Iridium-catalysed isomerising trialkylsilylation of methyl oleateq

Article abstract of DOI:10.1016/j.jorganchem.2013.06.002

Monounsaturated fatty acids from vegetable oils are attractive substrates for applications as renewable feedstock in polymer industry. Applying the concept of transition metal-catalysed isomerising functionalisation, their nearly inaccessible internal C]C double bond can be transformed into products with a functional group in the terminal position. In this work, methyl oleate is shown to undergo [Ir(OMe)( cod)]2-catalysed dehydrogenative silylation with triethylsilane to give terminal vinylsilanes 4 in 69% yield. Independent preparation of reference substances is helpful in identifying the desired products in gas chromatograms of complex reaction mixtures.

Full text of DOI:10.1016/j.jorganchem.2013.06.002

Journal of Organometallic Chemistry 744 (2013) 144e148
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Iridium-catalysed isomerising trialkylsilylation of methyl oleateq
*
Thimo Huber, Doris Firlbeck, Herbert M. Riepl
Hochschule Weihenstephan-Triesdorf, Fachgebiet für Organische und Analytische Chemie, Schulgasse 16, D-94315 Straubing, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Monounsaturated fatty acids from vegetable oils are attractive substrates for applications as renewable
feedstock in polymer industry. Applying the concept of transition metal-catalysed isomerising func-
tionalisation, their nearly inaccessible internal C]C double bond can be transformed into products with
a functional group in the terminal position. In this work, methyl oleate is shown to undergo [Ir(O-
Me)(cod)]2-catalysed dehydrogenative silylation with triethylsilane to give terminal vinylsilanes 4 in 69%
yield. Independent preparation of reference substances is helpful in identifying the desired products in
gas chromatograms of complex reaction mixtures.
Received 14 March 2013
Received in revised form
31 May 2013
Accepted 5 June 2013
Keywords:
Isomerizing silylation
Unsaturated fatty acids
Silicon
Ó 2013 Elsevier B.V. All rights reserved.
Iridium
1. Introduction
equilibrium mixture of positional and geometrical isomers by a
number of double bond migration and isomerisation steps. From
Unsaturated fatty acid methyl esters derived from natural oils
are an attractive feedstock for the fabrication of higher-value
chemicals because of their increasing availability due to the large-
scale production of biodiesel fuel [1]. The majority of these natu-
rally occurring unsaturated fatty esters are characterised by an in-
ternal double bond at the site of carbon number nine or higher [2],
methyl oleate being the most prominent example.
this rapidly interconverting pool of all possible isomers (31 in the
case of methyl oleate) only one is specifically trapped out of the
equilibrium by selective functionalisation. In many cases, the
transition metal complex plays a double role in catalysing both of
the abovementioned steps. According to Le Chatelier’s principle,
this combination of equilibrium isomerisation and irreversible
functionalisation can achieve high yields of the desired product
even though the respective positional isomer is present in only
minimal amounts [6]. For oleic acid and its esters, such processes
have been known since 2005, when Behr et al. reported on a
Obviously, the position of the double bond deep inside the mole-
cule is unfavourablewith respect to use of this renewable feedstock in
the polymer industry [3]. A double bond situated at the
u-terminal
end of the carbon chain would be desirable, for reasons of both
accessibilityand atomeconomy. Forexample, methyl ricinoleate from
castor oil is pyrolysed in high tonnage to yield methyl undecenoate, a
rhodium-catalysed
palladium-catalysed
Jiménez-Rodriguez et al. [8]. Iridium-catalysed
u
-selective hydroformylation [7]. In addition,
-alcoxycarbonylation was presented by
-hydroboration
u
u
terminally unsaturated fatty ester, which is transformed into
u
-ami-
was reported by Ghebreyessus and Angelici [9]. More recently,
Gooßen et al. commented on silver-catalysed selective functional-
noundecanoic acid and subsequently polymerised to the polyamide
nylon 11 (RilsanÒ) [4]. This is not a generalisable case, however, on
account of the special chemistry of ricinoleic acid. The amount of
othernatural fattyacids with terminaldoublebonds, suchasoropheic
acid from the leaves of Orophea enneandra or isanic acid from Olaca-
ceae, is negligible, as the relevant plant sources are scarce [5].
