Palladium complexes with N-heterocyclic carbene ligands as catalysts for the alkoxycarbonylation of olefins

Article abstract of DOI:10.1021/om300959f

Palladium catalysts, generated from Pd(OAc)₂ and 2 equiv of N,N-dialkylbenzimidazolium iodide, are effective for the alkoxycarbonylation of olefins in high yields (>90%). Alkoxycarbonylation of 1-hexene in dimethylacetamide is achieved within 24 h at 110 C using 1 mol % catalyst, 1000 psi CO, and ethanol. Reactions can be prepared in air, without the need of an acid additive to produce ethyl 2-methylhexanoate and ethyl heptanoate in approximately a 2:1 ratio.

Full text of DOI:10.1021/om300959f

Note
pubs.acs.org/Organometallics
Palladium Complexes with N‑Heterocyclic Carbene Ligands As
Catalysts for the Alkoxycarbonylation of Olefins
†
Gina M. Roberts, Philip J. Pierce, and L. Keith Woo*
Department of Chemistry, Iowa State University, Ames, Iowa 50011-3111, United States
ABSTRACT: Palladium catalysts, generated from Pd(OAc)2
and 2 equiv of N,N-dialkylbenzimidazolium iodide, are
effective for the alkoxycarbonylation of olefins in high yields
(
>90%). Alkoxycarbonylation of 1-hexene in dimethylaceta-
mide is achieved within 24 h at 110 °C using 1 mol % catalyst,
000 psi CO, and ethanol. Reactions can be prepared in air, without the need of an acid additive to produce ethyl 2-
1
methylhexanoate and ethyl heptanoate in approximately a 2:1 ratio.
INTRODUCTION
Carbonylation reactions have been used to synthesize a wide
variety of valuable polymer and oxygenate material. Interest in
■
1
Pd-phosphine catalysts for polymerization and alkoxycarbony-
lation of olefinic materials was piqued in the early 1980s with
the discovery that cationic palladium-phosphine complexes
could effectively copolymerize α-olefins with CO in the
2
−6
presence of methanol.
Subsequent studies of ligand
influence indicated that bidentate tertiary phosphines favored
polymerization (i.e., generation of polyketones), whereas
monodentate tertiary phosphines favored single-insertion
3,7−10
alkoxycarbonylation products.
While monodentate phos-
phines, such as triphenylphosphine, provided good yields of
methyl propanoate from ethylene, longer chain olefins resulted
in poor regioselectivity with mixtures of branched and linear
1
3,17
phosphines.
To date, very few Pd-NHC-catalyzed carbon-
7
,11
12
ylation reactions have been reported. Examples are limited to
carbonylation of aryl halides, oxidative carbonylation of
phenolic and amino compounds, and copolymerization of
esters.
Pringle,
Improvements by the research groups of Drent,
1
2,13
14,15 16
Tooze,
and Cole-Hamilton have led to new
Pd ligands (Chart 1) that greatly enhanced the linear selectivity
18−22
ethylene with CO.
Herein, we report our preliminary
results for the first known alkoxycarbonylation of olefins using
bis(benzimidazolylidene)palladium complexes (4a−c). These
studies illustrate the systematic differences between NHC and
phosphine ligands in alkoxycarbonylation.
Chart 1. Chelating Phosphine Ligands for
Alkoxycarbonylation of Olefins
RESULTS AND DISCUSSION
■
Simple N-alkyl-substituted benzimidazolium salts and the
corresponding palladium catalysts were readily prepared as
summarized in Scheme 1. Symmetrically substituted salts 3a−c
were synthesized from benzimidazole and 1° or 2° alkyl halides
23
by heating in CH CN with base.
3
Formation of the corresponding benzimidazolylidene com-
plexes was accomplished via adapted procedures by heating
Pd(OAc) and the salt in a minimal amount of DMSO in
2
for alkoxycarbonylation. Although these ligands were chelating,
the enhanced steric bulk of these phosphines effectively
resulted in conversion of terminal and internal C5−C14 olefins
to the corresponding linear ester products (eqs 1 and 2).
In seeking to develop more robust carbonylation catalysts, we
evaluated palladium N-heterocyclic carbene complexes. Use of
NHC ligands avoids the oxygen sensitivity and multistep ligand
syntheses that are often associated with the use of
24,25
air.
