Chinese Chemical Letters xxx (2014) xxx–xxx

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Original article

Synthesis of ester-capped carbosilane dendrimers via a hybriddivergent–convergent method

Yu-Zhong Niu a,b,*, Lin Zhang b, Shu-Jie Liang a, Deng-Xu Wang b, Sheng-Yu Feng b,**a School of Chemistry and Materials Science, Ludong University, Yantai 264025, Chinab Key Laboratory of Special Functional Aggregated Materials, Ministry of Education, School of Chemistry and Chemical Engineering, Shandong University,

Ji’nan 250100, China

A R T I C L E I N F O

Article history:

Received 24 June 2014

Received in revised form 30 July 2014

Accepted 25 August 2014

Available online xxx

Keywords:

Synthesis

Characterization

Ester-capped

Carbosilane dendrimers

Divergent–convergent method

A B S T R A C T

A series of novel ester-capped carbosilane dendrimers (G0-COOCH3–G2-COOCH3) were designed and

successfully synthesized via a hybrid divergent–convergent method through a facile hydrosilylation

reaction. The structures of these dendrimers were confirmed by FTIR, 1H NMR, and HRMS analyses.

� 2014 Yu-Zhong Niu and Sheng-Yu Feng. Published by Elsevier B.V. on behalf of Chinese Chemical

Society. All rights reserved.

Contents lists available at ScienceDirect

Chinese Chemical Letters

jo u rn al h om epag e: ww w.els evier .c o m/lo cat e/cc le t

1. Introduction

Dendrimers are highly branched macromolecules with well-defined architectures and surface functionality that are obtainedfrom iterative stepwise procedures. They have been one of themost extensively studied materials during the last decade becauseof their controllable nanosize, unique topological structure, andversatile physicochemical properties, and were widely used in thefields of catalysis, medicinal chemistry, and nanotechnologies [1–3]. In recent years, there has been a rapid growth in the discoveryand application of new dendrimers. Carbosilane dendrimers areamong the most widely used because of their high flexibility,catalytic inertness, and accessibility [4]. These dendrimerstypically possess low glass transition temperatures and relativelylow viscosities. They are kinetically and thermodynamically stableowing to the low polarity of the Si–C bond and its high bondstrength (306 kJ/mol), which is very similar to that of C–C bond(345 kJ/mol) [5]. In addition, the availability and versatility ofmany synthetic reactions in organosilicon chemistry enable thefacile synthesis of these dendrimers. As a result, the synthesis,

* Corresponding author at: School of Chemistry and Materials Science, Ludong

University, Yantai 264025, China.** Corresponding author.

E-mail addresses: niuyuzhong@126.com (Y.-Z. Niu), fsy@sdu.edu.cn (S.-Y. Feng).

Please cite this article in press as: Y.-Z. Niu, et al., Synthesis of ester-cmethod, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2

http://dx.doi.org/10.1016/j.cclet.2014.08.003

1001-8417/� 2014 Yu-Zhong Niu and Sheng-Yu Feng. Published by Elsevier B.V. on be

characterization, and application of different types of carbosilanedendrimers have attracted wide attention [6–8].

Carbosilane dendrimers are generally synthesized by thedivergent method. They are built stepwise from the central core

that possesses alkenyl groups through the reiteration of sequential

hydrosilylations with chlorosilanes and alkenylations with

Grignard reagents, which allow the construction of dendrimers

with various generations [9]. The synthetic route of carbosilane

dendrimers offers high flexibility and versatility as the hydro-

silylation and alkenylations reagents can be varied accordingly.

Especially, the hydrosilylation reaction, an addition of a hydro-

silane unit (Si–H) to a double bond, has been proved to be high

yielding and selective and represents a powerful tool for the rapid

and efficient synthesis of carbonsilane dendrimers [10]. In our

previous studies, a series of acetyl, N,N-dimethylaniline, and

platinum-capped carbosilane dendrimers were synthesized [11–

13]. However, the main drawback of these dendrimers is that such

functional groups exhibit relative low reactivity, which may limit

further functionalization and application of the carbosilane

dendrimers. The synthesis of carbosilane dendrimers with reactive

functional groups at periphery is still a great challenge.In the present study, a series of ester-capped carbosilane

dendrimers were synthesized via a hybrid divergent–convergent

method and their structures were confirmed by FTIR, 1H NMR, and

HRMS analyses. The peripheral reactive ester functional groups can

apped carbosilane dendrimers via a hybrid divergent–convergent014.08.003

half of Chinese Chemical Society. All rights reserved.

