Palladium-catalyzed decarbonylative dehydration of fatty acids for the production of linear alpha olefins

Article abstract of DOI:10.1002/adsc.201301109

A highly efficient palladium-catalyzed decarbonylative dehydration reaction of carboxylic acids is reported. This method transforms abundant and renewable even-numbered natural fatty acids into valuable and expensive odd-numbered alpha olefins. Additionally, the chemistry displays a high functional group tolerance. The process employs a low loading of palladium catalyst and proceeds under solvent-free and relatively mild conditions.

Full text of DOI:10.1002/adsc.201301109

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DOI: 10.1002/adsc.201301109
Palladium-Catalyzed Decarbonylative Dehydration of Fatty Acids
for the Production of Linear Alpha Olefins
a
a
a
a
Yiyang Liu, Kelly E. Kim, Myles B. Herbert, Alexey Fedorov,
a,
a,
Robert H. Grubbs, * and Brian M. Stoltz *
The Warren and Katharine Schlinger Laboratory for Chemistry and Chemical Engineering, Division of Chemistry and
Chemical Engineering, California Institute of Technology, 1200 E. California Blvd, MC 101-20, Pasadena, CA 91125, USA
Fax : (+1)-626-395-8436; e-mail: rhg@caltech.edu or stoltz@caltech.edu
a
Received: December 9, 2013; Published online: January 13, 2014
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201301109.
Abstract: A highly efficient palladium-catalyzed de-
carbonylative dehydration reaction of carboxylic
acids is reported. This method transforms abundant
and renewable even-numbered natural fatty acids
into valuable and expensive odd-numbered alpha
olefins. Additionally, the chemistry displays a high
functional group tolerance. The process employs
a low loading of palladium catalyst and proceeds
under solvent-free and relatively mild conditions.
numbered olefins are largely inaccessible and prohibi-
[
5]
tively expensive. These odd-numbered olefins are
valuable building blocks in the synthesis of various
fine chemicals such as lepidopteran insect phero-
mones, but are currently far too costly to be practi-
[
6]
cal. Therefore, the development of an efficient and
economic process for fatty acid decarbonylative dehy-
dration is highly desirable.
Many strategies to convert fatty acids to alpha ole-
fins have been pursued. Lead tetraacetate-mediated
[7]
Keywords: decarbonylative dehydration; fatty
acids; large-scale process; linear alpha olefins; pal-
ladium; renewable feedstocks; Z-selective metathe-
sis
oxidative decarboxylation is a classical method. Al-
ternative protocols that avoid stoichiometric toxic re-
agents have also been developed, such as Kolbe elec-
[
8]
trolysis and silver-catalyzed oxidative decarboxyla-
[
9]
tion. However, these reactions proceed through
highly reactive radical intermediates, and thus suffer
from low yields due to many side reactions. A more
recent approach entails transition metal-catalyzed de-
Linear alpha olefins represent an important class of carbonylative dehydration of fatty acids. Various tran-
[
10]
[11]
industrial chemicals with a wide range of applications. sition metals including rhodium, iridium, palladi-
[
12]
[13]
They are used as co-monomers for ethylene polymeri- um, and iron have been shown to catalyze decar-
[1]
zation as well as precursors to plasticizers, lubri- bonylative dehydration reactions. To date, palladium
[
2]
cants, and surfactants. Currently, these olefins are has demonstrated the highest activity, and catalyst
[
3]
mainly produced by oligomerization of ethylene
loadings as low as 0.01 mol% have been reported in-
[12a] [12b]
which, in turn, is derived from petroleum. As the dependently by Miller
and Kraus
(Scheme 1A).
worldꢀs oil reserves continue to diminish, the develop- Unfortunately, their methods require very high tem-
ment of renewable feedstocks for the production of peratures (230–2508C). In addition, it is necessary to
alpha olefins will become increasingly important. One distill the olefin product from the reaction mixture as
obvious choice is ethylene from biomass-derived etha- soon as it is formed in order to prevent double bond
[4]
nol. A potentially more direct method is the decar- isomerization, and therefore only volatile olefins can
bonylative dehydration of long-chain fatty acids. The be produced this way. Decarbonylation processes
latter route is particularly attractive because fatty under milder conditions have been developed inde-
[
12c]
[12d]
acids are inexpensive and readily available starting pendently by Gooßen
and Scott
