Novel smart ligand for immobilizing a highly efficient Rh-catalyst

Article abstract of DOI:10.1039/b101506k

Novel highly efficient Rh-based catalysts with a good regioselectivity in olefin hydroformylation have been prepared by deliberately choosing 11-mercaptoundecanoic acid as a ligating molecule to [Rh(cod)Cl]₂ (cod = cyclooctadiene) and a simple recovery process for their recycling was investigated.

Full text of DOI:10.1039/b101506k

Novel smart ligand for immobilizing a highly efficient Rh-catalyst
Jin-Kyu Lee,* Tae-Jong Yoon and Young Keun Chung
School of Chemistry and Center for Molecular Catalysis, Seoul National University, Seoul 151-747,
Korea. E-mail: jinklee@snu.ac.kr
Received (in Cambridge, UK) 15th February 2001, Accepted 11th May 2001
First published as an Advance Article on the web 8th June 2001
Novel highly efficient Rh-based catalysts with a good
regioselectivity in olefin hydroformylation have been pre-
pared by deliberately choosing 11-mercaptoundecanoic acid
as a ligating molecule to [Rh(cod)Cl]2 (cod = cycloocta-
diene) and a simple recovery process for their recycling was
investigated.
be repeated several times without any significant degradation of
the complexes (Scheme 1).
The catalytic activity of both 1 and 2 for the hydro-
formylation of arylolefins was checked, and the large differ-
ences in their solubility (depending on the pH) were used to
recover the catalyst after the reaction.‡ The water-soluble Rh
catalyst 1, in general, showed a high catalytic activity with the
exception of a-methylstyrene (Table 1, entry 1), probably due
to its steric bulkiness preventing the effective binding of the
disubstituted styrene onto the Rh catalysts. Although the
hydroformylation of aromatic olefins showed very good
regioselectivity, the electron donating or withdrawing sub-
stituents on styrene (Table 1, entries 2–6) did not give rise to any
further enhancement of selectivity. Unlike other reported
hydroformylation catalysts,1 compound 1 showed a very high
degree of catalytic activity even towards long alkyl olefins such
as oct-1-ene, undec-1-ene, and dodec-1-ene (Table 2). It is
expected that the long alkyl chains in 11-mercaptoundecanoic
acid themselves behave like a surfactant to enhance the
solubility of the long aliphatic olefins in the aqueous catalyst
phase. The low regioselectivity resulting from the hydro-
formylation of aliphatic terminal olefins suggests that re-
gioselectivity is related not only to steric but also to electronic
factors. After the reaction, the products were easily separated by
extraction with Et2O and the aqueous layer containing catalyst
1 could be reused without any further treatment. Reaction
results from the consecutive usage (five times) of aqueous
Catalyst recovery processes are becoming more important,
especially when expensive and toxic heavy metal complexes are
employed. Owing to these economic and environmental factors,
many attempts have been made to recover homogenous
catalysts and recycle them. The homogeneous catalysts are
usually recovered by thermal operations such as distillation,
which normally leads to thermal stress on the catalyst and
seldom yields quantitative recovery. Since the separation of
catalyst from products is much simpler in two-phase (or
biphasic) systems incorporating a water-soluble catalyst,1–3
water-soluble organometallic compounds have been studied for
many years. They are normally prepared by incorporating
highly polar functional groups such as sulfonate salt4,5 carbox-
ylate salt,1 amino,6 or hydroxide7 into the ligands. Recently, Rh-
based catalysts have attracted much attention due to their high
activity and selectivity, and some attempts to immobilize these
expensive catalysts have also been reported.6,8–13 In this
communication, we will introduce a simple synthetic method of
preparing novel water-soluble Rh-based hydroformylation
catalysts and discuss their unique solubility and reactivity,
which make catalyst recycling much simpler.
[Rh(m-S(CH2)10CO2Na)(cod)]2 1 was prepared in 89% yield
as a yellow–orange solid by treating [Rh(m-Cl)(cod)]2 with
11-mercaptoundecanoic acid disodium salt† (Scheme 1).
Complex 1 is a very stable solid in air and soluble in water,
slightly soluble in MeOH, and insoluble in most organic
solvents. This complex shows a very sensitive change of
solubility in water when pH values are varied, compared to
other known water-soluble organometallic compounds that
usually contain sulfonate groups and show no pH-dependent
solubility.4,5 When an aqueous solution of 1 was treated with
acid to pH < 4, [Rh(m-S(CH2)10CO2H)(cod)]2 2 was quickly
precipitated as a yellow–orange powder from the aqueous
solution, this powder being very soluble in polar organic
solvents such as THF. Complex 2 can also be quantitatively
converted back into 1 by treating with a base such as NaOH
aqueous solution. These interconversions between 1 and 2 can
Table 1 Hydroformylation results of substituted styrenes by employing
water-soluble catalyst 1
Conversion
yield (%)b
Entry
Olefin
b/la
1
2
3
4
5
6
a-Methylstyrene
4-Bromostyrene
4-Chlorostyrene
Styrene
4-Methylstyrene
4-Vinylanisole
~ 3
100
100
100
100
100
87/13
88/12
91/9
97/3
92/8
a Structural ratios of aldehyde products were measured by 1H NMR
spectroscopy (b: 9.62 ppm, l: 9.78 ppm); b stands for a branch form (e.g.
acetophenone) and l for linear form (e.g. phenylacetaldehyde). b De-
termined by 1H NMR spectroscopy.
Scheme 1 Synthesis of novel sulfur bridged [Rh] catalysts.
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Chem. Commun., 2001, 1164–1165
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b101506k

