Functionalization of silica gel by ultrasound-assisted surface Suzuki coupling

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Functionalization of silica gel by ultrasound-assisted surface Suzuki coupling Anastasia Kuvayskaya, Aleksey Vasiliev ⇑ Department of Chemistry, East Tennessee State University, PO Box 70695, Johnson City, TN 37614, USA

a r t i c l e

i n f o

Article history: Received 12 April 2019 Revised 9 July 2019 Accepted 12 July 2019 Available online xxxx Keywords: Silica gel Suzuki coupling Aryl halides Ultrasound

a b s t r a c t Mesoporous silica gel was functionalized by various organic functional groups using thiol-ene coupling of surface thiol groups with 4-vinylphenylboronic acid followed by Suzuki coupling with aromatic halides. For better performance, the synthesis was conducted under sonication. The presence of surface functional groups was confirmed by thermoanalysis, FT-IR spectroscopy and characteristic reactions of these groups. Solid-phase conditions of the synthesis eliminate the risk of side reactions of boronic acids. Ó 2019 Elsevier Ltd. All rights reserved.

Introduction Grafting of organic molecules onto silica surfaces is an important way to the synthesis of hybrid materials that found many applications in chemistry and chemical engineering [1]. The most common procedure for immobilization of organic molecules is the reaction of functionalized organosilanes with surface silanol groups of silica gel [2]. Other examples of surface reactions include Grignard reactions with chlorinated silica surface [3], esterification of silanol groups [4], surface hydrosilylation [5], reactions with surface amino groups [6] or thiol groups [7], etc. Most of these methods involve water- or air-sensitive reagents or catalysts and require anhydrous media. However, some important molecules (like aminoacids or metal salts) are insoluble in many organic solvents that makes it difficult to immobilize them. Cross-coupling reactions are useful tool in the synthesis of organic materials with unique characteristics [8]. These reactions require mild conditions, they can proceed in the presence of water [9] and result in high yields of the products. Significant advantage of cross-coupling reactions is tolerance to broad range of functional groups. Recently Chan-Lam coupling was successfully applied for functionalization of silica surface by various aromatic, aliphatic and heterocyclic groups [10]. One of the most interesting cross-coupling reactions between phenylboronic acids and aromatic halides leading to wide range of biaryl compounds is Suzuki-Miyaura reaction [11]. Catalyst for ⇑ Corresponding author. E-mail address: [email protected] (A. Vasiliev).

Suzuki coupling can be homogeneous as well as heterogeneous when catalytically active sites are located on the surface of porous support [12]. Immobilized site-isolated Pd-catalysts [13] or Pdnanoparticles [14] were highly active in Suzuki coupling between various substrates. Other kind of solid state Suzuki coupling is the use of immobilized boronic acids [15]. In particular, this approach was successfully used for the synthesis of pharmaceutically active compounds [16]. Numerous examples of heterogeneous cross-coupling reactions are presented in review [17]. Various researchers reported a notable activating effect of ultrasonic conditions on Suzuki coupling [18]. Under sonication, TONs increased almost nine fold as compared to the thermal conditions. The optimal temperature for ultrasound-assisted Suzuki coupling was found to be 60 °C. Thus, the objective of this work was the modification of surface of mesoporous silica by various functional groups using ultrasound-assisted heterogeneous Suzuki coupling reaction. Results and discussion Phenylboronic acid-functionalized silica gel was chosen as a starting material for modification of silica surface. To present time, various phenylboronic acid-functionalized materials were used in solid-phase extraction columns [19], enrichment of fructose [20] and glycopeptides [21], biocompatible cotton fiber-based sorbents [22] and flame retardant coatings [23]. In most reports, they were obtained using hydrosilylation or thiol-ene click chemistry. The first attempt of phenylboronic acid immobilization was based on hydrosilylation of silica surface containing SiH groups

https://doi.org/10.1016/j.tetlet.2019.07.028 0040-4039/Ó 2019 Elsevier Ltd. All rights reserved.

