Arenediazonium salts immobilized in imidazolium ionic liquids as electrophilic partners in the Pd(OAc)2-catalyzed Matsuda-Heck arylation

Article abstract of DOI:10.1016/j.tetlet.2011.02.021

The utility of arenediazonium salts immobilized in imidazolium-ILs [BMIMPF₆ and BMIMBF₄] for facile, high yielding, synthesis of olefins via the Pd(OAc)₂-catalyzed Matsuda-Heck arylation reaction has been demonstrated. The reaction can also be performed as a two-step process in the IL starting from ArNH₂, by sequential in-situ diazotization-arylation. Simple product isolation and the recycling/re-use of the IL are additional advantages of this one-pot method.

Full text of DOI:10.1016/j.tetlet.2011.02.021

Tetrahedron Letters 52 (2011) 1733–1737
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Tetrahedron Letters
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Arenediazonium salts immobilized in imidazolium ionic liquids as
electrophilic partners in the Pd(OAc)2-catalyzed Matsuda–Heck arylation
⇑
Rajesh G. Kalkhambkar, Kenneth K. Laali
Department of Chemistry, University of North Florida, 1, UNF Drive, Jacksonville, FL 32224, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The utility of arenediazonium salts immobilized in imidazolium-ILs [BMIMPF6 and BMIMBF4] for facile,
high yielding, synthesis of olefins via the Pd(OAc)2-catalyzed Matsuda–Heck arylation reaction has been
demonstrated. The reaction can also be performed as a two-step process in the IL starting from ArNH2, by
sequential in-situ diazotization–arylation. Simple product isolation and the recycling/re-use of the IL are
additional advantages of this one-pot method.
Received 29 December 2010
Revised 2 February 2011
Accepted 3 February 2011
Available online 26 February 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Matsuda–Heck arylation
Imidazolium ionic liquids
Arenediazonium salts
Olefin synthesis
One-pot diazotization–arylation
In comparison to aryl halides and oxygen-based electrophiles
that are widely used in Pd-catalyzed cross coupling reaction, the
utility of diazonium salts as electrophilic partners has not been
as widely exploited. Seminal early studies of ArNþ2 with transition
metals by Matsuda and coworkers1a–e attracted attention and led
to further investigations aimed at improving and expanding the
earlier work. The progress in the field was reviewed in 2006 by
Roglans et al.2 More recent studies in this area include: the use
of diazonium tosylates, prepared via resin-NO2/TsOH, for Heck ary-
lation of styrene with Pd(OAc)2/EtOH,3 stereoselective method for
the preparation of b,b-disubstituted acrylates using MeCN or alco-
hol solvents with or without a base,4 the intramolecular version of
Heck–Matsuda reaction applied to the synthesis of benzofuran and
indoles,5a Heck arylation of enol ethers with arenediazonium salts
as a new approach to the synthesis of flavanoids,5b and Heck–Mat-
suda arylation of 2-aza and 2-oxo-substituted acrylates.5c
A limited number of studies have explored the use of ILs in the
Heck reaction but these employ aryl halides. They include the use
of phosphonium ILs,6a–c imidazolium ILs along with ultrasound,7
and application of an ester functionalized imidazolium IL.8a In a
brief study, Kabalka et al reported styrene dimerization promoted
by diazonium salt in imidazolium IL.8b
In continuation of our studies on electrophilic and onium ion
chemistry in ionic liquids,9a–j and building on our earlier studies
focusing on diazonium ion chemistry in ILs,10 we report here a fac-
ile Matsuda–Heck process with ArNþ2 BF4ꢀ salts as electrophilic part-
ners in [BMIM][BF4] and [BMIM][PF6] as solvent, with catalytic
amounts (10–20 mol %) of Pd(OAc)2 without an external base, un-
der mild conditions in high yields with high stereoselectivity
(when applicable). The reactions were conveniently carried out in
a one-pot process with easy product isolation and recycling and re-
use of the IL. A variety of styrenes, as well as methyl crotonate and
ethyl cinnamate were employed as representative olefins for this
study. The product characterization data are gathered in the
accompanying supplementary file (for experimental procedures
see Ref. 11).
The arylation reactions with styrenes (Fig. 1) were performed in
[BMIM][PF6] as solvent. The isolated yields of corresponding trans-
stilbenes were in the 82–76% range (see Table 1).
It is possible to perform this reaction starting from the anilines
in a two step/one pot diazotization–arylation protocol (Fig. 2,
Table 1). The use of a nitrosonium salt for in situ diazotization is
convenient because NOX salt can be immobilized in imidazolium
N2BF4
R'
Pd(OAc)2
R
R'
R
BMIMPF6
70-80oC
Where R = H, 4-CH3, 4-F
R' = H, 4-CH3, 4-OCH3, 4-Cl, 4-Br, 4-F
⇑
Corresponding author. Tel.: +1 904 620 1503; fax: +1 904 620 3535.
E-mail address: kenneth.laali@UNF.edu (K.K. Laali).
Figure 1.
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.02.021

