Three step auto-tandem catalysed hydroesterification: Access to linear fruity esters from piperylene

Article abstract of DOI:10.1016/j.molcata.2015.06.019

A convenient and selective access to saturated hexanoic esters via hydroesterification of piperylene with synthesis gas and methanol is presented. This is the first three step auto-tandem hydroesterification, which is 100% atom economic proceeding under mild conditions. Our optimisations revealed Pd₂(dba)₃/1,2-dtbpmb as the best catalytic system. Besides, the reaction also tolerates several alcohols, which offers a broad range of fruity esters. In addition, we present insights into the reaction sequence, investigating whether the reaction proceeds via two- or three-step reaction cascade.

Full text of DOI:10.1016/j.molcata.2015.06.019

Accepted Manuscript
Title: Three step auto-tandem catalysed hydroesterification:
Access to linear fruity esters from piperylene
Author: Peter Neubert Matthias Steffen Arno Behr
PII:
S1381-1169(15)00231-9
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2015.06.019
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Journal of Molecular Catalysis A: Chemical
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11-6-2015
15-6-2015
Please cite this article as: Peter Neubert, Matthias Steffen, Arno Behr,
Three step auto-tandem catalysed hydroesterification: Access to linear
fruity esters from piperylene, Journal of Molecular Catalysis A: Chemical
http://dx.doi.org/10.1016/j.molcata.2015.06.019
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Three step auto-tandem catalysed hydroesterification:
Access to linear fruity esters from piperylene
Peter Neuberta, Matthias Steffena, Arno Behra*arno.behr@bci.tu-dortmund.de
aDepartment of Biochemical and Chemical Engineering, Chair of Technical
Chemistry, Technical University Dortmund, Emil-Figge-Str. 66, 44227 Dortmund,
Germany.
*Corresponding author.
1

Graphical Abstract
3 steps
vs.
O
R
O
+ CO/H2 and ROH
R = -Me, -Et, -Bu, -iPr
Y= up to 79%, l:b 12:1
2 steps
Copyright pinnaple: 123rf/Maksym Narodenko
First three step auto-tandem catalysed hydroesterification of piperylene with
synthesis gas provides access to linear fruity esters in high selectivities.
2

Highlights
- first auto-tandem isomerisation hydroesterification reaction cascade
- highly active palladium catalysts provide access to fruity esters
- high linear/branched rations up to 12.6:1
- further insights into the reaction cascade by applying different intermediates
3

Abstract
A
convenient and selective access to saturated hexanoic esters via
hydroesterification of piperylene with synthesis gas and methanol is presented. This
is the first three step auto-tandem hydroesterification, which is 100% atom economic
proceeding under mild conditions. Our optimisations revealed Pd2(dba)3/1,2-dtbpmb
as the best catalytic system. Besides, the reaction also tolerates several alcohols,
which offers a broad range of fruity esters. In addition, we present insights into the
reaction sequence, investigating whether the reaction proceeds via two- or three-step
reaction cascade.
4

