Fabrication of silicon embedded isomeric chalcone linkers using [CuBr(PPh3)3]

Article abstract of DOI:10.1016/j.poly.2016.10.001

The positional isomeric units of substituted chalcones were synthesized using [CuBr(PPh₃)₃] as source of catalyst. The alkynes (1a–9a) were fabricated with azides resulting into triazole moiety containing triethoxysilane substituents (1b–9b) which were further converted into silatranes 1c–9c. The synthesized compounds were confirmed by spectroscopic analytical studies like NMR (1H, 13C), IR, mass recognition studies confirm the silatrane product formation.

Full text of DOI:10.1016/j.poly.2016.10.001

Accepted Manuscript
Fabrication of silicon embedded isomeric chalcone linkers using
[CuBr(PPh3)3]
Gurjaspreet Singh, Jasbhinder Singh, Gurpreet Kaur, Jandeep Singh
PII:
S0277-5387(16)30492-2
DOI:
http://dx.doi.org/10.1016/j.poly.2016.10.001
POLY 12250
Reference:
Polyhedron
To appear in:
Received Date:
Revised Date:
Accepted Date:
30 May 2016
28 September 2016
1 October 2016
Please cite this article as: G. Singh, J. Singh, G. Kaur, J. Singh, Fabrication of silicon embedded isomeric chalcone
linkers using [CuBr(PPh3)3], Polyhedron (2016), doi: http://dx.doi.org/10.1016/j.poly.2016.10.001
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Fabrication of silicon embedded isomeric chalcone linkers using
[CuBr(PPh3)3]
Gurjaspreet Singha,* Jasbhinder Singh,a Gurpreet Kaur,b Jandeep Singh,c,*
a,* Associate Professor
Department of Chemistry
Panjab University, Chandigarh
b Assistant Professor
Department of Chemistry
Khalsa College for Women, Ludhiana
c Assistant Professor
Department of Chemistry
Lovely Professional University, Phagwara (Punjab)
* Corresponding authors
a,*Corresponding Author
Email: gjpsingh@pu.ac.in, Phone: +91-0172-2534428
b,*Corresponding Author
Email: singhjandeep@gmail.com, Phone: +91 9815967272
1

Fabrication of silicon embedded isomeric chalcone linkers using
[CuBr(PPh3)3]
Gurjaspreet Singha,* Jasbhinder Singh,a Gurpreet Kaur,a Jandeep Singh,b,*
Graphical Abstract
Graphical Synopsis
The positional isomeric units of substituted chalcones were synthesized
using [CuBr(PPh3)3] as source of catalyst.
2

Fabrication of silicon embedded isomeric chalcone linkers using
[CuBr(PPh3)3]
Abstract
The positional isomeric units of substituted chalcones were synthesized using [CuBr(PPh3)3] as
source of catalyst. The alkynes (1a–9a) were fabricated with azides resulting into triazole moiety
containing triethoxysilane substituents (1b–9b) which were further converted into silatranes 1c–
9c. The synthesized compounds were confirmed by spectroscopic analytical studies like NMR
(1H, 13C), IR, Mass recognition studies confirm the silatrane product formation.
Keywords
Chalcone
Silatrane
CuAAC
Cu(I) catalysis
Click chemistry
3

1. Introduction
Cu(I) assisted click reaction is highlighted by its modularity, high efficiency (nearly
100% yield in most case), biocompatibility and tolerance toward numerous functional groups.
The catalysis generates 1,2,3–triazole moiety which is distinctly stable and inert under
physiological condition [1–3]. One of the potential use of the 1,2,3–triazole ring is as linker of
biomolecules, following combinatorial approach, thus optimizing various procedures in
medicinal chemistry [4,5]. The utility of click chemistry in the field of medicinal chemistry
mainly focuses on the early stage of drug discovery. Therefore, click chemistry now plays dual
roles as linker and bioisostere. Further, the former one becomes more prominent in many areas
that include medicine, biology and material study; due to the fact that the concept of click
chemistry was originally proposed to be “linking” for small fragments [6,7].
The advancement in this field of research is uprising with the use of Copper–Catalyzed Azide
Alkyne Cycloaddition (CuAAC) reaction. This technique allows selective and efficient post
functionalization of triethoxysilanes [8]. The integration of triethoxysilanes with heterocyclic
triazole can extend its physical, chemical and mechanical properties, and provide robust route
drug discovery, polymer chemistry, and material science [9,10].
The significance of Cu(I) catalyst in click silylation chemistry is impeccable and its
bonding with any kind of moiety can yield plethora of products [11,12]. The [CuBr(PPh3)3]
generated fusion of silicon reactant with organic moiety like cross aldol product (chalcones) can
produce new substrates with modified and enhanced properties. Chalcones are chemically β–
hydroxyaldehydes, also known as aldol products, that result into β–unsaturated aldehydes which
are commercially significant for the production of saturated high molecular weight aldehydes.
These chemicals are significant precursors for the synthesis of solvents, perfumes, flavouring
agents, plastic, dye and disinfectant manufacturing intermediates, pharmaceutics, and molecular
ion detection studies. Industrialization of aldol condensation produces certain alcohols from
lower aldehydes by hydrogenation of the alkenals [13–16]. Along with, a series of silicon
embedded materials have been synthesized and utilized for their activity as transition metal ion
sensors [17].
The binding of specific and non–covalent guest species by a specific organic host
molecule is classified as ‘Molecular recognition’. Its origin has been traced back to macrocyclic
4

