On the ethenolysis of natural rubber and squalene

Article abstract of DOI:10.1039/c1gc15265c

We report here the ethenolysis of squalene and natural rubber utilizing (NHC)(NHC_ewg)RuCl₂(= CRR') and Grubbs-Hoveyda complexes. 0.01 mol% [Ru] per double bond are sufficient for extensive squalene cleavage, resulting in the formation of numerous terminal olefins, which were identified by GC/MS. The depolymerization of natural rubber requires 0.1 mol% [Ru] and leads to the formation of various oligomeric isoprenes, several of which (n = 2-6) were isolated and characterized.

Full text of DOI:10.1039/c1gc15265c

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COMMUNICATION
On the ethenolysis of natural rubber and squalene†‡
Stefanie Wolf and Herbert Plenio*
Received 10th March 2011, Accepted 13th May 2011
DOI: 10.1039/c1gc15265c
We report here the ethenolysis of squalene and natural rub-
ber utilizing (NHC)(NHCewg)RuCl (= CRR’) and Grubbs–
primarily provided by estates in tropical Asia, with Thailand,
17
Indonesia and Malaysia being the main producers. The high
degree of stereoregularity (with respect to double bond geometry
and head-to-tail orientation of the repeat units) of natural rubber
offers excellent chances for controlled polymer degradation.
2
Hoveyda complexes. 0.01 mol% [Ru] per double bond
are sufficient for extensive squalene cleavage, resulting in
the formation of numerous terminal olefins, which were
identified by GC/MS. The depolymerization of natural
rubber requires 0.1 mol% [Ru] and leads to the formation
of various oligomeric isoprenes, several of which (n = 2–6)
were isolated and characterized.
18
The cross metathesis reaction of polyolefins with ethene
ethenolysis) depolymerizes macromolecules to smaller isoprene
(
oligomers with terminal double bonds. Due its highly regular
stereochemistry, natural rubber should render a small set of
terminal olefins (Scheme 3) upon cleavage.
19
Early pioneering studies by Alimuniar et al. and Wagener
The enforced shift from a crude oil-based chemistry to a
chemistry relying on renewable resources has initiated vigorous
efforts to explore alternatives—primarily plant based substi-
tutes. Consequently, the interest in the conversion of biomass
20
et al. on the ethenolysis of polyisoprenes noted the formation
of ill-defined mixtures of partially-depolymerized rubber using
tungsten-based olefin metathesis catalysts. The limited success
met so far in the ethenolysis of natural rubber is primarily
due to the lower reactivity of the trialkylated double bonds.
Therefore, the dialkylated double bonds in oleic acid (esters)
1
into useful chemicals has grown enormously. Numerous cat-
alytic transformations have been tested on biomass derived
2–4
saccharides, vegetable oils, animal fats and small terpenes.
21
are much easier targets. We recently developed a series of
new (NHC)(NHCewg)RuCl (CRR¢) complexes that are highly
active in the formation of sterically-encumbered olefins via RCM
Currently, the most attractive target for olefin metathesis in
this context is the conversion of plant oils, primarily methyl
2
5
,6
7–12
oleate, utilizing various approaches, such as ethenolysis,
2
2–25
13
14
reactions.
The key features of such complexes are NHC
cross metathesis or enyne metathesis. Polyolefins such as
ewg
23
ligands acting as leaving groups during catalyst initiation. Un-
like other NHC ligands, NHCewg are characterized by electron-
withdrawing substituents, which render their electron donation
natural rubber, which is isolated from the latex of Hevea
15,16
brasiliensis and various other tropical plants,
have been
neglected—despite the fact that the annual production (2007)
26,27
6
comparable to that of trialkylphosphines.
The high thermal
of natural rubber amounts to 9.7 ¥ 10 t. The crude latex is
stability and excellent reactivity of such complexes in olefin
metathesis reactions prompted us to test them in the ethenolysis
of trisubstituted olefins in squalene and natural rubber.
Organometallic Chemistry, Petersenstr. 18, Technische Universit a¨ t
Darmstadt, 64287, Darmstadt, Germany.
E-mail: plenio@tu-darmstadt.de
We first used the linear triterpene squalene (C30
H
50), which is
29
available from olive oil or shark liver, as a model substrate for
natural rubber. This is justified as the bond connectivity, double
bond substitution and stereochemistry of squalene and natural
rubber are very similar. Furthermore, squalene and the partially-
cleaved ethenolysis products are well suited for extensive product
analysis via GC/MS. This facilitates the evaluation of product
distribution and enables the optimization of catalysts for later
use in natural rubber (NR) and liquid natural rubber (LNR)
ethenolysis.
