High loading polymer reagents based on polycationic Ultraresins. Polymer-supported reductions and oxidations with increased efficiency

Article abstract of DOI:10.1016/j.tet.2004.05.104

Ultraresin 1 was prepared from highly branched polyethylene imine (M _n=10,000) via reductive cross-linking with terephthaldialdehyde. Following quaternization with methyl iodide, the polycationic Ultraresin 2, with iodide as a counterion, was obtained. These novel resins combine low swelling with high mechanical stability. By anion exchange polycationic Ultraresins carrying borohydride (3) and periodate (22) were generated and were investigated as very high loading polymer reagents. Ultra-borohydride resin 3 had a reducing activity of up to 12 mmol/g depending on the substrate. It proved successful in diverse reductions including those of aldehydes, ketones, and nitroolefines. The resin was employed in the reductive amination of aldehydes with an excess of amines, which were removed by the use of a scavenger resin. Periodate resin 22 was obtained with an active loading of up to 5.4 mmol/g and was employed in oxidations of sulfides, diols, hydroquinones, and hydrazines.

Full text of DOI:10.1016/j.tet.2004.05.104

Tetrahedron 60 (2004) 8703–8709
High loading polymer reagents based on polycationic Ultraresins.
Polymer-supported reductions and oxidations with
increased efficiency
a
Michael Barth, Syed Tasadaque Ali Shah and Jorg Rademann
a
a,b
,
*
¨
a
¨
¨
Institute for Organic Chemistry, Eberhard-Karls-University Tubingen, Auf der Morgenstelle 18, 72076 Tubingen, Germany
¨
FMP-Research Institute for Molecular Pharmacology, Robert-Rossle-Str. 10, 13125 Berlin, Germany
b
Received 17 November 2003; accepted 15 January 2004
Available online 27 July 2004
Abstract—Ultraresin 1 was prepared from highly branched polyethylene imine (Mn¼10,000) via reductive cross-linking with
terephthaldialdehyde. Following quaternization with methyl iodide, the polycationic Ultraresin 2, with iodide as a counterion, was
obtained. These novel resins combine low swelling with high mechanical stability. By anion exchange polycationic Ultraresins carrying
borohydride (3) and periodate (22) were generated and were investigated as very high loading polymer reagents. Ultra-borohydride resin 3
had a reducing activity of up to 12 mmol/g depending on the substrate. It proved successful in diverse reductions including those of
aldehydes, ketones, and nitroolefines. The resin was employed in the reductive amination of aldehydes with an excess of amines, which were
removed by the use of a scavenger resin. Periodate resin 22 was obtained with an active loading of up to 5.4 mmol/g and was employed in
oxidations of sulfides, diols, hydroquinones, and hydrazines.
q 2004 Elsevier Ltd. All rights reserved.
1. Introduction
removed by filtration, the products can be easily analyzed
and further transformed in solution. They are especially
suitable for automated parallel synthesis. They allow
preparation of complex libraries by multi-step syntheses in
solution, they can be utilized in automated and in flow-
through systems, finally they can be employed to transform
single compounds as well as complex mixtures obtained by
split-mix combinatorial synthesis.
Complex organic molecules can be constructed in homo-
geneous solution, however, for diversity-oriented purposes
it is often advantageous to employ a multiple phase system
which greatly facilitates isolation and separation procedures
as well as the removal of excess reagents and the
completion of reactions. Solid phase synthesis is the most
widely applied example for multiple phase systems in
combinatorial chemistry, possessing significant advantages
in comparison to homogeneous single phase synthesis.
Many advanced polymer reagents have been developed for
demanding reactions over recent years. One significant
limitation of current polymer reagents, however, remains
their relatively low loading (0.5–2 mmol/g for most
commercial reagents). Due to their high price, polymer
reagents are not very economical confining the method to
small scale or microchemistry applications. High loading
and economically produced resins would increase the
efficiency and the atom economy5 of polymer-supported
methods considerably. In addition, in high loading and low
swelling reagents the concentration of the polymer-
supported reactants is enhanced, thus reducing the amount
of solvents and of polymer backbone employed. Higher
concentrations of the supported reactants should accelerate
reaction rates and increase yields.
