BN- and BO-Doped Inorganic–Organic Hybrid Polymers with Sulfoximine Core Units

Article abstract of DOI:10.1002/chem.201903289

While polysulfones constitute a class of well-established, highly valuable applied materials, knowledge about polymers based on the related sulfoximine group is very limited. We have employed functionalized diaryl sulfoximines and a p-phenylene bisborane as building blocks for unprecedented BN- and BO-doped alternating inorganic–organic hybrid copolymers. While the former were accessed by a facile silicon/boron exchange protocol, the synthesis of polymers with main-chain B–O linkages was achieved by salt elimination.

Full text of DOI:10.1002/chem.201903289

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Accepted Article
Title: BN- and BO-Doped Inorganic–Organic Hybrid Polymers with
Sulfoximine Core Units
Authors: Felix Brosge, Thomas Lorenz, Holger Helten, and Carsten
Bolm
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BN- and BO-Doped Inorganic–Organic Hybrid Polymers with
Sulfoximine Core Units
Felix Brosge,§[a] Thomas Lorenz,§[b] Holger Helten,*[b] and Carsten Bolm*[a]
In memory of Prof. Dr. Dieter Enders
Abstract: While polysulfones constitute a class of well-established,
highly valuable applied materials, the knowledge about polymers
based on the related sulfoximine group is very limited. We have
employed functionalized diaryl sulfoximines and a p-phenylene
bisborane as building blocks for unprecedented BN- and BO-doped
alternating inorganic–organic hybrid copolymers. While the former
were accessed by a facile silicon/boron exchange protocol, the
synthesis of polymers with main chain B–O linkages was achieved
by salt elimination.
phthalimid- and benzyl-protected NH-sulfoximines 4 and 6,
respectively, were targeted first. The synthetic sequences are
shown in Scheme 1. The preparation of 1 started from known
diarylsulfide 3,[12] which was imidated and oxidized by adopting a
protocol reported by Luisi, Bull and others[13] to give 4 in 62%
yield. Noteworthy, we applied aqueous ammonia as nitrogen
source instead of the originally suggested ammonium carbamate.
Polysulfones are a family of plastic materials that are noted for
their high thermal and oxidative stability.[1] They are being used
within fluid handling components, steam sterilizable biomedical
moldings as well as in a range of chemical process and
automotive applications.[2] Some of us recently reported a series
of BN-doped inorganic–organic hybrid polymers,[3-6] including the
first poly(p-phenylene iminoborane), which can be regarded as a
BN-analogue of poly(p-phenylene vinylene) (PPV).[3d] A dapson-
type diaryl sulfone was also incorporated into a polymeric
material.[3e]
Formal exchange of a sulfonyl oxygen by a nitrogen atom
converts a sulfone into a sulfoximine. The latter compounds are
relevant in asymmetric synthesis[7] and applications in
medicinal[8] and crop protection chemistry.[9] Functionalizing the
sulfoximine nitrogen allows a fine-tuning of physicochemical
properties, which proved useful in drug design and bioactivity
adjustment.[10] Surprisingly, sulfoximines have only once been
applied as building blocks in polymers.[11] In that study, Takata et
al. used Friedel-Crafts reactions to prepare polysulfoximines
with molecular weights (Mn) of ca. 13 000. Here we describe the
synthesis and characterization of the first inorganic–organic
hybrid polysulfoximines.
In light of previous work,[12] sulfoximines 1 and 2 were
identified as suitable organic starting materials. Both compounds
were N-methylated, thereby confining the reactive anchor sites
of the molecules to the free arylic amino and hydroxyl groups.
With the vision to allow future variations of the N-substituent,
Scheme 1. Syntheses of key intermediates 1 and 2.
N-Methylation under standard Eschweiler-Clark conditions
afforded sulfoximine 5 (21%), which was deprotected with
hydrazine in ethanol to give 1 in 61% yield. Following an
analogous reaction sequence, sulfoximine 2 was prepared by
imidation/oxidation of 6[14] to give 7 (64%) followed by N-
methylation with MeI in KOH/DMSO[15] providing 8 in 48% yield
and sequential debenzylation with BBr3 (98%)
Targeting a polymer formation by silicon/boron exchange,
bis(silylated) sulfoximine 9 was prepared, in analogy of literature
precedence,[16] by treatment of 1 with a mixture of Me3SiCl and
Et3N in THF at 45 °C for 24 h. The coupling partner for 9 was
bis(bromoborane) 10[3d] (Tip = 2,4,6-triisopropylphenyl). Two co-
polycondensation reactions were performed (Scheme 2). In both
cases, a 1:1 ratio of 9 and 10 was applied. In the first experiment
(trial 1), the mixture was kept in dichloromethane for 3 days at
ambient temperature. Trial 2 involved o-difluorobenzene (o-DFB)
as the solvent and heating the mixture to 80 °C for 24 h. The
resulting alternating copolymers 11 were then purified by
precipitation from concentrated solution with hexane and
subsequent drying in vacuo. The identities of copolymers 11,
which were obtained as off-white solids, were unambiguously
[a]
[b]
F. Brosge, Prof. Dr. C. Bolm
Institute of Organic Chemistry, RWTH Aachen University
Landoltweg 1, 52074 Aachen (Germany)
E-mail: carsten.bolm@rwth-aachen.de
T. Lorenz, Priv.-Doz. Dr. H. Helten
Institute of Inorganic Chemistry, RWTH Aachen University
Landoltweg 1, 52074 Aachen (Germany)
New address: Institute of Inorganic Chemistry, Julius-Maximilians-
Universität Würzburg
