Synthesis of diisocyanides with phenolic groups and their polymerization to helically chiral poly(quinoxaline-2,3-diyl)s

Article abstract of DOI:10.1002/pola.27580

J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 1320-1329 The synthesis of new diisocyanide monomers with one or two phenolic groups is described. Their polymerization ability is affected mainly by steric crowding of the diisocyanide functions by adjacent

Full text of DOI:10.1002/pola.27580

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Synthesis of Diisocyanides with Phenolic Groups and their
Polymerization to Helically Chiral Poly(quinoxaline-2,3-diyl)s
Johannes Scholz,1,2 Michael Reggelin2
1Novaled GmbH, Tatzberg 49, 01307 Dresden, Germany
2
€
€
€
Clemens-Schopf-Institut fur Organische Chemie und Biochemie, Technische Universitat Darmstadt, Alarich-Weiss-Straße 4,
64287 Darmstadt, Germany
Correspondence to: M. Reggelin (E-mail: re@punk.oc.chemie.tu-darmstadt.de)
Received 10 November 2014; accepted 5 February 2015; published online 4 March 2015
DOI: 10.1002/pola.27580
ABSTRACT: The development of synthetic routes which lead to
five new diisocyanide monomers with one or two phenolic
groups is described. Their polymerization behavior is studied
with Pd- and Ni-based initiators, as well as under microwave
irradiation. The polymerizability is mainly dominated by steric
effects as is concluded from experiments using different pro-
tecting groups. Chiroptical properties of these new polymers
are studied by CD-spectroscopy. After deprotection, helically
chiral poly(quinoxalin-2,3-diyl)s are obtained which display a
Brønsted function attached to a stereolabile biaryl axis whose
configuration should be influenced by the chiral polymer back-
C
V
bone.
2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A:
Polym. Chem. 2015, 53, 1320–1329
KEYWORDS: functionalization of polymers; helically chiral poly-
mers; heteroatom containing polymers; poly(quinoxalin-2,3-
diyl)s; supramolecular structures
configurationally flexible (tropos)11 biaryl axes under the
influence of a helical backbone. For low molecular weight
systems Reetz et al. showed that the combination of tropos
biaryl axes with other stereogenic elements can result in
highly stereoselective catalysis.12 Two scenarios for selective
catalysts based on this strategy are plausible: A strong ther-
modynamic preference of one conformational diastereomer
or a higher reactivity of one of the possible atropisomers.
INTRODUCTION Macromolecular soluble catalysts allow for
conducting reactions under homogenous conditions in com-
bination with process advantages like easy separation (ultra-
filtration or precipitation) and reusability.1–5 In addition,
synthetic polymers exhibiting single-handed helical main
chain chirality as the dominant stereogenic element offer the
possibility to introduce stereochemical function like asym-
metric induction in the course of stereogenic reactions.6,7 In
previous studies we focused on the application of poly(triar-
ylmethylmethacrylate)s6,7 and poly(isocyanate)s7 as ligands
for transition metals as a proof of principle (reaching up to
60 %ee in Pd-catalyzed allylic substitutions).7 Moreover, we
were able to obtain pyridine-N-oxide functionalized poly
(triarylmethylmethacrylate)s as active but rather stereounse-
lective organocatalysts.8 From these experiments we deduced
a lack of stereocontrol exerted by the helical backbone over
the catalytic microenvironment for at least two reasons. As
the most serious problem, besides its distance to the helical
backbone, we considered the vast numbers of chiral
propeller-like conformations of the triarylmethyl moiety9,10
within our approach (Chart 1). More selective catalysts
should be accessible by a reduction of possible diastereo-
morphic conformations (at the catalytically active sites)
together with an enhanced control of these conformers by
the helical backbone. Towards this end, we decided to study
The class of poly(quinoxaline-2,3-diyl)s (PQs) introduced by
Ito et al.13 raised our attention because their constitution
meets the aforementioned needs. Uniformly configured PQs
are prepared by screw-sense selective living polymerization
of aromatic diisocyanides.14–17 Moreover, their helical super-
structure was proven by X-ray structural analysis of a living
pentamer.15 The transition-metal catalyzed polymerization
tolerates a wide variety of functional groups, and aryl-
substituted PQs have proven to be configurationally stable.18
The collinear alignment of the biaryl axes to the helical back-
bone should secure an efficient transfer of chirality to the
catalytically active sites.
