Synthetic approach towards 'click' modified chalcone based organotriethoxysilanes; UV-Vis study

Article abstract of DOI:10.1039/c4ra08724k

The efficient linkage of a conjugate chalcone to n-propyltriethoxysilanes (nPTES) via a 1,2,3-triazole with good yields is reported. The synthesis involves a Claisen-Schmidt condensation reaction followed by a copper(i) catalyzed azide-alkyne cycloadditio

Full text of DOI:10.1039/c4ra08724k

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Graphical Abstract
Synthetic approach towards ‘Click’ modified chalcone based organotriethoxysilanes;
UV–Vis study
Gurjaspreet Singh,* Jandeep Singh, Satinderpal Singh Mangat, Aanchal Arora
Department of Chemistry and Centre of Advanced Studies, Panjab University, Chandigarh,
160014, India.
*Corresponding Author
Email: gjpsingh@pu.ac.in, Phone: +91ꢀ0172ꢀ2534428
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Synthetic approach towards ‘Click’ modified chalcone based organotriethoxysilanes;
UV–Vis study
Abstract
The efficient linkage of conjugate chalcone to n–propyltriethoxysilanes (nPTES) via 1,2,3ꢀ
triazole is being reported, in good yield which involves Claisen–Schmidt condensation
followed by Copper (I) catalyzed azide–alkyne cycloaddition (CuAAC) reaction as synthetic
intermediary routes. Two different approaches followed for syntheses, restrict us to follow
single pathway exclusively as an efficient route for organotriethoxysilanes (OTES)
preparation. The performance of click reaction under thermal reaction conditions has also
been optimized using [CuBr(PPh3)3] catalyst. The photoelectronic spectroscopy study in UV–
Vis region show significant absorption maxima in range of 300–325 nm. Moreover, the
solvatochromic aspects showing the effect of change in solvent polarity on absorption
maxima is being investigated for the first time on functionalised OTES.
Keywords: CuAAC, chalcone, click silylation, γꢀpropyltriethoxysilane, solvatochromism
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Introduction
The modification in organic segment of hybrid silica precursors have manifolded their utility
in the field of drug discovery,1 catalysis,2 surface coating of materials,3,4 polymer formation,5
ion detecting fluorescent probes,6,7 HPLC packing,8 and in nano chemistry.9 The synthetic
approach leading to generation of polyfunctional triethoxysilanes (PfTES) acting as precursor
to these materials follow different pathways such as cross–coupling reaction,10–12
hydrosilylation,13,14 and transmetalation reaction.15 These reaction methodologies require the
use of expensive metals, show limited functional group tolerance and result in moderate to
good yield. The major shortcoming in these conventional methodologies is the purification of
‘hydrolytically unstable’ PfTES and control of regioselectivity.
To override these limitations, a new technique was pioneered in 2001 by Sharpless and
Meldal,16–18 and exploited by Cattoen et al.,19,20 to fine tune nearly all functional groups
hooked to azide linked nPTES using Cu(I), thereby expanding the scope and utility of this
methodology. This technique follows CuAAC reaction of azide–alkyne fragments to 1,2,3ꢀ
triazole21 with efficient conversion of above 80% and 100% control over regioselectivity.22
Numerous catalytic systems have been reported for click reaction but the use of
[CuBr(PPh3)3]/THF–TEA system has been promising as efficient catalytic scheme for the
synthesis of functionalised OTES.23–28 Further, 1,2,3ꢀtriazolyl heterocycle proves to be an
important pharmacore29 associated with immense medicinal importance as antimycobacterial
activity,30,31
antituberculosis,32
antiinflammatory,33
antiangiogenic,34
antiviral,35
anticancerous as histone deacetylate inhibitor36,37 and anti–HIV activity.38
The combination of smaller fragments to assemble as larger unit with enhanced
pharmacological activity forms an important aspect of synthetic biochemistry and is
acquiring constant attention with increasing benefits in medicinal chemistry. The
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applicability of combinatorial approach has led to generation of substituted chalcone based
moieties that act as primary precursor to flavonoids and isoflavonoids, that are abundantly
distributed in edible plants and are associated to be essential cancer chemo–preventive food
components.39–41 A chalcone unit comprises of two aromatic rings connected by a three
carbon chain as α,β unsaturated carbonyl group (Figure 1). They are considered as
pharmacologically relevant entities known to exert pathogenic activity41 along with
antitumorigenic,43 antiinflammatory,44 antiangiogenic,45 antioxidant,46 antituberculosis,47
antimalarial,48 and anti–HIV properties.49 With this perspective to synthesize material with
merged activity of both functionalities, the integration of pharmacore chalcone moiety onto
medicinally vibrant 1,2,3–triazole was carried out.
O
(E)
Figure 1: Chemical structure of chalcone unit consisting of two aromatic rings linked by
threeꢀcarbon chain forming α–β unsaturated carbonyl moiety
Experimental
General material 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), γꢀchloropropyltriethoxysilane (ClPTES)
(Aldrich), propargyl bromide (80% wt. solution in toluene) (Aldrich), sodium azide
(SDFCL), potassium carbonate (THOMAS BAKER), N,Nꢀdimethylformamide (SDFCL),
were used as supplied. Acetophenone (SDFCL), 2ꢀhydroxyacetophenone (SDFCL), 3ꢀ
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hydroxyacetophenone (SDFCL), 4ꢀhydroxyacetophenone (SDFCL), salicylaldehyde
(Aldrich), 3ꢀhydroxybenzaldehyde (SDFCL), 4ꢀhydroxybenzaldehyde (SDFCL), pꢀ
methoxyacetophenone (HIMEDIA), 2,4ꢀdimethoxyacetophenone (HIMEDIA) were used as
supplied for synthesis of terminal alkynes 3a–3i and 5a–5i. γꢀazidopropyltriethoxysilane
(AzPTES) was synthesized according to procedure known in literature.20
Synthesis of compounds 2a(i–iii) and 2b(i–iii)
To a uniformly stirred solution of 1a/1b (2g, 16.40 mmol, 1 equiv) in 15 ml of DMF cooled
in ice bath to –5 ℃ K2CO3 (6.76 g, 47.2 mmol, 3 equiv) was added. To this stirring mixture,
propargyl bromide (2.15 g, 1.61 ml, 18.06 mmol, 1.1 equiv) was slowly injected dropwise
within 5 min. After complete addition of reactants, the temperature of reaction mixture was
slowly raised to 30 ℃ in 1 h and stirred at this temperature for 14 h. The reaction mixture was
quenched by addition of ice cold water and filtered the solid product (in case of 2a(i, iii);
2b(i–iii)). In case of low melting solid 2a(ii), ethyl acetate was used for extraction. The
combined organic layers were dried over anhydrous MgSO4 and vacuum evaporation of
solvent resulted into the formation of desired compound. The solid alkynes were
recrystallized by dissolving in minimum amount of absolute ethanol.
Synthesis of compounds 3a–3i/5a–5i
The compounds 2a/2b (1 equiv) and 1c(a–c) (1 equiv) were independently dissolved in
minimum amounts of absolute ethanol till clear solution. In another round bottom flask, KOH
(0.02 g, 0.36 mmol) was dissolved in ethanol and slowly added solution of 2a/2b and 1c(a–c)
respectively. The reaction was stirred for 4 h and monitored the end point using TLC
(hexane:ethyl acetate (8:2)). The aldol condensation mixture of 2a and 1c(a–c) yielded
compounds 3a–3i while mixture of 2a and 2b yielded compounds 5a–5i. On completion, the
reaction was quenched by ice cold water, extracted with methylene chloride and washed
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twice with brine solution. The combined organic phases were dried over anhydrous MgSO4
and vacuum evaporation of solvent afforded the desired product.
