Synthesis of bis(3-{[2-(allyloxy)ethoxy]methyl}-2,4,6-trimethylbenzoyl) (phenyl)phosphine oxide - A tailormade photoinitiator for dental adhesives

Article abstract of DOI:10.3762/bjoc.6.26

Because of the poor solubility of the commercially available bisacylphosphine oxides in dental acidic aqueous primer formulations, bis(3-{[2-(allyloxy)ethoxy]methyl}-2,4,6-trimethylbenzoyl)(phenyl)phosphine oxide (WBAPO) was synthesized starting from 3-(c

Full text of DOI:10.3762/bjoc.6.26

Synthesis of bis(3-{[2-(allyloxy)ethoxy]methyl}-2,4,6-
trimethylbenzoyl)(phenyl)phosphine oxide – a tailor-
made photoinitiator for dental adhesives
Norbert Moszner*1, Iris Lamparth1, Jörg Angermann1, Urs Karl Fischer1,
Frank Zeuner1, Thorsten Bock1, Robert Liska2 and Volker Rheinberger1
Full Research Paper
Open Access
Address:
Beilstein Journal of Organic Chemistry 2010, 6, No. 26.
1Ivoclar Vivadent AG, Bendererstrasse 2, FL-9494 Schaan,
Liechtenstein and 2Institute of Applied Synthetic Chemistry, Division
of Macromolecular Chemistry, Vienna University of Technology,
Getreidemarkt 9/163/MA, A-1060 Vienna, Austria
doi:10.3762/bjoc.6.26
Received: 23 November 2009
Accepted: 03 March 2010
Published: 15 March 2010
Email:
Norbert Moszner* - norbert.moszner@ivoclarvivadent.com
Guest Editor: H. Ritter
* Corresponding author
© 2010 Moszner et al; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
adhesives; dental polymers; dimethacrylates; photoinitiator; radical
polymerization
Abstract
Because of the poor solubility of the commercially available bisacylphosphine oxides in dental acidic aqueous primer formulations,
bis(3-{[2-(allyloxy)ethoxy]methyl}-2,4,6-trimethylbenzoyl)(phenyl)phosphine oxide (WBAPO) was synthesized starting from
3-(chloromethyl)-2,4,6-trimethylbenzoic acid by the dichlorophosphine route. The substituent was introduced by etherification with
2-(allyloxy)ethanol. In the second step, 3-{[2-(allyloxy)ethoxy]methyl}-2,4,6-trimethylbenzoic acid was chlorinated. The formed
acid chloride showed an unexpected low thermal stability. Its thermal rearrangement at 180 °C resulted in a fast formation of
3-(chloromethyl)-2,4,6-trimethylbenzoic acid 2-(allyloxy)ethyl ester. In the third step, the acid chloride was reacted with
phenylphosphine dilithium with the formation of bis(3-{[2-(allyloxy)ethoxy]methyl}-2,4,6-trimethylbenzoyl)(phenyl)phosphine,
which was oxidized to WBAPO. The structure of WBAPO was confirmed by 1H NMR, 13C NMR, 31P NMR, and IR spectroscopy,
as well as elemental analysis. WBAPO, a yellow liquid, possesses improved solubility in polar solvents and shows UV–vis absorp-
tion, and a high photoreactivity comparable with the commercially available bisacylphosphine oxides. A sufficient storage stability
was found in dental acidic aqueous primer formulations.
Introduction
Self-etching enamel-dentin adhesives (SEAs) are used in re- monomers, such as polymerizable phosphonic or phosphoric
storative dentistry to achieve a strong bond between the filling acids and crosslinking dimethacrylates, such as 2,2-bis[(4-(2-
composites and dental hard tissues. The main components of hydroxy-3-methacryloyloxypropoxy)phenyl]propane (Bis-
currently used SEAs include strongly acidic adhesive GMA) or triethylene glycol dimethacrylate (TEGDMA) [1]. In
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Beilstein Journal of Organic Chemistry 2010, 6, No. 26.
