Polymer precursors from catalytic reactions of natural oils

Article abstract of DOI:10.1039/c1gc16094j

Dimethyl 1,19-nonadecanedioate is produced from the methoxycarbonylation of commercial olive, rapeseed or sunflower oils in the presence of a catalyst derived from [Pd₂(dba)₃], bis(ditertiarybutylphosphinomethyl)benzene (BDTBPMB) and methanesulphonic acid (MSA). The diester is then hydrogenated to 1,19-nonadecanediol using Ru/1,1,1-tris-(diphenylphosphinemethyl)ethane (triphos). 1,19-Nonadecadienoic acid is hydrogenated to short chain oligoesters, which can themselves be hydrogenated to 1,19-nonadecanol by hydrogenation in the presence of water.

Full text of DOI:10.1039/c1gc16094j

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PAPER
Polymer precursors from catalytic reactions of natural oils†
Marc R. L. Furst,a Ronan Le Goff,a Dorothee Quinzler,b Stefan Mecking,b Catherine H. Bottinga and
David J. Cole-Hamilton*a
Received 2nd September 2011, Accepted 10th November 2011
DOI: 10.1039/c1gc16094j
Dimethyl 1,19-nonadecanedioate is produced from the methoxycarbonylation of commercial
olive, rapeseed or sunflower oils in the presence of a catalyst derived from [Pd2(dba)3],
bis(ditertiarybutylphosphinomethyl)benzene (BDTBPMB) and methanesulphonic acid (MSA).
The diester is then hydrogenated to 1,19-nonadecanediol using Ru/1,1,1-tris-
(diphenylphosphinemethyl)ethane (triphos). 1,19-Nonadecadienoic acid is hydrogenated to short
chain oligoesters, which can themselves be hydrogenated to 1,19-nonadecanol by hydrogenation in
the presence of water.
metathesis with dimethyl maleate, methyl propenoate or acry-
Introduction
lonitrile gives C11 difunctionalised products. However, in all of
As oil feedstocks dwindle, there will be a need to find al-
these reactions only half the C atoms from methyl oleate end up
ternative fuels and also alternative feedstocks that can be
in the desired diesters, the remainder giving hydrocarbons (self-
used to produce the many chemicals that enhance our lives.
metathesis) or monofunctionalised products (cross-metathesis).
Polymeric materials have revolutionised our lives over the last
An alternative is to use a reaction in which the double bond
100 years, not only replacing scarce, expensive and sometimes
toxic metals, cloths, etc., but also allowing the development
of wholly new applications that were not contemplated until
in an unsaturated ester, such as methyl oleate is isomerised to
the thermodynamically least favoured terminal position in the
hydrocarbon chain, where it is trapped by a tandem reaction.
plastics were introduced on a large scale.1 Cellulose esters, used
Attempts to use hydroformylation as the trapping reaction have
as thermoplastic materials, and vulcanised natural rubber2 were
amongst the first man made plastics to be re-discovered, being
already know in ancient Mesoamerica.3 These polymers were
only been partially successful with low w selectivity,10 but we
have recently reported that, using methoxycarbonylation as the
trapping reaction, very high selectivity (>95%) towards dimethyl
based on sustainable feedstocks, but they were soon superseded
1,19-nonadecanedioate can be obtained.11
in importance by petroleum derived polymers. A return to new
Two of us have recently used this methoxycarbonylation
polymer feedstocks, once again based on natural feedstocks,
reaction as part of a route to aliphatic polyesters with melt-
may occur in the very long term. This applies, amongst others,
ing and crystallization temperatures in the range of typical
to polyesters and polyamides as well as polyethylene, a small
thermoplastics starting from the nature-derived ester, methyl
portion of which is already produced from renewable resources
oleate.12,13 This process involves the methoxycarbonylation of
today.4
methyl oleate to give dimethyl 1,19-nonadecanedioate11 1 with
Polyamides and polyesters are usually derived from
monomers which are functionalised in the a and w positions,
so one possible route to bioderived analogues would be to make
very high selectivity and isolated yields of over 90%. The key
to this remarkable reaction, which involves isomerisation of
the double bond backwards and forwards along the chain,
such monomers from fatty acid esters, such as methyl oleate
but its only being trapped by carbonylation when it is in the
(methyl Z-octadec-9-enedioate). One way to do this is to use
thermodynamically least favoured w-position relative to the
metathesis.5–9 Self-metathesis gives the C18 diester, while cross
ester group, is the use of a palladium based catalyst modi-
fied by the ligand bis(ditertiarybutylphosphinomethyl)benzene
(BDTBPMB) 5 (Fig. 1).11,14,15
aEaStCHEM, School of Chemistry, University of St. Andrews, St.