compounds, which are capable of selective binding to alkali ions in supramolecular chemistry
[18]. In general, the ion sensing is the recognition process initiated by synthetic or natural
receptors, so called ionophores (Figure 1) [19,20]. The utility of these molecular cavities, or
hosts in which ions can fit and bind selectively and reversibly, is a step forward to improve the
selectivity of chemical sensors. Metal chelators and crown ethers are the forerunner of these
types of selective host molecules. Further, development in chemosensors lead to more
sophisticated organic molecules like podants, cryptands, spherands, calix, or chalcones [21].
Owing to the credibility of [CuBr(PPh3)3] for molecular stitching from readily available
substrates, silicon capped chalcones isomers 1c–9c were synthesized.
ION
IONOPHORE
BINDING SITES
FLUOROPHORE
Figure 1: Sensing of transition metal ions using ionophore
Moreover, the silicon compounds have increased importance owing to easy production of
mesoporous silica particles. Click chemistry has been magnificently applied to the field of
synthesis of functionalized silica packings for chromatographic technique thereby proving a
novel facile and efficient strategy for creation of stationary phases. The advantage of silatranes
over triethoxysilanes is the controlled adherence to various materials like glass slides or optical
fibers as a chemical linker generating different types of sensors [22].
2. Experimental
2.1 General experiment
5

Caution! Azide compounds are explosive to heat and shock. Great care and protection is
required for handling these compounds.
2.1.1 Materials and methods
All the syntheses were carried out under dry nitrogen atmosphere using vacuum glass
line. The organic solvents used were dried and purified according to the standard procedure and
stored under dry nitrogen atmosphere. Bromotris(triphenylphosphine)copper(I) (Aldrich), γ-
chloropropyltriethoxysilane (ClPTES) (Aldrich), propargyl bromide (80% wt. solution in
toluene) (Aldrich), sodium azide (SDFCL), potassium carbonate (THOMAS BAKER), 2-
hydroxyacetophenone (SDFCL), 3-hydroxyacetophenone (SDFCL), 4-hydroxyacetophenone
(SDFCL), salicylaldehyde (Aldrich), 3-hydroxybenzaldehyde (SDFCL), 4-hydroxybenzaldehyde
(SDFCL) and triethanolamine (SDFCL) were used as supplied for synthesis of compounds 1c–
9c. γ-azidopropyltriethoxysilane (AzPTES) was synthesized according to procedure known in
literature [23].
Melting points were uncorrected and measured in a Mel Temp II device using sealed
capillaries. Infrared spectrum was obtained neat on a Thermo Scientific Fischer spectrometer.
Multinuclear NMR (1H, 13C) spectra were recorded on a Bruker advance II 400 NMR
spectrometer (in CDCl3) at 298K. Mass spectroscopy data of synthesized compounds 3c, 5c and
8c was recorded on Waters QQ–TOF micro Mass Spectrometer. CHN analyses were obtained on
a Perkin Elmer Model 2400 CHNS elemental analyzer. Exact accurate weighing of samples was
carried out on Mettler Toledo XP–6 instrument.
2.2 Synthesis of chalcone substituted silatranes (1c–9c)
General procedure for the synthesis of different positional isomers chalcones involved
following methodology. The alkyne (i-iii) was synthesized by reaction of hydroxyl benzaldehyde
(o, m and p) with propargyl bromide, followed by aldol condensation with ketones to give alkyne
substituted isomeric chalcones (1a–9a). 3–Azidopropyltriethoxysilane was then reacted with
(1a–9a) resulting into formation of chalcone isomers of triethoxysilanes (1b–9b) [16c].
To uniformly stirred solution of triethanolamine (0.29 g, 1.96 mmol), and potassium
hydroxide (cat. amt.) in toluene (30 ml), slowly added (1b–9b) (1.0 g, 1.96 mmol) dropwise
o
within 2 min. The mixture was refluxed for 360 min at 110 C and after completion of reaction
6

