Application of Thiol− Ene Chemistry to the Preparation of Carbosilane

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Application of Thiol-Ene Chemistry to the Preparation of Carbosilane-Thioether Dendrimers Christiana Rissing and David Y. Son* Department of Chemistry, Southern Methodist UniVersity, Dallas, Texas 75275-0314 ReceiVed February 20, 2009

The application of thiol-ene chemistry to the synthesis of new carbosilane-thioether dendrimers is presented in this work. The dendrimers are prepared in a divergent fashion starting with tetravinylsilane as a core, followed by a succession of alternating thiol-ene and Grignard reactions. Vinyl-terminated dendrimers up to the fifth generation were isolated in excellent yields (78-94%). Products were characterized by multinuclear NMR spectroscopy, dynamic light scattering, gel permeation chromatography, and MALDI-TOF mass spectrometry. Introduction Dendrimers have been of long-standing interest due to their highly branched three-dimensional structure and monodispersed composition.1-5 More specifically, the study of organosilicon dendrimers has increased since their initial introduction in the 1990s,6-9 resulting in their widespread use in numerous applications including catalysis, host-guest chemistry, and liquid crystalline materials.10-14 The most common organosilicon dendrimers are the carbosilane dendrimers, consisting of silicon and carbon atoms along the backbone. Carbosilane dendrimers are stable and robust, easy to peripherally functionalize, and can be synthesized from a variety of core molecules. The preparation of carbosilane dendrimers generally involves the Pt-catalyzed hydrosilylation of a multivinylated or -allylated core with a chlorosilane followed by nucleophilic substitution with a Grignard or * To whom correspondence should be addressed. E-mail: [email protected]. (1) Bosman, A. W.; Janssen, H. M.; Meijer, E. W. Chem. ReV. 1999, 99, 1665–1688. (2) Fischer, M.; Vo¨gtle, F. Angew. Chem., Int. Ed. 1999, 38, 885–905. (3) Borges, A. R.; Schengrund, C.-L. Curr. Drug Targets: Infect. Disord. 2005, 5, 247–254. (4) Boas, U.; Christensen, J. B.; Heegaard, P. M. H. J. Mater. Chem 2006, 16, 3785–3798. (5) Fre´chet, J. M. J.; Tomalia, D. A. Dendrimers and Other Dendritic Polymers; Wiley & Sons: New York, 2001. (6) van der Made, A. W.; van Leeuwen, P. W. N. M. Chem. Commun. 1992, 1400–1401. (7) Zhou, L. L.; Roovers, J. Macromolecules 1993, 26, 963–968. (8) van der Made, A. W.; van Leeuwen, P. W. N. M.; de Wilde, J. C.; Brandes, R. A. C. AdV. Mater. 1993, 5, 466–468. (9) Seyferth, D.; Son, D. Y.; Rheingold, A. L.; Ostrander, R. L. Organometallics 1994, 13, 2682–2690. (10) Krska, S. W.; Son, D. Y.; Seyferth, D. In Silicon-Based Polymers: The Science and Technology of Their Synthesis and Applications; Chojnowski, J., Jones, R. G., Ando, W., Eds.; Chapman & Hall: New York, 2000; pp 615-641. (11) Son, D. Y. In The Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; John Wiley & Sons: New York, 2001; Vol. 3, pp 745-803. (12) Frey, H.; Schlenk, C. Top. Curr. Chem. 2000, 210, 69–129. (13) Wander, M.; Gebbink, R. J. M. K.; van Koten, G. In SiliconContaining Dendritic Polymers; Dvornic, P. R., Owen, M. J., Eds.; Springer: New York, 2009; pp 197-235. (14) Shibaev, V.; Boiko, N. In Silicon-Containing Dendritic Polymers; Dvornic, P. R., Owen, M. J., Eds.; Springer: New York, 2009; pp 237283.