SEAs, water is primarily used as the solvent or co-solvent. In this paper, we report the detailed synthesis of bis(3-{[2-
Thus, especially in the case of one-bottle adhesives, the meth- (allyloxy)ethoxy]methyl}-2,4,6-trimethylbenzoyl)(phenyl)phos-
acrylates may undergo hydrolysis of the methacrylate ester phine oxide WBAPO (Scheme 1) and its use as a tailor-made PI
groups in the presence of the strongly acidic adhesive in SEAs.
monomers. Therefore, we have synthesized new crosslinkers
[2], such as N,N'-diethyl-1,3-bis(acrylamido)propane
(DEBAMP), or new strongly acidic monomers, such as 2,4,6-
trimethylphenyl 2-[4-(dihydroxyphosphoryl)-2-oxa-
butyl]acrylate [3] or 1,3-bis(methacrylamido)propane-2-yl
dihydrogen phosphate (BMAMHP) [4], which show improved
hydrolytic stability under acidic aqueous conditions. The
visible-light (VL) photoinitiators (PIs) in current SEAs are
based on mixtures of camphorquinone (CQ) and tertiary amines
Scheme 1: Bisacylphosphine oxide with improved solubility in polar
solvents.
(A) [5]. The CQ-A PIs belong to bimolecular hydrogen abstrac-
tion PI systems: CQ shows a broad absorption spectrum
between 400 and 500 nm (λmax = 468 nm). The VL-excited CQ
Results and Discussion
forms an excited state complex with the amine co-initiator,
which generates a ketyl and an α-aminoalkyl radical by elec- Synthesis and characterization of WBAPO
tron and subsequent proton transfer. The aminoalkyl radical The synthesis of bisacylphosphines and their oxides often start
may initiate the polymerization of the monomers present, while either from P,P-dichlorophenylphosphine (PhPCl2) [13] or free
the ketyl radical is mainly deactivated by dimerization or phenylphosphine [14]. We chose to use the PhPCl2-route since
disproportionation [6]. However, in SEAs, the acid-base reac- primary phosphines have an unpleasant smell, a high toxicity
tion of acidic monomers with the basic amine co-initiators of and are very air-sensitive. Furthermore, phenylphosphine is
the PI system may significantly impair the formation of initi- difficult to access commercially and is very expensive. The syn-
ating radicals. Moreover, especially in the aqueous medium, the thesis of WBAPO started from 3-(chloromethyl)-2,4,6-
polar radical ions are well solvated by the surrounding medium, trimethylbenzoic acid, which was synthesized by chloromethyl-
thus inhibiting the proton transfer. If proton transfer occurs, ation of 2,4,6-trimethylbenzoic acid with a mixture of paraform-
both non-ionic and therefore rather hydrophobic species are aldehyde and hydrochloric acid, followed by treatment of the
kept in the solvent cage, which reduces the photoinitiating formed hydroxymethyl compound with concentrated hydro-
activity [7]. Therefore, in order to improve the performance of chloric acid in a one-pot reaction [15]. In the first step of the
SEAs, amine-free PIs were developed. In this context, we were WBAPO synthesis (Scheme 2), 3-(chloromethyl)-2,4,6-
able to show that benzoyltrimethylgermane [8] and dibenzoyl- trimethylbenzoic acid was coupled with 2-(allyloxy)ethanol
diethylgermane (DBDEG) [9,10] can be used as VL PIs for the using potassium hydroxide in excess of 2-(allyloxy)ethanol as
photopolymerization of dimethacrylate resins, dental adhesives solvent at 70 °C to afford 3-{[2-(allyloxy)ethoxy]methyl}-
or composites and undergo an α-cleavage with the formation of 2,4,6-trimethylbenzoic acid 1 as an off-white powder in 62%
benzoyl and germyl radicals, which may initiate the free-radical yield. The main problem of this step was the separation of the
polymerization of the monomers present. In addition, bisacyl- large amount of residual 2-(allyloxy)ethanol. This was success-
phosphine oxides, such as commercially available bis(2,4,6-tri- fully accomplished by washing its toluene solution with water
methylbenzoyl)phenylphosphine oxide (BAPO), seem to be a followed by recrystallization of the crude product from cyclo-
suitable alternative because their absorption tails out into the hexane.