was allowed to cool at room temperature. The solvent volume in mixture was reduced to 3 ml by
vacuum evaporation and addition of 10 ml of n–pentane resulted into separation of slightly
coloured solid. The solid so obtained was stirred for 2 days, filtered and washed with 2 x 5 ml n–
pentane to afford (1c–9c).
2.2.1
3–(2–(–(1–(3–(silatranyl)propyl)–1H–1,2,3–triazol–4–yl)methoxy)phenyl)–1–
phenylprop–2–en–1–one (1c): Empirical formula: C27H32N4O5Si; Yield: 88%; Anal. Calcd: C,
62.3; H, 6.2; N, 10.8; Found: C, 62.1; H, 6.1; N, 10.3; M. Pt. = 133 – 135 ℃; IR (neat, cm–1):
2946, 2872, 1671, 1596, 1490, 1447, 1383, 1295, 1235, 1179, 1085, 1033, 1012, 908, 839, 750,
696, 614, 542. NMR (400 MHz, CDCl3, 25 ℃) δH = 7.93 – 7.81 (m, 2H), 7.64 (t, J = 7.8 Hz,
2H), 7.54 (d, J = 7.7 Hz, 2H), 7.49 – 7.36 (m, 4H), 6.96 (d, J = 7.6 Hz, 1H), 5.14 (s, 2H), 4.23 (t,
J = 7.3 Hz, 2H), 3.64 (t, J = 5.3 Hz, 6H), 2.80 (t, J = 5.3 Hz, 6H), 1.97 – 1.76 (m, 2H), 0.26 –
0.21 (m, 2H); δC = 188.7, 159.7, 143.5, 137.6, 131.7, 129.5, 127.6, 127.1, 127.0, 126.4, 122.4,
118.9, 61.1, 56.4, 52.3, 49.9, 25.5, 12.3.
2.2.2 3–(3–(–(3–(silatranyl)propyl)–1H–1,2,3–triazol–4–yl)methoxy)phenyl)–1–phenylprop–2–
en–1–one (2c): Empirical formula: C27H32N4O5Si: Yield: 85%; Anal. Calcd: C, 62.3; H, 6.2;
N, 10.8; Found: C, 62.3; H, 6.1; N, 10.7; M. Pt. = 162 – 164 ℃ (decom); IR (neat, cm–1): 2938,
2873, 1655, 1596, 1484, 1447, 1254, 1215, 1163, 1121, 1085, 1033, 1015, 937, 908, 761, 687,
614, 580, 541; NMR (400 MHz, CDCl3, 25 ℃) δH = 7.97 (d, J = 6.0 Hz, 2H), 7.70 (d, J = 15.7
Hz, 1H), 7.60 (s, 2H), 7.52 – 7.35 (m, 4H), 7.25 (s, 2H), 7.20 – 7.06 (m, 2H), 6.99 (d, J = 10.1
Hz, 1H), 5.18 (s, 1H), 4.68 (s, 1H), 4.35 – 4.14 (m, 2H), 3.67 (t, J = 5.8 Hz, 6H), 2.73 (t, J = 5.8
Hz, 6H), 2.00 – 1.83 (m, 2H), 0.46 – 0.17 (m, 2H); δC = 189.4, 160.8, 158.9, 144.3, 132.5, 128.5,
122.6, 122.3, 121.8, 117.1, 62.1, 62.1, 57.4, 53.1, 50.9, 26.1, 12.7.
2.2.3
3–(4–(–(1–(3–(triethoxysilyl)propyl)–1H–1,2,3–triazol–4–yl)methoxy)phenyl)–1–
phenylprop–2–en–1–one (3c): Empirical formula: C27H32N4O5Si; Yield: 89%; Anal. Calcd: C,
62.3; H, 6.2; N, 10.8; Found: C, 62.2; H, 6.1; N, 10.5; M. Pt. = 87 – 88 ℃ (decom); IR (neat,
cm–1): 2929, 2880, 1661, 1589, 1509, 1438, 1348, 1315, 1255, 1215, 1173, 1121, 1095, 1054,
1015, 939, 912, 790, 768, 731, 687, 657, 619, 544; NMR (400 MHz, CDCl3, 25 ℃) δH = 7.92
(d, J = 2.8 Hz, 2H), 7.43 (ddd, J = 29.6, 19.2, 5.2 Hz, 6H), 7.20 (d, J = 6.3 Hz, 1H), 6.94 (d, J =
8.7 Hz, 2H), 5.17 (s, 2H), 4.33 – 4.12 (m, 2H), 3.64 (t, J = 5.8 Hz, 6H), 2.70 (t, J = 5.8 Hz, 6H),
7