organolithium reagent.15 Subsequent generations are added by alternating the hydrosilylation and nucleophilic substitution steps. Although this methodology has long been established, we sought an alternative synthetic procedure for the following reasons: (1) the hydrosilylation steps often require strictly controlled reaction conditions in terms of time and temperature, (2) the chlorosilanes are typically volatile and highly moisturesensitive, and (3) we have found the quality of commercial Pt catalysts to vary widely. An alternative procedure that could additionally introduce electron-rich heteroatoms to the dendrimer interior would alter the polarity of the interior, potentially leading to expanded application of carbosilane dendrimers in areas such as metal encapsulation and stabilization. Thiol-ene chemistry has recently been shown to be a valuable synthetic tool, particularly in the area of materials chemistry.16-20 Thiol-ene reactions are simply the sulfur version of the hydrosilylation reaction and involve the addition of a sulfur-hydrogen bond across a double or triple bond. Depending on the reaction conditions and the unsaturated substrates, the thiol-ene reaction can proceed by either a freeradical or an ionic mechanism. The user-friendliness of the thiol-ene reaction is readily apparent: moisture and air do not need to be excluded, expensive transition metal catalysts are unnecessary, a wide variety of functional groups and solvents (including water) are well tolerated, and high yields and clean products are common.20-23 Because of these advantageous attributes, an increasing number of researchers now consider the thiol-ene reaction a “click” reaction.17,24-26 (15) Roovers, J.; Ding, J. In Silicon-Containing Dendritic Polymers; Dvornic, P. R., Owen, M. J., Eds.; Springer: New York, 2009; pp 3174. (16) Campos, L. M.; Meinel, I.; Guino, R. G.; Schierhorn, M.; Gupta, N.; Stucky, G. D.; Hawker, C. J. AdV. Mater. 2008, 20, 3728–3733. (17) Dondoni, A. Angew. Chem., Int. Ed. 2008, 47, 8995–8997. (18) Li, Q.; Zhou, H.; Wicks, D. A.; Hoyle, C. E. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 5103–5111. (19) Lu, H.; Carioscia, J. A.; Stansbury, J. W.; Bowman, C. N. Dent. Mater. 2005, 21, 1129–1136. (20) Hoyle, C. E.; Lee, T. Y.; Roper, T. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 5301–5338. (21) Rissing, C.; Son, D. Y. Organometallics 2008, 27, 5394–5397. (22) Rim, C.; Lahey, L. J.; Patel, V. G.; Zhang, H.; Son, D. Y. Tetrahedron Lett. 2009, 50, 745–747. (23) Griesbaum, K. Angew. Chem., Int. Ed. Engl. 1970, 9, 273–87. (24) Gress, A.; Volkel, A.; Schlaad, H. Macromolecules 2007, 40, 7928– 7933.

10.1021/om9001395 CCC: $40.75  2009 American Chemical Society Publication on Web 04/30/2009

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Scheme 1. Synthesis of Carbosilane-Thioether Dendrimers by Thiol-Ene and Grignard Reactions (Vi ) CHdCH2)

The systematic incorporation of sulfur throughout a carbosilane dendrimer structure is unexplored. Frey and co-workers27 utilized the thiol-ene reaction for the end-group functionalization of carbosilane dendrimers. The introduction of thioether groups into carbosilane dendrimers by nucleophilic substitution of chloromethyl groups with thiolates was reported separately by Krska and Seyferth,28 and Omotowa and Shreeve.29 However, in all of these cases, the dendrimers were prepared using (25) Campos, L. M.; Killops, K. L.; Sakai, R.; Paulusse, J. M. J.; Damiron, D.; Drockenmuller, E.; Messmore, B. W.; Hawker, C. J. Macromolecules 2008, 41, 7063–7070. (26) Chan, J. W.; Yu, B.; Hoyle, C. E.; Lowe, A. B. Chem. Commun. 2008, 4959–4961. (27) Lorenz, K.; Frey, H.; Stu¨hn, B.; Mu¨lhaupt, R. Macromolecules 1997, 30, 6860–6868. (28) Krska, S. W.; Seyferth, D. J. Am. Chem. Soc. 1998, 120, 3604– 3612.

the traditional method, and the introduction of sulfur was primarily a peripheral functionalization. We recently reported the synthesis of branched, multifunctional organosilicon molecules using thiol-ene chemistry.21 The present Article describes the extension of this work to the divergent synthesis of new carbosilane-thioether dendrimers (Scheme 1). During the early stages of this investigation,21 Hawker and co-workers30 reported the synthesis of organic dendrimers using photoinitiated thiol-ene chemistry. Our synthetic method is based on an alternating thiol-ene/Grignard substitution sequence and thus parallels the traditional carbosilane dendrimer synthesis. Tetravinylsilane was used as the core, and the resulting first through (29) Omotowa, B. A.; Shreeve, J. n. M. Macromolecules 2003, 36, 8336– 8345. (30) Killops, K. L.; Campos, L. M.; Hawker, C. J. J. Am. Chem. Soc. 2008, 130, 5062–5064.

