Highly regioselective Heck reactions of heteroaryl halides with electron-rich olefins in ionic liquid

Article abstract of DOI:10.1016/j.jorganchem.2005.03.008

Palladium-catalyzed Heck reactions of the heteroaryl halides, halopyridines, bromoquinoline and bromothiophenes, with the electron-rich olefins vinyl ethers and allyl alcohol were shown to give essentially only the branched olefins in an imidazolium ionic liquid, whereas in molecular solvents a mixture of regioisomers was formed. The method obviates the need for aryl triflates and stoichiometric inorganic salt additives, providing an easy entry to functionalized heteroaromatics incorporating acetyl and 2-allyl alcohol functionalities.

Full text of DOI:10.1016/j.jorganchem.2005.03.008

Journal of Organometallic Chemistry 690 (2005) 3546–3551
www.elsevier.com/locate/jorganchem
Highly regioselective Heck reactions of heteroaryl halides
with electron-rich olefins in ionic liquid
a
Wen Pei , Jun Mo , Jianliang Xiao
b
b,
*
a
College of Chemical Engineering and Materials, Zhejiang University of Technology, Hangzhou 310014, China
b
Liverpool Centre for Materials and Catalysis, Department of Chemistry, University of Liverpool, Liverpool L69 7ZD, UK
Received 2 February 2005; received in revised form 4 March 2005; accepted 4 March 2005
Available online 19 April 2005
Dedicated to Professor Eric Derouane on the occasion of his 60th birthday
Abstract
Palladium-catalyzed Heck reactions of the heteroaryl halides, halopyridines, bromoquinoline and bromothiophenes, with the
electron-rich olefins vinyl ethers and allyl alcohol were shown to give essentially only the branched olefins in an imidazolium ionic
liquid, whereas in molecular solvents a mixture of regioisomers was formed. The method obviates the need for aryl triflates and
stoichiometric inorganic salt additives, providing an easy entry to functionalized heteroaromatics incorporating acetyl and 2-allyl
alcohol functionalities.
Ó 2005 Elsevier B.V. All rights reserved.
Keywords: Heck reaction; Regioselectivity; Ionic liquid; Heterocycles; Electron-rich olefins; Palladium catalysis
1
. Introduction
Although a great deal of progress has been made in
the Heck coupling reactions in the past several years,
the olefinic substrates have so far mostly been limited
to electron-deficient olefins [4]. With electron-rich ole-
fins, such as silanes, vinyl ethers, and enol amides, there
exits an important issue that has not been fully resolved
to date, that is the lack of regioselectivity in normal
intermolecular Heck reactions [4a,4d,4e–6]. Such reac-
tions usually give rise to a mixture of regioisomeric ole-
fins and hence are of only limited synthetic utility
(Scheme 1). The problem with the regioselectivity can
be addressed by using aryl triflates instead of halides
or stoichiometric amounts of halide scavengers when
aryl halides are used, thallium (I) or silver (I) salts being
most popular [6]. The effect of the triflates and halide
scavengers on the regioselectivity stems from their effect
on the reaction pathway. The Heck reaction is believed
to proceed via an ionic pathway leading to the branched
product and a neutral variant giving rise to the linear
olefin [4–7] (Scheme 2). One can easily envision that
Heteroaryl compounds have important biological
properties and many of their derivatives can be readily
accessed by metal catalyzed reactions [1,2]. Among the
various reactions catalyzed by metal complexes, the pal-
ladium catalyzed Heck reactions of heteroaryl halides
and alkenes have demonstrated their utility in a limited
number of instances [3]. However, as with most other
Heck reactions, these reactions are mainly concerned
with electron-deficient olefins [4]. We have recently
developed a method that allows electron-rich olefins to
be arylated by aryl halides in a highly regioselective
manner without recourse to any halide scavengers [5].
We now report results that extend the chemistry to
heteroaromatics.
*
Corresponding author. Tel.: +44 151 7942937; fax: +44 151
943589.
E-mail address: j.xiao@liv.ac.uk (J. Xiao).
7
0
022-328X/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.03.008

W. Pei et al. / Journal of Organometallic Chemistry 690 (2005) 3546–3551
3547
R
on from the work on normal aryl halides, we extended
the chemistry to heteroaryl compounds. The examples
presented below fall again in line with this hypothesis.