1.99 – 1.80 (m, 2H), 0.32 – 0.25 (m, 2H); δC =189.2, 159.9, 158.9, 143.5, 137.1, 132.6, 130.3,
128.6, 128.5, 122.9, 120.1, 115.4, 61.1, 57.5, 50.9, 33.0, 26.2, 13.1.
MS (ES+) Calcd for [M+H]+ 521.2; Found: 521.0.
2.2.4
3–(2–(–(1–(3–(triethoxysilyl)propyl)–1H–1,2,3–triazol–4–yl)methoxy)phenyl)–1–(4–
methoxyphenyl)prop–2–en–1–one (4c): Empirical formula: C28H34N4O6Si; Yield: 84%; Anal.
Calcd: C, 61.1; H, 6.2; N, 10.2; Found: C, 60.9; H, 6.3; N, 10.3; M. Pt. = 113 – 114 ℃ (decom);
IR (neat, cm–1): 2942, 2876, 1654, 1598, 1573, 1509, 1492, 1454, 1420, 1333, 1305, 1255,
1215, 1166, 1127, 1097, 1052, 1016, 1002, 939, 909, 834, 805, 757, 688, 589, 542; NMR (400
MHz, CDCl3, 25 ℃)
δH = 7.97 (d, J = 8.5 Hz, 1H), 7.79 – 7.33 (m, 5H), 7.26 (s, 1H), 6.93 (dd, J = 17.2, 8.3 Hz, 3H),
5.18 (s, 2H), 4.39 – 4.14 (m, 2H), 3.83 (s, 3H), 3.66 (t, J = 5.5 Hz, 6H), 2.71 (t, J = 5.5 Hz, 6H),
2.06 – 1.74 (m, 2H), 0.42 – 0.19 (m, 2H); δC = 190.0, 162.4, 161.6, 159.3, 155.9, 143.4, 141.9,
141.9, 135.5, 132.8, 132.0, 131.2, 129.4, 127.7, 122.0, 120.6, 118.9, 118.5, 116.9, 114.3, 61.2,
57.6, 51.6, 23.2, 17.3.
2.2.5
3–(3–(–(1–(3–(triethoxysilyl)propyl)–1H–1,2,3–triazol–4–yl)methoxy)phenyl)–1–(4–
methoxyphenyl)prop–2–en–1–one (5c): Empirical formula: C28H34N4O6Si; Yield: 79%; Anal.
Calcd: C, 61.1; H, 6.2; N, 10.2; Found: C, 61.0; H, 6.1; N, 10.1; M. Pt. = 173 – 175 ℃; IR
(neat, cm–1): 2949, 2872, 1654, 1597, 1508, 1406, 1342, 1310, 1248, 1238, 1167, 1097, 980,
943, 804, 770, 684, 645, 538; NMR (400 MHz, CDCl3, 25 ℃) δH = 8.00 – 7.89 (m, 1H), 7.57
(dt, J = 3.7, 2.6 Hz, 2H), 7.49 (d, J = 2.0 Hz, 1H), 7.47 (d, J = 1.5 Hz, 1H), 7.44 (s, 1H), 7.42 (d,
J = 1.4 Hz, 1H), 7.39 (dd, J = 3.8, 2.1 Hz, 1H), 7.26 (d, J = 2.0 Hz, 1H), 7.13 (d, J = 7.7 Hz, 1H),
6.96 (dd, J = 9.0, 1.6 Hz, 1H), 5.16 (s, 2H), 4.29 – 4.19 (m, 2H), 3.87 (s, 3H), 3.63 (t, J = 5.8 Hz,
6H), 2.70 (t, J = 5.8 Hz, 6H), 1.93 – 1.87 (m, 2H), 0.36 – 0.21 (m, 2H); δC = 190.6, 165.5, 160.6,
145.7, 138.6, 134.1, 132.9, 130.5, 124.9, 124.3, 123.7, 118.7, 116.3, 115.9, 60.5, 57.5, 54.6, 48.5,
20.3, 9.5.
MS (ES+) Calcd for [M+H]+ 551.2; Found: 551.1.
2.2.6
3–(4–(–(1–(3–(triethoxysilyl)propyl)–1H–1,2,3–triazol–4–yl)methoxy)phenyl)–1–(4–
methoxyphenyl)prop–2–en–1–one (6c); Empirical formula: C28H34N4O6Si; Yield: 83%; Anal.
Calcd: C, 61.1; H, 6.2; N, 10.2; Found: C, 61.2; H, 6.3; N, 10.1; M. Pt. = 76 – 77 ℃; IR (neat,
8