1. Introduction
Modern chemical industry heavily relies on fossil feedstocks, especially petroleum;
however, its availability is not endless. To overcome the emerging shortage of raw
materials, utilisation of new feedstocks such as fats and oils, terpenes and lignine is a
promising goal.[1, 2] On the other side, exploration of new fossil feedstocks[3]
extends the current supply of raw materials. In addition, the more efficient usage of
well-known but so far unused compounds and further development of established
technologies, such as homogeneously catalysed reactions are crucial for a more
responsible usage of feedstocks.[4-6]
A promising compound, which is already easily available from naphtha
steamcracking is piperylene (1,3-pentadiene). It exhibits potential chemical
properties, however, till now it is mainly used for hydrogenation to n-pentane followed
by subsequent isomerisation to iso-pentane which is used as a gasoline additive.[7]
Using this underrated compound, the approach of this contribution was to convert
piperylene into valuable ester compounds, by usage of common bulk chemicals.
A convenient access to esters from olefins is the transition metal catalysed reaction
of alkenes with carbon monoxide in the presence of an alcohol, which is referred to
as hydroesterification, hydrocarbalkoxylation or alkoxycarbonylation (Fig.1).
This reaction has been discovered by Walter Reppe in the nickel catalysed
production of acrylic acid.[8] In the following years, several other transition metals
turned out to be active in this reaction [9-12] and also several detailed contributions
on the hydroesterification mechanism have been published.[13-21] Particularly
palladium is a potential catalyst metal, which requires only mild reaction conditions
tolerating a magnitude of substrates and functional groups.[11, 22-24] Beside
terminal olefins it is also possible to derive linear esters from internal olefins, which
are obtained by previous isomerisation of the internal double bond to the -position.
α
This can be achieved with complexes containing feasible ligands such as bulky
adamantyl substituents bearing phosphines.[25] Another highly potential ligand is
1,2-bis-(di-tert-butylphosphinomethyl)benzol
(1,2-dtbpmb)
which
has
been
successfully applied by Cole-Hamilton et al. in the isomerising hydroesterification.[26-
28] This reaction has been expanded to several functionalised internal olefins,
providing access to a wide range of terminal esters.[29-32] This ligand is also used
5

for the industrial hydroesterification of ethene during the production of methyl
methacrylate.[33-37]
In addition to monoolefins it is also possible to use dienes as starting material in the
hydroesterification. A frequently used starting material is butadiene which is mainly
reacted with palladium and cobalt based catalysts.[38-42] Nevertheless, there are
also other other dienes like the renewable diene myrcene[43] and also piperylene
has already been subjected to hydroesterification.[44] However, till now the
piperylene hydroesterification suffers from harsh reaction conditions and the
formation of both, linear and branched esters with yields up to 42%, only.[45, 46]
2. Experimental
2.1 Analytics
1
Both H- and 13C-nuclear magnetic resonance (NMR) spectra were recorded in
deuterated chloroform on a Bruker Avance DRX 400 MH (Bruker Corp., Billerica,
Massachusetts, USA). Gas chromatography (GC) analysis of the reaction solutions
was carried out on a Hewlett-Packard gas chromatograph Series 6890 equipped with
a HP5 capillary column (coating: 5% diphenyl-95%-dimethoxy-polysiloxane; length 30
m; diameter 0.25 mm, thickness 0.25 mm). A flame ionisation detector (FID)
connected to an autosampler was used to detect individual components. The
qualitative assignment of the chromatographically determined peaks of the individual
components was carried out by comparing them to their respective pure substances.
The quantitative determinations were achieved using the method of internal standard.
The mass spectra were recorded by GC/MS. The mass spectrometer was a Hewlett
Packard 5973 with electron energy of 70 eV and a scan range [m/z] of 50-700.
2.2 Standard experimental procedure
Piperylene has been purchased from Haohua Chemical Corporation (Beijing/China)
and purified by complexation with copper(I) chloride and subsequent distillation,
giving pure piperylene.[47] All screening experiments were performed in a 20 ml
homemade multiplex reactor.[48] In an typical experiment 87.5 mg (0.10 mmol)
Pd2(dba)3 and 301.8 mg (0.75 mmol) 1,2-dtbpmb, 869.1 mg (12.75 mmol) piperylene,
98.0 mg (1.02 mmol) methanesulfonic acid and 10 ml methanol were placed in a
Schlenck-vessel. The mixture was placed in an ultrasonic bath for 15 min. The
mixture was transferred to an evacuated and flame-dried reactor and subsequently
6

