Use of an isotactic-propylene/hexene copolymer as a new, versatile, soluble support

Article abstract of DOI:10.1002/pola.27029

The use of isotactic-poly(propylene-co-hexene) (iPPH) as new polymeric scaffold for synthesis as well as a phase-selective, soluble polymer support for homogeneous catalysis is described. It was possible to functionalize olefin-terminated iPPH using stand

Full text of DOI:10.1002/pola.27029

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Use of an Isotactic-Propylene/Hexene Copolymer as a New,
Versatile, Soluble Support
1
1
2
1
Abbey Hicks, Binhong Lin, Philip L. Osburn, Christopher E. Hobbs
1
Department of Chemistry and Biochemistry, Angelo State University, San Angelo, Texas 76909
Department of Chemistry and Biochemistry, Bloomsburg University, Bloomsburg, Pennsylvania 17815
2
Correspondence to: C. E. Hobbs (E-mail: chobbs@angelo.edu)
Received 22 October 2013; accepted 12 November 2013; published online 26 November 2013
DOI: 10.1002/pola.27029
ABSTRACT: The use of isotactic-poly(propylene-co-hexene)
iPPH) as new polymeric scaffold for synthesis as well as a
Furthermore, it was demonstrated that an iPPH-supported
DMAP could serve as recoverable, recyclable catalyst.
(
a
†
phase-selective, soluble polymer support for homogeneous
catalysis is described. It was possible to functionalize olefin-
terminated iPPH using standard organic transformations. Each
derivative could be isolated and purified using typical precipi-
tations into a minimum amount of polar solvent, negating the
need for wasteful work up and chromatographic procedures.
Published 2013. J. Polym. Sci., Part A: Polym. Chem. 2014, 52,
600–605
KEYWORDS: green chemistry; organocatalysis; polymer-bound
DMAP; polymer chemistry; polymer-supported catalysis; polyo-
lefins; recyclable catalysts, supports; synthesis
INTRODUCTION The use of polymers as supports for homoge-
neous catalysts/reagents is still an important goal in synthetic
chemistry as it allows for catalyst recovery and reuse. It also
serves as an example of green chemistry as it reduces the
amount of environmentally detrimental waste generated in
catalytic processes. Some of the oldest examples utilized insol-
uble polymer supported-catalysts/reagents based off of cross-
product are separated in two different solvent phases. Under
special circumstances, however, solid/liquid techniques in
which precipitation of the products away from the catalyst
4
,12
can occur,
although this self-separation is not typical. On
the other hand, catalysts anchored to polymers such as
poly(ethylene glycol), noncrosslinked PS, and ring-opening
metathesis polymerization-based supports are almost always
separated as solids using solvent precipitations. This is
because such polymers typically lack the phase-selective sol-
1
–3
linked polystyrene (PS) resins.
Although this is a well-
seasoned principle, it still sees use. Recently, Toy et al. has
reported on a Rasta resin supported TBD as a recoverable cat-
1
,2
ubility required for liquid/liquid separations.
Recently,
4
though, Bergbreiter et al. have studied highly phase-
alyst for transesterifications. Although catalyst recovery can
1
0
selectively soluble linear PS derivatives.
be carried out using simple gravity filtration, the characteriza-
tion of such insoluble, resin-supported catalysts can be diffi-
cult, as solution-state NMR spectroscopy cannot be used.
While liquid/liquid and solid/liquid separations each have
their own advantages, in some cases, it may be desirable to
use a more versatile polymer support that can be recovered
using either system instead of relying on specifically engi-
neered reaction conditions in which product precipitation
On the other hand, various soluble polymers have been used
as supports for homogeneous catalysts. Bergbreiter et al. pio-
1
,2
neered the use of polyethylene oligomers as soluble sup-
ports, chemistry that is currently experiencing resurgence in
1
3
occurs. We show herein that commercially-available, isotac-
tic-poly(propylene-co-hexene) (iPPH) can serve as a useful
synthetic scaffold that can be transformed using standard,
organic reactions to provide iPPH derivatives that can be
easily isolated in good yield. Furthermore, these species
require little purification (i.e., no column chromatography or
wasteful workup procedures). We also demonstrate the
prowess of this material to serve as a new, versatile polymer
support through the preparation and use of a supported
DMAP catalyst that can be recovered and recycled using
both liquid/liquid and liquid/solid separations.
