Transformation of Unsaturated Fatty Acids/Esters to Corresponding Keto Fatty Acids/Esters by Aerobic Oxidation with Pd(II)/Lewis Acid Catalyst

Article abstract of DOI:10.1021/acs.jafc.7b02017

Utilization of renewable biomass to partly replace the fossil resources in industrial applications has attracted attention due to the limited fossil feedstock with the increased environmental concerns. This work introduced a modified Wacker-type oxidation for transformation of unsaturated fatty acids/esters to the corresponding keto fatty acids/esters, in which Cu²⁺ cation was replaced with common nonredox metal ions, that is, a novel Pd(II)/Lewis acid (LA) catalyst. It was found that adding nonredox metal ions can effectively promote Pd(II)-catalyzed oxidation of unsaturated fatty acids/esters to the corresponding keto fatty acids/esters, even much better than Cu²⁺, and the promotional effect is highly dependent on the Lewis acidity of added nonredox metal ions. The improved catalytic efficiency is attributed to the formation of heterobimetallic Pd(II)/LA species, and the oxidation mechanism of this Pd(II)/LA catalyst is also briefly discussed.

Full text of DOI:10.1021/acs.jafc.7b02017

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Article
Transformation of Unsaturated Fatty Acids/Esters to Corresponding Keto
Fatty Acids/Esters by Aerobic Oxidation with Pd(II)/Lewis Acid (LA) Catalyst
Ahmed M Senan, Sicheng Zhang, Miao Zeng, Zhuqi Chen, and Guochuan Yin
J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.7b02017 • Publication Date (Web): 18 Jul 2017
Downloaded from http://pubs.acs.org on July 19, 2017
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Journal of Agricultural and Food Chemistry
1 Transformation of Unsaturated Fatty Acids/Esters to
2 Corresponding Keto Fatty Acids/Esters by Aerobic Oxidation with
3 Pd(II)/Lewis Acid (LA) Catalyst
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Ahmed M. Senan, Sicheng Zhang, Miao Zeng, Zhuqi Chen, Guochuan Yin *
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School of Chemistry and Chemical Engineering, Key Laboratory of Material Chemistry for
Energy Conversion and Storage (Huazhong University of Science and Technology), Ministry of
Education, Hubei Key Laboratory of Material Chemistry and Service Failure, Huazhong
University of Science and Technology, Wuhan 430074, PR China.
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*Corresponding author (Tel: +86 027 8755 9436; E-mail: gyin@hust.edu.cn)
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ABSTRACT: Utilization of renewable biomass to partly replace the fossil resources in industrial
applications has attracted attention due to the limited fossil feedstock with the increased
environmental concerns. This work introduced a modified Wackerꢀtype oxidation for
transformation of unsaturated fatty acids/esters to the corresponding keto fatty acids/esters, in
which Cu2+ cation was replaced with common nonꢀredox metal ions, that is, a novel Pd(II)/Lewis
acid (LA) catalyst. It was found that adding nonꢀredox metal ions can effectively promote Pd(II)ꢀ
catalyzed oxidation of unsaturated fatty acids/esters to the corresponding keto fatty acids/esters,
even much better than Cu2+, and the promotional effect is highly dependent on the Lewis acidity
of added nonꢀredox metal ions. The improved catalytic efficiency is attributed to the formation
of heteroꢀbimetallic Pd(II)/LA species, and the oxidation mechanism of this Pd(II)/LA catalyst is
also briefly discussed.
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KEYWORDS: biomass valorization; unsaturated fatty acid; oxidation; Lewis acid;
palladium(II)
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INTRODUCTION
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With the rapid depletion of fossil feedstock, the exploitation of renewable biomass to partly
replace the fossil resources as the carbon source for chemical industry has attracted much
attention.1,2 One classic example is the extensive addition of biofuel in gasoline, in which biofuel
is produced through transesterification of plant oils with methanol to the corresponding methyl
esters.3 Because of the renewable availability and biodegradability, not only utilization as
biofuel, these plant oils, for example, vegetable oils have also been employed in the preparation
of drying oils for paints and coatings after transformation to conjugated derivatives,4ꢀ6 and
employed as the monomer of biopolymers after epoxidation.7ꢀ10 Because of the unsaturated C=C
bond in plant oils, utilization of unmodified vegetable oils as lubricants generally faces the aging
challenge. Accordingly, elimination of the C=C double bond through ketonization or formation
of chloro alkyoxy derivatives was reported to improve the quality of vegetable oil based
lubricants.11,12
While transesterification, isomerization and epoxidation of versatile vegetable oils have been
widely investigated and partly applied in industry, the studies on ketonization of unsaturated
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fatty acids and their derivatives are very limited until now. In literature, Sels and coworkers
reported a nitrous oxide based oxidation of unsaturated fatty acid esters and triacylglycerol
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mixtures to the keto fatty esters in the absence of solvent and metal ion catalyst; Knothe also
developed the synthesis of longꢀchain 1,2ꢀdioxo methyl esters of monounsaturated fatty acid
with potassium permanganate as oxidant. As a wellꢀknown protocol for olefin oxidation,
Wackerꢀtype oxidation with Pd(II) catalysts has also been reported in oxidizing unsaturated fatty
acid and its derivatives to the keto fatty acid and other derivatives with CuCl, or benzoquinone as
coꢀcatalysts.15ꢀ17
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For the Wackerꢀtype oxidations, Hosokawa et al.