Synthesis in solution, however, possesses indisputable
advantages in respect to the versatility of applicable
reactions, the ease of analytical monitoring, and the
accumulated knowledge of synthetic protocols. Thus, an
ideal synthetic strategy would combine these merits with the
advantages of solid-phase synthesis protocols, such as the
possibility of using reagents in large excess and of removing
them by filtration. This combination is realized in polymer-
assisted solution phase (PASP) synthesis either by using
scavenger resins or by the implementation of polymer
reagents.1–4 Polymer reagents can be used in excess and
Keywords: Combinatorial chemistry; Polymer reagents; Oxidation;
Reduction.
The first, but still important polymer reagents, were based
on anion exchange resins loaded with anionic reagents.
Classical ion exchange based polymer reagents, however,
*
Corresponding author. Tel.: þ49-30-94793275; fax: þ49-30-94793280;
e-mail address: rademann@fmp-berlin.de
0040–4020/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2004.05.104

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M. Barth et al. / Tetrahedron 60 (2004) 8703–8709
suffer from several drawbacks. Standard anion exchange
resins are obtained from cross-linked chloromethyl
polystyrene by nucleophilic substitution with trimethyl-
amine.6 The maximum loading of such resins is at first
limited by the weight of the polymer backbone. If the
chloromethyl groups are generated in the resin under
Friedel–Crafts conditions, the maximum loading is further
restricted by Lewis-acid catalyzed methylene cross-linking
of chloromethyl groups within the resin.
heterocycles.8 Low-crosslinked Ultraresins displayed large
swelling volumes, unless they were chemically derivatized
with a linker moiety. Especially when swollen in aqueous
hydrochloric acid, the free-amine resins had a swelling
volume of up to .100 ml/g resin. This enormous swelling
was attributed to the presence of multiple protonated
amines. Such very strongly extending resins were not
desirable for use as polycationic reagents, as they required
larger reaction volumes and led to reduced concentrations.
Furthermore, the chemical lability of the ammonium salt has
to be considered. Typically anion exchange resins display
the fishy odor of released trimethylamine. Thus for several
reasons, the molar activity of the supported reagents can
decrease considerably during storage and can be signifi-
cantly lower than the original loading as specified by the
supplier. Another general problem of polystyrene is its
chemcial lability under strong Lewis acidic and oxidative
conditions that have been documented thoroughly.7
Thus for use as polycationic polymer reagents, Ultraresins
had to be developed with reduced swelling yet retain good
accessibility. The swelling of polycationic Ultraresins could
be reduced by increasing the amount of the cross-linker. As
an undesired side-effect, high cross-linking ratios would
however reduce the loading. Thus, as an alternative, highly
branched PEI was investigated as the starting material for
very high loading resins.
Therefore, polycationic Ultrresins were prepared starting
from large, statistically branched polyethyleneimine
(Mn¼10,000; Mw¼25,000). This material contained a
mixture of 38% tertiary, 24% secondary, and 38% primary
amines and was reacted with terephthalic dialdehyde as the
cross-linker to yield the Ultraresin 1. The cross-linker ratio
was 42:1 per equivalent of the starting PEI (Scheme 1).
In principle, the maximum loading of carrier materials is
obtained in polymers constructed from low molecular
weight functional monomers such as polyvinylalcohol,
polyethylene imine, or polyvinylamine. Realizing this
fact, polyethylene imine (PEI) was selected as the starting
material for very high loading polymer supports (Ultra-
resins).8
Following the reported condensation–reduction sequence,
stacked sieves with calibrated mesh size were used to obtain
a size distribution of 90–180 mesh (80–170 mm) or .180
mesh (,80 mm), respectively (Fig. 1).
