Palladium-Catalyzed Alkoxycarbonylation of sec-Benzylic Ethers

Article abstract of DOI:10.1002/ejoc.201901592

Herein, we report the palladium-catalyzed synthesis of 3-arylpropionate esters starting from secondary benzylic ethers. With this investigation it could be shown that ethers are suitable starting materials in addition to the established carbonylation reactions of olefins, alcohols, or aryl halides.

Full text of DOI:10.1002/ejoc.201901592

A Journal of
Accepted Article
Title: Palladium-Catalyzed Alkoxycarbonylation of sec-Benzylic Ethers
Authors: Carolin Schneider, Ralf Jackstell, Bert Maes, and Matthias
Beller
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10.1002/ejoc.201901592
European Journal of Organic Chemistry
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Palladium-Catalyzed Alkoxycarbonylation of sec-Benzylic Ethers
Carolin Schneider,[a] Ralf Jackstell,[a] Bert U. W. Maes,[b] and Matthias Beller*[a]
Abstract: Herein, we report the palladium-catalyzed synthesis of 3-
arylpropionate esters starting from secondary benzylic ethers. With
this investigation it could be shown that ethers are suitable starting
materials beside the established carbonylation reactions of olefins,
alcohols or aryl halides.
While in the past most industrial and academic efforts were on the
carbonylation of olefins, recently there is an increasing interest in
the synthesis of carboxylic acid derivatives from sec- and tert-
alcohols,[12] partly because of their availability from renewables.
Due to their increased reactivity, benzylic and allylic alcohols have
been investigated mainly in the past. Notable examples for
palladium-catalyzed carbonylation of allylic alcohols were
disclosed by the groups of Alper and Miura as well as ours.[18]
Here, the specific catalyst system in both cases was based on
Pd(OAc)2 and PPh3 and showed superior performance to give
훽, 훾–unsaturated carbonyl compounds. Similar studies regarding
the carbonylation of benzyl alcohol and its analogues have been
achieved. In the latter case, the reactions often proceed through
the corresponding benzyl halides.[19] Furthermore, it is to mention
that Ibuprofen is produced on multi-ton scale from the
corresponding secondary benzylic alcohol via carbonylation.[20]
For both substrates the in situ generated benzylic and allylic
intermediates are highly stabilized, which enables easier C-O
bond cleavage.
Introduction
Homogeneous catalytic carbonylation reactions are known for
more than 80 years. Already in 1938, Otto Roelen discovered the
synthesis of aldehydes from alkenes and syngas
(hydroformylation), while he was trying to transfer the Fischer-
Tropsch synthesis into the industrial scale.[1] Shortly after, Walter
Reppe introduced carbonylations of alkynes and related
chemistry.[2]
Already in early years, these practical methodologies were
successfully introduced into industrial applications and today
especially hydroformylations represent the largest process in
homogeneous catalysis. In addition to aldehydes generated in the
oxo process,[3] methyl methacrylate is industrially realized from
methoxycarbonylation of ethylene in the so-called Lucite α
process.[4] Furthermore, in this context the Monsanto[5] or Cativa[6]
processes have to mentioned, too. In both cases acetic acid is
produced on bulk ton-scale starting from methanol and CO.
Over the years and especially from the academic point of view
various carbonylation reactions (e.g. hydroformylation,[7]
hydroxycarbonylation,[8]
alkoxycarbonylation[9]
or
amino-
carbonylation[10]) became an important tool and a straight-forward
method to produce all kinds of carboxylic acid derivatives. For
these reactions olefins,[11] alcohols[12] or aryl halides[13] constitute
standard starting materials and a large range of products such as
esters, acids, aldehydes, amides, and alcohols can be efficiently
accessed.[14,15]
Among the different types of olefins, terminal[9,11] and internal[16]
as well as sterically hindered and demanding alkenes were
studied.[17] These studies go along with finding ways to improve
the catalyst activity and/or controlling the (regio)selectivity which
Scheme 1: Carbonylation of (a) benzylic alcohol, (b) methyl tert-butyl
ether, and (c) this work, secondary benzylic ethers.