Meanwhile Yamamoto and Suginome showed that short poly-
mers of this class containing phosphorous atoms in a limited
number of repeating units at defined positions are very efficient
ligands for Pd-catalyzed, enantioselective hydrosilylation.19 This
Additional Supporting Information may be found in the online version of this article.
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CHART 1 Comparison of the constitution of poly(triarylmethylmethacrylate)s and poly(quinoxaline-2,3-diyl)s for improved catalyst
design. The shown poly(triarylmethylmethacrylate) gave up to 60 %ee when applied as ligand in Pd-catalyzed allylic substitutions
(Tr 5 trityl).7
work was extended to statistical, higher molecular weight
copolymers exhibiting these binding sites diluted along the poly-
meric chain, which were applied in further Pd-catalyzed asym-
metric transformations.20–25
to originate from increased steric hindrance of too bulky
protecting groups. In sharp contrast, the electron deficient
substituted monomer 2a turned out to be unstable on the
timescale of polymerization. By developing examples of a so-
called “electron-balanced” monomer like 2b by additional
introduction of alkoxy donor groups, we were able to solve
the stability issue. Polymerization of this monomer was suc-
cessful, but weak Cotton effects in the CD spectrum suggest
a detrimental influence of the pyridine moiety during the
Pd-catalyzed polymerization reaction. Replacement of one
phenyl substituent by a solubilizing group in ortho position
to the diisocyanide moiety led to an example of a so-called
second-generation monomer 3a (Chart 2, lower row).26 The
concomitant steric strain release resulted in improved poly-
merizability of the phenol substituted monomer and, in addi-
tion, CD spectroscopy indicated the formation of a helically
Using the PQs constitutional benefits, we focused on the
preparation of homopolymers with biaryl axes functionalized
by phenolic groups as organocatalytically active functions at
every repeating unit aiming at a high density of active sites
along the backbone.
In a previous study on functionalized, homopolymeric PQs
we described symmetrically substituted diisocyanid mono-
mers 1a–c, 2a, 2b (so called first generation) aiming at
Brønsted acid/base organocatalysis (Chart 2, upper row).26
Diisocyanides of type 1 are stable compounds but showed
almost no homopolymerization tendency, which we believe
CHART 2 Overview of diisocyanide monomer generations bearing protected organocatalytic functions (PG 5 protecting group).
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SCHEME 1 Synthesis of new second-generation monomers 3b, 3c, and 7 (precatalyst PEPPSI-IPr 15 see Ref. 29).
chiral polymer.26 Encouraged by this success, we aimed at
the development of new phenolic substituted PQs, further
exploring the scope of the design principle illustrated in
Chart 2. Following these lines, we herein describe our efforts
toward the synthesis of other second and new third-
generation monomers of type 3a and 4. Moreover, we report
on their screw-sense selective polymerization and the
removal of protecting groups to yield phenolic substituted
PQs.
protecting groups, because most other low molecular weight
silyl protecting groups did not survive the procedure of dii-
socyanide synthesis from benzothiadiazole derivatives and
bulky ones inhibit the polymerization reaction.26
Three new unsymmetrically substituted benzothiadiazoles
5a, 5b, and 6 were synthesized as precursors for their corre-
sponding diisocyanides 3b, 3c, and 7 via different routes
(Scheme 1). As described previously Suzuki coupling of ben-
zothiadiazole bromide 8 with (2-hydroxyphenyl)boronic acid
anhydride 9 yielded phenol 10,26 which was smoothly pro-
tected using MOM-Cl to give benzothiadiazole 5a. Unfortu-
nately, the originally intended introduction of the TMSE-
group in 5b by a Mitsunobu reaction of trimethylsilylethanol
and the phenol 10 failed. Therefore, we changed the syn-
thetic plan and prepared the boronic acid 1130 using a pro-
tocol for the synthesis of aryl alkyl ethers first described by
Wolter et al.31,32 After Suzuki coupling the TMSE-protected
benzothiadiazole 5b was isolated. Starting from benzothia-
diazole 12,26 displaying two solubilizing hexyloxy groups,
two successive Suzuki couplings with trimethylboroxine
and (2-hydroxyphenyl)boronic acid anhydride 9, followed
by protection of the phenol using MOM-Cl gave the
RESULTS AND DISCUSSION
Second-Generation Monomers
A first goal here was the installation of alternative protecting
groups, because the removal of the benzyl group after poly-
merization of monomer 3a (PG 5 Bn) was unsuccessful.26
The protecting group should not influence the polymeriza-
tion reaction, thus it should be small and conformationally
flexible. We decided to employ the MOM-group and the tri-
methylsilylethyl group (TMSE). Both can be removed under
mild conditions after the polymerization, by acid or via
fluoride ion induced decomposition, respectively.27,28 The
TMSE-group was chosen amongst the variety of other silyl
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TABLE 1 Polymerization of Second-Generation Monomers 3b, 3c, and 7
M/Ia
Polymer
Conditions
Yield (%)
Mp (g mol21
b,c
)
DP
PDId
gabs (1023
)
e
No.
