A further step to sustainable palladium catalyzed oxidation: Allylic oxidation of alkenes in green solvents

Article abstract of DOI:10.1016/j.apcata.2021.118349

The palladium catalyzed oxidation of alkenes with molecular oxygen is a synthetically important reaction which employs palladium catalysts in solution; therefore, a solvent plays a critical role for the process. In this study, we have tested several green solvents as a reaction medium for the allylic oxidation of a series of alkenes. Dimethylcarbonate, methyl isobutyl ketone, and propylene carbonate, solvents with impressive sustainability ranks and very scarcely exploited in palladium catalyzed oxidations, were proved to be excellent alternatives for the solvents conventionally employed in these processes, such as acetic acid. Palladium acetate alone or in the combination with p-benzoquinone efficiently operates as the catalyst for the oxidation of alkenes by dioxygen under 5–10 atm. For most substrates, the systems in green solvents showed better selectivity for allylic oxidation products as compared to pure acetic acid; moreover, the reactions in propylene carbonate solutions occurred even faster than in acetic acid.

Full text of DOI:10.1016/j.apcata.2021.118349

Applied Catalysis A, General 625 (2021) 118349
Contents lists available at ScienceDirect
Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
A further step to sustainable palladium catalyzed oxidation: Allylic
oxidation of alkenes in green solvents
Maíra dos Santos Costa, Amanda de Camargo Faria, Rayssa L.V. Mota, Elena V. Gusevskaya*
Departamento de Química, Universidade Federal de Minas Gerais, 31270-901 Belo Horizonte, MG, Brazil
A R T I C L E I N F O
A B S T R A C T
Keywords:
The palladium catalyzed oxidation of alkenes with molecular oxygen is a synthetically important reaction which
employs palladium catalysts in solution; therefore, a solvent plays a critical role for the process. In this study, we
have tested several green solvents as a reaction medium for the allylic oxidation of a series of alkenes. Dime-
thylcarbonate, methyl isobutyl ketone, and propylene carbonate, solvents with impressive sustainability ranks
and very scarcely exploited in palladium catalyzed oxidations, were proved to be excellent alternatives for the
solvents conventionally employed in these processes, such as acetic acid. Palladium acetate alone or in the
combination with p-benzoquinone efficiently operates as the catalyst for the oxidation of alkenes by dioxygen
under 5–10 atm. For most substrates, the systems in green solvents showed better selectivity for allylic oxidation
products as compared to pure acetic acid; moreover, the reactions in propylene carbonate solutions occurred
even faster than in acetic acid.
Alkenes, allylic oxidation
Green solvents
Molecular oxygen
Palladium
1. Introduction
robust ligands (mostly nitrogen-based ligands) [3–7,13,28,29] able to
stabilize reduced palladium in solutions ().
The Wacker process for the synthesis of acetaldehyde by the palla-
dium catalyzed oxidation of ethylene with dioxygen is one of the first
and the most important applications of transition metal catalysis in in-
dustry [1,2]. The chemistry was further extended to higher alkenes
opening a convenient pathway to many valuable compounds [3–18].
The use of environmentally benign dioxygen as the terminal oxidant is
an important advantage, especially within modern green chemistry re-
quirements. The main issue in palladium catalyzed oxidations consists in
ensuring a rapid re-oxidation of reduced palladium, otherwise the irre-
versible formation of inactive palladium metal would interrupt the
catalytic cycle. For this purpose, most applications use auxiliary redox
active co-catalysts. The original Wacker catalyst contains CuCl2 as the
co-catalyst and operates at high concentrations of chloride ions [1]. In
further developments, CuCl2 was substituted with less corrosive
halide-free alternatives, e.g., heteropoly acids, p-benzoquinone (BQ),
copper and iron salts [19–22]. In the last 15 years, palladium oxidation
chemistry is experiencing a renaissance [4] due to the finding the way to
avoid redox co-catalysts using special solvents or ligands. It was
discovered that the re-oxidation of palladium by dioxygen could occur in
dimethylsulfoxide (DMSO) [23,24] and dimethylacetamide (DMA)
media [25–27] (“palladium solo” systems) or with the use of oxidatively
Although the most desirable option for liquid phase reactions would
be neat conditions; solvents are often strongly required to dissolve the
reactants and to control some problematic issues with selectivity, mass
transfer, safety, and handling [30,31]. As the mass contribution of the
solvent in a whole chemical process is significant; finding more sus-
tainable solvent alternatives would represent an effective strategy to
reduce a negative environmental impact [30,31]. As palladium cata-
lyzed oxidations usually involve several simultaneously occurring redox
reactions, the choice of solvent is particularly crucial for the catalytic
activity and efficient catalyst turnover [11]. The industrial oxidation of
ethylene is performed in aqueous solutions; however, organic solvents
or mixed-solvent systems should be applied for higher alkenes due to
solubility problems. To the best of our knowledge, highly hazardous
organic solvents with low sustainability ranks, such as N,
N-dimethylformamide, tetrahydrofuran, dichloroethane, DMA, dime-
thoxyethane, and dioxane, still dominate the scenario of the palladium
catalyzed oxidation of alkenes in both laboratory and industry [11].
Acetic acid and DMSO, which are commonly employed in the palladium
catalyzed allylic oxidations and in the “palladium solo” systems, are also
considered as problematic solvents in modern solvent selection guides
[32,33].
* Corresponding author.
E-mail address: elena@ufmg.br (E.V. Gusevskaya).
https://doi.org/10.1016/j.apcata.2021.118349
Received 16 June 2021; Received in revised form 30 August 2021; Accepted 31 August 2021
Available online 3 September 2021
0926-860X/© 2021 Elsevier B.V. All rights reserved.