However, this problem can be circumvented by adapting the
concept of transition metal-catalysed isomerising functionalisation.
In this tandem reaction type, the substrate is converted into an
isation of the 4-position to yield
catalysed conjugate addition of aryl and amine groups to afford
arylated and -amino acids (Scheme 1) [11]. In the interim, papers
g-lactones [10] as well as rhodium-
b
-
b
have been published on polymerisation of the obtained monomers
[12], expansion of the range of substrates to commercial tri-
glycerides [13], mechanistic features [6e], and on improvement of
some of these processes using ionic liquids [14] and water as sol-
vent [15].
Nevertheless, to the best of our knowledge, there have been no
reports of an analogous process for the selective addition of silicon-
containing groups to fatty acids or esters, although there are in-
dications of a similar reaction with simpler olefins [16]. Along with
their expected high biocompatibility and low environmental
q
This paper is dedicated to Professor Wolfgang A. Herrmann on the occasion of
his 65th birthday.
* Corresponding author. Tel.: þ49 9421 187 302; fax: þ49 9421 187 211.
E-mail address: h.riepl@wz-straubing.de (H.M. Riepl).
0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.06.002

T. Huber et al. / Journal of Organometallic Chemistry 744 (2013) 144e148
145
RO
RO
RO
O
O
O
[catalyst]
[Rh] cat., NaBPh4
RO
30% (Gooßen 2011)
O
O
Ph
O
[Ag] cat.
[Ir] cat.,
51% (Gooßen 2010)
45% (Angelici 2006)
O
RO
...
B
O
O
[Pd] cat., CO, MeOH/H+
[Rh] cat., CO/H2
RO
RO
RO
OR
95% (Cole-Hamilton 2005)
26% (Behr 2005)
O
O
O
O
O
H
Scheme 1. Outline of important isomerising functionalisations of fatty acids and esters reported in the literature.
impact [17], such silicon oleochemicals would be of interest for
their wide range of applications as adhesives, coupling agents, lu-
bricants, plasticisers, and monomers for the production of silicon-
containing polymers [18]. Organosilicon compounds are predomi-
nantly prepared by direct synthesis [19] and hydrosilylation [20].
First results of hydrosilane addition to internally unsaturated fatty
acid esters were published as early as 1955 by Speier, followed by a
eluting from GC column at 8.8 min remained even after intensive
column chromatographic purification of 2. This contamination
turned out to be methyl dodecanoate (3), formed by methyl group
transfer from the mixed dialkylcuprate to the iodo ester 1, an un-
wanted side-reaction that is known from the literature [25b]. On
normal phase silica gel, 2 and 3 showed identical Rf values and thus
could not be separated by column and thin layer chromatography
(TLC). Roomi et al.’s suggestion of using reversed phase TLC for the
separation of saturated from unsaturated fatty acid esters [26]
proved viable for solving this problem as well. For 2 and 3, Rf
values were found to be 0.22 and 0.43, respectively, using RP-18
modified plates and pure acetonitrile as eluent. This difference
appeared promising enough for us to extend the scope of this
separation problem to preparative scale: the use of solid phase
extraction (SPE) cartridges with octadecyl-modified silica phases
served for this purpose. We were pleased to find that separations
were excellent even in large scale with a 10 g cartridge and more
than 230 mg of crude product, recovering up to 96% of pure ester 2
without any quantifiable contamination present. A discontinuous
gradient from acetonitrileewater (9:1) to pure acetonitrile proved
to be the eluent of choice.
few others, but no
u-substituted products whatsoever were ob-
tained [21]. In the last two decades, other reactions of silanes cat-
alysed by transition metal complexes have been revealed, notably
dehydrogenative silylation of terminal alkenes to produce vinyl-
silanes [22]. Herein, we offer an Ir(I)-catalysed dehydrogenative
silylation of methyl oleate with triethylsilane leading to good yields
of terminal products.
2. Results and discussion
2.1. Synthesis of reference compounds
Preliminary results of isomerising silylation reactions most
often showed complicated mixtures of products that were insep-
arable by conventional techniques and thus scarcely isolable for
NMR analysis. Therefore, our first step was to synthesise the
respective target compounds to have reference substances at hand
that allow for unequivocal identification in gas chromatographic
(GC) analyses of complex reaction mixtures. Consequently, a sam-
ple of methyl octadec-17-enoate (2), the positional isomer of
methyl oleate bearing a terminal double bond, was essential.