Monomer
1
2
3
4
5
6
7
40/1
40/1
80/1
40/1
150/1
40/1h
40/1
poly-3b
poly-3b
poly-3b
poly-3b
poly-3b
poly-3b
poly-3c
RT, 69 h
RT, 75 h
RT, 93 h
MWg, 2 h
MWg, 12 h
RT, 16 h
RT, 6 d
56f
84f
77f
64f
62f
86i
quant.j
12,700
8,100
34
21
93
21
247
29
27
1.14
1.10
1.14
1.19
1.33
1.27
1.14
0.26
0.27
0.19
0.19
0.08
0.06
0.20
33,300
8,100
86,900
10,600
11,400
8
40/1
40/1
poly-7
poly-7
RT, 4 d
75i
18,200
14,700
37
29
1.34
1.27
0.28
0.30
9
RT, 5 d
91f
a
f
Monomer to initiator 17 ratio.
Molar mass at peak maximum.
After precipitation from methanol.
b
c
g
h
i
Microwave irradiation at constant temperature 60 ꢀC.
Determined by GPC (PS standards).
Ni-based initiator 18 was used.
d
e
M
w/Mn.
After preparative GPC.
j
gabs was determined from the CD- and UV-spectrum at 347 nm (for
For analytics a small sample was purified via GPC.
poly-7 at 360 nm).
benzothiadiazole 6. The reductive opening of the MOM-
protected benzothiadiazole derivative 5a was achieved by
sodium borohydride/cobalt chloride.33 This protocol worked
even for the electron rich benzothiadiazole 6, but surpris-
ingly failed for the TMSE-protected derivative 5b. Fortu-
nately, the reduction with magnesium in methanol34
delivered the desired diamine 13b in 86% yield. Finally, for-
mylation35 and dehydratization provided the three new dii-
socyanide monomers 3b, 3c, and 7 as yellow oils in 61 to
79% yield for these two steps.
polymer was possible applying a M/I ratio of 150/1 (#5).
Under microwave irradiation a slight increase in the polydis-
persity index was observed. Employing the Ni-based initiator
1817 (#6) monomer 3b conversion was faster as compared
with the reaction with the Pd-based initiator 17 but not as
fast as under microwave assistance.
For yet unclear reasons the structurally similar TMSE-
protected monomer 3c displayed considerably slower poly-
merization rates but yielded a PQ with comparable GPC profile
(#7). Even the electron richest monomer 7 in this series
underwent smooth polymerization in five days, yielding PQs in
good yields and narrow PDIs (#8, 9). The slower reaction rate
in this case may be explained by additional steric hindrance,
caused by the biaryl axis which is not freely rotating as
These second-generation phenolic monomers with solubiliz-
ing groups attached to them were examined in homopolyme-
rization experiments with the Pd-based initiator 1716
(Table 1). To our delight, all new monomers successfully
polymerized with good yields and narrow polydispersity
indices. For monomer 3b polymerization reactions with M/I
ratios of 40/1 (#1, 2) and 80/1 (#3) were successful at
room temperature. Under microwave assisted conditions,26
we observed a faster monomer consumption (compare #1
and #4) and the preparation of a high molecular weight
1
concluded from its H-NMR-spectrum30 at room temperature.
All new second-generation PQs showed Cotton effects in
their circular dichroism (CD) spectra with local maxima at
around 295 nm and 345 nm (Figs. 1 and 2). Based on the
assignment in CD-spectra of structural similar PQs reported
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FIGURE 1 a) CD- (solid lines) and UV-spectra (dashed lines) of obtained PQs poly-3b (solvent for entry Table 1 #1, #2, #3, #5
MTBE, for entry #4 and #6 CHCl3). b) Comparison of CD- (solid lines) and UV-spectra (dashed lines) of PQs poly-3b, poly-3c and
poly-7 (solvent CHCl3).
by Ito et al.15 we conclude that a right-handed (P) helix has
been formed. Due to the lack of a suitable reference polymer
we could not conclude on the absolute value of the screw-
heating only slight, reversible changes in the intensities were
observed.