charged with 30 bar synthesis gas. The mixture was placed in an oil bath at reaction
temperature e.g. 110 °C for five hours. After cooling to room temperature the mixture
was depressurised and analysed by GC-FID.
2.3 Products
Products have identified by NMR and GC-MS and compared to known literature.
Hexanoic acid methyl ester (4) [49]
1H NMR (400.13 MHz, CDCl3, 25 °C, TMS):
δ [ppm] = 0.83 (t, J = 6.9 Hz, 3H, CH2-
CH3 ), 1.20-1.27 (m, 4H, CH2-CH2-CH3), 1.50-1.58 (m, 2H, CH2-CH2-COO-), 2.21 (t, J
=7.6 Hz, 2H, CH2-COO-), 3.64 (s, 3H, O-CH3). 13C {1H} (100.63 MHz, CDCl3, 25 °C,
TMS):
δ [ppm] = 14.3 (-CH3), 22.4 (CH2-CH3), 24.7 (CH2-CH2-CH3), 31.5 (CH2-CH2-
COOMe), 33.9 (CH2-COOMe), 51.7 (O-CH3), 174.8 (-COOMe). MS (m/z): 130 (M+
2%), 101 (10), 99 (22), 87 (37), 74 (100), 56 (27), 55 (18).
Hexanoic acid ethyl ester [50]
1H NMR (400.13 MHz, CDCl3, 25 °C, TMS):
δ [ppm] = 0.87 (t, J = 7.0 Hz, 3H; CH3-
CH2), 1.20-1.27 (m, 7H; O-CH2-CH3, CH2-CH2-CH3), 1.61 (quint, J = 7.6 Hz, 2H; CH2-
CH2-COO-), 2.27 (t, J = 7.8 Hz, 2H; CH2-COO-), 4.11 (q, J = 7.4 Hz, 2H; CO2-CH2-
CH3). 13C {1H} (100.63 MHz, CDCl3, 25 °C, TMS):
δ [ppm] = 14.3 (CH2CH3), 14.6
(CH2CH3), 22.7 (CH3CH2),31.7 (2C; CH2-CH2), 34.7 (-CH2-COO), 60.5 (COO-CH2-
CH3), 174.2 (COO-). MS (m/z): 144 (M+ 1%), 101 (27), 99 (51), 92 (20), 91 (35), 88
(100), 73 (30), 71 (28), 70 (33), 61 (25), 60 (41), 55 (21).
Hexanoic acid butyl ester [51]
1H NMR (400.13 MHz, CDCl3, 25 °C, TMS):
δ [ppm] = 0.87-0.95 (m, 6H; CH3CH2),
1.31-1.41 (m, 6H; CH2CH2), 1.56-1.62 (m, 4H; CH2CH2CH2), 2.29 (t, J = 7.8 Hz, 2H;
CH2CH2COO), 4.0 (t, J = 7.6 Hz, 2H; CO2CH2). 13C {1H} (100.63 MHz, CDCl3, 25 °C,
TMS):
δ [ppm] = 14.3 (2C, CH2CH3), 19.5 (2C, CH2CH3), 22.7 (-CH2), 31.7 (2C;
CH2CH2), 34.5 (CH2COO), 64.5 (COO-CH2), 174.4 (COO-CH2). MS (m/z): 172 (M+
1%), 117 (92), 99 (97), 87 (16), 73 (26), 71 (35), 61 (11), 60 (32), 57 (31), 56 (100),
55 (27).
Hexanoic acid isopropyl ester [52]
7

