Metathesis of renewable polyene feedstocks – Indirect evidences of the formation of catalytically active ruthenium allylidene species

Article abstract of DOI:10.1016/j.jorganchem.2017.04.018

Cross-metathesis (CM) of conjugated polyenes, such as 1,6-diphenyl-1,3,5-hexatriene (1) and α-eleostearic acid methyl ester (2) with several olefins, including 1-hexene, dimethyl maleate and cis-stilbene as model compounds has been carried out using (1,3-bis-(2,4,6-trimethylphenyl)-2-imidazolidinylidene)-dichloro(o-isopropoxyphenylmethylene)ruthenium (Hoveyda-Grubbs 2nd generation, HG2) catalyst. The feasibility of these reactions is demonstrated by the observed high conversions and reasonable yields. Thus, regardless of the relatively low electron density, =CH–CH= conjugated units of molecules, including compound 2 as a sustainable, non-foodstuff source, can be utilized as building blocks for the synthesis of various value-added chemicals via olefin metathesis. DFT-studies and the product spectrum of the self-metathesis of 1,6-diphenyl-1,3,5-hexatriene suggest that a Ru η¹-allylidene complex is the active species in the reaction.

Full text of DOI:10.1016/j.jorganchem.2017.04.018

Accepted Manuscript
Metathesis of renewable polyene feedstocks - indirect evidences of the formation of
catalytically active ruthenium allylidene species
Ervin Kovács, Péter Sághy, Gábor Turczel, Imre Tóth, György Lendvay, Attila
Domján, Paul T. Anastas, Róbert Tuba
PII:
S0022-328X(17)30244-9
10.1016/j.jorganchem.2017.04.018
JOM 19905
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 14 February 2017
Revised Date: 13 April 2017
Accepted Date: 15 April 2017
Please cite this article as: E. Kovács, Pé. Sághy, Gá. Turczel, I. Tóth, Gyö. Lendvay, A. Domján, P.T.
Anastas, Ró. Tuba, Metathesis of renewable polyene feedstocks - indirect evidences of the formation
of catalytically active ruthenium allylidene species, Journal of Organometallic Chemistry (2017), doi:
10.1016/j.jorganchem.2017.04.018.
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Graphical Abstract

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Metathesis of Renewable Polyene Feedstocks - Indirect Evidences of the
Formation of Catalytically Active Ruthenium Allylidene Species
Ervin Kovács,† Péter Sághy,† Gábor Turczel,† Imre Tóth,† György Lendvay,† Attila
§
†
‡
*
Domján, Paul T. Anastas and Róbert Tuba
†
‡
Institute of Materials and Environmental Chemistry and NMR Laboratory, Research Centre for
Natural Sciences, Hungarian Academy of Sciences, 1519 Budapest, P.O. Box 286, Hungary.
§Center for Green Chemistry and Engineering, Yale University, New Haven, Connecticut
06511, USA
Abstract
Cross-metathesis (CM) of conjugated polyenes, such as 1,6-diphenyl-1,3,5-hexatriene (1)
and α-eleostearic acid methyl ester (2) with several olefins, including 1-hexene, dimethyl
maleate and cis-stilbene as model compounds has been carried out using (1,3-bis-(2,4,6-
trimethylphenyl)-2-imidazolidinylidene)-dichloro(o-isopropoxyphenylmethylene)ruthenium
(Hoveyda-Grubbs 2nd generation, HG2) catalyst. The feasibility of these reactions is
demonstrated by the observed high conversions and reasonable yields. Thus, regardless of the
relatively low electron density, =CH–CH= conjugated units of molecules, including compound 2
as a sustainable, non-foodstuff source, can be utilized as building blocks for the synthesis of
various value-added chemicals via olefin metathesis. DFT-studies and the product spectrum of
the self-metathesis of 1,6-diphenyl-1,3,5-hexatriene suggest that a Ru η1-allylidene complex is
the active species in the reaction.
