The use of carboranylphosphite ligands in Rh-catalyzed hydroformylation of alkenes in supercritical carbon dioxide

Article abstract of DOI:10.1007/s11172-011-0316-3

Novel carborane-containing phosphites were synthesized. The efficiency of the ligands was tested in the Rh-catalyzed hydroformylation of styrene, hept-1-ene, and oct-1-ene in supercritical carbon dioxide (scCO₂) and toluene. The use of scCO₂ as the reaction medium allows one to attain higher conversions and regioselectivities compared to toluene.

Full text of DOI:10.1007/s11172-011-0316-3

Russian Chemical Bulletin, International Edition, Vol. 60, No. 10, pp. 2074—2077, October, 2011
2074
The use of carboranylphosphite ligands in Rhꢀcatalyzed
hydroformylation of alkenes in supercritical carbon dioxide
S. E. Lyubimov, P. V. Petrovskii, E. A. Rastorguev, T. A. Verbitskaya, V. N. Kalinin, and V. A. Davankov
A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
28 ul. Vavilova , 119991 Moscow, Russian Federation.
Fax: +7 (499) 135 5085. Eꢀmail: lssp452@mail.ru
Novel carboraneꢀcontaining phosphites were synthesized. The efficiency of the ligands was
tested in the Rhꢀcatalyzed hydroformylation of styrene, heptꢀ1ꢀene, and octꢀ1ꢀene in superꢀ
critical carbon dioxide (scCO2) and toluene. The use of scCO2 as the reaction medium allows
one to attain higher conversions and regioselectivities compared to toluene.
Key words: carboranylphosphites, hydroformylation, supercritical carbon dioxide, rhodium.
The study of regularities of chemical reactions cataꢀ
Results and Discussion
lyzed by transition metal complexes is an important asꢀ
pect of development of modern chemistry. Presently, conꢀ
siderable attention of researchers is concentrated on the
search for new efficient metallocomplex catalysts conꢀ
taining sterically bulky organophosphorus ligands.1—3
Icosahedral carboranes (C2B10H12) with a high steric facꢀ
tor are promising objects for the development of metalloꢀ
complex catalysts.4 We have recently shown that the use
of ligands of the phosphite type containing carborane subꢀ
stituents results in high conversions and enantioselectivity
in reactions of Rhꢀcatalyzed hydrogenation,5—11 Pdꢀcatꢀ
alyzed allyl substitution,12—14 and Suzuki—Miyaurа
Pdꢀcatalyzed crossꢀcoupling.15
Novel carboraneꢀcontaining phosphite ligands L1—L3
were synthesized by the oneꢀstep reaction of phosphoryꢀ
lating agent 1 with propanꢀ2ꢀol, 1,1,1,3,3,3ꢀhexafluoroꢀ
propanꢀ2ꢀol, and 2,6ꢀdimethylphenol (Scheme 1).
Scheme 1
One of the important processes of organometallic caꢀ
talysis is hydroformylation of alkenes, which gives aldeꢀ
hydes used in fragrance, food, and pharmaceutical indusꢀ
try and in manufacturing vanishes, paints, detergents, and
plasticizers.16—18 Since these reactions are carried out in
organic solvents in combination with with flammable and
explosive substances, the search for alternative media for
the process is needed. One of the most appropriate media
is supercritical carbon dioxide (scCO2) due to accessibiliꢀ
ty as well as environmental and fire safety of carbon diꢀ
oxide.19 The use of scCO2 as a medium provides a unique
possibility of handling of lowꢀboiling reactants and prodꢀ
ucts: carbon dioxide can readily be removed from the synꢀ
thesis products, whereas the isolation of products when
using organic solvents poses difficulties and requires furꢀ
ther fractionation. Despite the advantages of scCO2, there
have been few reports on its use in the metallocomplex
hydroformylation of alkenes.20—26 In this work, we present
for the first time the results of studies of novel carboranylꢀ
phosphite ligands in the Rhꢀcatalyzed hydroformylation
of alkenes in scCO2.
R = PriO (L1), (CF3)2CHO (L2), 2,6ꢀMe2C6H3O (L3)
i. RH, NEt3, PhMe
The protons of the methylene groups of carboraꢀ
nylphosphites L1—L3 are not equivalent (see Experimenꢀ
tal) because of a large size of the sevenꢀmembered heteroꢀ
cycle, which is also characteristic of a series of ligands
similar in structure.27,28
The efficiency of carboranylphosphites L1—L3 was
first studied in the Rhꢀcatalyzed hydroformylation of styꢀ
rene 2a in toluene with the formation of the catalysts in situ
from [(acac)Rh(CO)2] (acac is acetylacetonate) and
1 equiv. of the ligand (Scheme 2).
At the mole ratio substrate : catalyst = 200 : 1 it was
shown that for styrene 2a the maximum conversion within
4 h at 40 °C is observed when ligand L1 is used (Table 1,
entries 1—3). All ligands exhibit fairly high regioselectivity
towards the formation of branched product 3a (see
Scheme 2). When carboranylphosphite L1 is used, the
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2037—2040, October, 2011.
1066ꢀ5285/11/6010ꢀ2074 © 2011 Springer Science+Business Media, Inc.

