Direct alkoxysilylation of alkoxysilanes for the synthesis of explicit alkoxysiloxane oligomers

Protecting and Leaving Functions of Trimethylsilyl Groups in Trimethylsilylated Silicates for the Synthesis of Alkoxysiloxane Oligomers

The concept of protecting groups and leaving groups in organic synthesis was applied to the synthesis of siloxane-based molecules. Alkoxy-functionalized siloxane oligomers composed of SiO₄ , RSiO₃ , or R₂ SiO₂ units were chosen as targets (R: functional groups, such as Me and Ph). Herein we describe a novel synthesis of alkoxysiloxane oligomers based on the substitution reaction of trimethylsilyl (TMS) groups with alkoxysilyl groups. Oligosiloxanes possessing TMS groups were reacted with alkoxychlorosilane in the presence of BiCl₃ as a catalyst. TMS groups were substituted with alkoxysilyl groups, leading to the synthesis of alkoxysiloxane oligomers. Siloxane oligomers composed of RSiO₃ and R₂ SiO₂ units were synthesized more efficiently than those composed of SiO₄ units, suggesting that the steric hindrance around the TMS groups of the oligosiloxanes makes a difference in the degree of substitution. This reaction uses TMS groups as both protecting and leaving groups for SiOH/SiO^- groups.

Cubic Siloxanes with Both Si-H and Si-OtBu Groups for Site-Selective Siloxane Bond Formation

Cage-type siloxanes have attracted increasing attention as building blocks for silica-based nanomaterials as their corners can be modified with various functional groups. Cubic octasiloxanes incorporating both Si-H and Si-OtBu groups [(tBuO)_n H_8-n Si₈ O₁₂ ; n=1, 2 or 7] have been synthesized by the reaction of octa(hydridosilsesquioxane) (H₈ Si₈ O₁₂ ) and tert-butyl alcohol in the presence of a Et₂ NOH catalyst. The Si-H and Si-OtBu groups are useful for site-selective formation of Si-O-Si linkages without cage structure deterioration. The Si-H group can be selectively hydrolyzed to form a Si-OH group in the presence of Et₂ NOH, enabling the formation of the monosilanol compound (tBuO)₇ (HO)Si₈ O₁₂ . The Si-OH group can be used for either intermolecular condensation to form a dimeric cage compound or silylation to introduce new reaction sites. Additionally, the alkoxy groups of (tBuO)₇ HSi₈ O₁₂ can be treated with organochlorosilanes in the presence of a BiCl₃ catalyst to form Si-O-Si linkages, while the Si-H group remains intact. These results indicate that such bifunctional cage siloxanes allow for stepwise Si-O-Si bond formation to design new siloxane-based nanomaterials.

Silicone dendrons and dendrimers from orthogonal SiH coupling reactions

Iterative assembly of highly branched silicone dendrons and dendrimers, alternatively using Piers–Rubinsztajn and Pt-catalyzed hydrosilylation reactions, was achieved in high yield to give materials of up to 13 770 molecular weight by combining divergent and convergent approaches.

Synthesis of a multifunctional alkoxysiloxane oligomer

The siloxane bonds of oligomer 1 were not cleaved by hydrolysis, indicating the suitability of 1 for controlled hybrid preparation.

Siloxane-Bond Formation Promoted by Lewis Acids: A Nonhydrolytic Sol-Gel Process and the Piers-Rubinsztajn Reaction

Siloxane formation reactions of both the nonhydrolytic sol-gel process and Piers-Rubinsztajn reaction can be integrated as Lewis acid promoted siloxane syntheses without involving silanol groups. The former was developed in the field of inorganic materials chemistry and the latter was initiated in polymer chemistry. We have realized both reactions are quite similar, in terms of 1) the nonhydrolytic reaction, 2) the use of alkoxysilanes, 3) the group-exchange reactions competing with the siloxane formation, and 4) the proposed reaction mechanisms. This Minireview focuses on the above two reactions. The evolution of both reactions should realize a more sophisticated molecular design of siloxane compounds, which surely contributes to the development of advanced functional materials.