visible range of the spectrum [7]. BAPO undergoes a
monomolecular α-cleavage with the formation of two initiating In the second step, 1 was chlorinated with thionyl chloride in
radicals and shows a high photoinitiating reactivity with a good toluene as solvent by a procedure analogous to that described in
storage stability. Recently, a simple straightforward synthesis of the literature [16]. After distillation of the dark crude product,
BAPO was published [11]. However, the solubility of BAPO in 3-{[2-[allyloxy)ethoxy]methyl}-2,4,6-trimethylbenzoyl chloride
polar solvents and aqueous formulations is insufficiently low, 2 was obtained as a colorless liquid in 76% yield. However, 2
which limits the use of BAPO in water-based SEAs. In this showed unexpected limited thermal stability. It was found that 2
context, we were able to synthesize a number of new substi- decomposed with the formation of 3-(chloromethyl)-2,4,6-
tuted bisacylphosphine oxides, which show improved solubility trimethylbenzoic acid 2-(allyloxy)ethyl ester 4 slowly on
in aqueous compositions [12].
storage at room temperature over a few days or rapidly on
heating at 180 °C for 4 h. Obviously, this reaction is a thermal
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Beilstein Journal of Organic Chemistry 2010, 6, No. 26.
Scheme 2: Etherification of 3-(chloromethyl)-2,4,6-trimethylbenzoic acid and chlorination of 1.
rearrangement because an exchange of chlorine with the WBAPO was carried out by 1H NMR, 13C NMR, 31P NMR and
2-(allyloxy)ethoxy group took place as a result of heating IR spectroscopy, as well as by elemental analysis. The spectral
(Scheme 3). In this context, it should be noted that we also data are in agreement with the expected structure. For example,
found the exchange of chlorine in the cases of both the 2-(eth- the presence of the [2-(allyloxy)ethoxy]methyl substituent was
oxy)- and 2-(propoxy)ethoxy derivatives. For the successful supported by the presence of new signals compared to the spec-
purification of 2 by distillation both a short distillation time and trum of BAPO: a singlet for the benzyl protons at δ = 4.49 ppm
a relatively low bottom temperature are crucial. Furthermore, it and two multiplets for the vinyl protons at δ = 5.13–5.27 and
was found to be advantageous if the crude product was purged 5.83–5.93 ppm were evident in the 1H NMR spectrum
with nitrogen gas prior to distillation. The identity of com- (Figure 1). The 13C NMR spectrum of WBAPO showed a
pound 4 was established by an independent synthesis of the doublet arising from the carbonyl carbon atom at δ = 216.2 ppm
compound from 2-(allyloxy)ethanol and 3-(chloromethyl)- compared to 216.1 ppm in the case of BAPO, and the 31P NMR
2,4,6-trimethylbenzoyl chloride 3, prepared by chlorination of spectrum featured only one signal at δ = 6.58 ppm (BAPO:
3-(chloromethyl)-2,4,6-trimethylbenzoic acid with thionyl 6.98 ppm).
chloride.
The yield of the WBAPO synthesis was significantly lower
In the third step, the bisacylphosphine oxide WBAPO was compared to the high yield (>90%) in the case of the less substi-
prepared by the reaction of 2 with P,P-dichlorophenylphos- tuted bisacylphosphine oxide BAPO. Therefore, a number of
phine (Scheme 4). Thus, P,P-dichlorophenylphosphine model reactions were carried out to elucidate possible reasons
dissolved in THF was first lithiated with metallic lithium in a for this. We therefore investigated the formation of BAPO by
dry argon atmosphere in the presence of a small amount of the reaction of 2,4,6-trimethylbenzoylchloride with P,P-
naphthalene. The resulting dark green phenylphosphine dichlorophenylphosphine under analogous conditions. The lithi-
dilithium solution was then added to a THF solution of 2. THF ation step was carried out in the presence of different model
was evaporated from the intermediate bis(3-{[2-(allyloxy)eth- compounds for the reactive sites of the [2-(allyloxy)ethoxy]-
oxy]methyl}-2,4,6-trimethylbenzoyl)(phenyl)phosphine solu- methyl substituent, such as benzyl methyl ether, allyl ethyl ether
tion, the residue dissolved in toluene and oxidized with a or diethylene glycol dimethyl ether. In the case of benzyl
30 wt % hydrogen peroxide solution at 70 °C. After column methyl ether and allyl ethyl ether, BAPO was formed in a lower
chromatography, WBAPO was obtained as a yellow oil in 67% yield showing that benzyl and allyl groups had a negative effect
yield with a HPLC purity of 94–95%. The characterization of on the yield of BAPO. Probably, these groups initiate side reac-
Scheme 3: Rearrangement of 2 under formation of 4.