even isolated a few palladiumꢀcopper
heteroꢀbimetallic complexes which are active for olefin oxidations. Most recently, we
unexpectedly found that adding certain nonꢀredox metal ions like Sc3+ as Lewis acid can
significantly accelerate Pd(II)ꢀcatalyzed olefin oxidation even better than that using Cu2+ as coꢀ
catalyst.20 Similar promotional effects were also observed in Pd(II)ꢀcatalyzed olefin
isomerization, olefin oxidative coupling, alkane oxidative dehydrogenation, and nitrile
hydration.21ꢀ24 The promotional effect was attributed to the formation of heteroꢀbimetallic Pd(II)/
LA species which serves as the key active species in aboveꢀmentioned reactions, and binding of
Lewis acid to the Pd(II) species may have increased its positive net charge, thus improving its
redox potential for oxidation reactions.25 This Pd(II)/LA based catalyst strategy has offered a
new opportunity for versatile Pd(II)ꢀcatalyzed reactions, including traditional Wackerꢀtype
oxidations, 26,27 and related applications in biomass utilizations. Here, we report the application
of this Pd(II)/LA catalyst for the valorization of vegetable oils through oxidation of unsaturated
fatty acids/esters to the corresponding keto fatty acids/esters with oxygen as oxidant, and it was
found that the promotional effect of Lewis acids like Sc3+ is much better than that of Cu2+ as coꢀ
catalyst in this Pd(II)ꢀcatalyzed vegetable oil oxidation.
MATERIALS AND METHODS
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Materials. All chemical reagents were commercially available and used without further
purification. Sc(OTf)3: (Accela Chembio Co., Ltd., Shanghai, China). Pd(OAc)2: (Stream
Chemicals Inc., Newburyport, MA). Cu(OTf)2: (Alfa Aesar, Stoughton, MA). NaOTf,
Mg(OTf)2, Fe(OTf)2 and octadecane: (TCI (Shanghai) Development Co., Ltd., Shanghai, China).
Ca(OTf)2, Zn(OTf)2, Al(OTf)3, Y(OTf)3, and Yb(OTf)3: (Shanghai Dibai Chemical Co. Ltd.,
Shanghai, China). Methyl palmitoleate, methyl oleate, methyl linoleate, oleic acid, linoleic acid
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and 1,3,5ꢀtriꢀchlorobenzene: (Aladdin Ltd., Shanghai, China). Dimethyl sulfoxide (DMSO),
N,N′ꢀdimethylacetamide (DMA), N,N′ꢀdimethylformamide (DMF), acetonitrile (CH3CN),
toluene, nꢀoctane, cyclohexane and 1,4ꢀdioxane: (Sinopharm Chemical Reagent Co.,Ltd,
Shanghai, China). Nuclear magnetic resonance (NMR) was performed on an AV400 400MHz
instrument (Bruker, Beijing, China). Ultraviolet visible (UV/Vis) spectra were recorded on a
specord 205 Plus spectrophotometer (Analytik Jena, Jena, Germany) and gas chromatographyꢀ
mass spectrometry (GCꢀMS) analysis was performed on a 7890A GC/5975C mass spectrometer
(Agilent, Philadelphia, PA) using the following column and temperature program. HPꢀ5: 30 m ×
0.25 mm, i.d., 0.25 ꢁm (Agilent, Philadelphia, PA); the initial temperature of 150 °C was held
for 3 min, then raised at 5 °C/min to 200 °C, and again raised at 20 °C/min to 300 °C, finally
held for 2 min. Mass spectra were recorded at 70 eV in the electron ionization mode (EIꢀMS).
Internal standard method. Firstly, a series of calibration solutions (at least five) were
prepared, which contain the analytes that we need to find the response factors to the standard (nꢀ
octadecane in this study) in GC analysis. These calibration solutions should cover the
concentrations of the analytes for which the concentrations of future practical samples will fall
in. These calibration solutions were next injected one by one into GC for analysis, and the
detector responses (peak areas) to all of the analytes were recorded. The response factor of each
analyte in one calibration solution was calculated as the value of the weight ratio of the
analyte/nꢀoctadecane timing the peak area ratio of nꢀoctadecane/the analyte. The average
response factor of each analyte in five calibration solutions was employed as the GC response
factor of the analyte in coming practical analysis.
