Design, synthesis, and characterization of 6-[(trimethyl)silylethynyl]naphthalene-2-ethene: A new precursor for the preparation of high-refractive-index organic materials

Article abstract of DOI:10.1515/znb-2019-0199

A new polymerizable naphthalene derivative has been designed, prepared, and characterized by ¹H, ¹³C NMR, and MS. The new monomer synthesis has successfully been accomplished from a cheap commercially available raw material, in only four steps with good yields. The four steps can be easily scaled up for manufacturing purposes. It is anticipated that the new precursor can be very useful in the preparation of valuable materials with high refractive index for numerous opto-electronic applications.

Full text of DOI:10.1515/znb-2019-0199

Z. Naturforsch. 2020; 75(4)b: 359–363
Sacha Legrand*, Milja Hannu-Kuure and Ari Kärkkäinen
Design, synthesis, and characterization of 6-[(trimethyl)
silylethynyl]naphthalene-2-ethene: a new precursor for
the preparation of high-refractive-index organic materials
https://doi.org/10.1515/znb-2019-0199
materials as microelectronic devices due to their potential
pollution of the environment [16]. Polysiloxane polymers
also exhibit excellent opto-electronic applications [17].
These hybrid organic–inorganic materials can be easily
prepared using the sol-gel process.
Various polymeric materials having a high refrac-
tive index, such as thermoplastics for camera, pick-up,
projector lenses [18] and thermoset resins for eyeglasses
applications [19, 20], are now commercially available.
Nevertheless, their manufacturing procedures can be
challenging and very laborious. As a result, there is still
a high need of developing effective new materials, which
can be easily produced with excellent yields and scaled up
Received November 1, 2019; accepted February 1, 2020
Abstract: A new polymerizable naphthalene derivative
has been designed, prepared, and characterized by H,
1
13C NMR, and MS. The new monomer synthesis has suc-
cessfully been accomplished from a cheap commercially
available raw material, in only four steps with good yields.
The four steps can be easily scaled up for manufacturing
purposes. It is anticipated that the new precursor can
be very useful in the preparation of valuable materials
with high refractive index for numerous opto-electronic
applications.
Keywords: high refractive index; monomer; Sonogashira; without major difficulties for commercial purposes.
synthesis; Wittig.
Herein, we report the design, synthesis, and char-
1
acterization using H, 13C NMR, GC, and MS of a new
polymerizable monomer, which can be employed in the
preparation of very valuable materials having high refrac-
tive indices. The target molecule can be synthesized from
a cheap commercially available raw material, in only four
steps with high yields.
Badur et al. demonstrated the possibility to predict
the final refractivity of a polymer based on the refractive
indices of its components [21]. Indeed, high-n polymers
can be designed according to the Lorentz–Lorenz equa-
tion (Eq. (1))
1 Introduction
The needs for materials with high-refractive-index (high-
n) values are becoming more and more important. Such
polymers, with high optical transparency, are currently
required in a wide range of applications, including anti-
reflecting coatings [1], display and opto-electronic devices
[2], complementary metal oxide semiconductor image
sensors, and micro lens components for charge coupled
devices [3], to mention but a few.
Recently, Higashihara and Ueda reviewed macro-
molecules with high refractivities, together with their
application studies [4]. For example, sulfur-containing
polyimides [5], poly(arylene sulfide)s [6], polyphenylqui-
nolaxines [7], TiO2 inverse opal photonic crystals [8], poly-
mer–inorganic hybrid materials [9–12], polyimide hybrids
[13], and nanocomposites have been shown to exhibit
high-n values with good optoelectronic performances [14].
Halogen-containing high-refractive-index polymers can
also be found in the literature [15]. However, European
Union has limited the use of such halogen-containing
2[R]
1 +
V0
n =
(1)
[R]
1 −
V0
where n is the refractive index of the material, [R] the
molar refraction, and V0 the molecular volume of the
polymer repeating unit [22]. Therefore, the refractive
index value of a polymer should be high when its sub-
stituents have high molar refractivities and low molar
volumes, such as aromatic rings, halogen atoms (except
fluorine), and C=C or C≡C bonds [23]. Accordingly, a
polymer repeating unit which consists of an aromatic
ring substituted with both C=C and C≡C bonds should
have such property. A suitable example of a precursor
containing one naphthalene ring and both C=C and C≡C
bonds is depicted in Fig. 1.