Compared to alcohol carbonylations, similar transformations
applying ethers as starting material are much less explored. A
notable exception is the use of methyl tert-butyl ether (MTBE), an
important industrial bulk chemical and a representative for tert-
ethers (Scheme 1b).[17]
Based on our continuous interest in carbonylation, we became
attracted to explore the palladium-catalyzed carbonylation
reaction of benzyl ethers as starting material (Scheme 1c). Herein,
we show that those sec-ethers allow for efficient preparation of 3-
aryl-propionate. In general, they show comparable performance
to the established carbonylation reactions of olefins, alcohols, and
aryl halides. We believe these findings will broaden the substrate
class scope in carbonylation chemistry.
[a]
[b]
C. Schneider, Dr. R. Jackstell, Prof. Dr. M. Beller
Leibniz-Institut für Katalyse e.V.
Albert-Einstein Straße 29a, 18059 Rostock (Germany)
E-mail: matthias.beller@catalysis.de
Web: www.catalysis.de
Prof. Dr. B. U. W. Maes
University of Antwerp
Groenenborgerlaan 171, 2020 Antwerp (Belgium)
Supporting information for this article is given via a link at the end of
the document.
often is regulated by the corresponding ligand systems.
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10.1002/ejoc.201901592
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Results and Discussion
the tert-butyl substituents by phenyl or 2-pyridyl groups increased
the formation of 2a (4-45%) and 2a‘ (1-8%) significantly. Bidentate
ligand patterns which only contained phenyl substituents like L4
(dppf), L6 (Xantphos), and L9 (DPEphos) led to the highest yields
(46, 67, and 58%, respectively) under these conditions.
Interestingly, the regioselectivities only slightly changed to give
preferentially linear product 2a. It is worthwhile to mention that
bidentate ligands gave improved results compared to the
standard monodentate phosphines.
With these results in hand, we continued the optimization of our
reaction conditions (Pd source, acid, and ligand) (Table 1).
Xantphos (L6) as ligand was first tested in combination with
several palladium precursors, i.e. Pd-halides (Br, Cl) (entry 2),
Pd(dba)2 (entry 3), and Pd(cod)X2 (X = Br, Cl) (entry 4) were
compared. Notably, the use of Pd0 as catalyst precursor lowered
the yield to 36%, even so no reduction of the palladium center
was necessary. In case of tested PdII sources there was no further
influence observed and the yield of the desired product was
around 70% in total.
In the past three years, we developed specific phosphine ligands
with so-called built-in-base function for various palladium-
catalyzed alkoxycarbonylations.[21] Explicit incorporation of tert-
butyl and pyridine substituents on the phosphorous atom of
several bidentate phosphine ligands enhanced the rate of the
nucleophilic attack on the intermediate palladium acyl complex.[22]
Hence, we were interested in the behavior of such ligands in the
carbonylation of ethers.
Table 1. Palladium-catalyzed carbonylation of (1-methoxyethyl)benzene:
Optimization of the reaction conditions.
Scheme 2: Palladium-catalyzed carbonylation of (1-methoxyethyl)benzene:
Investigation of different phosphine ligands. Reaction conditions: 1.0 mmol 1a,
0.5 mol% Pd(OAc)2, 2 mol% L (bidentate) or 4 mol% (monodentate) ligand,
PTSA·H2O (0.4 mmol), 100 l MeOH, 2 mL methylethylketone (MEK), 130 °C,
25 bar CO, 18 h. Yields were determined by GC analysis using hexadecane as
internal standard. Selectivity’s are obtained by GC analysis, but due to the
measurement error range not for yields ≤ 5%.
Reaction conditions: a 1.0 mmol (1-methoxyethyl)benzene, 0.5 mol% palladium
precursor, 2 mol% L6, 0.3 mmol PTSA·H2O, 100 l MeOH, 2 mL MEK, 130 °C,
25 bar CO, 18 h. b 2 mol% L, 0.3 mmol acid, 2 mL toluene. c 100°C. d 160°C. e
10 bar. f 40 bar. Yields were calculated via GC analysis using hexadecane as
internal standard.