General procedure for catalytic oxidation of methyl oleate, 1, to a mixture of methyl oxo-
octadecanoate, 2, by Pd(OAc)2/Sc(OTf)3
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In a typical procedure, Pd(OAc)2 (1.34 mg, 0.006 mmol) and Sc(OTf)3 (5.9 mg, 0.012 mmol)
were dissolved in 2.7 mL of acetonitrile in a glass tube, next 1 (88.9 mg, 0.3 mmol) and 0.3 mL
of water were then added. The glass tube was placed into a 50 mL stainless autoclave which was
then charged with oxygen to 20 atm. (Caution: 20 atm of oxygen possesses potential explosion,
and can aid combustion of flammable materials; the reactions need to be carried out in a wellꢀ
ventilated fumehood, free from any source of ignition). Next, the reaction mixture was stirred at
80 °C in an oil bath for 18 h. After the reaction, the autoclave was cooled to room temperature
and carefully depressurized to normal pressure. Control experiments including using Pd(OAc)2
or Sc(OTf)3 alone as catalyst were carried out in parallel. After GC analysis of 2 using the
internal standard method, the solvent was removed by rotary evaporation and the residue was
purified by silica gel column chromatography (20 cm × 1.5 cm) using nꢀhexane/ethyl acetate
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(10:1, v/v, 500 mL) as eluent to afford the products as white solid in 92% yield. H NMR (400
MHz, CDCl3) δ 3.60 (s, 3H, ꢀCOOCH3), 2.31 (t, J = 7.4 Hz, 4H, ꢀCH2ꢀC(O)ꢀCH2ꢀ), 2.23 (t, J =
7.6 Hz, 2H, CH2ꢀCOOCH3), 1.61 – 1.42 (m, 6H, 3× ꢀCH2ꢀ), 1.26ꢀ1.15 (m, 18H, 9× ꢀCH2ꢀ), 0.81
(t, J = 6.3 Hz, 3H, CH2ꢀCH3).
Catalytic oxidation of oleic acid to a mixture of oxo-octadecanoic acid: General oxidation
procedures were followed for this reaction except using oleic acid (84.4 mg, 0.3 mmol).
Purification of the products by column chromatography (20 cm × 1.5 cm) using nꢀhexane/ethyl
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acetate/acetic acid (90:30:1, v/v/v) afforded the product as a pale yellow solid in 50% yield. H
NMR (400 MHz, CDCl3) δ 2.46 – 2.25 (m, 6H, ꢀCH2ꢀC(O)ꢀCH2ꢀ, CH2ꢀCOOH), 1.73 – 1.46 (m,
6H, 3× ꢀCH2ꢀ), 1.26 (s, 18H, 9× ꢀCH2ꢀ), 0.88 (t, J = 6.5 Hz, 3H, CH2ꢀCH3).
Catalytic oxidation of methyl palmitoleate to a mixture of methyl oxo-hexadecanoate:
General oxidation procedures were followed except using methyl palmitoleate (80.5 mg, 0.3
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mmol). The products were determined by 1H NMR analysis in 91% yield with internal standard
method. 1H NMR (400 MHz, CD3CN), δ 3.62 (s, 3H, ꢀCOOCH3), 2.39 (t, J = 7.4 Hz, 4H, ꢀCH2ꢀ
C(O)ꢀCH2ꢀ), 2.29 (t, J = 7.4 Hz, 2H, CH2ꢀCOOCH3), 1.60 – 1.48 (m, 6H, 3× ꢀCH2ꢀ), 1.35ꢀ1.23
(m, 14H, 7× ꢀCH2ꢀ), 0.90 (t, J = 6.8 Hz, 3H, CH2ꢀCH3).
Catalytic oxidation of methyl linoleate to a mixture of methyl dioxo-octadecanoate:
General oxidation procedures were followed except using methyl linoleate (88.8 mg, 0.3 mmol).
Purification of the products by column chromatography using nꢀhexane/ethyl acetate (10:1, v/v)
afforded the product as a light yellow solid in 83% yield. 1H NMR (400 MHz, CDCl3) δ 3.67 (s,
3H, ꢀCOOCH3), 2.53 – 2.14 (m, 4H, ꢀCH2ꢀC(O)ꢀC(O)ꢀCH2ꢀ), 2.13 – 1.84 (m, 2H, CH2ꢀ
COOCH3), 1.72 – 1.52 (m, 6H, 3× ꢀCH2ꢀ), 1.46 – 1.10 (m, 16H, 8× ꢀCH2ꢀ), 0.97 – 0.80 (m, 3H,
CH2ꢀCH3).