Preparation of Carbosilane-Thioether Dendrimers

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Table 1. General Characterization Data for Dendrimers dendrimer generation

expected mass

G1-OMe G1-Vi G2-OMe G2-Vi G3-OMe G3-Vi G4-OMe G4-Vi G5-OMe G5-Vi

920.31 872.37 3225.07 3081.26 10 139.38 9707.93 30 882.29 29 587.94 93 111.04 89 227.98

observed [M + Ag]+ 979.4 3189.5 9828.1 -

b

-

c

no. of terminal groups 12 12 36 36 108 108 324 324 972 972

sizea (nm) 0.76 1.4 2.7 4.3 8.2

% yield 100 83 73 82 85 94 71 78 46 87

a

b

Number-weighted average determined by dynamic light scattering. Broad signal obtained. c No signal obtained.

fifth generation carbosilane-thioether dendrimers were prepared in good to excellent yields (Table 1). The dendrimers were characterized using multinuclear nuclear magnetic resonance (NMR) spectroscopy, dynamic light scattering (DLS), gelpermeation chromatography (GPC), and MALDI-TOF analysis.

Results and Discussion The synthesis of the carbosilane-thioether dendrimers was achieved by alternating thiol-ene and nucleophilic substitution reactions starting with tetravinylsilane as a core (Scheme 1). Preparation of Methoxy-Terminated Dendrimers (Gn-OMe, n ) 1-5). All methoxy-terminated dendrimers (Gn-OMe, n ) 1-5) were synthesized by UV-initiated thiol-ene reactions of the corresponding vinyl-terminated dendrimers with commercially available 3-mercaptopropyltrimethoxysilane. The reactions were performed in methanol for G(1-2)-OMe, and a 1:2 THF/methanol mixture for subsequent generations. The light source was a General Electric 275 W sunlamp, with a total irradiation time of 3 h. The thiol-ene steps were clean and gave quantitative yields of crude products. With the exception of G1-OMe, all Gn-OMe dendrimers were purified by precipitation into small aliquots of methanol. Starting with the preparation of G3-OMe, 1 mol % benzophenone (as photoinitiator) and a 10% excess of the thiol were added to enhance completion of the reaction, thereby preventing long reaction times and eliminating potential side reactions such as cross-linking or disulfide formation. The photoinitiator and excess thiol were easily removed in the methanol precipitation step. Under these conditions, higher generation products G4-OMe and G5-OMe were synthesized successfully despite the rapid exponential increase in dendrimer size. However, the synthesis of G6-OMe was not achieved, presumably due to limiting steric congestion finally being reached. In this case, unreacted vinyl groups from G5-Vi were still present even after using 15% excess thiol and irradiating for 4 h. All of the thiol-ene reaction mixtures were normally characterized by a pale yellow color. Whenever benzophenone was used as a photoinitiator, the mixtures took on a more intense yellow coloration,31 which lessened after purification. The standard borosilicate reaction vessels used for the thiol-ene steps were routinely fitted with a condenser only to minimize the evaporation of solvent due to the small amount of heat generated by the UV lamps. Air was not excluded from the reactions, and the presence of oxygen had no significant negative effect.32 (31) Cramer, N. B.; Scott, J. P.; Bowman, C. N. Macromolecules 2002, 35, 5361–5365. (32) Cramer, N. B.; Davies, T.; O’Brien, A. K.; Bowman, C. N. Macromolecules 2003, 36, 4631–4636.

Preparation of Vinyl-Terminated Dendrimers (Gn-Vi, n ) 1-5). Subsequent reaction of the Gn-OMe dendrimers with an excess of freshly prepared vinylmagnesium bromide in THF gave Gn-Vi in good yields (n ) 1-5) (Table 1). The products were isolated by conventional aqueous acid workup and purified by precipitation into methanol. Preliminary results revealed that long reaction times (>2.5 h) or large excesses of the Grignard reagent (>50%) resulted in the formation of high molecular weight impurities, possibly due to coupling reactions involving vinylmagnesium bromide or the vinyl termini of the dendrimer product.7 Incomplete vinylation of Gn-OMe dendrimers usually resulted in the cross-linking of residual trimethoxysilyl groups during aqueous workup, leading to the formation of high molecular weight material. A minimum 2.5 h reflux period was established by monitoring the complete disappearance of the methoxy signal at 3.5 ppm in the 1H NMR spectrum. It should be noted that 1H NMR spectra of the crude products were relatively clean, suggesting that under the established conditions the Grignard steps proceeded with a minimum of side reactions. Characterizations. We observed a general increase in viscosity of the dendrimers with increasing generation. The first four generations of both types of dendrimers were isolated as liquids to viscous oils. The fifth generation dendrimers in both cases were collected as gummy materials. The pure products possess a pale yellow color and a slightly unpleasant odor. We observed the dendrimers to be soluble in common organic solvents such as diethyl ether, chloroform, and tetrahydrofuran. However, the solubility decreased with increasing solvent polarity and dendrimer size. Multinuclear NMR spectroscopy proved useful in confirming the general dendrimer structures. The NMR spectra of the Gn-OMe and the corresponding Gn-Vi dendrimers are very similar, with the exception of some key resonances. As an example, the 1H, 13C, and 29Si NMR spectra of G2-OMe and G2-Vi are shown in Figure 1. The -Si(OMe)3 signals in G2-OMe appear at 3.5, 50.6, and -42 ppm in the 1H, 13C, and 29Si spectra, respectively. In G2-Vi, the methoxy signal is absent, and vinylic signals are observed at 5.8-6.0 ppm in the 1 H NMR spectrum, and 134.4 ppm and 138.8 ppm in the 13C NMR spectrum. In addition to the changes in the proton and carbon NMR spectra, successful vinylation of a trimethoxy generation dendrimer is marked by a significant downfield shift of the end-group signals in the 29Si NMR spectra. Using NMR spectroscopy, we previously observed the formation of small amounts of Markovnikov product in the thiol-ene reactions of tetravinylsilane.21 Although Markovnikov addition products are undoubtedly present to a small extent throughout the dendrimer structures described here, these additions should not be considered defects, because dendrimer defects are typically defined as incompletely reacted branches. Although the vinyl-terminated dendrimers were stable to air and moisture, the Gn-OMe dendrimers exhibited a tendency to cross-link over time. As such, molecular weight and size characterizations were performed on the vinyl-terminated dendrimers only. Gel permeation chromatography (GPC) was used to confirm the monodispersity of the vinyl-terminated dendrimers and ensure the absence of high molecular weight impurities (Figure 2). GPC results for G1-Vi were unobtainable because its low molecular weight was at the low end of the instrument detection threshold. Furthermore, we did not obtain data for G5-Vi due to its insolubility in the GPC solvent (dimethylformamide). The GPC results for vinyl-terminated G2-G4 in Figure 2 indicate a steady increase in the dendrimer molecular