At the beginning of our investigations, different Pd(OAc)2 /ligand
combinations were tested for secondary benzylic ether
carbonylation reactions using (1-methoxyethyl)benzene as the
model substrate. In this benchmark reaction, 0.5 mol% Pd(OAc)2
was used as catalyst precursor in the presence of 2 or 4 mol% of
a bi- or monodentate phosphorous ligand, respectively, in
methylethylketone (MEK) as solvent. To ensure complete
esterification 2.5 equivalents of methanol were added. In general,
the mixture was heated at 130 °C for 18 hours at 25 bar of CO
pressure (Scheme 2).
Next, the influence of the palladium loading was studied under our
benchmark conditions. Here, different catalyst concentrations
between 0.25-3.00 mol% were tested. Surprisingly, no significant
influence was detected (yields: 60-70%) (data Tab. S1 ESI). This
behavior could also be observed for different Pd:Xantphos ratios.
While testing ratios between 1 and 6 equivalents, the product yield
varied in-between 60% to 70% (data Tab. S2 ESI). Finally,
variation of the solvent proved to be crucial for the improvement
of the reaction system. While performing the reaction exclusively
in methanol, the ether 1a is not consumed at all. However, in
toluene the yield of 2a and 2a´ could be improved to ≥ 99% in the
presence of L4 (dppf), L6 (Xantphos), and L9 (DPEphos) (Table
This systematic ligand screening pointed out the strong influence
of the substituents at the phosphorous atoms. Independent from
the core structure, ligands with bis-di-tert-butylphosphino groups
resulted in nearly no product formation. A stepwise substitution of
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1, entries 6-8). All further studies were performed using the least
expensive DPEphos (L9) because ester yields were comparable
in the presence of the most active ligands (L4, L6, and L9).
To allow for ether cleavage an acidic co-catalyst has to be added
(entry 9). Among the tested acids, PTSA·H2O exclusively gave
the desired reaction in moderate to excellent yields depending on
the amount, while trifluoroacetic acid and pyridinesulfonic acid did
not show any reaction (Table 1, entries 8-11). In case of
PTSA·H2O, amounts of 0.3-0.4 mmol led to optimal results.
Finally, the dependence of temperature, and pressure were
investigated. While at lower temperature (100°C) or pressure (10
bar) the catalyst activity dropped (entries 12, 14), comparable
product yields were obtained at 160 °C or 40 bar CO (entries 13,
15).
yield. Similarly, diverse benzylic ethers, which are substituted on
the aromatic ring with methyl- (1c-1e), methoxy- (1f-1h), or
chloride (1i-1k) in ortho-, meta- and para-position, respectively,
could be transformed. While the methyl- (2c-2e) and the methoxy-
(2f-2h) substituted products were isolated in good to excellent
yields, the ortho- and meta chloride-substituted compounds 2j
and 2k resulted in nearly no product formation. In contrast, the 1-
chloro-4-(1-methoxyethyl)benzene (1i) showed less inhibition and
yielded 2i in 65%.
Besides, the carbonylation of 1-isobutyl-4-(1-methoxyethyl)-
benzene (1l), which contains the core structure of ibuprofen, and
of 2-methoxy-6-(1-methoxyethyl)-naphthalene (1m), as a part of
the core structure of naproxen, were explored. To our delight, both
relevant substrates were transformed to the desired products (2l
and 2m) and could be isolated in good yields (93 and 90%). %).
In addition, (1-methoxypropyl)benzene was successfully tested
according to the possibility of forming an internal olefin as well as
providing the option of isomerization (SI, S3).
Table 2: Palladium-catalyzed carbonylation of sec-benzylic ethers: Substrate
scope.a
Encouraged by these results and to understand the mechanism
of this carbonylation process, we compared the reactivity of
different benzylic derivatives and styrene under identical reaction
conditions.