Please cite this article as: A. Kuvayskaya and A. Vasiliev, Functionalization of silica gel by ultrasound-assisted surface Suzuki coupling, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2019.07.028

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(route 1, Scheme 1). However, yields of the products in some cases were lower than at homogeneous hydrosilylation. Researchers explained this drawback by difficulty in access of catalytically active Pt colloidal particles to the surface SiH reaction sites. Other explanation was the hydrolysis of SiH groups by traces of water [24]. In the hydrosilylation experiment conducted at 40 °C, the total carbon contents (TOC) of silica gel decreased dramatically after 1 h of the reaction from 5.06 down to 2.56% (material 3). It followed by notable increase of BET surface area and total pore volume (up to 493.1 m2/g and 0.71 nm, respectively). Then after 4 h, TOC content slightly increased up to 4.0%. When the reaction was conducted at 80 °C, the loss of carbon after 1 h was almost the same (2.58%), but further increase of carbon content was insignificant and reached only 2.64% (see Suppl. Data, Table 3). In this case, existing hypotheses of low efficiency of surface hydrosilylation are not sufficient to explain defunctionalization of silica gel with the loss of carbon. This phenomena can be caused by a side reaction between boronic acid and surface SiH groups. This reaction is not sufficiently studied yet, however, reports about high reactivity of boronic acids to various silanes were published recently [25]. In both reports, mixtures of products were obtained. Thus, functionalization of silica gel by boronic acid using surface hydrosilylation was ineffective. The radical reactions between surface alkylthiol groups in sample 2 and 4-vinylphenylboronic acid were conducted with AIBN initiator in different solvents (route 2, Scheme 1). The study of the reaction conditions on the immobilization of phenylboronic acid showed that chloroform was the most efficient solvent. Starting material 2 contained 1.03 mmol/g of surface SH groups. After the reaction, 52.0% of SH groups reacted with VPBA giving the material 4 with contents of surface B(OH)2 functional groups of 0.53 mmol/g. The BET surface area of the material decreased from 575.7 to 262.3 cm3/g while pore volume decreased from 0.48 to 0.34 cm3/g. Surface density of B(OH)2 groups in 4 was 1.21 molecules/nm2. At the use of 1-propanol and acetonitrile, conversions

of SH groups were 39.8 and 45.6%, respectively (see Suppl. Data, Table 4). Comparing effectiveness of different methods of functionalization, thiol-ene coupling was selected for the synthesis of a material for further study. Optimal conditions for solid-phase Suzuki coupling were determined from results of a homogeneous reaction between phenylboronic acid and bromobenzene catalyzed by Pd(OAc)2. The reaction mixture was sonicated for 4 h at 60 °C in the presence of Cs2CO3 as a base. At these conditions, yield of biphenyl was 97%. Other Pd-catalysts tested, i.e., bis(benzonitrile)-dichloropalladium (II), tris(dibenzylideneacetone)dipalladium (0) and tetrakis-(triphenylphosphine)palladium (0), produced 82–93% yields. The presence of immobilized phenylboronic acid in 4 was evident from characteristic absorption bands in the FT-IR spectra (cm 1): 723 (øC-C, aromatic), 1364 (mB-O), 1608 (mC-C, aromatic), 3100 (mC–H, aromatic), 3288 (mB-OH) [26]. All these bands were absent in the spectrum of 2 (Fig. 1). The same conditions were used for heterogeneous Suzuki coupling. Yields of all surface biaryl compounds (conversions of B (OH)2 groups) were calculated from the data of elemental analyses on total organic carbon (TOC). Although the yields were lower due to steric limitations in pores of silica gel, their loading corresponded to nearly complete surface covering. Thus, the volume of an anchored unsubstituted biaryl molecule on samples 5 and 6 was equal to 0.29 nm3 (calculated using Spartan v.6 software) while they occupied surfaces of 0.87 and 0.92 nm2, respectively. Iodobenzene is more reactive than bromobenzene, thus, resulting in some higher surface loading (entry 5, Table 1). The conversion of sample 4 to 5 was studied by DSC and TGA methods (Fig. 2). At the heating of material 4, some mass loss up to 227 °C corresponded to physically and chemically adsorbed water. The first exothermic peak, identifying a surface reaction, appeared at 227 °C. It was expected that this thermal event was the decomposition of surface sulfide groups, as the bond CAS was the weakest (272 kJ/mol). Then an endothermic step was detected between 227 and 312 °C. This step is similar to the glass

Scheme 1. Functionalization of silica surface.