It is a well-known compound that has been synthesised in many
different ways [23], including the “acetylene zipper” method [24]
and the reaction of iodo esters with mixed dialkylcuprates [25].
We tried to reproduce this by preparing methyl 11-
iodoundecanoate (1) from methyl 11-bromoundecanoate and so-
dium iodide in refluxing acetone and subsequently reacting it with
a preformed mixed dialkylcuprate from hept-6-en-1-ylmagnesium
bromide and methylcopper (Scheme 2). Our problem was that a
substantial amount (9% calculated by GC peak area) of by-product
With pure 2 available, we were able to prepare methyl 18-(trie-
thylsilyl)octadec-17-enoate (4) by bis(1,5-cyclooctadiene)di-m-met
hoxydiiridium(I)-catalysed dehydrogenative silylation following a
procedure published by Lu and Falck [27] (Scheme 3). Vinylsilane 4
was of special interest to us as it would be the desired product formed
in a potential
u-selective isomerising dehydrogenative silylation
starting from internally unsaturated methyl oleate.
Good Z-diastereoselectivity of this reaction is described with
2,20-bipyridine-based ligands bearing sterically demanding tert-
butyl groups. In contrast to this, we used simple 2,20-bipyridine and
slightly prolonged reaction time, as we had to prepare both
geometrical isomers in substantial quantities. In this way, 4 was
obtained as an E/Z isomeric mixture in a ratio of about 1:4 (calcu-
lated by GC peak area). The two isomers were well separated by GC,
with retention times of 27.4 min and 27.6 min for (E)-4 and (Z)-4,
respectively. Their identical mass spectra showed no distinct peaks
in the range from m/z ¼ 202 to 348, indicating that silylation took
place on the terminal carbon. This could be confirmed by 1H NMR
analysis, in which 4 showed four signals with characteristic mul-
tiplets in the olefinic region, accounting for both alkene protons of
the E and Z isomer, respectively.
In order to be able to distinguish vinylsilanes 4 from the cor-
responding internally substituted isomers 5b, we also tried to
prepare these by [Cp*Ru(MeCN)3]PF6-catalysed hydrosilylation of
octadec-9-ynoic acid with triethylsilane (Scheme 4).
Hydrosilylation of long-chain internal alkynes seemed to be
somewhat difficult due to the lack of suitable catalysts, but Trost
and Ball’s procedure [28] turned out to meet our demands.
MgBr
5
O
O
NaI
Br
OMe
I
OMe
O
CuI, MeLi
9
9
1
O
OMe
OMe
15
9
+
2
3
(major product)
(minor product)
Scheme 2. Synthetic route to methyl octadec-17-enoate (2).

146
T. Huber et al. / Journal of Organometallic Chemistry 744 (2013) 144e148
were present in the reaction mixture. The majority of 69% consisted
O
O
[Ir(OMe)(cod)]2
,
+
HSiEt3
Et3Si
of two peaks eluting at 27.4 and 27.6 min. By comparison of GCeMS
data with our independently synthesised reference compounds,
these two were assigned to terminal vinylsilanes (E/Z)-4. Again,
quantifications are based on GC peak areas referred to one equiv-
alent of methyl oleate, while peaks resulting from reactions on
excess sacrificial alkene were excluded. The aforementioned
composition of the reaction system is a result of our efforts to
optimise the reaction conditions. Lowering the catalyst loading to
values of 5 mol% and below led to a significant decrease in product
yield, giving rise to less than 50% of 4. An even more negative effect
on product yield was observed when we tried to decrease the
amount of norbornene in the mixture. Generally, a temperature of
60 ꢀC was necessary to form reasonable quantities of products
within 24 h. However, markedly increasing the temperature caused
a complete breakdown of the catalyst system. Instead of trie-
thylsilane, we also tried to use hydrosilanes bearing more sterically
hindered alkyl substituents in order to enhance the silylation
selectivity for the terminal products. Unfortunately, only trace
amounts of products could be detected using triisopropylsilane and
tert-butyldimethylsilane.
