Synthesis of Double-Decker Silsesquioxanes from Substituted

Communication Cite This: Organometallics XXXX, XXX, XXX−XXX

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Synthesis of Double-Decker Silsesquioxanes from Substituted Difluorosilane Toru Tanaka,*,† Yasuharu Hasegawa,† Takashi Kawamori,† Rungthip Kunthom,‡ Nobuhiro Takeda,‡ and Masafumi Unno*,‡ †

Hitachi Chemical Company, Ltd., 48 Wadai, Tsukuba 300-4247, Japan Department of Chemistry and Chemical Biology, Graduate School of Science and Technology, Gunma University, Kiryu 376-8515, Japan

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‡

S Supporting Information *

ABSTRACT: A novel synthetic method for the construction of a double-decker silsesquioxane from fluorosilanes was developed. Phenyl-substituted double-decker silsesquioxane was prepared under mild conditions by coupling difluorodiphenylsilane and a tetrasiloxanolate precursor. A similar reaction was performed using difluorovinylsilane, and a divinyl double-decker silsesquioxane was obtained. The one-step reaction of a functional difluorosilane containing an aminopropyl group afforded a novel double-decker silsesquioxane with two amino groups complexed with BF3, which can react with carboxylic acid anhydrides to afford an amide product. This synthetic method using difluorosilane is tolerant of a wide range of functional groups and is applicable to the synthesis of polycyclic silsesquioxanes bearing amino groups, which are difficult to directly obtain from dichlorosilane. On the other hand, fluorosilane, an alternative to chlorosilane, has not been used as a synthetic precursor for the production of siloxane compounds because it is more stable than chlorosilanes and less susceptible to hydrolysis. On the basis of its stability, various compounds containing fluorosilanes have been synthesized,10−16 but only a limited number of examples have been reported by Klingebiel et al. using the formation of a siloxane bond from fluorosilane17,18 or detailing the implication of fluoride ions in the reaction.19 We particularly focused on the easy handling and apparently contradictory reactivity of fluorosilane and have investigated the preparation and reaction of fluorosilane as a novel method to synthesize polycyclic silsesquioxane. We previously demonstrated that hydroxyl groups of cyclic siloxane and fluoride group of BF3·Et2O exchange under mild conditions to yield an all-cis cyclic fluorosiloxane.20 Recently, we have also demonstrated the versatility of fluorosiloxane and reported the synthesis of a Janus cube octasilsesquioxane by coupling this all-cis cyclic fluorosiloxane with an all-cis cyclic siloxanolate (Scheme 1).21 To generalize this synthetic method, herein we applied it to the construction of doubledecker silsesquioxanes without any catalyst. To date, many reports of substituted monomers and the construction of polymers containing this structure have appeared.22,23 In this communication, we report the facile synthesis of a double-decker silsesquioxane by coupling of difluorosilane

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ell-defined polycyclic silsesquioxanes comprise a unique class of three-dimensional siloxane building blocks that can adopt cube, incomplete-cage, and ladderlike structures. This class has attracted significant attention as potential materials due to their excellent electrical insulating and prominent optical and mechanical properties in addition to their thermal stability.1−4 Over the past decades, numerous synthetic approaches have been explored to use these materials as small silica fillers in composite resins and to incorporate them as monomers targeting various types of inorganic/ organic hybrid polymers with novel properties.5−7 Precise construction of the silsesquioxane framework and controlled introduction of functional groups to the silsesquioxane moieties have become increasingly important, as they offer a straightforward synthetic pathway for tailor-made materials. Despite the recent remarkable progress of catalytic synthesis,8 siloxane bonds are formed via the condensation of silanols generated from chlorosilane hydrolysis in the general industrial process,9 and chlorosilane is the most common starting material for the synthesis of polycyclic silsesquioxanes and other siloxane compounds.1 In addition, chlorosilane directly reacts with alkoxysilane, silanol, and siloxanolate to afford siloxane bonds and various byproducts including alkyl chloride, hydrochloric acid, and sodium chloride. Although chlorosilane is quite versatile in the silicone industry, limitations exist, as it is highly sensitive toward reaction with moisture and generates corrosive hydrochloric acid as a byproduct and requires careful handling and high-performance corrosion-resistant equipment. © XXXX American Chemical Society