Pd(0)
Base
+
ArX
+
R
Ar
R
Ar
R = heteroatom, alkyl, -CH SiR' , -CH CH OH, etc.
2
3
2
2
Scheme 1.
2. Results and discussion
the ionic pathway would be rendered favorable when tri-
flates or halide scavengers are chosen.
We first targeted the Heck arylation of bromopyri-
dines with the benchmark electron-rich olefin butyl vinyl
ether. Following acidic hydrolysis, the resulting
branched olefins should readily lead to acetyl pyridines,
a class of compounds that are otherwise difficult to ac-
cess (Scheme 3). To test out if ionic liquids would offer
any advantage over a molecular solvent, the arylation
of 3-bromopyridine was conducted in the ionic liquid
1-butyl-3-methylimidazolium tetrafluoroborate ([bmim]
In a program aimed at developing homogeneous
catalysis in ionic liquids [5,8], we disclosed that highly
regioselective Heck arylation with aryl halides can be
accomplished in imidazolium ionic liquids [9], obviating
the need for aryl triflates or toxic and costly inorganic
additives [5]. The work originated from a hypothesis
that because ionic liquids are entirely composed of ions,
the ionic Heck pathway (Path B, Scheme 2) would be
made favorable when an arylation reaction is performed
therein, thus leading to the formation of branched ole-
fins. A similar argument was also invoked by Hallberg,
Larhed and coworkers [10,11] in recent studies of the
Heck reaction involving electron rich olefins. Following
[BF ]) alongside five normal solvents. The reaction was
4
carried out by heating a mixture of the bromide
(1.0 mmol), butyl vinyl ether (5.0 mmol), triethylamine
(1.6 mmol) in the presence of Pd(OAc) (4 mol%) and
2
1,3-bis(diphenylphosphino)propane (DPPP, 8 mol%) in
2 mL of a chosen solvent. The results are summarized
Ar
ArX
R
P
R
P
P
Ar
X
P
X
Ar
R
P
X
P
Pd
Pd
Pd
R
Ar
Path A
Base
P
P
P
P
X
H
Pd
Pd
Base.XH
Path B
+
X-
P
P
Ar
X
P
P
Ar
P
X
Pd
Pd
Pd
Ar
R
R
P
ArX
R
Ar
R
Scheme 2.
Pd(OAc)2
OBu
Br
Phosphine
OBu
+
+
OBu
NEt3
N
N
N
[bmim][BF4]
α
β
HCl
O
N
Scheme 3.

3548
W. Pei et al. / Journal of Organometallic Chemistry 690 (2005) 3546–3551
Table 1
Arylation of butyl vinyl ether by 3-bromopyridine in various solvents
Table 2
a
a
4
Heck arylation of vinyl ethers by heteroaryl halides in [bmim][BF ]
b
b
b
Entry
Solvent
Conversion (%)
Regioselectivity (a/b)
Entry
1
Substrate
Olefin
Product
Yield
88
1
2
3
4
5
6
a
[bmim][BF
Toluene
4
]
100
28
36
33
80
80
>99/1
61/39
65/35
62/38
71/29
68/32
O
N
Br
Br
N
N
COCH3
COCH3
Dioxane
Acetonitrile
DMF
2
3
4
5
O
O
O
81
75
69
91
N
DMSO
Br
COCH3
The reactions were conducted at 125 °C for 30 h in 2 ml of a chosen
N
N
solvent. See Section 4 for details.
b
1
Cl
COCH3
The product was analyzed by H NMR. When the b product could
not be detected by H NMR, a value of >99/1 was assigned.