Synthesis of compounds 4a–4i/6a–6i
In a 25 ml two neck round bottom flask, alkyne 3a–3i/5a–5i were dissolved in 1:1 solution of
THF:TEA (3 ml) till uniform solution followed by catalyst loading (0.02 mmol for 3a–3i and
0.04 mmol for 5a–5i). The slow and dropwise addition of AzPTES (1 equiv for 3a–3i and 2
equiv for 5a–5i) under carried out inert atmosphere. The temperature of reaction mixture was
raised slowly to 65 ℃ and stirred vigorously for 3 h. After completion of reaction, the
assembly was cooled to room temperature, filtered the mixture containing used Cu(I) catalyst
and vacuum evaporation of solvents resulted into desired nPTES 4a–4i/6a–6i.
Spectroscopic data for compounds 4a–4i
1ꢀphenylꢀ3ꢀ(2ꢀ((1ꢀ(3ꢀ(triethoxysilyl)propyl)ꢀ1Hꢀ1,2,3ꢀtriazolꢀ4ꢀyl)methoxy)phenyl)propꢀ
2ꢀenꢀ1ꢀone (4a): Yield: 91%, Empirical formula: C27H35N3O5Si; Anal. Calcd: C, 63.6; H, 6.9;
N, 8.2; Found: C, 63.4; H, 6.8; N, 8.0; IR (neat, cm–1): 2965, 2929, 2876, 1659, 1597, 1574,
1
1485, 1448, 1332, 1315, 1258, 1213, 1162, 1073, 1015, 907, 792, 726, 691, 646. H NMR
(400 MHz, CDCl3), δ = 8.00 (d, 3J = 15.8 Hz, 1H), 7.87 (d, 3J = 7.5 Hz, 1H), 7.55 (d, 3J = 6.3
3
Hz, 3H), 7.44 (d, 3J = 7.4 Hz, 2H), 7.38 (d, J = 7.7 Hz, 2H), 7.01 (d, 3J = 8.3 Hz, 1H), 6.92
3
3
(d, 3J = 8.1 Hz, 2H), 5.21 (s, 2H), 4.25 (t, J = 7.2 Hz, 2H), 3.69 (q, J = 7.0 Hz, 6H), 2.04 –
1.90 (m, 2H), 1.10 (t, 3J = 7.0 Hz, 9H), 0.57 – 0.41 (m, 2H). 13C NMR (101 MHz, CDCl3) δ =
189.8, 156.4, 142.5, 139.1, 137.3, 131.6, 130.7, 128.6, 127.5, 123.2, 122.0, 120.3, 111.7,
57.4, 55.1, 51.5, 23.3, 17.3, 6.4.
1ꢀphenylꢀ3ꢀ(3ꢀ((1ꢀ(3ꢀ(triethoxysilyl)propyl)ꢀ1Hꢀ1,2,3ꢀtriazolꢀ4ꢀyl)methoxy)phenyl)propꢀ
2ꢀenꢀ1ꢀone (4b): Yield: 89%, Empirical formula: C27H35N3O5Si; Anal. Calcd: C, 63.6; H,
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6.9; N, 8.2; Found: C, 63.3; H, 6.7; N, 8.3; IR (neat, cm–1): 2972, 2925, 2884, 1663, 1596,
1
1578, 1485, 1447, 1315, 1289, 1235, 1073, 1161, 1034, 1016, 955, 770, 688, 566. H NMR
3
(400 MHz, CDCl3) δ = 8.08 – 7.86 (m, 2H), 7.77 (d, J = 15.6 Hz, 1H), 7.59 – 7.52 (m, 2H),
3
7.51 – 7.46 (m, 2H), 7.36 – 7.31 (m, 1H), 7.27 (dd, J = 11.4, 5.3 Hz, 2H), 7.10 – 6.98 (m,
3
2H), 5.26 (s, 2H), 4.37 (t, J = 7.2 Hz, 2H), 3.81 (q, 3J = 7.0 Hz, 6H), 22.04 – 1.90 (m, 2H),
1.21 (t, 3J = 7.0 Hz, 9H), 0.68 – 0.53 (m, 2H). 13C NMR (101 MHz, CDCl3) δ = 189.3, 157.6,
156.8, 143.4, 137.0, 135.3, 131.8, 129.0, 127.5, 121.4, 120.7, 115.9, 113.5, 113.3, 66.9, 61.1,
57.5, 54.8, 51.5, 24.5, 23.1, 17.2, 6.4.
1ꢀphenylꢀ3ꢀ(4ꢀ((1ꢀ(3ꢀ(triethoxysilyl)propyl)ꢀ1Hꢀ1,2,3ꢀtriazolꢀ4ꢀyl)methoxy)phenyl)propꢀ
2ꢀenꢀ1ꢀone (4c): Yield: 90%, Empirical formula: C27H35N3O5Si; Anal. Calcd: C, 63.6; H, 6.9;
N, 8.2; Found: C, 63.4; H, 7.0; N, 8.1; IR (neat, cm–1): 2972, 2933, 2885, 1659, 1595, 1572,
1507, 1447, 1422, 1389, 1335, 1291, 1213, 1172, 1072, 1015, 956, 827, 778, 691, 657, 513.
3
3
1H NMR (400 MHz, CDCl3) δ = 7.92 (d, J = 7.3 Hz, 1H), 7.69 (d, J = 15.6 Hz, 1H), 7.51
3
3
3
3
(dd, J = 8.7, 4.1 Hz, 2H), 7.40 (t, J = 7.5 Hz, 1H), 7.35 (d, J = 7.8 Hz, 2H), 7.32 (d, J =
3
3.5 Hz, 1H), 7.17 (dd, J = 10.1, 6.0 Hz, 2H), 7.05 – 7.00 (m, 1H), 6.96 – 6.89 (m, 2H), 5.16
(s, 2H), 4.31 – 4.25 (m, 2H), 3.72 (d, 3J = 7.4 Hz, 6H), 2.00 – 1.90 (m, 2H), 1.14 (d, 3J = 7.0
Hz, 9H), 0.56 – 0.42 (m, 2H). 13C NMR (101 MHz, CDCl3) δ = 189.5, 158.5, 143.4, 137.4,
131.6, 129.1, 127.4, 126.8, 124.1, 121.8, 119.2, 114.3, 113.7, 61.1, 57.5, 54.8, 52.8, 23.2,
17.3, 6.4. MS (ES+) Calcd for [M+Na]+ 532.2; Found 532.3.
1ꢀ(4ꢀmethoxyphenyl)ꢀ3ꢀ(2ꢀ((1ꢀ(3ꢀ(triethoxysilyl)propyl)ꢀ1Hꢀ1,2,3ꢀtriazolꢀ4ꢀ
yl)methoxy)phenyl)propꢀ2ꢀenꢀ1ꢀone (4d): Yield: 88%, Empirical formula: C28H37N3O6Si;
Anal. Calcd: C, 62.3; H, 6.9; N, 7.8; Found: C, 62.2; H, 6.8; N, 8.0; IR (neat, cm–1): 2973,
2933, 2888, 1654, 1597, 1573, 1509, 1455, 1308, 1256, 1216, 1165, 1103, 1074, 1019, 907,
753, 725, 646, 614, 587, 541.1H NMR (400 MHz, CDCl3) δ = 8.05 – 7.97 (m, 2H), 7.91 (d, 3J
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= 8.8 Hz, 2H), 7.65 – 7.53 (m, 3H), 7.28 (t, 3J = 7.2 Hz, 1H), 7.03 (d, 3J = 8.3 Hz, 1H), 6.94
(d, 3J = 7.6 Hz, 1H), 6.88 (d, 3J = 8.8 Hz, 2H), 5.24 (s, 2H), 4.28 (t, 3J = 7.2 Hz, 2H), 3.79 (s,
3H), 3.71 (q, 3J = 6.9 Hz, 6H), 2.01 – 1.90 (m, 2H), 1.12 (t, 3J = 7.0 Hz, 9H), 0.63 – 0.39 (m,
2H). 13C NMR (101 MHz, CDCl3) δ = 188.1, 162.3, 142.6, 138.3, 131.1, 130.5, 129.8, 127.4,
125.1, 123.3, 121.9, 121.6, 120.3, 112.6, 111.7, 61.5, 57.5, 54.4, 51.6, 23.2, 17.3, 6.4.