1H NMR (400.13 MHz, CDCl3, 25 °C, TMS):
δ [ppm] = 0.89 (t, J =7.9 Hz, 3H;
CH3CH2), 1.28-1.61 (m, 12H; CH2CH3, CH2CH2, CH2CH2CH2), 2.25 (t, J = 7.7 Hz, 2H;
CH2CH2CO2), 5.11-5.19 (m, 1H; CO2CH(CH3)2). 13C {1H} (100.63 MHz, CDCl3, 25 °C,
TMS):
δ [ppm] = 14.3 (CH2CH3), 22.2 (2C; CH3CHCH3), 22.7 (CH3CH2), 31.7 (2C;
CH2CH2),34.5 (CH2-COO), 67.7 (COO-CH(CH3)2), 173.8 (COO2-CH). MS (m/z):
158 (M+ 1%), 143 (3), 117 (48), 116 (16), 102 (27), 99 (100), 92 (42), 91 (70), 87
(21), 74 (13), 73 (32), 71 (39), 61 (19), 60 (88), 59 (37), 56 (14), 55 (21).
3. Results and discussion
In our research on piperylene hydroesterification we were keen to obtain linear C6-
esters in a palladium catalysed reaction. These are well-known odoriferous
compounds with this structure, which have broad application in manufacture of
aromas and fragrances. Therefore, we developed a reaction cascade, which allows
convenient access to these linear esters under mild reaction conditions and with
100% atom efficiency. This concept is based on the fact that the main product in
diene hydrogenation is usually an internal olefin.[7, 53] On the other side the addition
of carbon monoxide for enhancing the selectivity in diene hydrogenation towards
monoolefins, is a known method.[54, 55] Bringing both concepts together in a single
pot, allows us to use inexpensive synthesis gas in the reaction, which we have
described earlier to be also feasible for hydroesterification reactions (Fig.2).[24]
The first step involves the hydrogenation of piperylene to either 1-, or 2-pentene
(step A), which can be easily catalysed by several transition metal catalysts, as in
this case, palladium complexes. Next to come is the isomerisation of 2-pentene,
which is usually the main occurring olefin from 1,4-additon during piperylene
hydrogenation, to linear 1-pentene (step B). This is also the initial step when using
internal olefins as a starting compound.[56, 57] This is followed by the actual
hydroesterification of the terminal pentene to give linear hexanoic acid methyl ester 4
(step C). However, steps B and C have to be fast enough to be rather favoured than
the side reaction B’, which is the complete hydrogenation to pentane. Therefore, an
easily switching catalytic system which is both, an average hydrogenation catalyst
and a highly active hydroesterification would work best.
The two catalytic steps proceed in the presence of only one catalyst precursor, which
does not need any addition of chemical trigger for switching its catalytic nature from
8

hydrogenation catalyst to hydroesterification catalyst. Regarding to dos Santos et al.
this is referred to as auto-tandem catalysis.[58]
We started our investigations with 0.75 mol% of different palladium precursors and a
ligand excess of 1:5, which prevents the reaction activity from decreasing owing to
catalyst decomposition. 10 eq. methanesulfonic acid work as an activator. In all
reactions the main product was the linear hexanoic acid methyl ester 4; however,
there are also little amounts of other saturated esters, which were quantified together
as “branched esters”. In an initial screening we have applied different palladium
precursos with 1,2-dtbpmb as a ligand (Table 1).
The initial precursor screening revealed that several palladium precursors are
feasible for the isomerising hydroesterification of piperylene. Using PdCl2 gave high
yields of esters; however, it also provided 14% of pentane (entry 1). Pd(II)-precursors
with weakly coordinating anions led to decreased yields (entries 2-4). The industrially
viable heterogeneous catalyst palladium on charcoal predominately acts as a
hydrogenation catalyst, which led to 31% of pentane and 14% esters, only (entry 5).
In addition, the linear:branched ratio is very low (2.5:1). The best precursor under
chosen conditions was Pd2(dba)3 which provided 47% of hexanoic acid methylester 4
and an excess of linear products of 7.8.1. In general, it can be said, that the
isomerisation and the hydroesterification are fast enough to overcome the fully
hydrogenation of the intermediary built pentenes. In the following, the influence of the
precursor concentration was investigated (Fig.3).
The initially chosen amount of 0.75 mol% precursor afforded a yield of 47% of ester
4. Decreasing the Pd-concentration led to decreasing activity of the catalyst system.
With the least amount of 0.35 mol% catalyst, the reaction gave 25% esters, only.
Furthermore, the formation of pentane did also decrease, which indicates, that the
fully hydrogenation is still not the favoured reaction, which supports the assumption
of carbon monoxide, preventing the olefin from being hydrogenated. On the other
side, increasing the catalyst amount to 0.85 mol%, did not give higher yields, but the
formation of pentane did slightly increase. Therefore, the catalyst concentration was
chosen to be 0.75 mol% in the following experiments.
9

