Liquid/liquid separation of polysiloxane-supported catalysts

Article abstract of DOI:10.1039/b601120a

Liquid/liquid separation after monophasic reactions is a viable way to use and recover polysiloxane-supported catalysts. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b601120a

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Liquid/liquid separation of polysiloxane-supported catalysts{
Melissa A. Grunlan,a Katherine R. Reganb and David E. Bergbreiter*b
Received (in Austin, TX, USA) 22nd January 2006, Accepted 3rd March 2006
First published as an Advance Article on the web 24th March 2006
DOI: 10.1039/b601120a
limited amount of the dye was used and a simple alkene was used
Liquid/liquid separation after monophasic reactions is a viable
way to use and recover polysiloxane-supported catalysts.
to convert the remaining Si–H into Si–octyl groups. This produced
a collection of four dye-labeled polysiloxanes 4–7 whose phase
selective solubility was subsequently tested.
Polymer supports used in synthesis most commonly are based on
organic materials and typically are recovered by filtration after a
reaction. Insoluble inorganic materials in the form of inorganic
metal oxides (e.g. functionalized silica gel) or meso-porous
supports (e.g. zeolites) are also widely used. Soluble inorganic
polymers like polysiloxanes or polyphosphazenes are well-known,
common materials1 but have received limited attention as supports
for catalysts.2–6 A recent report also described a polysiloxane-
supported organic catalyst.7 In all of these cases, precipitation or a
membrane separation is used to recover/re-use the supported
catalyst. Our success in developing liquid–liquid based separations
of soluble organic polymer-bound catalysts from products after a
homogeneous reaction8,9 and the excellent tunable solubility of
polysiloxanes suggested to us that inorganic polymers have
potential as soluble supports that could be separable from
products using liquid/liquid biphasic separations. This work
shows that commercially available polysiloxanes can be easily
modified and used to address the strategic issue of catalyst
separation.10
To test the feasibility of liquid/liquid biphasic separation for
recovery of 4–7, the dye-labeled polysiloxane was dissolved in a
The work reported herein uses polysiloxanes as soluble
inorganic polymer supports for an organocatalyst. Linear poly-
siloxanes (Si–O–Si) are widely used as fluids, surfactants, release
agents, and lubricants.11 These materials are low polarity,
hydrophobic polymers with very low glass transition temperatures
and are often available as viscous oils or gums soluble in many
organic solvents. Polysiloxanes possess good thermal, oxidative,
chemical, and biological stability and are commercially available
with silane (Si–H) functionality. Thus, it is surprising that they
have been relatively little used as catalyst supports.3–7
While polysiloxane-bound catalysts are known, separations of
these catalysts have generally not focused on using solubility as a
separation tool. To explore the potential of using polysiloxane
phase selective solubility in separation, we prepared several
polysiloxanes containing an azo dye. The azo dye served as a
surrogate for a catalyst and facilitated analysis of the phase
separability of these soluble polymers.12,13 As shown in Scheme 1,
a collection of three different commercially available dimethyl
polysiloxanes containing terminal or pendant silanes (1–3) were
hydrosilylated with a vinyl-containing azo dye. In one instance, a
aBiomedical Engineering Department, Texas A&M University, College
Station, TX 77843, USA. E-mail: mgrunlan@tamu.edu
bChemistry Department, Texas A&M University, College Station,
TX 77842, USA. E-mail: bergbreiter@tamu.edu; Fax: 979-845-4719;
Tel: 979-845-3437
{ Electronic supplementary information (ESI) available: Experimental
details for the synthesis of 4–8. See DOI: 10.1039/b601120a
Scheme 1 Synthesis of dye-labeled polysiloxanes 4–7.
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for immobilization and have been shown to catalyze the Michael
Table 1 Phase selective solubility for polysiloxanes (4–7) in biphasic
mixtures formed thermomorphically or by water addition
addition of thiols to a,b-unsaturated ketones both as monomers
and as polymer-supported species.14–16 Catalyst
Phase selective solubilitya
(% dye in heptane-rich phase)
8 (quinine
Polymer
Solvent mixture
attached to a,v-bis (hydrosilyl)polydimethylsiloxane 1) is equally
effective as a recyclable Michael addition catalyst for thiol
additions to a,b-unsaturated ketones and esters (eqn. 2). As