2. Results and discussion
In a recent publication, a collection of Ultraresins with
varied cross-linking were constructed from various PEIs and
were investigated in respect to their swelling volumes,
mobility and synthetic accessibility.9 Ultraresins con-
structed from short, prepondarably linear, PEIs were ideally
suited for solid phase organic synthesis of peptides and
Quaternization of the secondary and tertiary amines within
the resin network of 1 was attained by alkylation with
methyl iodide at rt (Scheme 2). The conversion was assessed
by elemental analysis of iodine in the washed and dried
resin sample yielding an iodine content of 47.5%. The
Scheme 1. Synthesis of Ultraresin 1 from highly branched polyethylene imine yielding a high-loading, low-swelling, and mechanically stable support material.

M. Barth et al. / Tetrahedron 60 (2004) 8703–8709
8705
synthesis in recent years.11,12 Standard resins are supplied
with nominal loadings of ca. 3 mmol/g. Before using these
resins in synthesis, it is usually advisable to determine the
actual reductive activity in a standardized test reaction, as
the active loading can be significantly lower than reported.
The reduction of 4-nitrobenzaldehyde was selected as a test
reaction for activity determination and indicated a reducing
activity of 8.0 mmol/g for resin 3 after 0.5 h (1 equiv.,
Table 2). By increasing the reaction time to 16 h the
reducing activity of resin 3 was even 12 mmol/g. Sub-
sequently, a selection of aromatic and aliphatic aldehydes,
activated carboxylic acids, aromatic and aliphatic ketones,
and a,b-unsaturated nitro-olefines were subjected to
polymer-supported reduction with reagent 3 (Scheme 3).
As the reaction rates depended strongly on the substrates,
reaction times and reagent excess were adapted in order to
achieve complete consumption of the starting materials
(Table 2). The products were analyzed for purity by HPLC
(214 nm) and by NMR-spectroscopy. Aldehydes reacted
much faster than ketones and were reduced selectively in the
presence of a double bond in a-b-position. On the contrary
the double bonds in nitroolefines were reduced without
affecting the nitro group. An a-ketoamide-containing fully
protected tetrapeptide13 was reduced to the norstatine
product 13 bearing four aliphatic side chains. The products
were obtained in good to excellent yields with only few
equivalents excess (Table 2), showing that the superior
loading of the Ultraresin 3 could be fully exploited in
polymer-assisted transformations.
Figure 1. Polycationic Ultraresin loaded with 12 mmol/g borohydride
(effective activity with 4-nitroaldehyde, rt, 16 h). The resin was sieved to a
particle size of 80–170 mm (90–180 mesh). The reaction vessel to the left
contains 1 mequiv. borohydride resin (82 mg).
Scheme 2. Preparation of polycationic Ultraresin 2 in the iodide form.
Loading of polycationic Ultraresin 2 with borohydride anions yielded the
reducing resin 3 with up to 12.0 mmol/g reductive activity.
Reductive amination was investigated by treatment of an
excess of the amine (1.3 equiv.) with an aldehyde followed
by addition of polycationic borohydride resin 3 (Scheme 3,
Table 3). A small excess of resin (2 equiv.) sufficed to
furnish clean products in good yields, that were obtained by
scavenging of the amine excess. Following to the reaction
the excess of the primary amine was removed by scavenging
with a polymer-supported aromatic aldehyde. Currently,
reductions of alternative substrates with resin 3 are under
investigation including amides, nitriles, nitro compounds,
and azides.
obtained polycationic Ultraresins 2 displayed unique
swelling properties, as well as chemical and mechanical
stability. The synthetic protocols allowed the preparation of
50 g of 2 without difficulties.
As the first polymer reagent based on the polycationic
Ultraresin, the borohydride resin 3 was generated by the
exchange of iodide anions from 2. The swelling volume of 3
was considerably reduced compared to the Ultraresins based
on short PEIs. The maximum swelling volume was found in
methanol with 10.7 ml/g. The swelling volume was
essentially constant when switching from DCM to water.
The swelling factor (volume of swollen resin / volume of
dry resin) was below 2 for most solvents (Table 1).