The influence of the ligand is crucial in isomerising hydroesterification reactions. On
the one side it stabilises the Pd(0)-species under acidic conditions and on the other
side it is in charge of the regioselectivity of the actual carbonylation step. Therefore,
we
have
investigated
the
influence
of
the
M:L
ratio
(
10

Table 2).
The variation of the M:L ratio had a distinct influence on the selectivity of the reaction.
A 1:1 ratio gave 25% of linear ester 4 but also 14% of branched esters, which
indicates a nonsufficient formation of the active isomerising species leading to lower
amounts of 1-pentene (entry 1). On increasing M:L ratios, the formation of the
branched esters dropped, along with an increasing yield of the preferred main
product 4. Furthermore, the formation of pentane did also decrease, which are both
favourable trends. However, the yield of the linear ester has a maximum at a ratio of
1:4, which was 56%. Higher ratios provided lower yields, which is probably caused by
blocking of the active species by the excess ligand.
Another important influence is the nature of the acid, which on the one hand activates
the catalyst and on the other hand stabilises the precursor. Therefore, we have
investigated the influence of different acids (Table 3).
Although, all acids have very weak coordinating anions, as well as they are all strong
acids, the nature of the acid heavily influenced the hydroesterification of piperylene.
By using p-toluenesulfonic acid, ester 4 is the main product, which was obtained in
29% yield. However, the reaction gave also 15% of pentane (entry 1). With
trifluoroacetic acid and trifluoromethanesulfonic acid the chemoselectivity of the
reaction switched, which resulted in the favoured formation of pentane instead of
formation of the linear ester (entries 2 and 3). The best results have been obtained
with methanesulfonic acid, which worked best by using 8 mol% (entry 6). Lowering
the acid amount led to higher excess of linear ester, however the overall yield
decreases while the yield of pentane increased (entries 4 and 5). In contrast higher
amounts of acid led to decreased formation of ester 4, while the pentane yield
remained stable (entry 7).
In addition to the acid, the temperature is a key factor for the activity of the catalyst.
However, too high temperatures may lead to ligand oxidation and decomposition of
the catalytic species. Therefore, we have conducted a detailed temperature
screening (Table 4).
In the examined temperature range, 110 °C worked best. At this temperature the
reaction provided 63% yield of ester 4, while the linear to branched ratio was 12:6:1.
Decreasing the temperature resulted in lower yields of 4 as well as decreasing
linear:branched ratio. In addition, at 90 °C there was a distinct rise of pentane
11

production, which indicated that the hydroesterification worked better at elevated
temperatures. On the other side, increasing the temperature higher than 110 °C, the
yield of linear ester 4 started to decrease, while the yield of the other esters remained
stable, which resulted in low selectivities. At a maximum temperature of 150 °C, there
was only little hydroesterification activity, as the main product was pentane.
Obviously these high temperatures led to catalyst decomposition.
A crucial fact in gas/liquid reactions is the gas pressure, which is preferably
somewhat elevated to ensure a sufficient saturation of the liquid phase. In the
hydroesterification of piperylene higher CO pressures are also beneficial for
suppressing the hydrogenation to pentane and it has also been shown, that high CO
pressures are advantageous for fast reaction of the terminal olefin, preventing its
back isomerisation to the thermodynamically more stable internal olefin.[26] In
contrast, carbon monoxide may act as a strong binding ligand, which may compete
for coordination sites with the phosphorus ligand. Therefore, we have investigated
the influence of the gas pressure (Fig.4).
The variation of the synthesis gas pressure revealed, that 40 bar CO/H2 provided the
highest ester yield, which was 68% and also the highest excess of the linear ester
(12.6:1). At lower pressures the tendency of the system to form more branched
esters increased, which resulted in the formation of 10% branched esters under 20
bar of synthesis gas. In contrast increasing the pressure to 50 bar had no remarkable
effect on the reaction.
By now, all reactions have been conducted for five hours. Hence, after optimisation of
the catalyst we focussed on the kinetics of the reaction. Therefore, we conducted
time dependent experiments (Fig.5).
Figure 5 shows that the maximum yield of hexanoic acid methyl ester is already
derived after 8 hours. The reaction gave 79% linear ester, which corresponds to an
excess of linear ester of 12.2:1. In addition; the reaction formed 10% of the by-
product pentane. During the reaction the formation of pentenes cannot be observed.
Obviously, the conversion to hexanoic acid methyl 4 ester proceeded very fast. This
might be caused by the very high excess of hydroesterification catalyst in comparison
to the in situ formed amount of pentenes, which are subsequently converted into
12

