Selective linear triene formation from different alkynes using Zr/Cu system

Article abstract of DOI:10.1021/ja8000387

Selective synthesis of linear trienes from three different alkynes was achieved in one-pot procedure using the Zr/Cu system. Zirconacyclopentadiene prepared from two different alkynes such as an alkyl-substituted alkyne and an aryl-substituted alkyne reac

Full text of DOI:10.1021/ja8000387

Published on Web 04/05/2008
Selective Linear Triene Formation from Different Alkynes
using Zr/Cu system
Ken-ichiro Kanno, Eri Igarashi, Lishan Zhou, Kiyohiko Nakajima, and
Tamotsu Takahashi*
Catalysis Research Center and Graduate School of Life Science, Hokkaido UniVersity, and
SORST, Japan Science and Technology Agency (JST), Kita-ku, Sapporo 001–0021, Japan, and
Department of Chemistry, Aichi UniVersity of Education, Igaya, Kariya 448–8542, Japan
Received January 3, 2008; E-mail: tamotsu@cat.hokudaii.ac.jp
Scheme 1
Synthesis of stereocontrolled π-conjugated oligoenes from
alkynes have been of growing interest in recent years.1–3 We
have recently reported selective linear triene formation4 or linear
tetraene formation from alkynes using zirconocene.5However,
there is no report on metal mediated selective linear triene
formation from different alkynes as shown in Scheme 1, to the
best of our knowledge.
Scheme 2
Oligomerization of alkynes using transition-metal complexes
very often provides cyclotrimerization products, benzene or
fulvene derivatives. In the case of benzene formation, we have
developed selective cyclotrimerization of three different alkynes
using zirconocene complexes.4,6 These reactions are applicable
for regioselective synthesis of highly substituted benzene and
pyridine derivatives from three different alkynes or nitriles.
During the course of our investigation for the formation of linear
oligoenes, we found the control of the coupling of alkynes in
the zirconocene/copper system.
Scheme 3
We report herein linear triene formation from different
alkynes using Zr/Cu system and tetraene formation is also
demonstrated as depicted in Scheme 2.
As shown in Scheme 2, two different alkynes can be
selectively coupled on Zr using Cp2ZrEt2 to give an unsym-
metrical zirconacyclopentadiene 1, as we reported previously.7
Differentiation of the two Zr-C bonds of the unsymmetrical
zirconacyclopentadiene was important. Acid treatment, reaction
with BuLi, or transmetalation to other transition metals of
unsymmetrical zirconacyclopentadienes did not give the selec-
tive cleavage of the Zr-C bond of the two different Zr-C bonds
in 1. When R1 is an alkyl group and R2 is an aryl one, the
Zr-C bond attached to R1 was cleaved with NCS with excellent
selectivity.8 The following transmetalation of the remained
Zr-C bond with CuCl gave 2.9 Insertion reaction of the third
alkyne to 2 produced linear triene 3 after hydrolysis.10 Before
hydrolysis subsequent insertion of the fourth alkyne afforded
the corresponding tetraene 4 after hydrolysis. In these insertion
steps, prevention of multiple insertions of alkynes and control
of the geometry of the formed double bonds are the key points
to obtain the desired oligoenes with high selectivity.
insertion to the other DMAD was obtained. At 50 °C, isomer-
ization at the copper-attached CdC bond was also observed,
and the triene 3a was obtained as a mixture of E/Z-isomers in
34% yield (Z/E ) 1:7.5). These results showed that the control
of the reaction temperature is very important to restrict the
number of the inserting DMAD molecules and prevention of
E,Z-isomerization.
Among the examined reagents for the ring opening of
zirconacyclopentadiene 1a, NCS gave the best results. When
EtOH or HOAc was used instead of NCS, the yield of triene
3a was significantly decreased. When NBS or NIS was used,
formation of the corresponding benzene derivative occurred after
addition of DMAD and CuCl.
When the reaction shown in Scheme 3 was quenched with
the other electrophiles instead of hydrolysis, the corresponding
triene derivatives were obtained as shown in Scheme 4. Iodine
quenching of the alkenylcopper 3a-Cu afforded iodotriene 3b.
Carbon-carbon bond formation is also possible. The reactions
with benzoyl chloride and allyl bromide afforded the corre-
sponding coupling products 3c and 3d, respectively.
This method was applied to the triene formation from three
different alkynes. The results were summarized in Table 1. The
triene formation was extended for that from three different
alkynes. With two different alkynes the corresponding unsym-
metrical zirconacyclopentadienes 1 were obtained in high yields
and with excellent selectivity as we reported previously.7 After
To verify the strategy mentioned above, the reactions with
tetraethylzirconacyclopentadiene 1a were examined as shown
in Scheme 3. Zirconacyclopentadiene 1a was mixed with NCS,
and the mixture was added with dimethyl acetylenedicarboxylate
(DMAD) and CuCl. The mixture was stirred at –20 °C for 3 h.
After hydrolysis, the desired linear triene 3a was obtained as a
single product in a good yield. On the other hand, when the
reaction was carried out at 0 °C or room temperature, a mixture
of the desired triene 3a and higher oligoenes formed by further
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5624 J. AM. CHEM. SOC. 2008, 130, 5624–5625
10.1021/ja8000387 CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Scheme 4
Scheme 5. Synthesis of Linear Tetraenes with Four Different
Alkynes
Table 1. Synthesis of Linear Trienes with Three Different Alkynes
skeleton has Z,Z,Z geometry, and the three alkynes were
connected in the expected order. As the same way, bis(p-fluoro-,
and p-methoxyphenyl)acetylenes, methyl propiolate, 3-butyn-
2-one, and dibenzoylacetylene were also applicable for the
reactions without significant loss of selectivity (entries 4–9).
Furthermore, the zirconium-mediated successive coupling
reaction was extended for linear tetraene formation as shown
in Scheme 5. The alkenylcopper intermediate 3-Cu prepared
as above was reacted with tosylacetylene at room temperature
to afford the corresponding tetraene 4a in moderate to good
yields with high regio- and stereoselectivity. The other tetraenes
4b and 4c were also obtained in 68 and 30% yields, respectively.
X-ray crystallographical analysis was carried out for tetraene
4a and confirmed the structure and stereochemistry.
Supporting Information Available: Detailed experimental
procedures, spectral data for new compounds, and results of X-ray
crystallographical analyses for 3e, and 4a. This material is available
free of charge via the Internet at http://pubs.acs.org.
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