The above results show that the internal double bond of methyl
oleate isomerises in the presence of catalyst [Ir(OMe)(cod)]2 and
that the terminal positional isomer selectively undergoes dehy-
drogenative trialkylsilylation to give the terminal vinylsilanes 4 in
good yield. Obviously, the iridium catalyst plays a double role in this
tandem process by promoting both isomerisation and silylation.
The use of norbornene as sacrificial alkene, which acts as hydrogen
acceptor, was crucial for this reaction. Unfortunately, more steri-
cally hindered silanes, for example triisopropylsilane, turned out to
be unreactive using this protocol. When additional catalysts or li-
gands were used that might facilitate isomerisation, any silylation
activity of the system collapsed.
OMe
OMe
15
15
N
N
(E/Z) - 4
2
Scheme 3. Synthesis of methyl 18-(triethylsilyl)octadec-17-enoate (4).
Theoretically speaking, hydrosilylation of such substrates can pro-
vide up to four isomers with the silyl group attached to either
carbon of the C]C double bond in E or Z geometry. In terms of
stereoselectivity, Ball and Trost reported extremely robust Z-
selectivity for the said ruthenium catalyst, whereas regioselectivity
was expected to be poor owing to the long alkyl chains at either
side of the double bond. In line with these expectations, 1H NMR of
acids 5a showed a single triplet for the alkene proton, while 13C
NMR showed double peaks for each olefinic carbon. This suggests
that we obtained 5a as
a mixture of two (Z)-configured
regioisomers with the triethylsilyl group attached to carbon num-
ber nine or ten. For GC analysis, we esterified acids 5a in methanol
to obtain methyl esters 5b. Surprisingly, the GC of 5b showed a
single peak eluting at 26.6 min, indicating that both regioisomers
have identical retention times in the system used. The mass spec-
trum of 5b revealed a distinct peak at m/z ¼ 253, which clearly
precludes substitution at carbons other than nine and ten.
2.2. Tandem isomerisation-silylation
When we tried to extend the method for dehydrogenative tri-
alkylsilylation [27] of terminal double bonds to long-chain internal
olefins, such as methyl oleate, it was to be expected that this re-
action would be too sluggish to be useful. Initially, we used exactly
the same composition as Lu and Falck, consisting of one equivalent
of methyl oleate, 5 mol% of iridium catalyst, 10 mol% of 2,20-
bipyridine-based ligand and three equivalents each of norbornene
and triethylsilane. After stirring at 60 ꢀC for 48 h, GC of the reaction
mixture showed a number of peaks in the region from 7.0 to
14.5 min, resulting from single and double silylation and eventual
hydrogenation of norbornene. These peaks are excluded from the
following quantification due to the large excess of sacrificial alkene
and silane used. In addition, the GC showed mainly peaks of
unreacted substrate and only a minor peak in the product region,
eluting from column at 26.5 min. This was quantified to constitute
15% (calculated by GC peak area, based on one equivalent of methyl
oleate) and identified as internal product 5b by comparison of
retention time and mass spectrum with the respective reference
compound. Indeed, this outcome is in accordance with Lu and
Falck’s assertion that 2,20-bipyridine-based ligands may inhibit any
isomerisation of olefin and thereby retain the double bond at the
initial position, making only unreactive internal olefin available for
silylation and giving rise to only 15% of product even after a pro-
longed period of heating.
3. Conclusion
The goal of this project was to establish an isomerising silylation
reaction starting from methyl oleate that would give a product in
which the trialkylsilyl group is on the terminal carbon. A system
comprising [Ir(OMe)(cod)]2 catalyst and norbornene gave rise to
the desired product in 69% yield. The identification of substances
was facilitated by independent preparation of reference com-
pounds. Our results suggest that also other monounsaturated fatty
acid esters, especially mixtures of positional isomers obtained by
partial hydrogenation of vegetable oils, can be similarly trans-
formed into the same terminal vinylsilanes 4. Studies are on the
way to improve the process by catalyst tuning and extend the scope
of the reaction by applying hydrosilanes with different polarity.
