Palladium-catalyzed alkoxycarbonylation of conjugated dienes under acid-free conditions: Atom-economic synthesis of β,γ-unsaturated esters

Article abstract of DOI:10.1002/anie.201404563

Carbonylation reactions constitute important methodologies for the synthesis of all kinds of carboxylic acid derivatives. The development of novel and better catalysts for these transformations is of interest for both academic and industrial research. Here, a benign palladium-based catalyst system for the alkoxycarbonylation of conjugated dienes under acid-free conditions has been developed. This atom-efficient transformation provides straightforward access to a variety of β,γ-unsaturated esters in good to excellent yields and often with high selectivities. As an industrially relevant example the (formal) synthesis of dimethyl adipate and ε-caprolactam from 1,3-butadiene is demonstrated.

Full text of DOI:10.1002/anie.201404563

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DOI: 10.1002/anie.201404563
Carbonylation
Palladium-Catalyzed Alkoxycarbonylation of Conjugated Dienes
under Acid-Free Conditions: Atom-Economic Synthesis of
b,g-Unsaturated Esters**
Xianjie Fang, Haoquan Li, Ralf Jackstell, and Matthias Beller*
Abstract: Carbonylation reactions constitute important meth-
odologies for the synthesis of all kinds of carboxylic acid
derivatives. The development of novel and better catalysts for
these transformations is of interest for both academic and
industrial research. Here, a benign palladium-based catalyst
system for the alkoxycarbonylation of conjugated dienes under
acid-free conditions has been developed. This atom-efficient
transformation provides straightforward access to a variety of
b,g-unsaturated esters in good to excellent yields and often with
high selectivities. As an industrially relevant example the
(formal) synthesis of dimethyl adipate and e-caprolactam from
1,3-butadiene is demonstrated.
Scheme 1. Synthesis of b,g-unsaturated esters by alkoxycarbonylation
reactions. TM=transition metal.
A
lkene carbonylations are among the most important
homogeneously catalyzed processes in industry.[1] Within
this class of reactions, alkoxycarbonylations, also called
hydroesterifications, represent a straightforward method for
the conversion of olefins, CO, and alcohols into the corre-
sponding esters.[2] In this respect, the transition-metal-cata-
lyzed carbonylation of allylic compounds is of considerable
interest for the synthesis of versatile b,g-unsaturated carbox-
ylic acid derivatives.[3] In the past, effective carbonylation
methods for reactions of allylic carbonates,[4] acetates,[5]
chlorides,[6] amines,[7] ethers,[8] phosphates,[5b,e,9] and alco-
hols[3e,6b,10] have been developed (Scheme 1a). Obviously,
a general drawback of all these reactions is the stoichiometric
generation of by-products. Alternatively, b,g-unsaturated
carboxylic acid derivatives can be synthesized by carbon-
ylation of conjugated dienes (Scheme 1b). Despite the
advantage of this more atom-efficient route, the carbon-
ylation of conjugated dienes has scarcely been explored in
academic laboratories. However, the selective alkoxycarbo-
nylation of 1,3-butadiene is of major industrial interest. This
substrate—produced in about 12 ꢀ 106 metric tons annually—
offers the possibility to produce bulk chemicals like adipic
acid and e-caprolactam via 3-pentenoic acid esters.[11]
In the early 1940s, Reppe first reported the reaction of 1,3-
butadiene to carbonylated vinylcyclohexene derivatives in the
presence of [Co2(CO)8] as a catalyst.[24] Later, Du Pont
reported the methoxycarbonylation of 1,3-butadiene to
methyl pentenoate by using a Co/Cu/Th catalyst at very
high pressure (810 bar).[12] In the late 1960s, Tsuji et al.[13]
described this reaction in the presence of a catalytic amount
of palladium chloride to give ethyl 3-pentenoate. While no
product yield was given in the original paper, Tsuji et al.[13b]
later reported an optimized yield of approximately 30% of
ethyl 3-pentenoate. Matsuda and co-workers also demon-
strated the use of cobalt catalysts in the presence of pyridines
for this reaction.[14] However, only low catalyst turnover
numbers (25–80) were achieved and high CO pressure was
needed. A systematic investigation of the palladium-cata-
lyzed carbonylation of 1,3-dienes was done by Knifton.[15]
Despite variation of different ligands and solvents, mainly 3,8-
nonadienoate esters (telomerization products) were obtained.
