1,4-cis-Hydrogenation of butyl sorbate in supercritical carbon dioxide

Article abstract of DOI:10.1007/s11172-018-2158-8

Hydrogenation of butyl sorbate into butyl Z-hex-3-enoate in supercritical carbon dioxide proceeds at high conversion and selectivity of 93–96% under catalysis with chromium hexacarbonyl at 180 °C and on using (methyl benzoate)chromium tricarbonyl as a catalyst at 160 °C.

Full text of DOI:10.1007/s11172-018-2158-8

Russian Chemical Bulletin, International Edition, Vol. 67, No. 5, pp. 923—926, May, 2018
923
Brief Communications
1,4-cis-Hydrogenation of butyl sorbate in supercritical carbon dioxide
A. A. Vasil´ev, I. V. Kuchurov, and S. G. Zlotin
N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences,
47 Leninsky prosp., 119991 Moscow, Russian Federation.
Fax: +7 (499) 135 5328. E-mail: vasiliev@ioc.ac.ru
Hydrogenation of butyl sorbate into butyl Z-hex-3-enoate in supercritical carbon dioxide
proceeds at high conversion and selectivity of 93—96% under catalysis with chromium hexa-
carbonyl at 180 °C and on using (methyl benzoate)chromium tricarbonyl as a catalyst at 160 °C.
Key words: hydrogenation, dienes, cis-alkenes, butyl sorbate, supercritical carbon dioxide,
chromium carbonyl complexes.
1,4-cis-Hydrogenation of conjugated dienes is recog-
nized as a convenient method for the stereocontrolled
preparation of monoolefins with various substitution pat-
tern at the double bond (see reviews1,2), which found
application in rational synthesis of pharmaceuticals, insect
pheromones, and fragrances. A unique type of the catalysts
for this purpose, for example, for hydrogenation of sorbic
acid esters 1 into Z-hex-3-enoates 2 with selectivity higher
than 90% (Scheme 1), turned out to be chromium carbonyl
complexes. According to the proposed mechanism,3,4
in the active complex A the diene acquires the cisoid
conformation and undergoes subsequent synchronous
1,4-addition of hydrogen thus providing stereospecific-
ity of the process. Carbonyl complexes of other metals
(Mo, W, and Co) catalyze hydrogenation of dienes with
low chemo- and stereoselectivity.5 In the 2000s, one more
Scheme 1
R = Bu (a), Me (b)
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 0923—0926, May, 2018.
1066-5285/18/6705-0923 © 2018 Springer Science+Business Media, Inc.

924
Russ. Chem. Bull., Int. Ed., Vol. 67, No. 5, May, 2018
Vasil´ev et al.
type of Cp*Ru complexes was found suitable for 1,4-cis-
hydrogenation.6,7
tion of all such isomeric dienes affords the same product,
although it proceeds at different rates due to steric rea-
sons.19—21
A substantial factor limiting the application of this
method in fine synthesis of sophisticated molecules is
the reaction temperature at which the initial complex
LnCr(CO)3 would dissociate into Cr(CO)3 active species
(see reviews1,2). The most available and stable chromium
hexacarbonyl Cr(CO)6 is catalytically active only at tem-
peratures of about 180 °C. Stable, accessible, and easy-to-
handle methyl benzoate complex (MeO2CPh)Cr(CO)3
(MBZ•Cr(CO)3) is active at 150—160 °C in hydrocarbon
solvents and at 120 °C in acetone. Naphthalene complex
(C10H8)Cr(CO)3 allows the reaction to proceed in THF
at 45 °C; however, this complex is poorly accessible, sen-
sitive to air, and is not good for storing.
