Selective ring opening of 4H-1,3,2-benzodioxasiline twin monomers

Article abstract of DOI:10.1039/c1nj20654k

4H-1,3,2-Benzodioxasilines represent a class of monomers, which are suitable for the recently developed concept of twin polymerization. Relating to the prediction of quantum chemical calculations we are now able to confirm the results for the first step o

Full text of DOI:10.1039/c1nj20654k

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LETTER
Selective ring opening of 4H-1,3,2-benzodioxasiline twin monomersw
¨
Patrick Kempe, Tina Loschner, David Adner and Stefan Spange*
Received (in Victoria, Australia) 28th July 2011, Accepted 30th August 2011
DOI: 10.1039/c1nj20654k
4H-1,3,2-Benzodioxasilines represent a class of monomers, which are
suitable for the recently developed concept of twin polymerization.
Relating to the prediction of quantum chemical calculations we are
now able to confirm the results for the first step of the reaction: the
selective ring opening of 4H-1,3,2-benzodioxasilines. An electrophilic
attack of iodotrimethylsilane leads firstly to a bond cleavage at the
Scheme 1 Twin polymerization of 1a to phenolic resin and silica.
oxymethylene group and not at the more stable Si–O–aryl bond,
yielding ring-opened species.
In previous works the twin polymerization of different monomers,
which were combinations of inorganic and organic moieties, was
reported.1–5 Especially the use of furfuryl and 2-hydroxybenzyl
ethers of silicon, titanium or tungsten led to monomers, which
were polymerized to inorganic–organic nanocomposites in a
simple cationic catalysis process without any further requirements
in reaction control (Scheme 1).1–5
Scheme 2 Selective ring opening reaction of 1a by an electrophilic
attack on the oxymethylene group.
Quantum chemical calculations of the cationic polymerization
process of 2,20-spirobi[4H-1,3,2-benzodioxasiline] (1a) proposed
a selective ring opening of the bidentate 2-hydroxybenzyl
alcohol (salicyl alcohol) at the oxymethylene group by electrophilic
attack of a proton as the first step in the cationic catalysis.4 Here
we report on our experimental results, which prove the predicted
selective ring opening of 1a and further 4H-1,3,2-benzodioxasilines
(Scheme 2).
consumed all ring-opened reactive benzyl iodide species. This
reaction was investigated with three different 4H-1,3,2-benzodioxa-
silines: 1a,4 2,2-dimethyl-4H-1,3,2-benzodioxasiline (2a),7 and
the new compound 2,2’-spirobi[8-tert-butyl-6-methyl-4H-
1,3,2-benzodioxasiline] (3a).
Spiro compounds 1a and 3a are chiral substances, each
consisting of two enantiomers (racemic mixture). The
1H NMR signals of the diastereotopic hydrogens of the
methylene bridges of 1a and 3a have a quite small difference
in their chemical shifts, and due to geminal coupling (2J) two
close doublets with a strong roofing effect occur (see ESIw).
Iodotrimethylsilane was used in nearly equimolar ratios
relative to the present number of 4H-1,3,2-benzodioxasiline
rings. At 25 1C the solid substances 1a and 3a were dissolved in
d2-dichloromethane (CD2Cl2) before iodotrimethylsilane was
added. The reaction mixture was analyzed directly via NMR
spectroscopy. The liquid substance 2a was reacted without
prior dissolving and only samples of the reaction mixture were
diluted in CD2Cl2 for the NMR analyses. 1H, 13C and 29Si
NMR spectra were taken immediately after addition of
iodotrimethylsilane and at further time intervals; an overview
of the product spectra is given in Fig. 1.
The search for electrophiles, which are suitable to perform a
mechanistic investigation, included different difficulties. The use
of Brønsted acids would lead to the formation of silanols and
therefore to additional side reactions, such as condensation
and hydrolysis. On the other hand, electrophilic CH3I and
(CH3)3SiCl were not reactive enough to open the ring of
4H-1,3,2-benzodioxasilines. The use of (CH3)2SO4 led to an
immediate polymerization resulting in phenolic resins, because
the anion was not able to form a stable bond with the reactive
benzyl cation (see ESIw).