†
Electronic supplementary information (ESI) available: Experimental
1 13
procedures, synthesis, GC, HPLC, H and C NMR traces, and mass
spectra. See DOI: 10.1039/c1gc15265c
‡
Ethenolysis of natural rubber or liquid natural rubber: The rubber (5 g)
was placed in a 250 mL B u¨ chi miniclave and toluene (100 mL) added
with stirring (dissolving natural rubber requires overnight treatment).
Catalyst 1 (63.1 mg, 0.073 mmol, 0.1 mol% per double bond) was
added. The reactor was purged with ethene for 5 min, the ethene pressure
◦
adjusted to 7 bar and the reactor heated to 120 C for 3 h. After the
reaction, the reactor was allowed to cool to rt and ethyl vinyl ether
(
0.5 mL) was added for catalyst deactivation. The reaction mixture
The ethenolysis of squalene dissolved in toluene was initially
carried out in the presence of 0.1 mol% of ruthenium complex
1 per double bond. This leads to the full conversion of squalene
and the formation of numerous terminal olefins. GC traces
of the ethenolysis products of squalene display thirteen major
was transferred to a millipore cell and filtered over a nanofiltration
-1
membrane (MWCO 500 Dalton, flow: 1 mL min , Dp = 3 bar) to remove
residual polymers. After filtering off ca. 80 mL of the solution, toluene
(
100 mL) was added and a volume of ca. 100 mL filtered through the
membrane. The toluene was carefully removed from the permeate to
obtain ca. 3.2 g of residue composed of various oligoisoprenes, which
were purified by column chromatography (silica, pentane).
30
products (Scheme 1). With the aid of GC-MS, all of these
2
008 | Green Chem., 2011, 13, 2008–2012
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Scheme 2 Ruthenium complexes tested in squalene ethenolysis (Ind =
-phenylindenylid-1-ene, NHC N,N¢-bis(2,4,6-trimethylphenyl)-
imdazolinylidene).
3
=
the related benzylidene complexes (catalysts 7–10; Table 1, en-
tries 22–27). Grubbs–Hoveyda complex 12 shows an ethenolysis
activity comparable to those of complexes 1 and 2. However, the
amount of secondary products formed with 12 is higher than
with 1 or 2.
Scheme 1 Ethenolysis of squalene. The bold sum formulas denote
the six primary products; eight additional compounds result from
4 8
ethenolysis of the primary products. All products (except for C H and
28
1
C
6
H
10) were identified by GC or GC/MS.
GC and H NMR studies show that the amount of ethenolysis
by-products formed with complexes 1 and 2 is small, as long as
the reaction solution contains less than 10% weight of squalene.
This is in line with the ethenolysis of methyl oleate, but compared
to this substrate the cleavage of the squalene double bonds is
retarded, while the secondary reaction should occur with equal
ease in methyl oleate and squalene ethenolysis. Nonetheless, the
concentration of squalene in the reaction solvent appears to be a
critical factor. Increasing the amount of squalene to 20% weight
in toluene leads to higher conversions, but this also occurs at the
cost of significantly enhanced side reactions, such as secondary
were identified (except for the two volatile products: C
4
H
8
and C 10). As expected for squalene conversions, the primary
6
H
cleavage products (which require only a single ethenolysis
reaction on squalene) are more abundant than the ethenolysis
products that result from cleavage of the primary products. The
larger amount of C28
H46 than C23H38 indicates that ethenolysis
reactions preferably occur in the periphery of squalene and are
31
less likely in the middle of the chain.
In order to optimize squalene ethenolysis, twelve different
ruthenium complexes (Scheme 2) were tested in a variety of
solvents at different reaction temperatures, squalene concentra-
tions and ethene pressures. Following an extensive optimization
32
olefin metathesis and double bond isomerization.
The knowledge obtained in the optimization of squalene
ethenolysis was applied to the analogous reactions of natural
33,34
(
Table 1), a catalyst loading of only 0.01 mol% [Ru] per double
rubber (SVR 3 L) and liquid natural rubber.
The major
bond was found to be sufficient for a 75% conversion of squalene
by utilizing catalyst 2 over 3 h at 120 C, with toluene being
difference between these two materials is that squalene is a
small molecule while the latter is a high molecular weight
polymeric material. This is also the reason why it is much easier
to study and optimize the ethenolysis efficiency of catalysts
in squalene reactions. Consequently, the best catalysts for
squalene ethenolysis, 1 and 2, were applied to the ethenolysis
of NR and LNR. In order to control the amount of secondary
reactions, several such ethenolysis experiments were carried out
◦
the preferred solvent. Notably with catalyst 1, as little as 0.005
mol% (50 ppm) of [Ru] per double bond enabled 35% squalene
conversion (Table 1, entry 11). In contrast to RCM reactions em-
23,24
ploying (NHC)(NHCewg)RuCl
2
(CHR) complexes,
the ideal
reaction times for successful ethenolysis reactions were relatively
short at elevated temperatures, while longer reaction times led
to an increase of secondary products. Obviously, the desired
ethenolysis reaction occurs rapidly and other products are not
in C
6
D as the reaction solvent. The crude reaction mixture
6
1
after the ethenolysis reaction was directly probed by H NMR
spectroscopy. Fortunately, the different types of olefinic protons
are easily discerned, based on their chemical shifts. 4.7 ppm ((–
7
formed in significant amounts (Fig. 1). As noted previously,
the effect of ethene pressure is less pronounced, but the reaction
slows down at less than 3 bar of ethene and then produces
significant amounts of other products.