Third-Generation Monomers
sense excess. For a qualitative comparison Kuhn’s dissymme-
36,37
With the intention to develop a polymeric pendant to well-
known and successful BINOL-based organocatalysts,38 we
developed the so called third-generation monomers. Towards
this goal a new synthetic route was necessary, which allows
for the installation of a 2,20-biphenolic structure during the
synthesis of the key benzothiadiazole 19 (Scheme 2). The
known nitrobenzene 2039 was reduced to the corresponding
diamine, which was subsequently protected by the action of
SO2 to yield the benzothiadiazole 21. A Negishi coupling was
used to introduce the solubilizing hexyl group furnishing
benzothiadiazole 22. Subsequent cleavage of the methyl
ether and selective bromination delivered benzothiadiazole
23, which was subjected to Suzuki cross coupling with bor-
onic ester 24. The PEPPSI-IPr 15 catalyzed reaction yielded
50% of the benzothiadiazole 19 with the desired biphenolic
structural motif as a common precursor for the third-
generation monomers.
try factor gabs
for polymers poly-3b and poly-3c were
calculated at the local maximum at 347 nm, for polymer
poly-7 the maximum at 360 nm was used respectively. We
observed the trend that an increase in molecular weight of
polymer poly-3b is accompanied by a decrease of the CD sig-
nal intensities at 295 nm and 347 nm and thus a decreased
gabs (Fig. 1, #1 vs. #3, or #4 vs. #5). The same trend was
observed by Yamada et al. for Ni-catalyzed polymerizations.17
It is possible that the directing influence of the chiral initia-
tor group (S,S)Ar* on the growing chain end decreases upon
increase of the molecular weight. This decrease is also
observed upon switching from room temperature polymer-
ization to microwave assisted polymerizations (Fig. 1, #1 vs.
#3). Slight racemization under these conditions seems plau-
sible, even though this was not observed during polymeriza-
tion
of
3,6-bis(p-tolyl)
substituted
diisocyanide.26
Remarkably, all new polymers showed good configurational
ꢀ
ꢀ
stability in the range of 210 C to 50 C as was investigated
As protecting groups MOM-ether and a bridging methylene
group were used (Scheme 3). The room temperature
by temperature dependent CD-spectroscopy (Fig. 2). Upon
FIGURE 2 Temperature-dependent CD-spectra of PQs in CHCl3: a) poly-3b (Table 1, #4), b) poly-7 (Table 1, #8).
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SCHEME 2 Synthesis of the benzothiadiazole 19 featuring a biphenolic structure.
1H-NMR spectrum of benzothiadiazole 25a30 showed an AB-
spin system for the MOM-group methylene protons thus
indicating a hindered rotation around the biaryl axis. For the
methylene bridged biphenole 25b this behavior was not
observed. Both benzothiadiazoles were reluctant towards
reductive desulfurization by CoCl2/NaBH4, but with Mg in
MeOH the diamines 26a and 26b were readily obtained.
Finally, formylation35 and dehydratization yielded the new
diisocyanide monomers in good (4a) or moderate (4b)
yields.
monomer/initiator ratios of 20/1 (#3) or 25/1 (#4) com-
plete conversion was observed after extended reaction times
(up to 10 days). Despite different monomer/initiator ratios
the GPC profile of all PQs obtained showed no differences. In
all cases oligomers with a length of about 10 repeating units
and narrow PDI were isolated, regardless of the monomer’s
conversion. This lack of reactivity towards homopolymeriza-
tion might be explained by increased steric hindrance due to
limited rotation around the monomers threefold ortho-sub-
stituted biaryl axis. Consequently, the phenolic moiety in
monomer 4a may be locked in such a way, that its OMOM-
group interacts with one isocyano group thus hampering the
consecutive insertion reaction during polymerization. In the
methylene bridged monomer 4b this problem obviously does
not exist. Here the attached phenolic moiety is kind of tied
down by the bridging protecting group and therefore the iso-
cyano groups are more accessible, which results in a rela-
tively fast homopolymerization reaction (#5). This
The homopolymerization of these third-generation mono-
mers was studied again employing the Pd-based complex 17
(Table 2). Monomer 4a was not fully consumed in a typical
polymerization experiment (#1) and only oligomers were
obtained as judged from GPC data. Even conducting the poly-
merization reaction under forcing microwave irradiation did
not lead to full monomer 4a consumption (#2). With smaller
SCHEME 3 Synthesis of third-generation monomers 4a und 4b.
