Abstract
Laddersiloxanes, that is, ladder silsesquioxanes with defined structures, could be obtained by stepwise synthesis starting from cyclic silanols. These compounds were shown to have high thermal stability. As an extension of the previous work, the first nonacyclic ladder silsesquioxanes were synthesized by the reaction of bicyclic silanol with tricyclic tetrachloride, which were obtained from cyclic silanols. The structure was confirmed by spectral measurements, and the spectral features of a series of ladder polysilsesquioxanes with determined structures were analyzed.
1. Introduction
Recently, the interest in ladder silsesquioxanes has been growing mainly because of their high thermal stability and their application to functional materials [1–3]. To study the relationship between the structure and properties, we prepared ladder silsesquioxanes, determined their structures, and investigated the properties. We referred to these ladder silsesquioxanes as “laddersiloxanes,” and we reported the syntheses and crystallographic analysis of tricyclic laddersiloxanes [4, 5]; pentacyclic laddersiloxanes [6]; bi-, tri-, tetra-, and pentacyclic laddersiloxanes with an all anti conformation [7]; extendible pentacyclic laddersiloxanes [8] and heptacyclic laddersiloxanes by a stereocontrolled approach [9]. As an extension, herein, we report the synthesis of the first nonacyclic ladder silsesquioxanes.
2. Experiments
Preparative recycle-type high-performance liquid chromatography (HPLC) was carried out by using a JAI LC-908 HPLC with a Chemco 7-ODS-H column (20 × 250 mm). The Fourier-transform nuclear magnetic resonance (NMR) spectra were obtained by using a JEOL model Λ-500 (1H NMR at 500.00 MHz, 13C NMR at 125.65 MHz, and 29Si NMR at 99.25 MHz). The chemical shifts were reported as δ units (ppm) relative to SiMe4, and the residual solvent peaks were considered as the standard. Electron impact mass spectrometry was performed with a JEOL JMS-DX302. The infrared spectra were measured with a Shimadzu FTIR-8700.
2.1. Preparation of Bicyclic Ladder Silanol (4)
A solution of (i-PrPhSiCl)2O [6] (1.02 g, 2.66 mmol) in pyridine (8 mL) was added dropwise to [i-Pr(OH)SiO]4 (1.02 g, 2.45 mmol) [10–12] in pyridine (8 mL) for 3 h at 0°C. The mixture was stirred for an additional 20 min at 0°C. The reaction mixture was added to saturated aqueous NH4Cl and hexane, and two phases were separated. The aqueous phase was extracted with hexane. The organic phase was then washed with saturated aqueous NH4Cl, then dried over anhydrous magnesium sulfate, and concentrated. The crude product was separated by dry column chromatography (eluent: hexane/Et2O = 9 : 1), followed by the separation using recycle-type HPLC (eluent: MeOH/THF = 9 : 1) to give bicyclic ladder silanols (4, 1.00 g, 56%) (isomeric mixture). They were identified by the comparison with authentic sample [4, 5].
2.2. Reaction of Tricyclic Laddersiloxanes Tetrachloride with Bicyclic Ladder Silanols
A solution of bicyclic ladder silanols (4) (1.00 g, 1.38 mmol) in pyridine (4 mL) was added dropwise to a solution of tetrachloro-tricyclic laddersiloxanes (5) [6] (0.552 g, 0.600 mmol) in pyridine (6 mL) for 13 min at room temperature. The mixture was stirred for 5 d at 100°C. The reaction mixture was added to saturated aqueous NH4Cl and hexane, and two phases were separated. The aqueous phase was extracted with hexane. The combined organic phase was washed with saturated aqueous NH4Cl, then dried over anhydrous magnesium sulfate, and concentrated. Ethanol was added to the concentrate, and the resulting (i-PrSiO1.5)8 (26 mg, 6%) was obtained by filtration. The filtrate was separated by dry column chromatography (eluent: hexane/Et2O = 9 : 1), followed by separation with recycle-type gel permeation chromatography (GPC) (eluent: THF) to give nonacyclic ladder silsesquioxanes (isomeric mixture) (6) (98 mg, 7%). 6: MS (70 eV) m/z (%) 2134 (M+-i-Pr, 5), 28 (100). IR (NaCl) ν 3072, 3051, 2947, 2895, 2868, 1593, 1466, 1429, 1387, 1366, 1259, 1115, 1034, 999, 920, 889, 719, 702 cm−1.