Fortunately, the use of iodotrimethylsilane was suitable to
open the ring of the used 4H-1,3,2-benzodioxasilines yielding
benzyl iodides,6 which were stable long enough for the used
analyzing procedures before
a polymerization process
After the ring-opening reaction with iodotrimethylsilane the
1H NMR signals of the iodomethylene hydrogens of 1b, 2b, and
3b are shifted to a higher field compared to the oxymethylene
groups of 1a, 2a, and 3a. In the 13C NMR spectra a strong
shift to high field of Dd E 63 ppm occurs, due to the strong
heavy-atom effect of iodine. The 13C NMR signals of the
Chemnitz University of Technology, Strasse der Nationen 62, 09111
Chemnitz, Germany. E-mail: stefan.spange@chemie.tu-chemnitz.de;
Fax: +49 (0)371 531-21239; Tel: +49 (0)371 531-21230
w Electronic supplementary information (ESI) available: Mass spectro-
grams, further reaction information, additional NMR analyses and
information on derivatization reactions. See DOI: 10.1039/c1nj20654k
c
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Fig. 1 NMR spectra of educts 1a, 2a, and 3a in CD2Cl2 and their ring-opened products 1b, 2b, and 3b in their reaction mixtures are depicted. 1H, 13
C
and 29Si NMR spectra are arranged from the left to the right and are referenced to CD2Cl2 [d(1H/13C) = 5.32/54.0 ppm] and tetrakis(trimethylsilyl)silane
(TTSS, TMS4Si) [d(29SiMe3) = ꢀ9.8 ppm]. Magnifications of the spectra are depicted in the ESIw as well as NMR data of side products.
c
2736 New J. Chem., 2011, 35, 2735–2739
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iodomethylene groups have chemical shifts of d = 1–5 ppm
and are very close to the signals of the trimethylsilyl groups,
they were distinguished by means of 1H/13C-HMQC NMR
spectroscopy (for 2D NMR signal assignment see ESIw). 29Si
NMR proves conversion to ring-opened benzyl iodides, too
(conventional labeling is used: M, D, T, Q).8
Besides the selective ring-opening reaction the occurrence
of a small amount of side reactions cannot be excluded.
Iodotrimethylsilane is also able to interact with the surface
of the used glass flasks, small moieties of trimethylsilyl species
such as trimethylsilanol (TMSOH), hexamethyldisiloxane
(TMS2O), and tetrakis(trimethylsiloxy)silane (TMSO4Si) are
able to form (see ESIw). Another side reaction represents the
polymerization tendency of unstable benzyl iodide species,
especially in higher concentrations and without cooling. The
polymerization process leads to dimer and oligomer species
and finally to the formation of phenolic resins and involves the
condensation of reactive hydroiodic acid, which will lead to
further side reactions and can act as a catalyst. Typically, the
methylene group of phenolic resins has a chemical NMR shift
of d(1H) E 3.9 ppm in CD2Cl2. When mass spectrometry was
applied for the characterization of the ring-opened benzyl
iodides only very soft ionization conditions led to a product
spectrogram equivalent to the NMR spectra, otherwise dimers
were observed or even no more clear spectrograms were
obtained (see ESIw). Redox processes probably lead to the
formation of spurious iodine, which explains the increasing
coloration intensity (red-brownish) of the reaction mixture.
Side reactions of iodotrimethylsilane and its sub-equimolar
use also explain remaining small amounts of educts (1a, 2a, 3a)
and only single ring-opened intermediate species (in the case of
1a and 3a). That is why exact knowledge about the time-,
solvent- and concentration-dependence of this reaction had to
be explored in order to obtain the ring-opened benzyl iodide in
nearly quantitative yields.
Fig. 2 HMQC NMR spectrum of 2b/mim, the reaction product of
2b and 1-methylimidazole. The 2D NMR spectrum and the additional
1H and 13C NMR spectra were taken from a CD2Cl2 solution
(d(1H/13C) = 5.32/54.0 ppm). Magnified 1H, 13C, and 29Si NMR
spectra and a mass spectrogram are depicted in the ESI.w
be avoided for the work with the benzyl iodides. Therefore,
1-methylimidazole is a convenient aprotic non-ionic nucleophile,
which attacks the electrophilic iodomethylene group forming an
imidazolium salt with an iodide anion. The obtained oil is
comparable to an ionic liquid (2b/mim, see the HMQC spectrum
in Fig. 2). In this way the polymerization tendency is vanished
and the only molecular instability of the imidazolium cation is
due to the possible hydrolysis of the siloxy-groups.