Catalytic oxidation of linoleic acid to a mixture of dioxo-octadecanoic acid: General
oxidation procedures were followed except using linoleic acid (84.2 mg, 0.3 mmol). Purification
of the product by column chromatography using nꢀhexane/ethyl acetate/acetic acid (90:30:1,
v/v/v) afforded the products as a light yellow solid in 47% yield. 1H NMR (400 MHz, CDCl3) δ
2.48 – 2.20 (m, 4H, ꢀCH2ꢀC(O)ꢀCH2ꢀ), 2.17 – 1.92 (m, 2H, CH2ꢀCOOH), 1.78 – 1.48 (m, 6H, 3×
ꢀCH2ꢀ), 1.45 – 1.05 (m, 16H, 8× ꢀCH2ꢀ), 1.06 – 0.73 (m, 3H, CH2ꢀCH3).
Catalytic kinetics of methyl oleate oxidation by Pd(OAc)2/Sc(OTf)3: General oxidation
procedures were followed using 1 (88.9 mg, 0.3 mmol) in the designed interval period, and the
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yields of isomerization and oxidation products were analyzed by H NMR and GC using the
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internal standard method, respectively.
RESULTS AND DISCUSSION
To investigate the oxidations of unsaturated fatty acids and their ester derivatives by Pd(II)/LA
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catalyst, methyl oleate, 1, was initially employed as a model compound using Pd(OAc)2 as
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catalyst, Sc(OTf)3 as Lewis acid additives, and oxygen as oxidant. First, the effects of solvents
on this reaction were investigated and the results are summarized in Table 1. It was found that
the CH3CN/H2O mixture (90:10, v/v) provided the best catalytic efficiency for the oxidation of 1
to the keto fatty esters, a mixture of methyl oxoꢀoctadecanoate, 2. After reaction at 80 ˚C for 18 h
under 20 atm of oxygen, 62% conversion with 39% yield of 2 was achieved by GC analysis with
1% equiv. of Pd(OAc)2 and 2% equiv. of Sc(OTf)3 as Pd(II)/LA catalyst; extending the reaction
time to 40 h further offered 96% conversion with 88% yield of 2 (Table 1, entries 1 and 2). The
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remaining byproducts were the mixtures of isomerization products, 3, as identified by H NMR
analysis. Similar isomerizations of common olefins by Pd(II)/LA catalyst were also observed
previously in our laboratory.21 Increasing the Pd(OAc)2 loading from 1% to 2% equiv. with
Sc(OTf)3 to 4% equiv. improved the conversion up to 100% with 98% yield of 2 after 18 h
reaction, and an isolated yield of 92% was achieved under current conditions (Table 1, entry 3).
Further increasing the catalyst loading to 4% equiv. completed the oxidation in 4.5 h, giving 98%
yield of 2 (Table 1, entry 4). In another test, keeping the concentration of the Pd(II)/LA
unchanged, increasing the methyl oleate from 100 mM to 200 mM, corresponding to 1% equiv.
Pd(OAc)2 loading, achieved 98% conversion of substrate with 95% GC yield or 90% isolated
yield of 2 in 40 h (Table 1, entry 5), clearly indicating the high activity of this Pd(II)/LA catalyst
in vegetable oil oxidation. However, in the case of using oxygen balloon as the oxidant source,
the activity of this Pd(II)/LA system is very sluggish, giving only 87% of conversion with 2%
yield of 2 (Table 1, entry 6), suggesting that the pressurized oxygen is essential for the
regeneration of the active Pd(II)/LA catalyst in the catalytic cycle. In another control experiment,
using acetonitrile alone as the solvent (in the absence of water), also provided 100% conversion
of methyl oleate under air, however, there were no 2 detected, only giving the isomerization
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products 3 (Table 1, entry 7). In nonpolar solvents like THF, toluene, nꢀoctane and cyclohexane,
extremely low yields were obtained with noticeable palladium black formation after the reactions
(Table 1, entries 8ꢀ11), implicating the inefficient reꢀoxidation of palladium(0) by dioxygen
under these conditions. In addition, these solvents except THF are immiscible with water, which
apparently block the formation of the active Pd(II)/LA species for oxidation. On the other side,
the low catalytic efficiency in polar aprotic solvents like DMA, DMF and DMSO without
palladium black formation can be attributed to their coordination to the Pd(II) species (Table 1,
entries 12ꢀ14), leading to the reduced oxidizing power of the Pd(II) species for olefin oxidation.