Andrews, Fife, KY16 9ST, Scotland, UK. E-mail: djc@st-and.ac.uk;
Fax: +44 1334 463808; Tel: +44 1334 463805
bLehrstuhl fu¨r Chemische Materialwissenschaft, Universita¨t Konstanz,
Fachbereich Chemie, Universita¨tsstr. 10, 78457, Konstanz, Germany.
E-mail: stefan.mecking@uni-konstanz.de; Fax: +39 7531 885152;
Tel: +39 7531 882593
† Electronic supplementary information (ESI) available: Spectra of
compounds (1), (2) and (3). See DOI: 10.1039/c1gc16094j
Fig. 1 Ligands used in these studies: BDTBPMB 5 and triphos 6.
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Dimethyl 1,19-nonadecanedioate 1 was reduced to 1,19-
nonadecanediol 2 using LiAlH4 or by catalytic hydrogenation.
Condensation of the diester with the diol gave the polyester with
ester groups separated by 17 carbon atoms on the carboxylic
acid side and 19 carbon atoms on the alcohol derived side.12,13
Similar chemistry was carried out starting from the unsaturated
C23 ester, methyl erucate.
In this paper, we report our attempts to render the syntheses
of these monomers even more biocompatible. In particular, we
have developed the synthesis of dimethyl 1,19-nonadecanediaote
1 directly from biological feedstocks such as olive, rapeseed
and sunflower oils, so as to remove the need for extraction
and purification of methyl oleate. We also report catalytic
hydrogenation of the diester to 1,19-nonadecanediol 2 as a
cleaner alternative to LiAlH4 reduction. Finally we report the
synthesis of oligomers of 19-hydroxynonadecanoate 4 by partial
hydrogenation of 1,19-nonadecanedioic acid. These reactions
are outlined in Scheme 1.
sisted laser desorption time of flight mass spectrometry. MALDI
MS was acquired using a 4800 MALDI-TOF/TOF Analyser
(ABSciex, Foster City, CA) equipped with a Nd:YAG 355 nm
laser and calibrated using a mixture of peptides. The sample
(0.5 mL), dissolved in the appropriate solvent, was applied to
the MALDI target along with a-cyano-4-hydroxycinnamic acid
matrix (0.5 mL, 10 mg mL-1 in 50 : 50 acetonitrile : 0.1% aq.
TFA) and allowed to dry. The spot was analysed in positive
MS mode over the appropriate mass range, by averaging
1000 laser spots. Elemental analyses were performed by the
University of St Andrews Microanalytical Service. [Pd2(dba)3],
[Ru(acac)3], 1,1,1-tris-(diphenylphosphinemethyl)ethane, zinc
(Sigma Aldrich), bis(ditertiarybutyl-phosphinomethyl)benzene
(Lucite International), and methanesulfonic acid (Alfa Aesar)
were used as supplied. Natural oils were obtained from a local
supermarket and used as supplied. They were analysed for their
different carboxylic acid contents by a literature method.18
Dimethyl 1,19-nonadecanedioate
In
a glovebox, [Pd2(dba)3] (549 mg, 0.6 mmol) and
bis(ditertiarybutylphosphinomethyl)benzene (2367 mg, 6 mmol)
were introduced into a steel autoclave and sealed. Degassed
methanol (30 mL), natural oil (10 mL) and methanesulfonic
acid (0.78 mL, 12 mmol) were added to the autoclave by cannula.
