Synthesis and characterization of aryltriphenylsilyl ethers and crystal structure of 2,4,6-tri-methylphenyl triphenylsilyl ether

Article abstract of DOI:10.1080/00958972.2013.783700

Synthesis and structures of aryl triphenylsilyl ether derivatives are reported. Reactions of chlorotriphenylsilane, (C₆H₅) ₃SiCl (1), with sodium salts of sterically hindered phenol derivatives (2a-2i) were investigated. A

Full text of DOI:10.1080/00958972.2013.783700

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Synthesis and characterization of
aryltriphenylsilyl ethers and crystal
structure of 2,4,6-tri-methylphenyl
triphenylsilyl ether
a
Saliha Begeç a , Adem Kiliç b , Fatma Yuksel b , Sümeyya Alataş
&
Nilay Kan a
a Faculty of Science and Arts, Department of Chemistry , Inönü
University , Malatya , Turkey
b Department of Chemistry , Gebze Institute of Technology ,
Gebze , Turkey
Accepted author version posted online: 12 Mar 2013.Published
online: 15 Apr 2013.
To cite this article: Saliha Begeç , Adem Kiliç , Fatma Yuksel , Sümeyya Alataş & Nilay Kan
(2013) Synthesis and characterization of aryltriphenylsilyl ethers and crystal structure of 2,4,6-
tri-methylphenyl triphenylsilyl ether, Journal of Coordination Chemistry, 66:8, 1459-1466, DOI:
10.1080/00958972.2013.783700
To link to this article: http://dx.doi.org/10.1080/00958972.2013.783700
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Journal of Coordination Chemistry, 2013
Vol. 66, No. 8, 1459–1466, http://dx.doi.org/10.1080/00958972.2013.783700
Synthesis and characterization of aryltriphenylsilyl ethers
and crystal structure of 2,4,6-tri-methylphenyl triphenylsilyl
ether
SALIHA BEGEÇ*†, ADEM KILILJ, FATMA YUKSEL‡, SÜMEYYA ALATAކ and
NILAY KAN†
†Faculty of Science and Arts, Department of Chemistry, Inönü University, Malatya,
Turkey
‡Department of Chemistry, Gebze Institute of Technology, Gebze, Turkey
(Received 26 November 2012; in final form 4 January 2013)
Synthesis and structures of aryl triphenylsilyl ether derivatives are reported. Reactions of chlorotri-
phenylsilane, (C6H5)3SiCl (1), with sodium salts of sterically hindered phenol derivatives (2a–2i)
were investigated. Aryltriphenylsilyl ethers (C6H5)3SiOAr (3–8) were obtained from these reactions.
Structures of 3–8 were defined by IR, 1H, 13C, 29Si NMR spectroscopy, elemental analysis, and
ESI-MS spectra. The molecular structure of 2,4,6-tri-methylphenyl triphenylsilyl ethe (6) were
determined by single-crystal X-ray diffraction.
Keywords: Aryltriphenylsilyl ether; Chlorotriphenylsilane; 29Si NMR; Phenol
1. Introduction
Organosilicon chemistry is a rapidly developing field of science [1]. Organosilicon
reagents have a range of applications in organic synthesis [2]. Silicon–oxygen bond-form-
ing reactions are one of the most important reactions leading to useful siloxane polymers
[3]. Monomeric organosilicon compounds are important in a range of applications apart
from silicone chemistry, which is the largest consumer of silanes [4]. Polymers containing
silicon–oxygen bonds in the main chain are important [5, 6] and silyl ethers are among the
most widely used protecting groups for hydroxyl functionality in organic synthesis [7–9].
Silicon–oxygen compounds also find applications in stereoselective transformations of
organic compounds [10]. They also play an important role in inorganic synthesis as precur-
sors in the preparation of sol–gels and other condensed siloxane materials. Therefore,
compounds containing a Si–O bond are important.