Taken together, a reasonable polarity of solvent like that of acetonitrile is essential for efficient
unsaturated fatty acid oxidation by the Pd(II)/LA catalyst as well as those for common olefin
oxidations.20
1
The identification of 2 was conducted by H NMR analysis, which disclosed that the
methylene groups nearby the oxidized carbonyl group of 2 have the chemical shift around 2.4
ppm (Figure 1A), while 3 have the chemical shifts of the olefinic C=C bond between 5.3 and 5.5
ppm (Figure 1B), as previously reported.6,13 Further identification of 2 by GCꢀMS analysis
disclosed a series of products including methyl 5ꢀoxoꢀoctadecanoate, methyl 10ꢀoxoꢀ
octadecanoate, methyl 9ꢀoxoꢀoctadecanoate, methyl 8ꢀoxoꢀoctadecanoate, and methyl 17ꢀoxoꢀ
octadecanoate with the relative contents of 7%, 70%, 8%, 11% and 3%, respectively.
Next, using CH3CN/H2O (v/v, 90/10) as solvent, various nonꢀredox metal salts as Lewis acids
were investigated to promote Pd(OAc)2ꢀcatalyzed methyl oleate oxidation, and the results are
summarized in Table 2. In the absence of Lewis acid, using Pd(OAc)2 alone as catalyst was
ineffective for methyl oleate transformation (Table 2, entry 1). The promotional effects of Ba2+
and Ca2+ were very limited, giving only 4% and 8% yield of 2 (Table 2, entries 2 and 3), while
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other divalent metal ions like Fe2+ and Zn2+ apparently improved the catalytic efficiency of
Pd(OAc)2 catalyst, providing 28% and 46% yield of 2, respectively (Table 2, entries 4 and 5).
Pd(II)/Cu(II) system is the classic Wackerꢀtype catalyst for olefin oxidation;27 however adding
Cu(OTf)2 provided only a slightly higher yield (56%) of 2 than that (46%) of adding Zn2+ as
Lewis acid (Table 2, entry 6), while adding Cu(OAc)2 did not generate any promotional effect.
Significantly, adding trivalent, nonꢀredox metal ions like Al3+, Y3+, and Sc3+ greatly improved
the yield up to 75%, 94% and 98%, respectively (Table 2, entries 7ꢀ9), much higher than that of
adding Cu2+. As mentioned, Cu(II) salts were popularly employed as coꢀcatalyst for regenerating
the active Pd(II) species from reduced Pd(0) in Wackerꢀtype oxidations;27 here, attempting to
further improve the catalytic efficiency of this Pd(II)/Sc(III) system, additional Cu(OAc)2,
Cu(TFA)2, CuCl2, CuBr2 or Cu(OTf)2 were added to the system using 1 mol% Pd(OAc)2 with 2
mol% Sc(OTf)3 catalyst loading, however, a sharply decreased yield was obtained in each case,
giving only 2ꢀ12% yield of 2. Even adding BQ or DMA to this Pd(II)/Sc(III) system in
acetonitrile also caused the yields dropping to 38% or 32%, respectively. Similar inhibitory
effect of CuCl2 were observed by Kaneda in their PdCl2ꢀcatalyzed internal olefin oxidations in
DMA, in contrast to its acceleratory role in terminal alkene oxidations.28 In that case, the
inhibitory effect of CuCl2 was attributed to inhibit πꢀcoordination of an olefin to the Pd species
through the complexation of Cu with Pd.
In another test in Table 1, 12% equiv. of NaOTf were employed as Lewis acid, which contains
the identical amount of OTfꢀ anion as those in 4% equiv. of Sc(OTf)3, however, it was inactive
for methyl oleate transformation (Table 2, entry 12). Clearly, the promotional effect originated
from the added metal ions as Lewis acid rather than the OTfꢀ anion. Consistent with this, the
stronger the Lewis acid employed and the larger the promotional effect generally observed. In
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particular, the much higher yield of 2 (94% or 98%) obtained by adding Al3+ or Y3+ than that
(56%) by adding Cu2+ in this study clearly highlighted the Lewis acid properties of added nonꢀ
redox metal ions rather than their redox properties in this Pd(II)ꢀcatalyzed vegetable oil
oxidation. In the textbook mechanism of Wackerꢀtype oxidation, the role of the Cu2+ cation is
assigned to reꢀoxidize the reduced Pd(0) back to the active Pd(II) species to achieve the catalytic
cycle, thus highlighted its redox properties.29 Together with previous studies,20 the results
demonstrated here further support that the Lewis acid properties of the Cu2+ cation may have
played significant roles in Wackerꢀtype oxidations, even though it is not distinguishable from its
redox properties in Pd(II)/Cu(II) system.