M. dos Santos Costa et al.
Applied Catalysis A, General 625 (2021) 118349
In this study, our efforts have been focused on the development of
“greener” solvent alternatives for the palladium catalyzed allylic
oxidation of alkenes by dioxygen. Several compounds with high sus-
tainability ranks and very scarcely exploited in palladium catalyzed
oxidations, such as dimethylcarbonate (DMC), diethylcarbonate (DEC),
methyl isobutyl ketone (MIBK), and propylene carbonate (PC) were
tested for this purpose as reaction media for these reactions. Some
mixed-solvent systems showed better results in terms of selectivity than
pure HOAc and, for some alkenes, even allowed to dispense redox co-
catalysts, with the palladium regeneration being performed directly by
dioxygen. Pd(OAc)2 alone or in the combination with BQ efficiently
operates as the catalyst for the allylic oxidation of alkenes containing
terminal, internal or endocyclic C-C double bonds. The substrate scope
includes a series of biomass-based natural compounds, i.e., cis-jasmone,
limonene, β-caryophyllene, β-ionone and β-citronellene, as well as 1-
octene and cyclohexene as model molecules.
Compound 4b (two isomers): MS (70 eV, EI): m/z (%): trans isomer
(shorter GC retention time): 152 (76), 134 (35) [M+–HOAc], 119 (88)
[M+–HOAc–CH3], 109 (100), 105 (29), 93 (30), 92 (30), 91 (58) 84
(73), 79 (21), 77 (18); cis isomer (longer GC retention time): 152 (59),
134 (53) [M+–HOAc], 119 (100) [M+–HOAc–CH3], 109 (88), 105 (32),
93 (29), 92(30), 91(76), 84 (94), 79 (24), 77 (18). For NMR data see
[36].
Compound 4c (two isomers): MS (70 eV, EI): m/z (%): trans isomer
(shorter GC retention time): 152 (35), 134 (71) [M+–HOAc], 119 (100)
[M+–HOAc–CH3], 109 (59), 106 (35), 105 (41), 93 (52), 92 (54), 91
(94), 84 (18), 79 (35), 77 (18), 68 (24), 67 (23); cis isomer (longer GC
retention time): 152 (6), 134 (47) [M+–HOAc], 119 (88)
[M+–HOAc–CH3], 106 (100), 105 (41), 93 (38), 92 (44), 91 (71), 84
(23), 79 (47), 77 (22), 68 (35), 67 (37) [37]. For NMR data see [38].
Compound 4d: MS (70 eV, EI): m/z (%): 152 (38), 134 (56)
[M+–HOAc], 119 (88) [M+–HOAc–CH3], 106 (43), 105 (43), 93 (50),
92 (62), 91 (100), 84 (31), 79 (38), 68 (56), 67 (44). For NMR data see
[38].
2. Experimental
Compound 5b (two isomers): MS (70 eV, EI): m/z (%): first isomer
(shorter GC retention time): 202 (10) [M+-HOAc], 187 (22) [M+-HOAc-
CH3], 173 (17), 159 (34), 146 (18), 145 (28), 133 (27), 132 (18), 131
(54), 120 (16), 119 (42), 118 (67), 117 (58), 107 (23), 106 (19), 105
(61), 93 (37), 92 (22), 91 (100), 81 (17), 79 (51), 77 (36), 69 (40), 67
(29), 65 (16), 60 (20), 55 (26); second isomer (longer GC retention
time): 202 (9) [M+-HOAc], 187 (20) [M+-HOAc-CH3], 173 (18), 159
(32), 146 (17), 145 (28), 133 (25), 132 (15), 131 (53), 120 (15), 119
(40), 118 (60), 117 (52), 107 (24), 106 (19), 105 (66), 93 (37), 92 (21),
91 (100), 81 (20), 79 (51), 77 (34), 69 (35), 67 (28), 65 (16), 60 (20), 55
(27). For NMR data see [39].
All reagents and solvents were acquired from commercial suppliers.
Cis-jasmone [cis-3-methyl-2-(2-pentenyl)ꢀ 2-cyclopenten-1-one], 1-
octene, cyclohexene, (R)-(+)-limonene, β-caryophyllene [(-)-trans car-
yophyllene], β-ionone, p-benzoquinone (BQ) and Pd(OAc)2 were
received from Sigma-Aldrich. (-)-β-Citronellene [dihydromyrcene, (R)-
(ꢀ )ꢀ 3,7-dimethyl-1,6-octadiene] was acquired from Fluka. BQ was
purified by column chromatography (silica) using dichloromethane as
the eluent. Dimethyl carbonate (DMC) (anhydrous, ≥ 99%), diethyl
carbonate (DEC) (99%), methyl isobutyl ketone (MIBK) (≥ 98.5%) and
propylene carbonate (PC) (anhydrous, 99.7%), were purchased from
Sigma-Aldrich. Glacial acetic acid used as a solvent was from VETEC.
Catalytic tests were run in a homemade autoclave with magnetic
stirring and a valve dip tube for sampling without the depressurization.
Typically, the solution (total volume of 10 mL) of the alkene (0.20 M),
palladium acetate (5–10 mM), BQ (if any, 50 mM), and internal standard
(bornyl acetate, 0.10 M) was transferred into the reactor. At the end of
the run, the autoclave was cooled to room temperature and oxygen was
slowly vented out. CAUTION: reactions under superatmospheric pres-
sure of oxygen can cause an unforeseen explosion. These reactions must
only be conducted using the appropriate equipment and with rigorous
safety precautions. The reaction progress was monitored by gas phase
chromatography (GC) analysis of the aliquots periodically sampled from
the reactor (GC- Shimadzu GC2010 instrument, FID, Rtx-Wax 30 m or
Rtx-5MS 30 m columns). Conversion and selectivity calculations were
based on the reacted substrate. Bornyl acetate was used as the internal
standard. Average reaction rates were calculated between 0 and ca. 40%
conversions.