The next target of research was the preparation of an
oxidizing polymer. Polycationic Ultraresins such as 2 could
be especially valuable as carriers for oxidative species as the
oxidative sensitivity of polystyrene has been documented
well and has limited the extended use of polymer-supported
oxidants so far. Recently, this group during the synthesis of
supported IBX-resin has reported on the partial oxidation of
polystyrene by treatment with tetrabutylammonium oxone
at 80 8C14 and by oxoammonium salts.15
Table 1. Swelling properties of borohydride resin 3
Solvent
Resin volume (ml/g)
Swelling factor
none (dry resin)
Methanol
DCM
Water
THF
5
1
Before the loading of resin 2 with oxidative species, the
iodide anions had to be exchanged for chloride in order to
avoid the formation of iodine by oxidation. For this purpose,
2 was washed with aqueous HCl (1 M) until no residual
iodide could be detected in the washing solution by the
addition of hydrogen peroxide. The resin 21 obtained was
then treated with sodium periodate solution, washed, and
dried yielding periodate resin 22 (Scheme 4). The presence
of periodate was assessed qualitatively by IR and by
reaction of a resin sample with sulfuric acid and
10.7
8.2
8.8
6.0
7.5
7.0
2.14
1.64
1.76
1.2
1.5
1.4
DMF
Toluene
Polystyrene-supported borohydride is one of the classical
polymer reagents. Introduced in 1961,10 it has been
employed broadly in polymer-assisted solution phase

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M. Barth et al. / Tetrahedron 60 (2004) 8703–8709
Table 2. Reactions employing reducing Ultraresin 3
Product
Starting material
Equivalentsa
1
Time (h)
Yield (%)
98
Purity (%)
100
4
0.5
1
5
1.3
98
100
6
7
2
1
4
2
100
100
99
99
8
2
3
95
90
9
1.6
4
4
100
99
81
10
16
100b
11
4
16
98
100
12
13
4
8
16
16
100c
65c
100
90
14
15
2
2
2
2
77d
52d
100
93
As determined by 1H NMR-spectroscopy.
a
Calculated for an effective loading of 8 mmol/g.
19% reduced to the corresponding saturated alcohol.
Obtained as mixture of 2 diastereomers.
Reduction to the saturated nitroalkane.
b
c
d
o-phenylenediamine leading to a red precipitate.16 The
oxidative activity of resin 22 was measured by the partial
oxidation of an excess of 1,4-hydroquinone to quinone 25 as
quantified by NMR-spectroscopy. The active loading in this
reaction was 5.4 mmol/g resin. This value exceed the
maximum loading obtainable with the periodate ion. The
supported periodate ion thus can be assumed to perform a
two step oxidation via the intermediary iodate (Scheme 5,
Table 4).
could be used as polymer reagents in a broad selection of
oxidation and reduction reactions. By selecting highly
branched high molecular weight PEI as starting material a
polymer was obtained that was chemically and mechani-
cally stable and was tailored to swelling volumes well-
suited for use as anion-carrying polymer reagents. The resin
was produced with narrow particle size distribution and low
swelling factors in most solvents. Thus, it might be useful in
continuous flow reactors that allow operation with low back
pressures.
The economic preparation of the support is noteworthy. All
starting materials are inexpensive industrial bulk products.
Resin production can be up-scaled with ease and does not
require the addition of emulgators and multiple phase
3. Conclusions
The original Ultraresin concept was successfully extended
towards the generation of polycationic Ultraresins 2 which

M. Barth et al. / Tetrahedron 60 (2004) 8703–8709
8707
Scheme 5. The oxidizing resin 22 was employed for the conversion of
various sulfides, hydroquinones, diols, and hydrazines yielding products
23-28.
systems as for polystyrenes. In addition, the obtained
Ultraresins are chemically and mechanically robust; the
lability towards oxidants and Lewis acids is strongly
reduced in comparison with standard support materials.
