Pyrazoles as ligands for the Rh-catalyzed hydroformylation of alkenes in supercritical carbon dioxide

Article abstract of DOI:10.1007/s11172-012-0331-z

Activity of pyrazole ligands in hydroformylation of a number of unsaturated terminal substrates in supercritical carbon dioxide (scCO₂) in the presence of Rh-catalyst was studied. The ligands without free NH group at position 1 of the pyrazole ring were found to be active. The use of scCO ₂ as the reaction medium served to reach a higher conversion and regioselectivity of hydroformylation as compared to those in toluene.

Full text of DOI:10.1007/s11172-012-0331-z

2356
Russian Chemical Bulletin, International Edition, Vol. 61, No. 12, pp. 2356—2359, December, 2012
Brief Communications
Pyrazoles as ligands for the Rhꢀcatalyzed hydroformylation
of alkenes in supercritical carbon dioxide
S. E. Lyubimov, E. A. Rastorguev, and V. A. Davankov
A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
2
8 ul. Vavilova, 119991 Moscow, Russian Federation.
Fax: +7 (499) 135 5085. Eꢀmail: lssp452@mail.ru
Activity of pyrazole ligands in hydroformylation of a number of unsaturated terminal subꢀ
strates in supercritical carbon dioxide (scCO ) in the presence of Rhꢀcatalyst was studied. The
2
ligands without free NH group at position 1 of the pyrazole ring were found to be active. The use
of scCO as the reaction medium served to reach a higher conversion and regioselectivity of
2
hydroformylation as compared to those in toluene.
Key words: pyrazoles, hydroformylation, supercritical carbon dioxide, rhodium.
One of the most important catalytic processes is hyꢀ
rhodium or cobalt complexes with organophosphorus
ligands (several examples are given in Refs 5—11). Amine
ligands are easier available and more stable on storage,
droformylation of alkenes leading to the preparation of
aldehydes, which are used in perfumery, food and pharꢀ
maceutical industries, in the production of varnishes, dyes,
detergents, and plastics stabilizers.1 The reaction is comꢀ
1
2,13
but they are not yet widely used in hydroformylation. An
exception is the use of rhodium complexes of tris(pyrꢀ
azolyl)methanesulfonate ligand in hydroformylation of
,2
3
monly carried out by the Wilkinson method in the presꢀ
1
4
ence of rhodium complexes with phosphorusꢀcontaining
ligands. Taking into account fireꢀhazard and explosive
character of the process (hydrogen, carbon(II) oxide), the
search of alternative media for carrying out the hydroꢀ
formylation process is an actual problem.
hexꢀ1ꢀene in organic solvents. In the present work, we
were able for the first time to show a possibility of efficient
application of pyrazole ligands in the Rhꢀcatalyzed hyꢀ
droformylation of alkenes in scCO2.
One of the most promising alternative approaches conꢀ
Results and Discussion
sists in the use of supercritical carbon dioxide (scCO ), an
2
available, environmently friendly, and incombustible solꢀ
vent. However, until the present time this solvent was
only used for hydroformylation of alkenes catalyzed by
Pyrazoles 1—4 were used as ligands, including those
4
with fluorinated substituents, which, as it is known, can
1
5
increase the solubility of the catalysts in scCO2.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2334—2336, December, 2012.
066ꢀ5285/12/6112ꢀ2356 © 2012 Springer Science+Business Media, Inc.
1