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Beilstein Journal of Organic Chemistry 2010, 6, No. 26.
it proved very difficult to separate the residual phosphine from
the corresponding phosphine oxide. Accordingly, we used tem-
peratures of 45, 50 or 56 °C and determined the area ratios of
the WBAPO peak to the peak of the corresponding phosphine.
These were found to be 3:5, 4:1 or 8:1 by HPLC. Only tempera-
tures higher than 60 °C ensure a fast and complete oxidation of
the bisacylphosphine.
Finally, different batches of WBAPO were investigated by
LC-MS during the scale-up of the synthesis. The results show
that a side-chain extended WBAPO with a molecular weight of
867 g/mol (Scheme 5) was formed in all cases as the main
impurity in amounts of 0.5–5%. The structure of this com-
pound was also clearly confirmed by the 13C and 1H NMR
spectroscopic measurements of a WBAPO sample that specific-
ally contained about 70% of this side product. The 13C NMR
spectrum of this sample showed three carbonyl signals: a singlet
at δ = 170.4 ppm and two doublets at δ = 215.9 and 216.5 ppm.
The singlet arises from the carbonyl group of the ester, whereas
Scheme 4: Synthesis of WBAPO starting from P,P-
dichlorophenylphosphine and 2.
tions during the lithiation step. Furthermore, it was found that the two doublets can be assigned to the carbonyl groups of the
the temperature of the phosphine oxidation significantly influ- differently substituted and therefore non-equivalent benzoyl
enced the yield and the purity of the formed phosphine oxide as moieties. Moreover, in the 1H NMR spectrum the signal inten-
Figure 1: 1H NMR spectra (400 MHz, CDCl3) of BAPO and WBAPO.
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Beilstein Journal of Organic Chemistry 2010, 6, No. 26.
Scheme 5: Structure of the main impurity in isolated WBAPO.
sities of CH3, =CH (aromat), CH2 (benzyl) and CH2O protons acetonitrile shows UV–vis absorption, which tails out into the
were increased compared to pure WBAPO. In addition, a new visible range of the spectrum. WBAPO showed almost the same
triplet assignable to the CO-O-CH2 protons was found at δ = long wavelength absorption maximum (λmax) of 368 nm and
4.40 ppm. The complete separation of this side compound by extinction coefficient (ε) of 8850 dm2/mol compared to BAPO
repeated column chromatography would be very difficult and (λmax = 369 nm, ε369 = 8820 dm2/mol). The long wavelength
expensive. However, it will probably show similar photochem- bis(benzoyl)phosphine oxide absorption between ~360 and
ical properties compared to WBAPO and therefore its separ- 400 nm can be generally assigned to symmetry forbidden n-π*
ation is not necessary.
transitions, which are responsible for α-cleavage and formation
of free radicals [17]. After excitation with light in the near
UV–vis, the excited triplet state undergoes cleavage of the
carbon-phosphorus bond, thereby producing two highly effi-
Properties of WBAPO, photopolymerization
and adhesives
WBAPO is a liquid that exhibits improved solubility in polar cient initiating radicals: a benzoyl and a phosphinoyl radical.
solvents compared to the solid BAPO. For example, the solu- CQ absorbs light in the region of 400–500 nm with a low
bility of WBAPO in ethanol is about 50% and in acetone >50% absorption coefficient due to the n-π* transitions of the dicar-
compared to the solubility of BAPO of 3% in ethanol or 13% in bonyl group.
acetone. As demonstrated in Figure 2, WBAPO dissolved in
Figure 2: UV–vis absorption spectra of WBAPO and CQ dissolved in acetonitrile (10−3 mol/L).
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Beilstein Journal of Organic Chemistry 2010, 6, No. 26.
Photo-DSC is a unique method for comparing the performance
of different PIs. Therefore, the photopolymerization of a
common dental dimethacrylate resin based on mixtures of Bis-
GMA (42 wt %), UDMA (37 wt %), TEGDMA (21 wt %) and
the PI WBAPO or BAPO (2.38 mmol/100 g resin) was studied
by photo-DSC using a blue LED (emission spectrum:
380–515 nm, λmax = 460 nm) as irradiation source. The Photo-
DSC plots (Figure 3) confirmed the same photoinitiating
activity of the two PIs taking into consideration the experi-
mental accuracy of the DSC method.