cm–1): 2935, 2873, 1655, 1598, 1510, 1483, 1443, 1420, 1310, 1252, 1169, 1122, 1096, 1033,
1016, 938, 909, 833, 778, 697, 615, 573, 542; NMR (400 MHz, CDCl3, 25 ℃) δH = 7.97 (d, J =
8.9 Hz, 1H), 7.67 (d, J = 2.7 Hz, 1H), 7.61 (s, 1H), 7.52 (d, J = 5.0 Hz, 1H), 7.25 (dd, J = 10.4,
4.9 Hz, 2H), 7.14 (dd, J = 11.9, 4.7 Hz, 1H), 7.03 – 6.82 (m, 3H), 5.16 (s, 2H), 4.24 (t, J = 7.4
Hz, 2H), 3.83 (s, 3H), 3.64 (t, J = 5.7 Hz, 6H), 2.73 (t, J = 5.7 Hz, 6H), 1.93 – 1.87 (m, 2H), 0.36
– 0.10 (m, 2H); δC = 187.5, 163.1, 158.5, 157.5, 143.3, 142.5, 141.1, 130.6, 130.0, 114.0, 61.7,
60.9, 59.2, 57.2, 56.6, 53.6, 52.3, 49.8, 27.9, 14.0.
2.2.7
3–(2–(–(1–(3–(triethoxysilyl)propyl)–1H–1,2,3–triazol–4–yl)methoxy)phenyl)–1–(2,4–
dimethoxyphenyl)prop–2–en–1–one (7c): Empirical formula: C29H36N4O7Si: Yield: 84%;
Anal. Calcd: C, 60.0; H, 6.3; N, 9.7; Found: C, 59.8; H, 6.2; N, 9.5; M. Pt. = 117 – 118 ℃
(decom); IR (neat, cm–1): 2946, 2872, 1655, 1598, 1452, 1414, 1251, 1209, 1160, 1015, 881,
834, 751, 680, 585, 542; NMR (400 MHz, CDCl3, 25 ℃) δH = 7.69 (d, J = 8.6 Hz, 1H), 7.56 (d,
J = 16.1 Hz, 3H), 7.43 (d, J = 15.7 Hz, 2H), 7.30 – 7.22 (m, 2H), 6.94 (td, J = 7.9, 1.9 Hz, 1H),
6.52 – 6.47 (m, 1H), 6.43 (d, J = 2.1 Hz, 1H), 5.15 (s, 2H), 4.30 – 4.22 (m, 2H), 3.85 (s, 3H),
3.80 (s, 3H), 3.68 (t, J = 5.8 Hz, 6H), 2.73 (t, J = 5.8 Hz, 6H), 2.00 – 1.84 (m, 2H), 0.46 – 0.24
(m, 2H); δC = 190.5, 162.9, 142.8, 141.5, 132.5, 131.7, 130.3, 130.1, 129.7, 122.7, 114.8, 104.9,
98.4, 61.9, 58.3, 55.5, 55.3, 52.3, 46.1, 27.4, 13.3.
2.2.8
3–(3–(–(1–(3–(triethoxysilyl)propyl)–1H–1,2,3–triazol–4–yl)methoxy)phenyl)–1–(2,4–
dimethoxyphenyl)prop–2–en–1–one (8c); Empirical formula: C29H36N4O7Si; Yield: 78%;
Anal. Calcd: C, 60.0; H, 6.3; N, 9.7; Found: C, 59.9; H, 6.1; N, 9.6; M. Pt. = 74 – 75 ℃; IR
(neat, cm–1): 2938, 2873, 1654, 1597, 1486, 1449, 1415, 1249, 1210, 1159, 1084, 1014, 937,
909, 828, 757, 697, 541; NMR (400 MHz, CDCl3, 25 ℃) δH = 7.69 – 7.47 (m, 3H), 7.36 – 7.20
(m, 3H), 6.96 – 6.94 (m, 2H), 6.43, (s, 1H), 5.18 (s, 2H), 4.68 (s, 1H), 4.59 – 4.55 (m, 2H), 3.81,
(s, 3H), 3.78 (s, 3H), 3.56 (t, J = 5.8 Hz, 6H), 2.61 (t, J = 5.8 Hz, 6H), 1.84 – 1.77 (m, 2H), 0.25
– 0.20 (m, 2H); δC = 190.6, 164.1, 160.3, 159.9, 143.6, 141.8, 132.8, 132.1, 13000, 128.6, 125.3,
122.9, 122.4, 115.1, 105.2, 62.1, 58.6, 55.8, 55.6, 52.6, 48.9, 26.2, 13.2.
MS (ES+) Calcd for [M+H]+ 581.2; Found: 581.0.
2.2.9
3–(4–(–(1–(3–(triethoxysilyl)propyl)–1H–1,2,3–triazol–4–yl)methoxy)phenyl)–1–(2,4–
dimethoxyphenyl)prop–2–en–1–one (8c): Empirical formula: C29H36N4O7Si; Yield: 85%;
9