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Figure 1. 1H (top), 13C (middle), and 29Si (bottom) NMR spectra of G2-OMe (a) and G2-Vi (b) in CDCl3.

Figure 2. GPC results for vinyl-terminated dendrimers G3 and G4.

weights and also confirm their relative monodispersity. Each of the analyzed dendrimers possessed a calculated PDI of approximately 1. In the MALDI-TOF MS analyses of the vinyl-terminated dendrimers (see Supporting Information), molecular ion peaks (M + Ag+) were observed with good resolution for G1-Vi and G2-Vi. Resolution decreased for G3-Vi, and the mass spectrum of G4--Vi showed only a broad and poorly resolved peak above the noise level. No signal at all was observed for G5-Vi. The difficulties associated with the larger dendrimers

were attributed to their large masses and broad isotopic distributions.33 The [M + Ag]+ peak was present for G(1-3)-Vi, and the measured m/z values corresponded closely to the calculated values (Table 1). Even in the case of G4-Vi, the broad peak signal at m/z ≈ 30 000 was in general agreement with the calculated mass value of 29 588 amu. In the mass spectrum of G3-Vi, an additional smaller peak of lower mass was observed at m/z 9647. The difference of 181 amu from the major molecular ion peak at m/z 9828 suggests that one Si-vinyl group of G2-Vi remained unreacted in the preparation of G3-OMe, resulting in G3-Vi possessing 105 vinyl end-groups (106 including the unreacted vinyl group) instead of the ideal 108. Such an incompletely reacted dendrimer would possess a mass 184 units less than theoretical. Additional less intense peaks in the spectrum of G3-Vi suggest the existence of further incompletely reacted products, but in lesser amounts. For many applications, it is of interest to know the approximate sizes of dendrimers in solution. As a final characterization, the hydrodynamic diameters of G(1-5)-Vi were measured in THF using dynamic light scattering techniques. The number-weighted averages of dendrimer sizes are listed in Table 1 and display a general increase from 0.76 nm for G1-Vi to 8.2 nm for G5-Vi. (33) ESI MS experiments were equally uninformative.

Preparation of Carbosilane-Thioether Dendrimers Scheme 2. Functionalization of G3-Vi with 2-Mercaptoethanol

Peripheral Functionalization. Because of the large number of end groups on a dendrimer, peripheral functionalization can significantly affect properties such as solubility and crystallinity. As mentioned in the Introduction, Frey and co-workers utilized the thiol-ene reaction to peripherally functionalize carbosilane dendrimers with perfluoroalkyl groups.27 Because of the commercial availability of a large number of functionally diverse thiols, the thiol-ene reaction is an attractive method for attaching various functional groups to the periphery of dendrimers.21 As a representative example, we reacted G3-Vi with 2-mercaptoethanol in THF (Scheme 2). After a 4 h irradiation period (without photoinitiator), hydroxyl-functionalized G3-OH was isolated as a pale yellow gum in 68% yield after purification. G3-OH was insoluble in water, but soluble in THF and DMSO. In conclusion, we have demonstrated that new carbosilanethioether dendrimers can be synthesized using thiol-ene chemistry as a key synthetic step. Relatively mild conditions minimize the occurrence of side reactions and lead to simple nonchromatographic purification methods. The high yields, robustness, and practical simplicity of the thiol-ene reaction are attractive features of this synthesis. Moreover, the presence of the thioether functionality introduces these carbosilane dendrimers as potential substrates for additional investigations, including sulfur oxidation chemistry and encapsulation applications.