Compound 5c: MS (70 eV, EI): m/z (%): 202 (13) [M+-HOAc], 187
(26) [M+-HOAc-CH3], 173 (24), 159 (36), 146 (21), 145 (34), 133 (30),
132 (20), 131 (63), 120 (16), 119 (39), 118 (31), 117 (48), 107 (23), 106
(20), 105 (63), 93 (39), 92 (22), 91 (100), 81 (15), 79 (51), 77 (34), 69
(39), 67 (27), 65 (16), 60 (15), 55 (24). For NMR data see [39].
Compound 5d: MS (70 eV, EI): m/z (%): 205 (3) [M+-CH3], 131 (29),
123 (24), 121 (40), 119 (28), 109 (44), 107 (55), 106 (33), 105 (53), 96
(30), 95 (60), 93 (89), 91 (88), 82 (23), 81 (48), 79 (100), 77 (42), 69
(64), 67 (51), 55 (40).
Compound 6b: MS (70 eV, EI): m/z (%): 250 (0.5) [M+], 235 (8)
[M+-CH3], 190 (27) [M+-HOAc], 175 (100), 157 (20), 147 (37), 134
(22), 133 (22), 131 (28), 119 (20), 105 (27), 91 (28). For NMR data see
[40].
Compound 6c: MS (70 eV, EI): m/z (%): 188 (4) [M+], 173 (100)
[M+-CH3], 145 (13), 144 (11), 130 (16), 129 (27), 128 (16), 115 (12).
For NMR data see [40].
Compound 6d: MS (70 eV, EI): m/z (%): 193 (1) [M+-CH3], 177 (75),
165 (45), 135 (1314), 124 (910), 123 (100), 109 (7), 107 (78), 95 (810),
91 (47), 79 (46), 69 (34).
Reaction products were analyzed/identified by gas chromatog-
raphy/mass spectrometry (GC-MS, Shimadzu QP2010-PLUS equipment
operating at 70 eV) or by NMR spectroscopy (Bruker 400 MHz, CDCl3,
TMS) after the isolation by a column chromatography (silica gel 60;
eluents: hexane and ethyl acetate).
Compound 7b: 1H NMR (400 MHz, CDCl3, 25 ◦C, TMS): δ =
5.57–5.64 (m, 1 H), 5.38, (t, J = 4.5, 1 H), 4.89 (d, J = 14.3 Hz, 1 H),
4.86 (d, J = 9.7 Hz, 1 H), 4.38 (s, 2 H), 2.00–2.10 (m, 1 H), 1.99 (s, 3
H), 1.90–2.00 (m, 2 H), 1.57 (s, 3 H), 1.25–1.30 (m, 2 H), 0.93 ppm (d,
J = 4.4 Hz, 2 H); 13C NMR (400 MHz, CDCl3, 25 ◦C, TMS): δ = 171.2,
144.6, 130.2, 130.1, 113.0, 70.6, 37.7, 36.3, 25.7, 21.2, 20.4, 14.16
ppm. NMR data are in accordance with those published in [41].
Compound 1b (two isomers): MS (70 eV, EI): m/z (%): cis isomer
(shorter GC retention time): 180 (16), 177 (15), 162 (53) [M+-HOAc],
151 (20), 149 (68), 147 (23), 133 (100), 123 (91), 119 (25), 105 (66), 93
(18), 91 (60), 79 (27), 77 (32), 65 (19), 60 (28), 55 (15); trans isomer
(longer GC retention time): 222 (0.02) [M+], 180 (18), 162 (50) [M+-
HOAc], 151 (21), 147 (22), 133 (41), 123 (100), 119 (18), 105 (40), 91
(40), 77 (19), 65 (9), 55 (12). For NMR data see [34].
3. Results and discussion
Compound 1c: MS (70 eV, EI): m/z (%): 164 (100) [M+], 149 (68),
135 (61), 131 (46), 122 (75), 121 (35), 110 (54), 109 (25), 107 (45), 105
(27), 93 (81), 91 (76), 81 (21), 79 (95), 77 (55), 67 (18), 65 (20), 55
(34). For NMR data see [34].
Cis-jasmone (1a), a natural compound found in the jasmine essential
oil, is commonly utilized as fragrance component in many commercial
products. We have reported recently the first palladium catalyzed
oxidation of this alkene by dioxygen [34]. In the present work, which is
directed to finding green reaction media for allylic oxidations, we have
chosen cis-jasmone as a model substrate.
Compound 2b: MS (70 eV, EI): m/z (%):128 (4) [M+], 113 (4) [M+-
CH3], 85 (10), 71 (21), 59 (19), 58 (100).
Compound 3b: MS (70 eV, EI): m/z (%): 140 (11) [M+], 98 (81), 97
(31), 83 (30), 81 (27), 80 (49), 79 (100), 70 (27). For NMR data see [35].
The oxidation of cis-jasmone in the solutions of HOAc containing
catalytic quantities of Pd(OAc)2 and BQ gave mainly allylic acetate 1b
2

M. dos Santos Costa et al.
Applied Catalysis A, General 625 (2021) 118349
Table 1
a
Oxidation of cis-jasmone (1a) with dioxygen catalyzed by Pd(OAc)2
.