A survey of the patent literature reveals significant work on
the palladium-catalyzed methoxycarbonylation of 1,3-dienes
by Shell,[16] Du Pont, and DSM.[11a,17] The latter companies, as
well as Rhone Poulenc[18] disclosed a positive influence of
added acids or quaternary onium salts on selectivity, con-
version, and stability of the palladium catalyst. In addition,
a Shell patent reported that by controlling the polarity of the
reaction medium higher reaction rates can be achieved.[19]
In line with our interest in industrially relevant carbon-
ylation reactions, we performed a systematic study on the
methoxycarbonylation of 1,3-butadiene.[20] Examination of
the influence of different reaction parameters on product
yield and selectivity demonstrated the importance of chelat-
ing phosphine ligands and benzoic acids as additives to get
good results. Until today, basically all of the published catalyst
systems for carbonylation of dienes suffer from drawbacks
such as the need for harsh reaction conditions and/or
[*] X. Fang,[+] H. Li,[+] Dr. R. Jackstell, Prof. Dr. M. Beller
Leibniz-Institut fꢀr Katalyse e. V. an der Universitꢁt Rostock
Albert-Einstein-Str. 29a, 18059 Rostock (Germany)
E-mail: matthias.beller@catalysis.de
[+] These authors contributed equally to this work.
[**] This research was founded by the Bundesministerium fꢀr Bildung
und Forschung (BMBF) and the State of Mecklenburg-Vorpom-
mern. We thank Dr. W. Baumann, Dr. D. Michalik, Dr. C. Fisher,
S. Buchholz, and S. Schareina for their excellent technical and
analytical support. We are grateful to Dr. D. Banerjee for helpful
discussions and providing some 1,3-dienes.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201404563.
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additives, for example, acids, narrow substrate scope, rela-
tively low product yield, and limited selectivity. In this regard,
the development of improved and acid-free catalyst systems
for this reaction is of high importance and constitutes
a challenging and relevant topic for academic and industrial
research.
gas pressure for the model reaction using Xantphos as the
ligand of choice. As shown in Table 1, the yield of 3aa was
significantly affected by the molar ratio of 1a to 2a.
Consequently, as the molar ratio of 1a to 2a increased to
1.2:1, the yield of 3aa increased to 94% (entries 1 and 2).
Lowering the catalyst loading revealed an optimal loading of
Herein, we present an efficient palladium-based catalyst
system for the selective alkoxycarbonylation of conjugated
dienes under relatively mild reaction conditions. Notably, the
various mono-, di-, and tri-b,g-unsaturated esters were
obtained in high yield with good selectivity under acid-free
conditions.
Table 1: Palladium-catalyzed alkoxycarbonylation of isoprene (1a) with
benzyl alcohol (2a): Investigation of reaction conditions.[a]
At the start of this study, we investigated the alkoxycar-
bonylation of isoprene (1a) and benzyl alcohol (2a) as
a model reaction in the presence of [Pd(cod)Cl2] and different
phosphine ligands (L1–L14; Scheme 2). The application of
Entry Catalyst
pCO [bar] T [8C] Yield [%][b] Selectivity[c]
1[d]
2
[Pd(cod)Cl2]
50
50
50
50
50
50
50
50
100
100
100
100
100
100
100
100
100
80
80
94
71
0
0
87
0
91
87
64
92
80
93:7
93:7
94:6
–
–
94:6
–
94:6
93:7
97:3
93:7
88:12
[Pd(cod)Cl2]
[Pd(cod)Cl2]
Pd(OAc)2
3[e]
4
5
6
[Pd(acac)2]
PdCl2
7
8
[Pd(dba)2]
[Pd(CH3CN)2Cl2]
9
[{Pd(cinnamyl)Cl}2] 50
10
11
12
[Pd(cod)Cl2]
[Pd(cod)Cl2]
[Pd(cod)Cl2]
50
40
20
100
100
[a] Reaction conditions: 1a (1.2 mmol), 2a (1.0 mmol), [Pd] (1.0 mol%),
Xantphos (2.0 mol%), CO (50 bar), toluene (2 mL), 1008C, 20 h.
[b] Yield determined by GC analysis using isooctane as the internal
standard. [c] The ratios of isomers were determined by GC analysis.