The aim of the present work is verification of 1,4-cis-
hydrogenation in supercritical carbon dioxide (sc-CO2),
which was not previously used for this purpose. In more
conventional cases, it was indicated that the improved
solubility of molecular hydrogen in sc-CO2 compared to
organic solvents can facilitate the process.8—16 We antici-
pated that higher concentration of hydrogen in sc-CO2
could affect dissociation of the starting chromium carbonyl
complex, which could allow the reaction temperature to
be reduced. In the present work, model butyl sorbate (1a)
and two available complexes, namely, Cr(CO)6 and
MBZ•Cr(CO)3, were employed. Previously, hydrogena-
tion of diene 1a into butyl Z-hex-3-enoate (2a) in the
presence of Cr(CO)6 at 195 °C was patented.17
The experiment showed (Table 1) that the Cr(CO)6-
catalyzed hydrogenation in sc-CO2 at 180 °C afforded
a mixture of products containing 91% of 3Z-monoolefin 2a
and a lot of unreacted less active 2E,4Z-diene 1a (entry 1).
On lowering the temperature to 160 °C, the conversion
dropped dramatically (entry 2). More active MBZ•Cr(CO)3
complex at both temperatures provided practically full
conversion of all diene isomers (entries 3 and 4), the purity
of the product being 93—96%. Lowering the temperature
to 140—150 °C caused noticeable deceleration of the reac-
tion (entries 5 and 6).
A comparison of the obtained results with the known
data on hydrogenation of related methyl sorbate 1b in
organic solvents (see Table 1, entries 7—9) indicates that
behavior of sc-CO2 in the studied process is analogous to
that of the saturated hydrocarbons, which do not affect
dissociation of chromium carbonyl pre-catalyst. Supposing
that solubility of hydrogen in sc-CO2 is higher, one may
conclude that hydrogen molecule does not participate in
the primary dissociation of LnCr(CO)3 into the Cr(CO)3
species and this dissociation occurs due to thermal effect.
The dissociation temperature is known to be reduced by
the application of polar solvents (see Ref. 1).
The pressure value in the system in our experiments
must be of extreme attention. Previously,13—16 hydrogena-
tion in liquid and supercritical carbon dioxide was multi-
ply performed, although at temperatures essentially lower.
It was reported24 that the pressure of pure CO2 in the
constant volume reactor at maintaining the density of the
medium of d ≈ 0.8 g cm–3 at 20 °C and 70 atm on raising
the temperature above the critical point up to 140, 150,
160, and 180 °C should equate 658, 708, 757, and 855 atm,
The starting butyl sorbate (1a) was obtained by conver-
sion of sorbic acid into its chloride followed by treatment
with n-butanol in pyridine.18 The thus prepared sample
was the mixture of 2E,4E- (80%), 2E,4Z- (15%), and
2Z,4E-isomers (5%). According to the recognized mech-
anism of the process (see Scheme 1), 1,4-cis-hydrogena-
Table 1. Carbonylchromium-catalyzed hydrogenation of alkyl sorbates 1a,b in supercritical carbon dioxidea (entries 1—6) and in
hydrocarbon solventsb (entries 7—9)
Entry
Diene
Pre-catalyst
Solventc
T/°C
Product composition (%)
2
3
4
2E,4Z-1d 2E,4E-1
е
1
2
3
4
5
6
7
8
9
1a
1a
1a
1a
1a
1a
1b
1b
1b
Cr(CO)6
sc-CO2
sc-CO2
180
160
180
160
150
140
180
160
150
91
60
1
—
5
—
8
14
1
0.5
9
9
—
—
—
—
26
—
Cr(CO)6
—
1
MBZ•Cr(CO)3
MBZ•Cr(CO)3
MBZ•Cr(CO)3
MBZ•Cr(CO)3
Cr(CO)6
MBZ•Cr(CO)3
MBZ•Cr(CO)3
sc-CO2
93
sc-CO2
96
3
0.5
—
—
0.3
1
—
sc-CO2
36
1
1
3.7
3
0.1
54
61
2.6
—
sc-CO2
29
Hexane
93.4
96
Cyclohexane
Cyclohexane
98.8
1.1
—
a The starting pressures at 20 °C are 40 atm H2 + 70 atm CO2; 2 h.
b See Refs 3, 4, 22, and 23, ~50 atm H2, 2—3 h.
c In amount of 6.7 mol.% (entries 1—6).
d Total content of 2E,4Z- and 2Z,4E-isomers, the content of the latter did not exceed 1%.
e Trace amounts.