1734
R. G. Kalkhambkar, K. K. Laali / Tetrahedron Letters 52 (2011) 1733–1737
Table 1
Pd(OAc)2-catalyzed arylation of olefins with arenediazonium salts
Entry
1
Alkenes
Diazonium salts
Product
Time (h)
6.5
Isolated yield (%)
88, 78a
N2BF4
N2BF4
N2BF4
2
3
5
7
83, 71a
O
85
O
Cl
N2BF4
4
5
6
8
8
8
81
85
78
Cl
Br
F
N2BF4
Br
N2BF4
F
N2BF4
7
8
8
6
83
83
F
F
F
N2BF4
O
F
F
O
N2BF4
9
6
80
76
77
F
F
F
Br
Cl
N2BF4
10
11
5.5
7.5
F
F
Br
N2BF4
Cl

R. G. Kalkhambkar, K. K. Laali / Tetrahedron Letters 52 (2011) 1733–1737
1735
Table 1 (continued)
Entry
Alkenes
Diazonium salts
Product
Time (h)
6.5
Isolated yield (%)
F
N2BF4
12
13
14
78
82
80
F
F
F
N2BF4
5
5
F
N2BF4
F
N2BF4
O
15
7
82
O
a
Reported yield for a one-pot in situ diazotization–arylation.
O
N2BF4
NH2
Styrene
O
Pd(OAc)2
Pd(OAc)2
O
R'
R
R'
O
R
NOBF4
BMIMBF4
70-80oC
BMIMBF4
70-80oC
Where R = H, 4-CH3
Where R' = H, 4-CH3, 4-OCH3, 4-Cl, 4-F
Figure 2.
Figure 4.
ILs.10b,12 In the two representative cases studied, isolated yields for
the two-step process were slightly lower (see Table 1).
The arylation reactions with ethyl cinnamate (Fig. 2 and Table 2)
and methyl crotonate ( Fig. 3 and Table 3) were carried out in
[BMIM][BF4] as solvent.
plementary data), and NMR analysis of the products showed only
traces of another isomer (ꢁ2%). With p-OMe (as was also observed
in Ref. 4) notable amounts of the Z isomer were present (in our case
the E/Z ratio was 60:40 by 1H NMR). With methyl crotonate, in the
studied examples (see Fig. 4) only the E isomers were observed by
NMR. In this regard, observation of a four-bond Me/H coupling
(ꢁ1.5 Hz) in the alkene products is noteworthy (see Supplementary
data, also including corroboratory NOE and NOESY data).
In summary, the ionic liquid version of Mastuda–Heck arylation
reported here offers a number of advantages over the existing pro-
cedures. High yields and selectivity can be achieved in a simple
procedure by using catalytic amounts of Pd(OAc)2, without the
need to employ MeCN or ROH solvents and without an added base.
In a 2009 report, Pastre and Correia4 studied the Heck arylation of
p-methoxybenzenediazonium tetrafluoroborate with methyl cinna-
mate in various solvents (MeCN, MeOH, EtOH, iPrOH, and 1,2-pro-
panediol) under reflux or at rt, employing Pd(OAc)2 and Pd2(dba)3
as catalyst. Variable E/Z ratios (from 98:2 to 60:40) were obtained
depending on the conditions. In the present study in the IL, with
R0 = H, 4-Me, 4-Cl, 4-Br, and 4-F (seeFig. 3) the reactions were highly
E selective (verified by NOED and NOESY measurements; see Sup-
N2BF4
O
O
Pd(OAc)2
O
O
R'
R'
BMIMBF4
70-80oC
Where R' = H, 4-OCH3, 4-Cl, 4-Br, 4-F
Figure 3.