5
–8
the literature.
Numerous other types of polymer supports
have been studied and most of them are typically recovered
1
–10
using either a solid/liquid or liquid/liquid technique.
Although useful, many of the polymers used lack versatility;
they are mostly limited to one type of recovery technique.
For instance, since amorphous materials such as polyisobu-
1
,2,8
11
tylene
and atactic-polypropylene
cannot be precipi-
tated, they are typically restricted to liquid/liquid
separations, in which the supported catalyst/reagent and
†
Published 2013. This article is a U.S. Government work and is in the public domain in the USA.
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EXPERIMENTAL
mixture was allowed to cool to room temperature and was
then precipitated into MeOH (10 mL). The suspension was
separated by vacuum filtration in which 0.155 g of 5 was
isolated as a white powder in 89% yield.
General Experimental Procedures
Olefin-terminated samples of isotactic-poly(propylene-co-hex-
ene) (iPPH) were donated by Baker Hughes. All other
reagents were purchased from commercial suppliers
21
FTIR (neat, cm ) 2954.63, 2917.52, 2871.51, 2098.29,
(
Aldrich, Alfa-Aesar, or VWR) and were used without further
1
1
458.18, 1376.47; H NMR (400 MHz, CDCl ) d 3.25–3.01
3
1
13
purification. H and C NMR spectra were obtained on an
Agilent 400-MHz spectrometer operating at 399.7962 and
(m, 2H), 1.75–0.8 (m, 230H).
1
00.5288 MHz, respectively. All peaks are reported in ppm
N-Ethyl-N-propargyl-dansyl (5)
and are referenced to the residual CHCl peak in CDCl . FTIR
spectra were obtained on a Perkin Elmer Spectrum 100
spectrometer. The phase-selective solubility of 6 was deter-
mined using literature procedures
3
3
To a 25-mL, round-bottomed flask equipped with a magnetic
stir bar and a rubber septum was added dansyl chloride (0.5
g, 1.85 mmol), aminoethane (0.24 mL, 3.71 mmol), and
CH Cl (12 mL). This reaction mixture was allowed to stir at
1
0,14
on a Photon Technol-
2
2
ogy International fluorometer. Chromatographic purification
of 10 was performed on a Grace Reveleris automated flash
chromatography system equipped with simultaneous UV–vis
and ELSD detection.
room temperature for 20 min. At this time, CH Cl (30 mL)
2
2
was added to this mixture and the organic layer was washed
with three 25 mL portions of water, one 25 mL portion of
brine, and then dried over Na SO followed by filtration and
2
4
removal of the solvent under reduced pressure to give 0.37
g of N-ethyl dansyl as a yellow solid in 74% yield.
Hydroxy-Terminated iPPH (2)
To a 250-mL, round-bottomed flask, equipped with a mag-
netic stir bar and a rubber septum was added olefin-
terminated iPPH 1 (4 g, 2 mmol) and THF (20 mL). While
1
H NMR (400 MHz, CDCl ) d 8.55 (d, J 5 8.5 Hz, 1H), 8.30–
3
8.24 (m, 2H), 7.60–7.50 (m, 2H), 7.19 (d, J 5 7.9 Hz, 1H),
4.49 (t, J 5 6.1 H, 1H), 2.95 (q, J 5 14.2, 7.2 Hz, 2H), 2.89 (s,
6H), 1.03 (t, J 5 7.6 Hz, 3H); C NMR (100 MHz, CDCl ) d
3
152.07, 130.41, 129.69, 129.36, 123.18, 118.64, 115.15,
45.40, 38.37, 15.13, 0.03. This material was used without
further purification.