Catalytic kinetics further displayed the high activity of this Pd(II)/LA catalyst in oxidation of
methyl oleate, and it also provided fruitful mechanistic information. As shown in Figure 2, in 4 h
reaction at 80 ˚C, using 2% equiv. Pd(OAc)2 loading with 4% equiv. of Sc(OTf)3 achieved 56%
conversion of 1, however, the isomerization products 3 were obtained as the main products in
34% yield, and the yield of 2 was only 21%. With the reaction proceeding, the yield of 3 dropped
gradually, meanwhile the yield of 2 increased gradually up to 98% in 18 h with 100% conversion
of 1. Clearly, the isomerization reaction is competitive with the oxidation, and it proceeds faster
than oxidation. Remarkably, with the reaction proceeding, the resulting isomerization products 3
can be eventually oxidized to 2 as the terminal products, thus leads to the high selectivity of the
keto fatty esters. It is worth mentioning that as shown in Figure 2, in the absence of Lewis acid,
Pd(OAc)2 alone as catalyst is very sluggish for both isomerization and oxidation reactions.
Figure 3 displays the influence of the Sc(III)/Pd(II) ratio on the Pd(II)/LA catalyzed methyl
oleate oxidation. In the absence of Sc(OTf)3, Pd(OAc)2 alone as the catalyst is almost inactive
for methyl oleate transformation in 18 h at 80 ˚C, whether for isomerization or oxidation. The 1:1
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ratio of Sc(III)/Pd(II) provided 41% conversion of 1 with dominant 2 (30% yield) and minor 3
(5% yield). Increasing the ratio of Sc(III)/Pd(II) to 3:1 improved the conversion up to 62% with
45% yield of 2 and 13% yield of 3. However, as displayed in Figure 3, further increasing the
ratio of Sc(III)/Pd(II) caused the yield of 2 to decrease, whereas the yield of 3 increased linearly,
even though it still improved the conversion of 1. For example, at the ratio of 8:1, after 18 h
reaction, it provided 76% conversion of 1 with only 31% yield of 2 but 28% yield of 3. The
selectivity of the keto fatty esters 2 also revealed a decreased trend with further increase of the
Sc(III)/Pd(II) ratio after 3:1.
Table 3 shows the oxidations of different unsaturated fatty acids/esters to the corresponding
keto fatty acids/esters by this Pd(II)/LA catalyst. As described above, oxidation of 1 provided
92% isolated yield of 2 under 20 atm of oxygen at 80 ˚C in 18 h. Under the current conditions,
oxidation of methyl palmitoleate and methyl linoleate gave the corresponding keto fatty esters, a
mixture of methyl oxoꢀhexadecanoate in 83% yield and a mixture of methyl dioxoꢀoctadecanoate
1
in 91% yield, respectively, which were determined by H NMR analysis. Not only the
unsaturated fatty esters, but also oleic acid and linoleic acid can be oxidized by this Pd(II)/LA
system to afford a mixture of oxoꢀoctadecanoic acid in 50% yield and a mixture of dioxoꢀ
octadecanoic acid in 47% yield, respectively. The relatively low activity of this Pd(II)/LA
catalyst in oxidizing unsaturated fatty acids is possibly related to the interaction of the carboxylic
acid with the Pd(II) and/or Lewis acid, which may block the formation of the active Pd(II)/LA
species for oxidation.
As presented above, adding nonꢀredox metal ions like Sc3+ can sharply promote the oxidations
of different unsaturated fatty acids/esters by Pd(OAc)2 catalyst to the corresponding keto fatty
acids/esters, which is similar to previous reports for common olefin oxidations.17 In previous
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studies, through UVꢀVis, H and C NMR characterizations, it was found that, in acetonitrile,
adding nonꢀredox metal ions to Pd(OAc)2 leads to the formation of heteroꢀbimetallic Pd(II)/LA
species having two acetate bridges.20ꢀ22 As a result, binding of Lewis acid to the Pd(II) species
makes it more positively charged as observed by the downshift of the chemical shift of the
1
13
acetate bridge in H and C NMR spectra, which leads to its improved catalytic efficiency in
olefin oxidations. In addition, similar heteroꢀbimetallic Pd(II)/LA salts with acetate bridges were
widely reported, including LA = Ba2+, Sr2+, Ca2+, Mn2+, Co2+, Ni2+, Cd2+, Nd2+, Zn2+ and Ce4+,
and some of those were successfully identified by Xꢀray single crystal characterizations.30ꢀ33
Here, in the identical CH3CN/H2O solvent, formation of the Pd(II)/LA species, for example,
Pd(II)/Sc(III) species, should happen as well as those in previous studies, and is responsible for
vegetable oil oxidations. It is worth mentioning that, a few heteroꢀbimetallic Pd(II)/Cu(II)
complexes were isolated by Hosokawa and Murahashi, and they are active catalyst for olefin
oxidations.18,19
Accordingly, a similar Pd(II)/Sc(III) species catalyzed oxidation can be proposed for this
unsaturated fatty acids/esters oxidation as shown in Figure 4. By taking 1 for example, in the first