The general reaction conditions for Pd-phosphine carbon-
ylation of olefins commonly involve (1) the use of alcohol as
solvent; (2) the addition of strong Brønsted acids as activating
Received: October 12, 2012
Published: February 19, 2013
©
2013 American Chemical Society
2033
dx.doi.org/10.1021/om300959f | Organometallics 2013, 32, 2033−2036

Organometallics
Note
Scheme 1. Synthesis of Benzimidazolium Salts 3a−c and Pd-
NHC Complexes 4a−c
°C and 1000 psi CO with a 4-fold excess of 1-hexene to EtOH
(Table 1, entry 5). These reactions could be prepared in air
using reagents as received from the supplier without reduction
in yield. The transformation of 1-hexene under these conditions
resulted in two major products, ethyl 2-methylhexanoate
(
branched) and ethyl heptanoate (linear) in a 2:1 (b:l) ratio.
Although high yields of ester product were obtained,
modification of the NHC structure as well as the pyridinium
acids was explored for possible improvement of regioselectivity
(
Table 2). Increasing the steric bulk of the N-bound
additives; and (3) the use of phosphine steric bulk to control
product selectivity. Specifically, alkoxycarbonylation with
ligands from Chart 1 was optimized with methanol as the
solvent and methane sulfonic acid (MSA), which presumably
generates an active hydrido-Pd(II)-phosphine com-
Table 2. Effect of Acid and NHC Ligand Structure on
Alkoxycarbonylation
a
4
,7,15,16,26
plex.
However, attempts to carbonylate 1-hexene
entry
[Pd]
acid
yield % (b:l)
with 4a under similar conditions resulted only in catalyst
decomposition, via elimination of the NHC ligands from Pd.
These results led us to examine systematically the Pd-NHC
reaction conditions.
Initially it was assumed that the failure of 4a to carbonylate
hexene under conditions similar to those used for eqs 1 and 2
was due to the strength of MSA and the poor solubility of the
catalyst in alcohol. Therefore we employed a milder acid
1
2
3
4
5
6
7
8
9
4a
4a
4a
4a
4a
4b
4b
4c
4a
4a
4a
4a
4a
4a
PMS
88 (2.0)
94 (1.6)
90 (1.8)
90 (1.6)
83 (1.8)
87 (2.0)
94 (1.6)
84 (1.9)
76 (1.8)
85 (1.7)
81 (1.7)
91 (1.7)
trace
2-PicMS
4-PicMS
LMS
PTF
PMS
LMS
PMS
(
pyridinium mesylate, PMS) and DMA as a cosolvent (Table
MSA
10
11
12
13
14
15
16
17
PTSA
Ph3B
nonanoic acid
ZnCl2
3a
95 (1.0)
21 (1.1)
trace
Pd(OAc)2
Pd(OAc)2
4a
3a
none
none
100 (1.6)
a
Table 1. Optimization of Alkoxycarbonylation Conditions
for Catalyst 4a
Conditions: 14 mmol of EtOH, 56 mmol of 1-hexene, 1 mol % 4a, 15
a
mol % acid, 5 mL of DMA, 1000 psi CO, 24 h.
entry
PMS (mol %)
EtOH/1-hexene
T (°C)
yield % (b:l)
1
2
3
4
5
6
7
5
10
15
10
15
15
15
1:4
1:4
1:4
1:4
1:4
1:1
4:1
100
100
100
110
110
110
110
32 (2.3)
44 (2.4)
58 (2.4)
80 (1.8)
88 (2.0)
substituents of the NHC ligand (4b) or increasing the electron
donation of the NHC backbone (4c) indicated that ligand
structure did not have a substantial influence on yield or
selectivity. Both 4b and 4c maintained similar conversions and
selectivity to those observed for 4a. These results brought into
question the nature of the active species and the lability of the
NHC ligands during catalysis. Analogous concerns with the
lability of the NHC ligand(s) arose for Rh-NHC-catalyzed
hydroformylations and Pd-NHC-catalyzed aryl halide carbon-
b
22 (2.1)
b
6 (2.1)
a
Conditions: 14 mmol of EtOH, 56 mmol of 1-hexene, 1 mol % 4a,
and 5 mL of DMA, 1000 psi CO, 24 h. 14 mmol EtOH, 14 mmol 1-
hexene. 14 mmol of 1-hexene and 56 mmol of EtOH used.
b
c
19
ylations. In examining this issue with our system, it was found
that alkoxycarbonylation did not occur in the absence of NHC
ligands. For example, alkoxycarbonylation was ineffective with
PMS and Pd reagents such as Pd (dba) or Pd(OAc) , under
1
). Under conditions of excess EtOH to 1-hexene (4:1), a 1
mol % loading of 4a was moderately active for carbonylation in
the presence of 15 mol % PMS and DMA. Moreover,
maintaining reagent ratios and increasing the catalyst loading
to 5 mol % did not significantly increase the yield of ester
product, even with prolonged reaction times. Notable yield
increase was seen when the EtOH to hexene ratio was reversed.