ester 4. However, this has to be clarified by further investigations on the tandem
reaction mechanism.
After optimisation of the reaction parameters, we have also investigated the scope of
the reaction, as we have conducted the reaction in presence of different alcohols
(Fig.6)
The isomerising hydroesterification of piperylene tolerated several alcohols. The best
results have been obtained with methanol. The longer chain alcohols led to
decreased yields while isopropanol gave only low yields.
The last aspect of our investigations covers insights into the reaction pathway of the
isomerising hydroesterification of piperylene with synthesis gas. The formation of the
saturated linear ester can basically follow two different reaction pathways (Fig. 7).
One possibility comprises three reaction steps, which starts with an initial
hydrogenation of the diene, forming either 1-, or 2-pentene, which are subsequently
isomerised to only 1-pentene, finally reacting to the linear ester 4 (route I). The other
pathway covers two steps, only. In an initial hydroesterification of piperylene it forms
3-hexenoic acid methyl ester, which undergoes hydrogenation to give linear ester 4
(route II).
To get deeper insights, we have applied different starting materials in the
hydroesterification of piperylene. The optimised reaction conditions for the
hydroesterification of piperylene with synthesis gas provided 79% of hexanoic acid
methyl ester 4 (path A). In a comparative experiment, using only carbon monoxide
did not provide ester 4, but 3-hexenoic acid methyl ester 8 in low yields.
First of all, we have investigated route I which covers the formation of pentenes as a
first step. If 1-pentene 3 was used as starting material, it provided hexanoic acid
methyl ester 4 in high yields, no matter synthesis gas or pure carbon monoxide have
been used (C and D). The application of 2-pentene 2, requires an initial isomerisation
to the terminal pentene 3, to give the linear ester 4. In this case the reaction also led
to high yields of 4 with both, synthesis gas or carbon monoxide, however, there was
slightly formation of pentane under synthesis gas atmosphere (E and F). This
13

indicates that the proposed reaction route I is possible. For further confirmation, we
have applied 3-hexenoic acid methyl ester 8 in the reaction to see whether the
saturated ester 4 can be obtained by a two-step reaction sequence. Converting 8
with hydrogen only, led to quantitative formation of hexanoic acid methylester (route
G), however, pure carbon monoxide atmosphere gave exclusively pimelic acid
dimethylester 9 (route H). This compound has not been observed in all foregoing
hydroesterifications with piperylene. Finally, the application of synthesis gas with
hexenoic acid methyl ester resulted in the formation of 56% pimelic acid
dimethylester 9, and 16% hexanoic acid, only. As a consequence, it can be said that
the hydroesterification of piperylene with synthesis gas proceeds via route I owing to
the totally different product distribution on route II.
4. Conclusion
In this contribution we have presented the first three step auto-tandem catalysed
hydroesterification using piperylene as starting material. This provides an efficient
and atom economic access to linear hexanoic acid esters which are used as fruity
odoriferous compounds. With the catalyst system derived in situ from 0.75 mol%
Pd2(dba)3 and the ligand 1,2-dtbpmb the reaction allows up to 79% yield of hexanoic
acid methyl ester. In addition, the reaction tolerates several alcohols, giving access to
different fruity compounds. Finally, we have clarified the course of the reaction
pathway, which proceeds via a three-step reaction cascade, involving hydrogenation,
isomerisation and hydroesterification.
Acknowledgements
Digital Specialty Chemicals (Toronto/Canada) is gratefully acknowledged for donation
of the ligand 1,2-dtbpmb. Umicore AG & Co. KG (Hanau/Germany) is gratefully
acknowledged for donation of palladium precursor.
14