Am Hubland, 97074 Würzburg (Germany)
E-mail: holger.helten@uni-wuerzburg.de
Supporting information for this article is given via a link at the end of
the document.
§
These authors contributed equally.
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ascertained by NMR spectroscopy. Their molecular mass
similar outcome resulted when o-difluorobenzene was used as
solvent at a reaction temperature of 80 °C. Also in this case, the
conversion was slow, taking 5 weeks in total. Although these
results showed that a Si/B exchange could, in principle, be
applied to accomplish a B–O bond formation starting from 10,
the slow rate of this process proved unfavorable for its
application to co-polycondensation reactions. Therefore, we
decided to investigate B–O bond formation between 10 and the
parent free phenol (13).
distributions
were
determined
by
gel
permeation
chromatography (GPC, Table 1). For both samples, the 1H NMR
spectrum showed a shift of the NH-Signal from δ = 3.77 ppm in 9
to the aromatic region in 11 (7.25 ppm), which was also
observed in previously prepared related BN polymers.3d
Scheme 3. Model reactions testing the feasibility of Si/B exchange and salt
elimination reactions in the formation of alternating copolymers with B–O
linkages.
Scheme 2. Polycondensation reaction of sulfoximine 9 and bisborane 10 to
give alternating copolymer 11.
While initial attempts without base remained unsatisfying,
the presence of triethylamine had a tremendously positive effect,
leading to a clean and quantitative coupling providing 14 within
24 hours at room temperature. Compound 14 was then isolated
by filtration and characterized by multinuclear NMR
spectroscopy. The 11B{1H} NMR spectrum showed a resonance
at δ = 47.3 ppm, which is in the expected range for the
suggested constitution.
The GPC analyses revealed number average molecular
weights of Mn = 9 750 (trial 1) and 11 830 (trial 2), according to
polymerization degrees of DPn = 13 and 15, respectively. The
polydispersity indices were close to 2, as expected for step-
growth polycondensation processes.
Table 1. GPC data of polymers 11 and 15 (against polystyrene standards).
Encouraged by this result, the aforementioned conditions
were applied in the copolymerization of sulfoximine 2 with
bisborane 10 (Scheme 4). Within 3 days in the presence of Et3N,
the dichloromethane solution became highly viscous (trial 1).
However, after work-up the GPC analysis revealed that the
product was of relatively low molecular weight (Mn = 2 400,
DPn = 3; Table 1). Consequently, in the next experiment (trial 2)
the solvent was changed to o-DFB, and then the reaction
temperature was raised to 80 °C. Pleasingly, in this manner,
after 24 h the molecular weight (Mn) of the resulting polymer 15
was increased to 5 300, revealing an average chain length of
DPn = 7 (Table 1).[18]
Mn
MW
PDI
1.91
2.44
1.52
1.84
DPn
13
15
3
11 (trial 1)[a]
11 (trial 2)[b]
15 (trial 1)[a]
15 (trial 2)[b]
9 750
11 830
2400
5300
18 600
28 900
2970
9740
7
[a] Prepared in dichloromethane (DCM), r.t., 3 d. [b] Prepared in o-difluoro-
benzene (o-DFB), 80 °C, 24 h.
Next, copolymers with main chain B–O linkages[17] were
targeted. Hypothesizing that such products could be accessed
by analogous Si/B exchange reactions as applied before in the
synthesis of 11, organic starting materials with silylated phenolic
hydroxyl groups became of interest. In order to get an estimate
of the feasibility of such an approach, a prior model reaction
between 10 and trimethylsilylated phenol 12 was performed
(Scheme 3). In the first experiment, the reaction was run in
dichloromethane at room temperature. As hypothesized, product
14 was indeed formed as revealed by 1H and 11B NMR
spectroscopy. The initial presence of two doublets (8.04 ppm
1
and 7.88 ppm) in the H NMR spectrum suggested a stepwise
formation of 14. However, the entire process was very slow, and
Scheme 4. Polycondensation reaction of sulfoximine 2 and bisborane 10 to
give alternating copolymer 15.
even after 4 weeks the conversion was not yet complete. A
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In summary, we have prepared the first inorganic–organic
hybrid
sulfoximine-containing
polymers
as
alternating
copolymers with B=N and B–O linkages. While our Si/B
exchange polycondensation protocol was successful in the
former case, for the synthesis of polymers with B–O linkages in
the main chain a salt elimination approach proved to be
favorable. In view of the recently demonstrated advantageous
effect of the formulation of dapsone-type drugs into polymer
conjugates for anti-inflammatory purposes[19] on the one hand,
and the well-established biomedical activity of many boron-
containing polymers[20] on the other hand, we are currently
exploring the biomedical potential of our novel sulfoximine–
B=N/B–O hybrids in detail.
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Conflicts of interest
There are no conflicts to declare
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Entry for the Table of Contents
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Fit us together: BN- and BO-doped
alternating inorganic–organic hybrid
copolymers were synthesized using
functionalized diaryl sulfoximines and
p-phenylene bisborane as monomer
units.
F. Brosge, T. Lorenz, H. Helten,* C.
Bolm*
Page No. – Page No.
BN- and BO-Doped Inorganic–
Organic Hybrid Polymers with
Sulfoximine Core Units
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