It was found that 4a (1 mol %) and PMS (15 mol %) could
achieve overall ester product yields of 88% within 24 h at 110
2
3
2
similar conditions that gave high conversion with 4a. However,
Pd(OAc)2 became active for carbonylation (32% yield of
products) in the presence of 3a (15 mol %), producing a nearly
1:1 ratio of branched to linear products. Catalyst 4a also
remained active when PMS was replaced with a similar loading
of dimethylbenzimidazolium iodide (3a), resulting in a 95%
yield of products and an improved linear selectivity (b:l = 1:1).
2
034
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Organometallics
Note
The presence of Pd was necessary, as salt 3a alone was inactive
for carbonylation in the absence of a palladium source.
Table 4. Alkoxycarbonylation of Various Olefins with
Catalyst 4a
a
Improvement of the linear selectivity by replacing the acid
additive PMS with 3a suggested that altering the acid source
might be key to catalytic performance (Table 2). Initially, the
influence of the pyridinium acid structure was explored. Neither
the presence of a single methyl group on the pyridinium ring
entry
substrate
1-pentene
acid
yield % (b:l)
1
2
3
4
5
6
7
8
PMS
85 (1.8)
99 (1.6)
100 (1.6)
16
1-pentene
1-octene
cyclohexene
cyclohexene
ethyl 4-pentenoate
styrene
PMS
(
2-picolinium, 2-PicMS; 4-picolinium, 4-PicMS) nor the
83
inclusion of 2,6-methyl groups (lutidinium, LMS) resulted in
a significant change in product selectivity. It was also noted that
similar yield and selectivity were maintained if the mesylate
anion was switched to triflate. Therefore, the range of acids was
expanded to include Lewis acids (ZnCl , Ph B), nonanoic acid,
100 (2.1)
b
PMS
PMS
27 (2.2)
b
ethyl acrylate
31
a
Conditions: 14 mmol of EtOH, 56 mmol of olefin, 1 mol % 4a, 15
mol % PMS, 5 mL of DMA, 1000 psi CO, 24 h. Large amount of
polymer product was detected.
2
3
b
and sulfonic acids (MSA; p-toluene sulfonic acid, PTSA), none
of which provided an improvement to selectivity or yield.
Surprisingly, reevaluation of the catalytic system revealed that
quantitative alkoxycarbonylation could be obtained without any
acid additive.
Since the optimized system was not dependent upon acid for
catalysis, further evaluation of the importance of the cosolvent
was undertaken. Addition of DMA was key to achieving high
carbonylation yields. Yields of 8% or less occurred at 110 °C
after 3 d in the absence of DMA. However, use of DMA also
produced small amounts (<3%) of side products, N,N-
dimethylheptanamide and N,N-dimethyl-2-methylhexanamide,
which resulted from addition of DMA to hexene. To eliminate
side product formation, other solvents were evaluated. Use of
improved over systems that included an acid additive such as
PMS. When using compounds containing a vinylic functionality
(e.g., styrene and ethyl acrylate), a large amount of polymeric
material was formed.
SUMMARY
■
We have shown that Pd benzimidazolylidene complexes are
robust and high-yielding alkoxycarbonylation catalysts in the
absence of acid additives. While many reports indicate analogies
between phosphines and NHC ligands, it is evident from our
preliminary work that Pd-NHC alkoxycarbonylation systems
are quite disparate from those of Pd-phosphines. The nature of
the active Pd intermediates remains unclear, but there is strong
evidence that the NHC is vital to reactivity. Future studies will
focus on identifying catalytic intermediates in hopes of better
understanding the systematic and mechanistic differences
between Pd-NHC and Pd-phosphine complexes as well as
developing the scope and applicability of these catalysts in a
variety of carbonylation reactions.
THF or CH CN as cosolvent resulted in a lower overall yield,
3
but an improvement in linear selectivity and reduction in side
products (Table 3).
