Nonhydrolytic Synthesis of Branched Alkoxysiloxane Oligomers Si[OSiH(OR)₂]₄ (R=Me, Et)

Abstract

Beyond silanol: A branched siloxane oligomer bearing terminal dialkoxysilyl groups was nonhydrolytically synthesized by direct alkoxysilylation of a tetraalkoxysilane with a chlorodialkoxysilane in the presence of the Lewis acid BiCl3 (see scheme). The reaction proceeds without the formation of intermediate silanol groups, and provides a selective route for siloxane-based oligomers. Silicon alkoxides are ideal starting materials for the preparation of silica-based materials.1 The development of synthetic methods toward various alkoxysiloxane oligomers with finely controlled structures and reactivities is important for the synthesis of materials with defined compositions, structures, morphologies, and functionalities.2, 3 However, examples of the rational synthesis of alkoxysiloxane oligomers are limited. The controlled formation of SiOSi bonds is a key step in the synthesis of alkoxysiloxane oligomers.2–4 A conventional synthesis involves hydrolysis of silicon alkoxides or chlorosilanes to form silanol groups and a subsequent condensation reaction. Although the reaction of chloroalkoxysilanes with organosilanols have been reported,3, 4 the number of molecules synthesized by using this reaction are quite limited because the reaction is restricted to the cases where the resulting organosilanols are stable. The formation of silanol groups during the reaction causes unwanted side reactions, such as self-condensation of silanols and subsequent hydrolysis of terminal alkoxy groups. Therefore, the synthesis of siloxane oligomers that contain alkoxy groups with defined structures is a challenge in circumventing the problem of side reactions. The formation of siloxane bonds without using silanols has attracted much interest.5–16 Alkoxysiloxanes can be synthesized by the reaction between chlorosilane and sodium alkoxysilanolates.5 On the other hand, the use of alkoxysilanes as a precursor with a Lewis acid catalyst is the most promising pathway.8–15 The formation of siloxane bonds proceeds with the generation of RX (X=Cl,8–10 Br,10 I,10 AcO,11 or H12–15). No competing reagents, such as compounds containing silanol groups, H2O, or HCl, are involved in the overall process. Oligosiloxanes with defined structures that contain more than one alkoxy group are difficult to synthesize,14, 15 as side reactions, such as ligand exchange, compete with the formation of siloxane bonds.8, 9, 13, 14 Herein we report the nonhydrolytic synthesis, with suppressed side reactions, of branched alkoxysiloxanes Si[OSiH(OR)2]4 (R=Me (1), Et (2)), which possess both reactive SiOR and SiH groups. The reaction proceeds by direct alkoxysilylation of tetraalkoxysilanes with ClSiH(OR)2 in the presence of BiCl3 (Scheme 1). BiCl3, a weak Lewis acid, was chosen as the SiOR groups would not be retained when conventional Lewis acids, such as AlCl3 or FeCl3, were used.9, 10 Si(OtBu)4 and Si(OCHPh2)4 were chosen as precursors because the stable carbocations (tBu+ and Ph2HC+, respectively) probably formed in the reaction should enhance siloxane formation while suppressing other competing side reactions. In addition, the bulky substituents should provide the benefit of higher stability toward hydrolysis than conventional SiOMe and SiOEt groups.1a Nonhydrolytic synthesis of branched alkoxysiloxane oligomers Si[OSiH(OR)2]4. A conventional synthesis of 1 and 2 is by ambient hydrolysis of a tetraalkoxysilane and subsequent alkoxysilylation with a chloroalkoxysilane. However, unwanted hydrolysis of the terminal alkoxy groups and generation of other oligomers by condensation will inevitably occur. The isolation of an unstable tetrahydroxysilane intermediate (monosilicic acid, Si(OH)4) is also impractical. With our method, Si(OtBu)4 and Si(OCHPh2)4 can be used as precursors instead of Si(OH)4. The 1H