(Scheme 1B).
materials derived from many natural sources. Since Although their reactions proceed at 1108C, much
natural fatty acids contain an even number of carbon higher palladium catalyst loading (3 mol%) and an
atoms, their corresponding alpha olefins will be odd- expensive, high-boiling-point solvent (DMPU) are re-
numbered after decarbonylative dehydration. More- quired. We envisioned that by judicious choice of the
over, conventional ethylene oligomerization processes ligand set for palladium and other parameters, the
[
3]
deliver only even-numbered alpha olefins, and odd- most advantageous aspects of these two systems could
130
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Palladium-Catalyzed Decarbonylative Dehydration of Fatty Acids
Scheme 1. Palladium-catalyzed decarbonylative dehydration. (A) High temperature processes (Miller, Kraus). (B) Low tem-
perature processes (Gooßen, Scott). (C) This research.
be combined. Herein we report a palladium-catalyzed in yield (90%; entry 8). Finally, we examined
decarbonylative dehydration using low catalyst load- a number of protic additives (entries 9–12), and found
ing under relatively mild and solvent-free conditions that (t-Bu) biphenol gave the optimal overall perfor-
4
to produce alpha olefins in good yield and high selec- mance to furnish the highest yield of alpha olefin
tivity (Scheme 1C).
(entry 12).
At the outset of our study, we examined the palla-
One major limitation of using pre-formed stearic
dium-catalyzed decarbonylative dehydration of stearic anhydride as the substrate is that only half of the mol-
acid 1a in neat acetic anhydride as the dehydrating ecule can be converted to the olefin while the other
agent (Table 1). A preliminary survey of phosphine li- half is wasted as stearic acid, so the maximum theo-
gands revealed Xantphos to be an optimal and unique retical yield based on the acid is 50%. Control experi-
ligand for the transformation (entries 1–4). It is be- ments also revealed that the build-up of acid in the
lieved that acetic anhydride converts the stearic acid reaction mixture was responsible for olefin isomeriza-
into stearic anhydride in situ, which then undergoes tion and erosion of alpha selectivity. We envisioned
oxidative addition by Pd(0) to initiate the catalytic that portionwise addition of acetic anhydride to the
[12]
cycle. Considering that acetic anhydride itself could reaction mixture and distillation of acetic acid in situ
compete with stearic anhydride for oxidative addition could transform stearic acid back into its anhydride,
at the metal center, we sought to simplify the system thereby reducing acid concentration and increasing
by using pre-formed stearic anhydride 1a’ alone. To olefin yield and selectivity (Table 2). When two por-
our surprise, the decarbonylation of neat stearic anhy- tions of acetic anhydride were added to the reaction
dride with the same catalyst was exceedingly slow mixture with immediate distillation of acetic acid,
(
only 12% yield, 120 TON, in 2 h; entry 5). Compar- once every 1.5 h, the olefin product was obtained in
ing the two systems, we found that the former had 69% yield based on stearic acid and 62% selectivity
one equivalent of acid in it, while the latter was acid- (entry 1). Furthermore, when three portions of acetic
free. Thus, we posited that acid might play a role in anhydride were added, once every hour, the olefin
promoting reactivity. Consequently we added 1 mol% product was obtained in 67% yield and 86% alpha se-
isophthalic acid to the system, and the yield rose to lectivity (entry 2). Finally, when six portions of acetic
22% (entry 6). When the ligand-to-metal ratio was re- anhydride were added, once every half hour, the
duced from 4:1 to 1.2:1, the yield dramatically in- olefin product was obtained in 67% isolated yield and
creased to 92%, but the selectivity dropped to 31% 89% alpha selectivity (entry 3). The selectivity trend
(
entry 7). Since it has been shown previously that tri- clearly shows that the more frequently acetic anhy-
[
12a]
phenylphosphine can inhibit olefin isomerization,
dride is added to dehydrate stearic acid, the higher is
we used PdCl ACHTUNGTRENNUNG( PPh ) instead of PdCl AHCUTNGERNNUG( nbd) (nbd= the alpha selectivity. This is a remarkable result in
2 3 2 2
norbornadiene), and we were delighted to observe an that it is possible to maintain high selectivity without
increase in selectivity to 54% with negligible erosion
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131