Run
Co-solvent
T
Time
(h)
Conversion
(%)
Selectivity for
allylic oxidation
(%)
Rateb
(vol%)
None
(◦C)
80
1b
total
(mM hꢀ 1
300
)
1c
2c
3
0.5
2
75
96
63
98
59
99
52
96
62
96
25
90
25
95
20
90
26
88
44
98
18
90
34
90
82
80
85
82
78
76
85
85
55
59
83
85
85
86
83
86
85
83
89
83
90
85
90
86
93
95
93
96
85
86
95
96
65
67
96
96
95
96
93
97
95
95
96
95
98
96
98
98
DMC (50)
DMC (50)
DMC (50)
None
80
80
60
60
60
60
60
60
60
60
70
0.5
2
252
236
104
124
100
50
0.5
4
4
1
4
5
1
4
6d
7e
8f
9
DMC (50)
DMC (50)
DMC (50)
DMC (70)
DEC (50)
MIBK (50)
MIBK (70)
1
8
1
8
1
40
8
1
52
8
10
11
12
1
88
4
1
36
8
1
70
6
a
Conditions: [cis-jasmone] = 0.20 M, [Pd(OAc)2] = 10 mM, gas phase – O2, 10 atm, solvent – HOAc. Conversion and selectivity calculations were based on the
reacted substrate. The rest of the mass balance was mainly due to compound 1c. DMC – dimethylcarbonate; DEC – diethylcarbonate; MIBK – methyl isobutyl ketone. b
Average rate calculated between 0 and ca. 40% conversion. c [p-benzoquinone] = 50 mM. d [cis-jasmone] = 0.40 M. e[Pd(OAc)2] = 5 mM. f O2 - 5 atm.
(80% selectivity, ca. 95% trans) along with a mixture of other isomeric
allylic acetates having the double bond in different positions of the side
pentenyl fragment (Table 1, run 1). Along with the allylic oxidation
products (total selectivity of 95%), the isomer of cis-jasmone, compound
1c, containing two conjugated C-C double bonds was also formed with
ca. 5% selectivity (Table 1). All oxidation products were derived from
agglomeration into inactive metal. For comparison, the same reaction in
the absence of BQ was run in pure HOAc (Table 1, run 5 vs. run 4).
Although no palladium metal was detected, selectivity for allylic
oxidation dropped to 67%, with the rest of the mass balance being due to
high molecular weight products which could not be detected by GC. This
result confirmed the essential role of DMC as the co-solvent in sup-
porting the catalytic cycle at the cis-jasmone oxidation.
–
the reaction of the disubstituted exocyclic C C bond, whereas the
–
–
endocyclic tetrasubstituted C C bond remained intact. A possible
The reactions with a doubled amount of cis-jasmone or with a half
amount of palladium acetate gave nearly the same yield of allylic ace-
tates, which means that a turnover number was also doubled, thus
illustrating the catalyst stability (Table 1, runs 6 and 7 vs. run 4). The
rate positively depended on the palladium concentration (nearly first
order) and on the dioxygen pressure (also nearly first order) implying
that the regeneration of palladium is probably the slowest reaction of the
catalytic process (Table 1, run 4 vs. run 7 and run 8, respectively). The
process was successfully performed under 5 atm of dioxygen (Table 1,
run 8); however, under 1 atm the catalyst regeneration was not efficient
and palladium metal appeared on the reactor walls (nor shown in
Table 1). The proportion of DMC in the solvent mixture could be
increased to 70 vol% without a negative impact on the reaction selec-
tivity, albeit the reaction was decelerated (Table 1, run 9 vs. run 4).
Diethyl carbonate (DEC) and methyl isobutyl ketone (MIBK) were
also tested as the reaction media for the cis-jasmone oxidation in the
“palladium solo” system (Table 1, runs 10, 11 and 12). Both DEC and
MIBK have excellent rankings in solvent sustainability guides occupying
the positions comparable with those of ethanol and water [32,33]. The
replacement of DMC by DEC did not affect the reaction performance
(Table 1, run 10 vs. run 4). In the MIBK/HOAc mixture, the reaction
–
–
–
reason for low reactivity of the endocyclic C C bond could be its
–
–
deactivation by the allylic C O group. The same reaction performed in
a 50 vol% mixture of HOAc and dimethylcarbonate (DMC) was slightly
slower; however, it showed nearly the same product selectivity (Table 1,
run 2 vs. run 1). DMC is a green biodegradable solvent with low toxicity
and much better sustainability score in the solvent selection guides than
acetic acid [33].
To our surprise, the oxidation of cis-jasmone was catalytic with
respect to palladium in the mixed DMC/HOAc solvent even without BQ
(Table 1, run 3). At 80 ◦C, the total selectivity for allylic oxidation was
lower than with BQ (86% vs. 96%) due to the contribution of high
molecular weight products which could not be detected by GC. How-
ever, the selectivity could be improved at lower temperature, 60 ◦C
(Table 1, run 4). Although the reaction occurred slower, it could be
nearly completed in 4 h with 85% selectivity for the main product 1b
and 96% total selectivity for the allylic acetates. Thus, under the applied
conditions, Pd(OAc)2 operated as a sole catalyst without the assistance
of additional redox co-catalysts (i.e., “palladium solo” system). Species
of the reduced palladium seem to be stabilized in the solution suffi-
ciently for the direct re-oxidation by dioxygen before their
3

M. dos Santos Costa et al.
Applied Catalysis A, General 625 (2021) 118349
Table 2
a
Oxidation of alkenes with dioxygen catalyzed by Pd(OAc)2
.
