Efficient Palladium-Catalyzed Alkoxycarbonylation of Bulk Industrial Olefins Using Ferrocenyl Phosphine Ligands

Article abstract of DOI:10.1002/anie.201700317

The development of ligands plays a key role and provides important innovations in homogeneous catalysis. In this context, we report a novel class of ferrocenyl phosphines for the alkoxycarbonylation of industrially important alkenes. A basic feature of our ligands is the combination of sterically hindered and amphoteric moieties on the P atoms, which leads to improved activity and productivity for alkoxycarbonylation reactions compared to the current industrial state-of-the-art ligand 1,2-bis((di-tert-butylphosphino)methyl)benzene). Advantageously, palladium catalysts with these novel ligands also enable such transformations without additional acid under milder reaction conditions. The practicability of the optimized ligand was demonstrated by preparation on >10 g scale and its use in palladium-catalyzed carbonylations on kilogram scale.

Full text of DOI:10.1002/anie.201700317

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201700317
German Edition: DOI: 10.1002/ange.201700317
Ligand Design
Efficient Palladium-Catalyzed Alkoxycarbonylation of Bulk Industrial
Olefins Using Ferrocenyl Phosphine Ligands
Kaiwu Dong, Rui Sang, Xianjie Fang, Robert Franke, Anke Spannenberg, Helfried Neumann,
Ralf Jackstell, and Matthias Beller*
Abstract: The development of ligands plays a key role and
provides important innovations in homogeneous catalysis. In
this context, we report a novel class of ferrocenyl phosphines
for the alkoxycarbonylation of industrially important alkenes.
A basic feature of our ligands is the combination of sterically
hindered and amphoteric moieties on the P atoms, which leads
to improved activity and productivity for alkoxycarbonylation
reactions compared to the current industrial state-of-the-art
ligand
1,2-bis((di-tert-butylphosphino)methyl)benzene).
Advantageously, palladium catalysts with these novel ligands
also enable such transformations without additional acid under
milder reaction conditions. The practicability of the optimized
ligand was demonstrated by preparation on > 10 g scale and its
use in palladium-catalyzed carbonylations on kilogram scale.
T
he synthesis of novel ligands and the corresponding metal
complexes provides the basis for basic innovations in catalysis,
organic synthesis, and the industrial production of numerous
fine and bulk chemicals. Illustrative examples include poly-
merizations, organometallic coupling reactions, hydrogena-
tions, and metathesis. Clearly, most of the homogeneous
processes developed in the chemical industry over the last
decades make use of new ligands and their respective
Figure 1. Representative ligands developed for the palladium-catalyzed
methoxycarbonylation of ethylene.
fact, the industrial alkoxycarbonylation catalyst is based on
t
Pd(OAc)2 and the specific ligand d bpx (L5; Figure 1,
[
3]
bottom). Despite substantial work in industry and aca-
[5]
demia, this ligand is unique regarding its activity and
selectivity. To develop alternative efficient catalysts for such
transformations in a rational manner, we thought that the
improvement of rate-limiting steps of the given catalytic cycle
or the prevention of catalyst deactivation reactions should be
specifically addressed (see the Supporting Information,
Scheme S1). To solve these problems, metal–ligand coopera-
[
1]
catalysts. This is especially true for olefin carbonylation
[2]
reactions, which are conducted on bulk scale. As a repre-
sentative example, methyl methacrylate, a monomer for the
manufacture of resins and plastics, is produced by the Lucite
a-process, which combines ethylene methoxycarbonylation
and subsequent condensation with formaldehyde (Figure 1,
top). This technology was patented in the late 1980s and was
commercialized on multi-100.000 ton scale in 2008.
Nowadays, it is generally accepted that Pd/phosphine
complexes give better results than other metal complexes in
[
6]
tivity has become a powerful tool in recent years. Pioneering
work on this concept was done by Drent and co-workers at
[
3]
[
7]
Shell in the area of alkyne carbonylation. Surprisingly, for
[
2c,8]
the more significant olefin alkoxycarbonylation,
this
[
2c,4]
[9]
alkoxycarbonylation reactions under mild conditions.
In
concept had not been investigated until very recently.
Representative ligands developed for ethylene methoxy-
[
9–13]
carbonylation are depicted in Figure 1.