1
N
N
COCH3
Br
in Table 1. To our delight, the vinyl ether was com-
pletely arylated in [bmim][BF ] to give essentially exclu-
sively the a substituted product; the H NMR spectrum
of the reaction mixture after removing the ionic liquid
showed no sign of the linear olefin. In contrast, none
of the reactions in the five molecular solvents afforded
an a/b regioselectivity near to that observed in
O
N
N
4
1
6
7
O
O
89
82
S
Br
S
COCH3
COCH3
Br
S
N
N
N
S
N
N
N
Br
Br
Br
COCH3
O
[
tion was faster and furnished cleaner product as well,
bmim][BF ]. Additionally, in the ionic liquid, the reac-
8
9
77
72
71
4
O
1
as judged by H NMR of the crude products. Theses re-
COCH3
O
sults are reminiscent of those we obtained in the aryla-
tion of vinyl ethers and other electron-rich olefins by
various aryl bromides and iodides; several lines of evi-
dence suggested that those arylation reactions proceeded
via the ionic pathway [5b]. The high a regioselectivity
observed here suggests that the ionic mechanism is also
operative in the arylation by heteroaryl halides in the io-
nic liquid. At the suggestion of one referee, the arylation
was also examined in neat water; but a conversion of
only ca. 2% was observed under similar conditions. This
is at least partly due to the insolubility of the substrates
and catalyst in the aqueous phase. However, Hallberg
and coworkers [10b] have recently found that a mixture
of DMF and water is capable of promoting high regiose-
lectivity in the arylation of vinyl ethers by aryl bromides.
With these results in hand, the arylation of halopyri-
dines, bromoquinoline and bromothiophenes was then
carried out. Table 2 summarizes the results obtained.
The methyl ketone compounds were obtained by acidic
treatment of the Heck coupling products. All the reac-
tions led to exclusive formation of the a arylated olefins,
providing the first examples of highly regioselective,
intermolecular arylation of electron-rich olefins with
heteroaryl halides. The relatively electron-rich 3-bro-
mopyrindines have previously been reported to couple
with butyl vinyl ether in acetonitrile to afford a mixture
of regioisomeric olefins, with the a substituted being the
major component; but 2- and 4-bromopyrindines failed
to react [3a]. Exclusive formation of an a product was
observed with a 3-pyridyl triflate, consistent with the io-
nic pathway being facilitated with a good leaving group
COCH3
10
O
a
All reactions gave >99% conversion and >99/1 a/b regioselectivity
as determined by H NMR. See Section 4 for details.
1
b
Isolated yield.
mopyrindine equally regioselectively, affording aryl
methyl ketones in isolated yields of over 70% after
hydrolysis.
To further explore the scope of this highly regioselec-
tive Heck arylation method, we applied it to the cou-
pling of allyl alcohol, aiming to prepare heterocyclic
allyl alcohol derivatives (Scheme 4). Arylation of allyl
alcohols by aryl halides generally leads to carbonyl
products via isomerization of the initially formed c-
substituted allyl alcohol [13]. This is also true with
bromothiophenes [14]. In fact, there appear to be only
two reports in the literature, describing the preferential
internal arylation of allyl alcohols and homoallyl alco-
hol with 1-naphthyl triflate; complex mixtures were
formed with the corresponding iodide [15]. Remarkably,
under the conditions described above, allyl alcohol cou-
pled with halopyridines, bromoquinoline and bromothi-
ophenes, affording the b-substituted allyl alcohols in
excellent regioselectivities and isolated yields, demon-
strating again the utility of the current method in con-
trolling regioselectivity in the Heck reaction (Table 3).
The similarity between the regioselectivity obtained by
Cabri with aryl triflates in DMF [15] and that we ob-
[12]. In addition to butyl vinyl ether, three other vinyl
ethers (entries 8–10) were shown to couple with 3-bro-
tained with aryl halides in [bmim][BF ] suggests that this
4
regiocontrol stems from a promoting effect of the ionic

W. Pei et al. / Journal of Organometallic Chemistry 690 (2005) 3546–3551
3549
Pd(OAc)2
Phosphine
OH
ArX
+
OH
+
Ar
OH
+
Ar
O
Ar
NEt3
bmim][BF4]
[
β
γ
Scheme 4.
Table 3
4. Experimental
a
4
Heck arylation of allyl alcohol by heteroaryl halides in [bmim][BF ]
b
Entry
1
Substrate
Product
Yield
93
All reactions were carried out under a nitrogen atmo-
sphere. 1-Butyl-3-methylimidizolium tetrafluoroborate
Br
S
OH
OH
S
([bmim][BF ]) was prepared according to the literature
4
Br
method [16]. Following vacuum-drying at 80 °C for 8
h, the ionic liquid was stored under nitrogen at ambient
temperature. Butyl vinyl ethers, halopyridines, bromo-
quinoline, bromothiophenes, Pd(OAc)2 and 1,3-bis
2
3
4
89
86
82
S
S
N
OH
OH
N
Br
(diphenylphosphino)propane (DPPP) were purchased
from Lancaster and Aldrich and were used as received.