*Corresponding author: Sacha Legrand, Optitune Oy, Kaitoväylä 1,
FIN-90590 Oulu, Finland, Fax: +358 (0)8 556 2613,
Phone: +358 (0)405620050, E-mail: sacha.legrand@optitune.com
Milja Hannu-Kuure and Ari Kärkkäinen: Optitune Oy, Kaitoväylä 1,
FIN-90590 Oulu, Finland

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360ꢀ ꢀS. Legrand et al.: Design, synthesis, and characterization of 6-[(trimethyl)silylethynyl]naphthalene-2-ethene
yield [31]. Selective oxidation of this primary alcohol in the
presence of MnO2 gave the aldehyde 3 in excellent yield. A
Wittig reaction between this aldehyde and Ph3PCH3Br in
the presence of the base KOtBu yielded the vinyl deriva-
tive 4 [32]. Finally, the Sonogashira coupling was used
to introduce the C≡C moiety [33]. The treatment of the
bromo derivative 4 with Me3SiC≡CH in the presence of the
palladium catalyst PdCl2, the co-catalyst CuI, the ligand
PPh3, and the base triethylamine (TEA) gave the desired
Fig. 1:ꢀExample of a polymer repeating unit with substituents
having high molar refractivities and low molar volumes.
Moreover, polymerization of such monomers can target molecule 5 in quantitative yield. The purity of the
easily occur via radical polymerization, since the C=C monomer 5 was found to be >98% according to GC meas-
moiety can react together with an azo-based radical initia- urements. The main impurity was determined by GC-MS
tor such as 2,2′-azobis(2-methylpropionitrile), 1,1′-azobis and was found to be Me3Si–C≡C–SiMe3. This impurity
(cyclohexanecarbonitrile), 2,2′-azobis(2-methylpropane), was most probably formed by the intermolecular reaction
or 4,4′-azobis(4-cyanovaleric acid), for example. This type of the radical Me3Si–C≡C˙ with itself, which was created
of radical polymerization process has been shown to be during the Pd cycle of the Sonogashira coupling mecha-
very effective in the preparation of macromolecules with nism. The general synthetic pathway of 6-[(trimethyl)sily-
high commercial values, such as water-soluble polymers lethynyl]naphthalene-2-ethene (5) is shown in Scheme 1.
for the paper industry [24–27], polysiloxanes for opto-
Importantly, all the steps presented in Scheme 1, the
electronic applications [28, 29], and promising chiral reduction and the oxidation steps, the Wittig reaction,
stationary phases for the pharmaceutical industry [30]. and the Sonogashira coupling, can be easily scaled up for
However, despite the relative simplicity of the general production purposes. The alkyne group present in 5 was
chemical structure depicted in Fig. 1, the preparation of not deprotected because of our envisaged applications.
such precursors and their applications have been poorly The largest possible number of silicon atoms should be
reported in the literature. Therefore, there is an urgent present in our polymer backbone made partly from 5 for
need to develop such monomers for valuable applications our opto-electronic purposes. In addition, our composi-
in organic, polymer, or material chemistry.
tion comprises a siloxane prepolymer with a backbone
exhibiting chemical groups, which are capable of being
deprotonated in an aqueous base solution. Consequently,
it is necessary to keep the alkyne group protected with
Si(CH3)3 and to avoid its deprotection during the polymeri-
2 Results and discussion
Our synthesis started with the commercially available ester zation process, which would result in a loss of one silicon
methyl 6-bromo-2-naphthoate (1), which was treated with atom.
diisobutylaluminum (DIBAL) at low temperature (−78°C),
giving the corresponding primary alcohol 2 in quantitative substituted with one or several ethynyl moieties protected
It is important to mention that naphthalene derivatives
O
O
(i)
(ii)
OH
O
H
Br
98%
90%
Br
Br
3
1
2
(iii)
71%
(iv)
Br
Si
99%
4
5
Scheme 1:ꢀReactions condition: (i) DIBAL, THF, −78°Cꢁ→ꢁRT; (ii) MnO2, THF, RT; (iii) Ph3PCH3Br, KOtBu, THF, −20°Cꢁ→ꢁRT; (iv) PdCl2, CuI, PPh3,
Me3SiC≡CH, TEA, RTꢁ→ꢁ50°C.