Experimental Section Materials and Equipment. (3-Mercaptopropyl)trimethoxysilane (Alfa Aesar) and methanol (VWR) were distilled before use. 2-Mercaptoethanol (Alfa Aesar), vinyl bromide (Aldrich), and benzophenone (Fisher) were used as received. THF (Mallinckrodt) was dried over activated 3 Å molecular sieves. Ultraviolet radiation was provided by a General Electric (GE) 275 W sunlamp bulb with a total light output of 9.28 W and a maximum emission at 365 nm.34 All irradiation reactions were carried out in 50 or 100 mL single-necked round-bottomed flasks at a distance of 13 cm from the radiation source. NMR data were obtained on a JEOL ECA500 NMR spectrometer. Elemental analyses were obtained using a CE Elantech Thermo-Finnigan Flash 1112 elemental analyzer. Dynamic light scattering (DLS) was conducted with a Malvern Zetasizer Nano-S equipped with a 4 mW, 633 nm He-Ne laser, and an Avalanche photodiode detector at an angle of 173°. Gel permeation chromatography (GPC) was conducted in DMF with a Viscotek GPC pump and light scattering detection; columns: ViscoGel I-series G3000 and G4000 mixed bed columns. MALDITOF mass spectrometry was provided by the Washington University (34) Dryja, T. P.; Kimball, G. P.; Albert, D. M. InVest. Ophthalmol. Visual Sci. 1980, 19, 559–562.

Organometallics, Vol. 28, No. 11, 2009 3171 Mass Spectrometry Resource with support from the NIH National Center for Research Resources (grant no. P41RR0954) and was performed using an ABI 4700 TOF analyzer and an ABI VoyagerDE STR TOF analyzer. G1-OMe. This compound was synthesized according to ref 21. G1-Vi. Magnesium turnings (3.3 g, 140 mmol) were added to a 250 mL three-necked round-bottomed flask equipped with a stir bar, septum, dry ice condenser, and addition funnel. The reaction vessel was flushed with N2 gas. The magnesium turnings were covered with THF, and a crystal of iodine was added. A small amount of vinyl bromide dissolved in THF (∼2 mL) was added to initiate the reaction, followed by addition of THF (75 mL). The remaining solution of vinyl bromide (9.6 mL total, 140 mmol) in THF (25 mL) was added dropwise to the reaction mixture to maintain a gentle reflux. The resulting Grignard reagent was refluxed for 1 h and allowed to cool to room temperature. A solution of G1-OMe (8.0 g, 8.7 mmol) in THF (25 mL) was added dropwise to the mixture. After addition, the reaction mixture was refluxed for 2.5 h and then cooled to room temperature. The reaction mixture was added to chilled saturated aqueous NH4Cl (100 mL) and poured into a separatory funnel by gravity filtration through a plug of glass wool. The aqueous layer was extracted with