1736
R. G. Kalkhambkar, K. K. Laali / Tetrahedron Letters 52 (2011) 1733–1737
Table 2
Pd(OAc)2-catalyzed arylation of olefins with arenediazonium saltsa
Entry
Alkenes
Diazonium salts
Product
Time (h)
11
Isolated yield (%)
OEt
N2BF4
O
O
1
77
O
OEt
N2BF4
O
O
O
2
12
76
O
O
E/Z isomer ratio
(60:40)a
OEt
N2BF4
O
O
O
3
4
5
13
14
12
75
89
83
Cl
Cl
F
OEt
N2BF4
O
O
O
F
OEt
N2BF4
O
O
O
Br
Br
a
Determined by 1H NMR.
Table 3
Pd(OAc)2-catalyzed arylation of olefins with arenediazonium salts
Entry
Alkenes
Diazonium salts
Product
Time (h)
Isolated yield (%)
68
O
O
N2BF4
1
O
O
6.5
7
O
O
O
O
N2BF4
N2BF4
O
2
3
81
75
O
O
O
O
O
5
O
O
O
O
O
N2BF4
O
4
5
8
66
68
Cl
Cl
O
N2BF4
O
4.5
F
F

R. G. Kalkhambkar, K. K. Laali / Tetrahedron Letters 52 (2011) 1733–1737
1737
Soc., Perkin Trans. 2 2002, 2, 953; (d) Sarca, V. D.; Laali, K. K. Green Chem. 2004, 6,
245; (e) Laali, K. K.; Sarca, V. D.; Okazaki, T.; Brock, A.; Der, P. Org. Biomol. Chem.
2005, 3, 1034; (f) Sarca, V. D.; Laali, K. K. Green Chem. 2006, 8, 615; (g) Laali, K.
K. Ionic Liquids in Organic Synthesis, ACS Symp. Ser. 950 In Malhotra, S. V., Ed.;
ACS: Washington DC, 2006. chapter 2; (h) Hubbard, A.; Okazaki, T.; Laali, K. K.
Aust. J. Chem. 2007, 60, 923; (i) Pavlinac, J.; Laali, K. K.; Zupan, M.; Stavber, S.
Aust. J. Chem. 2008, 61, 946; (j) Pavlinac, J.; Zupan, M.; Laali, K. K.; Stavber, S.
Tetrahedron 2009, 65, 5625.
Easy product isolation/recovery and reuse of the IL could make this
method economically viable for scale-up.
Acknowledgments
Research support to K.L. from University of North Florida is
gratefully acknowledged. We thank Dr. Nelson Zhao of this depart-
ment for NMR assistance and the reviewer for very helpful
suggestions.
10. (a) Laali, K. K.; Okazaki, T.; Bunge, S. D. J. Org. Chem. 2007, 72, 6758; (b)
Hubbard, A.; Okazaki, T.; Laali, K. K. J. Org. Chem. 2008, 73, 316.
11. General procedure: The desired alkene (1 mmol) and the diazonium salt
(2 mmol) were introduced at rt into an oven-dried Schlenk tube charged
with [BMIM][PF6] or [BMIM][BF4] ionic liquid (ꢁ4 mL) under
a nitrogen
atmosphere. Upon efficient magnetic stirring (for 10–20 min) the diazonium
salt was immobilized in the IL, forming a semi-homogenous mixture, which
was then charged with Pd(OAc)2 (10–20 mol %) under nitrogen. The reaction
mixture was stirred at 70–80 °C under nitrogen and the progress of the
reaction was monitored by TLC. The brown-colored reaction mass was cooled
to rt and the products were extracted with dry diethyl ether (4 times). Removal
of solvent under vacuum furnished the crude products which were
chromatographed with hexane–ethyl acetate mixture (80:20) to afford the
pure Heck coupling products which were characterized by GC–MS and NMR.
The brown colored IL was dried overnight under vacuum at 50 °C and reused in
the next 2–3 run, after which it was set aside for recovery and recycling as
outlined below.
Procedure for recycling of ionic liquid: The combined brown-colored ionic liquids
recovered from several set of experiments were dissolved in MeCN, and filtered
through celite to remove insoluble black particles. After removal of solvent
from the filtrate under vacuum, the recycled IL was dried overnight under
vacuum at 50 °C and reused in subsequent runs.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.02.021.
References and notes
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sonicated at 50 °C for 45 min and then stirred under nitrogen at 60–70 °C for
2 h. The reaction was monitored by TLC for the disappearance of aniline,
following which the styrene (0.5 mmol) and Pd(OAc)2 (20–30 mol %) were
introduced and stirring was continued at 70–80 °C for 7–8 h. Upon completion,
the reaction mass was cooled to rt, the products were extracted in diethyl ether
(4 times) and the solvent was removed under vacuum to furnish the crude
products which were chromatographed with hexane–ethyl acetate mixture
(80:20) to afford pure products which were characterized by GC–MS and NMR
(see Table 1, entries 1–2).
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