1ꢀ(4ꢀmethoxyphenyl)ꢀ3ꢀ(3ꢀ((1ꢀ(3ꢀ(triethoxysilyl)propyl)ꢀ1Hꢀ1,2,3ꢀtriazolꢀ4ꢀ
yl)methoxy)phenyl)propꢀ2ꢀenꢀ1ꢀone (4e): Yield: 90%, Empirical formula: C28H37N3O6Si;
Anal. Calcd: C, 62.3; H, 6.9; N, 7.8; Found: C, 62.1; H, 6.7; N, 7.7; IR (neat, cm–1): 2975,
2938, 2889, 1659, 1600, 1484, 1389, 1251, 1168, 1075, 906, 831, 785, 724, 646, 616, 541. 1H
3
3
NMR (400 MHz, CDCl3) δ = 7.97 (d, J = 8.8 Hz, 2H), 7.68 (d, J = 15.6 Hz, 1H), 7.58 (s,
3
3
3
1H), 7.46 (d, J = 15.6 Hz, 1H), 7.26 (t, J = 7.9 Hz, 1H), 7.19 (dd, J = 10.5, 4.9 Hz, 2H),
3
3
3
6.97 (dd, J = 8.1, 2.1 Hz, 2H), 6.91 (d, J = 8.8 Hz, 2H), 5.19 (s, 2H), 4.30 (t, J = 7.2 Hz,
3
3
2H), 3.81 (s, 3H), 3.73 (q, J = 7.0 Hz, 6H), 2.01 – 1.90 (m, 2H), 1.14 (t, J = 7.0 Hz, 9H),
0.62 – 0.44 (m, 2H). 13C NMR (101 MHz, CDCl3) δ = 187.6, 162.5, 157.6, 142.7, 135.6,
131.1, 129.9, 127.5, 121.8, 121.3, 120.7, 115.7, 113.3, 112.9, 57.5, 54.5, 51.5, 17.3, 6.4.
1ꢀ(4ꢀmethoxyphenyl)ꢀ3ꢀ(4ꢀ((1ꢀ(3ꢀ(triethoxysilyl)propyl)ꢀ1Hꢀ1,2,3ꢀtriazolꢀ4ꢀ
yl)methoxy)phenyl)propꢀ2ꢀenꢀ1ꢀone (4f): Yield: 87%, Empirical formula: C28H37N3O6Si;
Anal. Calcd: C, 62.3; H, 6.9; N, 7.8; Found: C, 62.4; H, 6.7; N, 7.5; IR (neat, cm–1): 2973,
2933, 2884, 1656, 1600, 1507, 1422, 1390, 1257, 1216, 1165, 1074, 958, 905, 787, 725, 646.
1H NMR (400 MHz, CDCl3) δ = 7.98 (d, 3J = 8.8 Hz, 2H), 7.75 (m, 2H), 7.63 (d, 3J = 4.4 Hz,
3
3
3
1H), 7.54 (d, J = 8.6 Hz, 1H), 7.42 (d, J = 9.6 Hz, 1H), 7.37 (s, 1H), 7.06 (d, J = 8.6 Hz,
3
3
1H), 6.98 (d, J = 8.6 Hz, 1H), 6.92 (d, J = 8.8 Hz, 1H), 5.22 (d, 3J = 14.5 Hz, 2H), 4.33 (t,
3J = 7.1 Hz, 2H), 3.82 (s, 3H), 3.76 (q, 3J = 7.0 Hz, 6H), 2.09 – 1.91 (m, 2H), 1.17 (t, 3J = 7.0
Hz, 9H), 0.61 – 0.50 (m, 2H). 13C NMR (101 MHz, CDCl3) δ = 187.6, 162.3, 142.4, 131.1,
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130.3, 129.7, 129.0, 127.7, 127.3, 122.0, 118.8, 114.1, 112.9, 61.1, 57.4, 54.5, 51.5, 23.2,
17.3, 6.4. HRMS (ES+) Calcd for [M+Na]+ 562.2245; Found 762.2288.
1ꢀ(2,4ꢀdimethoxyphenyl)ꢀ3ꢀ(2ꢀ((1ꢀ(3ꢀ(triethoxysilyl)propyl)ꢀ1Hꢀ1,2,3ꢀtriazolꢀ4ꢀ
yl)methoxy)phenyl)propꢀ2ꢀenꢀ1ꢀone (4g): Yield: 88%, Empirical formula: C29H39N3O7Si;
Anal. Calcd: C, 61.1; H, 6.9; N, 7.4; Found: C, 60.9; H, 6.7; N, 7.5; IR (neat, cm–1): 2970,
2925, 2880, 2839, 1687, 1597, 1577, 1507, 1422, 1310, 1248, 1212, 1108, 1073, 1018, 827,
1
759, 643. H NMR (400 MHz, CDCl3) δ = 7.84 – 7.64 (m, 3H), 7.64 – 7.53 (m, 1H), 7.53 –
7.44 (m, 2H), 7.43 – 7.15 (m, 1H), 7.07 – 6.99 (m, 1H), 6.96 – 6.89 (m, 2H), 6.55 – 6.34 (m,
3
2H), 5.22 (s, 2H), 4.30 (td, J = 7.2, 3.1 Hz, 2H), 3.83 (s, 3H), 3.79 (s, 3H), 3.86 – 3.76 (m,
3H), 3.76 (q, 3J = 7.0 Hz, 6H), 2.08 – 1.87 (m, 2H), 1.12 (t, 3J = 7.0 Hz, 9H), 0.60 – 0.42 (m,
2H). 13C NMR (101 MHz, CDCl3) δ = 190.8, 163.2, 143.0, 141.8, 132.8, 131.9, 130.6, 130.3,
130.0, 122.9, 115.1, 105.1, 98.7, 62.2, 58.6, 52.6, 24.2, 18.3, 7.5.
1ꢀ(2,4ꢀdimethoxyphenyl)ꢀ3ꢀ(3ꢀ((1ꢀ(3ꢀ(triethoxysilyl)propyl)ꢀ1Hꢀ1,2,3ꢀtriazolꢀ4ꢀ
yl)methoxy)phenyl)propꢀ2ꢀenꢀ1ꢀone (4h): Yield: 89%, Empirical formula: C29H39N3O7Si;
Anal. Calcd: C, 61.1; H, 6.9; N, 7.4; Found: C, 61.0; H, 6.8; N, 7.2; IR (neat, cm–1): 2970,
2924, 2886, 1689, 1658, 1597, 1576, 1508, 1462, 1438, 1421, 1358, 1254, 1210, 1106, 1159,
1
1072, 1022, 826, 799, 721, 695, 644, 568, 541. H NMR (400 MHz, CDCl3) δ = 7.65 (d, 3J =
8.4 Hz, 1H), 7.55 (d, 3J = 13.8 Hz, 2H), 7.46 (d, 3J = 7.8 Hz, 2H), 7.31 (d, 3J = 15.7 Hz, 2H),
6.92 (d, 3J = 8.0 Hz, 2H), 6.47 (d, 3J = 8.3 Hz, 2H), 6.41 (s, 1H), 5.16 (s, 2H), 4.27 (t, 3J = 7.2
Hz, 2H), 3.81 (s, 3H), 3.78 (s, 3H), 3.72 (dd, 3J = 13.4, 6.6 Hz, 6H), 2.02 – 1.90 (m, 2H), 1.13
3
(t, J = 6.7 Hz, 9H), 0.61 – 0.43 (m, 2H). 13C NMR (101 MHz, CDCl3) δ = 189.5, 163.0,
159.3, 158.8, 142.5, 140.8, 131.7, 131.1, 128.9, 127.6, 124.3, 121.9, 121.4, 114.1, 104.2,
61.1, 57.5, 54.7, 54.5, 51.5, 23.2, 17.3, 6.4.