diethyl ether (2 × 50 mL), and the combined organic layers were washed with deionized water (2 × 50 mL) and once with saturated aqueous NaCl. After being dried over anhydrous MgSO4, most volatiles were removed before precipitation of the crude product as an oil by pouring the concentrated solution into methanol (∼100 mL). Drying the pure product in vacuo afforded G1-Vi as a pale yellow oil (6.3 g, 83%). 1H NMR (500 MHz, CDCl3): δ 0.84 (t, 3J ) 8.3 Hz, -CH2SiVi, 8H), 0.95 (t, 3J ) 8.6 Hz, -SiCH2CH2S, 8H), 1.62 (quintet, 3J ) 7.9 Hz, -CH2CH2CH2, 8H), 2.50-2.54 (m, -CH2SCH2, 16H), 5.79 (dd, 2J ) 6.0 Hz, 3J ) 24.1 Hz, -CHdCHH, 12H), 6.08-6.16 (m, -CHdCHH, 24H). 13C NMR (125 MHz, CDCl3): δ 12.4 (-CH2SiVi), 13.2 (-SiCH2CH2), 23.9 (-SiCH2CH2S), 27.1 (-CH2CH2CH2), 35.6 (-SCH2CH2CH2), 134.4 (-CHdCH2), 134.8 (-CHdCH2). 29Si NMR (99 MHz, CDCl3): δ -19.4 (-SiVi), 2.68 (-SiCH2CH2S). Anal. Calcd for C44H76S4Si5: C, 60.48; H, 8.77. Found: C, 60.23; H, 8.97. G2-OMe. G1-Vi (2.0 g, 2.3 mmol), (3-mercaptopropyl)trimethoxylsilane (5.1 mL, 28 mmol), and methanol (30 mL) were added to a 50 mL single-necked round-bottomed flask equipped with a stir bar and condenser. The reaction mixture was irradiated for 3 h, and the reaction progress was monitored by 1H NMR spectroscopy. The initial reaction mixture was concentrated via rotary evaporation. Methanol (20 mL) was added to the residue with stirring for about 30 min. The resulting mixture was allowed to settle, and the solvent was decanted from the oil. This purification procedure was repeated with another 20 mL aliquot of methanol. The pure product was dried in vacuo to afford G2-OMe as a pale yellow oil (5.4 g, 73%). 1H NMR (500 MHz, CDCl3): δ0.68-0.84 (m, CH2Si(OCH3), CH2CH2CH2Si, 32H), 0.92-0.99 (m, SiCH2CH2S, 32H), 1.53-1.57 (m, CH2CH2CH2Si, 8H), 1.64-1.70 (quintet, 3J ) 7.7 Hz, -CH2CH2SiOCH3, 24H), 2.50-2.54 (m, -CH2SCH2, 64H), 3.55 (s, -SiOCH3, 108H). 13C NMR (125 MHz, CDCl3): δ 8.69 (-CH2Si(OCH3)3), 11.9 (CH2CH2CH2Si), 13.2 (-SiCH2CH2S), 22.9 (-SiCH2CH2S), 23.9, 27.1 (-CH2CH2CH2), 27.6, 35.0, 36.0 (-SCH2CH2CH2), 50.6 (-SiOCH3). 29Si NMR (99 MHz, CDCl3): δ -41.9 (-SiOCH3), 2.74, 3.05 (-SiCH2CH2). Anal. Calcd for C116H268O36S16Si17: C, 43.14; H, 8.36. Found: C, 43.13; H, 8.31. G2-Vi. This compound was prepared from G2-OMe (4.2 g, 1.3 mmol), vinyl bromide (4.3 mL, 61 mmol), magnesium turnings (1.5 g, 61 mmol), and THF (125 mL total) using the preparative procedure for G1-Vi. The product was obtained as a pale yellow oil (3.3 g, 82%). 1H NMR (500 MHz, CDCl3): δ 0.68-0.99 (m, -CH2Si, 64H), 1.54-1.66 (m, -CH2CH2CH2, 32H), 2.49-2.58 (m, -CH2SCH2, 64H), 5.79 (dd, 2J ) 6.3 Hz, 3J ) 24.6 Hz,