Anal. Calcd: C, 60.0; H, 6.3; N, 9.7; Found: C, 60.1; H, 6.2; N, 9.4; M. Pt. = 100 – 101 ℃; IR
(neat, cm–1): 3139, 2874, 1649, 1597, 1507, 1460, 1422, 1328, 1302, 1283, 1246, 1210, 1161,
1123, 1084, 1015, 938, 909, 826, 802, 761, 686, 589, 541; NMR (400 MHz, CDCl3, 25 ℃) δH =
8.12 (d, J = 8.9 Hz, 1H), 7.66 – 7.50 (m, 2H), 7.45 – 7.22 (m, 2H), 7.09 – 6.91 (m, 2H), 6.77 (dd,
J = 43.3, 22.5 Hz, 1H), 6.56 (dd, J = 20.2, 9.4 Hz, 2H), 5.12 (dd, J = 28.8, 10.9 Hz, 2H), 4.23 (s,
2H), 3.81 (s, 3H), 3.77 (s, 3H), 3.32 (t, J = 6.1 Hz, 6H), 2.45 (t, J = 9.6 Hz, 6H), 1.81 – 1.67 (m,
2H), 0.50 – 0.13 (m, 2H); δC = 189.3, 160.0, 141.2, 131.9, 130.1, 128.3, 124.8, 121.5, 115.2,
105.8, 98.6, 59.1, 57.2, 56.5, 55.9, 55.5, 49.8, 26.8, 14.2.
2.3 Time dependent hydrolytic stability study
To pre-weighed and dry aluminium sheet 30 mg of each triethoxysilane 1b–9b silatrane
1c–9c was uniformly spread out under inert atmosphere and set it up on µg weighing balance to
consider weight change. The sheet was let open at room temperature; monitored and studied the
gain of weight by sample. The moisture absorption of silatranes 1b–9b and 1c–9c was monitored
with time using IR spectroscopy as well.
3. Results and Discussions
3.1 Synthesis and characterization
Organotriethoxysilanes (OTES) have always fascinated a large bunch of, synthetic
organic chemists owing to their abundant group transferring properties, material fabricators for
selective hydrolysis and drug designers for creating a novel medication. Since numerous
applications and little known stability towards hydrolysis, OTES have not been able to prove
worthy of extensive application in water based systems. Silatranes are another class of silicon
with silicon protected under ‘Helmet’ of –N– atom from trialkoxyamine. The synthetic approach
towards silatranes was processed under inert conditions (moisture free) to maximum yield.
The synthetic procedure followed for the generation of silicon capped compounds 1c–9c
is summarized in Scheme 1. The final product was generated in this four–step route involves a)
Nucleophilic substitution reaction yielding terminal alkyne b) Claisen–Schmidt Condensation
resulting into chalcone formation c) CuAAC reaction creating substituted 1,2,3–Triazole d)
Transetherification of triethoxysilane with triethanolamine producing silatranes 1c–9c.
10

Scheme 1: Schematic illustration for synthesis of chalcone substituted silatranes from cost
effective raw materials
3.2 Optimization of reaction conditions
Furthermore, the silatrane (1c) formation was optimized under specific conditions
involving hydrous/anhydrous conditions using open and closed glassware assembly, using
Schlenk apparatus, simultaneously to confirm the product formation (Table 1). The reaction did
not follow a smooth pathway when the system was open, while for a closed system the product
yield was higher. Furthermore, the protic solvent system failed to produce good product yield
while aprotic non–polar solvents proved to be most efficient solvent system for silatrane
formation. In addition to this, the studies were extended towards utility of DMF as solvent for
silatrane generation and it was established that polar aprotic solvents were effective in producing
moderate yield but the separation was a big challenge as the product was soluble in DMF.
Therefore, optimized reaction conditions for the synthesis of silatranes 1c–9c are as per entry 6
in Table 1.
Entry
Conditions
Duration
Glassware
Solvent
Product
11

(min)
240
600
360
240
600
360
240
600
360
Assembly
Open
Yielda (%)
1
2
3
4
5
6
7
8
9
78 ℃, st
78 ℃, st
Ethanol
Ethanol
Ethanol
Toluene
Toluene
Toluene
DMF
11
07
27
43
31
89
15
26
57
Open
78 ℃, st
Close
Open
110 ℃, st
110 ℃, st
110 ℃, st
118 ℃, st
118 ℃, st
118 ℃, st
Open
Close
Open
Open
DMF
DMF
Close
a Isolated product yield of silatrane: Determined by 1H NMR analysis of a crude sample.
st = stirring
Table 1: Optimization of conditions for chalcone based silatrane formation
3.3 Role of [CuBr(PPh3)3]
The importance of Cu(I) in manufacture of 1,2,3–triazole, lie in coupling of alkyne to
azide, which was achieved only by [CuBr(PPh3)3] as catalyst with excellent product conversion,
in anhydrous conditions. There have been numerous Cu(I) catalytic systems used for 1,4–
cycloaddition reaction, but none proved its worth for efficient use in case of triethoxysilanes
(Table 2). Among the set of catalysts explored, [CuBr(PPh3)3] shows excellent product yield for
click silylation chemistry.
Stability of
Cu(I) catalyst
Stable in inert
conditions
Amount of
Reaction
Cycloaddition
yield (%)
S. No.
Cu(I) source
catalyst required time (min)
1.
CuCl2, PPh3, SO2
3.0 mol
600
84
CuSO4, Sodium
ascorbate, H2O
[CuBr(PPh3)3]
2.
3.
in situ formation
8.0 mol
1200
13
stable
0.01 mol
180
91
Table 2: Catalytic systems for synthesis of triethoxysilanes
12