Br
N
N
1
13
N
N
N
The products were characterized by H and C NMR,
MS and HRMS, and by comparison of their spectra
with the literature data and/or those of authentic sam-
ples when available.
Cl
OH
5
75
95
Br
OH
6
N
4
.1. Heck arylation in ionic liquid
a
All reactions gave >99% conversion and >99/1 b/c selectivity as
determined by H NMR. See Section 4 for details.
1
b
An oven-dried, two-necked round-bottom flask con-
Isolated yield.
taining a stir bar was charged with a heteroaryl halide
1.0 mmol), Pd(OAc) (0.04 mmol), DPPP (0.08 mmol),
and [bmim][BF ] (2 mL) under nitrogen at room temper-
(
2
4
liquid on the dissociation of the halide anion from
Pd(II), thus facilitating the ionic pathway. The yield
with 3-chloropyridine was relatively low, however.
ature. Following degassing three times, butyl vinyl ether
(5.0 mmol) and triethylamine (1.6 mmol) were added.
The mixture was heated and stirred at 125 °C for 30 h.
After cooling to room temperature, aqueous HCl (10
mL, 10%) was added and the mixture stirred for 1 h.
3
. Conclusions
The solution was then treated with saturated Na
(
2
CO
10 mL) for a few min, without which the subsequent
3
The results presented here demonstrate again that
extraction would not work, and extracted with CH Cl
2
2
Pd-DPPP in combination with an imidazolium ionic li-
quid solvent provides an excellent catalytic system,
with which highly regioselective Heck arylation of elec-
tron rich olefins can be accomplished without recourse
to aryl triflates or stoichiometric silver and thallium
additives. Specifically, we showed that halopyridines,
bromoquinoline and bromothiophenes regioselectively
couple with vinyl ethers to give acetyl heterocycles fol-
lowing hydrolysis, and with allyl alcohol to give 2-allyl
alcohol-functionalized heteroaromatics. The introduc-
tion of these functionalities allow the heterocycles to
be further elaborated leading to compounds of poten-
tially interesting bioactivities. Whilst the detailed mech-
anism of these arylation reactions and those we
reported previously [5] remains to be delineated, we be-
lieve that the arylation proceeds via the ionic pathway
and the ionic liquid solvent plays an essential role in
making this possible.
three times. The combined organic layers were washed
with water and brine, dried with MgSO and evaporated
under reduced pressure. The residue was finally purified
by flash chromatography (SiO , hexane/EtOAc = 8/2) to
give the desired heteroaryl methyl ketone. The Heck ary-
lation of allyl alcohol was performed in a similar
manner.
4
2
4.2. Compounds isolated
1
2-Acetylpyridine. H NMR (CDCl
): d = 2.73 (s, 3H),
7.49 (m, 1H), 7.84 (m, 1H), 8.04 (m, 1H), 8.69 (d, 1H);
3
1
3
C NMR (CDCl
149.2, 153.9, 200.4; CI-MS, m/z (%): 122 [(M + H) ,
): d = 26.1, 122.0, 127.4, 137.1,
3
+
+
NO (M + H) : 122.0606.
100]; HRMS Calcd for C
Found: 122.0606.
H
8
7
1
3-Acetylpyridine. H NMR (CDCl
): d = 2.64 (s, 3H),
7.35 (m, 1H), 8.24 (m, 1H), 8.78 (s, 1H), 9.17 (d, 1H);
3

3550
W. Pei et al. / Journal of Organometallic Chemistry 690 (2005) 3546–3551
1
3
1
C-NMR (CDCl ): d = 27.1, 124.0, 132.6, 135.8, 150.3,
2-(Quinolin-3-yl)allyl alcohol. H NMR(CDCl3):
d = 1.98 (br, 1H), 4.67 (m, 2H), 5.56 (d, J = 0.6 Hz,
1H), 5.68 (d, 1H), 7.55 (m, 1H), 7.68 (m, 1H), 7.81 (m,
3
+
53.9, 197.1; CI-MS, m/z (%): 122 [(M + H) , 100], 108
1
+
13); HRMS Calcd for C H NO (M + H) : 122.0606.