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1ꢀ(2,4ꢀdimethoxyphenyl)ꢀ3ꢀ(4ꢀ((1ꢀ(3ꢀ(triethoxysilyl)propyl)ꢀ1Hꢀ1,2,3ꢀtriazolꢀ4ꢀ
yl)methoxy)phenyl)propꢀ2ꢀenꢀ1ꢀone (4i): Yield: 87%, Empirical formula: C29H39N3O7Si;
Anal. Calcd: C, 61.1; H, 6.9; N, 7.4; Found: C, 61.3; H, 6.8; N, 7.2; IR (neat, cm–1): 2962,
1
2922, 2884, 1658, 1601, 1509, 1443, 1417, 1257, 1166, 1074, 1010, 864, 786, 695, 541. H
3
NMR (400 MHz, CDCl3) δ = 7.96 (d, 3J = 16.0 Hz, 1H), 7.66 (d, J = 8.6 Hz, 1H), 7.55 (dd,
3
3
3J = 10.0, 2.1 Hz, 2H), 7.46 (d, J = 15.9 Hz, 1H), 7.42 – 7.28 (m, 1H), 7.24 (dd, J = 12.0,
3
3
5.1 Hz, 1H), 7.17 (dd, 3J = 13.9, 8.1 Hz, 1H), 7.01 (d, J = 8.3 Hz, 1H), 6.91 (t, J = 7.6 Hz,
3
4
3
1H), 6.47 (dd, J = 8.6, 2.2 Hz, 1H), 6.40 (d, J = 2.2 Hz, 1H), 5.23 (s, 2H), 4.26 (t, J = 7.2
3
Hz, 2H), 3.78 (s, 3H), 3.77 (s, 3H), 3.72 (q, 3J = 7.0 Hz, 6H), 2.04 – 1.92 (m, 2H), 1.12 (t, J
= 7.0 Hz, 9H), 0.60 – 0.43 (m, 2H). 13C NMR (101 MHz, CDCl3) δ = 189.8, 163.1, 159.4,
156.2, 142.9, 136.1, 131.8, 130.3, 127.5, 126.7, 123.8, 121.7, 121.4, 120.3, 111.9, 104.2,
97.6, 61.9, 57.5, 54.7, 54.5, 51.6, 23.2, 17.3, 6.4. HRMS (ES+) Calcd for [M+Na]+ 592.2455;
Found 592.2365.
Spectroscopic data for compounds 6a–6i
1,3ꢀbis(2ꢀ((1ꢀ(3ꢀ(triethoxysilyl)propyl)ꢀ1Hꢀ1,2,3ꢀtriazolꢀ4ꢀyl)methoxy)phenyl)propꢀ2ꢀenꢀ
1ꢀone (6a): Yield: 90%, Empirical formula: C39H58N6O9Si2; Anal. Calcd: C, 57.8; H, 7.2; N,
10.4; Found: C, 57.5; H, 7.1; N, 10.2; IR (neat, cm–1): 3072, 3024, 2921, 2891, 1652, 1597,
1482, 1448, 1371, 1327, 1287, 1216, 1162, 1105, 1050, 1015, 925, 872, 834, 820, 748, 676,
633, 584. 1H NMR (400 MHz, CDCl3) δ = 8.00 (d, 3J = 7.3 Hz, 2H), 7.67 (s, 2H), 7.60 (d, 3J
= 8.6 Hz, 2H), 7.48 (d, 3J = 7.1 Hz, 2H), 7.44 (s, 2H), 7.03 (d, 3J = 8.6 Hz, 2H), 5.25 (s, 4H),
4.37 (t, 3J = 7.2 Hz, 4H), 3.81 (dd, 3J = 14.4, 7.4 Hz, 12H), 2.08 – 1.96 (m, 4H), 1.21 (t, 3J =
7.0 Hz, 18H), 0.65 – 0.53 (m, 4H). 13C NMR (101 MHz, CDCl3) δ = 189.5, 159.3, 143.5,
137.4, 131.6, 130.9, 129.2, 127.5, 127.1, 127.1, 126.5 (m), 126.5, 119.1, 114.2, 61.1, 57.5,
52.7, 51.5, 23.2, 21.6, 17.3, 6.5.
10

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1ꢀ(2ꢀ((1ꢀ(3ꢀ(triethoxysilyl)propyl)ꢀ1Hꢀ1,2,3ꢀtriazolꢀ4ꢀyl)methoxy)phenyl)ꢀ3ꢀ(3ꢀ((1ꢀ(3ꢀ
(triethoxysilyl)propyl)ꢀ1Hꢀ1,2,3ꢀtriazolꢀ4ꢀyl)methoxy)phenyl)propꢀ2ꢀenꢀ1ꢀone
(6b):
Yield: 90%, Empirical formula: C39H58N6O9Si2; Anal. Calcd: C, 57.8; H, 7.2; N, 10.4; Found:
C, 57.6; H, 7.0; N, 10.5; IR (neat, cm–1): 2972, 2929, 2885, 1660, 1598, 1484, 1448, 1389,
1
1286, 1236, 1162, 1071, 954, 783, 757, 696, 542. H NMR (400 MHz, CDCl3) δ = 7.58 (dd,
3
3
3J = 13.8, 7.4 Hz, 2H), 7.48 (d, J = 15.9 Hz, 1H), 7.39 (dd, J = 19.2, 11.6 Hz, 2H), 7.32 –
7.13 (m, 2H), 7.05 (dd, 3J = 12.8, 8.3 Hz, 2H), 7.01 – 6.87 (m, 2H), 6.81 (s, 1H), 5.24 (s, 2H),
3
3
3
5.14 (s, 2H), 4.28 (t, J = 7.1 Hz, 2H), 4.04 (t, J = 7.1 Hz, 2H), 3.71 (td, J = 13.8, 6.9 Hz,
12H), 1.94 (dd, 3J = 15.4, 7.6 Hz, 2H), 1.88 – 1.67 (m, 2H), 1.28 – 0.94 (m, 18H), 0.56 – 0.47
(m, 2H), 0.46 – 0.32 (m, 2H). 13C NMR (101 MHz, CDCl3) δ = 191.3, 157.7, 155.9, 141.2,
135.5, 132.2, 131.2 , 130.8, 129.6, 129.0, 128.5, 127.5, 126.7, 121.9, 120.7, 115.9, 113.2,
112.1, 61.9, 61.1, 57.5, 51.4, 23.1, 17.3, 6.4.
1ꢀ(2ꢀ((1ꢀ(3ꢀ(triethoxysilyl)propyl)ꢀ1Hꢀ1,2,3ꢀtriazolꢀ4ꢀyl)methoxy)phenyl)ꢀ3ꢀ(4ꢀ((1ꢀ(3ꢀ
(triethoxysilyl)propyl)ꢀ1Hꢀ1,2,3ꢀtriazolꢀ4ꢀyl)methoxy)phenyl)propꢀ2ꢀenꢀ1ꢀone
(6c):
Yield: 89%, Empirical formula: C39H58N6O9Si2; Anal. Calcd: C, 57.8; H, 7.2; N, 10.4; Found:
C, 57.5; H, 7.1; N, 10.2; IR (neat, cm–1): 2972, 2929, 2885, 1654, 1597, 1508, 1482, 1449,
1
1422, 1389, 1330, 1292, 1239, 1172, 1071, 1026, 954, 858, 752, 542. H NMR (400 MHz,
3
CDCl3) δ = 7.68 – 7.59 (m, 2H), 7.58 – 7.50 (m, 2H), 7.46 (d, 3J = 7.5 Hz, 3H), 7.29 (t, J =
3
3
3
7.9 Hz, 1H), 7.13 (d, J = 8.3 Hz, 1H), 7.05 (t, J = 7.5 Hz, 1H), 6.98 (t, J = 8.3 Hz, 2H),
5.30 (s, 2H), 5.23 (s, 2H), 4.36 (t, 3J = 6.7 Hz, 2H), 4.17 (t, 3J = 7.2 Hz, 2H), 3.87 – 3.69 (m,
12H), 2.08 – 1.97 (m, 2H), 1.93 – 1.83 (m, 2H), 1.20 (dt, 3J = 10.1, 3.5 Hz, 18H), 0.63 – 0.55
(m, 2H), 0.54 – 0.42 (m, 2H). 13C NMR (101 MHz, CDCl3) δ = 192.6, 160.1, 156.7, 143.6,
143.3, 142.1, 132.8, 130.4, 130.0, 129.8, 128.1, 125.4, 122.8, 121.4, 115.2, 113.1, 67.9, 63.0,
62.1, 58.5, 52.5, 24.1, 18.2, 11.3, 7.4. MS (ES+) Calcd for [M+Na]+ 833.4; Found 833.5.
