A new highly efficient method for the preparation of phenyl-containing siloxanes by condensation of phenylsilanols in liquid ammonia

Abstract

A new simple and highly efficient method for the synthesis of phenylsiloxanes in a liquid inorganic medium, namely, ammonia at room temperature or 100 °C at elevated pressure, has been suggested. The method that requires no organic solvent in the reaction medium can be considered as a new “green” method for the preparation of organosiloxanes. The compounds were obtained in almost quantitative yields and were fully characterized by a set of physicochemical analysis methods: ¹H, ¹³C, ²⁹Si NMR, IR-spectroscopy, HRMS ESI, GPC and elemental analysis.

Introduction

Phenyl-containing siloxanes are used widely as heat-resistant polymeric materials (Brandt et al., 1990, Li et al., 2005, Morariu et al., 2011, Shi et al., 2019, Temnikov and Muzafarov, 2020, Zlatanic et al., 2017), high-porous organic–inorganic sorbents (Gou et al., 2018, Seo et al., 2019, Shao et al., 2018), and materials with high refractive index (Kim et al., 2010) and high radiation stability (Pankratova and Rudnev, 2006).

The main method for the preparation of phenylsiloxanes involves the hydrolytic polycondensation (HPC) of phenylsilanes containing various hydrolyzable groups. It is a multistage heterogeneous process, which makes it difficult to control the structure of the resulting products as well as to achieve high conversion rates. Usually, the products obtained after hydrolysis then undergo condensation under drastic conditions at elevated temperatures (Brown et al., 1960, Harreld et al., 2002, Petrova et al., 2015, Petrova et al., 2014, Zhang et al., 2008). The HPC process requires the presence of organic solvents and catalysts. The solvents and reaction products require purification after HPC.

Phenylsilanols are often used to obtain organosilicon compounds of this class, too. Phenyl-containing silanols are very stable compared to other organosilanols with smaller organic substituents at the silicon atom. For this reason, they can undergo homofunctional condensation in the presence of catalysts. The homocondensation process of these compounds becomes more difficult with an increase in the number of phenyl substituents at the silicon atom. For example, it is very difficult to obtain hexaphenyldisiloxane in a quantitative yield by homocondensation of triphenylsilanol (Gilman et al., 1950, Tondeur et al., 1981).

Therefore, many studies search for simple and efficient methods of condensation of hydroxyl-containing phenyl-containing monomers.

During the past decades, there has been a growing interest in compressed gas media, including condensed gases and supercritical fluids, as new solvents for chemical reactions (Liu et al., 2016, Ohashi, 2006, Pigaleva et al., 2016). The main properties of such fluids, including their dissolving ability, can be tuned and controlled by varying the pressure (the extent of medium compression). It is very important that such media can be removed instantly from the reaction zone on decompression. As a result, the reaction products do not require isolation from a solvent. The gas used in the synthesis requires almost no purification after the reaction and can be reused without considerable loss.

Ammonia is one of the basic compounds in modern chemical industry. It is obtained on a wide scale by fixing atmospheric nitrogen whose reserves are inexhaustible (Pomerance, 2020, Rafiqul et al., 2005, Samaroo et al., 2020). The field of use and distribution of ammonia is huge, from refrigeration technology and fertilizer production to ammonia liquid in every pharmacy. Since the XIX century, ammonia has been used actively as a solvent in organic synthesis (Cao et al., 2009, Jorapur and Shimada, 2012, Kim et al., 2013, Shi et al., 2013). Moreover, ammonia is used widely in organosilicon chemistry for the synthesis of silicon-nitrogen-containing compounds, namely, silazanes. In this approach, ammonium mainly serves as a reagent, while we use it as a solvent and a catalyst.