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17

Figure Captions
OR'
OR'
[cat.]
+ CO + HOR'
R
R
+
R
O
O
branched
product
linear
product
Fig.1. General hydroesterification of a monolefin with synthesis gas and an alcohol R’OH.
Hydrogenation
Isomerisation
Hydroesterification
O
H2
CO/MeOH
[Pd]
+
O
[Pd]
[Pd]
3
1
2
3
4
A
B
C
B'
H2/[Pd]
B'
H2/[Pd]
+ small amounts of
branched esters
5
Fig.2. Reaction cascade for the linear hydroesterification of piperylene with synthesis gas and methanol.
18

80
60
40
20
0
pentenes2/3
pentane 5
linear ester 4
branched esters
conversion of
piperylene 1
0.85
0.75
0.65
0.55
0.45
0.35
precursor concentration [mol%]
Fig.3. Influence of the precursor concentration on the hydroesterification of piperylene.a
a: Pd2(dba)3, 1,2-dtbpmb, M:L = 1:5, n(piperylene) = 12.75 mmol, n(methanesulfonic acid) = 1 mmol, T = 130 °C, t
= 5 h, p = 30 bar CO/H2 1:1,10 ml methanol.
conversion of
piperylene 1
branched esters
pentane 5
linearester 4
80
60
40
20
0
20
30
40
50
Synthesis gas pressure [bar]
Fig.4. Variation of synthesis gas pressure.a
a: 0.75 mol% Pd2(dba)3, 1,2-dtbpmb, M:L = 1:4, n(piperylene) = 12.75 mmol, n(methanesulfonic acid) = 8 mol%,
= 110 °C, = 5 h, CO/H2 1:1,10 ml methanol.
T
t
19

100
80
60
40
20
0
linear ester 4
pentane 5
branched esters
piperylene content
0
2
4
6
8
10
12
14
Fig.5 Kinetics of the hydroesterification of piperylene.a
a: 0.75 mol% Pd2(dba)3, 1,2-dtbpmb, M:L = 1:4, n(piperylene) = 12.75 mmol, n(methanesulfonic acid) = 8 mol%,
= 110 °C, = 5 h, = 40 bar CO/H2 1:1,10 ml methanol.
T
t
p
branched
esters
conversion
conversion
of 1
pentane 5
pentane
pentenes
pentenes
linear ester
linear ester
other esters
100
80
60
40
20
0
methanol
ethanol
butanol
isopropanol
Fig.6. Scope of the hydroesterification of piperylene.a
a: 0.75 mol% Pd2(dba)3, 1,2-dtbpmb, M:L = 1:4, n(piperylene) = 12.75 mmol, n(methanesulfonic acid) = 8 mol%,
= 110 °C, = 8 h, = 40 bar CO/H2 1:1,10 ml alcohol.
T
t
p
20

C
, CO/H2, 90%
3
D
, CO, 96%
E
, CO/H2, 86%
2
F
CO, 95%
I
O
A CO/H2, 79%
1
4
B
CO, 0%
O
II
B
CO, 5%
16%
O
G, H2, 100%
8
O
J
, CO/H2
56%
O
H
O
9
CO, 82%
O
O
Fig.7. Investigation of the reaction network.a
a 0.75 mol% Pd2(dba)3, 1,2-dtbpmb, M:L = 1:4, n(olefin) = 12.75 mmol, n(methanesulfonic acid) = 8 mol%,
110 °C, = 8 h, = 40 bar gas as indicated (synthesis gas: 1:1 mixture),10 ml methanol.
T =
t
p
21