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DOI: 10.1039/C4RA08724K
3ꢀ(2ꢀ((1ꢀ(3ꢀ(triethoxysilyl)propyl)ꢀ1Hꢀ1,2,3ꢀtriazolꢀ4ꢀyl)methoxy)phenyl)ꢀ1ꢀ(3ꢀ((1ꢀ(3ꢀ
(triethoxysilyl)propyl)ꢀ1Hꢀ1,2,3ꢀtriazolꢀ4ꢀyl)methoxy)phenyl)propꢀ2ꢀenꢀ1ꢀone
(6d):
Yield: 87%, Empirical formula: C39H58N6O9Si2; Anal. Calcd: C, 57.8; H, 7.2; N, 10.4; Found:
C, 57.5; H, 7.0; N, 10.1; IR (neat, cm–1): 2973, 2926, 2884, 1660, 1594, 1577, 1485, 1438,
1389, 1328, 1273, 1240, 1164, 1099, 1071, 1007, 954, 867, 782, 750, 721, 696, 679, 593,
1
3
3
541. H NMR (400 MHz, CDCl3) δ = 7.68 (d, J = 15.7 Hz, 1H), 7.60 (d, J = 4.3 Hz, 2H),
3
3
7.55 (d, J = 7.2 Hz, 1H), 7.45 (s, 1H), 7.40 – 7.31 (m, 2H), 7.23 (dd, J = 9.5, 6.4 Hz, 2H),
3
3
3
7.15 (dd, J = 15.6, 7.7 Hz, 2H), 6.97 (d, J = 8.1 Hz, 1H), 5.26 – 5.04 (m, 4H), 4.29 (t, J =
7.1 Hz, 4H), 3.78 – 3.65 (m, 12H), 2.04 – 1.83 (m, 4H), 1.22 – 1.05 (m, 18H), 0.59 – 0.43 (m,
4H). 13C NMR (101 MHz, CDCl3) δ = 188.9, 157.6, 156.9, 143.6, 142.5, 138.5, 135.3, 131.0,
128.9, 128.7, 128.4, 127.4, 121.3, 120.8, 120.5, 118.9, 116.0, 113.1, 61.1, 57.5, 51.5, 23.2,
17.3, 6.4.
1,3ꢀbis(3ꢀ((1ꢀ(3ꢀ(triethoxysilyl)propyl)ꢀ1Hꢀ1,2,3ꢀtriazolꢀ4ꢀyl)methoxy)phenyl)propꢀ2ꢀenꢀ
1ꢀone (6e): Yield: 83%, Empirical formula: C39H58N6O9Si2; Anal. Calcd: C, 57.8; H, 7.2; N,
10.4; Found: C, 57.5; H, 7.1; N, 10.2; IR (neat, cm–1): 3019, 2977, 2881, 1651, 1593, 1521,
1
1484, 1390, 1325, 1214, 1161, 1075, 1033, 1015, 742, 669, 665, 627, 542. H NMR (400
MHz, CDCl3) δ = 8.05 (d, 3J = 15.8 Hz, 1H), 7.60 (s, 1H), 7.57 (s, 2H), 7.54 – 7.45 (m, 2H),
7.39 (dd, 3J = 8.5, 1.9 Hz, 1H), 7.36 – 7.27 (m, 2H), 7.13 (dd, 3J = 8.1, 2.3 Hz, 1H), 7.05 (d,
3
3
3J = 8.3 Hz, 1H), 6.95 (t, J = 7.5 Hz, 1H), 5.26 (s, 2H), 5.19 (s, 2H), 4.29 (q, J = 7.1 Hz,
3
4H), 3.81 – 3.64 (m, 12H), 2.02 – 1.88 (m, 4H), 1.13 (dd, J = 13.6, 6.9 Hz, 18H), 0.53 (dd,
3J = 16.5, 8.5 Hz, 4H). 13C NMR (101 MHz, CDCl3) δ = 189.3, 157.5, 156.5, 139.2, 138.8,
131.2, 130.4, 128.7, 128.3, 127.5, 123.2, 121.8, 120.5, 118.6, 113.1, 111.8, 61.6, 61.2, 57.5,
51.6, 23.2, 17.3, 6.4.
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1ꢀ(3ꢀ((1ꢀ(3ꢀ(triethoxysilyl)propyl)ꢀ1Hꢀ1,2,3ꢀtriazolꢀ4ꢀyl)methoxy)phenyl)ꢀ3ꢀ(4ꢀ((1ꢀ(3ꢀ
(triethoxysilyl)propyl)ꢀ1Hꢀ1,2,3ꢀtriazolꢀ4ꢀyl)methoxy)phenyl)propꢀ2ꢀenꢀ1ꢀone
(6f):
Yield: 84%, Empirical formula: C39H58N6O9Si2; Anal. Calcd: C, 57.8; H, 7.2; N, 10.4; Found:
C, 57.9; H, 7.0; N, 10.1; IR (neat, cm–1): 2972, 2935, 2885, 1660, 1572, 1508, 1484, 1438,
1389, 1289, 1242, 1172, 1071, 1010, 954, 783, 722, 696, 542. 1H NMR (400 MHz, CDCl3) δ
= 7.62 – 7.55 (m, 3H), 7.54 (d, 3J = 8.5 Hz, 2H), 7.51 – 7.26 (m, 3H), 7.16 (dd, 3J = 14.0, 6.4
3
3
Hz, 2H), 6.96 (d, 3J = 8.6 Hz, 2H), 5.20 (d, J = 8.4 Hz, 4H), 4.30 (t, J = 7.2 Hz, 4H), 3.73
3
3
(q, J = 7.0 Hz, 12H), 2.03 – 1.88 (m, 4H), 1.14 (t, J = 7.0 Hz, 18H), 0.63 – 0.39 (m, 4H).
13C NMR (101 MHz, CDCl3) δ = 189.0, 159.3, 157.5, 143.6, 142.5, 138.9, 131.1, 129.3,
128.7, 127.5, 127.1, 121.9, 120.4, 119.0, 118.4, 114.2, 113.2, 61.2, 57.5, 51.5, 23.2, 17.3, 6.5.