The demonstrated applications of the novel resin include a
reducing borohydride resin 3 which was generated with an
active loading of up to 12.0 mmol/g, significantly higher
than for conventional borohydride exchange resins. The
resin performed successfully in reductions of aldehydes,
ketones and in reductive aminations. The polymer-sup-
ported periodate resin was obtained with an active loading
of up to 5.4 mmol/g and very efficient in the oxidation of
sulfides, hydroquinones, hydrazines, and in periodate-
cleavage of diols.
Scheme 3. The borohydride resin 3 was employed for the conversion of
various aldehydes, ketones and nitro-olefines and for reductive amination
yielding products 4-20.
Table 3. Reductive aminations with resins 3: following to 3 h, 2 equiv. of
borohydride resin 3 was used and reacted 16 h at room temperature.
Scavenging of the primary amine with 10 equiv. of formyl-PS (0.57 mmol/
g) for 6 h at 40 8C
As found for applications in solid phase synthesis, the
accessibility of the novel Ultraresin-based polymer reagents
3 and 22—despite of their high loading—remain fully
accessible for soluble reactants. This is shown not only by
the determined activity of the reagents but as well by the
efficiency in less-favored transformations. Therefore, Ultra-
resin-based polymer reagents should be a viable alternative
to current support materials.
Product
Amine
Aldehyde
Yield Purity
(%)
(%)
16
85
99
17
18
83
94
91
94
4. Experimental
4.1. General procedures
Polyethylene imine (PEI) (Mn¼10,000, Mw¼25,000) was
obtained from Sigma Aldrich. Solvents were purchased in
HPLC grade or freshly distilled before use. Formyl
polystyrene resin was a gift from Merck Biosciences,
19
20
92
72
90
85
¨
Laufelfingen, Switzerland. All reactions were carried out in
glass tubes. The NMR measurements were conducted on a
Bruker Avance 400 MHz spectrometer. The IR spectra were
measured on a Bruker Vector 22 FT-IR spectrometer
employing a split-pea ATR unit. For HPLC analysis a
Beckman Gold system was used with a diode array detector
and an analytic reversed phase column (Nucleosil 100 C-18,
5 mm, 2£250 mm, Fa. Grom, Herrenberg) operated with
acetonitril–water mixtures containing 0.1% trifluoroacetic
acid.
4.1.1. Synthesis of ultra resin (1). PEI 10,000 (21 g) was
dissolved in THF (150 ml) in a 500 ml round-bottom flask.
Terephthalic dialdehyde (11.9 g, 0.088 mol, dissolved in
125 ml THF) was added rapidly to the stirred solution. The
reaction mixture solidified spontaneously and was shaken
for one hour. The cross-linked polymer was washed with
THF, crushed, and suspended again in a mixture of THF
Scheme 4. The oxidizing periodate resin 22 was obtained via the chloride-
loaded polycationic Ultraresin 21.

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M. Barth et al. / Tetrahedron 60 (2004) 8703–8709
Table 4. Oxidations employing the periodate resin 22
Product
Starting material
Product
Equivalentsa
1.63
Time (h)
1b
Yield (%)
94
Purity (%)
97
23
24
2.2
0.5c
100
100
25
26
1.4
2.1
1d
100
100
99
93
0.33d
27
28
2.1
2.1
10d
91
88
93
85
4c
a
Calculated for an effective loading of 3 mmol/g.
In MeOH.
In DCM.
In DCM/MeOH 2:1.
b
c
d
(300 ml) and methanol (150 ml). Sodium borohydride
(6.7 g, 0.17 mol) was added and the mixture was stirred
for 2 h. To remove excess sodium borohydride the polymer
was transformed in its hydrochloride form by treatment with
1 N hydrochloric acid for one hour. The polymer was passed
through a frit, washed with water and was stirred in 2 N
sodium hydroxide solution for 15 min to convert the
polymer to the free amine form 1. After washing with
water, THF and dichloromethane (DCM) the polymer was
dried in vacuo at 60 8C for 5 h. The polymer was sieved to
get particles of 90–180 mesh (80–170 mm). Yield 28 g
(90%) of 1. Elemental analysis C 53.1%, H 9.9%, N 21.1%,
Cl 0.1%. Loading of amine 15 mmol/g. FT-ATR-IR: 798,
1114, 1460, 1508, 2817, 2937, 3220.
sodium borohydride. The resin was dried in vacuo at 80 8C
for 5 h to yield 16 g of borohydride exchange Ultraresin 3 as
a white polymer. FT-ATR-IR: 1070, 1456, 1578, 2223,
2814, 2939. The polymer was stored at 4 8C and the hydride
content was stable for 3 months.