Table 2 Hydroformylation results of long alkyl olefins by catalyst 1
Notes and references
† A mixed solution of NaOH (3 M, 3.06 ml) and HS(CH2)10CO2H (1.00 g,
4.58 mmol) in 10 ml of THF was stirred at room temperature for 30 min to
precipitate the salt Na+2S(CH2)10CO22Na+. The precipitate was filtered
off, and then washed with THF in order to eliminate excess NaOH. 0.53 g
Conversion
yield (%)
Entry
Olefin
b/l
1
2
3
Oct-1-ene
Undec-1-ene
Dodec-1-ene
50/50
48/52
78/22
100
100
92
2
(2.00 mmol) of dried Na+2S(CH2)10CO2 Na+ salt was added to a stirred
solution of [Rh(m-Cl)(cod)]2 (0.49 g, 1.00 mmol) in dried MeOH (10 ml).
The mixed solution was refluxed with vigorous stirring for 30 min. Solvent
was evaporation to dryness, and the residual solid was rinsed with cold
CH2Cl2 (3 3 10 ml) and crystallized from H2O–THF mixed solvent. Solid
crystals were filtered off and vacuum dried to give 0.80 g of pure complex
1 (89% yield). 1H NMR (300 MHz, D2O): d 2.18 and 2.3 (br, total 8H, cod
–CH2–), 4.3 (br, 4H, cod NCH–), 2.19 (t, 2H, 2O2CCH2–), 1.27 (m, 12H,
internal –CH2–), 2.53 (t, 2H, –SCH2–), 1.51(m, 4H, –SCH2CH2– and
–O2CCH2CH2–). Anal. Found: C, 50.66; H, 7.16; S, 7.12%. Calc. for
Catalytic reaction and analysis process was as for Table 1.
solutions containing catalyst 1 did not indicate any differences
in catalytic activity. Further detailed study on the long-term
stability of catalyst 1 is under investigation.
C
38H64O4S2Na2Rh2: C, 50.69; H, 7.12%; S, 7.12%. MS (FAB, negative):
901 (M+).
When organic-soluble compound 2 was used as a catalyst, the
reactions were carried out in homogeneous THF solution. All
the reaction results were the same as for the case of catalyst 1,
thereby confirming the high catalytic activity of the new Rh-
based catalysts in either a homogeneous or a heterogeneous
system. However, the recovery process of catalyst 2 from THF
solution was different. It was isolated by treating with NaOH
solution to precipitate as 1, followed by filtering off and
dissolving it in water. It was then treated with HCl solution
again to precipitate as compound 2, then filtered off and dried
for recycling. Recycled catalyst 2 showed the same activity, but
there was always a small amount of weight loss during the
filtration and drying procedures. From the practical point of
view of recycling, therefore, it is much easier and simpler to use
compound 1 in a biphasic (water/organic bilayer) system.
We are quite certain that the present catalyst should exhibit a
high degree of regioselectivity toward various olefins. Further
experiments to obtain a detailed mechanistic understanding and
the application of these novel Rh-catalysts to other types of
catalytic reactions such as hydrosilylation and stereoselective
hydrogenation reactions, are currently under investigation.
This work was supported by the Korean Science and
Engineering Foundation through the Center for Molecular
Catalysis at Seoul National University. We are grateful to Dr J.
W. Han for his kind technical assistance. T. J. Yoon is also
grateful to the BK21 fellowship.
‡ Neat substituted styrenes (2.5 mmol) over 5.0 mL of the aqueous solution
of catalyst 1 (1.3 3 1023 M) were hydroformylated with pressurized H2/CO
(500 psi, 2+1 ratio of H2+CO) gas mixtures in a stainless steel autoclave
reactor at 55 °C for 22 h. Structural ratios of aldehyde products were
measured by 1H NMR spectroscopy. (b: 9.62 ppm, l: 9.78 ppm)
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