CH
(CH
2
)(CH
3
)C CH
2
), 4.90–5.08 ppm (–CH CH
2
), 5.1–5.2 ppm
CH–CH ) are
2
–CH C(Me)C) and 5.75–5.90 ppm (CH
2
2
In general, the (NHC)(NHCewg)RuCl
plexes (catalysts 1–6; Table 1, entries 1 –21) perform better than
2
(indenylidene) com-
the resonances in the olefinic region associated with the regular
squalene and natural rubber cleavage products, while resonances
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a
Table 1 Screening results for squalene ethenolysis
Conditions
◦
d
Number
Catalyst
Catalyst (mol%)
Temperature/ C
Time/h
Solvent
Conversion (%)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
1
0.1
0.1
0.05
0.01
0.01
0.01
0.01
0.01
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.01
100
100
100
100
100
120
120
120
120
140
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
140
120
120
120
20
6
6
3
1
3
1
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
C
C
C
C
C
C
C
6
6
6
6
6
6
6
D
D
D
D
D
D
D
6
6
6
6
6
6
6
99
99
82
33
36
65
32
65
25
20
35
45
3
5
1
2
3
33
1
75
48
33
6
6
55
1
1
1
1
1
1
1
1
Toluene
Toluene
Toluene
Toluene
Toluene
CH
Pentane
1
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
9
1
b
c
1
1
1
2 2
Cl
1
1
C
6
H
12
5
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
3
3
6
0.005
0.01
0.01
2
4
8
0.01
8
0.005
0.005
0.01
0.005
0.005
0.01
7
9
10
10
11
12
12
3
35
15
0.005
0.01
e
85
a
Reaction conditions: Squalene (100 mg) was dissolved in the respective solvent (5 mL), the designated amount of [Ru] was added, the ethene pressure
7 bar) applied and the reaction mixture heated to the designated temperature for the designated time. 8% weight squalene. 20% weight squalene.
b
c
(
d
The conversion was calculated according to conversion = squaleneend/squalenestart. However, the real TON of the catalyst will be significantly higher,
e
since only ethenolysis reactions on squalene itself are considered, while the ethenolysis of the primary products is not taken into account. Under
these reaction conditions, more by-products are formed.
Fig. 1 GC trace of the squalene ethenolysis reaction mixture using complex 1. The major peaks were assigned with the help of GC/MS (retention
time 3.4–4 min: toluene, 14.3 min: tetradecane internal reference).
2
010 | Green Chem., 2011, 13, 2008–2012
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between 5.3–5.5 ppm represent signals of secondary products
with disubstituted double bonds.
An analysis of the H NMR spectra provides clear evidence
of several small oligoisoprenes. Future work will be directed
towards exploring the up-scaling of the ethenolysis reaction with
1
15
natural rubber andgutta-percha; tyre rubber especiallyappears
of the cleavage of NR and LNR. However, the amount of
products resulting from secondary reactions, such as double
to be an interesting reactant in this respect. Furthermore, the
utilization of the oligoisoprenes for the synthesis of flavors,
odorants, pheromones and pharmaceutically active compounds
will be explored.
32
bond isomerization and secondary olefin metathesis, is slightly
higher than in the squalene reactions. More importantly, the
quantity of catalyst needed for efficient cleavage reactions
amounts to ca. 0.1 mol% per double bond. This is only partly
due to the polymeric nature of the rubber, and it is likely that the
need for an increased catalyst loading is related to impurities in
Acknowledgements
This work was supported by the DFG via Pl 178/13-1. We wish
to thank Weber & Schaer GmbH & Co. KG, Hamburg for a
generous donation of natural rubber.
15–17
NR and LNR.
Nonetheless, the relatively high ethenolysis efficiency enables
facile multigram depolymerization reactions in toluene using 0.1
mol% of complexes 1 or 2 by applying the conditions reported
for squalene.
Notes and references
Following the filtration of the crude reaction mixture over a
nanofiltration membrane, a number of ethenolysis products can
be separated (Scheme 3) by simple gravity column chromatogra-
phy. Oligomers (n = 3–6) were obtained in >90% purity (HPLC,
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2
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