Сarboranylphosphite ligands
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 10, October, 2011
2075
Scheme 2
regioselectivity (see Table 1, entries 5 and 10), and the use
of the increased pressure results in the complete converꢀ
sion of styrene 2a.
Hydroformylation of heptꢀ1ꢀene (2b) involving ligand
L1 revealed that this process is insensitive to the reaction
conditions. The variation of temperature and carbon diꢀ
oxide pressure changes the content of linear isomer (4b,
see Scheme 2); however, the same regioselectivity is obꢀ
served in all cases (see table 1, entries 11—13). Carboranylꢀ
phosphites L2 and L3 allow one to obtain close selectivity
in the formation of the linear product (see Table 1, entries
14—15). In all cases, the complete conversion is achieved
within 3 h. The close result was obtained for the hydroꢀ
formylation of octꢀ1ꢀene (2c, see Table 1, entries 16—18).
So, ligands L1—L3 allow one to attain quantitative conꢀ
version within 3 h, and the regioselectivity with respect to
linear isomer 4c is 62—66%.
Thus, carboraneꢀcontaining phosphites were used for
the first time in the Rhꢀcatalyzed hydroformylation of
terminal alkenes. These ligands can provide high converꢀ
sion in hydroformylation at low temperature (40 °С) within
3—4 h. It was shown that higher values of regioselectivity
and conversion can be attained in a medium of scCO2
compared to toluene.
R = Ph (a), nꢀC5H11 (b), nꢀC6H13 (c)
i. H2/CO (1 : 1, 35 atm), [(acac)Rh(CO)2]/L.
temperature increase from 40 to 60 °C increases converꢀ
sion but considerably decreases regioselectivity (see Table
1, entries 1 and 4). The replacement of toluene by scCO2
(total pressure 200 atm, 40 °C) and the use of the same
pressure of synthesis gas (35 atm) make it possible to atꢀ
tain a high or even complete conversion, and in the case of
ligand L1 the regioselectivity can be increased from 88 to
96% (see Table 1, entries 1—3 and 5—7). We also investiꢀ
gated the influence of the reaction temperature and СО2
pressure on the conversion and regioselectivity. When highꢀ
er temperatures (60 °C) are used, the regioselectivity in
scCO2 decreases (see Table 1, entries 7 and 8), although
this decrease is not so significant as in toluene (see Table 1,
entries 1 and 4). The decrease in the total pressure from
200 to 150 atm at the same pressure of synthesis gas
(35 atm) decreases both conversion and regioselectivity
(see Table 1, entries 5 and 9). The reaction carried out at
the total pressure 200 and 250 atm gives nearly the same
Experimental
1
H, 11B, 19F, and 31Р NMR spectra were recorded on
a Bruker Avance 400 instrument (working frequencies 400.13,
Table 1. Rhꢀcatalyzed hydroformylation of compounds 2a—cа
Entry
Compound
Ligand
Medium
Total
pressure,
P/atm
T/°C
t/h
Conversion
of compound 2
(%)
Ratio
of products
3 : 4
1
2
3
4
5
6
7
8
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2b
2b
2b
2b
2b
2c
2c
2c
L1
L2
L3
L1
L1
L2
L3
L1
L1
L1
L1
L1
L1
L2
L3
L1
L2
L3
Tolueneb
Tolueneb
Tolueneb
Tolueneb
skСО2
skСО2
skСО2
skСО2
skСО2
skСО2
skСО2
skСО2
skСО2
skСО2
skСО2
skСО2
skСО2
skСО2
35
35
35
40
40
40
60
40
40
40
60
40
40
40
40
60
40
40
40
40
40
4
4
4
4
4
4
4
4
4
4
3
3
3
3
3
3
3
3
74
52
50
95
95
88 : 12
89 : 11
89 : 11
69 : 31
96 : 4
89 : 11
93 : 7
90 : 10
91 : 9
35
200
200
200
200
150
250
200
250
200
200
200
200
200
200
100
100
100
80
100
100
100
100
100
100
100
100
100
9
10
11
12
13
14
15
16
17
18
95 : 5
40 : 60
40 : 60
40 : 60
34 : 66
36 : 64
38 : 62
34 : 66
36 : 64
а The mole ratio [Rh] : L : substrate is 1 : 1 : 200, and the pressure of synthesis gas is 35 atm (PH : PCO = 1 : 1).
2
b The volume is 2 mL.