Hydrogenation of diene in sc-CO2
Russ. Chem. Bull., Int. Ed., Vol. 67, No. 5, May, 2018
925
was added dropwise. Gas evolution was observed. The mixture
was stirred at ambient temperature for 2.5 h, the solvent was
removed in vacuo, and the residue was vacuum distilled to afford
sorbic acid chloride in the yield of 3.0 g, yellow liquid, b.p. 105 °C
(30 Torr) (cf. Ref. 18: b.p. 80—82 °C (20 Torr)). Further on, the
solution of sorbic chloride (3.0 g, 23 mmol) in CH2Cl2 (20 mL)
was added to a stirred ice-cooled solution of n-butanol (1.85 g,
25 mmol) and pyridine (2.72 g, 34.5 mmol) in CH2Cl2 (30 mL),
and the mixture was stirred at ambient temperature for 2 h. The
mixture was successively washed with water, cold 5% HCl, water,
cold 5% NaOH, twice with NaHCO3 (aq.), and dried with CaCl2.
The solvent was removed in vacuo and the residue was vacuum
distilled to afford the title product 1a in the yield of 2.4 g (53%
on two steps), b.p. 125—128 °C (30 Torr), nD28 1.4851 (cf. Ref. 26:
b.p. 108—110 °C (14 Torr)). 1H NMR (CDCl3, 300 MHz), δ,
2E,4E-isomer: 0.93 (t, 3 H, J = 7.6 Hz); 1.40 (sext, 2 H,
J = 7.6 Hz); 1.64 (quint, 2 H, J = 7.6 Hz); 1.85 (d, 3 H, J = 5.5 Hz);
4.13 (q, 2 H, J = 7.0 Hz); 5.77 (d, 1 H, J = 15.8 Hz); 6.16 (m, 2 H);
7.24 (dd, 1 H, J = 15.0 Hz, J = 9.9 Hz); 2E,4Z-isomer, charac-
teristic signals: 1.89 (d, 3 H, J = 6.6 Hz); 5.87 (d, 1 H,
J = 15.0 Hz); 5.93 (m, 1 H); 6.15 (t, 1 H, J = 11.7 Hz); 7.63 (dd,
1 H, J = 15.4 Hz, J = 11.4 Hz); 2Z,4E-isomer, characteristic
signals: 5.55 (d, 1 H, J = 11.7 Hz); 6.53 (t, 1 H, J = 11.0 Hz);
7.37 (m, 1 H). Spectral data for 2E,4E-1a and similar geometric
isomers of linear alka-2,4-dienoic acid esters are in a good agree-
ment with published data.26—28
Hydrogenation of butyl sorbate (1a) (general procedure).
A stainless steel 2 cm3 autoclave was charged with diene 1a
(50 mg, ~0.3 mmol) and pre-catalyst (0.02 mmol, 6.7 mol.%),
namely, Cr(CO)6 (4.4 mg) or MBZ•Cr(CO)3 (5.3 mg). The
autoclave was purged with hydrogen, and the hydrogen pressure
was adjusted to 40 atm. The system was then fed with liquid CO2
to the total pressure of 110 atm. The reaction mixture was heated
with magnetic stirring at temperature specified in Table 1 for 2 h.
The autoclave was cooled to room temperature, carefully decom-
pressed, and opened. The content was rinsed off with CH2Cl2,
the washings were passed through a silica gel pad, and analyzed
by GC. The sample obtained in entry 4 (Table 1) was addition-
ally studied by 1H NMR and GC-MS.
respectively, which is dangerous for practical implementa-
tion. We specially measured the pressure inside the auto-
clave that at 20 °C initially contained hydrogen at a pres-
sure of 40 atm and then was fed with CO2 up to a total
pressure of 110 atm. It was found that on heating to 140,
150, 160, and 180 °C the pressure in the system was equal
to 167, 170, 175, and 190 atm, respectively. These values
are essentially lower than those for pure CO2 and relate to
the growth of hydrogen partial pressure without noticeable
contribution of CO2.
In summary, the chromium carbonyl-catalyzed 1,4-cis-
hydrogenation of conjugated dienes can be accomplished
in supercritical carbon dioxide. Although the application
of sc-CO2 did not allow us to reduce the reaction tem-
perature, one should take into account other advantages
of this solvent such as low cost, non-flammability, and
ecological safety.
