A new pathway for the preparation of diaryl acetylenes

Article abstract of DOI:10.1016/S0040-4039(03)00832-3

A simple method for the preparation of diaryl acetylenes in yields up to 61% from aryl bromides and 1-bromo-2-chloroethane using the palladium catalyst (dppb)Pd(OAc)₂ under PTC conditions (solid KOH/18-crown-6) has been developed.

Full text of DOI:10.1016/S0040-4039(03)00832-3

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 3911–3913
A new pathway for the preparation of diaryl acetylenes
Edgars Abele,* Ramona Abele, Pavel Arsenyan and Edmunds Lukevics
Latvian Institute of Organic Synthesis, 21 Aizkraukles Street, Riga LV-1006, Latvia
Received 26 February 2003; revised 21 March 2003; accepted 28 March 2003
Abstract—A simple method for the preparation of diaryl acetylenes in yields up to 61% from aryl bromides and 1-bromo-2-
chloroethane using the palladium catalyst (dppb)Pd(OAc)2 under PTC conditions (solid KOH/18-crown-6) has been developed.
© 2003 Elsevier Science Ltd. All rights reserved.
Conjugated systems have been a highly prominent
research subject for over two decades because of their
unique optical and electronic properties, and environ-
mental stability.1 Alkynes are important and in many
cases convenient intermediates for the preparation of
various synthetic precursors for the construction of
valuable materials.2 Diaryl acetylenes can be seen as
components in mixed triple bond–arene conjugated
oligomers, polymers, and dendrimers.3 A number of
methods have been used for the preparation of substi-
tuted alkynes.4 One of the most common methods for
bond formation between an acetylene and an arene is
the Sonogashira coupling of aryl halide with a terminal
acetylene catalyzed by palladium complexes and copper
iodide. Silylated acetylenes and acetylene gas have
served as a source of acetylenes in the Sonogashira
reaction terminal acetylenes.4,5 The acetylene–arene
coupling can also be realized by reaction of aryl bro-
mide with stannyl acetylenes in the presence of a palla-
dium catalyst.6
The high activity of a Pd catalyst with 1,4-bis-
(diphenylphosphino)butane (dppb) as ligand was
demonstrated in Ref. 8. The presence of solid KOH/18-
crown-6 is necessary for the successful dehydrohalo-
genation of 1-bromo-2-chloroethane.9 The best results
were obtained using the following catalytic system solid
KOH/18-crown-6/Pd(OAc)2/dppb in toluene in a molar
ratio of 5.0:0.1:0.05:0.125.10 We propose that initially
the aryl bromide forms an adduct with palladium cata-
lyst (dppb)Pd(ArBr)2 (Scheme 2). In a second step, the
subsequent Heck reaction11 of vinyl chloride formed
from 1-bromo-2-chloroethane under PTC conditions
with the aryl bromides 1–6 gives the intermediate
ArCHꢀCHCl, which undergoes elimination of HCl in
the presence of base and leads to arylacetylenes. Then
the arylacetylenes reacted with the second equivalent of
aryl bromide in the presence of palladium catalyst to
afford the desired diarylacetylenes 7–12 in yields up to
61% (Table 1). This step was proved experimentally.
The reaction of phenylacetylene with bromobenzene in
system Pd(OAc)2/dppb/solid KOH/18-crown-6/PhMe
at 100°C led to diphenylacetylene as a main product in
40% yield. The formation of aryl bromides homocou-
pling products (Ar–Ar) (5–20%) was also detected. In
this way 1,2-diphenyl acetylene 7 was obtained in 47%
yield from bromobenzene. Lower yields were obtained
As part of our ongoing research to create new materi-
als,7 we now report on a new pathway for the prepara-
tion of diaryl acetylenes. 1-Bromo-2-chloroethane was
used as a source of ‘acetylene’ in the Sonogashira
coupling. The diaryl acetylenes 7–12 were formed in the
presence of one equivalent of 1-bromo-2-chloroethane
but in a mixture of other products in low yields. To
increase the selectivity of this reaction we optimized the
aryl bromide 1–6:1-bromo-2-chloroethane ratio. It was
found that a twofold excess of 1-bromo-2-chloroethane
to aromatic bromide is the optimal amount (Scheme 1).
Keywords: acetylene; aryl bromides; palladium catalysis; phase trans-
fer catalysis.
* Corresponding author. Tel.: (371) 7551822; fax: (371) 7550338;
e-mail: abele@osi.lv
Scheme 1. Palladium catalyzed synthesis of diarylethynes in
the system BrCH2CH2Cl/solid KOH/Pd(OAc)2/dppb/18-
crown-6/toluene at 100°C.
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)00832-3

3912
E. Abele et al. / Tetrahedron Letters 44 (2003) 3911–3913
Scheme 2. Mechanism of palladium catalyzed synthesis of diarylethynes under PTC conditions.
Table 1. Palladium catalyzed synthesis of diarylethynes 7–
12 from aryl bromides 1–6 in the system BrCH2CH2Cl/
solid KOH/Pd(OAc)2/dppb/18-crown-6/toluene at 100°C
Science (Grant Nos. 166 and 187). We thank Dr. J.
1
Popelis for H and 13C NMR studies.
ArBr
Ar
Reaction time
(h)
Products
References
(yield, %)a
7 (47)5a,c–e
8 (26)5a,b
9 (37)5e,12
10 (30)13
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Acknowledgements
This work was supported by the Latvian Council of

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