2076
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 10, October, 2011
Lyubimov et al.
128.4, 376.5, and 161.98 MHz, respectively) relative to Me4Si,
BF3•OEt2, CF3COOH, and 85% Н3РО4 in D2О. Elemental
analysis was carried out at the Laboratory of Organic Microanalꢀ
ysis of the A. N. Nesmeyanov Institute of Organoelement Comꢀ
pounds, Russian Academy of Sciences. 2ꢀChloroꢀ5,6ꢀcarboranoꢀ
1,3,2ꢀdioxaphosphepane (1) and [Rh(acac)(CO)2] were syntheꢀ
sized according to published procedures.29,30
Synthesis of ligands L1—L3 (general procedure). Triethyꢀ
lamine (0.28 mL, 2.0 mmol) and the corresponding alcohol or
phenol (2.0 mmol) were added to a solution of 2ꢀchloroꢀ5,6ꢀ
carboranoꢀ1,3,2ꢀdioxaphosphepane (0.537 g, 2.0 mmol) in toluꢀ
ene (15 mL). The mixture was stirred for 5 min at 20 °С, heated
to boiling, and cooled to room temperature. A precipitate of
NEt3•HCl was filtered off. The products were purified by flash
chromatography on a column with silica gel (benzene as eluent).
2ꢀIsopropyloxyꢀ5,6ꢀcarboranoꢀ1,3,2ꢀdioxaphosphepane (L1).
Viscous colorless oil. The yield was 0.49 g (84%). Found (%):
C, 28.86; H, 7.29; P, 10.52. C7H21PO3B10. Calculated (%):
C, 28.76; H, 7.24; P, 10.60. 31P{H} NMR (CDCl3), δ: 135.11.
1H NMR (CDCl3), δ: 1.27 (d, 6 H, (CH3)2, J = 6.2 Hz);
1.50—3.46 (m, 10 H, BH, carb.); 4.20 (dd, 2 H, CH2, J = 13.0 Hz,
J = 9.0 Hz); 4.20 (m, 1 Н, CHO); 4.73 (t, 2 Н, CH2, J = 13.1 Hz).
11B {H} NMR (CDCl3), δ: –3.88 (s, 2 В); –(9.04—13.25)
(m, 8 В).aaaa
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Received June 1, 2011;
in revised form June 27, 2011