Please cite this article as: A. Kuvayskaya and A. Vasiliev, Functionalization of silica gel by ultrasound-assisted surface Suzuki coupling, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2019.07.028

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transition in polymer materials, however, in this case it could be explained by continuous decomposition of surface organic groups. The second exothermic reaction was detected at 253 °C. Most of the mass loss occurred between 300 and 500 °C, which corresponded to the second step of decomposition at 312–484 °C. In this range of temperatures, surface mercaptopropyl groups decomposed [27]. Chemical immobilization of phenylboronic acid was evident from absence of an endothermic peak at 190 °C corresponding to melting point of physically adsorbed VPBA. Similar endothermic step in biaryl-modified material 5 was shifted to lower temperatures in the range of 220–271 °C. It showed lower thermal stability of surface biaryl groups as compared to phenylboronic acid. The second exothermic step was also shifted to 234 °C. During this step, both materials lost about 1% of their mass. The second step of decomposition of 5 ended above 600 °C. It was clear that iodobenzene reacted with B(OH)2 groups as no endothermic peak at 188 °C indicating its boiling point was detected. In addition, no significant mass loss occurred at this temperature. Based on TGA data, total mass loss above 220 °C was 14.2%. This result was in a good agreement with the data of elemental analysis of samples 2, 4 and 5. Considering calculated loading of surface

Fig. 1. FT-IR spectra of samples 2 and 4.

Table 1 Suzuki coupling on the silica surface. Sample

Aryl halide

TOC, %

Yield, %

Loading, mmol/g

Surface density, molecules/nm2

BET surface area, cm3/g

4 5

None

I

8.77 11.26

– 52.1

– 0.28

– 1.15

262.3 146.1

Br

10.96

45.8

0.24

1.09

132.4

11.64

51.4

0.27

1.00

162.2

9.85

19.4

0.10

0.30

201.6

10.99

34.8

0.18

0.46

235.2

O

10.09

18.4

0.10

0.27

224.3

OH

9.95

24.7

0.13

0.55

143.8

11.35

40.4

0.21

0.58

219.1

10.43

20.5

0.11

0.31

215.5

8.96

3.4

0.02

0.06

194.7

6

O

7

OH I O

8

I O

9

OH

Cl 10

Br 11

Cl

Cl

O

12

Br

Br 13

OH Br O

14

NH2 Cl Please cite this article as: A. Kuvayskaya and A. Vasiliev, Functionalization of silica gel by ultrasound-assisted surface Suzuki coupling, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2019.07.028

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A. Kuvayskaya, A. Vasiliev / Tetrahedron Letters xxx (xxxx) xxx

functional groups, total amount of organic phase in sample 5 was found also 14.2%. Substituted arylhalides also reacted with the surface boronic acid, although the yields of their surface products in most cases were lower. It is known that in homogeneous reactions, EWGs in aryl halides activate them, while EDGs deactivate [28]. No clear effect of substituents was noted in the surface reactions of aryl iodides and aryl bromides (entries 7, 8, 10, 12 and 13, Table 1). Regarding aryl chlorides, 4-chlorophenylacetic acid and 2,4dichlorophenol were rarely used in Suzuki coupling due to their low activity in cross-coupling reactions [29]. However, they also formed immobilized products with moderate yields (entries 9 and 11, Table 1). Very low yield in the reaction with 4-chlorobenzamide might be explained by low electron-withdrawing effect of an amide group in combination with low reactivity of chlorides in general (entry 14, Table 1). Due to relatively low content of organic molecules on the surface, their corresponding absorption bands in the FT-IR spectra had very weak intensity and presented as shoulders on more intensive bands of silica gel. For immobilized carboxylic acids, their weak mC=O bands in the FT-IR spectra were found as shoulders at 1684 and 1699 cm 1, respectively (entries 7 and 9, Table 1). In addition, some weak bands in the region of absorption of carboxylic groups were detected. The presence of carbonyl groups in immobilized ketones was detected by their weak mC=O bands at 1714 and 1698 cm 1, respectively (entries 10 and 12, Table 1). These bands are partially hidden by a strong band of hydrated silica gel at 1657 cm 1. However, the presence of functional groups was evident from their characteristic reactions (Scheme 2). All functionalized materials (except for sample 14) did not contain nitrogen in their structure. The presence of carboxylic groups in samples 7 and 9 was determined using diisopropylcarbodiimide-assisted hydrazide formation. Carbonyl groups in 10 and 12 were tested using Brady’s reagent. The presence of hydroxyl groups in samples 11 and 13 was confirmed by the reaction with 3,5-dinitrobenzoyl chloride. The products of characteristic reactions were analyzed on nitrogen contents (Table 2).