Under the used reaction conditions the nearly quantitative
conversion of 1a and 2a into 1b and 2b occurred in about 5–20 min
(see Fig. 1 and the experimental part). A nearly quantitative
conversion of 3a into 3b is achieved after hours (1H NMR
of 3b in Fig. 1 was taken after 11 h). Being sterically blocked
and, due to its lower solubility, more dilute, 3a showed a
slower reaction rate and therefore undesired side reactions of
iodotrimethylsilane increased slightly.
Substitution of iodide of 2b with an ionic nucleophile like an
acetate ion could be fulfilled using tetra-n-butylammonium
acetate, which is advantageously soluble in less polar solvents.
The obtained benzyl acetate can be purified, but it is still able
to polymerize, as described in the ESI.w
A selective ring-opening reaction at the oxymethylene group
of the three 4H-1,3,2-benzodioxasilines (1a, 2a, and 3a) was
evidenced by the use of electrophilic iodotrimethylsilane and
previous quantum chemical calculations were confirmed.4 With
this work, the initial step of the cationic twin polymerization is
considered to be investigated sufficiently enough, the following
process of polymerization is part of our future work. As shown
in this article the Si–O–aryl bonds are stable in the initial phase
of the twin polymerization, which is regarded as a reason why
phase separation of both polymers (organic/inorganic) is
suppressed and nanocomposites can be obtained at the end
of the polymerization process.4
At 25 1C and at the used concentrations 1b and 2b were only
stable for some hours and polymerized slowly to phenolic
resins. Diluting and freezing down the reaction mixture
decelerated this process. At 25 1C and at the used concentration
3b was stable for several days and almost no polymerization
species was observed after 5 days. This difference to 1b and 2b is
due to the full substitution of all ortho- and para-positions of
the phenolic ring. Electrophilic attack and substitution with
benzylic species is therefore rather improbable.
In order to stabilize the ring-opened benzyl iodides they
must be further derivatized. Among different possibilities,
nucleophilic substitution of iodide yielded stable molecules
that were isolated as shown in the ESI.w Of course only aprotic
nucleophiles were tested, because the liberation of hydrogen
iodide interferes and could cleave the siloxy groups. Ionic
nucleophiles often require strong polar solvents, which are to
The not yet described ring-opened benzyl iodides 1b, 2b, and
3b were clearly identified by 1H, 13C, 29Si NMR, 1H/13C-HMQC
NMR, and mass spectrometry (see Fig. 1, for mass spectrograms
and further NMR details see ESIw).
The obtained benzyl iodides offer the opportunity of further
modification via substitution of iodide in order to stabilize the
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benzylic group or to adjust the molecule to a desired
application—as shown in the ESIw, three new derivative
substances were obtained. Nucleophilic attack onto the silicon
atom next to the phenoxy group should lead to ortho quinone
methide intermediates that are of synthetic interest in cycloaddition
reactions,9 but polymerization must be avoided.
(0.086 mol) of 2-tert-butyl-6-(hydroxymethyl)-4-methylphenol
was dissolved in 100 ml toluene. At 80 1C 6.55 g of
tetramethylorthosilicate (0.043 mol) was dropped into the
solution and 20 ml of tetra-n-butylammonium fluoride (1 M
in THF) was added. The reaction mixture was kept at 80 1C
for the next few hours while resulting methanol was removed
slowly under reduced pressure. Colorless crystals were
obtained after evaporation of toluene and recrystallization
using hexane. Yield: 52% of theory (0.022 mol, 9.23 g).