submitted to Journal of Organometallic Chemistry

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INTRODUCTION
Biological materials containing conjugated bonds, such as conjugated polyunsaturated
fatty acids, terpenes and chromophores, are abundant in nature.1 Polyenes are highly unsaturated
conjugated compounds with characteristic color and antioxidant properties.2 They have a wide
range of applications, including molecular wires in nano-devices and “light harvesting materials”
in photosynthesis.3 These chemicals are often underutilized as renewable feedstock because of
their special conjugated electron systems. This is especially true for their use in metathesis
reactions, as the available catalyst systems have not been optimized yet to conjugated
compounds. Olefin metathesis is fundamentally one of the most versatile synthetic
methodologies in homogeneous catalysis and it has initiated new industrial avenues leading to
innovative materials, petrochemicals and pharmaceuticals since its invention about half a century
ago.4 In the last twenty years many chemical technologies have been developed on the basis of
olefin metathesis, which also became dominant in several sustainable and green chemical
processes.5 The application of olefin metathesis in the field of green chemistry, especially in the
utilization of renewable feedstocks, is emerging.6
Even though many transition metals were found to catalyse metathesis reactions, the most
widely used and investigated systems still employ Ti, W, Mo and Ru. The latter has the highest
reactivity with olefinic bonds, while remaining tolerant towards moisture, oxygen and several
functional groups. To date, Ru appears to be the most frequently selected transition metal for
metathesis reactions.7
The synthesis of polyenes was carried out mainly by the Wittig reaction in the past.8 The
longest polyene synthetized so far contains eighty carbon atoms and twenty-seven conjugated
double bonds.9 However, research on polyenes and π-conjugated oligomers has dramatically
expanded by the development of transition metal catalysed cross-coupling reactions and
analytical techniques, allowing the well-designed synthesis of linear, conjugated molecular rods
with accurate characterisation.10 An emerging alternative way for the synthesis of polyenes is the
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metathesis of conjugated olefins.11
However, not only the fabrication of conjugated systems but also their breakdown into
valuable building blocks via olefin metathesis can be substantial. For example, α-eleostearic acid
as a carbon-dioxide neutral renewable material, containing three conjugated double bonds (at
carbons 9-cis, 11-trans and 13-trans), can be the green alternative for the replacement of
important petro-based chemicals such as nylon monomers.12 One source of such a conjugated
compound is tung oil, which is non-edible and can be produced from the seeds of tung plants
grown on waste or marginal lands.13 Unlike other vegetable oils, such as soybean or sunflower
oils (containing mainly oleic acid), tung oil consists of the desired α-eleostearic acid in up to 80
% concentration, that makes this bio-based raw material suitable for the synthesis of a wide
range of chemicals via olefin metathesis. For example, the simple ethenolysis of α-eleostearic
acid can provide methyl dec-9-enoate and C4–C10 hydrocarbons including butadiene and 1-
hexene (Scheme 1). These chemicals are widely used as additives in cosmetics and in the
production of synthetic rubber or high- and low density polyethylene (HDPE/LDPE).14 The cross
metathesis of conjugated polyenes is an alternative way for the utilization of =CH–CH= building
blocks for the synthesis of many other value added chemicals.
Scheme 1. Tentative scheme of ethenolysis of α-eleostearic acid methyl ester (2)
In this short communication, we are reporting our preliminary results on the synthetic and
mechanistic investigation of the ruthenium catalysed metathesis reactions of polyenes using 1,6-
diphenyl-1,3,5-hexatriene (1) and α-eleostearic acid methyl ester (2) as model compounds.
Synthesis of polyene homologs. The self-metathesis of 1,6-diphenyl-1,3,5-hexatriene (1)
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has been investigated in toluene solution using Hoveyda-Grubbs 2nd (HG2) generation catalyst at
room temperature. During the reaction, the colour of the mixture changed gradually from green
to orange, meanwhile the formation of a precipitate was observed. The precipitation may occur
due to the formation of n > 3 homologs, which are expected to have lower solubility than the n ≤
3 species. It is well-known that the solubility of polyacetylene decreases steadily with the chain
growth of the oligomer.15 As the reaction mixtures could not be investigated in one pot because
of the precipitation, the components were hydrogenated off to obtain fully soluble, saturated
α,ω-phenyl substituted hydrocarbon chains. Then, assuming that the components of the reaction
mixture retain comparative response factors, GC-MS analyses were carried out and the
approximate values of the conversion and yields were estimated from the peak areas of the total
ion current (TIC) chromatograms. These indicated a relatively high conversion of the starting
material 1 (92%) and the formation of 1(1-6) compounds in reasonable yields of (1(1): 20%;
1(2): 21%; 1(5): 25%; 1(6): 15%). Homologs 1(4), 1(7) and 1(8) were also detected in traces.