Generally, three approaches are used for the synthesis of silyl ethers. Silyl ethers are
most commonly prepared from chlorosilanes (R4ÀxSiClx) by reaction with alcohols or
alkoxides under acidic or basic reaction conditions [11]. The second method involves
*Corresponding author. Email: saliha.begec@inonu.edu.tr
Ó 2013 Taylor & Francis

1460
S. Begeç et al.
cleavage of Si–N or Si–S bonds with alcohols or water and requires substrates that
undergo such alcoholysis reactions. The third method, dehydrogenative coupling reaction
between Si–H and O–H functional groups, is user friendly and proceeds under mild
reaction conditions being catalyzed by various metal complexes [12].
Sterically hindered phenols are biologically active compounds [13]. Especially, 2,6-di-
tert-butyl-4-methylphenol (butylated hydroxy toluene, BHT) derivatives are efficient
antioxidants [14]. Here we report the synthesis and characterizations of aryl triphenylsilyl
ethers via the reaction of chlorotriphenylsilane (1) with sterically hindered phenol
derivatives. These compounds could find potential applications as effective antioxidants.
2. Experimental
All synthetic steps were carried out under N2 or Ar in predried glassware using Schlenk
techniques [15] with previously dried solvents. Tetrahydrofuran (THF), used as solvent,
was distilled under argon from sodium benzophenone, prior to use. Starting materials were
commercially available (Aldrich, Merck, Alfa Aesar, and Fluka) and used without purifica-
tion. Reactions were monitored by using silica gel 60 F254 precoated Thin Layer Chroma-
tography plates (Merck, Kieselgel 60, 0.25 mm thickness) and separating conditions were
determined. The separation of products was carried out by column chromatography using
silica gel (Merck, Kieselgel 60, 230–400 mesh; for 3 g crude mixture, 100 g silica gel was
used in a column, 3 cm in diameter and 60 cm in length).
1
All isolated new compounds have been characterized by elemental analysis, H, 13C,
29Si NMR spectrometry, FT-IR, and mass spectrometry. Microanalysis was carried out on
a LECO 932 CHNS-O apparatus. Melting points were measured in open capillary tubes
with an Electrothermal-9200 melting point apparatus and were uncorrected. IR spectra
were recorded on an ATI Unicam Mattson 1000 FT-IR spectrophotometer in KBr disks
1
and were reported in cmÀ1. H, 13C, and 29Si NMR spectra were recorded in CDCl3 using
a Varian INOVA spectrometer operating at 500 MHz (1H), 125.6 MHz (13C), and
1
99.3 MHz (29Si). The H, 13C, and 29Si NMR chemical shifts were measured using SiMe4
(δ = 0) as an internal standard. Mass spectra were obtained by a Bruker MicROTOF LC/
MS spectrometer using electrospray ionization (ESI) method.
2.1. Synthesis and characterization of phenyl triphenylsilyl ether derivatives (3–8)
2.1.1. Synthesis of 3. Small pieces of metallic sodium (0.15 g, 6.52 mM) were added to
the solution of 2,4-di-methylphenol (2a) (0.48 g, 3.92 mM) in 25 mL of THF under Ar and
the solution was stirred until cessation of hydrogen gas evolution, an indication of comple-
tion of the reaction. Excess sodium was removed by filtration and the solution of sodium-
2,4-di-methylphenoxide was frozen with
a
liquid nitrogen–acetone mixture
(À95 °C 5 °C). To this solution, chlorotriphenylsilane (1 g, 3.40 mM) in 10 mL of THF
was slowly added and the resulting mixture was allowed to warm to ambient temperature
with continued stirring. The reaction mixture was vigorously stirred at room temperature
for 48 h, precipitated NaCl was filtered off, and solvent was removed under vacuum. The
residue was chromatographed on silica gel: 100 g (eluent : acetone/n-hexane 1 : 2). 2,4-di-
methylphenyl triphenylsilyl ether (3), as white crystals, was obtained in 11% yield, m.p.
= 86–88 °C (Rf = 0.879 dichloromethane/n-hexane 1 : 2). IR (KBr, cmÀ1): υ(CH aryl) = 3066,

Triphenyl silyl ethers
1461
υ(CH al) = 2916, υ(CC aryl) = 1428, υ(Si–C) = 1265, υ(Si–O–C) = 1118, 1027. NMR (CDCl3, δ,
1
ppm): H, 2.10 (s, 3H, 4–CH3), 2.15 (s, 3H, 2–CH3), 6.48–6.83 (m, 3H, Ar–O), 7.56–7.59
(m, 15H, Ar–H). 13C, 151.2, 135.8, 135.4, 134.0, 130.5, 130.2, 128.5, 127.9, 127.7,
126.9, 118.4, 20.5, 17.0. 29Si, –15.23 (s). ESI-MS m/z: 403.2 [M+Na]+. Found: C, 82.32;
H, 6.5. Calcd (%) for C26H24SiO (380.56): C, 82.10; H, 6.31.