NMR spectrum of 1 (Figure 1), which was synthesized from Si(OtBu)4, shows signals at δ=4.27 and 3.58 ppm (intensity ratio 1.0:6.2), which can be assigned to Ha and Hb,17 respectively. The 13C NMR spectrum of 1 (Figure S1 in the Supporting Information) shows a signal at δ=49.9 ppm, which is assigned to Cb. The 29Si NMR spectrum (Figure 1) shows signals at δ=−110.5 and −65.4 ppm (intensity ratio 1.0:4.3), which correspond to Q4(Si(OSi)4) and T1(SiH(OSi)(OMe)2), respectively. Further evidence for the structure of 1 is obtained from the 29Si–1H HMBC spectrum (Figure 1).18 The signals of Q4 and Ha show a correlation, which indicates the SiOSiH connectivity. The signals for T1 and Hb also show a correlation that arises from the SiOCH connectivity. The direct bonding of Ha to the Si atom T1 was confirmed by the presence of doublet correlation signals. The high-resolution EI-MS spectrum shows a peak at m/z 425.0242, which corresponds to [M−MeO−]+ (calcd for C7H25O11Si5+: 425.0243), thus confirming the selective formation of 1.19 29Si–1H HMBC spectrum of 1 synthesized from Si(OtBu)4. A gas-phase product that was obtained during purification of 1 by solvent evaporation shows a signal at δ=l.61 ppm in the 1H NMR spectrum (Figure S3 in the Supporting Information) and signals at δ=68.0 and 34.6 ppm in the 13C NMR spectrum (Figure S4 in the Supporting Information), which can be assigned to the formation of tBuCl.20 This result also confirms that the reaction shown in Scheme 1 took place. When Si(OCHPh2)4 was used as a precursor instead of Si(OtBu)4, all the NMR spectra (Figure S5–7 in the Supporting Information) and HRMS data (m/z 425.0244) of a crude sample before distillation (in this case, 1 could not be isolated because of the similar boiling points of Ph2CHCl and 1) confirmed the formation of 1, thus indicating that Si(OCHPh2)4 can also be used in the synthesis. The 1H NMR spectrum of the crude sample (Figure S6 in the Supporting Information) shows a multiplet signal at δ=7.19–7.35 ppm (10 H), and a singlet at δ=6.10 ppm (1 H); the 13C NMR spectrum (Figure S7 in the Supporting Information) shows signals at δ=64.3, 127.8, 128.0, 128.6, and 141.3 ppm, which correspond to chlorodiphenylmethane (Ph2CHCl).20 The formation of an alkyl chloride is consistent with the behavior of the reaction with Si(OtBu)4. The synthesis of Si[OSiH(OEt)2]4 (2) from Si(OtBu)4 was also investigated because the SiOEt group exhibits a different hydrolysis behavior and is frequently used in sol–gel processes. The 1H NMR spectrum of 2 (Figure 2) shows signals at δ=4.33, 3.86 and 1.24 ppm (intensity ratio 0.9:4.0:6), which can be assigned to Ha, Hb, and Hc,21 respectively. The 13C NMR spectrum (Figure S8 in the Supporting Information) also shows signals at δ=58.3 and 18.2 ppm, which can be assigned to Cb and Cc, respectively. These results show that the SiH and SiOEt groups are retained in the product. The 29Si NMR spectrum of 2 (Figure 2) shows signals at δ=−111.0 and −68.3 ppm (intensity ratio of 1:4.1), which can be assigned to Q4 and T1, respectively. Further evidence for 2 is obtained from the 29Si–1H HMBC spectrum (Figure 2). The signals of T1 and Hb show a correlation that arises from SiOCH, in addition to an SiOSiH correlation. The high-resolution EI-MS spectrum shows a peak at m/z 523.1335, which corresponds to [M−EtO−]+ (calcd for C14H39O11Si5+: m/z 523.1339), which confirms the selective formation of 2. 