The autoclave was purged with CO and the CO pressure was set
to 30 bar. The autoclave was heated to 80 ◦C for 22 h. After
cooling, venting and opening, the yellow powder was dissolved
by adding dichloromethane (20 mL) and the yellow solution was
filtered through a plug of cotton wool. The solvent was removed
on a rotary evaporator until a white precipitate appeared. Cold
◦
methanol (0 C) was added and the mixture was stirred in an
ice bath for 20 min before filtration. The remaining deep orange
solvent was again evaporated on a rotary evaporator until a
precipitate appeared, which was cooled in an ice bath for 20
min, and filtered. The desired diester was obtained as a white
powder in different isolated yields, depending on the starting
natural oil; olive oil: 6.93 g (74.7%); sunflower oil: 3.36 g (36.8%);
rapeseed oil: 6.44 g (69.3%). Yields are based on the amount
of unsaturated esters contained in the oil. Elemental analysis:
found C 71.10%, H 11.21%; C21H40O4 requires C 70.74%, H
11.31%. 1H-NMR (400 MHz; CDCl3): d 3.66 (s, 6H, –O–CH3),
2.29 (t, J = 7.6 Hz, 4H, –CH2–CO–O–CH3), 1.61 (quintet, J =
7.3 Hz, 4H, –CH2–CH2–CO–O–CH3), 1.24 (s, 26H, alkyl chain).
13C-NMR (100 MHz; CDCl3): d 174.49 (s, Ccarbonyl), 51.58 (s,
Scheme 1 (i) Formation of dimethyl 1,19-nonadecanedioate 1 by
methoxycarbonylation of natural oils catalysed by Pd/H+/BDTPMB
5; (ii) hydrogenation using Ru/triphos 6; (iii) hydrolysis. For conditions
see the experimental and Table 2.
During the course of this work a preliminary communication
has been published16 on using the same system for the catalytic
carbonylation of high oleic acid sunflower oil directly to dimethyl
1,19-nonadecanedioate. We show that it is not necessary to use
high oleate oils to obtain pure materials, but much cheaper
vegetable oils can be used. The same communication16 also
briefly describes the hydrogenation of the diester to the diol
using a catalyst devised by Milstein and co-workers.17
C
methoxy), 34.26 (s, –CH2–Ccarbonyl), 29.81 (s), 29.78 (s), 29.74 (s),
29.60 (s), 29.40 (s), 29.30 (s), 25.11 (s, –CH2–CH2–Ccarbonyl). These
data are consistent with those described in the literature.12
1,19-Nonadecanedioic acid13
Experimental
Dimethyl 1,19-nonadecanedioate (400 mg, 1.12 mmol) was
introduced into a round bottom flask. Dioxane (30 mL), distilled
water (20 mL) and hydrochloric acid (36% in water, 1 drop,
10 mg, 0.28 mmol) were added and the mixture was left to
reflux overnight. After cooling, the solvent was removed on a
rotary evaporator. Distilled water (100 mL) was added and the
heterogeneous mixture stirred fast before being filtered through a
Bu¨chner funnel. The white solid was washed with water (50 mL)
and dried under vacuum. The desired diacid was obtained as
All reactions were performed using standard Schlenk line and
glovebox techniques. Solvents were degassed with nitrogen.
1H and 13C nuclear magnetic resonance (NMR) spectra were
recorded at 298 K on a Bruker 300 MHz or 400 MHz spectrom-
eter for 1H-NMR and 75 MHz or 100 MHz for 13C-NMR, using
the residual solvent peak to reference the spectra to tetramethyl-
silane at d 0 ppm. IR spectra were recorded on a Perkin Elmer
Spectrum GX FT-IR spectrometer using KBr discs. Matrix as-
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a white powder (230 mg, 62%). Elemental analysis: found C
69.62%, H 10.91%; C19H36O4 requires C 69.47%, H 11.05%.
1H-NMR (300 MHz; THF): d 10.54 (br s, 2H, –CO2H), 2.21
(t, J = 7.4 Hz, 4H, –CH2–CO2H), 1.57 (quintet, J = 7.2 Hz,
4H, –CH2–CH2–CO2H), 1.30 (s, 30H, alkyl chain). 13C-NMR
(75 MHz; THF): d 174.41 (s, –CO2H), 34.23 (s, –CH2–CO2H),
30.62-30.12 (m, alkyl chain).