1H NMR (CD2Cl2, d = 5.32 ppm): 7.06 (2 ꢁ 1H, d, aryl),
Experimental
Analytical methods
2
6.74 (2 ꢁ 1H, d, aryl), 5.15/5.03 (4H, 2d, 2 ꢁ CH2O, J =
NMR spectroscopy. The depicted 1H, 13C, 29Si and
1H/13C-HMQC NMR spectra were recorded at 25 1C on a
Bruker Avance 250, the residual peak of the used solvent
CD2Cl2 was used as reference, d(1H/13C) = 5.32/54.0 ppm; as
29Si reference TTSS was added just before the measurement,
13.8 Hz), 2.27 (2 ꢁ 3H, s, 2 ꢁ aryl-CH3), 1.36 (2 ꢁ 9H, s, 2 ꢁ
ꢀ
t-Bu) ppm. 13C NMR (CD2Cl2, d = 54.0 ppm): 149.8
(Caryl–O), 139.5, 131.5, 127.8, 126.6, 125.0, 67.0 (CH2O),
35.1 t-Bu(Cq), 30.1 t-Bu(CH3), 21.1 (aryl-CH3) ppm. 29Si
ꢀ
NMR (CD2Cl2, TTSS d = ꢀ9.8 ppm): ꢀ79.3 ppm (Q0).
Further characterization, such as X-ray crystallographic data
will be published separately.10
d(29Si(CH3)3)
=
ꢀ9.8 ppm. Different experiments of
29Si NMR were used according to the investigated molecular
structure (1a, 2b: DEPT45, d1 = 5 s, CNST2 = 9 Hz; 1b, 2a,
3a: ig30, d1 = 30 s; 3b: ig30, d1 = 20 s). For the analyses of the
diastereotopic hydrogens of 1a spectra were recorded on a
Varian Unity Inova 400 spectrometer at 25 1C, as reference the
residual peak of the used solvent CDCl3 (d = 7.26 ppm) was
used (Fig. S3, ESIw).
Ring-opening reactions of 1a, 2a, and 3a with
iodotrimethylsilane
Bis(2-iodomethylphenoxy)-bis(trimethylsiloxy)silane (1b).
Under dry argon 0.884 g (3.246 mmol) of 1a was dissolved
in 1.945 g of CD2Cl2. At 25 1C 1.297 g (6.485 mmol) of
iodotrimethylsilane was added at once to the stirred solution.
After 15 min 1H NMR proved a nearly quantitative formation
of 1b, as depicted in Fig. 1. At 25 1C and at the used
concentration (c1b = 2.27 mol lꢀ1) 1b is only stable for some
Mass spectrometry. Mass spectrograms (Fig. S1, ESIw) of 1b
have been recorded on a Bruker micrOTOF-Q II spectrometer
using the electrospray ionization method (ESI-MS). ESI-MS
was also applied for analyzing 2b, 3b and the imidazolium
derivative of 2b (2b/mim), a Bruker Esquire with an ion trap
detector at TU Dresden was used.
1
hours and polymerizes slowly to a phenolic resin. H NMR
(CD2Cl2, d = 5.32 ppm): 7.50–7.44 (2 ꢁ 1H, m, aryl),
7.39–7.24 (2 ꢁ 2H, m, aryl), 7.14–7.05 (2 ꢁ 1H, m, aryl),
4.63 (2 ꢁ 2H, s, 2 ꢁ CH2I), 0.37 (2 ꢁ 9H, s, 2 ꢁ Si(CH3)3)
ppm. 13C NMR (CD2Cl2, d = 54.0 ppm): 152.00 (Caryl–O),
131.09, 130.06, 130.04, 123.24, 119.94, 2.14 (Si(CH3)3),
Substances
Tetramethylorthosilicate was used as purchased from Fluka/
Sigma Aldrich Chemie GmbH (498%).
Tetra-n-butylammonium fluoride (1 M solution in THF)
was used as purchased from ABCR GmbH & Co. KG.
Toluene and hexane were dried over sodium and freshly
distilled prior use.
1.72 (CH2I) ppm. 29Si NMR (CD2Cl2, TTSS
d =
ꢀ9.8 ppm): +13.5 (Si(CH3)3, M1), ꢀ102.9 (‘SiO4’, Q2) ppm. m/z:
673.06 [M + H]+, 694.94 [M + Na]+, 710.91 [M + K]+.
2-Iodomethylphenoxy-trimethylsiloxy-dimethylsilane (2b).