3172 Organometallics, Vol. 28, No. 11, 2009 -CHdCHH, 36H), 6.07-6.16 (m, -CHdCHH, 72H). 13C NMR (125 MHz, CDCl3): δ 12.0, 12.4 (-CH2SiVi), 13.2 (-SiCH2CH2), 23.9 (-SiCH2CH2S), 27.2 (-CH2CH2CH2), 27.6, 35.6 (-SCH2CH2CH2), 36.1, 134.4 (-CHdCH2), 134.8 (-CHdCH2). 29Si NMR (99 MHz, CDCl3): δ -19.4 (-SiVi), 2.80, 3.08 (-SiCH2CH2S). Anal. Calcd. for C152H268S16Si17: C, 59.15; H, 8.75. Found: C, 59.18; H, 9.08. G3-OMe. This compound was prepared from G2-Vi (2.4 g, 0.78 mmol), (3-mercaptopropyl)trimethoxylsilane (5.6 mL, 30 mmol), benzophenone (0.056 g, 0.31 mmol), and a 1:2 THF/ methanol solution (45 mL) using the preparative procedure for G2-OMe, except the precipitation procedure was performed three times with methanol (3 × 25 mL). The pure product was dried in vacuo to afford G3-OMe as a viscous pale yellow oil (6.7 g, 85%). 1 H NMR (500 MHz, CDCl3): δ 0.68-0.76 (m, CH2Si(OCH3), CH2CH2CH2Si, 104H), 0.93-0.99 (m, SiCH2CH2S, 104H), 1.54-1.71 (m, CH2CH2CH2Si, 104H), 2.51-2.58 (m, -CH2SCH2, 208H), 3.56 (s, -SiOCH3, 324H). 13C NMR (125 MHz, CDCl3): δ 8.59 (-CH2SiOCH3), 11.9, 12.1, 13.2 (-SiCH2CH2S), 22.8 (-SiCH2CH2S), 23.8, 27.0 (-CH2CH2CH2), 27.5, 27.7, 34.9 (-SCH2CH2CH2), 35.8, 36.1, 50.5 (-SiOCH3). 29Si NMR (99 MHz, CDCl3): δ -42.0 (-SiOCH3), 2.99 (-SiCH2CH2). Anal. Calcd for C368H844O108S52Si53: C, 43.53; H, 8.38. Found: C, 43.52; H, 8.51. G3-Vi. This compound was prepared from G3-OMe (5.0 g, 0.49 mmol), vinyl bromide (5.3 mL, 75 mmol), magnesium turnings (1.8 g, 75 mmol), and THF (125 mL total) using the preparative procedure for G1-Vi. The product was obtained as a viscous pale yellow oil (4.5 g, 94%). 1H NMR (500 MHz, CDCl3): δ 0.68-0.96 (m, -CH2Si, 208H), 1.51-1.66 (m, -CH2CH2CH2, 104H), 2.50-2.54 (m, -CH2SCH2, 208H), 5.80 (dd, 2J ) 5.8 Hz, 3J ) 24.6 Hz, -CHdCHH, 108H), 6.06-6.17 (m, -CHdCHH, 216H). 13 C NMR (125 MHz, CDCl3): δ 12.0, 12.3 (-CH2SiVi), 13.2 (-SiCH2CH2), 23.9 (-SiCH2CH2S), 27.2 (-CH2CH2CH2), 27.6, 27.9, 35.6 (-SCH2CH2CH2), 36.0, 36.3, 134.4 (-CHdCH2), 134.8 (-CHdCH2). 29Si NMR (99 MHz, CDCl3): δ -19.4 (-SiVi), 3.06 (-SiCH2CH2S). Anal. Calcd for C476H844S52Si53: C, 58.80; H, 8.75. Found: C, 58.30; H, 9.23. G4-OMe. This compound was prepared from G3-Vi (2.0 g, 0.21 mmol), (3-mercaptopropyl)trimethoxylsilane (4.5 mL, 24 mmol), benzophenone (0.045 g, 0.25 mmol), and a 1:2 THF/ methanol solution (45 mL) using the preparative procedure for G2-OMe, except the precipitation procedure was performed three times with methanol (3 × 25 mL). The pure product was dried in vacuo to afford G4-OMe as a viscous pale yellow oil (4.6 g, 71%). 1 H NMR (500 MHz, CDCl3): δ 0.69-0.76 (m, CH2Si(OCH3), CH2CH2CH2Si, 320H), 0.93-0.97 (m, SiCH2CH2S, 320H), 1.54-1.71 (m, CH2CH2CH2Si, 320H), 2.52-2.54 (m, -CH2SCH2, 640H), 3.59 (s, -SiOCH3, 972H). 13C NMR (125 MHz, CDCl3): δ 8.72 (-CH2SiOCH3), 12.1, 13.2 (-SiCH2CH2S), 22.9 (-SiCH2CH2S), 24.0, 27.2 (-CH2CH2CH2), 27.7, 35.0 (-SCH2CH2CH2), 36.0, 50.6 (-SiOCH3). 29Si NMR (99 MHz, CDCl3): δ -41.9 (-SiOCH3), 3.10 (-SiCH2CH2). Anal. Calcd for C1124H2572O324S160Si161: C, 43.65; H, 8.38. Found: C, 43.70; H, 8.53. G4-Vi. This compound was prepared from G4-OMe (3.9 g, 0.13 mmol), vinyl bromide (4.0 mL, 57 mmol), magnesium turnings (1.4 g, 57 mmol), and THF (125 mL total) using the preparative procedure for G1-Vi. The product was obtained as a viscous pale yellow oil (3.0 g, 78%). 1H NMR (500 MHz, CDCl3): δ 0.68-0.95 (m, -CH2Si, 640H), 1.53-1.68 (m, -CH2CH2CH2, 320H), 2.50-2.54 (m, -CH2SCH2, 640H), 5.79 (dd, 2J ) 5.45 Hz, 3J ) 24.1 Hz, -CHdCHH, 324H), 6.06-6.16 (m, -CHdCHH, 648H). 13 C NMR (125 MHz, CDCl3): δ 12.1, 12.4 (-CH2SiVi), 13.3 (-SiCH2CH2), 23.9 (-SiCH2CH2S), 27.2 (-CH2CH2CH2), 27.7,