ꢁꢀꢀꢀ
S. Legrand et al.: Design, synthesis, and characterization of 6-[(trimethyl)silylethynyl]naphthalene-2-etheneꢂ
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Si
Br
(i)
(ii)
Br
91%
71%
Si
6
7
8
Scheme 2:ꢀReactions condition: (i) PdCl2, CuI, PPh3, Me3SiC≡CH, TEA, RTꢁ→ꢁ50°C. (ii) TBAF, THF, RT.
1
13
with the Si(CH3)3 group can be easily deprotected anyway. A spectrometer (200 MHz for H and 50 MHz for C). CDCl3
typical example is shown in Scheme 2. The dibromo deriva- was used as a solvent, and the signal of the solvent served
tive 6 was first treated with Me3SiC≡CH in the presence of as the internal standard. Chemical shifts were expressed
the palladium catalyst PdCl2, the co-catalyst CuI, the ligand in ppm, followed by their multiplicity (s, singlet; t, triplet;
PPh3, and the base TEA, yielding the di(alkyne)-containing d, doublet; m, multiplet) and number of protons. The mass
group 7 in excellent yield. Deprotection of the ethynyl moie- spectra of positive ions obtained by the electron impact
ties in 7 was successfully accomplished after treatment with (EI, 70 eV) were measured on the Varian Saturn ws GC-MS
tetrabutylammonium fluoride (TBAF) at room temperature, instrument. GC measurements were performed using
yielding 2,6-diethynylnaphthalene (8) in good yield.
Agilent 6890N apparatus equipped with a TCD detector.
We are now aiming to use the new monomer 5 with The reactions were performed in an inert atmosphere (N2).
other suitable precursors for the preparation of new valu- TEA was dried over CaH2, and THF was dried over Na/
able polysiloxanes. By using the sol-gel technology and benzophenone before use. The other starting materials
a radical polymerization process, new hybrid organic– employed were purchased from commercial suppliers and
inorganic polymers will be prepared in suitable organic were used without further purification.
solvents. After their deposition (using, for example, spin-
coating techniques) on specific substrates such as silicon
4.1 6-Bromonaphthalene-2-methanol (2)
wafers, the resulting films will then be characterized by
an ellipsometer in order to determine both the thickness
Methyl 6-bromo-2-naphthoate (1) (50 g, 188.6 mmol) was
and the refractive properties of the final material [34].
dissolved in THF (350 mL). DIBAL (1.0 m in hexane, 452 mL,
452 mmol) was added dropwise to the reaction mixture at
Tꢀ=ꢀ−78°C. The reaction mixture was stirred at Tꢀ=ꢀ−78°C for
3 Conclusion
1 h and then at room temperature for additional 1 h. The
reaction mixture was then cooled to Tꢀ=ꢀ−30°C, and MeOH
We report the design, synthesis, and characterization, (60 mL) was added dropwise. Potassium sodium tartrate
including H and 13C NMR, together with MS, of a new salt (concentrated aqueous solution, 470 mL) was added in
1
polymerizable precursor. By means of GC measure- portions, and the reaction mixture was stirred at room tem-
ments, we also prove the very high purity of the isolated perature for 30 min. Then the phases were separated, and
monomer. We anticipate the high value of this new pre- the aqueous phase was extracted with EtOAc (2ꢀ×ꢀ100 mL).
cursor for the preparation of polymers with high refrac- The combined organic layers were washed with brine,
tivities. Importantly, the preparation of this new monomer dried over Na2SO4, and filtrated. The solvents were evapo-
can be achieved in good yield and can be easily scaled up rated to give 6-bromonaphthalene-2-methanol (2) as a pale-
for manufacturing purposes. Our current work consists yellow solid (44 g, 98%), which was used without further
1
of the preparation, characterization, and applications purification in the next step. – H NMR (200 MHz, CDCl3,
studies of macromolecules based on the new monomer 20°C): δꢀ=ꢀ7.91 (s, 1H, Har), 7.62–7.25 (m, 5H, Har), 4.90 (s, 2H,
presented in this study.
