A highly active cationic ruthenium complex for alkene isomerisation: ACatalyst for the synthesis of high value molecules

Full text of DOI:10.1002/cctc.201300396

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DOI: 10.1002/cctc.201300396
A Highly Active Cationic Ruthenium Complex for Alkene
Isomerisation: A Catalyst for the Synthesis of High Value
Molecules
Simone Manzini, David J. Nelson, and Steven P. Nolan*[a]
Dedicated to Professor Carl D. Hoff on the occasion of his 65th birthday.
The isomerisation of terminal alkenes to form internal alkenes
is an extremely useful process, employed in industrial process-
es to equilibrate feedstocks or to achieve high-value molecules
starting from easily accessible precursors. Moreover, this trans-
formation is also widely applied in synthetic chemistry;[1] in
particular, this can be used to obtain vinyl derivatives staring
from the corresponding allylic moiety, which can often be
easier to introduce. Several such processes have been reported
Scheme 1. Synthesis of 1.
in the literature.[2] Homogenous ruthenium-based systems are
known, including the use of [RuCl(PCy3)H(CO)(L)] complexes
(L=a phosphane or N-heterocyclic carbene) which can isomer-
ise alkenes through a relatively facile addition/b-hydride elimi-
nation mechanism.[3] However, these are not particularly robust
species, especially in the presence of moisture or air.[4] For syn-
thetic purposes, such species are typically produced in situ
through decomposition of ruthenium carbene complexes
using reagents such as alcohols[5] or vinyloxy(trimethyl)silane;[6]
high catalyst loadings are typically necessary using these pro-
cedures (ꢀ5–10 mol%). Grotjahn has recently employed an ef-
ficient cationic bifunctional ruthenium complex in alkene iso-
merisation.[7,8] Krompiec et al. have utilised [RuCl(CO)H(PPh3)3]
to isomerise N-allyl compounds.[9] Consorti et al. employed the
same species as part of a tandem isomerisation/metathesis
study,[10] obtaining a distribution of products. Schmidt and
Snapper have conducted metathesis/isomerisation sequences
by converting the metathesis catalyst to an isomerisation cata-
lyst in situ.[11]
viously reported, we proceeded to evaluate its activity in iso-
merisation reactions. Initial experiments were conducted with
1-octene and 1. After optimisation, it was possible to reach
high conversions with only 300 ppm 1 at 608C in THF (1:1 sol-
vent/substrate) (Table 1). However, when attempts were made
to extend this methodology to a wider range of compounds,
poor conversions were obtained.
Table 1. Isomerisation of 1-octene using 1.[a]
Entry
Complex 1
[mol%]
Solvent[b]
t
[h]
Conv.[c]
[%]
1[d]
2
3
4
5
6
7
8
0.2
0.2
0.1
0.1
0.1
0.1
0.1
0.1
0.06
0.03
neat
neat
neat
toluene
DCM
IPA
THF
THF
THF
THF
24
24
24
24
24
24
24
12
24
24
48
66
69
50
76
98
91
92
90
87
Recently, we reported the synthesis of [RuCl(PPh3)2(3-phenyl-
indenyl)] (1), which was originally obtained from the decompo-
sition of metathesis pre-catalyst precursor M10.[12] 1 can be
easily prepared by means of a simple one-pot procedure using
cheap and readily available starting materials (Scheme 1).
1 was revealed to be a highly active yet air- and moisture-
stable catalyst for a number of synthetically useful reactions,
including alcohol racemisation,[12] alcohol oxidation,[13] ketone
reduction[14] and SÀS, SÀSi and SÀB bond formation.[15]
9
10
[a] General conditions: substrate and 1 heated to 608C with stirring for
the specified time. [b] 1:1 v/v with 1-octene. [c] Determined by 1H NMR
integration. [d] 0.9 mol% KHMDS added.
