Selective synthesis of 1-aryl-2-ferrocenylethylene by cross-metathesis

Article abstract of DOI:10.1246/cl.2001.812

Novel synthesis of π-conjugated molecules by cross-metathesis reaction of vinylferrocene with a series of vinylarenes was investigated with a molybdenum-based Schrock catalyst (CHCMe₂Ph)Mo(N-2,6-i-Pr₂C₆H₃)[OCMe(C F₃)₂]₂. The cross-metathesis reactions occurred successfully and the cross-metathesis product, i.e., heterodimers, were readily obtained selectively, together with only small amounts of the corresponding self-dimers.

Full text of DOI:10.1246/cl.2001.812

812
Chemistry Letters 2001
Selective Synthesis of 1-Aryl-2-ferrocenylethylene by Cross-Metathesis
Tomohiro Yasuda, Jiro Abe, Tomokazu Iyoda, and Tadashi Kawai*
Department of Applied Chemistry, Graduate School of Engineering, Tokyo Metropolitan University,
Hachioji, Tokyo 192-0397
(Received May 1, 2001; CL-010397)
Novel synthesis of π-conjugated molecules by cross-
metathesis reaction of vinylferrocene with a series of vinyl-
arenes was investigated with a molybdenum-based Schrock cat-
alyst (CHCMe2Ph)Mo(N-2,6-i-Pr2C6H3)[OCMe(CF3)2]2. The
cross-metathesis reactions occurred successfully and the cross-
metathesis product, i.e., heterodimers, were readily obtained
selectively, together with only small amounts of the correspon-
ding self-dimers.
Here we report the cross-metathesis reaction between
vinylferrocene and a series of vinylarenes to give π-conjugated
molecules with a homogeneous well-defined metathesis
Schrock catalyst. Ferrocene is a typical redox-active molecule
and has been widely applied for functional molecules such as
dendrimer,24 C-T complex with TCNQ,25 mixed-valence prop-
erty of diferrocenes26 and so on. Much attention should be
given to such compounds due to their unique properties, e.g.,
magnetic, electric and nonlinear optical properties.
Numerous studies on metathesis of a broad range of
alkenes have been reported for various kinds of heterogeneous
and homogeneous catalysts.1 The advent of well-defined tran-
sition metal catalysts, especially the ruthenium benzylidene
complexes developed by Grubbs et al.2 and the molybdenum
imido alkylidene complexes developed by Schrock et al.3 has
dramatically increased their application in organic synthesis.
They are now extensively used in various kinds of metathesis
reactions, such as acyclic diene metathesis (ADMET),4 ring-
closing metathesis,5 ring-opening metathesis6 and cross-
metathesis1,7 using various kinds of reactants.8–15
A Schrock catalyst, 2,6-diisopropylphenylimido-neophylidene-
molybdenum(VI)-bis(hexafluoro-t-butoxide) was used without
further purification.
Vinylferrocene, 2-vinylthiophene, 2-vinylfuran and 4-
vinylbiphenyl were prepared in high yield by Wittig reaction in
accordance with a procedure27–29, that we adopted with modifi-
caton.
All manipulations were performed using standard Schlenk
techniques under an atmosphere of argon. Toluene dried and
distilled from Na was used as a solvent. 1H NMR, FAB-MS,
Gel Permeation Chromatography and elemental analysis were
used for the analysis of metathesis products.
As for the cross-metathesis reactions, many studies
between acyclic compounds have been reported, e.g., the syn-
thesis of natural products, but only a few studies between aro-
matic compounds and acyclic compounds1,16–19 have so far
been reported. The simplified reaction scheme for the cross-
metathesis of two α-olefins is depicted in Scheme 1.
Vinylferrocene and vinylarenes undergo self-metathesis at
reasonable rate, but surprisingly the cross-metathesis between
vinylferrocene and vinylarenes occurred efficiently with high
heterodimer selectivity as shown in Table 1.
In the cross-metathesis between olefins with similar struc-
ture and reactivity, the product distribution of heterodimer and
two homodimers is about 2 : 1 : 1, as expected.20–22 Crowe et
al. reported interesting results on selective cross-metathesis to
give heterodimer between styrene and α-olefins the presence of
a Schrock catalyst (Figure 1).18
There are several studies related to self- and cross-metathe-
sis of styrene. However, self-metathesis reactions of vinyl-
arenes and vinyl derivatives of aromatic heterocycles, and their
cross-metathesis reactions have not been adequately investigat-
ed.23 The success of application of metathesis to the chemistry
of vinylarenes and vinyl aromatic heterocycles will provide
new approaches to the synthesis of valuable materials, which
will unfold developments of new fields.
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
813
The reaction products were the heterodimer and two
homodimers. The heterodimer selectivity exceeded 80% and
nearly 100% selectivity was achieved for the system of vinyl-
ferrocene and vinylnaphthalene. No side-reaction products and
oligomers were detected. The homodimer of vinylarene
(Homo-2) was formed in selectivity more than that of vinylfer-
rocene (Homo-1). Although we could not get satisfactory het-
erodimer yields, higher yields will be achieved by investigating
more suitable reaction conditions, e.g., reaction temperature,
removal of ethylene formed, etc.
4
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7
8
The geometry of the double bond of heterodimers was
determined by 1H NMR and 100% trans isomer was found. An
ORTEP diagram of 1-ferrocenyl-2-(2-naphthyl)ethylene is
shown in Figure 2. The geometry of the double bond is trans
isomer. Naphthalene ring and cyclopentadiene ring are on the
same plane that suggests the delocalization of π electrons.
9
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Several explanations for the selective dimer formation have
been made.16–18 Crowe et al. reported that the reaction of stable
benzylidene carbene and terminal olefin with greater basicity is
key to the reaction.18 Since two olefins are aromatic com-
pounds in our systems, similar amounts of both the correspon-
ding carbenes will be formed. The yield curve of homodimer in
self-metathesis of vinylarene as a function of reaction time was
similar to that of heterodimer in vinylarene and vinylferrocene
system. This suggested that both reactions occurred at similar
reaction rates. Thus, selective heterodimer formation may be
caused by: (i) selective reaction between ferrocenylidene car-
bene and vinylarene enhanced by the smaller size of the vinyl-
arene, and (ii) selective reaction between arenilidene carbene
and vinylferrocene, enhanced by the greater basicity of the
vinylferrocene.
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In conclusion, we succeeded for the first time to get a
series of interesting π-conjugated molecules, 1-aryl-2-ferro-
cenylethylene selectively by cross-metathesis in the presence of
a Schrock catalyst. A study of the mechanism of high het-
erodimer selectivity is currently in progress.
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This work was supported by Grant-in-Aid for Science
Research from the Ministry of Education, Science, Sports and
Culture, Japan (10650773).
Reference and Notes
1
2
3
K. J. Ivin and J. C. Mol, “Olefin Metathesis and Metathesis
Polymerization”, Academic Press, London 1997.
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3875(1990).
30 The crystal data of 1-ferrocenyl-2-(2-naphthyl)ethylene are
as follows; C22H18Fe, fw = 338.23 Monoclinic, Space
Group P21/c(# 14), a = 13.377(2), b = 9.9889(10), c =
12.111(1) Å, β = 91.762°, Dcalc = 1.39 g cm–3, V =
1617.6(3) Å3, Z = 4, R = 0.046, Rw = 0.036 for 2514 reflec-
tion with I > 3σ(I).