.
stirring at room temperature, BH SMe (0.57 mL, 0.46
3
2
1
3
mmol) was added via syringe and the reaction mixture was
allowed to stir for 1 h. At this time, an aqueous solution of
NaOH (6 mL, 4 M) was added to the reaction mixture, fol-
lowed by H2O2 (8 mL, 30%). The heterogeneous reaction
mixture was allowed to stir for 1 h. At this time, THF (10
mL) was added to dissolve all of the material, which was
then precipitated into MeOH (50 mL). The suspension was
separated by vacuum filtration in which 3.6 g of 2 was iso-
lated as a white powder in 90% yield.
To a 25-mL, round-bottomed flask equipped with a magnetic
stir bar, a rubber septum, and a water-jacketed reflux con-
denser was added N-ethyl dansyl (0.2 g, 0.76 mmol), K CO3
2
(
0.99 g, 7.2 mmol), and DMF (4 mL). This mixture was
allowed to stir at room temperature for 10 min. At this time,
an 80% toluene solution of propargyl bromide (0.133 g, 0.9
mmol) was added and the flask was placed on an oil bath
1
H NMR (400 MHz, CDCl ) d 3.61–3.33 (m, 2H), 1.75–0.8 (m,
3
228H).
ꢀ
regulated at 75 C and allowed to stir for 12 h. At this time,
iPPH-Mesylate (3)
the reaction was allowed to cool to room temperature fol-
lowed by the addition of ether and water. The organic layer
was isolated and washed with three 25 mL portions of
water, one 25 mL portion of brine, and then dried over
To a 25-mL, round-bottomed flask equipped with a magnetic
stir bar and a rubber septum was added 2 (2.9 g, 1.45
mmol) followed by CH Cl (42 mL). While stirring, the reac-
2
2
tion mixture was charged with triethylamine (1.38 mL, 9.95
mmol) followed by methanesulfonylchloride (0.72 mL, 9.3
mmol). This reaction mixture was allowed to stir at room
temperature for 45 min. Upon completion, the reaction mix-
ture was precipitated into MeOH (30 mL). The suspension
was separated by vacuum filtration in which 2.75 g of 3 was
isolated as a white powder in 95% yield.
Na SO followed by filtration and removal of the solvent
2 4
under reduced pressure to give 0.22 g of 5 as a yellow oil
1
(that crystallized upon standing) in 80% yield, based on H
NMR analysis. 5 was used without any further purification.
1
H NMR (400 MHz, CDCl ) d 8.54 (d, J 5 8.6 Hz, 1H), 8.32–
3
8.19 (m, 2H), 7.59–7.47 (m, 2H), 7.21–7.14 (m, 1H), 4.29 (d,
J 5 2.3 Hz, 2H), 3.42 (q, J 5 14.5, 7.4 Hz, 2H), 2.87 (s, 6H),
1
13
H NMR (400 MHz, CDCl ) d 4.18–3.98 (m, 2H), 2.99 (s, 3H),
2.10 (t, J 5 2.4 Hz, 1H), 1.15 (t, J 5 7.1 Hz, 3H); C NMR
3
1.75–0.8 (m, 228H).
(100 MHz, CDCl
) d 134.7, 130.46, 128.15, 127.99, 123.19,
3
122.19, 119.63, 113.64, 115.16, 73.21, 45.49, 41.15, 38.47,
iPPH-Azide (4)
35.14, 15.18, 13.06.
To a 25-mL, round-bottomed flask equipped with a magnetic
stir bar, water-jacketed reflux condenser, and a rubber sep-
General Procedure for CuAAC Reactions with
iPPH-Azide (4)
tum was added 3 (0.175 g, 0.088 mmol), KN
3
(0.036 g,
0
.438 mmol), and a 1:1 mixture of toluene and DMF (2 mL).
To a 25-mL, two-necked, round-bottomed flask equipped
with a magnetic stir bar, water-jacketed reflux condenser,
and a rubber septum was added 4 (1 equiv), alkyne
ꢀ
This mixture was placed on an oil bath regulated at 100 C
and was allowed to stir for 1 h. At this time, the reaction
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(2 equiv), CuBr (10 mol %), and PMDETA (10 mol %). This
flask was then charged with a 1:1 mixture of toluene and
DMF and sparged with N for 15 min. The reaction mixture
2
ꢀ
was then placed on an oil bath regulated at 105 C and was
allowed to stir for 1 h. At this time, the reaction mixture was
allowed to cool to room temperature and was then precipi-
tated into MeOH (10 mL). The suspension was separated by
vacuum filtration and the triazole product was isolated as a
light brown or white powder.