3ꢀ(2ꢀ((1ꢀ(3ꢀ(triethoxysilyl)propyl)ꢀ1Hꢀ1,2,3ꢀtriazolꢀ4ꢀyl)methoxy)phenyl)ꢀ1ꢀ(4ꢀ((1ꢀ(3ꢀ
(triethoxysilyl)propyl)ꢀ1Hꢀ1,2,3ꢀtriazolꢀ4ꢀyl)methoxy)phenyl)propꢀ2ꢀenꢀ1ꢀone
(6g):
Yield: 89%, M.Pt: 83 ℃, Empirical formula: C39H58N6O9Si2; Anal. Calcd: C, 57.8; H, 7.2; N,
10.4; Found: C, 57.6; H, 6.9; N, 10.2; IR (neat, cm–1): 2982, 2884, 1620, 1554, 1457, 1368,
1321, 1218, 1160, 1052, 1025, 1007, 922, 820, 752, 725, 663, 618, 541. 1H NMR (400 MHz,
CDCl3) δ = 8.08 – 7.99 (m, 1H), 7.98 – 7.88 (m, 2H), 7.63 – 7.54 (m, 3H), 7.39 (td, 3J = 7.4,
2.9 Hz, 1H), 7.32 – 7.25 (m, 1H), 7.01 – 6.92 (m, 4H), 5.26 (s, 2H), 5.22 (s, 2H), 4.32 – 4.26
(m, 4H), 3.83 – 3.62 (m, 12H), 1.98 – 1.91 (m, 4H), 1.23 – 1.07 (m, 18H), 0.55 – 0.48 (m,
4H). 13C NMR (101 MHz, CDCl3) δ = 188.0, 161.9, 161.0, 160.8, 157.5, 143.5, 143.3, 139.6,
139.4, 138.3, 132.0, 131.3, 131.1, 130.5, 129.7, 129.4, 128.3, 121.8, 120.3, 114.6, 113.5,
112.9, 111.8, 62.7, 62.3, 61.6, 61.2, 58.6, 57.5, 52.7, 51.6, 24.3, 23.2, 18.4, 17.3, 7.5, 6.4.
3-(3-((1-(3-(triethoxysilyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)-1-(4-((1-(3-
(triethoxysilyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)prop-2-en-1-one (6h): Yield:
87%, Empirical formula: C39H58N6O9Si2; Anal. Calcd: C, 57.8; H, 7.2; N, 10.4; Found: C,
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DOI: 10.1039/C4RA08724K
57.5; H, 7.1; N, 10.1; IR (neat, cm–1): 3019, 2994, 2886, 1521, 1455, 1322, 1217, 1163, 1033,
1
3
3
929, 742, 670, 542. H NMR (400 MHz, CDCl3) δ = 7.94 (d, J = 8.7 Hz, 1H), 7.81 (d, J =
3
3
8.8 Hz, 1H), 7.62 (d, J = 15.4 Hz, 1H), 7.55 (d, 3J = 16.2 Hz, 1H), 7.44 (s, 1H), 7.38 (dd, J
3
3
3
= 9.1, 6.4 Hz, 1H), 7.21 (t, J = 6.6 Hz, 1H), 7.12 (d, J = 7.8 Hz, 1H), 6.98 (d, J = 8.7 Hz,
3
3
2H), 6.94 – 6.87 (m, 2H), 5.20 (d, J = 12.1 Hz, 4H), 4.29 (t, J = 7.1 Hz, 4H), 3.77 – 3.64
3
(m, 12H), 2.02 – 1.88 (m, 4H), 1.13 (t, J = 7.0 Hz, 18H), 0.53 – 0.42 (m, 4H). 13C NMR
(101 MHz, CDCl3) δ = 186.1, 160.7, 157.5, 142.3, 131.0, 130.4, 130.6, 131.0, 129.6, 128.7,
127.3, 120.9, 115.6, 113.4, 112.9, 61.1, 57.3, 51.1, 23.1, 17.3, 6.3.
1,3-bis(4-((1-(3-(triethoxysilyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)prop-2-en-1-
one (6i): Yield: 89%, Empirical formula: C39H58N6O9Si2; Anal. Calcd: C, 57.8; H, 7.2; N,
10.4; Found: C, 57.6; H, 7.1; N, 10.2; IR (neat, cm–1): 2982, 2884, 1653, 1550, 1456, 1321,
1
1249, 1051, 1017, 1006, 923, 820, 750, 726,663, 542. H NMR (400 MHz, CDCl3) δ = 7.93
(d, 3J = 8.5 Hz, 1H), 7.93 (d, 3J = 8.5 Hz, 1H), 7.83 (d, 3J = 8.7 Hz, 1H), 7.74 (d, 3J = 8.5 Hz,
1H), 7.61 – 7.54 (m, 1H), 7.50 (d, 3J = 8.5 Hz, 1H), 7.40 (d, 3J = 7.6 Hz, 1H), 7.35 – 7.19 (m,
3
1H), 7.21 (s, 1H), 7.04 – 6.92 (m, 1H), 6.97 (ddd, J = 13.1, 11.2, 5.5 Hz, 2H), 5.37 – 4.97
(m, 2H), 5.34 – 5.02 (m, 2H), 4.29 (t, 3J = 7.0 Hz, 2H), 4.29 (t, 3J = 7.0 Hz, 2H), 3.72 (q, 3J =
7.0 Hz, 6H), 3.72 (q, 3J = 7.0 Hz, 6H), 2.08 – 1.66 (m, 2H), 2.01 – 1.91 (m, 2H), 1.14 (t, 3J =
3
6.9 Hz, 9H), 1.14 (t, J = 6.9 Hz, 9H), 0.54 – 0.45 (m, 2H), 0.59 – 0.38 (m, 2H). 13C NMR
(101 MHz, CDCl3) δ = 194.6, 188.8, 186.8, 161.9, 160.9, 158.9, 142.5, 130.9, 129.5, 128.9,
127.5, 127.1, 61.2, 57.3, 51.3, 23.1, 17.3, 6.4.
Result and discussion
Synthetic approach
Synthesis of conjugate chalcone based organotriethoxysilanes (OTES) was efficiently
achieved in a three step route. First step involves the synthesis of propynyloxybenzaldehydes
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2a(i–iii) and propynyloxyacetophenones 2b(i–iii) by reaction of oꢀ, mꢀ and pꢀisomers of
hydroxylbenzaldehyde and hydroxylacetophenone, respectively, with propargyl bromide in
presence of K2CO3 as base. The strong base helps in easy proton abstraction from hydroxyl
group and enhances the rate of forwarding reaction. Second step follows Claisen–Schmidt
condensation reaction of 2a with i) substituted acetophenones 1c(a–c) resulting into chalcone
substituted terminal alkynes 3a–3i, and ii) propynyloxyacetophenones 2b(i–iii) resulting into
chalcone substituted terminal di–alkynes product 5a–5i. The aldol condensation reaction was
carried out under strongly basic conditions provided by KOH, using ethanol as solvent. After
vigorous stirring for 4 h, the reaction progress was monitored by TLC (hexane:ethyl acetate
(8:2)). On completion, the reaction mixture was quenched by acidic (2N HCl) ice cold water
(till pH of 4 was attained) and the product was isolated by methylene dichloride in good
yield. Third and the final step of mono–OTES 4a–4i and di–OTES 6a–6i synthesis via
CuAAC reaction of chalcone based terminal alkynes with AzPTES was proceeded with
[CuBr(PPh3)3]/THF–TEA system at 65 ℃ for 3 h under inert conditions. The final products
have 1,2,3–triazole linking chalcone to nPTES were synthesized in good yield (Scheme 1,
Table 1).