4. Experimental section
In the course of our investigations the question arose how
simple omission of the bipyridine ligand would affect the reaction
system. We were pleased to find that this measure dramatically
boosted the reaction. Best results were then obtained with 24 h of
stirring at 60 ꢀC and a catalyst loading of 8 mol%. After that time, a
total of 12% of unreacted substrate methyl oleate and its positional
isomers, 6% of hydrogenation product methyl stearate, 8% of
internally silylated products 5b and 6% of unidentified products
4.1. General remarks
All reactions and manipulations were carried out under an
argon atmosphere (Westfalen, 5.0) using standard Schlenk tech-
niques and flame-dried glassware, except where otherwise noted.
All solvents used during this work were of p.a. grade, anhydrous
solvents were purchased from VWR, deuterated solvents from
Deutero. [Ir(OMe)(cod)]2 was from Umicore, [Cp*Ru(MeCN)3]PF6
was from Strem. Methyl 11-bromoundecanoate (95%), magnesium
granules (98%), 7-bromo-1-heptene (97%), methyllithium solution,
methyl oleate (99%), triethylsilane (99%) and norbornene (99%)
were from Sigma Aldrich. 2,20-Bipyridine (99%) and octadec-9-
ynoic acid (98%) were from Alfa Aesar. Copper(I) iodide (99.999%)
and anhydrous sodium iodide (99%) were from ABCR. Boron
-
[Cp*Ru(MeCN)3]+PF6
HSiEt3
O
O
SiEt3
7
OH
OH
7
7
6
5a
Scheme 4. Preparation of internally silylated acids 5a.

T. Huber et al. / Journal of Organometallic Chemistry 744 (2013) 144e148
147
trifluoride diethyl ether complex (for synthesis) was from Merck.
All commercial reagents were used as received without further
purification. 1H and 13C NMR spectra were recorded on a JEOL ECS
400 spectrometer. Chemical shifts are given as dimensionless
solution of 1 (1.47 g, 4.51 mmol) in tetrahydrofuran (20 mL) was
added via syringe. That mixture was allowed to stir at ꢂ78 ꢀC for 1 h
and at room temperature for 2 h, and the reaction was quenched by
adding saturated aqueous ammonium chloride (15 mL). After
addition of diethyl ether (50 mL), two layers and a brown insoluble
formed which was filtered off. The organic phase was separated and
the royal blue aqueous phase was extracted with diethyl ether. The
combined organic fractions were washed with brine, dried over
anhydrous magnesium sulphate, filtered and concentrated under
reduced pressure. Flash column chromatography of the residual
brown oil with n-hexaneediethyl ether (95:5) yielded the crude
ester mixture (0.56 g, 42%). A 0.23 g portion was separated from the
by-product 3 by passing over a 10 g SPE cartridge. Step gradient
elution from acetonitrileewater (9:1) to pure acetonitrile yielded
pure ester 2 as a white solid (0.20 g, 96%) after concentration under
reduced pressure. TLC (n-hexaneediethyl ether 9:1): Rf 0.43. RP-
d
values and are frequency-referenced to the peak of tetrame-
thylsilane (
d
¼ 0 ppm). Coupling constants J are reported in hertz.
The multiplicity of the signals is indicated as “s”, “brs”, “t”, “q”, “qui”
or “m” for singlet, broad singlet, triplet, quartet, quintet or multi-
plet, respectively. GCeMS data were obtained on an Agilent Tech-
nologies HP 6890 GC system with HP 5973 mass selective detector
using an HP-5 ms capillary column (30 m ꢁ 0.25 mm ꢁ 0.25
mm), a
helium carrier gas flow rate of 1 mL minꢂ1 and a time program
starting from 50 ꢀC, hold for 1 min, heating to 100 ꢀC at a rate of
15 ꢀC minꢂ1, heating to 200 ꢀC at a rate of 25 ꢀC minꢂ1, hold for
14 min, heating to 300 ꢀC at a rate of 25 ꢀC minꢂ1, hold for 5 min.