Dansyl-Labeled iPPH (6)
Precipitation into methanol followed by isolation by vacuum
filtration provided 0.072 g of 6 in 80% yield.
SCHEME 1 Syntheses of iPPH derivatives.
ꢀ
mmol) and THF (10 mL). This solution was cooled to 290 C
1
H NMR (400 MHz, CDCl ) d 8.55 (d, J5 8.4 Hz, 1H), 8.27
3
in an acetone/N (l) bath. The cooled mixture was allowed to
2
(
7
3
d, J5 8.7 Hz, 1H), 8.18 (d, J5 7.1 Hz, 1H), 7.58–7.48 (m, 2H),
.43 (s, 1H), 7.22–7.17 (m, 1H), 4.62 (s, 2H), 4.26–3.92 (m, 2H),
.35 (q, J5 14.2, 7.5 Hz, 2H), 2.89 (s, 6H), 1.75–0.8 (m, 230H).
stir for 20 min. At this time, nBuLi (1.6 M, 6.6 mL, 10.56
mmol) was added via syringe and the reaction mixture was
allowed to stir for 2 h while warming to 220 C. Upon reach-
ꢀ
ing this temperature, an 80% toluene solution of propargyl
bromide (2.98 g, 13.9 mmol) was added via syringe and the
reaction mixture was allowed to slowly warm to room tem-
perature and was stirred for 24 h. At this time, the reaction
iPPH-Supported Pyridine (8)
Precipitation into methanol followed by isolation by vacuum
filtration provided 0.18 g of 8 in 89% yield.
1
was quenched by the addition of H O. The solvent was
removed under reduced pressure followed by the addition of
2
3
H NMR (400 MHz, CDCl ) d 7.46 (s, 1H), 7.41 (d, J 5 7.8 Hz,
H), 6.4 (d, J 5 7.8 Hz, 2H), 5.1 (s, 2H), 4.4–4.1 (m, 2H),
.75–0.8 (m, 273H).
2
1
CH Cl (50 mL). This organic layer was washed with three 25
2
2
mL portions of water and one 25-mL portion of brine. The
organic layer was dried over Na SO , filtered and the solvent
iPPH-Supported DMAP (11)
2
4
Precipitation into methanol followed by isolation by vacuum
filtration provided 11 as a powder that was then dissolved
in THF and passed through a short alumina plug to yield
was removed under reduced pressure to give 0.85 g of 10 in
85% yield. 10 could be purified by automated flash column
chromatography (0–100% ethyl acetate in hexane gradient),
but, in most cases, was used without further purification.
0.18 g of 11 in 90% yield.
1
1
H NMR (400 MHz, CDCl ) d 8.21 (broad s, 3H), 6.59 (broad
H NMR (400 MHz, CDCl ) d 8.4 (d, J 5 6.6 Hz, 2H), 6.6
3
3
s, 2H), 4.64 (s, 2H), 4.29–3.98 (m, 2H), 3.1 (s, 3H), 1.75–0.8
(d, J 5 6.7 Hz, 2H), 4.0 (d, J 5 2.4 Hz, 2H), 3.0 (s, 3H), 2.2
1
3
(m, 273H).
(t, J 5 2.7 Hz, 1H); C NMR (100 MHz, CDCl
07, 73, 40, 36.
) d 154, 150,
3
1
4
-(N-Methyl-N-propargylamino)pyridine (10)
Boc-Protection of 2,6-Dimethylphenol Catalyzed by 11
Using Solid/Liquid Separation for Catalyst Recovery
To a 5-mL, round-bottomed flask, equipped with a stir bar
and a rubber septum was added 11 (0.1 g, 0.05 mmol), 2,6-
To a 25-mL, round-bottomed flask, equipped with a magnetic
stir bar and rubber septum was added MAP (1.0 g, 9.26
dimethylphenol (0.122 g, 0.998 mmol), Boc O (0.23 g, 1
2
mmol), and THF (2 mL). This mixture was allowed to stir for
25 min followed by the addition of MeOH (10 mL), which
caused 11 to precipitate. This suspension was subjected to
centrifugation followed by decantation of the product-
containing solvent phase. Pure product was isolated by
removal of the solvent under reduced pressure while 11 was
reused by the addition of fresh solvent and substrates. 11
could be reused four times with an average product yield of
1
0
94%. Product spectral data matched literature examples.