29Si–1H HMBC spectrum of 2 synthesized from Si(OtBu)4. The reaction of chlorotrimethoxysilane (ClSi(OMe)3) with Si(OCHPh2)4 was also investigated. The 13C NMR spectrum (Figure S9 in the Supporting Information) of the crude sample shows several signals around δ=51.1–51.4 ppm, which correspond to the SiOCH3 groups. Signals for these groups also appear around 3.57 ppm in the 1H NMR spectrum (Figure S10 in the Supporting Information). The 13C NMR spectrum shows signals at δ=64.4, 127.9, 128.1, 128.8, and 141.5 ppm, and the 1H NMR spectrum shows signals at δ=6.28 and 7.05–7.44 ppm. These signals indicate the generation of chlorodiphenylmethane by siloxane bond formation and thus suggest that the same reaction scheme can successfully be applied to ClSi(OMe)3. However, the branched alkoxysiloxane like 1 or 2 was not obtained in this case. The 29Si NMR spectrum of the product (Figure 3) shows multiple signals at δ=−78.2, −85.6 to −85.8, and −93.6 ppm, which can be assigned to Si(OMe)4 ,22 the silicon atom in 3, and a mixture of linear alkoxysiloxanes, such as octamethoxytrisiloxane (4; Scheme 2 a).17 These various alkoxysilanes were formed by transesterification and ligand exchange in addition to the formation of siloxane bonds (Scheme 2 b). The difference between Si(OMe)3 and SiH(OMe)2 groups is presumably due to the larger steric hindrance of Si(OMe)3 and the weaker electron-withdrawing effect of the hydrogen atom attached to the silicon atom. 29Si NMR spectrum of the sample obtained from the Si(OCHPh2)4/ClSi(OMe)3/BiCl3 system. Products and proposed competing reactions with siloxane bond formation in the Si(OCHPh2)4/ClSi(OMe)3/BiCl3 system. We also examined the use of Si(OiPr)4 in order to investigate the reactivity of the alkoxy groups. The 13C NMR spectrum (Figure S11 in the Supporting Information) of the crude sample from the Si(OiPr)4/ClSiH(OMe)2/BiCl3 reaction system shows many signals between δ=25.2–25.7, 66.1–68.4, and 50.2–52.1 ppm, which can be assigned to OCH(CH3)2 and OCH3 groups, respectively. The 29Si NMR spectrum of the product (Figure S12 in the Supporting Information) shows several signals down to δ=−80.2 ppm, which can be assigned to ClaSiHb(OMe)c(OiPr)d (a+b+c+d=4), thus showing that ligand exchange and transesterification occur prior to siloxane bond formation.9 In this case, the 13C and 1H NMR spectra of the crude product (Figures S11 and S13 in the Supporting Information) did not show the signals that correspond to 2-chloro-2-methylpropane, thus indicating that siloxane bond formation did not occur. Although alkoxy groups that can generate stable carbocations are known to enhance siloxane bond formation in nonhydrolytic sol–gel processes and related reactions,7, 9, 10 iPr+ is less stable than tBu+ and Ph2HC+,24 and therefore the expected reaction did not occur. A plausible reaction mechanism (Scheme 3) is proposed on the basis of previous studies on the reaction mechanisms between SiX (X=Cl, Br, I, or H) and a Lewis acid,7–10, 13, 23 and consists of the following steps: 1) The SiCl bond is activated by the BiCl3 catalyst; 2) the silyloxonium cation is formed by nucleophilic attack of the alkoxysilane; 3) the cation rearrangement reaction is driven by the stability of tBu+ or Ph2HC+ and subsequent attack of Cl− on the carbocation; 4) the siloxane bond is formed by elimination of R′Cl; 5) compounds 1 and 2 are formed by repeating steps (1) to (4). Proposed reaction mechanism for the siloxane formation. See main text for descriptions of each step. As observed for the reaction systems of Si(OCHPh2)4/ClSi(OMe)3/BiCl3 and Si(OiPr)4/ClSiH(OMe)2/BiCl3, siloxane formation competes with unwanted transesterification and/or ligand-exchange reactions (Scheme S1 in the Supporting Information). Thus, the most important criterion for the success of direct alkoxysilylation to obtain 1 and 2 is preferential siloxane formation. For this purpose, the following factors are crucial: 1) the stability of carbocations obtained from starting alkoxysilanes, 2) molecular structures of the silylating agents, and 3) the use of appropriate Lewis acid catalysts. A previously proposed reaction mechanism for nonhydrolytic formation of amorphous gels7–9 involves competition between siloxane formation and ligand exchange. In this study, we achieved selective siloxane formation and obtained 1 and 2. We believe that our findings