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TABLE 2 Polymerization of Third-Generation Monomers 4a and 4b
M/Ia
Polymer
Conditions
Yield (%)
Mp (g mol21
b,c
)
DP
PDId
gabs (1023
)
e
No.
Monomer
1
2
3
4
40/1
40/1
20/1
25/1
poly-4a
poly-4a
poly-4a
poly-4a
RT, 5 df
26g
18g
78g
73g
5,800
4,200
4,200
5,000
12
9
1.14
1.12
1.11
1.14
0.33
0.19
0.36
0.33
MW, 12 hf,h
RT, 10 di
RT, 7 d
9
10
5
40/1
poly-4b
RT, 45 h
80j
13,000
39
1.22
0.38
a
f
Monomer to initiator 17 ratio.
Incomplete monomer conversion.
After precipitation from methanol.
b
c
g
h
i
Molar mass at peak maximum.
Determined by GPC (PS standards).
Microwave irradiation at constant temperature 60 ꢀC.
d
e
M
w/Mn.
Reaction time not optimized.
After preparative GPC.
j
gabs was determined from the CD- and UV-spectrum at 358 nm (for
poly-4b at 357 nm).
interpretation is supported by the already mentioned AB-
spin system in the 1H-NMR-spectra for the methylene group
of monomer 4a and the singlet for the methylene bridge in
monomer 4b.30 Obviously, only in the latter case the configu-
rational lability of the biaryl axis leads to a time averaged
signal. The isolated PQ poly-4b showed a GPC profile with
molecular weight and PDI comparable to PQs derived from
second-generation monomers. Considering the short
polymerization time the synthesis of high molecular weight,
homopolymeric PQs employing monomer 4b seems very
promising.
the (S,S)-configured initiator 17 induces right-handed helical
structures (P) during the polymerization. The CD intensities
of all new PQs are similar, except for the polymer obtained
with microwave assistance (Table 2, #2). Comparison of the
gabs values at 358 nm reveals that in this case the intensity
was decreased by a factor about almost 2. As described
above, this was also the case for the second-generation poly-
mers, although much less pronounced. The small differences
in the CD- and UV-spectra of poly-4a and poly-4b (Fig. 3,
right) may be attributed to different conformations, which
are accessible for a MOM-protected or a dioxepine bridged
biphenolic motif in the PQs, respectively. Despite their rather
low molecular weight these new polymers showed a surpris-
The CD-spectra of all new third-generation PQs showed Cot-
ton effects (Fig. 3, left). Based on the same reasoning put
forward for the second-generation polymers, we assume that
ꢀ
ꢀ
ing configurational stability in the range of 210 C to 50 C
as judged from their temperature dependent CD-spectra (Fig.
FIGURE 3 a) CD- (solid lines) and UV-spectra (dashed lines) of MOM-protected PQs poly-4a obtained under different polymeriza-
tion conditions. b) Comparison with poly-4b (Table 2, #3 and #5, solvent CHCl3).
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bathochromic shift of the maximum at longest wavelength in
their CD-spectra was observed (e.g. poly-4a 357 nm to
373 nm in poly-29). We noticed decrease of the CD inten-
sities accompanied by a decrease of the gabs for deprotected
polymers poly-27 and poly-28 compared with their parent
polymers. Whether this is due to a competitive racemization
process or a material property is not clear at the moment. In
contrast, for poly-29, we observed an increase of the gabs. If
this can be attributed to different conformations upon cleav-
age of the protecting groups of the 2,20-biphenolic motif or
other reasons are responsible is not clear either. Please note
that these deprotected polyquinoxalines showed no racemi-
zation in the range of 210 ꢀC to 50 ꢀC upon investigation
with temperature dependent CD-spectroscopy (Fig. 6). Even
PQ poly-29 derived from lower molecular weight precursor
poly-4a is stable in this temperature range (Fig. 7).