Fig. 2. DSC and TGA thermograms of samples 4 and 5.

Scheme 2. Characteristic reactions of functionalized silica gel.

Table 2 Products of reactions of surface functional groups. Sample

Product NH

7 Ar

NH

O

9

O

Loading, mmol/g

14.8

0.04

NH

NO 2

0.22

22.2

0.04

0.29

50.0

0.05

0.25

68.7

0.09

0.53

42.9

0.09

0.28

90.9

0.10

O 2N

N

10

Yield, %

0.23

O 2N

NH

Ar

NO 2

N contents, %

Ar

NH

NO2

O 2N 11

NO 2

O O

Ar

NO 2

Cl

N

12 Ar

Br

NH O 2N

NO2

13 Ar

O O

NO 2

NO2

Please cite this article as: A. Kuvayskaya and A. Vasiliev, Functionalization of silica gel by ultrasound-assisted surface Suzuki coupling, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2019.07.028

A. Kuvayskaya, A. Vasiliev / Tetrahedron Letters xxx (xxxx) xxx

In summary, the ultrasound-assisted reaction of Suzuki coupling was successfully used for functionalization of silica surface. This approach makes possible obtaining a large variety of functionalized materials with various surface properties and characteristics. Another advantage of this procedure is a reduced risk of side reactions, e.g., homocoupling of boronic acids in the presence of oxygen due to their anchoring to the surface of silica gel. Aprotic conditions of the reaction also prevent deboronation [30]. Acknowledgment This work was supported by ETSU Honors College (Student-Faculty Collaborative Grant). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.tetlet.2019.07.028. References [1] (a) M. Faustini, L. Nicole, E. Ruiz-Hitzky, C. Sanchez, Adv. Funct. Mater. 28 (2018) 1704158; (b) D. Urbano, B. Daniel, C. Avelino, Chem. Soc. Rev. 42 (2013) 4083–4744; (c) M. Vallet-Regi, M. Colilla, B. Gonzalez, Chem. Soc. Rev. 40 (2011) 596–607. [2] (a) J.-W. Park, Y.J. Park, C.-H. Jun, Chem. Commun. 47 (2011) 4860–4871; (b) S. Fujita, M.P. Kapoor, S. Inagaki, Adv. Materials Res. 13 (2009) 141–169. [3] (a) J.E. Lim, C.B. Shim, M. Kim, B.Y. Ji, J.E. Lee, Yie, Angew. Chem. Int. Ed. 43 (2004) 3839–3842; (b) O. Vassylyev, G.S. Hall, J.G. Khinast, J. Porous Mater. 13 (2006) 5–11. [4] (a) Y. Lu, R. Wang, W. Ren, Y. Zhang, Polym. Composit. 39 (2018) 3586–3593; (b) O. Vassylyev, J. Chen, G.S. Hall, J.G. Khinast, Micropor. Mesopor. Mat. 92 (2006) 101–108. [5] P.J. Pedersen, J. Henriksen, C.H. Gotfredsen, M.H. Clausen, Tetrahedron Lett. 53 (2008) 6220–6223. [6] M. Qiao, X. Liu, J.-W. Song, T. Yang, M.-L. Chen, J.-H. Wang, J. Mater. Chem. B 6 (2018) 7703–7709. [7] (a) R. Gobel, P. Hesemann, A. Friedrich, R. Rothe, H. Schlaad, A. Taubert, Chem. Eur. J. 20 (2014) 17579–17589; (b) T. Simerly, T. Milligan, R. Mohseni, A. Vasiliev, Tetrahedron Lett. 49 (2012) 5297–5301. [8] (a) A. Suzuki, Angew. Chem. Int. Ed. 50 (2011) 6722–6737; (b) L.-C. Campeau, N. Hazari, Organometallics 38 (2019) 3–35; (c) A. Biffis, P. Centomo, A. Del Zotto, M. Zecca, Chem. Rev. 118 (2018) 2249– 2295.

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Please cite this article as: A. Kuvayskaya and A. Vasiliev, Functionalization of silica gel by ultrasound-assisted surface Suzuki coupling, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2019.07.028