Normal and reversed phase TLC were performed on Merck
aluminium sheets coated with silica gel 60 and silica gel 60 RP-18
F254s, respectively. Compounds were visualised by dipping in 1 M
sulphuric acid solution and subsequent heating. Flash column
chromatographic purifications were conducted on silica gel 60
TLC (acetonitrile): Rf 0.22. 1H NMR (400 MHz, CDCl3):
d 5.80 (1H,
ddt, J ¼ 16.9, 10.1, 6.6 Hz), 5.00e4.89 (2H, m), 3.65 (3H, s), 2.28 (2H,
t, J ¼ 7.4 Hz), 2.05e1.99 (2H, m), 1.60 (2H, qui, J ¼ 7.3 Hz), 1.38e1.20
(24H, m). 13C NMR (100 MHz, CDCl3):
d 174.4, 139.3, 114.1, 51.5, 34.2,
(40e63
m
m) obtained from VWR. Solid phase extraction was per-
33.9, 29.7 (3), 29.6, 29.5, 29.3, 29.2, 29.0, 25.0. GC: tR 16.6 min. EIMS
(70 eV): m/z (%) 296 (1), 264 (51), 222 (28), 180 (19), 111 (19), 97
(45), 87 (59), 74 (83), 55 (100), 41 (58).
formed using ChromabondÒ C18 ec cartridges from Machereye
Nagel.
4.2. Synthesis of reference compounds
4.2.3. Methyl 18-(triethylsilyl)octadec-17-enoate 4
Adapting Lu and Falck’s procedure, terminal olefin 2 (41 mg,
0.14 mmol), [Ir(OMe)(cod)]2 (5 mg, 0.01 mmol), 2,20-bipyridine
(3 mg, 0.02 mmol) and norbornene (40 mg, 0.43 mmol) were dis-
solved in anhydrous tetrahydrofuran (1 mL). After stirring for
5 min, triethylsilane (0.14 mL, 0.91 mmol) was added dropwise via
syringe, and the reaction mixture was stirred at 40 ꢀC for 2 h.
Concentration under reduced pressure and flash column chroma-
tography of the residual oil with n-hexaneediethyl ether (9:1) gave
the vinylsilanes 4 as an E/Z-diastereomeric mixture (41 mg, 72%).
TLC (n-hexaneediethyl ether 9:1): Rf 0.37. 1H NMR (400 MHz,
Following literature-known procedures [25b,c], methyl octadec-
17-enoate (2) was prepared in two steps.
4.2.1. Methyl 11-iodoundecanoate 1
A
suspension of methyl 11-bromoundecanoate (2.88 g,
10.30 mmol) and sodium iodide (4.77 g, 31.82 mmol) in acetone
(100 mL) was heated under reflux for 4 h. After cooling to room
temperature, the white salt was filtered off. The filtrate was
concentrated under reduced pressure, then water (50 mL) was
added. The mixture was extracted with dichloromethane. The
combined organic extracts were washed successively with 10 wt%
aqueous sodium thiosulphate and brine, dried over anhydrous
magnesium sulphate, filtered and concentrated under reduced
pressure to yield 1 as a light yellow oil (3.18 g, 95%) that was used
without further purification. TLC (n-hexaneediethyl ether 9:1): Rf
CDCl3):
d
6.35 (0.9H, dt, J ¼ 14.3, 7.3 Hz), 6.00 (0.1H, dt, J ¼ 18.7,
6.3 Hz), 5.51 (0.1H, d, J ¼ 18.7 Hz), 5.36 (0.9H, d, J ¼ 14.1 Hz), 3.65
(3H, s), 2.28 (2H, t, J ¼ 7.6 Hz), 2.14e2.04 (2H, m), 1.65e1.56 (2H, m),
1.36e1.20 (24H, m), 0.98e0.84 (9H, m), 0.61e0.51 (6H, m). 13C NMR
(100 MHz, CDCl3): d 174.4, 150.5, 148.9, 125.5, 124.9, 51.5, 37.1, 34.2,
32.0, 29.9, 29.7 (2), 29.5 (2), 29.3, 29.2, 25.0, 7.6, 7.5, 4.8, 3.6. GC: tR
27.4, 27.6 min. EIMS (70 eV): m/z (%) 410 (1), 381 (100), 349 (7), 201
(5), 117 (59), 115 (15), 87 (27), 59 (16).
0.35. 1H NMR (400 MHz, CDCl3):
d 3.64 (3H, s), 3.16 (2H, t,
J ¼ 7.0 Hz), 2.28 (2H, t, J ¼ 7.5 Hz), 1.83e1.76 (2H, m), 1.59 (2H, qui,
J ¼ 7.4 Hz), 1.38e1.23 (12H, m). 13C NMR (100 MHz, CDCl3):
d 174.4,
51.5, 34.2, 33.6, 30.5, 29.4, 29.3, 29.2, 28.6, 25.0, 7.4. GC: tR 12.2 min.