may also contribute to the understanding of nonhydrolytic sol–gel processes for silica and metal oxides. In order to clarify the essential role of BiCl3 in the reaction, we carried out the reaction without the use of the Lewis acid catalyst. When Si(OCHPh2)4 was allowed to react with ClSiH(OMe)2 without the addition of BiCl3, 1 was not formed, even after a longer reaction time (12 h).25 This result indicates that BiCl3 accelerates the siloxane formation. Previous reports show that the presence of BiCl3 does not lead the primary alkoxy groups (SiOMe, SiOEt) in the products to form siloxane bonds.10 On the other hand, siloxane bond formation prior to the occurrence of side reactions was reported to occur for the SiOiPr group when B(C6F5)3 was used as catalyst.13 However, the selective synthesis of alkoxysiloxane oligomers such as 1 and 2 is impossible with B(C6F5)3 because primary alkoxy groups in the products have a strong tendency to form extended siloxane networks.14 In conclusion, we have demonstrated the direct alkoxysilylation of alkoxysilanes catalyzed by BiCl3 occurs without the formation of silanols. Alkoxysiloxanes 1 and 2 were synthesized nonhydrolytically by the reaction of stable tetraalkoxysilanes that possess bulky alkoxy groups (Si(OR′)4 R′=tBu, CHPh2) with silylating agents (ClSiH(OR)2). Stable carbocations (tBu+, Ph2HC+) and molecular structures of silylating agents are important in the BiCl3-catalyzed siloxane formation prior to the occurrence of other competing reactions. The conventional methods that involve hydrolysis are impractical for the synthesis of branched alkoxysilanes 1 and 2; our approach represents a new strategy for the synthesis of various oligomeric silicon alkoxides that can be applied to a wide variety of sol–gel reactions and hybrid materials. Further investigations into the versatility of direct alkoxysilylation together with applications of this synthetic methodology to alkoxysiloxane oligomers toward hybrid silica materials are in progress. Compound 1 was synthesized in a one-pot procedure. Solutions of BiCl3 (0.29 g, 0.92 mmol) in acetonitrile (15 mL) and Si(OtBu)4 (5.9 g, 18.4 mmol) in hexane (20 mL) were added to a solution of ClSiH(OMe)2 in a 200 mL Schlenk flask. ClSiH(OMe)2 was prepared from HSiCl3 (25 g, 185 mmol) and MeOH (15 mL) at 0 °C (see the Supporting Information for details). Although a certain amount of HSi(OMe)3 was present in the silylating agent, the reaction was not affected because the compound does not contain SiCl groups. The overall molar ratio was Si(OtBu)4/HSiCl3/MeOH/BiCl3=1:10:20:0.05. The mixture was stirred for 3 h at RT. The solvents, excess ClSiH(OMe)2, HSi(OMe)3, and tBuCl were removed under reduced pressure. Compound 1 (2.2 g, 4.8 mmol) was isolated by vacuum distillation. NMR spectra were recorded before and after the distillation. Compound 2 was also synthesized in a one-pot procedure. Solutions of BiCl3 (0.29 g, 0.92 mmol) in acetonitrile (15 mL) and Si(OtBu)4 (5.9 g, 18.4 mmol) in hexane (20 mL) were added to a solution of ClSiH(OEt)2 in a 200 mL Schlenk flask. ClSiH(OEt)2 was prepared from HSiCl3 (25 g, 185 mmol) and EtOH (21.6 mL) at 0 °C (see the Supporting Information for details). Although a certain amount of HSi(OEt)3 was present in the silylating agent, the reaction was not affected. The molar ratio was Si(OtBu)4/HSiCl3/EtOH/BiCl3=1:10:20:0.05. The mixture was stirred for 3 h at RT, after which time pyridine (29.8 mL) and EtOH (10.8 mL) were added and the mixture was stirred for 1 h for ethoxylation of the SiCl groups formed by ligand exchange. After the reaction, volatile components were removed under reduced pressure and extracted with hexane. Compound 2 (0.68 g, 1.2 mmol) was isolated by vacuum distillation. Detailed facts of importance to specialist readers are published as ”Supporting Information”. 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