Yiyang Liu et al.
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[a]
Table 1. Effects of catalyst, ligand, and additive.
[
[
[
a]
b]
c]
Conditions: A) 1 equiv. 1a (5 mmol), 2 equiv. Ac O; B) 1 equiv 1a’ (5 mmol).
Determined by H NMR with methyl benzoate as internal standard. Alpha=2a/
YꢂA=YieldꢂAlpha.
2
1
A
H
U
G
R
N
U
G
having to distill the olefin product out of the reaction sidering the significant thermodynamic driving force
mixture as it forms. for isomerization into conjugation with the aromatic
With the optimized conditions in hand, we explored ring. Carboxylic acids with a- or b-substituents were
the decarbonylative dehydration of a variety of car- considerably less reactive (entries 14–16). Neverthe-
boxylic acid substrates, as shown in Table 3. Common less, catalyst turnovers up to 400 could be achieved
saturated fatty acids with carbon numbers from 12 to for these challenging substrates. Compared to previ-
[
12a]
[12b]
18 all provided the corresponding olefin in good yield ous reports by Miller
and Kraus,
our reaction
and high alpha selectivity (entries 1–4). In particular, exhibits a much broader scope, does not require distil-
volatile olefins were formed with exceptionally high lation of the olefin products to maintain high selectiv-
selectivities (entries 3 and 4). Terminally functional- ity, and is compatible with various heteroatom-con-
ized fatty acids were also competent substrates, and taining functional groups.
functional groups such as esters, chlorides, imides,
Since this process requires no solvent and low cata-
silyl ethers, ketones, terminal olefins, and substituted lyst loading, it can be readily scaled up (Scheme 2). In
aromatics were all well tolerated (entries 5–13). Nota- a laboratory setting, a 100-mmol scale (28.4 g stearic
bly, allylbenzene derivative 2k was formed with 91% acid or 23.0 g 10-acetoxydecanoic acid) decarbonyla-
alpha selectivity (entry 11), which is impressive con- tive dehydration was easily carried out in a 100-mL
132
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Adv. Synth. Catal. 2014, 356, 130 – 136

