Terminally functionalized polyisobutylene oligomers as soluble supports in catalysis

Article abstract of DOI:10.1039/b312368e

A new phase selective hydrocarbon soluble polymer support is described.

Full text of DOI:10.1039/b312368e

Terminally functionalized polyisobutylene oligomers as soluble supports
in catalysis†
David E. Bergbreiter* and Jun Li
Department of Chemistry, PO Box 30012, Texas A&M University, College Station, Texas, USA.
E-mail: bergbreiter@tamu.edu; Fax: 001 979 845 4719; Tel: 001 979 845 3437
Received (in Corvallis, OR, USA) 7th October 2003, Accepted 10th November 2003
First published as an Advance Article on the web 27th November 2003
A new phase selective hydrocarbon soluble polymer support is
described.
Soluble polymer supports are of increasing interest in synthesis and
catalysis.1 Most past attention on polymer supports has focused on
polystyrene with catalysts/reagents/ligands on pendant groups.2
Other soluble polar and nonpolar supports which are useful in
catalysis and have tunable phase selective solubility that is altered
by the identity of alkyl substituent groups have been described.3–8
We and others have shown that such polymers can be recovered by
liquid/liquid separations.3–10 Terminally functionalized polymers
are soluble alternatives to polymer supports with pendant groups.
Poly(alkene oxide) (PEG) is the most common example of such a
support.11 These linear polyethers are easier to characterize and can
be recovered in the polar phase of a thermomorphic mixture.3
However, PEG supports are more often recovered by less efficient
solvent precipitation schemes. Solvent precipitation can be avoided
by using terminally functionalized polyethylene (PE) oligomers.
1
Fig. 1 H NMR spectra of polyisobutylene oligomers: a) 1, with terminal
PE oligomers are recoverable by precipitation and filtration without
vinyl groups; b) 2, with a terminal –CH2OH group; and c) 3, with a terminal
excess solvent addition.12 However, PE’s insolubility at < 50 °C
–CH2OSO2CH3 group. The expansions overlaid on these spectra cover 0.4
and solubility only at > 75 °C in nonpolar solvents limit its utility.
d in each case.
Here we describe an alternative to either PEG or PE – poly-
isobutylene (PIB). Suitable terminally functionalized polyisobuty-
materials. Of particular note is the facility with which these
lenes are easily prepared from commercially available materials.
materials can be analyzed by 1H NMR spectroscopy. As shown in
These readily soluble oligomers and the catalysts attached to them
Fig. 1, the 1H NMR spectra of these oligomers’ end groups are as
can be easily separated from products by liquid/liquid separation
well resolved as those of a small molecule, simplifying analysis of
using a nonpolar solvent after a reaction.
the course of a reaction.
The PIB oligomers we used were obtained from BASF as
To be effective as a support, polyisobutylene (PIB) oligomer
separation from the products of a reaction has to be simple and
oligomers with a d.p. of ca. 20 or 40 with terminal vinyl groups. As
shown in Scheme 1, such groups were converted into ligands for
effective. A nominal value for an effective separation is 200 : 1 ( <
transition metal catalysts or into catalyst precursors using simple
0.5% loss of catalyst/ligand per stage). Higher separation numbers
transformations. Such ligands have loadings of ca. 1 or 0.5 mmol of
are more desirable. By preparing PIB oligomers with dansyl or azo
dye labels (8 and 9) we have shown that PIB supports are
ligands/g, loadings that are comparable to polar PEG-based
selectively separated into one phase of a thermomorphic biphasic
mixture. For example, in 20 mL of a thermomorphic 1 : 1 (v : v)
heptane–90% aq. EtOH mixture, 30 mg of 8 was selectively soluble
in the heptane-rich phase to an extent of > 99.6%. Fluorescence
analysis of a similar solution containing 5 mg of 9 showed that 9
was selectively soluble in the heptane-rich phase to the extent of >
300 : 1.
Scheme 1 Synthesis routes to terminally functionalized polyisobutylene
oligomers containing ligands and catalysts.
To demonstrate the utility of PIB as a soluble polymer support,
we have looked at several known catalytic reactions. Specifically,
we briefly examined three different Pd-catalysts (eqns. (1) and (2)).
The first of these was a Pd(II) catalyst or catalyst precursor – the
terminally bound pincer sulfur–carbon–sulfur SCS-Pd(II) species
10.13 This Pd catalyst was attached to the PIB oligomer via a
† Electronic supplementary information (ESI) available: experimental
details for the synthesis and use of the PIB oligomers and catalysts. See
http://www.rsc.org/suppdata/cc/b3/b312368e/
42
C h e m . C o m m u n . , 2 0 0 4 , 4 2 – 4 3
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