Pyrazole ligands for hydroformylation
Russ.Chem.Bull., Int.Ed., Vol. 61, No. 12, December, 2012
2357
Scheme 1
R = Ph (a), 4ꢀMeꢀC H (b), 4ꢀBrꢀC H4 (c), 2ꢀnaphthyl (d),
6
4
6
t
(
CH ) Me (e), Bu (f)
2 4
Reagents and conditions: i. H : CO = 1 : 1, 25 atm.
2
Hydroformylation of styrene (5a) in toluene at 50 C
with the in situ formation of the catalysts starting from
entry 10). An increase in the temperature to 60 C resulted
in complete conversion, however, in this case a decrease
in regioselectivity was observed (see Table 1, cf. entries 10
and 11). The use of 4ꢀbromostyrene (5c) as the substrate
allowed us to obtain the complete conversion and a high
regioselectivity at 50 C (see Table 1, entry 12). In the case
of 2ꢀvinylnaphthalene (5d), the complete conversion was
observed within 4 h at 60 C, however, a decrease in temꢀ
perature allowed us to insignificantly increase regioselecꢀ
tivity (see Table 1, entries 13 and 14). The hydroformylaꢀ
tion of heptꢀ1ꢀene (5e) (see Scheme 1) proceeded with the
quantitative conversion and led to a predominant formaꢀ
tion of the linear isomer 7 (see Table 1, entry 15) rather
than a branched product. In the experiment with hydroꢀ
formylation of 3,3ꢀdimethylbutꢀ1ꢀene (5f), it was found
that an increase in temperature promoted the increase in
conversion, with the formation of only linear isomer 7
being observed in all the cases (see Table 1, entries 16
and 17).
[
Rh(CO) Cl] and two equivalents of the ligand has been
2 2
chosen as a model reaction (Scheme 1). The reaction proꢀ
ceeded with a low (29%) conversion only in the presence
of the catalytic system based on pyrazole 2 containing
4
ꢀfluorophenyl substituent, whereas the other catalysts
turned out to be inefficient (see Table 1, entries 1—4). In
scCO styrene could be converted in the presence of the
2
catalyst based on ligands 1 and 2 at the same temperatures
and a pressure of the synthesis gas of 25 atm, whereas
pyrazoles 3 and 4 containing free NH group have proved
inactive (Table 1, entries 5—8). More bulky pyrazole
ligand 2 showed the same regioselectivity as the ligand 1,
but a considerably lower conversion. An increase in temꢀ
perature led to a small decrease in regioselectivity of the
process (see Table 1, cf. entries 5 and 9).
The rhodium catalyst with ligand 1, which showed the
best result, was studied in hydroformylation of other unꢀ
saturated substrates in scCO (see Scheme 1). Thus, high
2
conversion and regioselectivity were reached in hydroꢀ
formylation of 4ꢀmethylstyrene (5b) at 50 C (see Table 1,
Pyrazole ligand 1 was also used in the Rhꢀcatalyzed
hydroformylation of (R)ꢀlimonene (Scheme 2).
Table 1. Results of hydroformylation of unsaturated substrates (4 h)
Entry
Ligand
R
Medium
P/atm
T/C
Conversion (%)
Selectivity 6/7
1
2
3
4
5
6
7
8
9
1
2
3
4
1
2
3
4
1
1
1
1
1
1
1
1
1
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Toluene
Toluene
Toluene
Toluene
scСО2
scСО2
scСО2
scСО2
scСО2
scСО2
scСО2
scСО2
scСО2
scСО2
scСО2
scСО2
scСО2
25
25
25
25
200
200
200
200
200
200
200
200
200
200
200
200
200
50
50
50
50
50
50
50
50
60
50
60
50
50
60
50
50
60
0
29
0
0
100
25
—
82/18
—
—
a
92/8
92/8
—
a
a
0
0
a
—
a
100
85
100
100
80
100
100
76
88/12
92/8
85/15
92/8
90/10
86/14
40/60
0/100
0/100
a
10
11
12
13
14
15
16
17
4ꢀMe—C H
4ꢀMe—C H
4ꢀBr—C H
2ꢀNaphthyl
2ꢀNaphthyl
6
4
4
a
6
a
6
4
a
a
a
(CH ) CH
2
4
3
t
a
Bu
Bu
t
a
100
a
The pressure of the synthesis gas was 25 atm.

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Russ.Chem.Bull., Int.Ed., Vol. 61, No. 12, December, 2012
Lyubimov et al.
Table 2. Results of hydroformylation of (R)ꢀ8 in scCO2
followed by the addition of the corresponding substrate 5
(
P
2 mmol). The autoclave was filled with the synthesis gas (25 atm,
: PCO = 1 : 1) and then with carbon dioxide using a High
Entry Ligand
P
atm
T
/C
t/h
Converꢀ Selectivity
H2
Pressure Equipment manually operated pump until the pressure
reached 200 atm. The mixture was heated over 5 min to the
corresponding temperature with stirring using a magnetic stirrer.
After a required period of time (see Table 1 and 2), the autoclave
was cooled to 5 C (10 min), the reaction gases were slowly
/
sion (%)
9/10
1
2
1
1
200a
200
60
70
5
7
61
100
100/0
100/0
a
a
The pressure of the synthesis gas was 25 atm.
1
released. The reaction mixture was analyzed by H NMR.
The spectroscopic characteristics of the hydroformylation prodꢀ
2
3
ucts of 2ꢀphenylpropanal and 3ꢀphenylpropanal (R = Ph),
ꢀ(4ꢀmethylphenyl)propanal and 3ꢀ(4ꢀmethylphenyl)propanal
Scheme 2
2
2
3
24
(
R = 4ꢀMePh), 2ꢀ(4ꢀbromophenyl)propanal and 3ꢀ(4ꢀbromoꢀ
2
5
phenyl)propanal (R = 4ꢀBrPh), 2ꢀ(2ꢀnaphthyl)propanal
and 3ꢀ(2ꢀnaphthyl)propanal (R = naphthyl) , octanal and
2
3
2
3
26
2
7
28
ꢀmethylheptanal (R = (CH ) CH ), 4,4ꢀdimethylpentanal,
2 4 3
2
9
ꢀ(4ꢀmethylcyclohexꢀ3ꢀenyl)butanal (9) agreed with the literꢀ
ature data.
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at 60 C within 5 h with good conversion (Table 2, enꢀ
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Received June 15, 2012