Boc-Protection of 2,6-Dimethylphenol Catalyzed by 11
Using Liquid/Liquid Separation for Catalyst Recovery
To a 5-mL, round-bottomed flask, equipped with a stir bar
and a rubber septum was added 11 (0.1 g, 0.05 mmol), 2,6-
dimethylphenol (0.122 g, 0.998 mmol), Boc O (0.23 g, 1
mmol), and heptane (2 mL). This mixture was allowed to
2
1
FIGURE 1 H NMR spectrum of olefin-terminated 1 showing
the signals corresponding to the terminal, disubstituted olefin.
stir for 25 min followed by the addition of CH CN (2 mL),
3
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FIGURE 2 Photograph illustrating the phase-selective solubility of
(for heptane) compared to the selectivity of low molecular
6
weight 5 (for acetonitrile) in a mixture of heptane and acetonitrile.
SCHEME 2 Synthesis of dansyl-labeled iPPH 6.
Olefin terminated oligomer 1 could be transformed into alco-
hol
Scheme 1). Analysis by H NMR spectroscopy showed com-
plete disappearance of the olefinic proton signals at 4.72 and
.65 ppm and the appearance of a new multiplet at 3.6–3.3
2 using typical hydroboration/oxidation conditions
which resulted in a biphasic mixture. Pure product was iso-
lated by removal of the polar solvent layer, followed by sol-
vent removal under reduced pressure while 11 was reused
by the addition of fresh substrates. 11 could be reused five
times with an average product yield of 93%. Product spec-
1
(
4
ppm. Treatment of 2 with methanesulfonyl chloride in
dichloromethane afforded mesylate 3 in quantitative conver-
sion, as evidenced by the disappearance of the proton signals
corresponding to 2 and the appearance of a multiplet and a
1
0
tral data matched literature examples.
RESULTS AND DISCUSSION
1
singlet at 4.19–3.98 and 2.9 ppm, respectively, in the H NMR
iPPH oligomers (with a nominal M of 2000 and a statistical
spectrum. 3 served as a useful intermediate that could
undergo a typical substitution reaction to provide azide 4, as
confirmed by the disappearance of the singlet at 2.9 ppm and
an upfield shift of the multiplet from 4.19–3.98 to 3.28–3.05
n
propylene/hexene ratio of ꢁ17/1) are commercially available
1
4
from Baker Hughes. These species are soluble in a range of
solvents (such as heptane, chloroform, THF, and toluene)
which enables their characterization by solution state spectro-
21
ppm. 4 was further characterized by a peak at 2098 cm
1
scopic methods (i.e., H NMR spectroscopy), like their amor-
corresponding to the azide stretch in the FTIR spectrum. It is
important to note that each of these reactions proceeded to
1
1
phous counterparts. Unlike these formless materials, iPPH
exists as a semicrystalline solid, rendering its purification and
isolation simpler (vide infra). This material contains, exclu-
sively, an olefin terminus (Fig. 1) that can be converted into
many other functional groups using standard organic transfor-
1
completion, as evidenced by H NMR spectroscopy, but the
yields are less than quantitative due to some mass loss occur-
ring in the isolation/purification. Furthermore, the spectra
1
7,18
obtained were similar to compounds previously reported.
Advantageously, these derivatives were all isolated as solids
8
,9,11,12,15,16,22
mations that are similar to existing procedures.
SCHEME 3 Preparation of iPPH-supported pyridine 9 and DMAP 11.