shown in Table 2, using a latent biphasic liquid/liquid separation to
recover the polymeric catalyst was effective in recovery/re-use of
this catalyst through 5 cycles in several examples of this reaction.
As has been noted previously, the isolated yield of thioether
product increased through the first few cycles in some cases. This
reflects the fact that some thioether product was lost to the
heptane-rich phase in the first few cycles. This effect is absent once
the heptane-rich phase is saturated with the low molecular weight
product.
4
5
6
7
4
5
6
7
a
heptane/DMF
heptane/DMF
heptane/DMF
heptane/DMF
heptane/aq. EtOH
heptane/aq. EtOH
heptane/aq. EtOH
heptane/aq. EtOH
97.6
99.7
94.2
98.8
99.6
99.5
99.5
99.3
Phase selective solubilities were measured after 2 phase separation
cycles. Further phase separation cycles led to modest improvements
in phase selectivity (e.g. 6 in heptane/DMF had 97.7% phase
selective solubility after 6 phase separation cycles).
thermomorphic mixture of heptane and DMF or heptane and
EtOH. The first solvent mixture is a thermomorphic solvent
mixture that is biphasic at room temperature and monophasic at
elevated temperature. To test separations, a heptane solution of the
dye-labeled polymer was mixed with an equal volume of DMF,
heated to 70 uC, and cooled to 25 uC. The second solvent mixture
is a latent biphasic mixture. The dye-labeled polymer is soluble in
an equivolume mixture of heptane and EtOH but can be recovered
on addition of ,20 vol% water because the water addition
produces a biphasic mixture with a less dense heptane phase. In all
cases, the initially formed polymers 4–7 were subjected to several
phase separation cycles. The first few cycles presumably removed
any unreacted methyl red (or oligosiloxanes). An analysis of
the heptane and the polar phases by UV-vis spectroscopy for
the azo dye-labeled polysiloxane during the third phase separation
served as our criteria for phase selective solubility. The results
of these studies are listed in Table 1 below and show that
all these polymers were phase selectively soluble in heptane. All
of these polymers have phase selective solubility that is
adequate for catalyst recovery/reuse in the heptane/EtOH–H2O
system and 5 and 7 had almost as good phase selective solubility in
heptane/DMF.
ð1Þ
ð2Þ
The goal of this study was to show that liquid/liquid separations
are a viable way to recover/re-use polysiloxane-bound catalysts
after a monophasic reaction. In this case, quinine was a convenient
vinyl-containing starting material. However, quinine is a chiral
catalyst too and in one case the product thioether, 3-thiophenyl-
cyclohexanone, is chiral. While we did not expect to see high levels
of asymmetric induction in this case, we did examine the product’s
e.e. It was modest (20%) as expected.
While dye-labeled polymers used as surrogates for polymer-
bound catalysts simplify testing of liquid/liquid separation
strategies for catalyst recovery,12,13 real catalytic reactions have
to be studied to show that catalyst recovery/re-use is feasible. To
do this, we immobilized quinine, a cinchona alkaloid, on the
siloxane support (eqn. 1). Cinchona alkaloids have vinyl groups
Support of this work by the Robert A. Welch Foundation and
the National Science Foundation (CHE-0446107) is gratefully
acknowledged. We also thank Vikram C. Purohit for assistance in
determining the e.e. of 3-thiophenylcyclohexanone.
Table 2 Product yields for Michael addition reactions catalyzed by 8 in latent biphasic mixtures over 5 cycles
Michael donor
Michael acceptor
Product yield (cycle)a
C6H5SH
C6H5SH
C6H5SH
p-CH3OC6H4SH
p-HO2CC6H4SH
a
H2CLCHCOCH3
2-cyclohexen-1-one
H2CLCHCO2Et
H2CLCHCO2Et
H2CLCHCOCH3
83 (1); 92 (2); 100 (3); 100 (4); 100 (5)
81 (1); 100 (2); 100 (3); 97 (4); 98 (5)
44 (1)b; 82 (2); 70 (3); 86 (4); 95 (5)
45 (1)c; 66 (2); 55 (3); 42 (4); 42 (5)
88 (1); 86 (2); 94 (3); 90 (4); 85 (5)
Reactions were carried out at room temperature in 50 : 50 mixtures of heptane and EtOH using 10 mol% 8 as catalyst. After the reactions
were complete (GC assay), addition of 10 vol% H2O perturbed this system to form a biphasic mixture. The yields are for pure products
characterized by 1H NMR spectroscopy that were isolated from the aq. EtOH phase after removal of the EtOH and water at reduced pressure.
b
Some loss of the more volatile products may have occurred during these workups. The yield of Michael addition product using quinine
(1 mol%) not immobilized on 1 was 57%. The yield of Michael addition product using quinine (1 mol%) not immobilized on 1 was 58%.
c
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