Run
Substrate
Co-solvent
(vol%)
T
Time
(h)
Conversion
(%)
Products/Selectivity
(%)
Rateb
(◦C)
(mM hꢀ 1
)
1
2
3
4
5
1-Octene (2a)
1-Octene (2a)
1-Octene (2a)
1-Octene (2a)
Cyclohexene (3a)
DMC (50)
MIBK (50)
MIBK (70)
None
60
60
70
60
60
6
8
95
73
96
99
47
52
59
63
24
30
90
55
98
96
2b (30) allylic acetates (70)
2b (26) allylic acetates (74)
2b (25) allylic acetates (65)
2b (20) allylic acetates (50)
3b (100)
80
120
60
6
4
120
30
DMC (50)
4
8
3b (100)
6
7
Cyclohexene (3a)
Cyclohexene (3a)
DMC (50)
MIBK (50)
70
60
4
3b (100)
70
12
8
3b (100)
4
3b (100)
8
3b (100)
8
Limonene (4a)
DMC (50)
DMC (50)
DMC (50)
DMC (50)
80
60
60
80
6
4b (25) 4c (11) 4d (3)
5b (28) 5c (10) 5d (25)
6d (50)
60
4
9
β-Caryophyllene (5a)
β-Ionone (6a)
24
6
10
11
66
48
β-Citronellene (7a)
6
7b (30)
a
Conditions: [substrate] = 0.20 M, [Pd(OAc)2] = 10 mM, gas phase – O2, 10 atm, solvent – HOAc. Conversion and selectivity calculations were based on the
reacted substrate. DMC – dimethylcarbonate; DEC – diethylcarbonate; MIBK – methyl isobutyl ketone.
b
Average rate calculated between 0 and ca. 40% conversion.
–
occurred slower as compared to DMC and DEC but with the same
selectivity (Table 1, runs 11 vs. runs 4 and 10). A slight temperature
increase (from 60 to 70 ◦C) allowed to accelerate the reaction and to
decrease the HOAc content in the MIBK reaction mixture (Table 1, run
12). Main product 1b was obtained with 86% selectivity at 90% con-
version for 6 h, with the total selectivity for the allylic oxidation being
98%. As a positive result, it can be noted that the runs in mixed solvents
showed better selectivity for main acetate 1b as compared with the re-
action in pure acetic acid (Table 1, runs 1 and 5). It seems that the C-C
double bond isomerization in cis-jasmone and other undesirable trans-
formations are less favored in the solutions with lower acidity. As re-
ported in our previous work [34], the mixtures of allylic acetates
obtained from cis-jasmone have a pleasant smell and could be utilized in
flavor formulations directly.
acetates indicated a significant isomerization of the C C bond in 1-
–
octene under the applied conditions. The oxidation of cyclohexene
under similar conditions resulted exclusively in allylic acetate 3b
(Table 2, runs 5, 6 and 7). The re-oxidation of palladium was also effi-
cient as no Pd mirror was detected on the reactor walls. However, the
conversion of cyclohexene became stagnated at ca. 60% in the DMC and
at ca. 30% in MIBK solutions.
–
–
The oxidation of trisubstituted endocyclic C C bonds in limonene
(4a) and β-caryophyllene (5a) gave corresponding allylic acetates.
However, the selectivity for allylic oxidation was only ca. 40% in both
reactions (Table 2, runs 8 and 9). In the run with β-caryophyllene, the
–
–
product of the endocyclic C C bond epoxidation (epoxide 5d) was also
identified as one of the main products (25% selectivity). The molecule of
the next substrate, β-ionone (6a), contains two C-C double bonds: the
endocyclic tetrasubstituted and exocyclic disubstituted ones. β-Ionone
has showed a relatively high reactivity in the mixed DMC/HOAc solvent
being almost completely converted for 6 h at 60 ◦C (Table 2, run 10).
The reaction gave epoxide 6d with 50% selectivity, originated by the
Encouraged by the results obtained with cis-jasmone, we have
applied the “palladium solo” system in mixed solvents for other alkenes
(Table 2). First, we tested 1-octene (2a) and cyclohexene (3a) as model
–
molecules. The oxidation of the terminal C C bond in 1-octene
–
–
–
occurred smoothly in both DMC/HOAc and MIBK/HOAc mixtures
(Table 2, runs 1, 2 and 3). No formation of palladium metal occurred in
these runs indicating the effective catalyst regeneration. Three isomeric
allylic acetates with different locations of the alkene and acetate func-
tionalities were detected as main products with up to 75% total selec-
tivity, along with smaller amounts (20–30%) of 2-octanone 2b. The
selectivity for oxidation in mixed solvents was much higher than in pure
acetic acid (Table 2, run 4). The formation of several isomeric allylic
oxidation of the endocyclic C C bond, along with a complex mixture of
unidentified products. The only identified product formed from β-cit-
ronellene (7a) was allylic acetate 7b derived from the oxidation of the
–
trisubstituted internal C C bond (Table 2, run 11).
–
In all reactions shown in Table 2, the palladium re-oxidation was
performed directly by dioxygen, i.e., Pd(OAc)2 operated as the sole
catalyst. Although all the alkenes demonstrated a reasonable reactivity
in the “palladium solo” system, most of the reactions were either low
4

M. dos Santos Costa et al.
Applied Catalysis A, General 625 (2021) 118349
Table 3
Oxidation of cyclohexene (3a) with dioxygen catalyzed by Pd(OAc)2/BQ in different solventsa.
Run
Solvent
HOAc
Time
(h)
Conversion
(%)
Selectivity for 3b
Rateb
(vol%)
(%)
(mM hꢀ 1
)
1c
2
DMC
HOAc
DMC
DMC
DMC
MIBK
MIBK
PC
50
100
30
2
4
6
6
8
8
6
4
4
100
100
100
100
90
97
91
184
104
94
3
97
4
20
98
80
5
10
98
52
6
20
50
100
100
95
48
7c
8
20
80
60
50
100
96
104
100
9
PC
20
98
a
Conditions: [cyclohexene] = 0.20 M, [Pd(OAc)2] = 10 mM, [p-benzoquinone (BQ)] = 50 mM, gas phase – O2, 10 atm, 60 ◦C. Conversion and selectivity cal-
culations were based on the reacted substrate. DMC – dimethyl carbonate; DEC – diethyl carbonate; MIBK – methyl isobutyl ketone; PC – propylene carbonate.
b
Average rate calculated between 0 and ca. 40% conversion.
c
70 ◦C.