4.1.4. Reduction of carbonyl compounds and nitro-
olefines to products 4–15. The following reduction of
4-chlorobenzaldehyde is representative for the reduction of
carbonyl compounds and nitroalkenes. 4-Chlorobenzalde-
hyde (23.6 mg, 0.168 mmol) was dissolved in 2 ml
methanol. After adding 28 mg of borohydride Ultra resin
3 (0.22 mmol with an effective loading 8 mmol/g) the
reaction mixture was shaken for one hour at room
temperature. The completion of the reaction was indicated
by thin layer chromatography. After adding 4 ml of DCM
the resin was removed by filtration and washed for three
times with 2 ml of DCM. The solvent was removed by
evaporation to get pure 4-chlorobenzylalcohol. The purity
was checked by 1H and 13C NMR-spectroscopy and HPLC
analysis (detection wavelength 214 nm).
4.1.2. Synthesis of polycationic Ultraresin 2. Ultraresin 1
(20 g) was swollen in N,N-Dimethylformamide (DMF)
(500 ml) and methyl iodide (426 g, 3 mol) was added. After
shaking for 24 h at room temperature, the quaternized
polymer was transferred to frit (glass filter), washed with
DMF and DCM and dried in vacuo at 60 8C for 5 h. Yield:
42.7 g of a pale yellow polymer. Elemental analysis C
31.3%, H 5.8%, N 9.2%, I 47.5%. FT-ATR-IR: 1460, 1612,
2784, 2949, 3402.
4.2. General procedure for the reductive amination to
amines 16–20
4.1.3. Loading of polycationic Ultraresin 2 with boro-
hydride anions yielding reducing resin 3. Quaternized
ultra resin (2, 32 g) was swollen in water and a 1 M aqueous
solution of sodium borohydride was poured over the resin
for 15 min. After repeating this procedure for two times the
resin was washed with distilled water until free from excess
5-Methoxytryptamine (25.2 mg, 0.133 mmol) and cyclo-
hexanecarboxyaldehyde (11.4 mg, 0.102 mmol) were dis-
solved in 2 ml methanol. After shaking for 3 h at room
temperature borohydride Ultra resin 3 (51 mg, 2 equiv.) was
added and the reaction mixture was shaken for 16 h at room
temperature. The resin was removed by filtration, washed

M. Barth et al. / Tetrahedron 60 (2004) 8703–8709
8709
three times with 2 ml of DCM. To remove excess amine a
formyl polystyrene resin (0.57 mmol/g, 10 equiv.) was
used. The mixture was shaken for 6 h at 40 8C. After
filtration of the resin and washing with 15 ml of DCM the
solvent was evaporated to get the secondary amine 16.
by evaporation to furnish 23. The purity was checked by 1H
and 13C NMR-spectroscopy and HPLC analysis (detection
wavelength 214 nm).
1
Purity was checked by H and 13C NMR-spectroscopy and
HPLC analysis (detection wavelength 214 nm).
References and notes
4.2.1. Loading of polycationic Ultraresin 2 with chloride
anions yielding resin 21. Quaternized ultra resin (2, 2 g)
was swollen in water and a 1 M aqueous solution of
hydrochloric acid was added then the reaction mixture was
shaken for 30 min at room temperature. After repeating this
procedure for five times the resin was washed with distilled
water until free from excess hydrochloric acid. The resin
was dried in vacuo overnight to yield 1.8 g of chloride
exchange Ultra resin 21 as a white polymer. FT-ATR-IR:
1021, 1472, 1638, 2821, 2969, 3376.