[d] 1a (1.0 mmol). [e] [Pd] (0.5 mol%), Xantphos (1.0 mol%). acac=
acetylacetonate
1 mol% of [Pd] (entry 3). Several palladium(II) and palla-
dium(0) precursors were also investigated. Interestingly,
when standard palladium precatalysts such as Pd(OAc)2,
[Pd(acac)2], and [Pd(dba)2] were used, no conversion was
observed (entries 4, 5, and 7). However, the use of other
palladium chloride precursors resulted in desired product 3aa
in 87–91% yield with high selectivity (entries 6, 8, and 9). This
crucial effect of chloride ions for the successful alkoxycarbo-
nylation is not yet completely understood. The yield of the
desired ester 3aa significantly decreased when lowering the
reaction temperature (entry 10). Furthermore, lowering the
CO pressure to 40 bar resulted in a similar yield and
selectivity compared to 50 bar of CO, but at 20 bar CO the
product yield and selectivity decreased (entries 11 and 12).
With optimized reaction conditions established (Table 1,
entry 10), we examined the general scope of this acid-free
alkoxycarbonylation process with respect to aliphatic alcohols
(Scheme 3). A variety of substituted primary aliphatic alco-
hols gave the corresponding carbonylative products in
excellent yields with high selectivity. Interestingly, menthol
and heterocyclic alcohols proved to be efficient coupling
partners and gave the corresponding esters (3ag, 3ak, 3al,
and 3am) in excellent yields with good selectivities, too. A
broad range of functional groups is tolerated, including
reactive alkene (3an), alkyne (3ao), and benzyl (3aa)
groups, which provide useful handles for further synthetic
Scheme 2. Palladium-catalyzed alkoxycarbonylation of isoprene (1a)
with benzyl alcohol (2a): Influence of the ligand. Reaction conditions:
1a (1.0 mmol), 2a (1.0 mmol), [Pd(cod)Cl2] (1.0 mol%), monodentate
Ligand (4.0 mol%), bidentate ligand (2.0 mol%), CO (50 bar), toluene
(2 mL), 1008C, 20 h. Yield (Y) determined by GC analysis using
isooctane as the internal standard. The ratios of isomers were
determined by GC analysis. S=selectivity. Ad=adamantyl, cod=
1,5-cyclooctadiene; Cy=cyclohexyl.
the monodentate ligands L3 and L5, did not lead to any
conversion. Notably, using L1, L2, and L4 provided the
desired product 3aa without acid, albeit in low yields. When
commercially available cataCXium PtB L6 was used, the
desired product 3aa was obtained in good yield, however with
low selectivity. Then, commercially available bidentate
ligands were tested. Interestingly, all of bidentate ligands
exhibited no activity in the formation of the desired product
except Xantphos (L12), which was identified as the most
promising ligand to afford the desired product 3aa in good
yield with high selectivity.
Next, to improve the reaction, we evaluated the influence
of critical reaction parameters such as the molar ratio of 1a to
2a, catalyst loading, palladium precursors, temperature, and
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Scheme 4. Palladium-catalyzed alkoxycarbonylation of conjugated
dienes (1) benzyl alcohol (2a). Reaction conditions: 1a (1.2 mmol),
2a (1.0 mmol), [Pd(cod)Cl2] (1.0 mol%), Xantphos (2.0 mol%), CO
(40 bar), toluene (2 mL), 1008C, 20 h. Yield is that of the isolated
product. The ratios of isomers were determined by GC analysis. [a] 1b
(12 mmol), 2a (10 mmol).
Next, we evaluated the scope of conjugated 1,3-dienes
using benzyl alcohol (2a) as a standard coupling partner
(Scheme 4). From an industrial point of view it is important
that 1,3-butadiene furnish the corresponding product (3ba) in
good yield. Furthermore, sterically crowded conjugated
dienes 1c and 1e were smoothly transformed into the
corresponding b,g-unsaturated esters in good yields and
with excellent selectivities (3ca and 3ea). The cyclic con-
jugated diene 1d was also efficiently transformed into the
corresponding cyclic b,g-unsaturated ester in good yield and
with excellent selectivity (3da). Notably, the use of the
renewable diene myrcene (1 f) led to the desired functional-
ized b,g-unsaturated ester in excellent yield though low
selectivity (3 fa).
Considering the importance of diesters and triesters,
which are widely applied in industry as alternative plasti-
cizers,[21] we turned our attention to this class of compounds
by using diols and glycerol as coupling reagents. As shown in
Table 2, the reactions of isoprene (1a) with the open-chain
diol 2q and cyclic diol 2r gave the corresponding products in
very good yields (entries 1 and 2). Even the triester 3as was
obtained in good yield by triple carbonylation of 1a with
glycerol 2s (entry 3). Moreover, the cyclic diene 1d also
proved to be an efficient coupling partner for this dialkoxy-
carbonylation reaction and smoothly transformed into corre-
sponding diesters in good yields (entries 4 and 5).