Table 1 Cross-coupling reactions using PIB-bound Pd catalystsa
Cycle Cycle Cycle Cycle
terminal –CO2H group. The PIB-bound 10 was then used to carry
out Heck chemistry (eqn. (3)). As was true for SCS-Pd(II) species
on other polymers, this catalyst was only effective for aryl iodides.
However, the catalyst could be used through three cycles without
any detectable loss of activity by cooling a reaction mixture to room
temperature and separating the heptane rich phase containing the
catalyst from the polar phase containing the product. The products
were isolated from the polar phase, purified by chromatography and
identified by 1H and 13C NMR spectroscopy. Recycling simply
involved adding fresh substrate(s) solution to the recovered heptane
phase and reheating.
Substrate
Acceptor
1
2
3
4
cinnamyl acetate
cinnamyl acetate
iodobenzene
morpholine
diethylamine
phenylacetylene 48%
phenylacetylene 35%
81%
78%
95%
99%
95%
97%
99% 100%
96% 100%
99%
4-iodoanisole
97%
4A-iodoacetophenone phenylacetylene 74%
95% 100%
98% 99%
100% 100%
iodobenzene
iodobenzene
methyl acrylate 63%
acrylic acid 50%
a Allylic substitutions were carried out in a monophasic EtOH–heptane
mixture at 25 °C using 1 mol% (PIB-PPh2)4Pd. Cycle 5 is not shown but
recycling was equally effective in cycle 5. Increased yields in cycles 1–3
reflect saturation of the heptane phase by product; Sonogashira reactions
were carried out at 70 °C in a monophasic 90% EtOH–heptane mixture
using 1 mol% (PIB-PPh2)4Pd; Heck reactions were carried out at 100 °C in
a monophasic DMA–heptane mixture using 1 mol% PIB-SCS-Pd (10).
These yields are isolated yields
terminally functionalized PEG oligomers for support of catalysts.
The derivatization and chemistry of these oligomers can be easily
followed by conventional spectroscopy. The activity of the
catalysts attached to the oligomers is analogous to that of other
soluble polymer or low molecular weight catalysts. Such supports
contain only unreactive C–C and C–H bonds (except for the
ligating or catalysts species) and should thus be useful as soluble
supports for a wide range of catalysts. Finally, these oligomers’
nonpolar character ensures that they are readily recoverable in the
nonpolar phase by a liquid/liquid separation after either cooling of
a thermomorphic solvent mixture or by perturbing a latently
biphasic solvent mixture.
Support of this work by the Petroleum Research Fund, the Robert
A. Welch Foundation and the National Science Foundation (CHE-
0010103) is gratefully acknowledged as is a gift of the PIB
oligomers from Dr Rocco Paciello of BASF.
Notes and references
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Phosphine-ligated Pd(0) catalysts are generally more reactive
than SCS-Pd(II) species.13 Using the reactions shown in eqn. (2),
we have attached Pd(0) catalysts to phosphine ligands at the end of
a polyisobutylene chain. The resulting phosphine-complexed Pd
catalysts were successfully used in two sorts of Pd(0) chemistry –
Sonagashira alkyne–arene couplings and allylic substitution (eqns.
(4) and (5) respectively). The results of these reactions and catalyst
recycling in this chemistry are detailed in Table 1. Recycling
followed the general protocol described above. In these cases,
recycling required rigorous oxygen free conditions. Otherwise
oxidation of the phosphine ligand and formation of Pd black
occurred.
In summary, the studies here show that polyisobutylene
oligomers are a nonpolar, phase selectively soluble alternative to
C h e m . C o m m u n . , 2 0 0 4 , 4 2 – 4 3
43