To increase the substrate scope and to continue to avoid
the use of any additives, a systematic variation of the system
was required. We proposed that the use of a base in previous
studies[13,14] was necessary to remove the chloride from com-
plex 1, forming the active species in situ. Consequently, a cat-
To extend the utility of complex 1, with the aim of establish-
ing a more stable, accessible and active system than those pre-
[a] S. Manzini,+ Dr. D. J. Nelson,+ Prof. Dr. S. P. Nolan
EaStChem, School of Chemistry
ionic version of
1 was synthesised (Scheme 2). Adding
University of St Andrews
North Haugh, St. Andrews, Fife, KY15 4DX (UK)
E-mail: snolan@st-andrews.ac.uk
1.1 equivalents of sodium tetrakis-[3,5-bis(trifluoromethyl)phe-
nyl]borate (NaBArF) to a DCM solution of 1 at room tempera-
ture furnished analytically-pure 2 in excellent yield after 1 h.
Complex 2 was found to be stable to air and moisture in the
solid state.
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201300396.
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E-isoeugenol quantitatively. The same trend is shown in the
isomerisation of estragol to E-anethole (Table 2, entry 2). With
electron-poor alkenes it was necessary to increase the catalyst
loading to 0.5 mol%. Sterics also play an important role in the
isomerisation. Hindered substrates with substituents close to
the double bond affected the reactivity and stereoselectivity
(Table 2, entries 5, 11, 12 and 15). Interestingly, the catalyst
shows reduced activity with allyl ethers (Table 2, entries 10 and
11), and in the presence of allylamine derivatives it is necessary
to increase the catalyst loading to 1 mol% (Table 2, entries 13
and 14). Notably, N,N-allylbenzylamine spontaneously rear-
ranged to the corresponding imine as often reported in the lit-
erature (Table 2, entry 13).[2] Another interesting result is the
isomerisation of 1,4-cyclohexadiene, where it isomerises to the
corresponding 1,3-isomer, and then is dehydrogenated to form
benzene as the final product (Table 2, entry 16).
Scheme 2. Synthesis of complex 2.
Initially, the activity of 2 was evaluated with different sub-
strates under batch conditions for 16 h at 1108C (Table 2).
Complex 2 was revealed to be remarkably active with a variety
of different terminal alkenes, and in particular with aliphatic
(Table 2, entries 7, 8 and 9) and electron-rich alkenes such as
eugenol (Table 2, entry 4), from which it is possible to achieve
Encouraged by these results with complex 2, we decided to
quantify its performance by evaluating its turnover number
(TON) and turnover frequency (TOF) in some reactions with
model substrates of important resources: 1-octene (oil feed-
stock), methyl-1-undecenoate (fatty esters feedstock) and allyl-
benzene (essential oils feedstock) (Table 3). Complex 2 revealed
very high activity, which could be appealing for industrial pro-
cesses. In addition, the cationic nature of 2 could be suitable
for heterogenisation of the catalyst for use in flow systems.
Table 2. Isomerisation of terminal alkenes in bulk conditions with
complex 2.
Entry[a] Substrate
Product
Conv.[d]
[%]
E:Z
ratio[c]
1
2
95
93
12:1
17:1
Table 3. Performance of complex 2 with model compounds.
Entry
Substrate
TON[a]
3
4
5
95[b]
>99
18:1
18:1
7:1
1
2
3
76250
53750
45000
>99
[a] General conditions: substrate (2 mmol), 1; the reaction was performed
at 1108C for 16 h. The conversion was determined by 1H NMR
spectroscopy.
6
81[b]
9:1
7
8
>99
>99
5:1[f]
4:1[f]
Notably, 2 is very active with allylic arenes (Table 2, entries 1
to 6) which can be easily derivatised to yield high value mole-
cules used in various fields (such as medicinal chemistry, poly-
mer chemistry, fragrances, pesticides, etc.).[16] The groups of
Bruneau and Fogg have explored the functionalisation of iso-
eugenol, isosafrole and anethole by cross-metathesis to yield
valuable products. We therefore combined the isomerisation
reaction with olefin metathesis, to perform ethenolysis. Al-
though the 1-arylprop-1-ene products from isomerisation can
be deployed directly in metathesis reactions, the styrene prod-
ucts produced by ethenolysis can be further functionalised by
various reactions, such as epoxidation, dihydroxylation or oxi-
dative cleavage, for example. The preparation of useful aro-
matic compounds from natural sources is a key challenge in
modern chemistry, owing to the high cost and long term un-
certainty of fossil fuel supplies.[17]
9
95
1.9:1[f]
10
11
70
82
24:1
1:1.4
12
13
87
7:1
–
99[c]
14
15
16
>99[c]
39[c]
6:5:1
–
–
78[c]
[a] Conditions: substrate (2 mmol), 1 (0.25 mol%) heated at 1108C for
16 h. [b] Catalyst loading increased to 0.5 mol%. [c] Catalyst loading in-
creased to 1 mol%. [d] Determined by 1H NMR spectroscopy. [e] Deter-
mined by GC. [f] Further isomerisation was detected (<10%).
