Synthesis and Applications of Siloxane Boronic Acids and Siloxane Boronates

Abstract

<p> Synthesis of new biocompatible siloxane surfactants, where hydrophobic siloxanes are modified by biocompatible hydrophilic functional groups (e.g., triethoxysilane (TES), polyethylene glycol (PEO), and carboxylic acid) is a research area of increasing interest. In this research project we have developed a new class of these biocompatible surfactants, based on siloxanes, as the hydrophobic part, and boronic acids as the hydrophilic part. The reasons for choosing boronic acidlboronates as a modifying agents include: their pH sensitivity, biocompatibility, possible interactions with sugars, and because of a broader general utility in synthesis. The promise of these properties combined with the hydrophobicity, flexibility and many other important features of siloxanes encouraged us to initiate these syntheses. </p> <p> We have explored different synthetic strategies to prepare siloxane boronic acid surfactants, including Grignard reactions and metal-catalyzed hydroboration reactions. Nevertheless, the main approach that was investigated is metal-catalyzed hydrosilylation reactions of vinylphenylboronic acid. Two different approaches were developed to prepare the target compounds: (1) metal-catalyzed hydrosilylation using non-protected vinylphenylboronic acid and (2) metal-catalyzed hydrosilylation using protected vinylphenylboronic acid that can be removed under gentle conditions. The protected compounds underwent hydrosilylation smoothly, but led after separation using column chromatography to the unprotected compounds in moderate yield. The conversion of the hydrosilylation of unprotected boronic acids was quite good, but the compounds underwent decomposition during chromatography. Thus, the two approaches are complementary, depending on whether pure molecules are required for further synthetic elaboration, or a mixture of materials is suitable for practical application as surfactants. </p> <p> The amphiphilic nature of these siloxane boronic acid surfactants was studied and the compounds were found to be surface active. The limited solubility of our compounds in H20 prevented us from studying their surface tension properties. However, their solubility in chloroform enabled us to study their interfacial properties. </p>