Y.-Z. Niu et al. / Chinese Chemical Letters xxx (2014) xxx–xxx2

G Model

CCLET-3079; No. of Pages 4

be further functionalized using appropriate synthetic proceduresto expand the application of carbosilane dendrimers.

2. Experimental

Fourier transform infrared spectra (FTIR) were recorded on aBruker Tensor 27 spectrophotometer (Bruker, Switzerland) in therange of 400–4000 cm�1 with a resolution of 4 cm�1 byaccumulating 32 scans. The data were treated with an OPUSspectroscopy software of version 6. 1H NMR was measured inchloroform on a Bruker AVANCE-400 NMR Spectrometer, andchemical shifts were recorded in parts per million (ppm) withoutan internal reference. Molecular weights were determined by anAgilent Technologies 6510 Q-TOF LC–MS (HRMS). Methyl acrylate(MA), ethylenediamine (EDA), allylamine (AA), 1,1,3,3-tetra-methyldisiloxane (MHMH) and tetrahydrofuran (THF) were redis-tilled just before use. All other reagents used were of analytical-reagent grade. The starting materials carbosilane dendrimers G0,G1, and G2 were prepared according to the method described in

Scheme 1. The hybrid divergent–convergent synthesi

Please cite this article in press as: Y.-Z. Niu, et al., Synthesis of ester-cmethod, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2

Ref. [13]. DMAA was prepared by a Michael addition reaction of MAwith AA according to the procedure described in our previous work[14]. The synthetic routes of ester-capped carbosilane dendrimersare illustrated in Scheme 1.

2.1. Synthesis of MHM-DMAA

MHM-DMAA was prepared according to a similar methoddescribed in Ref. [14]. A suspension of 2.68 g (0.020 mol) of MHMH

and three drops of the Karstedt catalyst were added into the flaskand the mixture was heated slowly at 70 8C under argonatmosphere. Then 1.15 g (0.0050 mol) of DMAA was added dropwise with stirring and the reaction mixture was allowed to stir for12 h. After the reaction completed, the excess MHMH was removedunder reduced pressure and 1.70 g (0.0047 mol) of MHM-DMAAwas obtained. Yield: 94.10%.

FTIR (KBr, cm�1): y 2969, 2863 (CH3, CH2), 2110 (SiH), 1738(COOCH3), 1251 (Si–CH3), 1041 (Si–O–Si); 1H NMR (CDCl3): d�0.07 to 0.02 (m, 12H, SiCH3), 0.30–0.35 (t, 2H, SiCH2CH2CH2N),

s routes of ester-capped carbosilane dendrimers.

apped carbosilane dendrimers via a hybrid divergent–convergent014.08.003

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1.29 (m, 2H, SiCH2CH2CH2N), 2.29–2.33 (m, 6H, SiCH2CH2

CH2NCH2), 2.62–2.65 (m, 4H, NCH2CH2CO), 3.52 (s, 6H, OCH3),4.52 (s, H, SiH).

2.2. Synthesis of G0-COOCH3

G0-COOCH3 was synthesized via the hydrosilylation reactionbetween G0 and MHM-DMAA similar to the procedures describedelsewhere [11]. A sample of 0.17 g (0.0010 mol) G0, 1.34 g(0.0036 mol) MHM-DMAA, and three drops of the Karstedt catalystwere suspended in 30 mL THF under argon atmosphere. Themixture was stirred at 65 8C for 12 h, then the solvent andremaining MHM-DMAA were removed under reduced pressure,and 0.98 g (0.78 mmol) of G0-COOCH3 was obtained. Yield: 78.0%.

FTIR (KBr, cm�1): y 2952, 2873 (CH3, CH2), 1737 (COOCH3),1251 (Si–CH3), 1041 (Si–O–Si); 1H NMR (CDCl3): d 0.00–0.10(m, 39H, SiCH3), 0.38–0.48 (m, 12H, SiCH2CH2CH2Si), 0.50–0.59(m, 6H, SiCH2CH2CH2N), 1.24–1.45 (m, 12H, SiCH2CH2CH2Si,SiCH2CH2CH2N), 2.36–2.53 (m, 18H, SiCH2CH2CH2NCH2), 2.73–2.91(t, 12H, NCH2CH2CO), 3.65 (s, 18H, OCH3); HRMS: m/z 1256.7007[MH]+ (calcd. 1255.6864).