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an 80% conversion. Advantageously, this crude product could
be used without any further purification in a click reaction
between with 4 in the presence of CuBr and PMDETA. The for-
mation of 6 was confirmed by the appearance of signals at
7
.48 and 4.62 ppm and a shift in the signals from 3.28–3.05
1
ppm (ACH AN ) to 4.26–3.92 ppm in the H NMR spectrum
and the disappearance of the azide stretch at 2098 cm in
the FTIR spectrum. Using literature procedures,
2
3
2
SCHEME 4 Protection of 2,6-dimethylphenol with Boc O in the
21
presence of 11. Conditions a: Boc
mmol), 11 (0.05 mmol), THF (2 mL), catalyst separation: precip-
itation into methanol. Conditions b: Boc O (1 mmol), phenol
0.99 mmol), 11 (0.05 mmol), heptane (2 mL), catalyst separa-
2
O (1 mmol), phenol (0.99
9
,11
fluores-
cence spectroscopy showed that 6 has a phase selective solu-
bility of >98%, suggesting a small amount of leaching of
material into the polar phase. This selectivity can be observed
in Figure 2. This indicated that iPPH has potential as a recov-
2
(
tion: liquid/liquid extraction with acetonitrile (2 mL).
1,2
erable, polymer support in liquid/liquid separations.
via precipitation into a minimum amount of methanol fol-
lowed by filtration. This negates the need for wasteful gravity-
based liquid/liquid work up procedures or chromatographic
separations that are typically used.
We next turned our attention to the development of func-
tional iPPH derivatives. It was possible to use a “click” strat-
egy, similar to that shown in Scheme 2, for the preparation
of functionalized iPPH oligomers that have the potential to
A useful method for the determination of a polymer’s phase
selective solubility involves the use of catalyst/reagent surro-
gates that can be anchored to the polymer chain. The use of
chromophores or fluorophores as surrogates allows for quali-
tative observation and quantitative measurement of this selec-
tivity. A dansyl-labeled iPPH derivative was obtained through a
serve as recoverable reagents, ligands, or catalysts. Alkyne 7
6
(
prepared using literature procedures ) could be clicked
onto the end of 4 in the presence of CuBr and PMDETA to
provide iPPH-pyridine derivative 8 (Scheme 3). However,
since our goal in this report is the preparation and use of
supported species that can act as recoverable reagents and
organocatalysts, we became interested in the synthesis of an
iPPH-supported DMAP. DMAP (and its derivatives) has been
shown to be more nucleophilic than pyridine and can there-
1
9
copper-catalyzed azide/alkyne cycloaddition (CuAAC ) reac-
tion between azide-terminated 4 and 5. The preparation of 6
began with the synthesis of “clickable” dansyl 5 by first sub-
jecting dansyl chloride to a reaction with aminoethane in
CH Cl for 20 min (Scheme 2). This was followed by a depro-
1
7,18,20
fore be a more useful catalyst.
2
2
tonation with K CO and reaction with propargyl bromide in
DMF to provide alkyne 5 in good yield. The formation of 5
was confirmed by the appearance of a doublet at 4.19 ppm, a
2
3
DMAP derivative 11 was prepared using the atom economi-
cal, click strategy outlined in Scheme 3. First, 4-(N-methyl-N-
propargylamino)pyridine 10 was prepared by
a low-
quartet at 3.42 ppm, a singlet at 2.87 ppm, and two triplets at
temperature deprotonation of 4-(N-methylamino)pyridine 9
1
2
.10 and 1.15 ppm in the H NMR spectrum. Although this
ꢀ
with butyllithium at 290 C. This mixture was allowed to
product was recovered from the reaction mixture in high yield,
integration of the doublet at 4.19 ppm and the diminished tri-
plet (corresponding to the amino proton of the intermediate
ꢀ
warm to 220 C at which point, propargyl bromide was
added. Formation of 10 was confirmed by the appearance of
doublets at 8.3, 6.6, and 4.0 ppm, a singlet at 3.0 ppm and a
1
sulfonamide) at 4.49 ppm in the H NMR spectrum revealed
1
triplet at 2.3 ppm in the H NMR spectrum. Although it was
FIGURE 3 FTIR spectra for the Boc-protection of 2,6-dimethylphenol catalyzed by 11: cycle 1 (᭿ with solid line), cycle 2 (᭿ with
dotted line), cycle 3 (᭜ with dotted line), and cycle 4 (~ with a dashed line), compared to the uncatalyzed reaction (dotted line).