While monoden-
[
*] Dr. K. Dong, R. Sang, Dr. X. Fang, Dr. A. Spannenberg,
Dr. H. Neumann, Dr. R. Jackstell, Prof. Dr. M. Beller
Leibniz-Institut fꢀr Katalyse e.V. an der Universitꢁt Rostock
Albert-Einstein Strasse 29a, 18059 Rostock (Germany)
E-mail: matthias.beller@catalysis.de
tate phosphines afford esters, most bidentate phosphines give
high-molecular-weight polyketones. However, Drent and co-
[7,11]
[12]
workers
as well as the Cole-Hamilton group success-
fully demonstrated the benefits of sterically hindered and
electron-rich bidentate ligands compared to the conventional
Prof. Dr. R. Franke
Evonik Performance Materials GmbH
Paul-Baumann-Strasse 1, 45772 Marl (Germany)
and
Lehrstuhl fꢀr Theoretische Chemie
Ruhr-Universitꢁt Bochum
PPh -modified Pd catalyst system. Furthermore, the bite
3
angle of the diphosphine backbone is also crucial for the
activity and chemoselectivity of the catalyst. For example, Pd
catalysts based on L1 and L2 sluggishly gave ethylene/CO co-
oligomers and polyketones, while catalysts of L3 and L4 led to
44780 Bochum (Germany)
[11]
methyl propionate with > 97% chemoselectivity. Because
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
http://dx.doi.org/10.1002/anie.201700317.
t
of the success of L5 (d bpx), several groups in industry and
academia have focused on the use and modification of L5 in
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[
5]
recent years. For example, interesting analogues such as L6–
L10 were developed by Pringle and co-workers, and showed
[
13]
comparable activity.
Nevertheless, no significantly
improved activity was realized until very recently when we
[9]
introduced ligand L7.
Owing to its stability, ready availability, and facile
modification, ferrocene is an ideal choice for ligand back-
[14]
bones.
In fact, ferrocene-based ligands have been used
advantageously in a range of transition-metal-catalyzed
[
15]
[16]
[17]
reactions,
including cross-couplings,
Heck reactions,
[
18]
and enantioselective synthesis. Additionally, these ligands
have been found to be useful for several carbonylations of
aryl halides.
ylation of alkenes has only been scarcely investigated. For
example, in 2004, Butler and co-workers reported the syn-
thesis of a range of ferrocenylmethylphosphine ligands and
found that L11 showed comparable activity to L5 in ethylene
[19]
However, their application in the carbon-
[
20]
[21]
methoxycarbonylation. Despite this promising activity, the
tedious synthesis of L11 impeded its further application.
Herein, we report the synthesis of several novel ferrocenyl
ligands and their application in the palladium-catalyzed
alkoxycarbonylation of industrially important olefins
(
Figure 1).
Inspired by our previous work on alkoxycarbonylation,
[9]
we prepared novel ferrocenyl ligands that featured both
sterically hindered substituents and amphoteric moieties on
the P atoms (Figure 2). Following a general procedure,
n
Figure 2. Synthesis of novel ligands with heterocycles on the
P atoms.
ferrocene was lithiated with BuLi in heptane followed by
[
25]
the addition of tert-butyl(2-pyridyl)phosphine chloride. The
crude product was easily purified by crystallization in
methanol to give the pure ligand L15. Notably, the practic-
ability of this method was demonstrated by the synthesis of
L15 on > 15 g scale.
Aside from L15, other ferrocenyl ligands with five- and
six-membered heterocycles, including 2-quinolinyl, 1H-
pyrrol-2-yl, 2-furyl, and 2-thienyl moieties, were prepared.
As depicted in Figure 2, L16–L21 were synthesized in yields
of 46–71% following the general method described above. In
addition, ligand L22 with N,N-dimethylanilinyl groups on the
P atoms was also prepared in 60% yield. Instead of the tert-
butyl group, a 1-adamantyl moiety can also be successfully
incorporated to give L23 in 47% yield. To elucidate the steric
and electronic effects of substituents on the P atoms further
on, ligands L24 and L25 with di(2-pyridyl) and di(1-methyl-2-
imidazolyl) groups were synthesized. Finally, for comparing
related ligands, L26–L28, which are based on different
backbones, and monodentate ligand L29 were prepared in
moderate to high yields. Notably, all of these ligands were new
and fully characterized. In agreement with all the spectro-
scopic data, the molecular structures of ligands L15 and L16
Figure 3. Molecular structures of the palladium complexes a) Pd(L14)-
[22]
[25]
(NMM) and b) Pd(L15)(NMM). Hydrogen atoms are omitted for
clarity. Thermal ellipsoids set at 30% probability. The P-Pd-P bite angle
of Pd(L15)(NMM) (103.317(11)8) is similar to that of Pd(L14)(NMM)
(103.979(18)8). The PdÀP bond lengths of Pd(L15)(NMM) are
2.3305(3) and 2.3286(3) ꢂ while those of Pd(L14)(NMM) are 2.3409(5)
and 2.3215(5) ꢂ, respectively.