1. Rademann, J. Advanced polymer reagents: Powerful novel
tools in diversity-oriented synthesis. In Combinatorial
Chemistry II; Bunin, B. A., Morales, G., Eds., Academic:
San Diego, Vol. 03; pp 366–390.
2. Akelah, A.; Sherrington, D. C. Chem. Rev. 1981, 81, 557–587.
3. Kirschning, A.; Monenschein, H.; Wittenberg, R. Angew.
Chem. 2001, 113, 670–701, Angew. Chem., Int. Ed. 2001, 40,
650–679.
4. Ley, S. V.; Baxendale, I. R.; Bream, R. M.; Jackson, P. S.;
Leach, A. G.; Longbottom, D. A.; Nesi, M.; Scott, J. S.; Storer,
R. I.; Taylor, S. J. J. Chem. Soc., Perkin Trans. 1 2000, 23,
3815–4195.
4.2.2. Loading of polycationic Ultraresin 21 with
periodate anions yielding oxidizing resin 22. Quaternized
ultra resin (21, 1.8 g) was swollen in water and an aqueous
solution of sodium periodate (5 equiv.) was added. The
reaction mixture was shaken for one hour at room
temperature. After repeating this procedure for three times
the resin was washed with distilled water until free from
excess periodate and finally three times with acetone. The
resin was dried in vacuo overnight to yield 1.7 g of periodate
exchange Ultra resin 22 as a white polymer. FT-ATR-IR:
780, 841, 1473, 1654, 3022.
5. Trost, B. M. Angew. Chem. 1991, 107, 285–307, Angew.
Chem., Int. Ed. 1991, 30, 214–234.
6. Harland, C. E. Ion Exchange: Theory and Practice; The Royal
Society of Chemistry: Cambridge, 1994.
7. Gaylord, N. G.; Hoffenberg, D. S.; Matyska, B.; Mach, K.
J. Polym. Sci. A-1 1968, 6, 269–289.
8. Rademann, J.; Barth, M. Angew. Chem. 2002, 114,
3087–3090, Angew. Chem. Int. Ed. 2002, 41, 2975–2978.
9. Barth, M.; Rademann, J. J. Comb. Chem. 2004, 6, 340–349.
10. Sansoni, B.; Sigmund, O. Naturwiss 1961, 48, 598–599.
11. Gibson, H. W.; Bailey, F. C. J. Chem. Soc., Chem. Commun.
1977, 815.
4.3. General procedure for the oxidation of diols,
sulfides, quinols and catechols to products 23–28
12. Yoon, N. M.; Kyoung, B. P.; Young, S. G. Tetrahedron Lett.
2003, 24, 5367–5370.
The following oxidation of thioanisole to 23 is representa-
tive for the oxidation of diols, sulfides, quinols and
catechols. Thioanisole (15 mg, 0.12 mmol) was dissolved
in 1 ml methanol. After adding 65 mg of periodate resin
(effective loading 3 mmol/g) the reaction mixture was
shaken for 1 h at room temperature. The completion of the
reaction was indicated by thin layer chromatography. The
resin was filtered off and washed for three times with 2 ml of
DCM and two times with methanol, all solvent was removed
13. Weik, S.; Rademann, J. Angew. Chem. 2003, 115, 2595–2598,
Angew. Chem., Int. Ed. 2003, 42, 2491–2494.
14. Sorg, G.; Mengel, A.; Jung, G.; Rademann, J. Angew. Chem.
2001, 4532, 4535. Angew. Chem., Int. Ed. 2001, 40, 4395–
4397.
15. Weik, S.; Nicholson, G.; Jung, G.; Rademann, J. Angew.
Chem. 2001, 113, 1489–1492, Angew. Chem., Int. Ed. 2001,
40, 1436–1439.
16. Johar, G. S.; Fresenius, J. Anal. Chem. 1974, 269, 123.