2.1.2. Synthesis of 4. 3,4-di-methylphenol (2b) (0.48 g, 3.92 mM), Na (0.15 g, 6.52 mM),
and 1 (1 g, 3.40 mM) were used for the preparation of 4 as for 3. The residue was
chromatographed (silica gel: 100 g, eluent : dichloromethane/n-hexane 1 : 1) giving 3,4-di-
methylphenyl triphenylsilyl ether (4) in 19% yield, m.p. = 56–58 °C (Rf = 0.657 dichloro-
methane/n-hexane 1 : 3). IR (KBr, cmÀ1): υ(CH aryl) = 3066, υ(CH al) = 2920, υ(CC aryl) = 1427,
1
υ
(Si–C) = 1257, υ(Si–O–C) = 1116. NMR (CDCl3, δ, ppm): H, 2.26 (s, 3H, 4–CH3), 2.13 (s,
3H, 3–CH3), 6.61–6.89 (m, 3H, Ar–O), 7.38–7.50 (m, 15H, Ar–H). 13C, 152.9, 137.5,
135.7, 135.5, 135.2, 133.8, 130.2, 130.1, 129.8, 129.7, 127.9, 127.7, 116.8, 121.2, 19.8,
18.8. 29Si, –15.15 (s). ESI-MS m/z: Found: C, 82.24; H, 6.53. Calcd (%) for C26H24SiO
(380.56): C, 82.10; H, 6.31.
2.1.3. Synthesis of 5. 2,6-di-methylphenol (2c) (0.41 g, 3.36 mM), Na (0.15 g, 6.52 mM),
and 1 (1 g, 3.40 mM) were used for the preparation of 5 as for 3. The residue was
chromatographed (eluent : dichloromethane/n-hexane 1 : 2) and 2,6-di-methylphenyl triphe-
nylsilyl ether (5) was obtained in 11% yield, m.p. = 91–93 °C (Rf = 0.540 acetone/n-hexane
1 : 2) IR (KBr, cmÀ1): υ(CH aryl) = 3067, υ(CH al) = 2915, υ(CC aryl) = 1427, υ(Si–C) = 1268, υ(Si–
1
O–C) = 1116, 1028. NMR (CDCl3, δ, ppm): H, δ 2.21 (s, 6H, 2,6-di-CH3), 6.81–6.95 (m,
3H, Ar–O); 7.44–7.71 (m, 15H, Ar–H). 13C, 152.2, 135.5, 135.2, 134.1, 130.2, 129.8,
127.7, 121.8, 18.54. 29Si, –16.50 (s). ESI-MS m/z: 403.2 [M+Na]+. C, 82.10; H, 6.31.
Found: C, 82.08; H, 6.48. Calcd (%) for C26H24SiO (380.56): C, 82.10; H, 6.31.
2.1.4. Synthesis of 6. 2,4,6-tri-methylphenol (2d) (0.55 g, 4.04 mM), Na (0.18 g, 7.82 mM)
and 1 (1 g, 3.40 mM) were used for the preparation of 6 as for 3. The residue was chromato-
graphed (eluent : dicholoromethane/n-hexane 1 : 1) to give 2,4,6-tri-methylphenyl triphenyl-
silyl ether (6) in 49% yield, m.p. = 119–120 °C (Rf = 0.658 dichloromethane/n-hexane 1 : 2).
IR (KBr, cmÀ1): υ(CH aryl) = 3045, υ(CH al) = 2909, υ(CC aryl) = 1426, υ(Si–C) = 1234, υ(Si–O–
1
C) = 1116. NMR (CDCl3, δ, ppm): H, δ 1.92 (s, 6H, 2,6-di-CH3), 2.23 (s, 3H, 4-CH3), 6.72
(s, 2H, ArO–H), 7.28–7.67 (m, 15H, Ar–H), 13C, 149.9, 135.5, 135.3, 134.3, 130.8, 129.2,
128.4, 127.6, 20.5, 18.4. 29Si, –16.81 (s). ESI-MS m/z: 395 [M+]. Found: C, 82.18; H, 6.61
%. Calcd (%) for C27H27SiO (395.59): C, 82.15; H, 6.59.