H 13.17%; C19H40O2 requires C 75.94, H 13.42%. 1H-NMR (400
MHz; THF): d 3.46 (t, J = 6.5 Hz, 4H, –CH2–OH), 1.47 (quintet,
J = 6.7 Hz, 4H, –CH2–CH2–OH), 1.30 (s, 30H, alkyl chain).
13C-NMR (100 MHz; THF): d 62.54 (s, –CH2–OH), 34.11 (s,
–CH2–CH2–OH), 30.69, (s) 30.63 (s), 30.55 (s), 26.93 (s, –CH2–
CH2–CH2–OH).
Oligoesters were prepared using the first method described
above for the diol, starting from nonadecanedioic acid, in
anhydrous dioxane. The oligomers were obtained as white solids.
Elemental analysis: found C 75.83%, H 11.64%; C57H112O6 (i.e.
oligomers with 3 units terminated by –OH groups) requires C
1,19-Nonadecanediol
First method. [Ru(acac)3] (4.5 mg, 0.011 mmol), 1,1,1-tris-
(diphenylphosphinemethyl)ethane (14 mg, 0.022 mmol) and
dimethyl nonadecanedioate (393 mg, 1.103 mmol) were intro-
duced into an autoclave and sealed. The autoclave was purged
three times with nitrogen. Dioxane (10 mL) and water (1 mL)
were introduced via cannula. The autoclave was purged with H2
and the H2 pressure was set to 40 bar before heating to 220 ◦C
for 16 h. After cooling, depressurising and opening, the crude
mixture was transferred to a round bottom flask (50 mL). The
solvent was removed on a rotary evaporator. Dichloromethane
(10 mL) was added and the yellow solution containing white
crystals was stirred for 20 min before filtration. The desired
diol was obtained as a white powder (265 mg, 80%). Elemental
analysis: found C 75.93%, H 13.36%; C19H40O2 requires C 75.94,
1
76.62%, H 12.63%. Typical NMR data: H-NMR (400 MHz;
CDCl3): d 4.05 (t, J = 6.8 Hz, 1H, -CO-O-CH2-), 3.64 (t, J =
6.6 Hz, 1H, -CH2-OH), 2.28 (t, J = 7.6 Hz, 1H, –CH2–CO–O–
CH2–), 1.64–1.53 (m, 3H), 1.25 (s, 22H, alkyl chain), 0.88 (t, J =
6.9 Hz, saturated end group). 13C-NMR (100 MHz; CDCl3): d
64.56 (s, –CH2–CO–O–CH2–), 63.26 (s, –CH2–OH), 34.58 (s),
32.98 (s), 29.83–28.82 (m, alkyl chain), 26.10 (s), 25.90 (s), 25.19
(s). IR: 3450 cm-1 (nOH), 2918 cm-1 and 2850 cm-1 (nCH), 1735
cm-1 (nC O), 1630 cm-1, 1470 cm-1 (CH2), 1180 cm-1, 1060 cm-1,
720 cm-1 (CH2).
Results and discussion
1
H 13.42%. H-NMR (300 MHz; THF): d 3.45 (t, J = 6.4 Hz,
1,19-Nonadecandioate from natural oils
4H, –CH2–OH), 1.45 (quintet, J = 6.1 Hz, 4H, –CH2–CH2–
OH), 1.28 (s, 30H, alkyl chain). 13C-NMR (75 MHz; THF): d
62.53 (s, –CH2–OH), 34.11 (s, –CH2–CH2–OH), 30.68 (s), 30.62
(s), 30.55 (s), 26.93 (s, –CH2–CH2–CH2–OH). These data are
consistent with those described in the literature.12
Our previous studies on the hydrogenation of unsaturated C18
esters11 led us to investigate whether 1 could be obtained from
oils that contain substantial amounts of C18 fatty acid esters with
different amounts of unsaturation, reasoning that methanolysis
of the triglyceride esters and methoxycarbonylation should
occur in tandem under the reaction conditions. The oils were
obtained from a local supermarket and were chosen for their
different proportions of oleic, linoleic and linolenic acid esters
(Table 1).