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possible to purify 10 using automated flash column chroma-
tography, this was not necessary as it could be used in the
next reaction with no further purification. Reaction of 10
with azide 4 in the presence of CuBr and PMDETA in a 1:1
mixture of toluene and DMF provided supported DMAP 11,
which was isolated and purified via precipitation into metha-
nol followed by passing a THF solution of 11 through a short
pad of alumina. Preparation of 11 was confirmed by the dis-
organic transformations. Using typical click chemistry, we
were able to prepare an iPPH-supported DMAP that could be
used as a recoverable catalyst whose versatility was illus-
trated by the fact that both liquid/solid and liquid/liquid
separation strategies could be used for catalyst recovery and
reuse.
ACKNOWLEDGMENTS
21
appearance of the azide stretch at 2098 cm
spectrum. Furthermore, a shift in the proton signals from
.28–3.05 to 4.29–3.98 ppm and the appearance of broad
in the FTIR
The authors thank Paul K. Hanna (Baker Hughes) for the
generous donation of iPPH samples. Support of this work
by the Robert A. Welch Foundation (Grant AJ-0029), Amer-
ican Chemical Society-Petroleum Research Fund (Grant
3
signals at 8.2, 6.6 ppm and singlets at 4.7, and 3.1 ppm in
1
the H NMR served as positive identification of 11.
5
3245-UNI1), and Angelo State University is gratefully
acknowledged.
Nucleophilic organic bases such as DMAP have found wide
use as organocatalysts for many different types of reactions.
1
7
18
These include acylations, esterifications, and polymeriza-
REFERENCES AND NOTES
2
1
tions. As proof of principle, we were able to show that
iPPH-DMAP 11 could be used as a recoverable catalyst for
the Boc-protection of 2,6-dimethylphenol (Scheme 4). Fur-
thermore, the versatility of this support allowed us to
recover and reuse 11 under two different systems: (i) a clas-
sical solvent precipitation and (ii) a liquid/liquid extraction.
1
D. E. Bergbreiter, J. Tian, C. Hongfa, Chem. Rev. 2009, 109,
5
30–582.
2
3
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2
phenol were dissolved in THF and allowed to react at room
temperature 25 min. At this time, excess methanol was
added to the homogeneous reaction mixture, causing 11 to
precipitate as a solid. This suspension was then subjected to
centrifugation followed by decantation of the product-
containing, methanol-rich solvent layer. Pure product was
isolated by the removal of the solvent while the addition of
fresh solvent and substrate to the recovered catalyst facili-
tated its reuse. Under these conditions, 11 was reused four
times with an average yield of 94%.
6
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To illustrate the flexibility of this support, we showed that it
was also possible to run the same reaction under a liquid/
liquid separation system by taking advantage of the high
phase-selective solubility of iPPH. This first involved dissolu-
Hobbs, Tetrahedron Lett. 2013, 54, 970–974.
1
2
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1
3 Baker-Hughes: Available at: http://www.bakerhughes.com/
tion of 11, Boc O, and 2,6-dimethyl phenol in heptane. This
2
news-and-media/resources/technical-data-sheet/polypropylene-
homogeneous reaction mixture was allowed to stir at room
temperature for 25 min. At this time, the addition of acetoni-
trile resulted in a biphasic mixture in which 11 was isolated
in the heptane phase while the product was isolated in the
polar phase. To establish the effectiveness of supported
DMAP 11 as a catalyst for this reaction, FTIR traces of the
reaction mixture for each cycle compared to the uncatalyzed
reaction (control) are shown in Figure 3. Removal of the
polar phase (using a simple gravity separation), followed by
addition of fresh substrates facilitated the reuse of 11. Using
this liquid/liquid scheme, 11 could be recycled four times
with an average product yield of 93%.
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2
In conclusion, we have shown that commercially available
iPPH can serve as a new, useful synthetic scaffold for the
preparation of a small library of derivatives using standard
524.
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