2.1.5. Synthesis of 7. 4-tert-butyl-2-methylphenol (2f) (0.56 g, 3.41 mM), Na (0.16 g,
6.95 mM), and 1 (1 g, 3.40 mM) were used for the preparation of 7 as for 3. The residue
was chromatographed (eluent : dichloromethane/n-hexane 1 : 1) giving 4-tert-butyl-2-methyl-
phenyl triphenylsilyl ether (7) in 35% yield, m.p. = 127–128 °C (Rf = 0.682 dichlorometh-
ane/n-hexane 1 : 2). IR (KBr, cmÀ1): υ(CH aryl) = 3067, υ(CH al) = 2960, υ(CC aryl) = 1428, υ(Si–
1
C) = 1255, υ(Si–O–C) = 1116. NMR (CDCl3, δ, ppm): H, δ 1.14 (s, 9H, 4-But), 2.20 (s, 3H,
2-CH3), 6.71–6.79 (m, 3H, Ar–O) 7.27–7.59 (m, 15H, Ar–H). 13C, 151.1, 143.9, 135.9,

1462
S. Begeç et al.
135.7, 135.5, 135.2, 135.0, 130.2, 127.9, 127.8, 127.7, 123.2, 117.9, 34.0, 31.5, 17.4. 29Si,
–15.62 (s). ESI-MS m/z: 445.2 [M+Na]+. Found: C, 82.52; H, 7.27. Calcd (%) for
C29H30SiO (422.64): C, 82.34; H, 7.09.
2.1.6. Synthesis of 8. 2-tert-butyl-4-methylphenol (2g) (0.63 g, 3.83 mM), Na (0.17 g,
7.39 mM), and 1 (1 g, 3.40 mM) were used for the preparation of 8 as for 3. The residue
was chromatographed (eluent : dichloromethane/n-hexane 1 : 1) and 2-tert-butyl-4-meth-
ylbutylphenyl triphenylsilyl ether (8) was obtained in 32% yield, m.p. = 114–116 °C
(Rf = 0.676 dichloromethane/n-hexane 1 : 2). IR (KBr, cmÀ1): υ(CH aryl) = 3047, υ(CH
al) = 2945, υ(CC aryl) = 1427, υ(Si–C) = 1231, υ(Si–O–C) = 1115. NMR (CDCl3, δ, ppm): 1H,
1.59 (s, 9H, 2-But), 2.35 (s, 3H, 4-CH3), 6.77 (s, 3H, Ar–O), 7.24–7.85 (m, 15H, Ar),
13C, 151.7, 139.0, 135.5, 135.2, 133.8, 130.2, 130.0, 128.0, 127.7, 126.9, 119.8, 34.7,
29.8, 20.9. 29Si, –15.40. ESI-MS m/z: 445.2 [M+Na]+. Found: C, 82.54; H, 7.32 %. Calcd
(%) for C29H30SiO (422.64): C, 82.34; H, 7.09.
2.2. X-ray crystallography
Crystallographic data were collected on a Bruker SMART APEXII diffractometer using
MoKα radiation (λ = 0.71073 Å). Absorption correction by multi-scan was applied [16], the
structure was solved by direct methods and refined by full-matrix least squares against F2
using all data [17]. All non-H atoms were refined anisotropically. Hydrogen positions were
calculated geometrically at 0.95 Å (CH) and 0.98 Å (CH3) from the parent carbon; a riding
model was used during the refinement process, and the Uiso (H) values were constrained to
Table 1. Crystallographic data of 6.