Table 1: Shear bond strength (SBS, MPa) on dentin of experimental
aqueous SEAsa measured after storage of different SEAs at 42 °C.
PI
SBS after 0 d SBS after 14 d SBS after 28 d
CQ/EMBO
WBAPO
DBDEG
17.0 ± 4.7
30.3 ± 4.8
28.1 ± 3.3
9.7 ± 3.9
29.4 ± 3.2
32.2 ± 3.2
n.m.b
30.3 ± 2.7
29.2 ± 1.6
aSEA based on an aqueous mixture of the cross-linker DEBAMP (43 wt
%), the strongly acidic monomer BMAMHP (14 wt %) and the different
PIs (0.5 wt %); bnot measurable.
Because of the excellent performance of the synthesized expected improved solubility in polar solvents and the same
WBAPO, it has been used as part of the PI system in our current photochemical properties as the commercially available bisacyl-
SEA AdheSE® One F. This self-etching enamel-dentin adhesive phosphine oxides. Given its sufficient storage stability,
is mainly based on an aqueous mixture of the hydrolytically WBAPO can be used as efficient PI in dental acidic aqueous
stable cross-linker DEBAMP and the strongly acidic adhesive primer formulations.
monomer BMAMHP. For the investigation of the adhesive
properties, the shear bond strength of corresponding compos- Experimental
itions containing different PIs was measured as a function of 2,4,6-Trimethylbenzoic acid (Jiangsu Panoxi Chemical Co.,
storage time of the adhesive at 42 °C. The results (Table 1) Ltd., China) and 2-(allyloxy)ethanol (Kowa Europe GmbH,
showed that the efficiency of the CQ-A based adhesive Germany) were used without further purification. All other
decreased very rapidly. In contrast, the bonding properties of substances were purchased from Sigma-Aldrich (Switzerland).
the adhesives based on the bisacylphosphine oxide WBAPO or 3-(Chloromethyl)-2,4,6-trimethylbenzoic acid was prepared
DBDEG were not influenced by the stress test.
from 2,4,6-trimethylbenzoic acid by chloromethylation
according to the literature [15]. DBDEG was synthesized as
described previously [18].
Conclusion
WBAPO was synthesized via the dichlorophosphine route in a
satisfactory yield. Benzyl and allyl groups of the introduced DL-camphorquinone (CQ, Rahn, Switzerland), ethyl
[2-(allyloxy)ethoxy]methyl substituent probably initiate side p-dimethylaminobenzoate (EMBO, Fluka Chemie AG, Switzer-
reactions during the lithiation step and have a negative effect on land) and Irgacure® 819 (BAPO, Ciba Specialty Chemical,
the yield of the synthesis. WBAPO, a yellow liquid, showed the Switzerland) were used without purification. Bis-GMA and
Figure 3: DSC-plot of a mixture of Bis-GMA (42 wt %), UDMA (37 wt %), TEGDMA (21 wt %) and the PI WBAPO or BAPO.
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Beilstein Journal of Organic Chemistry 2010, 6, No. 26.
TEGDMA (Esschem, USA) and 1,6-bis(2-methacryloyloxyeth- IR (diamond ATR): ν = 3080 (w, =CH), 2980–2861 (m, CH2,
oxycarbonylamino)-2,2,4-trimethylhexane (UDMA, Ivoclar CH3), 1786 (vs, C=O), 1647 (w, C=C allyl), 1599 (w, C=C
Vivadent AG, Liechtenstein) were purchased from the suppliers aromat), 1449 (m, CH2, CH3), 1348 (m, CH3), 1095 (vs, COC),
noted above.
993, and 924 (=CH allyl), 785 cm−1 (vs, CCl). Anal. calcd. for
C16H21ClO3: C, 64.75; H, 7.13; Cl, 11.95. Found: C, 64.47; H,
7.15; Cl, 12.42.