Using a method very similar to that described previously12,13
for the methoxycarbonylation of methyl oleate, a one-pot
synthesis of 1 from natural oils has been successfully achieved,
affording the desired product as an analytically pure snowy
white powder, with NMR spectra as reported earlier.12,13 The
yields obtained from 10 mL of oil, as well as the % conversion
of C18 chains are shown in Table 1. The yield reduces in the
order olive oil > rapeseed oil > sunflower oil, reflecting the
differing amounts of triglyceride oleate present in the starting
oils. This one-pot process represents a significant enhancement
in the viability of production of 1 since it starts from standard
quality natural oils with no extra purification and avoids the
multiple reaction and purification steps usually required for
Second method. [Ru(acac)3] (4.5 mg, 0.011 mmol), 1,1,1-
tris-(diphenylphosphinemethyl)ethane (14 mg, 0.022 mmol),
dimethyl nonadecanedioate (393 mg, 1.103 mmol) and zinc
powder (8.6 mg, 0.132 mmol), were introduced into an autoclave
and sealed. The autoclave was purged three times with nitrogen.
Methanol (10 mL) was introduced via cannula. The autoclave
was purged with H2 and the H2 pressure was set to 70 bar.
The autoclave was heated to 140 ◦C for 16 h. After cooling,
venting and opening, dichloromethane (10 mL) was added and
the mixture transferred to a round bottom flask (50 mL). The
solvent was removed on a rotary evaporator. Dichloromethane
(10 mL) was added and the yellow solution containing white
crystals was stirred for 20 min. The two phase mixture was
allowed to settle for 20 min (the product stays in the upper
phase, the metallic zinc stays at the bottom). The upper phase
was decanted and filtered. The desired diol was obtained as a
white powder (270 mg, 81%). Elemental analysis: found C 76.03,
Table 1 Composition of natural oilsa and the yields of dimethyl 1,19-nonadecanedioate ,1, obtained
Methyl oleate (Aldrich)
Olive (supermarket)
Rapeseed (supermarket)
Sunflower (supermarket)
Oleate/%
Linoleate/%
>90
73
2
64
19
38
50
Linolenate/%
3
6.9
74.7 (102.3)
10
6.4
69.3 (108.3)
2
3.4
36.8 (96.8)
Diester/g from 10 mL of oil
9.0
Yield (from oleate)b/%
a Unsaturated C18 fraction only, the remaining fatty acid residues are from different chain lengths (mainly C16 and C23) and from stearic (saturated
C18) acids. b Yields are calculated assuming the molecular mass of the starting material is equivalent to glycerol trioleate (885.43 amu).
474 | Green Chem., 2012, 14, 472–477
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high purity methyl oleate as a feedstock. The yields suggest
that the isolated diester arises mainly from the oleate chains
in the oils. This suggestion is supported by GC analysis of the
crude reaction products. Due to the multitude of compounds
present and the often similar or identical masses and similar
fragmentation patterns, full assignments from GC-MS are
problematic. However, the main impurities are glycerol and
methyl esters of saturated fatty acids of different chain lengths
which are present as glycerol esters in the feed oil. Peak j in the
GC-MS traces shown in the ESI†, which is much more intense
in the products from sunflower oil than from olive oil, with
a slightly shorter retention time than that for dimethyl 1,19-
nonadecanedioate 1 (peak k) and a parent ion at 354 amu, can
be assigned tentatively to (isomeric) dimethyl nonadecenedioate.
This is relevant in view of the significant linoleate content of
many feedstocks (Table 1), and shows that this material is also
carbonylated to linear products (cf. ESI†). Further studies on
the methoxycarbonylation of methyl linoleate are currently in
progress and will be reported separately.19 Full GC-MS traces
with mass spectra are shown in the ESI†.