Table 4
Oxidation of alkenes with dioxygen catalyzed by Pd(OAc)2/BQ in dimethyl carbonate (DMC) solutionsa.
Run
Substrate
HOAc
T
Time
(h)
Conversion
(%)
Products/Selectivity
(%)
TOFb
(vol%)
(◦C)
(mM hꢀ 1
)
1
2
3
4
5
6
7
8
cis-Jasmone (1a)
cis-Jasmone (1a)
1-Octene (2a)
20
10
20
10
20
20
40
30
60
60
80
60
80
70
80
80
8
24
6
95
78
95
90
80
99
80
72
1b (90)
1b (87)
36
16
90
52
48
68
30
40
Allylic acetates (92)
3b (98)
Cyclohexene (3a)
Limonene (4a)
8
6
4b (40) 4c (47) 4d (5)
β-Caryophyllene (5a)
β-Ionone (6a)
8
5b (64) 5c (22)
6b (80) 6c (18)
7b (97)
8
β-Citronellene (7a)
8
a
Conditions: [substrate] = 0.20 M, [Pd(OAc)2] = 10 mM, [p-benzoquinone (BQ)] = 50 mM, gas phase – O2, 10 atm, solvent – DMC. Conversion and selectivity
calculations were based on the reacted substrate.
b
Average rate calculated between 0 and ca. 40% conversion.
selective towards the allylic oxidation or became stagnated at incom-
plete conversions (cyclohexene). To overcome these limitations, we
decided to add BQ in catalytic amounts to the system. Besides the
function to re-oxidize palladium during the catalytic cycle, BQ acts as a
ligand for palladium and is known to be often capable to improve the
catalyst performance. As for the regeneration of BQ, we have previously
found that the oxidation of hydroquinone (BQH2, the reduced form of
BQ) can be performed directly by molecular oxygen under 5–10 atm
[42]. Contrarily, under the atmospheric pressure, the reaction of BQH2
with molecular oxygen is slow and requires the presence of additional
5

M. dos Santos Costa et al.
Applied Catalysis A, General 625 (2021) 118349
Table 5
Oxidation of alkenes with dioxygen catalyzed by Pd(OAc)2/BQ in methyl isobutyl ketone (MIBK) solutionsa.
Run
Substrate
HOAc
T
Time
(h)
Conversion
(%)
Products/Selectivity
(%)
TOFb
(vol%)
(◦C)
(mM hꢀ 1
)
1
2
3
4
5
6
7
8
cis-Jasmone (1a)
cis-Jasmone (1a)
1-Octene (2a)
20
10
10
20
30
20
30
30
70
70
80
70
80
70
80
80
8
24
8
90
73
65
80
90
98
57
99
1b (87)
1b (85)
72
12
44
60
70
66
8
Allylic acetates (90)
3b (100)
Cyclohexene (3a)
Limonene (4a)
6
8
4b (38) 4c (51) 4d (4)
β-Caryophyllene (5a)
β-Ionone (6a)
6
5b (70) 5c (16)
6b (83) 6c (9)
7b (91)
24
8
β-Citronellene (7a)
48
a
Conditions: [substrate] = 0.20 M, [Pd(OAc)2] = 10 mM, [p-benzoquinone (BQ)] = 50 mM, gas phase – O2, 10 atm, solvent – MIBK. Conversion and selectivity
calculations were based on the reacted substrate.
b
Average rate calculated between 0 and ca. 40% conversion.
metal co-catalysts, such as copper or manganese salts [8,19,22].
The aim of our further study was to find optimal conditions for the
efficient and selective allylic oxidation of alkenes in green reaction
media using minimal amounts of acetic acid as the reactant. Cyclo-
hexene, the molecule with no options for the double bond isomerization,
was chosen as a model substrate (Table 3). Really, the addition of BQ
had a beneficial effect on the reaction, which was completed for 2 h to
give allylic acetate 3b in nearly quantitative yield (run 1 in Table 3 vs.
run 6 in Table 2). The oxidation of cyclohexene in DMC/HOAc mixtures
occurred slower than in pure acetic acid (Table 3, runs 3–5 vs. run 2).
However, the reaction was reasonably fast to be nearly completed in 8 h
at 60 ◦C even with only 10 vol% of HOAc (corresponding to ca. 9 molar
equivalents with respect to the substrate, Table 3, run 5). The selectivity
for allylic acetate 3b, was improved in DMC solutions as compared to
pure acetic acid allowing for nearly quantitative yields. MIBK can be also
used as a reaction medium for the oxidation of cyclohexene (Table 3,
runs 6 and 7). The reaction rate in MIBK solutions was lower then in
DMC at the same concentration of acetic acid (Table 3, run 6 vs. run 4).