Synthesis of 1
A suspension of 3-(chloromethyl)-2,4,6-trimethylbenzoic acid
(84.0 g, 0.395 mol) in 2-(allyloxy)ethanol (200.0 g, 1.96 mol) Synthesis of WBAPO
was added to a solution of potassium hydroxide (88.6 g, 1. Phenylphosphine dilithium
1.58 mol) in 2-(allyloxy)ethanol (606.7 g, 5.94 mol) and heated A solution of P,P-dichlorophenylphosphine (6.59 g, 36.8 mmol)
to 70 °C for 1 h. The mixture was poured in ice-water (600.0 g) in anhydrous tetrahydrofuran (THF, 10 mL) was added to a
and acidified with concentrated hydrochloric acid (pH < 1). stirred mixture of lithium (1.53 g, 221 mmol), naphthalene
After extraction with toluene (2 × 200 mL), the combined (0.064 g, 0.50 mmol) and anhydrous THF (40 mL) in a flame
organic layers were washed with water (5 × 200 mL), dried with dried flask under an argon atmosphere at room temperature.
anhydrous sodium sulfate, and the solvent evaporated (50 °C, After 22 h, the dark green solution was transferred into another
80 mbar). Recrystallization of the crude product from cyclo- flame dried flask flushed with dry argon via double ended
hexane yielded 1 as an off-white powder (68.5 g, 62%, mp: needle. This solution was directly used for the acylation step.
80–81 °C) which was used without further purification.
2. Bis(3-{[2-(allyloxy)ethoxy]methyl}-2,4,6-trimethyl-
1H NMR (CDCl3, 400 MHz): δ (ppm): 2.34, 2.38, and 2.43 (3 s, benzoyl)(phenyl)phosphine
3H each, CH3), 3.61–3.68 (m, 4H, O(CH2)2), 4.02 (dt, J = 2 (21.98 g, 74.1 mmol) was dissolved in anhydrous THF
5.3 Hz, 1.4 Hz, 2H, OCH2 allyl), 4.60 (s, 2H, OCH2 benzyl), (25 mL). This solution was added dropwise to the above
5.15–5.30, and 5.86–5.96 (2 m, 2H, 1H, HC=CH2), 6.90 (s, 1H, prepared phenylphosphine dilithium solution while the tempera-
=CH aromat), 11.0 (br. s, 1H, OH). 13C NMR (CDCl3, ture was kept between 30–35 °C in the dark, and subsequently
100 MHz): δ (ppm): 16.7, 19.7 and 19.8 (CH3), 67.1, 69.4, 69.7, stirred at ambient temperature. After 4 h, THF was removed at
and 72.2 (OCH2), 117.1 (=CH2), 129.9 (=CH allyl), 132.0, 40 °C under reduced pressure and the residue was used in the
132.1, 134.5, 135.0, and 139.6 (=C aromat), 134.6 (=CH subsequent oxidation step.
aromat), 175.4 (C=O). IR (diamond ATR): ν = 3100 (vbr, OH),
3010 (w, =CH), 2927 and 2878 (m, CH2, CH3), 1712 (vs, 3. Bis(3-{[2-(allyloxy)ethoxy]methyl}-2,4,6-trimethyl-
C=O), 1660 (w, C=C allyl), 1604 (w, C=C aromat), 1439 (m, benzoyl)(phenyl)phosphine oxide
CH2, CH3), 1352 (m, CH3), 1040 (vs, COC), 995 and 923 cm−1 For this stage all manipulations were carried out in brown glass
(s, =CH allyl). Anal. calcd. for C16H22O4: C, 69.04; H, 7.97. apparatus or under yellow light. The residue from the latter step
Found: C, 69.17; H, 7.95.
was dissolved in toluene (40 mL). Hydrogen peroxide solution
(30%, 4.17 g, 36.8 mmol) was added dropwise under vigorous
stirring while the temperature was kept at about 70 °C. After the
Synthesis of 2
1 (52.3 g, 0.188 mol) was suspended in a mixture of anhydrous solution was stirred at room temperature for 30 min, it was
toluene (300 mL) and N,N-dimethylformamide (1.4 mL). diluted with ethyl acetate (30 mL). The two layers were separ-
Thionyl chloride (33.5 g, 0.282 mol) was added at room tem- ated and the organic layer washed with 0.5 N sodium hydroxide
perature. After 2 h, toluene was distilled off and a stream of solution (5 × 20 mL) and once with brine (20 mL). After drying
nitrogen was bubbled through the crude product for 4 h before it with anhydrous sodium sulfate, the solvent was evaporated and
was purified by vacuum distillation (130 °C, 0.05 mbar) to give the crude product purified by column chromatography (silica
2 as a colorless liquid (42.6 g, 76%).
gel 60, n-heptane:ethyl acetate = 2:1 → 1:2) to give WBAPO as
a yellow oil (15.9 g, 67%).