step, 1 is ligated to the Pd(II)/Sc(III) species to generate the intermediate 4. Next, a water
molecule attacks on the ligated C=C bond of the vegetable oil followed by βꢀhydride
elimination.34ꢀ36 Then, the keto fatty esters 2 are released with the formation of the Hꢀ
Pd(II)/Sc(III) intermediate 5. This intermediate may interact with the second Sc3+ cation to
stabilize the Pd(II) hydride to form the Sc(III)•••HꢀPd(II)/Sc(III) intermediate 6; otherwise, the
intermediate 5 may be feasibly reduced to the inactive palladium(0).37,38 Dioxygen insertion of
the intermediate 6 leads to the formation of the HOOꢀPd(II)/Sc(III) intermediate 7.39ꢀ44 After
releasing the HOOꢀ anion, the Pd(II)/Sc(III) species is regenerated to achieve the catalytic
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cycle.20,22 Since the formation of the Pd(II)/Sc(III) species sharply promotes the vegetable oil
oxidation, two plausible roles of Sc3+ can be expected in this Pd(II)/LA species: 1) ligation of the
Sc3+ cation to the Pd(II) species may increase the positive charge of the Pd2+ cation as indicated
by the chemical shift of acetate bridge, making it more electrophilic, thus facilitating the C=C
bond coordination of the vegetable oil on the Pd2+ cation; and 2) because of the enhanced
electron transfer from the C=C bond of the unsaturated fatty esters to the Pd(II)/Sc(III) species, it
further promotes the water molecule attacking on the ligated C=C bond, thus accelerating the
oxidation of the vegetable oil. As shown in Figure 3, not only the 1:1 ratio of Sc(III)/Pd(II)
improved the vegetable oil oxidation, the ratio of 2:1 still obviously improved the oxidation
efficiency, and the ratio of 3:1 also slightly improved the efficiency, indicating the extra Sc3+
cation still benefited the Pd(II)ꢀcatalyzed oxidation. Accordingly, a third role of the Sc3+ cation
was proposed to stabilize the HꢀPd(II)/Sc(III) species through the formation of the Sc(III)•••Hꢀ
Pd(II)/Sc(III) intermediate III, which inhibits the reductive elimination of the HꢀPd(II) bond to
form the inactive Pd(0), and promotes the oxygen insertion into the HꢀPd(II) bond because of its
weakness through Sc3+ binding. Indeed, in the case of using a 1:1 ratio of the Sc(III)/Pd(II)
catalyst, there still was palladium black observed after the vegetable oil oxidation, while with 2:1
ratio of the Sc(III)/Pd(II) catalyst, there was no palladium black formation observed.
This work introduces the Pd(II)/LA catalyst for oxidation of renewable unsaturated fatty
acids/esters to the corresponding keto fatty acids/esters which can be applied as lubricants in
industry. It was found that adding nonꢀredox metal ions like Sc3+ to simple Pd(OAc)2 as the
Pd(II)/LA catalyst can sharply promote its oxidation efficiency, even much better than the classic
Pd(II)/Cu(II) catalyst, which highlights the Lewis acid properties of Cu2+ cation in this Pd(II)ꢀ
catalyzed oxidation of unsaturated fatty acids/esters. The observed promotional effects are
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obviously Lewis acidity dependent on the added nonꢀredox metal ions, and generally, a stronger
Lewis acid provided a better efficiency. As well as in previous studies, the promotional effect of
Lewis acid is attributed to the formation of the heteroꢀbimetallic Pd(II)/LA species which is
responsible for oxidations. The demonstrated Pd(II)/LA catalyst may offer new opportunities for
transforming versatile plant oils to the corresponding keto fatty acids, esters or other derivatives
in their industrial utilizations through catalytic oxidations.
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SUPPORTING INFORMATION
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308
Detailed NMR and GCꢀMS data for different vegetable oil oxidations. This material is
available free of charge via the Internet at http://pubs.acs.org.
309
ACKNODLEDGEMENTS
310
311
312
This work was supported by the National Natural Science Foundation of China (no.
21573082). The GCꢀMS analysis was performed in the Analytical and Testing Center of
Huazhong University of Science and Technology.
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44. Denney, M. C.; Smythe, N. A.; Cetto, K. L.; Kemp, R. A.; Goldberg, K. I. Insertion of
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FIGURE CAPTIONS
1
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Figure 1. H NMR identifications of products from methyl oleate oxidation by Pd(II)/LA
catalyst. (A) oxidation products, (B) isomerization products, and (C) original methyl oleate
substrate, in CDCl3.
436
437
438
Figure 2. Catalytic kinetics for oxidation of 1 to 2 by Pd(II)/Sc(III) catalyst. Reaction conditions:
CH3CN/H2O (v/v, 2.7 mL/0.3 mL), Pd(OAc)2 (2 mM), Sc(OTf)3 (4 mM), 1 (100 mM), O2 (20
atm), 80 ˚C.