Apart
from
our
model
substrate
(1-
methoxyethyl)benzene and the corresponding olefin (styrene), 1-
phenylethanol as well as (1-chloroethyl)benzene were tested in
the presence of the optimal catalyst system at three different
temperatures (Table 3).
Table 3: Comparison of palladium-catalyzed carbonylations of benzylic
derivatives and styrene.
Reaction conditions: 1.0 mmol reaction scale, 0.5 mol% Pd(OAc)2, 2 mol%
DPEphos, PTSA·H2O (0.3 mmol), 100 l MeOH, 2 mL toluene, 25 bar CO, 18 h.
Yields were determined by GC analysis [linear/branched] using hexadecane as
internal standard.
At 130-160 °C the reactivity for all substrates is comparable. In
addition, similar regioselectivities are observed except for the
benzylic chloride, which gave slightly more of the branched ester.
However at lower temperature (100 °C), styrene turned out to be
the most reactive substrate.
a
Unless otherwise noted, all reactions were performed in toluene (1.5 mL) at
130 °C for 18 h with 1 (1.0 mmol), Pd(OAc)2 (1.12 mg, 0.5 mol%), PTSA·H2O
(0.3 mmol), L9 (10.7 mg, 0.02 mmol), 100 l MeOH, and CO (25 bar). The yields
were isolated yields for the linear and branched mixture by column
chromatography and the ratio of isomers was determined by GC analysis
[linear/branched].
b
Non-isolated yields, determined by GC analysis [linear/branched] using
hexadecane as internal standard.
With optimized reaction conditions, the scope of the reaction was
studied (Table 2). The naphthalene analogue 1b of the model
compound was selectively transformed to 2b and isolated in good
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As a final point, the time-dependent conversion of styrene and (1-
methoxyethyl)benzene under similar reaction conditions was
compared. As shown in Figure 1, styrene is fully consumed within
the first 90 min to give 90-95% of the desired ester product
(linear+branched).
carbonylation of the unsaturated carbon-carbon double bond.
The elimination step is rate-determining in this process.
Scheme 3 Reaction pathway for the carbonylation of (1-methoxyethyl)-
benzene under the established reaction conditions.
Conclusions
In conclusion, we have shown that benzylic ethers can be used in
carbonylation reactions analogous to the corresponding alcohols
or styrenes. In the past, such ethers have been scarcely
investigated. We clearly demonstrate that their reactivity in
carbonylations is similar to styrenes, benzylic alcohols and
benzylic halides. Optimal results are obtained with catalyst
systems containing Pd(OAc)2 precatalyst, and DPEphos or dppf
or Xantphos as ligand, and PTSA as acid additive. With this
method, further valorizations of benzylic ethers including
renewable substrates can be envisioned.
Experimental Section
General considerations
All manipulations were carried out under an argon atmosphere using
Schlenk-techniques, unless stated otherwise. All glass devices used for
synthesis were dried and cooled under vacuum before use. Chemicals
were purchased from commercial sources and used as received, if not
stated otherwise. Oxygen-free and dry solvents were prepared by
distillation or using
Technologies.
a solvent purification system by Innovative
Figure
1
Kinetic investigations of styrene (upper) and (1-
methoxyethyl)benzene (lower) under the established conditions (20 mL
toluene at 130 °C with substrate (10.0 mmol), Pd(OAc)2 (0.5 mol%),
PTSA·H2O (3.0 mmol), L9 (2 mol%), 1 mL MeOH, and CO (25 bar)).
Catalytic experiments were performed in 4 mL screw cap vials, closed with
a polytetrafluoroethylene (PTFE)/white rubber septum (Wheaton 13 mm
Septa) and phenolic cap. The connection with the atmosphere was
achieved by a needle. The vials were placed inside a 300 mL Parr
autoclave and stirred with a magnetic stirring bar.