2.3. Synthesis of G1-COOCH3

Under argon atmosphere, a suspension of 0.24 g (0.0004 mol)G1, 1.34 g (0.0036 mol) MHM-DMAA, and three drops of theKarstedt catalyst in 30 mL THF was stirred at 65 8C for 16 h. Afterthe reaction is complete, the purification procedures similar tothose of G0-COOCH3 were used to produce 0.39 g (0.143 mmol)G1-COOCH3. Yield: 72.1%.

FTIR (KBr, cm�1): y 2953, 2873 (CH3, CH2), 1738 (COOCH3),1251 (Si–CH3), 1040 (Si–O–Si); 1H NMR (CDCl3): d 0.02–0.10(m, 84H, SiCH3), 0.38–0.45 (m, 24H, SiCH2CH2CH2Si), 0.51–0.59(m, 24H, SiCH2CH2CH2N), 1.25–1.35 (m, 18H, SiCH2CH2CH2Si),1.37–1.45 (m, 12H, SiCH2CH2CH2N), 2.40–2.46 (m, 36H,SiCH2CH2CH2NCH2), 2.75–2.79 (t, 24H, NCH2CH2CO), 3.66 (s,36H, OCH3); HRMS (FAB): m/z 2724.5228 [MH]+ (calcd.2723.5150).

2.4. Synthesis of G2-COOCH3

A solution of 0.65 g (0.0005 mol) G2, 3.63 g (0.01 mol) MHM-DMAA, five drops of the Karstedt catalyst in 100 mL THF wasstirred under argon atmosphere at 65 8C for 24 h. After the reactioncompleted, the purification procedures similar to that of G0-COOCH3 were used to produce 1.97 g (0.348 mmol) G2-COOCH3.Yield: 69.5%.

Scheme 2. The divergent synth

Please cite this article in press as: Y.-Z. Niu, et al., Synthesis of ester-cmethod, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2

FTIR (KBr, cm�1): y 2953, 2873 (CH3, CH2), 1738 (COOCH3),1251 (Si–CH3), 1041 (Si–O–Si); 1H NMR (CDCl3): d �0.03 to 0.08(m, 174H, SiCH3), 0.38–0.47 (m, 84H, SiCH2CH2CH2Si), 0.51–0.62(m, 24H, SiCH2CH2CH2N), 1.24–1.34 (m, 42H, SiCH2CH2CH2Si),1.40–1.43 (m, 24H, SiCH2CH2CH2N), 2.37–2.46(m, 72H,SiCH2CH2CH2NCH2), 2.74–2.79 (t, 48H, NCH2CH2CO), 3.65 (s, 72H, OCH3); HRMS (FAB): m/z 5660.1801 [MH]+ (calcd. 5659.1722).

3. Results and discussion

In general, the syntheses of carbosilane dendrimers withdifferent peripheral functional groups were commonly achievedby the divergent method. Based on this strategy, ester-cappedcarbosilane dendrimers could also be synthesized according to theprocedures described in Scheme 2 (G0-COOCH3 was chosen as arepresentative). As shown in Scheme 2, G0-NH2 can be firstsynthesized according to the procedures that described in ourprevious studies [11,13], and then the Michael addition of MA tothe –NH2 group in G0-NH2 leads to the formation of G0-COOCH3

[14]. However, the divergent synthesis strategy can be hindered byside reactions that yield incomplete or imperfect dendrimers[13,15]. During the hydrosilylation process of ATMS with MHMH

and G0 with MHM-ATMS, the amino groups should be protected bya trimethylsilyl group to reduce the side reactions between theamino group and the silane. An alcoholysis reaction should beemployed to deprotect the silyl group in order to obtain G0-NH2

[11]. Moreover, ATMS must be added in excess to ensure thecompletion of the reaction with MHMH due to its relative lowactivity. To overcome these drawbacks and the tedious protection–deprotection procedures, a hybrid divergent–convergent methodwas adopted. As illustrated in Scheme 1, G0, G1, and G2 were firstsynthesized by the divergent method, then the MHM-DMAA wasconvergently attached to the allyl group in G0–G2 to give thecorresponding G0-COOCH3–G2-COOCH3. Using this method,DMAA was obtained by the Michael addition of MA with AA innearly 100% yield [14]. The intermediate ester-capped compoundMHM-DMAA was prepared by the hydrosilylation of DMAA withMHMH in the presence of the Karstedt catalyst in excellent yield(94.1%). At last, G0-COOCH3–G2-COOCH3 can be produced by ahydrosilylation reaction of MHM-DMAA with G0, G1, and G2 ingood yields (78.0%, 72.1%, and 69.5%, respectively). Therefore, thehybrid divergent–convergent method provides simpler syntheticprocedures and higher yields compared to that of the divergentmethod, and was suitable for the synthesis of ester-cappedcarbosilane dendrimers.