3.4 Importance of embedded silicon compounds
The hydrolytic instability of OTES is due to the attack of water molecules (–:O:H) on
vacant d–orbitals and higher mobility of –:O:R facilitates removal of ethoxy group (Figure 2).
The rapidity in replacement of alkoxy groups by hydroxyl groups yields silica thus reducing the
applicability of OTES. However, this barrier is removed by conversion of OTES into silatranes
which are capped structures and prevent any attack on Si–center. The donation of a set of pπ
electrons to hollow d–orbitals modifies behavior at silicon centered rendering it stable (depends
on –R group) [13,24].
Figure 2: Hydrolysis of OTES in presence of moisture and hindered attack of water on silatranes
13

Exposure
Time
(min)
50
Moisture
Absorbed
(µmol)
33.8
Moisture
Absorbed
(µmol)
17.8
Silane
ID
%
Silatrane
ID
%
Entry
Hydrolysis
Hydrolysis
1
2
3
4
5
6
7
8
9
1b
2b
3b
4b
5b
6b
7b
8b
9b
2.03
0.41
1c
2c
3c
4c
5c
6c
7c
8c
9c
1.07
0.26
0.44
3.17
5.87
1.21
6.09
0.26
1.33
50
6.8
4.3
50
0.89
14.8
7.3
50
12.05
17.81
3.58
200.9
296.8
59.6
52.9
50
97.9
50
20.2
50
18.47
0.77
307.8
12.9
101.5
4.3
50
50
2.71
45.1
22.2
Table 3: Hydrolysis (moisture gain) study of triethoxysilanes 1b – 9b and silatranes 1c – 9c
330
1b
2b
300
3b
4b
270
5b
6b
240
7b
8b
9b
210
180
150
120
90
60
30
0
0
5
10
15
20
25
30
35
40
45
50
Time (min)
14

Figure 3: Plot for rate of hydrolysis (moisture absorbed) in µmols for silatranes 1b – 9b vs time;
mols of water absorbed = (w2 – w1)/18; where w1 is initial weight of sample at the beginning and
w2 is final weight of sample.
130
1c
120
110
100
90
80
70
60
50
40
30
20
10
0
2c
3c
4c
5c
6c
7c
8c
9c
0
5
10
15
20
25
30
35
40
45
50
Time (min)
Figure 4: Plot for rate of hydrolysis (moisture absorbed) in µmols for silatranes 1c – 9c vs time;
mols of water absorbed = (w2 – w1)/18; where w1 is initial weight of sample at the beginning and
w2 is final weight of sample.
15

Figure 5: IR monitored time dependent spectrum of silatrane 7c (time dependent hydrolysis)
3.5 Hydrolytic stability study
The hydrolytic stability of compounds 1c–9c was studied under normal atmospheric
conditions was compared with that od triethoxysilanes 1b–9b (Figure 3, 4). It was found that
most of the silatranes are stable towards air in comparison silanes 1b–9b. Compounds 5c and 7c
were hydrolysed rapidly within few minutes of exposure to air. The plot of gain of water
molecules from atmospheric air with time was studied to observe the amount of moisture
absorbed. The study indicated that silanes are 3-4 times more rapidly hydrolysed as compared to
silatranes. Only two compounds 5c and 7c gained 16 µmols of water as moisture each. The plot
of µmols of water (as moisture) absorbed vs time (min) becomes constant, after 40 min of
exposure for silanes 1b–9b and 30 min for penta coordinated compounds 1c–9c. Moreover, the
studies indicated that the hydrolysis depends on surface area of sample exposed per given mass
of sample and relative humidity of air in surroundings.
The hydrolytic study was well supported by time dependent Infrared Spectroscopy
technique which clearly indicated water gain with time. IR spectra of hydrolysed silatrane 7c
(Figure 5) (IR of 5c is nearly similar to 7c) was recorded with time that provides additional
information regarding stability of silatrane cage towards hydrolysis by the existence of
transannular N→Si bond by sharp and intense absorption bands in the region of 750 – 580 cm–1.
16

The probable explanation for hydrolysis of compounds 5c and 7c can be the tendency to form
extensive intermolecular hydrogen bonding with–OCH3 on interaction with water molecules [17,
25].
3.6 Chalcone substituted silatranes (1c–9c)
Owing to hydrolytic instability, the OTES 1b–9b were modified into silatranes 1c–9c
using triethanolamine as ligand and KOH as reaction promoter with toluene as solvent. The
reaction proceeded to completion at 110 ℃ within 360 min. The final compound 1c–9c were
separated by vacuum evaporation of toluene followed by the addition of 10 ml of n–pentane. The
solid filtrate that appeared after addition of n–pentane was filtered, dried under vacuum and
collected as fine creamish white coloured powder in good yield. All data of resulting silatranes
1c–9c have been compiled in Table 2.
17

Substrate
alkyne
(i–iii)
Aromatic
ketone
(a–c)
Triethoxysilanes
(1b–9b)/
Silatranes
(1c–9c)
M.Pt.
(℃)
Yield (%)
Yield (%)
O
133 –
135
O
OHC
O
162 –
164
O
OHC
O
O
87 – 88
CHO
18