The isomerisation of eugenol and estragole was performed
on up to a 3 g scale under neat conditions using standard glo-
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vebox and Schlenk techniques; material prepared in this way
was washed through silica with toluene, which was evaporated
to yield the desired product. The product retained some
colour, but was deployed directly in subsequent metathesis re-
actions; impurities remaining in this material did not compro-
mise further steps, validating a telescoped isomerisation-meta-
thesis procedure.
After initial screening of a number of metathesis-precata-
lysts, M20 was selected as the most efficacious catalyst for the
ethenolysis of isoeugenol and anethole (Figure 1).
Scheme 4. Plausible reaction mechanisms.
fins are the best substrates, being better able to co-ordinate
the ruthenium centre, explaining why bulky and/or electron-
poor olefins require higher catalyst loadings.
In summary, we have shown that complex 2 is a very useful
complex for alkene isomerisation. This complex can be pre-
pared in two straightforward and high-yielding steps from
commercially available materials, rendering it highly accessible.
No additives are required, rendering it appropriate for reac-
tions involving sensitive substrates that might not tolerate, for
example, the presence of base. In particular, we have shown
that this species can be applied towards the isomerisation of
essential oils to generate feedstocks that can be further elabo-
rated into high-value chemical products. Further investigations
into the applications of 1 and 2 are underway in our
laboratories.
Figure 1. Metathesis pre-catalysts explored in this study.
Ethenolysis reactions were conducted using 1,7-octadiene as
the solvent, resulting in the generation of ethene in situ from
the rapid and thermodynamically-favoured metathesis reaction
to form cyclohexene.[18] The system avoids the use of ethene
directly and consequently precludes the need for high-pres-
sure apparatus, making it a valuable laboratory methodology.
Optimised conditions allowed the corresponding styrene com-
pounds to be obtained in high yield (Scheme 3). These com-
pounds are potentially valuable feedstocks for further
functionalisation.
Experimental Section
A typical isomerisation protocol
In the glovebox, neat substrate and 2 were weighed into a vial.
The vial was sealed, removed from the glovebox, and stirred at
110 8C for 16 h. Upon cooling, the reaction was analysed by
1H NMR spectroscopy (and GC if required) to assess conversion and
E/Z stereoselectivity.
A typical ethenolysis protocol
An anhydrous and degassed solution of the phenylpropene sub-
strate and 1,7-octadiene was added to the precatalyst in a small
septum-fitted vial previously purged with inert gas. A syringe
barrel fitted with a balloon and a needle was attached to the vial,
which was stirred at room temperature for 2 h before being
quenched by the addition of 1 mL ethyl vinyl ether. The mixture
was then filtered through a pad of silica, followed by ethyl acetate.
The volatiles were removed and the crude material was analysed
Scheme 3. Functionalisation of allylarene feedstocks by means of
ethenolysis.
Although the primary aim of this manuscript is to disclose
this new methodology for the isomerisation of alkenes, some
comments must be made regarding the mechanism. The
mechanism most likely relies on either 1) formation of a ruthe-
nium hydride species in situ, which mediates the reaction
through a hydride addition/elimination mechanism[3] or 2) a
mechanism proceeding via a hydridoruthenium(h3-allyl) inter-
1
by H NMR spectroscopy with the use of diethyl malonate as inter-
nal standard.
mediate (Scheme 4). While mechanistic investigations are on- Acknowledgements
going, it is worthwhile to note that complex 3, which is the hy-
dride relative of 1, is far less active than 1 under the same con-
ditions. A simple change from chloride to hydride is, therefore,
not likely. In either mechanism, electron-rich, unhindered ole-
We thank the EPSRC and the ERC (Advanced Investigator Award
FUNCAT to SPN) for funding this work. We thank Umicore for the
gifts of materials.
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Keywords: green chemistry
metathesis · renewable resources · ruthenium
·
isomerisation
·
olefin
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Received: May 23, 2013
Published online on July 5, 2013
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