = 7.8 Hz, 1H), 7.55 (d, J = 7.7 Hz, 1H), 7.94 (d, J = 7.7 Hz, 1H),
8.06 (s, 1H). 13C NMR (75 MHz, CDCl3): d 24.4, 52.2, 103.9, 127.0,
128.6, 128.8, 129.7, 130.6, 138.6, 139.8, 166.7, 169.8. HRMS
C12H13NO3 calcd: 219.0895; found, 219.0899. N-Acetyl-1-(20-naph-
thyl)-ethenamine (3h). 1H NMR (300 MHz, CDCl3): d 2.03 (s, 3H),
4.94 (s, 1H), 6.30 (s, 1H), 6.73 (br s, 1H), 7.46–7.54 (m, 4H), 7.86–7.87
(m, 2H), 7.88–7.90 (m, 1H); 13C NMR (75 MHz, CDCl3): d 24.4,
103.6, 125.2, 125.4, 126.2, 126.7, 126.8, 128.4, 129.0, 131.0, 133.7,
136.7, 139.5, 169.0. HRMS
C14H13NO calcd: 211.0997; found,
211.0999. N-Acetyl-1-(30-isoquinolinyl)-ethenamine (3i). 1H NMR
(300 MHz, CDCl3): d 1.97 (s, 3H), 4.79 (s, 1H), 6.13 (s, 1H), 7.50–
7.53 (m, 1H), 7.62–7.68 (m, 1H), 7.72 (d, J = 8.1 Hz, 1H), 7.91 (s, 1H),
7.94 (d, J = 8.5 Hz, 1H), 8.20 (s, 1H), 8.76 (s, 1H). 13C NMR
(75 MHz, CDCl3): d 23.3, 104.1, 123.1, 126.9, 127.4, 129.1, 130.8,
131.0, 132.7, 135.5, 141.3, 151.7, 168.2. HRMS C13H12N2O calcd:
212.0950; found, 212.0955.