O
113 –
114
O
OHC
OCH3
O
173 –
175
O
OHC
OCH3
O
O
76 – 77
OCH3
CHO
19

O
H3CO
117 –
118
O
OHC
OCH3
O
H3CO
O
74 – 75
OCH3
OHC
O
O
H3CO
100 –
101
OCH3
CHO
Table 2: Synthesized triethoxysilanes (1b–9b) [21] silatranes (1c–9c) along with their yield and melting points for solids
20

3.6 Infra–Red spectroscopy
The neat vibrational spectroscopic data for all compounds was recorded in the range 4000
– 400 cm–1. All the resulting frequencies in the form of IR bands were in accordance with the
expected structure of synthesized compounds. The clear region in the range 2150 to 2050 cm–1
proved complete cyclization of AzPTES and alkynes 1a–9a to 1,2,3–triazole linked
triethoxysilanes 1b–9b. There were sharp absorption bands in the region 3072 – 2807 cm–1 and
1660 – 1438 cm–1 that corresponded to aromatic conjugation in chalcone substituent and 1,2,3–
triazole ring. The transannular N→Si bond in all the silatranes 1c–9c was signaled by the
presence of absorption bands in the region 770 – 538 cm–1. The –CH vibrations in FTIR spectra
of all triethoxysilanes 1c–9c were signaled near 2980 – 2700 cm–1.
3.7 NMR Spectroscopy for silatranes 1c–9c
1H NMR spectra of silatranes 1c–9c on keen observation revealed nearly similar trend in
signals as depicted in NMR study of triethoxysilanes 1b–9b. All the silatranes (1c–9c) displayed
a set of multiplets due to –CH2 unit of propyl chain. The –OCH2 linked to 1,2,3–triazole
heterocycle was largely de–shielded which appeared at δ 5.26 – 5.14 ppm. The aromatic ring
protons conjugated as chalcone were signaled as multiplets in the region δ 8.05 – 6.34 ppm.
Moreover, the –OCH3 protons appeared as singlets in the region δ 3.85 – 3.78 ppm.
The NMR spectra of silatrane differed from silane by the complete disappearance of a
3
quartet and triplet at δ 3.81 – 3.69 ppm and δ 1.21 – 1.10 ppm with J of 7.4 – 7.0 Hz; and
emergence of new triplets with δ 3.66 – 3.56 due to –NCH2CH2O– and at δ 2.80 – 2.61 with 3J of
5.8 Hz due to –NCH2CH2O– arms of atrane ring. Another major shift in NMR was observed for
methylene proton of –CH2Si in which signal shifted from the maximum of δ 0.68 ppm to the
minimum value of δ 0.17 ppm for some silatranes. Any other effect due to attachment of
different substituents at o–,m– and p– isomeric positions was negligible on propyl chain protons.
13
In C NMR spectra, the major shift was observed–N3C– of 1,2,3–triazole as it shifted
from δ 60.1 – 52.3 ppm and appeared in the region of δ 131.2 – 127.8 ppm. The methylene
carbon of propyl chain attached to silicon atom –C–Si– appeared as the most shielded carbon
atom at around δ 6.4 ppm for triethoxysilanes 1b–9b which was shifted by 7 – 10 ppm to δ 17.3
– 9.5 ppm for silatranes 1c–9c indicated generation of hypervalency at silicon centre. For
21

silatranes, the –OCCN– of cage structure appeared in the range δ 61.1 – 56.5 ppm and for –
OCCN– at δ 52.6 – 49.8 ppm.
3.8 Mass Spectroscopy
Mass spectral studies performed for one of the isomer of silatranes 3c, 5c and 8c along
with CHN analytical study was performed on all silatranes 1c–9c which proved the high purity
of all compounds. Molecular ion signals for compound 3c [Molecular Mass: 521.2], m/z 551.2
[M +H]+ for compound 5c [Molecular Mass: 551.2], and m/z 581.0 [M +H]+ for compound 8c
[Molecular Mass: 581.2], confirmed the proposed structure. Moreover, the elemental analysis
performed on all silatranes 1c–9c justified the purity of title compounds.
4. Conclusion
In current work, the silatranes 1c–9c were synthesized in good yield and prove to have
higher hydrolytic stability than their corresponding OTES. Moreover, CuAAC technique acted as
combinatorial scheme for joining together of silicon to chalcone group. Moreover, silatranes are
almost three times more stable than their corresponding substituted triethoxysilanes. The capped
silicon compounds pave a new pathway for the silicon family that can have potential in wide
range as synthetic decoration of alkyne based DNA, protein genomics, oligosaccharides, and
glycoconjugates.
Acknowledgments
We thank Mr. Avtar Singh and Mr. Manish Kumar for the NMR studies (SAIF, Panjab
University, Chandigarh).
Notes
The authors declare no competing financial interest.
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