CH2O). The spectroscopic data found were in accordance
with those published by Gingras and Collet [31].
4 Experimental section
4.2 6-Bromonaphthalene-2-carbaldehyde (3)
Flash chromatography was performed on silica gel (Merck The primary alcohol 2 (2.50 g, 10.5 mmol) was dissolved
60). NMR spectra were recorded on the Bruker DMX in THF (50 mL). MnO2 (4.58 g, 527 mmol) was added in

ꢀꢀꢀꢁ
362ꢀ ꢀS. Legrand et al.: Design, synthesis, and characterization of 6-[(trimethyl)silylethynyl]naphthalene-2-ethene
small portions at room temperature, and the reaction the target molecule 5 (11 g, 99%) as a slightly orange solid.
1
mixture was then stirred at this temperature for 24 h in a – H NMR (200 MHz, CDCl3, 20°C): δꢀ=ꢀ8.08 (s, 1H, Har),
dark environment. The reaction mixture was then filtered, 7.83–7.77 (m, 4H, Har), 7.66–7.65 (m, 1H, Har), 7.03–6.89 (dd,
and the solvent was evaporated to give the corresponding 1H, CHgem=CH2), 6.03–5.94 (d, 1H, CH=CHHcis), 5.50–5.44
aldehyde 3 (2.20 g, 90%) as a slightly brown solid, which (d, 1H, CH=CHtransH), 0.42 (s, 9H, (CH3)3Si). – 13C NMR
was used without purification in the next step. – 1H NMR (50 MHz, CDCl3 20°C): δꢀ=ꢀ136.91, 136.17, 133.29, 132.76,
(200 MHz, CDCl3, 20°C): δꢀ=ꢀ10.15 (s, 1H, CHO), 8.31 (s, 1H, 132.65, 129.04, 128.22, 128.20, 126.38, 124.04 (10ꢀ×ꢀCar),
Har), 8.07 (s, 1H, Har), 8.01–7.96 (app. d, 1H, Har), 7.89–7.82 120.53 (C=CH2), 115.00 (C=CH2), 105.63 (C≡C–Si), 95.00
(dt, 1H, Har), 7.69–7.64 (dd, 2H, Har). The spectroscopic (C≡C–Si), 0.00 ((CH3)3Si). – MS (EI, 70 eV): m/z (%)ꢀ=ꢀ250
data found were in accordance with those published by (100), 235, 220, 205, 177, 152, 128, 102, 87, 75, 51.
Granzhan and Teulade-Fichou [35].
4.5 2,6-Bis[(trimethyl)silylethynyl]
4.3 6-Bromonaphthalene-2-ethene (4)
naphthalene (7)
Ph3PCH3Br (71 g, 199 mmol) was mixed with THF (100 mL). 2,6-Dibromonaphthalene (6) (3 g, 10.5 mmol) was mixed
KOtBu (20.62 g, 184 mmol) was added in small portions together with PdCl2 (81 mg, 0.45 mmol), CuI (204 mg,
and the reaction mixture was stirred at room tempera- 1.05 mmol), and PPh3 (510 mg, 1.9 mmol). Me3SiC≡CH
ture for 15 min. The reaction mixture was then cooled to (4.36 mL, 31.4 mmol) was added at room temperature,
Tꢀ=ꢀ−20°C, and the aldehyde 3 (36 g, 153 mmol) dissolved followed by TEA (30 mL, 214 mmol). Then, the reaction
in THF (270 mL) was added dropwise. Then, the reac- mixture was stirred at Tꢀ=ꢀ50°C overnight. After cooling to
tion mixture was allowed to reach room temperature room temperature, the solution was filtrated, and the sol-
and was stirred overnight at this temperature. The reac- vents were evaporated to give an orange solid. The crude
tion was then quenched by the addition of deionized product was purified by flash chromatography on silica
water (200 mL). The phases were then separated, and the gel, using EtOAc–n-hexane (1:4, v/v) as an eluent to give
aqueous phase was extracted with EtOAc (2ꢀ×ꢀ150 mL). The the compound 7 (3.05 g, 91%) as a slightly orange solid.