In order to ensure the hydrosilylation reaction proceeds readilyand completely, excess MHM-DMAA was used in the reaction. In

esis routes of G0-COOCH3.

apped carbosilane dendrimers via a hybrid divergent–convergent014.08.003

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addition, the content of C55C groups and the steric hindrance of thedendrimer increase in later generations, and thus the amount ofMHM-DMAA, the Karstedt catalyst, and the reaction time need tobe increased to complete the reaction. Previous studies showedthat allyl derivatives allowed the synthesis of carbosilanedendrimers up to the fifth generation with unified molecularstructures [16]. Montealegre also successfully synthesized thethird generation carboxylate-terminated carbosilane dendrimerspossessing 32 terminal groups with a molecular weight of 6586[17]. The structures of G0-COOCH3–G2-COOCH3 and the interme-diate compounds were confirmed by FTIR, 1H NMR, and HRMSanalyses. The FITR spectrum of MHM-DMAA showed thecharacteristic adsorption bands of y(Si–H) and y(COOCH3) at2110 and 1738 cm�1, and the adsorption bands at 1251 and1041 cm�1 corresponding to d(Si–CH3) and y(Si–O–Si), respec-tively. After the hydrosilylation reaction of MHM-DMAA withG0–G2, the vibration peaks of y(Si–H) at 2110 cm�1 and y(C55C) at1630 cm�1 disappeared in the FTIR spectrum of G0-COOCH3–G2-COOCH3, and new adsorption peaks at 1041 and 1738 cm�1

attributed to y(Si–O–Si) and y(COOCH3) appeared, indicating thatG0-COOCH3–G2-COOCH3 had been successful synthesized. The 1HNMR spectrum of G0-COOCH3 exhibited resonances for Si–CH3

protons in the region of 0.00–0.10 ppm and for ester protons(OCH3) at 3.65 ppm. The resonances appeared in the range of 0.38–0.48, 0.50–0.59, 1.24–1.45, 2.36–2.53, and 2.73–2.91 ppm can beassigned to the protons of SiCH2CH2CH2Si, SiCH2CH2CH2N,SiCH2CH2CH2Si, SiCH2CH2CH2NCH2, and NCH2CH2CO, respectively.The 1H NMR spectrum of G1-COOCH3 and G2-COOCH3 are almostidentical to that of G0-COOCH3 for analogous protons, althoughbroader resonances are observed with the increased generations.The reason for this can be ascribed to the slightly differentchemical environments for the protons in different generationsand the restricted mobility of the respective protons in the outershells [14]. In the 1H NMR spectrum of G0-COOCH3–G2-COOCH3,the ratio of ester end groups (OCH3) to CH3Si groups was consistentwith the expected value based on the formula. Also, the completedisappearance of C55C group resonances in the spectrum of G0–G2further confirmed the formation of unified molecular dendrimersof G0-COOCH3–G2-COOCH3. Furthermore, the molecular weightsof G0-COOCH3–G2-COOCH3 were determined by HRMS, whichwere in good agreement with the theoretical calculation. Thisunambiguously confirmed the formation of G0-COOCH3–G2-COOCH3, further demonstrating that the desired ester-cappedcarbosilane dendrimers had been successfully synthesized.

4. Conclusion

In conclusion, a series of novel ester-capped carbosilanedendrimers (G0-COOCH3–G2-COOCH3) were successfully synthe-sized via a hybrid divergent–convergent method in good yield. Allthe targeted dendrimers were confirmed by FTIR, 1H NMR, and

Please cite this article in press as: Y.-Z. Niu, et al., Synthesis of ester-cmethod, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2

HRMS analyses. The synthesized dendrimers possess reactive esterfunctional groups, which can be further functionalized usingappropriate synthetic procedures to expand the application ofcarbosilane dendrimers.

Acknowledgments

The authors are grateful for the financial support by theNational Natural Science Foundation of China (No. 21307053),China Postdoctoral Science Foundation Funded Project (No.2013M541911), Promotive Research Fund for Excellent Youngand Middle-Aged Scientists of Shandong Province (No.BS2013CL044), and Natural Science Foundation of LudongUniversity (No. LY2011004).

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