Rissing and Son 35.6 (-SCH2CH2CH2), 36.1, 134.4 (-CHdCH2), 134.8 (-CHd CH2). 29Si NMR (99 MHz, CDCl3): δ -19.3 (-SiVi), 3.09 (-SiCH2CH2S). Anal. Calcd for C1448H2572S160Si161: C, 58.68; H, 8.75. Found: C, 58.30; H, 8.93. G5-OMe. This compound was prepared from G4-Vi (2.0 g, 0.067 mmol), (3-mercaptopropyl)trimethoxylsilane (5.6 mL, 30 mmol), benzophenone (0.06 g, 0.31 mmol), and a 1:2 THF/methanol solution (45 mL) using the preparative procedure for G2-OMe, except the precipitation procedure was performed three times with methanol (3 × 25 mL). The pure product was dried in vacuo to afford G5-OMe as a pale yellow gum (3.0 g, 46%). 1H NMR (500 MHz, CDCl3): δ 0.73-0.98 (m, SiCH2, 1936H), 1.57-1.69 (m, CH2CH2CH2Si, 968H), 2.52-2.55 (m, -CH2SCH2, 1936H), 3.59 (s, -SiOCH3, 2916H). 13C NMR (125 MHz, CDCl3): δ 8.73 (-CH2SiOCH3), 12.1, 13.2 (-SiCH2CH2S), 23.0 (-SiCH2CH2S), 24.1, 27.2 (-CH2CH2CH2), 27.7, 35.0 (-SCH2CH2CH2), 36.1, 50.6 (-SiOCH3). 29Si NMR (99 MHz, CDCl3): δ -41.8 (-SiOCH3), 3.12 (-SiCH2CH2). Anal. Calcd for C3392H7756O972S484Si485: C, 43.69; H, 8.38. Found: C, 43.58; H, 8.47. G5-Vi. This compound was prepared from G5-OMe (2.5 g, 0.027 mmol), vinyl bromide (2.6 mL, 36 mmol), magnesium turnings (0.90 g, 36 mmol), and THF (125 mL total) using the preparative procedure for G1-Vi. The product was obtained as a pale yellow gum (2.1 g, 87%). 1H NMR (500 MHz, CDCl3): δ 0.69-0.95 (m, -CH2Si, 1936H), 1.55-1.65 (m, -CH2CH2CH2, 968H), 2.52-2.55 (m, -CH2SCH2, 1936H), 5.78 (dd, 2J ) 5.45 Hz, 3J ) 24.1 Hz, -CHdCHH, 972H), 6.06-6.16 (m, -CHdCHH, 1944H). 13C NMR (125 MHz, CDCl3): δ 12.4 (-CH2SiVi), 13.3 (-SiCH2CH2S), 24.0 (-SiCH2CH2S), 27.2 (-CH2CH2CH2), 27.7, 35.6 (-SCH2CH2CH2), 36.1, 134.5 (-CHdCH2), 134.8 (-CHdCH2). 29Si NMR (99 MHz, CDCl3): -19.3 (-SiVi), 3.10 (-SiCH2CH2S). Anal. Calcd for C4364H7756S484Si485: C, 58.65; H, 8.75. Found: C, 58.10; H, 8.65. Functionalization of G3-Vi with 2-Mercaptoethanol (G3-OH). G3-Vi (0.30 g, 0.031 mmol) and 2-mercaptoethanol (0.25 mL, 3.6 mmol) were added to a 20 mL single-necked round-bottomed flask equipped with a stir bar and condenser. THF (10 mL) was added, and the reaction mixture was irradiated for 4 h. Most of the volatiles were removed via rotary evaporation, and the residue was poured into water to precipitate the product as an oil. The supernatant was decanted from the oil, and the product was dissolved in THF (30 mL). After being dried over anhydrous MgSO4, the solution was concentrated to afford G3-OH as a pale yellow gum (0.38 g, 68%). 1H NMR (500 MHz, DMSO-d6): δ 0.65-0.89 (m, -SiCH2CH2, 424H), 1.48 (-CH2CH2CH2, 104H), 2.52-2.55 (m, -CH2SCH2, 640H), 3.54 (-CH2OH, 216H), 4.68 (-CH2OH, 108H). 13C NMR (125 MHz, DMSO6): δ 11.7, 13.2 (-SiCH2CH2), 24.0, 27.1, 27.3 (-SiCH2CH2), 34.3 (-SCH2CH2OH), 35.4, 61.5 (-SCH2CH2OH). 29Si NMR (99 MHz, DMSOd6): δ 3.03, 4.14. Anal. Calcd for C692H1492S160Si53O108: C, 45.76; H, 8.28. Found: C, 45.51; H, 8.38.

Acknowledgment. We thank the Robert A. Welch Foundation (grant no. N-1375) and Southern Methodist University for financial support of this work. We also thank Prof. Brent Sumerlin’s research group for assistance with gel-permeation chromatography and dynamic light scattering measurements. Supporting Information Available: MALDI-TOF mass spectra for G(1-4)-Vi. This material is available free of charge via the Internet at http://pubs.acs.org. OM9001395