Received: December 12, 2018

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DOI: 10.1021/acs.organomet.8b00896 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

Because B(OCH3)3, the byproduct of difluorination, is inert toward most reactions, we expected that the coupling of difluorosilane with siloxanolate would proceed in the presence of B(OCH3)3. This may represent a synthetic advantage, especially when the separation of the difluorosilane and B(OCH3)3 is difficult by distillation. We generated 1b in situ via the reaction between methylvinyldimethoxysilane and BF3· Et2O in a 3:2 molar ratio and directly added the product to a suspension of tetraphenylsiloxanolate precursor 2 to obtain a vinyl-bifunctionalized double-decker silsesquioxane (3b) in 19% yield after simple purification. This supported our assumption that the one-pot synthesis of a bifunctional double-decker silsesquioxane would be possible after quantitative difluorination by BF3·Et2O. Although diamino-functionalized and well-defined silsesquioxanes have been previously reported,29−31 multiple steps are required to obtain the desired product, since amino groups cannot coexist with chlorosilane because of the possible formation of aminosilane. We next targeted the direct introduction of two amino groups to the double-decker silsesquioxane using our synthetic method, which should be feasible on the basis of previous reports that showed dialkyldifluorosilane and trialkylmonofluorosilane do not react with primary amines under completely dry conditions.32 When 3-aminopropylmethyldiethoxysilane was treated with excess BF3·Et2O under solvent-free conditions, a difluorosilane bearing a BF3-complexed amino propyl group (1c) was obtained in high yield after distillation. The 19F NMR spectrum showed a typical 1:1:1:1 quartet signal due to the B−F coupling at −151.9 ppm for the BF3 moiety and a singlet signal at −136.1 ppm from the SiF2 group.33 The formation of the BF3−amine complex and difluorination likely occurred spontaneously. Treatment of 1c with 2 in THF and subsequent purification by moderate-pressure liquid chromatography afforded a diamine silsesquioxane (3c) in 44% yield as a white powder. The structure of 3c was confirmed by NMR (1H, 13C, 19F, and 29 Si) spectroscopy, IR spectroscopy, and matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. The 19F NMR spectrum showed that the BF3complexed amine remained after the reaction and purification. In the 29Si NMR spectrum, signals corresponding to the cis and trans isomers of the typical double-decker framework were observed at −19.93, −80.02, −80.94, and −81.12 ppm and −19.93, −80.02, and −81.03 ppm, respectively.34,35 In the MALDI-TOF mass spectrum, the major mass peak at m/z 1289 was assigned to the sodiated parent species without a BF3 unit, indicating that decomplexation of the primary amine occurred during ionization. Single crystals suitable for X-ray crystallographic analysis were obtained by vapor diffusion of THF solution in a vial containing hexane between −20 and −25 °C. The molecular structure of trans-3c is shown in Figure 1. Refinement of the site occupancy suggests a statistical distribution of the two structures, and the ratio of A and B forms was 70:30. Because the forms are independent, the compound becomes trans in a combination of AA or BB and cis when an AB or BA combination is applied. Thus, the ratio of trans and cis isomers is calculated to be 100:0 to 40:60. The BF3-complexed amine is a potential hardener of epoxy resin due to its nucleophilic properties at elevated temperatures.33 To the best of our knowledge, the reactions of the BF3-complexed amine with a carboxylic acid or its anhydride have not been explored despite their industrial importance.

Scheme 1. Synthesis of Janus Cube Octasilsesquioxane

prepared from dialkoxysilane. The coupling of the functionalized difluorosilane with siloxanolates under mild conditions afforded a bifunctional double-decker silsesquioxane. Although the amino group formed a complex with BF3 during the preparation, its reactivity toward carboxylic acid anhydride was not significantly affected and it afforded the amide compound as a condensation product. The resulting bifunctional doubledecker silsesquioxanes are desirable building block monomers for various macromolecules. Previously, we reported that the coupling of fluorosilane with siloxanolate and the precise alignment of the four reaction sites were essential for the synthesis and isolation of Janus cube silsesquioxanes. On the basis of this synthetic approach, the construction of double-decker frameworks is possible when two siloxanolate groups on each side are in suitable positions to react with difluorosilane. Difluorination was performed following the reported method where an alkyl- or aryl-substituted dialkoxysilane is converted by BF3·Et2O to the corresponding difluorosilane in high yield.15 The reaction between diphenyldimethoxysilane and BF3·Et2O in a 3:2 molar ratio proceeded quantitatively between 15 and 30 °C under solvent-free conditions, followed by distillation to remove the volatile byproducts, B(OCH3)3 and diethyl ether, to yield Ph2SiF2 (1a) as a colorless liquid (Scheme 2). Scheme 2. Synthetic Route to Double-Decker Silsesquioxane