6
Empirical formula
Fw
C27H26OSi
394.57
T (K)
120(2)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
Triclinic
PÀ1
10.2419(3)
11.3407(3)
11.4646(5)
109.9160(10)
103.7490(10)
109.9990(10)
1077.81(6)
2
α (°)
β (°)
γ (°)
V (Å3)
Z
μ(mmÀ1) (Mo Kα)
Reflection collected
Reflection total
Reflection unique
Parameters
Rint (merging R value)
hmax (°)
0.124
14,791
3793
3557
265
0.0177
25.03
Tmin/Tmax
0.9449/0.9689
1.063
0.0324
Goodness-of-fit on F2
R [F2 > 2σ(F2)]
wR
0.0830

Triphenyl silyl ethers
1463
be 1.2 Ueq (CH) and 1.5 Ueq (CH3). The general-purpose crystallographic tool PLATON
[18] was used for structure analysis and presentation of the results. The figure was drawn
with DIAMOND (Version 3.1) [19]. Crystal structure and refinement data of 6 are
summarized in table 1.
3. Results and discussion
Reactions of chlorotriphenylsilane with an equimolar amount of sodium salts of sterically
hindered phenols (2a-2i) in THF gave aryl triphenylsilyl ether derivatives (3–8) (table 2).
Products were isolated from the reaction mixtures by column chromatography. The
1
structures of 3–8 identified by H, 13C, 29Si NMR, ESI-MS, elemental analyses, and FT-IR
spectroscopy were consistent with the analytical and spectroscopic data (mentioned in the
Experimental section). White solid 3–8 were stable in air and water.
Compounds 3–8 were obtained in low yield, from 11 to 49%. The nature of solvent and
reagents affects the yields which were low, perhaps a result of losses during purification of
the silyl ethers by column chromatography.
Compound 1 was also reacted with 2-tert-butyl-6-methylphenol (2g), 2,6-di-tert-butyl-4-
methylphenol (2h), and 2,4,6-tri-tert-butylphenol (2i). But, pure and defined compounds
could not be obtained from these reactions; steric hindrance plays an important role in
these reactions.
Although 6 was synthesized by others with high yield in the presence of (B(C6F5)3 as
catalyst, the structural elucidation was given with spectroscopic data only [20]. In this
study, the same compound was prepared without catalyst in moderate yield and structural
characterization was done by X-ray diffraction.
Table 2. Preparations of silyl ether derivatives.
THF, Na
Si
Cl
OH
O
Si
Rn
Rn
(1)
(2a-i)
(3-8)
Yield
(%)
Chromatographic
eluent ratios
Compound Rn
M.p. (°C)
Rf
3
4
5
6
7
8
2,4-Dimethyl
3,4-Dimethyl
2,6-Dimethyl
2,4,6-Dimethyl
86–88
11
0.879 [CH2Cl2-n-hexane Acetone2-n-hexane
(1 : 2)] (1 : 2)
0.657 [CH2Cl2-n-hexane CH2Cl2-n-hexane
56–58
91–93
19
11
49
35
32
(1 : 3)]
(1 : 1)
0.540 [Acetone2-n-
hexane (1 : 2)]
CH2Cl2-n-hexane
(1 : 2)
119–120
127–128
114–116
0.658 [CH2Cl2-n-hexane CH2Cl2-n-hexane
(1 : 2)] (1 : 1)
0.682 [CH2Cl2-n-hexane CH2Cl2-n-hexane
(1 : 2)] (1 : 1)
0.676 [CH2Cl2-n-hexane CH2Cl2-n-hexane
(1 : 2)] (1 : 1)
4-tert-Butyl-2-
dimethyl
2-tert-Butyl-4-
dimethyl

1464
S. Begeç et al.
3.1. Spectroscopic studies
The aryl and alkyl C–H stretching frequencies of 3–8 occur at 3045–3067 cmÀ1 and
2915–2960 cmÀ1, respectively. Si–C bonds had strong infrared absorbances at 1231–
1268 cmÀ1 [21–23]. These derivatives also showed strong intensity absorptions at 1115–
1118 from (Si–O–C) stretching vibrations. These data are in agreement with the literature
[23–25].