(Scheme 2, Figure 1)
Scheme 2. Self metathesis of 1, synthesis of polyene homologs via olefin metathesis.
1(5)*
1(2)*
1(1)*
1(6)*
1*
1(4)*
1(7)*
Figure 1. Total ion current (TIC) chromatogram plot of the hydrogenated self-metathesis
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reaction mixture of 1 using HG2 catalyst. (*hydrogenated derivatives)
The cross-metathesis (CM) of 1 with 1-hexene and dimethyl maleate using HG2 has also
been investigated. In the case of 1-hexene, a high (more than 90%) conversion of 1 was
estimated as judged by the TIC chromatogram (Figure S9). The reaction mixture contained a
wide range of hydrocarbons, including the expected alkylated benzenes (see supplementary
material) as major products. However, the reaction of 1 with dimethyl maleate under the same
condition revealed a rather low conversion with only the formation of the self-metathesis
products of 1 (Figure S7).
Following the preliminary studies described above, the metathesis of α-eleostearic acid
methyl ester (2) as a potential renewable material was investigated. The self-metathesis of 2
followed by one-pot hydrogenation has revealed the formation of the expected reaction products
including hydrocarbon and methyl ester homologs with reasonable yields (Figure S10). The
feasibility of the CM of 2 has been investigated with cis-stilbene (6) as a model compound.
Compound 6 is an ideal choice for a cross-coupling agent, as new side products are not provided
in the self-metathesis and the core structure of the active species is not affected in a reaction with
the catalyst. Thus, the rate and conversion of the CM reaction between 2 and 6 is supposedly
determined solely by 2.
The reaction mixtures were hydrogenated off on Pd/C in all cases to improve the
solubility of the components by saturation. Furthermore, by the elimination of the presence of Z
and E isomers of the reaction products, the chromatograms could be simplified. The CM reaction
of 2 with 6 followed by hydrogenation led to the formation of methyl 10-phenyldecanoate (3,
95%), 1,4-diphenylbutane (4, 35%) and hexylbenzene (5, 95%) as major products (Scheme 3,
Figure 2). The yield was determined by quantitative 13C NMR.16
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Scheme 3. Cross-metathesis (CM) of α-eleostearic acid methyl ester (2) with cis-stilbene (6).
6*
3
4
5
2*
Figure 2. TIC chromatogram of the CM reaction mixture of 2 and 6. (*hydrogenated
derivatives)
The product spectrum of the self-metathesis of 1 suggests that a Ru η1-allylidene
complex (8) is the active species in the reaction (Scheme 4). However, there is some concern
about the possible deactivation of the η1 species by transformation into inactive η3 structures 9
and/or 10, based on earlier studies. In these complexes the π-bond of the allyl moiety also
coordinates to the Ru atom, occupying an equatorial and an axial (9) or two equatorial (10)
coordination sites (Scheme 4). Specifically, a stable and catalytically inactive analogue of 10 has
been reported by Grubbs and co-workers.17
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3 - allylidene (equatorial)
η
3 - allylidene (axial)
η
1 - allylidene
η
1 - allylidene
η
N
N
Mes
N
N
N
N
Mes
Mes
Mes
Mes
Mes
N
N
Mes
Mes
-
R
R
R
R
Cl
Cl
Cl
Cl
Cl
Ru
Ru
Ru
Cl
Cl
Cl
Ru
+
R
R
7
8
9
10
a: R = H;
b: R = CH=CH-Ph
Scheme 4. Hypothetic competition between 7 and/or 9 and/or 10
They have synthetized complex 10 by the reaction of Grubbs 2nd generation (G2) catalyst
with phenyl acetylenes (Scheme 5), and found an unusual chlorine arrangement around the Ru
atom. The η1 - η3-allylidene conversion was investigated and the η1-allylidene species formation
o
was not observed even at 130 C. One of the chlorine atom is almost perpendicular to the Cl–
Ru=CR'R" plane in the suggested structures. However, according to recent X-Ray studies, the
highly active (i.e. Cl2Ru=CHPh or Cl2Ru=CH2) catalyst species contains both the chlorines in
equatorial positions close to planar with the carbene (=CHP or =CH2) ligands.18 The shift of one
of the Cl atoms to axial position may explain the catalytic inactivity of the stable allyl η3-
triphenyl-allylidene structure.