FIGURE 4 Temperature-dependent CD-spectra of PQ poly-4a
(#1, solvent CHCl3).
4). The small changes in the intensity upon heating were
found to be reversible.
CONCLUSIONS
A total of five new diisocyanide monomers (3b, 3c, 7, 4a,
4b) containing protected phenolic groups have been synthe-
sized. These monomers display one or two phenolic groups
and different alkyl groups to ensure stability and solubility.
All of these new diisocyanides show good stability in solu-
tion at least on the time scale of the polymerization experi-
ment. With the known initiators 17 (Pd-based) or 18 (Ni-
based) the second-generation monomers 3b, 3c, and 7 and
the third-generation monomer 4b yielded new PQs with
good yields and narrow PDI. Unfortunately, monomer 4a
yielded only short chain oligomers, most probably due to the
presence of an atropos biaryl-axis leading to increased steric
bulk. The new PQs showed CD-signals characteristic for the
formation of a preferentially one-handed helical backbone
during the polymerization. Comparison of the CD-spectra
intensities revealed that the Pd-based initiator 17 led to a
DEPROTECTION
Deprotection of the MOM-containing polymers poly-3b, poly-7,
and poly-4a was successful with BF3-etherate in anisole/
CH2Cl2 (Table 3) as judged by the absence of the methylene
protons of the MOM protecting group at 5.0 ppm in the 1H-
NMR spectra. Moreover, the observation of OH-stretching
vibrations in the IR spectra at around 3450 cm21 corroborates
this interpretation. Phenolic substituted PQs poly-27 and poly-
28 were soluble in common organic solvents e.g. CHCl3, THF,
and benzene. Poly-29, displaying a biphenolic structure per
repeating unit, turned out to be only soluble in polar solvents
or solvent mixtures containing THF, MeOH, DMSO.
Figure 5 shows the CD- and UV-spectra of the obtained PQs
after the deprotection step. For all deprotected polymers a
TABLE 3 Examples for Deprotection of MOM-Protected PQs.
gabs (1023 a
)
Product
Yield (%)b
Mp (g mol21
c,d
)
DP
PDIe
gabs (1023
)
a
No.
Educt polymer
DP
1
2
3
4
poly-3b (Table 1, #1)
poly-7 (Table 1, #8)
poly-4a (Table 2, #1)
poly-4a (Table 2, #3)
34
37
12
9
0.26
0.28
0.33
0.36
poly-27
poly-28
poly-29
poly-29
70
88
30
78
10,400
18,200
n.d.f
32
1.18
1.30
n.d.f
n.d.f
0.13
0.25
0.50
0.43
42
n.d.f
n.d.f
n.d.f
a
d
e
f
gabs was determined from the CD- and UV-spectrum at 353 nm (poly-
27), 362 nm (poly-28), 373 nm (poly-29).
Isolated yield after precipitation from appropriate solvent.
Determined by GPC (PS standards).
w/Mn.
Not measurable due to strong absorption on the GPC column.
M
b
c
Molar mass at peak maximum.
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FIGURE 7 Temperature-dependent CD-spectra of a PQ poly-29
(Table 3, #3, solvent CHCl3).
FIGURE 5 CD- (solid lines) and UV-spectra (dashed lines) of
PQs after deprotection (Table 3, entry #1–#4, solvent CHCl3).
microwave synthesizer, N. A. Dencher for experimental time on
his UV spectrometer, S. Foro (TU Darmstadt) for her careful
work on the X-ray structures, and H. Fueß (Department of
Material Science, TU Darmstadt) for measuring time on his dif-
fractometer. S. Immel and Alexander Proskurjakov are acknowl-
edged for carefully reading the manuscript.
higher screw-sense excess than Ni-based initiator 18. The
helical backbone is configurationally stable during deprotec-
tion of the phenolic groups and under thermal stress up to
ꢀ
50 C.
Possible applications of the new phenolic substituted PQs as
organocatalysts will now be studied in Brønsted acid medi-
ated transformations. Moreover, applications in metal medi-
ated or catalyzed transformations like Diels-Alder reactions
under Lewis acid control appear feasible. Since the phenolic
group allows further synthetic manipulations these polymers
may even serve as starting materials for example, polymeric
helically chiral phosphites.
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FIGURE 6 Temperature-dependent CD-spectra of deprotected PQs in CHCl3: a) poly-27 (Table 3, #1), b) poly-28 (Table 3, #2).
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