Tables
Table 1 Results of primary precursor screening.a
yield [%]
ester 4
conversion
entry
Precursor
lin:br.
branched
esters
[%]
pentenes pentane
1
2
3
4
5
6
7
8
PdCl2
62
28
44
32
46
54
40
61
2
1
0
1
1
4
2
0
14
4
41
20
33
20
10
34
28
47
5
3
4
3
4
6
3
6
8.2:1
6.7:1
8.3:1
6.7:1
2.5:1
5.7:1
9.3:1
7.8:1
Pd(OAc)2
Pd(acac)2
Pd(hfacac)2
Pd/C 10%
[Pd(allyl)Cl]2
Pd(cod)Cl2
Pd2(dba)3
8
9
31
10
7
8
a: 0.75 mol% precursor, 1,2-dtbpmb, M:L = 1:5, n(piperylene) = 12.75 mmol, n(methanesulfonic acid) = 1 mmol,
= 130 °C, = 5 h, = 30 bar CO/H2 1:1,10 ml methanol; lin:br = linear:branched ratio.
T
t
p
22

Table 2 Variation of the M:L-ratio.a
yield [%]
pentenes 2/3 pentane 5 ester 4 branched esters
conversion
entry
M:L ratio
lin:br.
[%]
1
2
3
4
5
1:1
1:3
1:4
1:5
1:6
52
62
70
60
51
1
0
0
0
1
12
10
8
25
14
8
1.8:1
5.5:1
9.3:1
9.4:1
14:1
44
56
47
42
6
8
5
5
3
a: 0.75 mol% Pd2(dba)3, 1,2-dtbpmb, n(piperylene) = 12.75 mmol, n(methanesulfonic acid) = 1 mmol,
= 130 °C, = 5 h, = 30 bar CO/H2 1:1,10 ml methanol; lin:br = linear:branched ratio.
T
t
p
23

Table 3 Variation of the acid.a
yield [%]
conc. conversion
entry
acid
lin:br.
branched
esters
[mol%]
[%]
pentenes 2/3 pentane 5 ester 4
1
2
3
4
5
6
7
pTsOH
TFA
8
8
8
5
3
8
9
49
27
33
66
70
70
44
0
1
1
0
1
0
0
15
17
22
12
11
8
29
7
5
2
3
4
5
6
3
5.8:1
3.5:1
2.3:1
TfOH
MSA
MSA
MSA
MSA
7
50
53
56
12.5:1
10.6:1
9.3:1
8
33
11:0:1
a: 0.75 mol% Pd2(dba)3, 1,2-dtbpmb, M:L = 1:4, n(piperylene) = 12.75 mmol,
1:1,10 ml methanol; lin:br = linear:branched ratio, TsOH =
TfOH = trifluoromethanesulfonic acid, MSA = methanesulfonic acid
T
= 130 °C, t = 5 h, p = 30 bar CO/H2
p
p-toluenesulfonic acid, TFA = trifluoroacetic acid,
24

Table 4 Variation of the temperature.a
yield [%]
temperature conversion
entry
lin:br.
[°C]
[%]
pentenes 2/3 pentane 5 ester 4 branched esters
1
2
3
4
5
6
7
90
62
68
75
76
70
47
21
1
1
0
0
0
0
0
11
6
45
55
63
61
56
30
5
5
6
5
6
6
6
1
9.0:1
9.2:1
12.6:1
10.2:1
9.3:1
5.0:1
5.0:1
100
110
120
130
140
150
7
9
8
11
15
a: 0.75 mol% Pd2(dba)3, 1,2-dtbpmb, M:L = 1:4, n(piperylene) = 12.75 mmol, n(methanesulfonic acid) = 8 mol%,
= 5 h, = 30 bar CO/H2 1:1,10 ml methanol; lin:br = linear:branched ratio.
t
p
25