Experimental
1H NMR spectra were recorded on a Bruker AM-300 instru-
ment in CDCl3 at a working frequency of 300.13 MHz. The
chemical shifts are given in the δ scale and referred to the re-
sidual proton signal. Gas chromatography was performed using
a Chrom 5 chromatograph, temperatures of both the injector and
flame ionization detector were 250 °C, a capillary column was
0.2×25000 mm in size, stationary phase was silicon SE-30.
Thermostat temperature was raised from 90 to 230 °C at a heat-
ing rate of 15 deg min–1. The mixture compositions were calcu-
lated using normalization method. Electron impact (EI, 70 eV)
mass spectra were measured on an Agilent 5977A quadrupole
instrument with sample injection via an Agilent 7890 gas chroma-
tograph.25 External calibration was performed automatically
using perfluorotributylamine (MS grade, Synquest Laboratories)
as a reference. Measurements were performed in full-scan mode
(scan range m/z 30—400; 6.7 Hz) with ionization energy set to
70 eV, source temperature set to 230 °C, and transfer capillary
temperature set to 300 °C. Chromatography separation for mass
spectra was carried out on an Agilent HP-5ms fused silica capil-
lary column (length, 30 m; diameter, 250 μm; film thickness,
0.25 μm; packing, 5% (phenyl)methylpolysiloxane) using helium
as a carrier gas with flow rate of 1 mL min–1. Injection port
temperature was set to 300 °C operating in a split mode at
a 10 : 1 ratio with a sample injection volume of 1 μL. Temperature
was programmed as follows: 60 °C maintaining for 4 min, linear
increase to 240 °C at a heating rate of 10 deg min–1, linear increase
to 300 °C at a heating rate of a rate of 30 deg min–1, maintaining
for 5 min. The spectra were processed using Agilent MassHunter
B.06.00 software package.
Butyl Z-hex-3-enoate (2a). 1H NMR (CDCl3, 300 MHz), δ:
0.94 (t, 3 H, J = 7.6 Hz); 0.99 (t, 3 H, J = 7.3 Hz); 1.39 (sext,
2 H, J = 7.6 Hz); 1.62 (quint, 2 H, J = 7.6 Hz); 2.06 (quint, 2 H,
J = 7.0 Hz); 3.08 (d, 2 H, J = 6.2 Hz); 4.09 (t, 2 H, J = 7.0 Hz);
5.55 (m, 2 H). MS, m/z: 170 [M]+, 114 [M – C4H8]+. An impu-
rity of butyl E-hex-2-enoate (3a), 1H NMR characteristic signals:
5.82 (d, J = 15.0 Hz), 6.95 (m); MS, m/z: 170 [M]+, 127
[M – C3H7]+, 115 [M – C4H7]+, 97 [M – BuO]+. An impurity
of butyl hexanoate (4a), MS, m/z: 172 [M]+, 117 [M – C4H7]+,
99 [M – BuO]+.
The authors are grateful to S. E. Lyubimov (A. N.
Nesmeyanov Institute of Organoelement Compounds of
the Russian Academy of Sciences) for help in measure-
ments of pressure in hydrogen—carbon dioxide system at
high temperatures.
Hydrogenation experiments were carried out in a 2 cm3 stain-
less steel autoclave equipped with a magnetic stirring bar. For
heating, an all-copper bath placed on a top of a temperature-
controlled hot-plate stirrer was used. Carbon dioxide was fed
using a Supercritical 24 high pressure pump (Scientific Systems,
Inc.) with an integrated cooling system using Peltier thermoelec-
tric modules. Carbon dioxide of 99.995% purity was used.
Butyl sorbate (1a), a mixture of 2E,4E-, 2E,4Z-, and 2Z,4E-
isomers in a 80 : 15 : 5 ratio. To a stirred ice-cooled mixture of
sorbic acid (technical grade, 3.0 g, 26.8 mmol), dichloromethane
(45 mL), and DMF (1 drop), oxalyl chloride (3.51 mL, 41 mmol)
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Received February 9, 2018;
in revised form February 26, 2018;
accepted March 23, 2018