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O
CHO
CHO
1c(aꢀc)
N3(CH2)3Si(OEt)3
THF:TEA (1:1)
Br
O
O
Ar
O
O
[CuBr(PPh3)3]
EtOH, KOH
2N HCl
4 h, rt
DMF , K2CO3
rt, 14 h
3 h, 65 o
C
Ar
Ar
HO
O
N
1a(iꢀiii)
3aꢀ3i
4aꢀ4i
2a(iꢀiii)
N
N
O
O
i
= ortho
ii = meta
iii = para
1c(a)
Ar =
(EtO)3Si
OCH3
OCH3
1c(b)
1c(c)
O
H3CO
CHO
CHO
Br
O
DMF , K2CO3
rt, 14 h
O
HO
1a(iꢀiii)
O
N3(CH2)3Si(OEt)3
THF:TEA (1:1)
2a(iꢀiii)
O
O
EtOH, KOH
+
[CuBr(PPh3)3]
3 h, 65 o
C
2N HCl
4 h, rt
O
O
O
O
N
N
N
5aꢀ5i
6aꢀ6i
N
N
N
Br
i
= ortho
ii = meta
iii = para
DMF , K2CO3
rt, 14 h
HO
1b(iꢀiii)
O
(EtO)3Si
2b(iꢀiii)
(EtO)3Si
Scheme 1: Synthesis of conjugate chalcone based nPTES (4a–4i) and (6a–6i)
Product ID/
Texture
Substrate
alkyne
Ar
Product
Yield
(%)
O
4a/
O
N
O
N
N
Dark red oil
91
OHC
Si(OEt)3
O
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O
N
N
N
4b/
Si(OEt)3
O
89
90
O
Chocolate
brown oil
O
OHC
O
O
4c/
Orange red
oil
O
O
N
N
N
Si(OEt)3
CHO
O
OCH3
4d/
Dark brown
oil
O
88
90
O
Si(OEt)3
N
N
OHC
N
O
OCH3
OCH3
O
4e/
Light brown
oil
Si(OEt)3
O
N
N
O
N
O
OCH3
OHC
O
O
4f/
Light brown
oil
O
H3CO
O
87
88
N
N
N
Si(OEt)3
OCH3
CHO
O
4g/
Thick yellow
oil
H3CO
O
N
O
N
N
OHC
OCH3
Si(OEt)3
OCH3
O
OCH3
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O
OCH3
4h/
Si(OEt)3
H3CO
Brown oil
89
87
O
N
OCH3
N
O
N
O
OCH3
OHC
OCH3
O
O
4i/
H3CO
O
O
H3CO
Dark red oil
N
N
N
Si(OEt)3
OCH3
CHO
Product ID/
Texture
Substrate
Substrate
Product
Yield
(%)
alkyne (A1) alkyne (A2)
O
O
6a/
O
N
O
N
O
N
O
N
N
N
Brown oil
90
90
OHC
(EtO)3Si
Si(OEt)3
O
N
Si(OEt)3
O
6b/
Dark brown
oil
N
O
O
N
N
N
N
O
O
(EtO)3Si
(EtO)3Si
OHC
O
N
6c/
Dark brown
solid
N
N
O
O
O
89
87
O
N
N
N
O
Si(OEt)3
CHO
O
6d/
O
O
N
Brown oil
O
O
O
OHC
N
N
N
(EtO)3Si
N
N
Si(OEt)3
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O
Si(OEt)3
N
O
N
N
O
N
N
83
84
6e/
O
N
O
O
Dark red oil
Si(OEt)3
OHC
O
N
6f/
Dark brown
oil
N
(EtO)3Si
N
O
O
O
O
Si(OEt)3
N
N
N
O
CHO
O
(EtO)3Si
6g/
Dark brown
solid
N
N
Si(OEt)3
N
N
N
N
O
O
O
89
87
O
OHC
O
O
N
N
N
(EtO)3Si
O
O
6h/
O
O
Brown oil
O
N
N
OHC
N
Si(OEt)3
O
O
6i/
Orange red
oil
O
N
N
N
O
(EtO)3Si
91
O
N
N
N
O
CHO
Si(OEt)3
Table 1: Synthesis of mono–nPTES (4a–4i) and di–nPTES (6a–6i) using ‘Click Silylation’ at
65 ℃ for 3 h
We proceeded in the two possible pathways (Scheme 2) for generation of chalcone based
triethoxysilane (4a) to investigate the affect on yield of product formed and rapidity in
hydrolysis of chalcone functionalised triethoxysilanes. In experiment, pathway 1 involves
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Claisen–Schmidt condensation of propynylbenzaldehyde 2 with differently substituted
acetophenones, followed by cycloaddition reaction with AzPTES resulting into OTES with
good yield and high purity. In comparison, following pathway 2, the first step involving click
reaction efficiently gives OTES with more than 90% yield, but the second step of chalcone
formation results into hydrolysis of triethoxysilanes as prominent reaction associated with
poor yield. This confirms the incapability of pathway 2 to synthesize nPTES and exclusively
follow pathway 1 for the efficient synthesis of nPTES.
CHO
O
O
N3(CH2)3Si(OEt)3
R
Pathway 2
CHO
Pathway 1
O
O
N
N
N
R
O
Si(OEt)3
O
N3(CH2)3Si(OEt)3
Not feasible
Feasible
O
R
R
O
N
N
N
Si(OEt)3
Scheme 2: Probable pathways followed for synthesis of OTES from a common scaffold
1
The synthesized OTES ((4a–4i) and (6a–6i)) clearly point outs in H NMR with two major
shifts i) 1.0 unit in triplet of –N3CH2CH2– after cyclization to heterocycle and ii) 5.0 units in
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the triplet at 2.5 ppm for terminal alkynes (4J = 2–3 Hz) 7.5 ppm. Silanes (6a–6i) exhibit
different multiplets arising due to each –CH2 unit of propyl chain arm in nPTES (NMR A and
NMR B, Figure 2). NMR A (6a) show unsplit multiplets of two propyl chains while NMR B
(6c) marks out the separate identity to each –CH2 unit of the two –N3CH2CH2CH2Si– chains.
In case of 13C NMR, each carbon of nPTES (6a–6i) appeared as separate singlet giving
unique identity to each carbon atom and show significant chemical shift. The E (trans)
geometry of the chalcone double bond was evident by the large olefinic coupling constant (J
= 14–16 Hz) between the relevant signals in the 1H NMR spectrum.
NMR A
NMR B
Figure 2: NMR A (6a) reveals unsplit multiplets due to two propyl chain arms; NMR B (6c)
shows isolated multiplets for each propyl chain arm
Previously reported by Cattoen et al,20 hybrid silica precursors were synthesized under
microwave conditions in excellent yield. We herein report optimized thermal reaction
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conditions (4a, Table 2) for synthesis of chalcone stapled to nPTES via 1,2,3ꢀtriazole. The
affect of variation in temperature, reaction time along with catalyst loading was keenly
examined. On performing reaction at room temperature with 0.01 mmol of catalyst loading,
only 20% of product was isolated. With increase in catalyst loading to 0.02 mmol and
extending time duration, no significant improvement in product yield was observed. But on
raising the temperature of reaction mixture to 65 ℃ and vigorous stirring for 3 h drastically
improved yield from 24% to 91%. Further raising temperature under similar reaction
conditions does not alter product yield. Therefore, thermally optimised reaction conditions
for OTES 4a–4i and 6a–6i following Click silylation are as per entry 7 in table 2.
Entry Catalyst (%)
Reaction conditions
rt, st
Reaction Duration (h) aYield (%)
1
1 x 10–5
1 x 10–5
1 x 10–5
1 x 10–5
1 x 10–5
1 x 10–5
1 x 10–5
1 x 10–5
1 x 10–5
1 x 10–5
1 x 10–5
3
3
3
3
1
3
3
5
3
3
3
20
24
76
81
45
86
91
91
91
85
91
2
rt, st
3
55 ℃, st
55 ℃, st
65 ℃, st
65 ℃, st
65 ℃, st
65 ℃, st
65 ℃, st
75 ℃, st
75 ℃, st
4
5
6
7
8
9
10
11
aDetermined by 1H NMR analysis of crude sample
Table 2: Optimization of reaction conditions under thermal environmental conditions
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Solvatochromism
As defined by Hantzschlater, solvatochromism refers to the change in the position, intensity,
and shape of absorption bands intensity maxima in UV–Vis spectroscopy with change of
solvent polarity.50 The major factor governing this phenomenon involves variation in solute–
solvent interactions with change in solvent polarity that affects position, shape and intensity
of absorption bands.51 The preferential solvation taking place in vicinity of solute molecule
alters the salvation shell arrangements, thereby shifting maxima of wavelength absorbed.
Negative solvatochromism corresponds to hypsochromic shift (blue shift) whereas reverse is
referred to positive solvatochromism or bathochromic shift (red shift).52 It is for the first time,
to the best of our knowledge, that solavotochromic effect is being reported for OTES.