Propylene carbonate (PC) is also recognized as an eco-friendly
alternative for traditional organic solvents due to its low toxicity,
biodegradability, low corrosivity and other attractive characteristics
which render PC an excellent position in modern solvent sustainability
guides, even better than MIBK and DMC [33,43]. In the present study,
we have demonstrated, for the first time to the best of our knowledge, a
high potential of PC as a green solvent for the palladium catalyzed
oxidation of alkenes. The allylic oxidation of cyclohexene proceeded in
the PC solutions at nearly the same rate as in pure HOAc and with even
better selectivity (Table 3, runs 8 and 9 vs. run 2). The reaction could be
completed in 4 h at 60 ◦C in the presence of 20 vol% of HOAc (Table 3,
run 9). A further reduction in the HOAc concentration was not possible
to perform due to the problems with the solubility of alkene as PC
= 64) is much more polar than HOAc ( = 6), MIBK ( = 13), and
DMC ( = 3). It should be mentioned that in all other solvents tested
(
ε
ε
ε
ε
(DMC, DEC and MIBK) no problems with miscibility of the components
were detected under the conditions employed. The reaction in the PC
solution occurred faster than in DMC and MIBK at the same concen-
trations of HOAc (Table 3, cf. runs 4, 6 and 9). No formation of acetate
esters from acetic acid and DMC, DEC or PC was detected under the
reaction conditions employed.
The scope of the substrates for the Pd/BQ catalyzed allylic oxidation
in green solvents was extended to several other alkenes with different
types of C-C double bonds. Representative data obtained by the opti-
mization of reaction variables are collected in Table 4 for DMC, Table 5
for MIBK and Table 6 for PC. For the comparison, the data on the
oxidation of these substrates in HOAc solutions under similar conditions
are shown in Table 7. The reaction of cis-jasmone (1a) gave allylic ac-
etate 1b as the main product in higher selectivity (ca. 90%) than in pure
HOAc (runs 1 and 2 in Table 4, vs. run 2 in Table 7). 1-Octene (2a) and
cyclohexene (3a) also gave almost exclusively the corresponding allylic
acetates in DMC solutions containing only 10–20 vol% of HOAc
(Table 4, runs 3 and 4). For all the substrates, lowering the HOAc con-
centration in DMC decelerated the reaction. For limonene (4a) and
β-caryophyllene (5a), the best balance was achieved at 20 vol% of HOAc
(Table 4, runs 5 and 6). For both substrates, only endocyclic double
bonds reacted with palladium to give a mixture of allylic acetates in a
total selectivity of ca. 90%. The terminal disubstituted double bonds in
both molecules 5a and 6a remained intact. A similar situation occurred
with β-ionone (6a) (Table 4, run 7). The main product 6b originated by
the oxidation of the endocyclic tetrasubstituted double bond, whereas
–
the disubstituted internal C C bond in a side chain remained intact
–
6

M. dos Santos Costa et al.
Applied Catalysis A, General 625 (2021) 118349
Table 6
Oxidation of alkenes with dioxygen catalyzed by Pd(OAc)2/BQ in propylene carbonate (PC) solutionsa.
Run
1
Substrate
HOAc
T
Time
(h)
Conversion
(%)
Products/Selectivity
(%)
Rateb
(vol%)
(◦C)
(mM hꢀ 1
)
cis-Jasmone (1a)
50
70
1
6
4
4
4
3
2
6
6
82
98
86
96
95
96
42
91
93
1b (28) 1c (70)
1b (50) 1c (48)
164
2
3
4
5
6
7
8
1-Octene (2a)
20
20
30
20
30
30
30
80
60
80
80
70
80
80
Allylic acetates (85)
120
126
102
120
40
Cyclohexene (3a)
Limonene (4a)
3b (98)
4b (13) 4c (53) 4d (24)
4b (12) 4c (54) 4d (28)
5b (81) 5c (12)
6b (82) 6c (10)
7b (90)
Limonene (4a)
β-Caryophyllene (5a)
β-Ionone (6a)
80
β-Citronellene (7a)
60
a
Conditions: [substrate] = 0.20 M, [Pd(OAc)2] = 10 mM, [p-benzoquinone (BQ)] = 50 mM, gas phase – O2, 10 atm, solvent – PC. Conversion and selectivity
calculations were based on the reacted substrate. b Average rate calculated between 0 and ca. 40% conversion.
–
(probably, due to the deactivation by the allylic C O group). Due to the
Promising results were also obtained for most of the substrates in PC
solutions (Table 6). However, high polarity of this solvent could cause
the problems with solubility or selectivity. In particular, the oxidation of
cis-jasmone (1a) was heavily complicated by a rapid substrate isomeri-
zation to give conjugated diene 1c. After 1 h, the reaction showed 82%
conversion and only 28% selectivity for allylic oxidation product 1b,
with the rest of cis-jasmone being converted into 1c (Table 6, run 1). At
longer reaction time, the selectivity for 1b was gradually increased due
to the oxidation of 1c by palladium and/or the shift of the isomerization
equilibrium towards the more reactive isomer, cis-jasmone 1a, and its
further oxidation. All other substrates, except β-caryophyllene (5a),
showed excellent performance in PC solutions and gave corresponding
allylic oxidation products with 80–98% selectivity at almost complete
conversions (Table 6). The content of HOAc in the PC solutions should
be at least 20–30 vol%, otherwise the problems with the alkene and/or
product solubility appear. For β-caryophyllene (5a), the bulkiest sub-
strate among the alkenes studied, the solubility problems were observed
even with 30 vol% of HOAc (Table 6, run 6). The reaction of β-car-
yophyllene showed high selectivity for allylic oxidation products 5b and
5c up to ca. 40% conversion, after that the reaction mixture turned
murky. It is worth mentioning that under the same conditions, the re-
actions with all alkenes were faster in PC solutions than in DMC and
MIBK. Moreover, the reactions in PC were faster than even in pure acetic
acid (cf. Table 6 and Table 7). It should be mentioned that for all sub-
strates, the allylic oxidation selectivity was higher or at least the same in
PC as well as in DMC and MIBK solutions as compared to the reactions
performed in pure acetic acid under similar conditions (Table 7 vs.