Scheme 5. Formation of η3-allylidene Ru NHC complex based on the study by Trnka and
Grubbs17
In order to understand the possibility of inhibition by conversion of the active η1 species
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(8) to η3-allylidene structures (9) and/or 10, we used methods of electronic structure theory, and
calculated the molecular geometries and relative energies of the candidate species.19 Density
functional theory was used in the calculations at the M06/LANL2DZ level proposed by Zhao and
Truhlar20 as appropriate to describe Grubbs catalysts, and with the CAM-B3LYP functional
combination using the basis sets SDD (for Ru, including also the pseudopotential describing the
core electrons) and 6-31G (d,p) (for C, N, H). Both functionals handle the dispersion effects
properly that are essential in the formation and energetics of various isomers and conformers of
the catalyst. As a test, we calculated the geometry of the triphenyl analog of 10 reported by
Grubbs and co-workers, and found that both methods provide very good agreement with the X-
ray structure. With the validated methods, we searched for the candidate structures, and
identified several conformers of the η1 species, differing only in the orientation of the allyl
moiety and of the phenyl substituent. The most stable conformer of complex 8 is structure A
shown in Figure 3. In addition to this, a η3 species 10 with an equatorial and an almost axial Cl
atom (B in Figure 3) has also been found at a realistic relative energy level (Figure 4). None of
our efforts led to success in finding structure 9 with π coordination at the axial position of Ru.
The structures and binding energies of the adduct 7 of 8 with olefins, C2H4 or 1 have also been
determined, (C in Figure 3).
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A: complex 8
B: complex 10
C: complex 7a
Figure 3. Calculated DFT structures for complexes 7a, 8 and 10.
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The relative energies of the complexes are shown in Figure 4 (the relative free energies at
300 K are essentially the same). The energy of 10 is referenced to that of 8, while the energy of
adduct 7 is measured from the sum of those of complex 8 and the olefin. One can see that the
olefin coordination is highly exothermic, while formation of the η3 structure is endothermic by
more than 40 kJ/mol. It is remarkable that the binding energy of the complex of 8 and 1 is
significantly larger than that of 8 and ethylene. It can be concluded from the energetics that
catalysts deactivation by the formation of a η3 species should not be significant when a 3-
phenylallylidene group is present (Figure 4). This is in good agreement with the observation
mentioned earlier, namely, that the self-metathesis of 1 proved to be facile.
Figure 4. The relative energies of complexes 7a, 7b, 8 and 10.
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In summary, this paper covers the results of an exploratory study on the metathesis of
polyenes including a renewable material. It can be concluded that the polyenes containing three
olefinic bonds undergo metathesis reaction despite the relatively low electron density on the
conjugated double bonds. Thus, a sustainable, non-foodstuff material such as α-eleostearic acid
can be converted into valuable chemicals. The electronic structure theory calculations showed
that catalyst deactivation is negligible for the ruthenium 3-phenyl-allylidene system, because the
formation of the possible inactive η3 species (10) is endothermic, while the addition of the next
olefin to the Ru (7a and 7b) is highly exothermic. The equilibrium is essentially shifted towards
the ruthenium η1- allylidene-olefin complexes (7a and 7b).
■ ASSOCIATED CONTENT
Supporting Information. Experimental procedures and characterization data. This material is
available free of charge via the Internet at http://www.sciencedirect.com
■ AUTHOR INFORMATION
Corresponding Authors
*E-mail: tuba.robert@ttk.mta.hu
■ ACKNOWLEDGEMENT
We thank the Hungarian Academy of Sciences and National Research, Development and
Innovation Office (NKFIH) for financial support under Grant Nos. OTKA NN 117986 and K
108966.
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