1
combined organic phases were dried over MgSO4, filtrated, – H NMR (200 MHz, CDCl3, 20°C): δꢀ=ꢀ7.95 (s, 2H, Har),
and evaporated. The crude product was then purified by 7.73–7.68 (app. d, 2H, Har), 7.52–7.48 (app. d, 2H, Har), 0.29
flash chromatography on silica gel, using EtOAc–n-hex- (s, 18H, 2ꢀ×ꢀ(CH3)3Si). The spectroscopic data found were
ane (1:4, v/v) as an eluent, to give the ethene-containing in accordance with those published by Yamaji et al. [37].
compound 4 (25.35 g, 71%) as a slightly yellow solid. – 1H
NMR (200 MHz, CDCl3, 20°C): δꢀ=ꢀ8.10 (s, 1H, Har), 7.76–7.73
(m, 4H, Har), 7.66–7.65 (app. d, 1H, Har), 7.03–6.89 (dd, 1H, 4.6 2,6-Diethynylnaphthalene (8)
CHgem=CH2), 6.03–5.94 (d, 1H, CH=CHHcis), 5.50–5.44 (d,
1H, CH=CHtransH). The spectroscopic data found were in The starting material 7 (3 g, 9.37 mmol) was dissolved
accordance with those published by Fakhouri et al. [36].
in THF (30 mL). TBAF (1.0 m in THF, 28.1 mL) was added
dropwise to the reaction mixture at room temperature.
The reaction was slightly exothermic, and the color of
the reaction mixture changed from deep brown to very
dark green. The reaction mixture was stirred at room tem-
perature for 2 h. The solvent was evaporated to give very
4.4 6-[(Trimethyl)silylethynyl]naphthalene-
2-ethene (5)
The bromo derivative 4 (10 g, 43 mmol) was mixed viscous dark oil, which solidified under high vacuum.
together with PdCl2 (187.3 mg, 1.05 mmol), CuI (408.6 mg, The crude product was purified by flash chromatography
2.1 mmol), and PPh3 (996 mg, 3.8 mmol). Me3SiC≡CH using CH2Cl2–n-hexane (1:1, v/v) as an eluent, yielding the
(12 mL, 986 mmol) was added at room temperature, fol- deprotected ethynyl-containing naphthalene 8 (1.17 g,
lowed by TEA (90 mL, 645 mmol). Then, the reaction 71%) as a slightly orange solid. – 1H NMR (200 MHz, CDCl3,
mixture was stirred at Tꢀ=ꢀ50°C overnight. After cooling 20°C): δꢀ=ꢀ8.00 (s, 2H, Har), 7.80–7.70 (app. d, 2H, Har), 7.60–
to room temperature, the solution was filtrated, and the 7.52 (app. d, 2H, Har), 3.20 (s, 2H, C≡CH). – MS (EI, 70 eV):
solvents were evaporated to give a brown solid. The crude m/z (%)ꢀ=ꢀ176, 163, 145 (100), 118, 103, 88, 73, 58, 43. The
product was purified by flash chromatography on silica spectroscopic data found were in accordance with those
gel, using EtOAc–n-hexane (1:4, v/v) as an eluent to give published by Kim et al. [38].

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S. Legrand et al.: Design, synthesis, and characterization of 6-[(trimethyl)silylethynyl]naphthalene-2-etheneꢂ
ꢂ363
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Acknowledgments: We thank Päivi Joensuu (University of
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