With the novel ligands in hand, the methoxycarbonylation
of ethylene was carried out under “process-related” condi-
[
3a]
tions. As shown in Table 1, the industrially used ligand L5
and its analogue L7 gave methyl propanoate (MeP) in
excellent yield after 30 and 10 min, respectively. In contrast,
dppf only led to MeP in moderate yield along with alternating
[
25]
were also confirmed by X-ray crystallography.
Furthermore, single crystals suitable for X-ray crystallog-
raphy of the palladium complexes Pd(L14)(NMM) and
Pd(L15)(NMM) were obtained by combining cyclopenta-
dienyl allyl palladium with the corresponding ligands and
[
20b]
oligomers after 20 h.
The ferrocenyl ligands L12–L14
performed much better, and needed 10 h for complete
conversion. Remarkably, using the new ferrocenyl ligands
L15 and L16 resulted in MeP in quantitative yield within
10 min. When the reaction was carried out with L17, the
desired product was afforded in 92% yield after 20 h.
[25]
N-methyl maleimide (NMM). As shown in Figure 3, both
complexes contain one ligand, which is coordinated to the
metal center through the P atoms with pseudo-square-planar
coordination geometry.
2
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Table 1: Palladium-catalyzed methoxycarbonylation of ethylene: Inves-
[
a]
tigation of various ligands.
[
b]
[b]
[b]
L
Yield [%]
L
Yield [%]
(t)
L
Yield [%]
(t)
[
c]
[c]
[c]
(t)
L5
L7
99 (30 min)
99 (10 min)
50 (20 h)
99 (10 h)
99 (10 h)
L16
L17
L18
L19
L20
L21
L22
99 (10 min)
92 (20 h)
64 (20 h)
72 (20 h)
95 (20 h)
91 (20 h)
4 (20 h)
L23
L24
L25
L26
L27
L28
L29
99 (10 min)
34 (20 h)
3 (20 h)
92 (20 h)
91 (20 h)
0 (20 h)
dppf
L12
L13
L14
L15
99 (10 h)
99 (10 min)
14 (20 h)
[
a] Reaction conditions: Pd(acac) (6.5 mg, 0.04 mol%), bidentate
2
(
0.16 mol%) or monodentate ligand (0.32 mol%), PTSA (61 mg,
0
.6 mol%), CO (30 bar), MeOH (20 mL), ethylene (1.5 g, 53.6 mmol).
[
[
b] Determined by GC analysis using isooctane as the internal standard.
c] Reaction time. acac=acetylacetonate, PTSA=para-toluenesulfonic
acid.
Similarly, the related ligands L18–L21 afforded MeP in
4–95% yield after 20 h. However, low yields or only traces of
6
the desired product were detected when L22, L24, or L25
were used, demonstrating the importance of both sterically
hindered and basic groups on the P atoms. L23 with a
1
-adamantyl group was also demonstrated to be a highly
active ligand. When L26 or L27 was used, the desired product
was afforded in high yield after 20 h. On the other hand,
xanthene-based ligand L28 showed no activity at all. Finally,
with monodentate ligand L29, MeP was observed in only 14%
yield.
Figure 4. Palladium-catalyzed methoxycarbonylation of ethylene with
various ligands a) at 238C and b) at 808C without additional acid.
To compare the efficiency of L5 and the ferrocenyl ligands
more clearly, the methoxycarbonylation of ethylene was
carried out at room temperature under otherwise identical
conditions. As shown in Figure 4a, ligands L15, L16, and L23
notably displayed unprecedented activity and afforded the
desired product in quantitative yield within 3 h (activity
order: L16 > L15 > L23). L16 performed particularly well and
gave almost full conversion within 1 h. Ligand L5 showed
significantly lower activity, and the yield of MeP was < 10%
in this case. L12–L14, which were active at 808C, did not give
the desired product under these milder conditions, which
revealed that both sterically hindered and amphoteric groups
on the P atoms are indispensable.
the catalyst (see Figure S1). These results suggest that the
crucial PdÀH complex in the catalytic cycle can be generated
efficiently under much milder conditions.