EXPERIMENTAL SECTION
General Considerations. All reagents and solvents were of
■
Pd-NHC-catalyzed alkoxycarbonylation of additional olefins
generally resulted in quantitative conversion of terminal olefins
to ester product (Table 4). It is also important to note that
alkoxycarbonylation of internal olefins was dramatically
reagent grade or higher and were used as received. 4,5-
2
7
Dimethylbenzimidazole and 1,3-diisopropylbenzimidazolium bro-
2
3
mide were synthesized following previously reported procedures.
Benzimidazolium salts (3a−c) and the corresponding catalysts (4a−c)
were synthesized without rigorous exclusion of air and water. The
carbonylation reactions were prepared in open air prior to sealing and
pressurizing the reactor. Routine NMR spectra were recorded on a
Varian VXR-400 spectrometer. Gas chromatography was performed
on an HP-6890 instrument fitted with a HP-5 capillary column (30 m
length, 0.25 mm internal diameter, 0.25 μm film thickness).
Table 3. Effect of Cosolvent on Alkoxycarbonylation with
a
Catalyst 4a
entry
acid
EtOH/1-hexene
solvent
DMF
yield % (b:l)
1
2
3
4
5
6
7
8
9
PMS
PMS
none
PMS
none
PMS
none
PMS
none
PMS
MSA
none
1:4
1:4
1:4
1:4
1:4
1:4
1:4
1:4
1:4
4:1
4:1
4:1
trace
Synthesis and Characterization of Benzimidazolium Salts 3a
2
3
CH CN
78 (1.4)
59 (1.4)
27 (1.0)
30 (1.1)
3
and 3c. As adapted from published reports, a flask was charged with
CH CN
CH CN (10 mL), benzimidazole (8.5 mmol), and K CO (1.18g, 8.5
3
3
2
3
THF
THF
−
mmol). MeI (4.0 equiv, 2.1 mL, 33.7 mmol) was added, and the
reaction was heated at reflux for 18 h. After reaction completion, all
volatiles were removed in vacuo. The residue was washed with
CH Cl , and the filtrates were collected. CH Cl was removed in
b
3 (1.0)
b
2
2
2
2
−
8 (1.1)
vacuo, resulting in a sticky residue. Removal of the remaining
benzimidazole and isolation of a solid product were achieved by
sonicating the residue with EtOAc (3 × 5 mL), decanting after each
sonication step. The solid was then filtered, and washed with EtOAc.
The white powder was dried under vacuum for at least 24 h before use.
DMA
DMA
DMA
DMA
DMA
89 (1.8)
100 (1.6)
c
1
1
1
0
1
2
6 (2.1)
c
7 (1.8)
c
28
10 (1.8)
Yield: 85−91%. NMR analysis for 3a matched published values.
1
1
,3,4,5-Tetramethylbenzimidazolium iodide (3c): H NMR (400
a
Conditions: 14 mmol of EtOH, 56 mmol of 1-hexene, 1 mol % 4a, 15
MHz, CDCl ) δ 10.92 (s, 1H, NCHN), 7.43 (s, 2H, Aryl-H), 4.20
3
b
mol % acid, 5 mL of DMA, 1000 psi CO, 24 h. Reaction ran in the
absence of DMA for 72 h. 14 mmol of 1-hexene and 56 mmol of
(s, 6H, NCH ), 4.29 (s, 6H, aryl-CH ).
3
3
c
Synthesis and Characterization of Benzimidazolium Salt 3b.
EtOH used.
After dissolving 1,3-diisopropylbenzimidazolium bromide (1.0 g, 3.5
2
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dx.doi.org/10.1021/om300959f | Organometallics 2013, 32, 2033−2036

Organometallics
Note
mmol) in acetone (5 mL), NaI (0.6 g, 4.0 mmol) was added. A white
precipitate immediately formed, and the solution was filtered through
Celite, washing with small portions of acetone. The filtrate was
collected and dried in vacuo. The resulting residue was washed with
dichloromethane, filtering again through Celite, and the filtrate was
dried in vacuo. Removal of the remaining benzimidazole and isolation
of a solid product were achieved by sonicating the residue with EtOAc
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(
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product yields.
AUTHOR INFORMATION
■
*
Corresponding Author
E-mail: kwoo@iastate.edu.
Notes
The authors declare no competing financial interest.
†
ERC CBiRC REU student.
ACKNOWLEDGMENTS
■
We thank the NSF for partial support of this work through
ERC award EEC-0813570 (the Center for Biorenewable
Chemicals, CBiRC, at Iowa State University) and CHE-
0
809901.
2
036
dx.doi.org/10.1021/om300959f | Organometallics 2013, 32, 2033−2036