The term ‘solvent polarity’ lacks an exact definition, although numerous attempts have been
made so far, but it broadly describes all of the intermolecular interactions of solute with
solvent. The important point concerning the so called ‘polarity of a solvent’ is the overall
solvation capability, which is the cumulative effect of all the solvent–solute interactions,
excluding those such as protonation, oxidation, reduction, complexation, etc., that might lead
to a chemical change of the solute.53 A number of scales have been established to quantify the
influence of solvent on chemical properties. These scales are based on some physicochemical
property, which could be an equilibrium constant, reaction rate constant, spectral shift using
absorption spectroscopy, etc.54 We followed the Dimroth and Reichardt polarity scale to
examine the effect of variation in polarity solvents on various PfTES.
The observed solvatochromism depends upon the chemical structure and physical properties
of the chromophore and their interaction with the solvent molecules which, in turn determines
the strength of the intermolecular solute/solvent interactions in the equilibrium ground state
and the FranckꢀCondon excited state. Typically, a large change in dipole moment upon
excitation exhibits strong solvatochromism. In addition, the ability of solute molecules to
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donate or to accept hydrogen bonds to or from surrounding solvent molecules in its ground
and FranckꢀCondon excited state determines further the extent and sign of its
solvatochromism. The pronounced shift in the position of the absorption bands is due to
solvent–induced change in electronic ground state structure from a less dipolar (in less polar
solvents) to a more dipolar chromophore (in high polar solvents) with increasing solvent
polarity.
Moreover, the ability of solute to form hydrogen bond with solvent molecules in ground state
and Franck Condon excited state too determines the extent of solvatochromism exhibited
which is a result of π–π* transitions. Typically, solvatochromic compounds can be described
by extreme resonance contributing structures. The change in absorption band with solvent
arises from variation in contribution of these conjugated π electronic systems.55
To examine solvatochromic effects on chalcone modified OTES, UV–Vis photoelectronic
study was performed using chloroform, acetonitrile and ethanol (with relative polarity of
0.26, 0.46, 0.65 as per Dimroth and Reichardt polarity scale) as solvent media (shown for 4b
in Figure 3, for other see supporting information) within the concentration range of 0.1 mM–
1 mM. Non–polar solvents proved to be inefficient owing to their incapability to solubilize
the compounds; while other polar solvents were ineffective as they have λmax cut off value
above absorption maxima intensity region. The effect on absorption maxima due to different
substituents and positional isomers was observed and plotted as shown in Figure 4. With
increase in polarity of a solvent, the shift in λmax value for triethoxysilanes was observed to be
both bathochromic and hypsochromic. Beginning with chloroform, the absorption maxima
for all nPTES lie in the region between 272–331 nm (4a–4i), with maximum moieties
absorbing in the region of 300 and 320 nm. With increase in polarity using acetonitrile,
bathochromic shift was observed for 4c, 4d, and 4h. Further increasing polarity using
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ethanol, the most polar of three, red and blue shifts were observed for 4a, 4b, 4e, 4f, 4g and
4i, and 4c, 4d and 4h, respectively (Table 3).
3.0
2.5
2.0
EtOH
MeCN
CHCl3
1.5
1.0
0.5
0.0
240
260
280
300
320
340
360
380
400
Wavelength (nm)
Figure 3: UV–Vis spectra of compound 4b in different solvents at 25 ℃ (1 mM)
Sample ID
λmax (nm)
Chloroform
Acetonitrile
Ethanol
4a
4b
4c
4d
4e
4f
4g
4h
4i
302 300,250 322
293 298,247 333
295 301,249 327
314
318
314
318
314
318
327
324
328
272
271
272
331
335
329
295
294
296
Table 3: The absorption maxima values of 4a–4i (0.1 mM–1 mM) in various solvents at 25
℃
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400
350
300
250
200
150
100
50
Chloroform
Acetonitile
Ethanol
0
4a
4b
4c
4d
4e
4f
4g
4h
4i
Sample ID
Figure 4: Plot depicting the effect of polarity change using CHCl3, CH3CN and C2H5OH on
OTES 4a–4i
Di nPTES 6a–6i exhibit distinct behavior for shift in λmax value (shown for 6c in Figure 5 for
other see supporting information) as compared to mono nPTES 4a–4i in these solvents. The
decrease or increase in solvent polarity using chloroform, acetonitrile and ethanol, the π
electron cloud remains conjugated and hence appear significantly in absorption spectra. With
increase in polarity of solvent using acetonitrile, the hypsochromic shift was observed for all
compounds except 6d, 6h and 6i which exhibit bathochromic shift (Figure 6, Table 4). On
further raising polarity of solvent system using ethanol, red shift in absorption maxima was
recorded for 6b, 6c, 6f, 6g and 6i while other significantly exhibit hypsochromic shift in
wavelength.
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EtOH
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
MeCN
CHCl3
260
280
300
320
340
360
380
400
420
Wavelength (nm)
Figure 5: UV–Vis spectra of compound 6c in different solvents at 25 ℃ (0.5 mM)
Sample ID
max (nm)
6a
6b
6c
6d
6e
6f
6g
6h
6i
λ
Chloroform 332,298 301
Acetonitrile 328,291 298
297
293
299
274
334
303
265
313
301
337
324
335
301
299
304
263
314
274
328
Ethanol
328
302
265 330,275
Table 4: Representation of absorption maxima values of 6a–6i (0.1 mM–1 mM) in various
solvents at 25 ℃
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400
350
300
250
200
150
100
50
Chloroform
Acetonitile
Ethanol
0
6a
6b
6c
6d
6e
6f
6g
6h
6i
Sample ID
Figure 6: Plot depicting the effect of polarity change using CHCl3, CH3CN and C2H5OH on
OTES 6a–6i
Solvatochromic study performed on a series of nPTES illustrating the effect of polarity
change show best activity for silanes using chloroform and acetonitrile as solvent media in
UV–Vis spectra. Moreover, on consideration of different polarity scale which alters the
polarity index of solvents used, there will be change in corresponding batho– and hypso–
chromic shifts observed. OTES being hydrolytically unstable have limited solubility and
solvent tolerance. Its dissolution in protonating solvent like ethanol may cause hindrance
towards absorption spectra for some OTES that may actually be active in other aprotic polar
solvents. The rapidity in hydrolysis of OTES tends to overcome the solubility in protic
solvents at high concentrations but at concentration of 10–3 M or lower, we can study their
photophysical properties.
Conclusions
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We have synthesized 1,2,3ꢀtriazolyl chalcone linked n–propyltriethoxysilanes (4a–4i and 6a–
6i) by an efficient and promising methodology. The optimized thermal reaction conditions for
CuAAC, using ‘[CuBr(PPh3)3]/THF–TEA’ system, prove their excellence for the product
formation at 65 ℃ in 3 h. The variation in substituent moiety and effect of positional
isomerism in nPTES considerably affected the absorption spectral properties as displayed in
UV–Vis spectra of final compounds. Solvatochromic study illustrated the dissimilarity in λmax
value arising as a result of increase in dipole moment, which confirmed the different behavior
of each o, m and p substituted TES in spectroscopic study. The blue shift in the absorption
bands seems to be due to some strong stabilising interactions of solvent for substituted OTES
while the red shift in the absorption band can correspond to an increased solvation of the
conjugated system. The extent of interaction of different isomers (oꢀ, mꢀ and pꢀ) with solvent
system and the degree of solvation determines the band shift observed in UV–Vis spectra.
Acknowledgments
We thank Mr. Avtar Singh for the NMR studies (SAIF, Panjab University, Chandigarh). One
of the authors, Jandeep Singh, thank Council of Scientific and Industrial Research (CSIR),
India for providing financial support in the form of CSIR–SRF(NET) fellowship.
Notes
The authors declare no competing financial interest.
References
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