EIMS (70 eV): m/z (%) 326 (1), 295 (39), 199 (100),167 (83), 149 (82),
83 (76), 69 (74), 55 (77).
4.2.4. Mixture of isomeric 9-(triethylsilyl)octadec-9-enoic and 10-
(triethylsilyl)octadec-9-enoic acids 5a
Adapting Trost and Ball’s procedure [28], octadec-9-ynoic
acid (90 mg, 0.32 mmol) was dissolved in anhydrous dichloro-
methane (1 mL). The mixture was cooled to 0 ꢀC and triethylsilane
(0.06 mL, 0.38 mmol) was added via syringe. Immediately after
[Cp*Ru(MeCN)3]PF6 (5 mg, 0.01 mmol) was added, the ice bath was
removed and the flask was stirred at room temperature for 2 h. The
crude reaction mixture was concentrated under reduced pressure
and purified by flash column chromatography with n-hexanee
diethyl ethereacetic acid (80:20:1) as eluent to yield pure acids 5a
as a light yellow oil (103 mg, 81%). TLC (n-hexaneediethyl ethere
4.2.2. Methyl octadec-17-enoate 2
Grignard reagent hept-6-en-1-ylmagnesium bromide was pre-
pared by suspending magnesium granules (0.22 g, 9.05 mmol) in
anhydrous tetrahydrofuran (20 mL) and subsequently adding 7-
bromo-1-heptene (1.63 g, 9.19 mmol) dropwise. The mixture was
refluxed for 30 min. In a separate flask, copper(I) iodide (1.72 g,
9.03 mmol) was suspended in anhydrous tetrahydrofuran (10 mL)
and cooled to ꢂ78 ꢀC in an isopropyl alcoholedry ice bath. Meth-
yllithium solution (1.6 M in diethyl ether, 5.65 mL, 9.04 mmol) was
added very slowly via syringe. The resultant mixture was stirred
at ꢂ78 ꢀC for 1 h and then slowly allowed to warm to 0 ꢀC,
whereupon a brownish suspension formed, which was immedi-
ately cooled to ꢂ78 ꢀC. After that, the aforementioned solution of
Grignard reagent in tetrahydrofuran was added via syringe. The
mixture thus obtained was stirred at ꢂ78 ꢀC for 1 h and then
allowed to warm to 0 ꢀC, whereupon a purple colouration
appeared. The mixture was then cooled again to ꢂ78 ꢀC and a
acetic acid 80:20:1): Rf 0.2. 1H NMR (400 MHz, CDCl3):
d 11.49 (1H,
brs), 5.96 (1H, t, J ¼ 7.2 Hz), 2.33 (2H, t, J ¼ 7.4 Hz), 2.08e2.03 (2H,
m), 1.96 (2H, brs), 1.68e1.56 (2H, m), 1.38e1.21 (20H, m), 0.94e0.85
(12H, m), 0.63 (6H, q, J ¼ 7.9 Hz). 13C NMR (100 MHz, CDCl3):
d 180.6
(2), 144.2, 143.9, 136.4, 136.1, 38.5, 38.4, 34.2, 32.2, 32.1, 32.0, 31.2,
31.1, 30.3, 30.2, 29.7, 29.6, 29.4 (2), 29.3, 29.2 (2), 29.1, 24.7, 22.8,
14.2, 7.7, 4.3. Esterification was necessary for GCeMS characteri-
sation: an aliquot of acids 5a was dissolved in methanol, mixed
with few drops of boron trifluoride diethyl etherate, heated to 70 ꢀC

148
T. Huber et al. / Journal of Organometallic Chemistry 744 (2013) 144e148
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Acknowledgements
The authors acknowledge financial support of the Fachagentur
Nachwachsende Rohstoffe (FNR, FKZ 22018008). Special thanks are
due to Andreas Rivas-Nass (Umicore AG, Precious Metals Chemis-
try), Peter Gigler and Jürgen Stohrer (Wacker Chemie AG, Con-
sortium für elektrochemische Industrie) for the donation of
chemicals and fruitful discussions.
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