The tetraphenylsiloxanolate precursor (2) was prepared according to methods described in the literature as the main product24,25 and further reacted with 1a in THF at 0 °C to yield an all-phenyl-substituted double-decker silsesquioxane (3a) in 45% yield after purification to remove byproducts, i.e. sodium fluoride26,27 (Scheme 2), a yield comparable to that from Ph2SiCl2 (55%).28 This shows that difluorosilane reacts in a manner similar to that of dichlorosilane to form four siloxane linkages. Thus, the coupling of fluorosilane and siloxanolate is a promising method to yield well-defined polycyclic silsesquioxanes and is not limited to the synthesis of Janus cube silsesquioxanes. B


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00896. General experimental details, NMR spectra, IR spectra, MALDI-TOF mass spectra, and X-ray crystallographic analysis (PDF) Accession Codes

CCDC 1881805 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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Figure 1. Crystal structure of trans-3c with the thermal ellipsoids shown at 50% probability. Hydrogen atoms and THF molecules are omitted for clarity. Color scheme: black, carbon; blue, silicon or nitrogen; red, oxygen; light brown, boron; light green, fluorine.

AUTHOR INFORMATION

Corresponding Authors

*E-mail for T.T.: [email protected]. *E-mail for M.U.: [email protected].

When 3c was reacted with acetic anhydride in DMSO-d6 at 80 °C for 3 h, new peaks at 22.5 and 168.9 ppm emerged in the 13 C NMR spectrum (for further data, see the Supporting Information), corresponding to the acetic amide groups (Scheme 3). Subsequently, 4c was purified by water extraction

ORCID

Masafumi Unno: 0000-0003-2158-1644 Notes

The authors declare no competing financial interest.

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Scheme 3. Amidation Reaction of BF3-Complexed Diamino Double-Decker Silsesquioxane

ACKNOWLEDGMENTS We thank Yoshihiro Amano, Chenxia Zhang, Tomoki Ezure, Yuki Arai, and Akihiro Unno of Hitachi Chemical Company, Ltd., for assistance with NMR and MALDI-TOF MS measurements. Dr. Keita Watanabe, Dr. Masaru Watanabe, Yasushi Goto, Dr. Toshihiko Takasaki, and Dr. Liping Shen of Hitachi Chemical are also thanked for their continuous support and helpful discussions. Finally, we thank Hitachi Chemical Company, Ltd., for supporting this research. This work was partially supported by the “Development of Innovative Catalytic Processes for Organosilicon Functional Materials” project (PL: K. Sato) from the New Energy and Industrial Technology Development Organization (NEDO).

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to remove the water-soluble fluoroborates and the structure of 4c was determined using NMR spectroscopy and MALDITOF mass spectrometry. The results indicate that the BF3complexed amine reacted with the carboxylic acid anhydride to afford the amide compound and bifunctional 3c can be used as a potential raw material for the synthesis of polyamides or polyimides. In summary, a substituted difluorosilane prepared from BF3· Et2O was reacted with a siloxanolate precursor to afford a double-decker silsesquioxane. With a functionalized difluorosilane, bearing a vinyl- or BF3-complexed amino group, as the starting material, bifunctionalized silsesquioxanes were synthesized under mild conditions. This method using a substituted difluorosilane as a precursor represents a general strategy for the synthesis of polycyclic silsesquioxanes, to which various functional groups can be introduced. Future studies will apply these double-decker silsesquioxanes to the precise synthesis of polymeric materials as functional building blocks.

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Synthesis of Double-Decker Silsesquioxanes from Substituted

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