439
440
441
Figure 3. The influence of the Sc(III)/Pd(II) ratio on the Pd(II)ꢀcatalyzed oxidation of 1.
Reaction condition: Pd(OAc)2 (1 mM), CH3CN/H2O (v/v, 2.7 mL/0.3 mL), O2 (20 atm), 18 h, 80
˚C.
442
443
Figure 4. Proposed mechanism for Pd(II)/LA catalyzed oxidation of unsaturated fatty
acids/esters.
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Table 1. Pd(II)/Sc(III) Catalyzed Methyl Oleate, 1, Oxidation to a Mixture of Methyl Oxoꢀ
octadecanoate, 2, in Different Solvents.a
Entry
Pd(II)/Sc(III) Solvent
(mM/ mM)
Time /h
Conv. /%
Yield %
1
1/ 2
1/ 2
2/ 4
4/ 8
2/ 4
2/ 4
2/ 4
2/ 4
2/ 4
2/ 4
CH3CN/ H2O 18
53
39
2
CH3CN/ H2O 40
96
88
3
CH3CN/ H2O 18
CH3CN/ H2O 4.5
CH3CN/ H2O 40
CH3CN/ H2O 18
100
100
98
98 (92)
4
98
5b
6c
7d
8c
9
95 (90)
88
2
CH3CN
18
18
100
28
N.D.
10
3
THF/ H2O
Toluene/ H2O 18
12
10
nꢀOctane/
18
15
1
H2O
11b
2/ 4
Cyclohexane/ 18
H2O
10
6
12
13
14
2/ 4
2/ 4
2/ 4
DMA/ H2O
18
9
4
DMF/ H2O
18
24
23
8
DMSO / H2O 18
13
a Conditions: organic solvent/water (v/v, 2.7 mL/0.3 mL), 1 (100 mM), O2 (20 atm), 80 ˚C, yield
b
was determined by GC analysis and the data in parentheses represent isolated yield. 1 (200
mM). c O2 balloon at 70 ˚C. d CH3CN alone as solvent under air. N.D. = Not detected.
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Table 2. Pd(II)ꢀCatalyzed Oxidation of 1 in the Presence of Different NonꢀRedox Metal Ions. a
Entry
Lewis acid
ꢀ
Conv.%
3.1
7
Yield%
N.D.
4
1
2
Ba(OTf)2
Ca(OTf)2
Fe(OTf)2
Zn(OTf)2
Cu(OTf)2
Al(OTf)3
Y(OTf)3
Sc(OTf)3
Yb(OTf)3
In(OTf)3
NaOTf
3
23
8
4
37
28
5
50
46
6
80
56
7
92
75
8
100
100
53
94
9
98 (92)
49
10
11
68
28
12b
3
trace
a
Conditions: CH3CN/H2O (v/v, 2.7 mL/0.3 mL), Pd (OAc)2 (2 mM), Lewis acid (4 mM), 1
(100 mM), O2 (20 atm), 80 ˚C, 18 h. Yield was determined by GC and the data in parentheses
represent isolated yield. b NaOTf 12 mM. N.D. = Not detected.
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Table 3. WackerꢀType Oxidation of Different Unsaturated Fatty Acids/Esters by Pd(OAc)2 with
the Sc(OTf)3 in CH3CN/H2O.a
Entry
1b
Substrate
Methyl oleate
Oleic acid
Time
18 h
18 h
18 h
24 h
24 h
Yield %
92
2 b
50
3c
Methyl palmitoleate
Methyl linoleate
Linoleic acid
91
4 b
83
5 b
47
a
Reaction Conditions: CH3CN/H2O (v/v, 2.7 mL/0.3 mL), Pd(OAc)2 (2 mM), Sc(OTf)3 (4
b
c
mM), unsaturated fatty acids or esters (100 mM), O2 (20 atm), 80 ˚C. Isolated yield. Yield
determined by 1H NMR analysis with internal standard.
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Figure 1
A methyl oxo-octadecanoate, 2
B isomerization products of methyl oleate, 3
C methyl oleate, 1
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Figure 2
Conv. of methyl oleate
Yield of isomerization products
Yield of keto fatty esters
Conv. of methyl oleate with Pd(II) alone
Selectivity of keto fatty esters
100
80
60
40
20
0
100
80
60
40
20
0
0
2
4
6
8
10 12 14 16 18 20
Reaction time (h)
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Figure 3
Conv. of methyl oleate
Yield of isomerization products
Yield of keto esters
100
80
60
40
20
0
100
80
60
40
20
0
Selectivity of keto esters
0
2
4
6
8
Ratio of Sc(III)/ Pd(II)
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