1
1HÀ, 13CÀ, 29Si–NMR data provided strong evidence for the structures of 3–8. In H–
NMR spectra all CH3 protons were singlets. The phenoxy protons were at δ = 6.48–6.79,
δ = 6.61–6.89, δ = 6.81–6.95, and δ = 6.71–6.79 as multiplets for 3, 4, 5, and 7,
respectively. The phenoxy protons were at δ = 7.56–7.59, δ = 7.38–7.50, δ = 7.44–7.71,
δ = 7.28–7.67, δ = 7.27–7.69, and δ = 7.24–7.85 as multiplets for 3, 4, 5, 6, 7, and 8,
respectively. The methyl protons resonate at δ = 2.10 (4-CH3), 2.15 (2–CH3) (in a 1 : 1
ratio); 2.26 (4–CH3), 2.13 (3–CH3) (in a 1 : 1 ratio), and 2.21 (2,6-di–CH3) as singlet for
3, 4, and 5, respectively.
The methyl protons resonate at δ = 2.23 (2–CH3) and 1.92 (4–CH3) (in a 2 : 1 ratio) for
6. For 7, the protons of the tert-butyl group at the para position and methyl group at the
ortho position gave singlets at δ = 1.14 and 2.20 (in a 3 : 1 ratio), respectively. The protons
of the methyl at the para position and tert-butyl group at the ortho position in 8 gave sing-
lets at δ = 2.35 and 1.59 (in a 1 : 3 ratio), respectively. These data were in agreement with
the literature [26, 27].
The 29Si NMR spectra of these compounds contain signals characteristic of silyl ether
derivatives between À15.15 and À16.81 ppm [28] (a sharp single peak at À15.23 for 3,
À15.15 for 4, À16.50 for 5, À16.81 for 6, À15.62 for 7, and À15.40 for 8).
Figure 1. View of the molecular structure for 6 with the atom-numbering scheme. Displacement ellipsoids are
drawn at 30% probability level. Hydrogens have been omitted for clarity. Selected bond parameters: Si–O: 1.6517
(10) Å; Si–C1: 1.8666(14) Å; Si–C7: 1.8697(14) Å; SiC13: 1.8681(14) Å; O–C19: 1.3814(17) Å; C1–Si–C13:
112.83(6)°; C7–Si–C13: 109.04(6)°; C1–Si–C7: 110.63(6)°; C1–Si–O: 105.12(6)°; C7–Si–O: 108.70(6)°; C13–Si–
O: 110.41(6)°; SiO–C19: 136.79(9)°.

Triphenyl silyl ethers
1465
X-ray crystal structure of 6. The data collection and refinement parameters of 6 are
presented in table 1. The molecular structure is shown in figure 1. Compound 6 shows a
distorted tetrahedral coordination sphere around Si. Selected bond parameters also are
given in figure 1. The three SiC bond lengths were 1.8666(14)–1.8697(14) Å and the
tetrahedral C–Si–C bond angles were 109.04(6)°, 110.63(6)°, and 112.83(6)°. The Si–O
distance is 1.6517(10) Å. The O–Si–C bond angle values were 105.12(6)°, 108.70(6)°, and
110.41(6)°. These values are close to those reported for triphenylsilanol [29–32]. The steri-
cal hindrance between the tris-methylphenoxy-substituent and the three phenyl groups is
reflected by a large Si–O–C19 bond angle of 136.79(9)°, comparable to those reported in
the ether isopropenyloxy[tris(pentafluorophenyl)]silane (138.9(1)°) [33] and tert-butoxytri-
phenylsilane (135.97 (12)°) [34], but larger than those reported in the 2-(methoxy)ethoxy
(triphenyl)silane (123.08°) [35].
Close investigation of crystal structures of 6 shows many intermolecular interactions
where separations between donors and acceptors are less than 3.2 Å. These intermolecular
π–π interactions are predominantly between phenyl groups.
4. Conclusion
Reactions of chlorotriphenylsilane with sterically hindered phenols were investigated at
room temperature. Five silyl ether derivatives (3, 4, 5, 7, and 8) from these reactions were
prepared at room temperature. Structure of 6 was clarified with X-ray diffraction.
Sterically hindered phenols are efficient antioxidants. Separately, silyl ether derivatives
are used as protecting groups in organic synthesis. Thus, these compounds could find
applications as effective antioxidants and protecting groups.
Supplementary material
CCDC-793401 contains the supplementary crystallographic data for compound 6. These
data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Acknowledgment
This work was financially supported by İnönü University Research Fund (İ.Ü.B.A.P: 2006/40).
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