decarbonylation of alcohols and the mechanism is quite well
understood.26 In addition, we have isolated [RuH2(CO)(triphos)]
from a reaction of [Ru(acac)3], and triphos in methanol under
hydrogen,27 and others have isolated the same complex from
the hydrogenation of levulinic acid under similar conditions.28
Water is then required to return the catalyst into an active form
by water–gas shift type chemistry (bottom half of Scheme 2).24
An alternative method for hydrogenating simple esters using 7 as
the catalyst involves carrying out the hydrogenation in methanol
in the presence of zinc, where good activities could be obtained
◦
at 140 C.21 Original suggestions that triphos is active because
it coordinates in a facial manner have been challenged by the
discovery of several other catalysts, which do not contain facially
coordinating ligands but which are active for ester and acid
hydrogenation.17,29,30 Indeed some of us have recently reported
that nonadecane-1,19-diol as well as tricosane-1,23-diol can be
formed in high yields and with more than 99% purity, using
dichlorobis[2-(diphenylphosphino)ethylamine]ruthenium as the
catalyst.13
Leitner and co-workers have reported the successful hydro-
genation of levulinic acid using the same catalyst under similar
conditions (added water, 220 ◦C, 40 bar) and have shown
that the acid is more successfully reduced than the methyl
ester.28 In some of their reactions, they added [NH4]PF6 which
will undoubtedly react with water to give HF and [O2PF2]-.31
This led us to investigate whether addition of acid might
enhance our hydrogenation reactions, which had shown variable
reproducibility depending upon the batch of triphos employed.
Under the high temperature conditions (220 ◦C in dioxane
containing 10% water), in the presence of catalytic amounts of
methanesulfonic acid (MSA), 2 was not produced as expected
(Table 2, entry 1). Furthermore, the reaction in the presence of
MSA could not be successfully carried out without any added
water (Table 2, entry 2). Using lower temperature conditions
(dry methanol, zinc powder (12% mol), 140 ◦C) and addition
of MSA, allows the reproducible production of 2 in high yield
(Table 2, entry 3). Full conversion to diol is also observed under
these conditions if zinc is omitted (Table 2, entry 4).
Hydrogenation of dimethyl 1,19-nonadecanedioate
The hydrogenation of esters and carboxylic acids, which
are amongst the more difficult hydrogenations to have been
reported,20 has traditionally been carried out using copper
chromite at high temperatures and pressures. The first homoge-
neous catalysts for this reaction, ruthenium complexes of 1,1,1-
tris-(diphenylphosphinemethyl)ethane (triphos, 6, Fig. 1) were
described by Elsevier and co-workers.21–23 Initially, the reactions
only worked with activated acids such as oxalic acid, which
could be hydrogenated to 1,2-ethanediol with high selectivity.21
Further developments by the same group,22,23 but also by workers
at Davy Process Technology24,25 extended the substrate scope to
cover other acids including simple aliphatic carboxylic acids.
High activities towards primary alcohols were achieved by
adding water (10% volume) and working at high temperature
(220 ◦C).24,25 It is thought that the high temperatures required to
obtain good rates lead to the decarbonylation of an aldehyde
or other reaction intermediate to give [RuH2(CO)(triphos)]
7, which is catalytically inactive (Top half of Scheme 2).24
Ruthenium carbonyl complexes are sometimes synthesised by
Afterwards, we attempted to carry out the same reactions in
the absence of MSA. Full conversion was obtained at 220 ◦C in
dioxane containing 50% water (Table 2, entry 5). As suggested in
Scheme 2, the role of water is to regenerate the poisoned catalyst
into an active species by a water–gas shift type reaction. In order
Table 2 Production of 2 by hydrogenation of 1 catalysed by Ru
complexes of 6a
MSA/
Water/% mmol
Yield/%
Entry
Solvent
T/◦C GC-(isolated)
1
2
3
4
5
6
7
8
Dioxane
Dioxane
Methanolb
Methanolc
Dioxane
Dioxane
Dioxane
Dioxane
10
0
0
0.111
0.111
0.111
0.111
0
0
0
220
220
140
140
220
220
220
220
<1
<1
>99–(81)
>99–(63)
>99–(76)
>99–(73)
18
0
50
10
0.6
0
0
13
Scheme 2 Proposed poisoning of the catalyst 7 by decarbonylation
of an aldehyde intermediate and the regeneration of the catalyst by
water–gas shift type reactions. Triphos, which occupies the other three
coordination sites, has been omitted for clarity.