(
Found: 122.0607.
7
8
1
3
1H), 8.09 (m,1H), 8.19 (s, 1H), 9.01 (s, 1H); C NMR
(CDCl ): d = 65.2, 115.3, 127.4, 128.1, 128.5, 129.3,
129.9, 131.6, 132.9, 144.8, 147.7, 149.4; CI-MS, m/z
1
4
-Acetylpyridine. H NMR (CDCl ): d = 2.36 (s, 3H),
3
3
1
3
7
.46 (m, 2H), 8.52 (m, 2H);
C-NMR (CDCl3):
+
(%) (5): 186 [(M + H) , 100], 170 (15); HRMS Calcd
d = 27.0, 121.3, 151.3, 198.2; CI-MS, m/z (%): 122
+
(M + H) , 100], 108 (15); HRMS Calcd for C H NO
+
for C H NO (M + H) : 186.0919. Found: 186.0917.
[
7
8
12 12
+
M + H) : 122.0606. Found: 122.0606.
(
1
3
-Acetylquinoline. H NMR (CDCl ): d = 2.74 (s,
3
3
(
H), 7.63 (m, 1H), 7.84 (m, 1H), 7.95 (m, 1H), 8.16
Acknowledgements
1
m, 1H), 8.70 (s, 1H), 9.43 (s, 1H); C NMR (CDCl3):
3
d = 27.2, 126.9, 128.2, 129.6, 129.8, 132.4, 137.7, 149.6,
We are grateful to the Zhejiang Association for Inter-
national Exchange of Personnel for financial support
(WP).
+
50.2, 197.1; CI-MS, m/z (%): 172 [(M + H) , 100],
1
1
58 (10), 130 (15); HRMS Calcd for C H NO
1
1
10
+
(
M + H) : 172.0762. Found: 172.0761.
1
2
-Acetylthiophene. H NMR (CDCl ): d = 2.57 (s,
3
13
3
H), 7.14 (m, 1H), 7.64 (m, 1H), 7.71 (m, 1H);
C
References
NMR (CDCl ): d = 27.3, 128.5, 132.9, 134.2, 145.0,
3
+
91.2; CI-MS, m/z (%): 127 [(M + H) , 100], 111 (43);
1
HRMS Calcd for C H OS (M + H) : 127.0218. Found:
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+
6
7
(
1
27.0215.
1
[3] (a) For some recent examples, see S.W. Wright, D.L. Hageman,
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3-Acetylthiophene. H NMR(CDCl ): d = 2.51 (s,
3
1
3
3
H), 7.31 (m, 1H), 7.54 (m, 1H), 8.04 (s, 1H);
C
(b) S.A. Karl, M.L. Vallin, A. Hallberger, J. Org. Chem. 66
(2001) 4340;
NMR (CDCl ): d = 27.4, 126.8, 127.3, 132.5, 143.1,
3
+
92.7; CI-MS, m/z (%): 127 [(M + H) , 100], 111 (50);
1
HRMS Calcd for C H OS (M + H) : 127.0218. Found:
(c) F. Berthiol, M. Feuerstein, H. Doucet, M. Santelli, Tetrahe-
dron Lett. 43 (2002) 5625, and references therein;
+
6
7
(
d) Y. Wang, X. Dong, R.C. Larock, J. Org. Chem. 68 (2003)
090;
1
27.0216.
-(Thiophene-2-yl)allyl alcohol. H NMR (CDCl ):
3
1
2
3
(
(
e) M. Sundermeier, A. Zapf, M. Beller, Eur. J. Inorg. Chem.
2003) 3513;
d = 1.89 (br, 1H), 4.43 (m, 2H), 5.19 (d, J = 0.3 Hz,
1
1
1
1
1
H), 5.44 (d, 1H), 6.93 (m, 1H), 7.03 (m, 1H), 7.14 (m,
1
(f) C.-H. Park, V. Ryabova, I.V. Seregin, A.W. Sromek, V.
Gevorgyan, Org. Lett. 6 (2004) 1159, and references therein;
(g) F. Berthiol, H. Doucet, M. Santelli, Tetrahedron Lett. 45
3
H); C NMR (CDCl ): d = 63.9, 110.2, 122.7, 123.6,
3
+
26.4, 139.9; 141.0; CI-MS, m/z (%): 141 [(M + H) ,
(
2004) 5633.