Under dry argon at 25 1C 1.519 g (7.591 mmol) of iodotri-
methylsilane was added at once to the stirred 1.372 g
(7.591 mmol) of 2a. After 5 min 1H NMR proved a nearly
quantitative formation of 2b, as depicted in Fig. 1. At 25 1C
and at the used reaction conditions 2b is only stable for some
2-tert-Butyl-4-methylphenol was used as purchased from
Acros Organics (99%).
Iodotrimethylsilane was used as purchased from ABCR &
Co. KG (97%, stabilized with copper).
D2-Dichloromethane (CD2Cl2) was purchased from
Deutero GmbH (99.6%) and dried with a molecular sieve of
3 A prior use.
1
hours and polymerizes slowly to a phenolic resin. H NMR
(CD2Cl2, d = 5.32 ppm): 7.40–7.33 (1H, m, aryl), 7.27–7.18
(1H, m, aryl), 7.00–6.90 (2H, m, aryl), 4.52 (2H, s, CH2I), 0.38
(6H, s, Si(CH3)2), 0.21 (9H, s, Si(CH3)3) ppm. 13C NMR
(CD2Cl2, d = 54.0 ppm): 153.6 (Caryl–O), 130.8, 130.2,
129.9, 122.3, 119.8, 2.4 (CH2I), 2.1 Si(CH3)3, 0.2 Si(CH3)2. 29Si
NMR (CD2Cl2, TTSS d = ꢀ9.8 ppm): +9.7 (Si(CH3)3, M1),
ꢀ12.4 (Si(CH3)2, D1) ppm. m/z: 398.0 [M + NH4]+.
Tetrakis(trimethylsilyl)silane (TTSS) was used as purchased
from ABCR & Co. KG (98%).
1-Methylimidazole (99%) was used as purchased from Carl
Roth GmbH & Co. KG.
Syntheses
2-tert-Butyl-6-(hydroxymethyl)-4-methylphenol (used for the
synthesis of 3a). 2-tert-Butyl-6-(hydroxymethyl)-4-methylphenol
was synthesized by hydroxymethylation of 2-tert-butyl-4-methyl-
phenol with formaldehyde.10,11
Bis(2-tert-butyl-6-iodomethyl-4-methylphenoxy)-bis(trimethyl-
siloxy)silane (3b). Under dry argon 0.242 g (0.586 mmol) of 3a
was dissolved in 1.639 g of CD2Cl2. At 25 1C 0.234 g
(1.171 mmol) of iodotrimethylsilane was added at once to the
stirred solution. After 11 h 1H NMR proved a nearly quantitative
formation of 3b, as depicted in Fig. 1. At 25 1C and at the used
concentration (c3b = 0.49 mol lꢀ1) 3b was stable for several days.
1a and 2a were synthesized as described in the literature.4,7
2,20-Spirobi[8-tert-butyl-6-methyl-4H-1,3,2-benzodioxasiline]
(3a) (modified synthesis of 1a)4. Under dry argon 16.71 g
c
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1H NMR (CD2Cl2, d = 5.32 ppm): 7.20 (2 ꢁ 1H, d, aryl), 7.16
(2 ꢁ 1H, d, aryl), 4.84 (2 ꢁ 2H, s, 2 ꢁ CH2I), 2.32 (6H, s,
Notes and references
1 S. Grund, P. Kempe, G. Baumann, A. Seifert and S. Spange,
Angew. Chem., 2007, 119, 636; S. Grund, P. Kempe, G. Baumann,
A. Seifert and S. Spange, Angew. Chem., Int. Ed., 2007, 46, 628.
2 A. Mehner, T. Ruffer, H. Lang, A. Pohlers, W. Hoyer and
S. Spange, Adv. Mater., 2008, 20, 4113.
3 S. Spange and S. Grund, Adv. Mater., 2009, 21, 2111.
4 S. Spange, P. Kempe, A. Seifert, A. A. Auer, P. Ecorchard,
H. Lang, M. Falke, M. Hietschold, A. Pohlers, W. Hoyer,
G. Cox, E. Kockrick and S. Kaskel, Angew. Chem., 2009,
121, 8403 (Angew. Chem., Int. Ed., 2009, 48, 8254).