a [Ru(acac)3] (4.5 mg, 0.011 mmol); 6 (14 mg, 0.022 mmol), 1 (393 mg,
1.103 mmol), H2 (40 bar), dioxane/water (10 mL); b Zn (8.6 mg,
0.132 mmol); c No Zn
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to investigate more about the role of water during the reaction,
the amount of water was successively decreased from 5 mL to
1 mL, 0.06 mL and 0 mL. We saw full conversion to diol when
a large excess of water in dioxane (50% to 10%, Table 2, entries
5 and 6) was employed, but the conversion of 1 to 2 decreased
dramatically if only a small amount (0.6%, Table 2, entry 7) of
water was added or if it was omitted entirely (Table 2, entry 8).
The large amounts of water required for hydrogenation reactions
in dioxane may suggest that the water is not only required for
catalyst regeneration as in Scheme 2, but also for ester hydrolysis
and that the acid formed may be the active species being
hydrogenated. To test this hypothesis, we prepared the diacid
3 by an acid catalysed hydrolysis of species 1 and hydrogenated
it in the absence of both water and MSA (conditions were the
same as for Table 1, entry 8). Surprisingly, we observed the
production of a white solid, for which NMR spectroscopy,
MALDI-TOF mass spectrometry (MTMS) and IR analyses
suggest that partial hydrogenation to 19-hydroxynonadecanoic
acid occurs, followed by a condensation esterification to give
oligoesters 4 (for spectra see ESI†). The IR spectrum shows that
be intermediates in the formation of the diol, we took one
of the samples of oligoester (see ESI† for its structure) and
hydrogenated it in dioxane/water (1 : 1) with a catalyst derived
from pure triphos. Full conversion to 2 was obtained, confirming
our previous hypothesis that water can aid in depolymerisation
reactions.
Three approaches to the hydrogenation of dimethyl 1,19-
nonadecanedioate 1 have now been described using three
different catalytic systems. The catalysts are shown in Fig. 3.
Each of them has some disadvantages. Catalysts 713 and 816
both require fairly sophisticated ligands, and 7 requires a large
amount of base (NaOMe). However, they both operate under
relatively mild conditions (50 bar, 100 ◦C, 22 h for 7 and 10 bar,
115 ◦C for 8) whereas, the simpler catalyst system 9 (this work),
formed in situ from readily available materials requires rather
higher temperatures (140–200 ◦C).
hydroxy (nOH = 3450 cm-1) and ester carbonyl groups (nCO
=
1735 cm-1) are present. Hydroxy end groups are also observed
1
Fig.
3
Catalysts used for the hydrogenation of dimethyl 1,19-
in the H-NMR spectrum, which shows that the average chain
nonadecanedioate to 1,19-nonadecanediol.
length of the isolated polymer is three units. The crude product
also contains diol. The MTMS suggests that the oligomers are
mostly short chains (up to four monomer units).
Detailed analysis of the MTMS shows that each chain length
of oligomer can have several different end groups. The peaks
corresponding to oligomers of chain lengths with four monomer
units are shown in Fig. 2. Well defined clusters of peaks are
observed and are separated by 14 mass units. The main signals
arise from oligoesters in which the terminal units are diol (2 ¥
OH) hydroxyacid (OH, CO2H), or diacid (2 ¥ CO2H).
Conclusions
Dimethyl 1,19-nonadecanedioate can be conveniently prepared
in high purity from the methoxycarbonylation of cheap and
readily available olive, rapeseed or sunflower oils. The diester
can be hydrogenated using Ru/triphos in the presence of water
to give 1,19-nonadecanediol. 1,19-Nonadecanedoic acid can by
hydrogenated in the absence of MSA and water but only to short
chain oligoesters. Such oligoesters can be hydrogenated to 1,19-
nonadecanediol if water is added. Here, water may have three
key roles: it may recycle inactive carbonyl compounds formed
from the active catalyst by water–gas shift type chemistry; it
may hydrolyse the starting diester to the carboxylic acid, thus
generating the acid required to promote the reaction; it may
assist in hydrolysing the intermediate oligoesters that might
otherwise precipitate during the reaction.
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Since hydrogenation of the 1,19-nonadecanedioic acid pro-
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