Scheme 3. Palladium-catalyzed alkoxycarbonylation of isoprene (1a)
with aliphatic alcohols (2). Reaction conditions: 1a (1.2 mmol), 2a
(1.0 mmol), [Pd(cod)Cl2] (1.0 mol%), Xantphos (2.0 mol%), CO
(40 bar), toluene (2 mL), 1008C, 20 h. Yield is that of the isolated
product. The ratios of isomers were determined by GC analysis.
[a] Used [Pd(cod)Cl2] (2.5 mol%), Xantphos (5.0 mol%), 1208C.
transformations. Notably, we demonstrated the utility of our
carbonylation protocol in the reaction of the allylic alcohol
geraniol (2p), an ingredient commonly used in perfumes and
flavors (3ap). Even tertiary alcohols underwent this trans-
formation, although in lower yields, probably because of the
increased steric effect (3ah).
By using the deuterated alcohol 2t instead of MeOH,
a similar yield and selectivity of the desired product was
obtained under optimal reaction conditions [Eq. (1)]. Here,
Finally, we were interested in demonstrating the useful-
ness of our procedure for the synthesis of adipic acid esters
and caprolactam. Hence, the synthesis of 3bb was scaled up to
30 mmol of 1,3-butadiene (1b; Scheme 5). Indeed, 77% yield
of the corresponding b,g-unsaturated ester was obtained.
Subsequent transformation of 3bb gave dimethyl adipate (4)
in high yield with excellent regioselectivity.[22] It should be
noted that this sequence also allows a straightforward prep-
the deuterium atom was found to be incorporated only at the
d-carbon atom of the product 3at, which makes a cyclo-
metallation mechanism more possible. However, we can
exclude a selective insertion of the more sterically hindered
À
double bond of the 1,3-diene into the Pd D bond, which
would also result in 3at.
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Table 2: Palladium-catalyzed di-and trialkoxycarbonylation of 1,3-dienes
(1) with diols and glycerol (2).[a]
unsaturated esters in good yields with often high selectivity.
Compared to previously known procedures the substrate
scope is enhanced and no additives such as acids, which might
cause corrosion problems are needed. Furthermore, we
reported the first catalytic di- and trialkoxycarbonylations
of 1,3-dienes utilizing easily accessible diols and glycerol.
These products are of interest as alternative plasticizers.
Combining the presented procedure with established carbon-
ylation reactions allows an efficient preparation of adipates
and e-caprolactam. We believe these procedures will inspire
chemists to use carbonylation reactions more frequently in
organic synthesis.
Experimental Section
Typical procedure for the preparation of 3: Avial (4 mL) was charged
with [Pd(cod)Cl2] (2.85 mg, 1 mol%), Xantphos (11.6 mg, 2 mol%),
and a stirring bar was added. Then, toluene (2 mL), the conjugated
diene 1 (1.2 mmol) and alcohol 2 (1 mmol) were injected by syringe.
The vial was placed in an alloy plate, which was transferred into an
autoclave (300 mL) of the 4560 series from Parr Instruments under
argon atmosphere. At room temperature, the autoclave was flushed
with CO three times, pressurized with CO to 40 bar, and finally the
pressure was increased to 90 bar by adding nitrogen. The reaction was
performed for 20 h at 1008C. After the reaction finished, the
autoclave was cooled to room temperature and the pressure was
carefully released and isooctane (150 mL) (internal standard) was
added to the solution. The yield and selectivity were measured by GC
analysis. After removing the solvent by vacuum, the residue was
directly purified by flash chromatography on silica gel (eluent:
heptane/ethyl acetate = 20:1) to give the desired product 3.
Received: April 22, 2014
Published online: July 1, 2014
[a] Reaction conditions: 1a (2.4 mmol), 2a (1.0 mmol), [Pd(cod)Cl2]
(1.0 mol%), Xantphos (2.0 mol%), CO (40 bar), toluene (2 mL), 1008C,
20 h. Yield is that of isolated product. The ratios of isomers were
determined by GC analysis. [b] [Pd(cod)Cl2] (5.0 mol%), Xantphos
(10.0 mol%), 1208C. [c] 1a (3.6 mmol), [Pd(cod)Cl2] (5.0 mol%), Xant-
phos (10.0 mol%), 1208C. [d] Reaction temperature: 1208C.
Keywords: carbonylation · conjugation · P ligands · palladium ·
synthetic methods
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