2 ꢁ aryl-CH3), 1.51 (2 ꢁ 9H, s, 2 ꢁ t-Bu), ꢀ0.06 (2 ꢁ 9H, s,
ꢀ
2 ꢁ Si(CH3)3) ppm. 13C NMR (CD2Cl2, d = 54.0 ppm): 148.61,
141.14, 132.34, 131.10, 130.50, 128.98, 35.46 (t-Bu(Cq)), 31.24
(t-Bu(CH3)), 21.18 (aryl-CH3), 4.84 (CH2I), 1.84 (Si(CH3)3) ppm.
ꢀ
29Si NMR (CD2Cl2, TTSS d = ꢀ9.8 ppm): +12.0 (Si(CH3)3,
M1), ꢀ109.0 (‘SiO4’, Q2) ppm. m/z: 830.40 [M + NH4]+.
1-(2-((1,1,3,3,3-Pentamethyldisiloxanyl)oxy)benzyl)-3-methyl-
imidazolium iodide (2b/mim). In a 50 ml flask at 25 1C 0.947 g
(5.25 mmol) of 2a are stirred and 1.020 g (5.10 mmol) of
iodotrimethylsilane were added with a syringe at once. After
5 min 10 ml of CH2Cl2 were added and a small sample
(2 drops) of the diluted mixture was analyzed via 1H NMR
spectroscopy. As the NMR data proved nearly quantitative
consumption of 2a to 2b 0.412 g (5.02 mmol) of 1-methylimidazole
was added to the solution and the reaction mixture was
continued to stir at W = 25 1C for 3 h. The solvent was
removed under vacuum, and the residue was washed 5 times
with 3 ml hexane. The obtained product is yellow oil. Yield:
B100% relative to the used 1-methylimidazole. 1H NMR
(CD2Cl2, d = 5.32 ppm): 9.87 (1H, s(br), N–CH–N),
5 F. Bottger-Hiller, R. Lungwitz, A. Seifert, M. Hietschold,
M. Schlesinger, M. Mehring and S. Spange, Angew. Chem., 2009,
121, 9039; F. Bottger-Hiller, R. Lungwitz, A. Seifert,
M. Hietschold, M. Schlesinger, M. Mehring and S. Spange, Angew.
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6 (a) U. Kruerke, Chem. Ber., 1962, 95, 174; (b) M. E. Jung and
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7.65–6.95 (6H, m), 5.46 (2H, s, CH2), 4.02 (3H, s, mimCH3),
ꢀ ꢀ
7 (a) M. Wieber and M. Schmidt, J. Organomet. Chem., 1963, 1, 93;
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0.26 (6H, s, Si(CH3)2), 0.07 (9H, s, Si(CH3)3) ppm. 13C NMR
(CD2Cl2, d = 54.0 ppm): 153.7 (C–O), 136.9 (N–CH–N),
131.8, 131.4, 123.9, 123.8, 122.5, 122.4, 119.5, 49.2 (CH2),
37.3 (mimCH3), 1.7 (Si(CH3)3), 0.0 (Si(CH3)2) ppm. 29Si NMR
ꢀ ꢀ
(CD2Cl2, d(TTSS, 29Si(CH3)3) = ꢀ9.8 ppm): +10.6 (M1),
8 F. Uhlig and H. C. Marsmann, in 29Si NMR, Some Practical
Aspects, from: Gelest Catalog: Silicon Compounds, Silanes &
Silicones, ed. B. Arkles and G. Larson, Gelest Inc., Morrisville,
PA, 2008, 2nd edn, pp. 208–222.
ꢀ11.0 (D1) ppm. m/z: 335.2 [M ꢀ I]+.
9 A. F. Barrero, J. F. Quılez del Moral, M. M. Herrador, P. Arteaga,
M. Cortes, J. Benites and A. Rosellon, Tetrahedron, 2006, 62, 6012.
10 unpublished results of our working group, in preparation
forpublication.
11 (a) J. Pikl, patent: US 2792428, 1957, DuPont, inv; (b) J. Pikl,
patent: GB 893896 (1962), Shell Int. Research, 1962.
Acknowledgements
Financial support by BASF SE and DFG is gratefully
acknowledged. We thank Dr Ingmar Bauer of TU Dresden
for mass spectrometry measurement of 2b, 2b/mim and 3b.
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New J. Chem., 2011, 35, 2735–2739 2739