The remaining product is the corresponding methyl ether, which
is observed due to the added methanol in the reaction. Then, the
cleavage of the ether is the limiting reaction step. The whole
process is finished within 180 min. In contrast using 1a, full
conversion needs a longer reaction time (270 min). Interestingly,
in the first phase of the reaction small amounts of styrene can be
observed here, too. This indicates this reaction is proceeding
through styrene as an intermediate.
General ether preparation
All ethers were prepared starting from the corresponding alcohol or the
ketone according to the general procedures.
Procedure 1, reducing the ketone to an alcohol: Reactions took place in a
round bottom flask that had been charged with the ketone (1 eq.) and was
dissolved in THF. Further NaBH4 (2 eq.) was dissolved in water, and this
solution was added to the THF dropwise under stirring at room
temperature. The final solution was stirred for 6h, HCl was added to the
reaction system. To accelerate the phase separation H2O was used and
the aqueous phase was extracted by ether. Organic phases were
combined and anhydrous Na2SO4 was used to remove the residual water.
The desired alcohol was obtained after the solvents were removed under
reduced pressure.
Based on all these studies as well as previous mechanistic
investigations, we propose the following reaction pathway for
the carbonylation of benzylic ethers (Scheme 3). Initially,
acid-catalyzed elimination of methanol produces styrene,
which is an intermediate in this reaction. Then final product
is formed from styrene via the palladium-catalyzed
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Procedure 2 conversion alcohols to ether: Reactions took place in a round
bottom flame-dried Schlenk flask and flushed with argon three times. NaH
(1.3 eq) was added and dissolved in anhydrous THF. The flask was placed
in an ice bath to cool and stirred for 15 minutes. The desired alcohol (1 eq)
was added drop-wise and the solution was stirred for an additional hour.
Then MeI (0.98 eq) was added in a drop-wise fashion at low temperature.
The solution was stirred at room temperature for 6h and the remaining NaH
was quenched with methanol. The product was obtained after extraction
with a water and ether mixture (1:1), drying with Na2SO4 and removing the
solvent under reduced pressure.
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Catalytic experiments
In a typical catalytic experiment, the reaction were performed in 4 mL glass
vials charged with Pd(OAc)2 (0.5 mol%), DPEphos (2.0 mol%) and
PTSA·H2O (0.3 mmol) rapidly weighed in the air. If used, solid substrates
(1 mmol) were also weighed in the air and added into the vial. The
atmosphere in the vial was then changed to argon and 1.5 mL toluene and
100 µl MeOH were added. Next, liquid substrates (1.0 mmol) were added
and the vials were placed in a metal plate inside a 300 mL autoclave. The
reactor was closed and pressurised with nitrogen (about 10 bar), which
was released again. This was carried out two times, before the same
procedure was done three times with CO (about 10 bar). After the last
release the autoclave was pressurised with 25 bar CO and then heated to
130 °C for 18 h inside an aluminium block. At the end of the reaction the
autoclave was placed into an ice bath to cool down and stop the reaction.
Finally, the pressure was released, the reactor flushed with N2 and opened.
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Acknowledgements
[14] M. Beller, Catalytic Carbonylation Reaction, Springer, Berlin, 2006.
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Financial support from Research Foundation Flanders (FWO)
(BioFact Excellence of Science project Grant No. 30902231) and
the Francqui Foundation is gratefully acknowledged.
Conflicts of interest
[17] J. Liu, K. Dong, R. Franke, H. Neumann, R. Jackstell, M. Beller, Chem.
Commun., 2018, 54, 122238-12241.
There are no conflicts to declare.
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Keywords: carbonylation • benzylic ether • palladium • ester •
carbon monoxide
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Carolin Schneider, Dr. Ralf Jackstell,
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Palladium-Catalyzed
Alkoxycarbonylation of sec-Benzylic
Ethers
Benzylic ether carbonylation: An alkoxycarbonylation reaction protocol for the
synthesis of 3-arylpropionate esters starting from sec-benzylic ethers and
demonstration of a comparable reaction behavior to established olefin, alcohol or aryl
halide systems.
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