Next, we applied our ferrocenyl ligands in the
Pd-catalyzed methoxycarbonylation of propylene under the
optimized reaction conditions (see Figure S2). As expected,
with ligands L15 and L16, propylene was carbonylated at
408C into the desired ester in quantitative yield and high
n-selectivity within 2 h. Again, only trace amounts of the
product were detected with L5 as the ligand. In addition, with
L15, the reaction still worked well at 238C and gave the
desired ester in high yield after 20 h. To the best of our
knowledge, this is the first example of propylene alkoxycar-
bonylation at such low temperatures.
A general drawback of current Pd-catalyzed alkoxycar-
bonylation processes is the requirement for acid cocatalysts,
which might lead to corrosion of the equipment. Indeed, the
C olefins are mainly obtained as coproducts of naphtha
4
combination of PdCl and our ferrocenyl ligands leads to an
cracking to produce ethylene. Hence, the Pd-catalyzed
methoxycarbonylation of Raffinate-1 was performed with
2
[23]
active catalyst system that allows for efficient ethylene
methoxycarbonylation without additional co-acids. As
shown in Figure 4b, L15 performed best in terms of activity
and gave MeP in almost quantitative yield within 1 h. With
ligands L16 and L23, the desired product was also obtained in
high yield after slightly longer reaction times (5 h). To
compare the activity of the corresponding catalysts, L5 was
also investigated, and MeP was detected in < 5% yield.
Encouraged by these results, we next carried out the reaction
under acid- and halide-free reaction conditions. Interestingly,
ligands L5 and L15 at 1208C for 20 h. High yields of C esters
5
were obtained with L15 within 10 h (see Figure S3). On the
other hand, with L5, a long induction period of 10 h was
observed, which was explained by inhibition by 1,3-buta-
[
24]
diene. This result demonstrates the durability of the new
catalyst, which is crucial in industry when dealing with bulk
chemicals with small amounts of impurities.
Finally, the effect of different alcohols was explored in the
alkoxycarbonylation of ethylene. With ligand L15, the
reaction was carried out at 808C in the corresponding alcohol.
a high yield of MeP was detected using Pd(OAc) and L15 as
2
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As illustrated in Figure S5, primary and secondary alcohols
were well tolerated, and the corresponding esters were
afforded in very high yields within 30 min.
thank the analytical team and acknowledge helpful discus-
sions with Prof. Haijun Jiao.
The attack of the nucleophile onto the palladium acyl
complex is believed to be the rate-determining step in Pd- Conflict of interest
[
5]
catalyzed alkoxycarbonylation reactions. We assume that
the pyridyl substituent facilitates this crucial elementary step
by internal deprotonation of the alcohol and subsequent
faster ester formation via hydrogen-bonded intermediates. To
understand the superiority of these novel catalysts, the
apparent activation energies of ethylene methoxycarbonyla-
tion with ligands L5 and L15 were investigated. The concen-
tration of ethylene was calculated by the gas consumption of
ethylene and CO, which was monitored on-line. The reaction
rate was measured at four different temperatures, and the
concentration of ethylene was plotted as a function of time
The authors declare no conflict of interest.
Keywords: carbonylation · ferrocenyl ligands · ligand design ·
olefins · palladium
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obtained free of charge from The Cambridge Crystallographic
Data Centre.
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Final Article published: && &&, &&&&
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Angewandte
Communications
Chemie
Communications
Ligand Design
K. Dong, R. Sang, X. Fang, R. Franke,
A. Spannenberg, H. Neumann,
R. Jackstell, M. Beller*
&&&& — &&&&
Efficient Palladium-Catalyzed
Alkoxycarbonylation of Bulk Industrial
Olefins Using Ferrocenyl Phosphine
Ligands
Very active: A novel class of ferrocenyl
phosphine ligands for the palladium-
catalyzed alkoxycarbonylation of indus-
trially important alkenes is reported.
Compared to currently applied catalysts,
the resulting palladium complexes
showed much higher activity under
milder conditions.
6
www.angewandte.org
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
These are not the final page numbers!