1H NMR (CDCl3, 400 MHz): δ (ppm): 2.33, 2.39, and 2.43 (3 s,
3H each, CH3), 3.61–3.68 [m, 4H, O(CH2)2], 4.02 (d, J = 1H NMR (400 MHz, CDCl3): δ (ppm): 2.10, 2.19, and 2.34 (3 s,
5.3 Hz, 2H, OCH2, allyl), 4.57 (s, 2H, OCH2 benzyl), 6H each, CH3), 3.51–3.56 [m, 8H, O(CH2)2], 3.98 (dt, J =
5.16–5.29, and 5.86–5.96 (2 m, 2H, 1H, HC=CH2), 6.91 (s, 1H, 5.6 Hz, 1.4 Hz, 4H, OCH2 allyl), 4.49 (s, 4H, OCH2 benzyl),
=CH aromat). 13C NMR (CDCl3, 100 MHz): δ (ppm): 16.5, 5.13–5.27, and 5.83–5.93 (2 m, 4H, 2H, HC=CH2), 6.84 (s, 2H,
19.1, and 19.8 (CH3), 66.8, 69.67, 69.69, and 72.2 (OCH2), =CH aromat), 7.37–7.42, 7.49–7.54 and 7.84–7.88 (3 m, 2H,
117.1 (=CH2), 130.1 (=CH allyl), 132.2, 132.6, 132.7, 138.1 1H, 2H, =CH phenyl). 13C NMR (100 MHz, CDCl3): δ (ppm):
and 140.5 (=C aromat), 134.7 (=CH aromat), 171.0 (C=O). 17.1, 19.4 and 19.9 (CH3), 66.5, 69.2, 69.6, and 72.2 (OCH2),
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Beilstein Journal of Organic Chemistry 2010, 6, No. 26.
117.0 (=CH2), 125.8 (d, JC-P = 74 Hz, =CP), 128.5 (d, 2JC-P = C=C allyl), 1603 (m, C=C aromat), 1447 (s, CH2, CH3), 1379
11 Hz, =CH), 130.7 (=CH allyl), 132.1 (d, 3JC-P = 8 Hz, =CH), (m, CH3), 1044 (vs, COC), 992, and 924 cm−1 (s, =CH allyl).
132.6, 134.8, 135.5 and 141.2 (=C aromat), 132.9 (d, 4JC-P = Anal. calcd. for C16H21ClO3: C, 64.75; H, 7.13; Cl, 11.95.
3 Hz, =CH), 134.7 (=CH aromat), 136.7 (d, 2JC-P = 41 Hz, Found: C, 64.83; H, 7.12; Cl, 11.86.
=C-C=O), 216.2 (d, JC-P = 58 Hz, C=O). 31P NMR (162 MHz,
CDCl3): δ (ppm): 6.58. IR (diamond ATR): ν = 3070 (w, =CH), Measurements
2930 and 2860 (m, CH2, CH3), 1735 (m, C=O), 1679 (m, C=C NMR spectroscopic measurements were recorded on a DPX-
allyl), 1656 and 1596 (m, C=C aromat), 1437 (m, CH2, CH3), 400 spectrometer (Bruker Biospin, 1H: 400 MHz, 13C:
1373 (m, CH3), 1204 (s, P=O), 1096 (vs, COC), 995 and 925 100 MHz, 31P: 162 MHz) in CDCl3 or dimethyl sulfoxide-d6 as
cm−1 (s, =CH allyl). Anal. calcd. for C38H47O7P: C, 70.57; H, the solvent using tetramethylsilane (TMS) as standard. A FT-IR
7.32; O, 17.32. Found: C, 70.23; H, 7.28; O, 18.06.
spectrometer 1600 (Perkin-Elmer) was used to record IR
spectra. Melting points were measured with a Melting Point
B-540 (Büchi). Elemental analyses were performed with an
Synthesis of 3
Compound 3 was prepared from 3-(chloromethyl)-2,4,6- elemental analyzer CHNS-O Typ EA 1108 (Fisons
trimethylbenzoic acid (21.27 g, 0.10 mol) and thionyl chloride Instruments). UV–vis spectra were recorded with a UV–vis
in the same way as described for 2. After vacuum distillation spectrometer Lambda 2 (Perkin Elmer) in acetonitrile.