Tables 4–6).
–
relatively low reactivity of β-ionone, reasonable reaction rates were
obtained only with 40 vol% of HOAc, otherwise the reaction required
for longer reaction times. The reaction with β-citronellene (7a) showed
excellent selectivity for allylic acetate 7b originated by the oxidation of
the trisubstituted C-C double bond. The terminal monosubstituted
olefinic bond was not involved in the oxidation. The only hydrogen in
–
–
the position allylic to the terminal C C bond in 7a is attached to the
tertiary carbon atom and therefore it is not easily abstractable, which
could be the reason of the low reactivity of this bond towards oxidation
under the applied conditions.
MIBK was also found to be an excellent green reaction medium for
the palladium catalyzed oxidation of alkenes (Table 5). Similar to that
observed in DMC, the reactions occurred slower at lower acetic acid
concentrations. Nevertheless, under optimized conditions, all the sub-
strates studied were oxidized in the presence of 10–30 vol% of HOAc at
reasonable rates to give corresponding allylic acetates with 85–100%
selectivity. Although in most cases, the reactions in MIBK solutions were
slower than in pure acetic acid, the selectivity for the allylic oxidation
was the same for substrates 4a and 7a and higher for the other alkenes
(cf. Table 5 and Table 7).
To check the possibility to obtain ketones from olefins, we performed
the reactions with cis-jasmone (1a) under the conditions of run 2 in
Table 4 and run 2 in Table 5 using 10 vol% of water instead of acetic acid
in DMC and MIBK, respectively. Due to miscibility problems, it was not
possible to use more than 10 vol% of water in these solvents. Unfortu-
nately, the attempts were not successful. Practically no olefin conversion
was observed in these reactions; moreover, the catalytic system was not
stable under such conditions, with palladium mirror being detected on
the walls of the reactor after the runs.
7

M. dos Santos Costa et al.
Applied Catalysis A, General 625 (2021) 118349
Table 7
Oxidation of alkenes with dioxygen catalyzed by Pd(OAc)2/BQ in HOAc solutionsa.
Run
Substrate
T
Time
(h)
Conversion
(%)
Products/Selectivity
(%)
Rateb
(◦C)
(mM hꢀ 1
)
1
2
3
4
5
6
7
8
cis-Jasmone (1a)
cis-Jasmone (1a)
1-Octene (2a)
70
60
80
60
80
70
80
80
4
8
7
4
4
8
7
8
97
94
1b (82) 1c (6)
1b (61) 1c (15)
250
188
100
104
80
100
100
96
Allylic acetates (75)
Cyclohexene (3a)
Limonene (4a)
3b (91)
4b (33) 4c (46) 4d (10)
5b (59) 5c (11)
6b (70) 6c (20)
7b (93)
β-Caryophyllene (5a)
β-Ionone (6a)
90
30
80
30
β-Citronellene (7a)
90
50
a
Conditions: [substrate] = 0.20 M, [Pd(OAc)2] = 10 mM, [p-benzoquinone (BQ)] = 50 mM, gas phase – O2, 10 atm, solvent – acetic acid. Conversion and
selectivity calculations were based on the reacted substrate.
b
Average rate calculated between 0 and ca. 40% conversion.
4. Conclusions
Investigation, Writing – original draft. Amanda de Camargo Faria:
Methodology, Investigation, Validation. Rayssa L.V. Mota: Methodol-
ogy, Investigation. Elena V. Gusevskaya: Conceptualization, Supervi-
sion, Project administration, Writing – review & editing.
As the solvent plays a central role in sustainability, employing sol-
vents with good sustainability ranks is the issue of a great importance for
reducing a negative environmental impact of the industrial process. In
this work we proposed several green reaction media for the palladium
catalyzed allylic oxidation of alkenes by dioxygen. Dimethylcarbonate
(DMC), methyl isobutyl ketone (MIBK), and propylene carbonate (PC),
the compounds with excellent positions in modern solvent selection
guides and very scarcely exploited in palladium catalyzed oxidations,
were proved to be excellent alternatives for conventionally used sol-
vents, such as HOAc. The systems with green solvents mostly showed
better selectivity than the reactions in pure HOAc and, for some alkenes,
even allowed to dispense the use of additional co-catalysts, e.g., p-ben-
zoquinone. Palladium acetate alone or in the combination with p-ben-
zoquinone efficiently operates as the catalyst for the allylic oxidation of
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgments
We acknowledge fellowships and financial support from Conselho
´
Nacional de Desenvolvimento Científico e Tecnologico (CNPq, Brazil)
˜
Coordenaçao de Aperfeicoamento de Pessoal de Nível Superior (CAPES,
–
alkenes with different types of C C bonds. The regeneration of p-ben-
–
˜
`
Brazil), Fundaçao de Amparo a Pesquisa do Estado de Minas Gerais
zoquinone was performed directly by dioxygen at 5–10 atm. We tested a
ˆ
(FAPEMIG, Brazil), and Instituto Nacional de Ciencia e Tecnologia de
broad range of substrates:
α
-alkenes and alkenes containing internal
´
´
Catalise em Sistemas Moleculares e Nanoestruturados (INCT-Catalise,
–
–
disubstituted, trisubstituted and tetrasubstituted C C bonds, including
Brazil).
several biomass-based challenging alkenes. In comparison with pure
acetic acid, the reactions in DMC and MIBK solutions were slower for
most of the substrates tested, whereas the reactions in PC solutions were
faster. Thus, we conclude that DMC, MIBK and PC are highly recom-
mended solvents for the palladium catalyzed allylic oxidation of alkenes,
in terms of both the catalyst performance and sustainability.
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