+
00], 125 (10); HRMS Calcd for C H OS (M + H) :
7
9
[
4] (a) For some recent reviews on the Heck reaction, see M. Larhed,
A. Hallberg, in: E.-I. Negishi (Ed.), Handbook of Organopalla-
dium Chemistry for Organic Synthesis, vol. 1, Wiley-Interscience,
New York, 2002, p. 1133;
41.0374. Found: 141.0378.
-(Thiophene-3-yl)allyl alcohol. H NMR(CDCl3):
d = 1.59 (br, 1H), 4.51 (m, 2H), 5.31 (d, J = 0.3 Hz,
1
2
1
3
(b) A.F. Littke, G.C. Fu, Angew. Chem. Int. Ed. 41 (2002) 4176;
1
H), 5.50 (d, 1H), 7.26-7.33 (m, 3H); C NMR
(
(
(
c) N.J. Whitcombe, K.K. Hii, S.E. Gibson, Tetrahedron 57
2001) 7449;
d) I.P. Beletskaya, A.V. Cheprakov, Chem. Rev. 100 (2000)
(
CDCl ): d = 65.5, 111.7, 126.1, 129.1, 131.1, 132.3,
3
+
41.4; CI-MS, m/z (%): 141 [(M + H) , 100], 140 (M ,
+
1
4
1
+
5), 125 (63); HRMS Calcd for C H OS (M + H) :
3009;
e) L.S. Hegedus, Transition Metals in the Synthesis of Complex
7
9
(
41.0374. Found: 141.0377.
-(Pyridin-2-yl)allyl alcohol. H NMR (CDCl3):
d = 1.54 (br, 1H), 4.58 (m, 2H), 5.51 (d, J = 0.5 Hz,
1
Organic Molecules, second ed., University Science Books, Sau-
salito, California, 1999 (Chapter 4.6).
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2
[
1
1
1
H), 5.79 (d, 1H), 7.22 (m, 1H), 7.63 (m, 1H), 7.71 (m,
3
(
[6] W. Cabri, I. Candiani, Acc. Chem. Res. 2 (1995) 28.
b) J. Mo, L. Xu, J. Xiao, J. Am. Chem. Soc. 127 (2005) 751.
1
H), 8.53 (m, 1H); C NMR (CDCl ): d = 66.6, 116.6,
3
[7] (a) M.M.S. Andappan, P. Nilsson, H. von Schenck, M. Larhed,
J. Org. Chem. 69 (2004) 5212;
20.5, 121.2, 123.0, 126.1, 137.1, 148.7; CI-MS, m/z
+
(%): 136 [(M + H) , 100], 121 (45); HRMS Calcd for
+
C H NO (M + H) : 136.1735. Found: 136.1735.
(
(
b) R.J. Deeth, A. Smith, J.M. Brown, J. Am. Chem. Soc. 126
2004) 7144;
8
10
1
2
-(Pyridin-3-yl)allyl alcohol.
H
NMR(CDCl3):
(c) H. von Schenck, B. Akermark, M. Svensson, J. Am. Chem.
Soc. 125 (2003) 3503;
d = 1.68 (br, 1H), 4.56 (m, 2H), 5.31 (d, J = 0.4 Hz,
(
d) F. Ozawa, A. Kubo, T. Hayashi, J. Am. Chem. Soc. 113
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b) L. Xu, J. Mo, C. Baillie, J. Xiao, J. Organomet. Chem. 687
(2003) 301;
1
1
1
H), 5.53 (d, 1H), 7.21 (m, 1H), 7.83 (m, 1H), 8.54 (m,
1
3
H), 8.69 (s, 1H); C NMR (CDCl ): d = 66.7, 118.6,
3
(
21.3, 124.2, 129.4, 134.8, 139.1, 148.6; CI-MS, m/z
+
[
(%): 136 [(M + H) , 100], 121 (55); HRMS Calcd for
+
C H NO (M + H) : 136.1735. Found: 136.1734.
(
8
10

W. Pei et al. / Journal of Organometallic Chemistry 690 (2005) 3546–3551
3551
(
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