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Palladium-Catalyzed Decarbonylative Dehydration of Fatty Acids
[a]
Table 2. Portionwise addition of acetic anhydride.
In summary, we have developed a highly efficient
palladium-catalyzed decarbonylative dehydration pro-
cess that converts carboxylic acids (e.g., fatty acids) to
linear alpha olefins in good yield and with high selec-
tivity. The reaction requires low palladium catalyst
loading and proceeds under solvent-free and relative-
ly mild conditions. In situ distillation of the olefin
product is not necessary, and a wide range of func-
tionalized and unfunctionalized carboxylic acids can
be transformed into their corresponding olefins. Pro-
cess development for large-scale production is under-
way and will be reported in due course.
Experimental Section
[
[
a]
b]
General Procedure
2
0 mmol 1a.
1
Determined by H NMR (isolated yield in parentheses).
A 15-mL round-bottom flask was charged with PdCl ACHTUNGTRENNUNG( PPh )
2 3 2
(
0.01 mmol,
0.05 mol%),
Xantphos
(0.012 mmol,
0
.06 mol%), (t-Bu) biphenol (0.1 mmol, 0.5 mol%), and
4
fatty acid substrate (20 mmol, 1 equiv.). The flask was
equipped with a distillation head and a 25-mL round-
bottom receiving flask. The closed system was connected to
a vacuum manifold, equipped with a needle valve and a digi-
tal vacuum gauge. The system was evacuated and refilled
with N2 (ꢂ3), and the first portion of acetic anhydride
round-bottom flask. Compared with small-scale reac-
tions, the large-scale ones afforded products in similar
yields and slightly higher alpha selectivity. When the
olefin is sufficiently volatile, it is distilled out together
with the acetic acid (Table 3, entries 3–6, 11, and 13).
Although distillation of olefin is not necessary in
(
20 mmol, 1 equiv.) was added via syringe through the
order to maintain high selectivity (e.g., Scheme 2A), septum that seals the top of the distillation head. The flask
it is convenient to do so for volatile olefins was lowered into an oil bath at 208C and gradually heated
(
Scheme 2B).
to 1328C in 23 min. When the oil bath temperature had
risen to 1228C, the needle valve was closed, switched to
vacuum, and the needle valve carefully and slowly opened
to allow distillation of acetic acid into a receiving flask,
which was cooled to À788C. When the oil bath temperature
reached 1308C, time was recorded as t=0. After distillation
ceased (about t=3 min), the needle valve was opened fully
and a vacuum of 1–5 mmHg was drawn. At t=30 min, the
The olefin products thus obtained are important
building blocks in chemical synthesis. For example, 8-
nonenyl acetate (2f) is a precursor to insect phero-
mones 4a (oriental fruit moth pheromone) and 4b
[
14]
[
15]
(
figure-of-eight moth pheromone). Aided by Z-se-
lective cross-metathesis catalyst 5, olefin 2f reacts
with 1-pentene or 1-hexene to afford pheromones 4a
system was refilled with N , and the second portion of acetic
2
or 4b in 58% and 48% yields, respectively, and >98% anhydride (2.8 mmol, 0.14 equiv.) was added via syringe.
[
6b]
Z-selectivity (Table 4).
The system was then gradually (t=35 min) resubjected to
Scheme 2. Large-scale decarbonylative dehydration of stearic acid and 10-acetoxydecanoic acid.
Adv. Synth. Catal. 2014, 356, 130 – 136
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133

Yiyang Liu et al.
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[a]
Table 3. Substrate scope study.
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Palladium-Catalyzed Decarbonylative Dehydration of Fatty Acids
[a]
Conditions: 20 mmol 1, 6 portions of Ac O, 1 +0.14+0.12+0.10+0.09+0.08 equiv.,
2
added every 30 min.
[
[
[
[
[
b]
c]
d]
e]
f]
Isolated yield (column chromatography).
1
Determined by H NMR.
Purified by distillation.
18.5 mmol 1e.
PdCl (nbd) (0.05 mol%), PPh (0.05 mol%), Xantphos (0.06 mol%), 1.5 h, 3 portions of
AHCTUNGTRENNUNG
2
3
Ac O (1+0.15+0.10 equiv.).
Single isomer observed.
2
[
[
g]
h]
PdCl ACHTNUGTRENN(NGU PPh ) (0.25 mol%), Xantphos (0.30 mol%), (t-Bu) biphenol (1 mol%), 2n:3n=
2
3
2
4
4
2
9:51.
-Methyldecanoic anhydride (10 mmol), no Ac O, PdCl AHCTUNTGERNNUNG( nbd) (1 mol%), Xantphos
2
[i]
2
(
1.1 mol%), salicylamide (2 mol%), 1608C, 10 mmHg distillation, 10 h, 3p:2p=73:27.
Table 4. Synthesis of pheromones 4a and 4b from olefin 2f.
tech Center for Catalysis and Chemical Synthesis, and Cal-
tech for financial support. Materia, Inc. is thanked for the
generous donation of ruthenium catalysts. Dr. David Vander-
Velde is acknowledged for NMR spectroscopy assistance. Dr.
Mona Shahgholi and Naseem Torian are acknowledged for
high-resolution mass spectrometry assistance. Dr. Scott Virgil
is acknowledged for helpful discussions.
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