3. Results and Discussions
3.1. Synthesis of Nonacyclic Ladder Silsesquioxanes
Our strategy for constructing a real ladder structure is based on the reaction of cyclotetrasiloxane units [10–12]. Recently, Gunji’s group reported the synthesis of ladder polysilsesquioxane starting from cyclotetrasiloxane units [13], showing that this unit is essential to obtain the real ladder structure. As can be seen from Scheme 1, all cis-cyclotetrasiloxanetetraol 1 was treated with dichlorodisiloxane to give tricyclic laddersiloxanes 2. Dephenylchlorination that was followed by hydrolysis afforded tricyclic tetraols, and then, a similar procedure could be applied again to extend ladder [6]. The obtained pentacyclic laddersiloxanes (mixture of isomers) were isolated by recycle-type reverse-phase HPLC, and the structure of one of the isomers was determined by X-ray crystallography. In this synthesis, tricyclic laddersiloxanes 2 were obtained in a good yield (85%), but the yield of pentacyclic laddersiloxane was not satisfactory (47%, mixture of five stereoisomers). This can be attributed to the generation of disadaptive isomers. Additional rings can be formed when the terminal hydroxyl groups are in the cis position. However, when two hydroxyl groups are in trans position, laddersiloxanes cannot be obtained. This explains why the yield of pentacyclic laddersiloxanes was not high.
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Therefore, in the case of heptacyclic laddersiloxanes 3, we separated and utilized (1R,3S)-disiloxanediol in order to obtain extendible products [8] (Scheme 2). By the reaction with (R,S)-disiloxanediol, only cis-diphenyl pentacyclic laddersiloxanes were obtained. These laddersiloxanes enabled us to obtain 3.
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Although the obtained heptacyclic laddersiloxanes 3 could be theoretically extendible, synthesis of nonacyclic laddersiloxanes from heptacyclic laddersiloxanes was unsuccessful because of the lack of enough supply of starting heptacyclic laddersiloxanes. Therefore, we devised alternative approach.
During the preparation of tricyclic laddersiloxane 2, we observed the generation of bicyclic diol 4 as a by-product. To obtain 4 in a higher yield, we treated 1 with 1 equiv. of dichlorodisiloxane. As shown in Scheme 3, the desired diol 4 was obtained in 56% yield. When we reacted 4 with tricyclic tetrachloride 5, which was prepared in the synthesis of pentacyclic laddersiloxanes [6], target nonacyclic ladder silsesquioxanes 6 were obtained as a mixture of isomers (7%) with octasilsesquioxane (6%). As in the case of heptacyclic laddersiloxanes, nonacyclic ladder silsesquioxanes were obtained as a viscous oil, and X-ray crystallographic analysis was impossible. Therefore, we determined the structure of 6 by spectroscopic analysis.
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The 29Si NMR spectrum of 6 in CDCl3 showed multiple peaks between −66.52 and −65.45 ppm and between −34.53 and −32.80 ppm. The peaks around −65 ppm were attributed to the internal silicon atom, and those around –34 ppm were attributed to the terminal Si(–Ph) atom. The chemical shifts of silicon () atoms in laddersiloxanes, whose structures were determined by X-ray analysis, are summarized in Table 1. The 29Si NMR value of 6 was in good agreement with those of other laddersiloxanes. In addition, the mass spectrum showed a peak at 2133 (M+–C3H6), and the isotope pattern was similar to the calculated one (Figure 1). From these results, the obtained product was identified as a nonacyclic ladder silsesquioxanes.
3.2. Spectral Features of Ladder Silsesquioxanes
The 29Si NMR chemical shifts of internal silicon atoms of laddersiloxanes are summarized in Table 1. The results indicate that 29Si NMR peaks of laddersiloxanes and ladder polysilsesquioxanes are observed in a narrow area and are independent of the number of rings, stereostructures, and terminal substituents.
The IR spectra are very useful for the characterization of ladder silsesquioxanes. As Yoon’s group has shown in their theoretical IR studies [14], ladder silsesquioxanes are characterized by two peaks around 1150 cm−1 and 1050 cm−1, while IR spectra of cage silsesquioxanes do not have a peak at 1050 cm−1 [14]. As shown in Figure 2, two peaks were detected in that region for all laddersiloxanes. Because heptacyclic and nonacyclic laddersiloxanes are a mixture of stereoisomers, these two peaks are rather broad and are comprised of several peaks. On the other hand, absorption peaks of ladder polysilsesquioxanes were sharp and symmetrical, showing the highly organized structure.
Acknowledgments
The authors acknowledge the generous gift of silicon compounds by Shin-Etsu Chemical, Momentive Performance Materials, and Azumax.