(85–86 °C, 0.1 mbar), 3 was obtained as colorless crystals
(14.0 g, 61%, mp: 201–202 °C, decomp.).
The photopolymerization of a dimethacrylate resin (Bis-GMA:
42 wt %, UDMA: 37 wt % and TEGDMA 21 wt %) was
1H NMR (400 MHz, CDCl3): δ (ppm): 2.36, 2.42 and 2.46 (3 s, studied by photo-DSC (Perkin Elmer DSC 7) using a blue LED
3H each, CH3), 4.62 (s, 2H, ClCH2), 6.95 (s, 1H, =CH). Bluephase (1200 mW·cm−2, Ivoclar Vivadent AG) and an ir-
13C NMR (100 MHz, CDCl3): δ (ppm): 16.2, 19.2 and 19.4 radiation time of 180 s at 37 °C.
(CH3), 40.1 (ClCH2), 130.4 (=CH), 132.1, 132.4, 133.0, 138.2
and 140.0 (=C), 170.3 (C=O). IR (diamond ATR): ν = 3070 (w, For the measurement of the shear bond strength of the corres-
=CH), 2980, 2910 and 2870 (w, CH2, CH3), 1791 (vs, C=O), ponding dentin adhesives, freshly extracted bovine lower
1596 and 1566 (m, C=C), 1423 (m, CH2, CH3), 1381 (s, CH3), incisors were embedded in unsaturated polyester resin
772 cm–1 (vs, C-Cl). Anal. calcd. for C11H12Cl2O: C, 57.17; H, (Castolite, Buehler, USA) in cylindrical molds. Flat dentinal
5.23; Cl, 30.68. Found: C, 57.56; H, 5.46; Cl, 29.82.
surfaces were ground with water-cooled P240-grit SiC followed
by P1000-grit SiC abrasive paper to expose the middle dentin of
the embedded teeth. After application of the SEA, solvent evap-
Synthesis of 4
To a solution of 2-(allyloxy)ethanol (5.11 g, 50.0 mmol) and oration, and light activation, a Teflon mould with a cylindrical
triethylamine (5.06 g, 50.0 mmol) in anhydrous methylene hole (3.00 mm in diameter and 4 mm in height, Guillotine
chloride (30 mL), 3 (11.56 g, 50.0 mmol) in anhydrous method [19]) was placed on the top of bonded surface and filled
methylene chloride (20 mL) was added at 0 °C. After 2 h the with two increments of the hybrid composite Tetric Ceram
white precipitate was filtered off and the filtrate was washed (Ivoclar Vivadent AG). The increments were light cured using a
with water (3 × 50 mL), dried with anhydrous sodium sulfate halogen lamp Astralis 10 (emission spectrum: 380–510 nm,
and the solvent removed under vacuum to give 4 as a colorless 1000 mW·cm−2, Ivoclar Vivadent AG) for 40 s each and the test
liquid (11.3 g, 76%). Compound 4 was also obtained by heating specimens were immersed in water at 37 °C for 24 h prior to
2 at 180 °C for 4 h and was the main component of its distilla- testing. Then the shear bond strength was measured using a
tion residue.
universal testing machine (Zwick Z010, Germany) at a cross-
head speed of 1.0 mm/min.
1H NMR (400 MHz, CDCl3): δ (ppm): 2.27, 2.36 and 2.38 (3 s,
3H each, CH3), 3.72–3.75 (m, 2H, OCH2 ether), 4.01–4.03 (m,
2H, OCH2 allyl), 4.48–4.50 (m, 2H, OCH2 ester), 4.62 (s, 2H,
ClCH2), 5.16–5.30, and 5.84–5.94 (2 m, 2H, 1H, HC=CH2),
6.90 (s, 1H, =CH aromat). 13C NMR (100 MHz, CDCl3):
δ (ppm): 16.2, 19.3 and 19.5 (CH3), 40.7 (ClCH2), 63.9, 67.8,
and 72.02 (OCH2), 117.3 (=CH2), 130.0 (=CH allyl), 131.9,
133.2, 134.3, 135.2, and 138.7 (=C aromat), 134.4 (=CH
aromat), 170.0 (C=O). IR (diamond ATR): ν = 3090 (w, =CH),
2949, 2919 and 2862 (m, CH2, CH3), 1722 (vs, C=O), 1647 (w,
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