Polymethylhydrosiloxane (PMHS) as Sustainable Reductant in the Titanocene Catalyzed Epoxide Hydrosilylation

Epoxide hydrosilylation is a chemical reaction that involves the addition of a silane compound (such as phenylsilane) to an epoxide. The most relevant representatives of this class of compounds are ethylene oxide and propylene oxide. Each of them is manufactured on an annual megaton scale, showcasing their high industrial relevance. This reaction may become important in the production of various alcohols because of its unique selectivity. During epoxide hydrosilylation, the silane compound adds to the epoxide in a way that its hydrogen atom is added to the more substituted carbon atom, resulting in the formation of a silylated alkoxide intermediate. Following the addition of an alkaline aqueous solution, the free alcohol is obtained. These kinds of “less-substituted” alcohols are called anti-Markovnikov alcohols because they contradict a rule concerning the addition of water to unsymmetrical olefins, which was first formulated by the Russian chemist Vladimir Markovnikov in 1869. To this day, this rule has severely limited the accessibility of these alcohols and has thus inspired the development of creative work-arounds.

The first hydrosilylation of epoxides to prepare anti-Markovnikov alcohols with a high degree of regio- and stereoselectivity was achieved by titanocene catalysis. The success of this approach is embedded in its metalloradical mechanism ensuring an opening of the epoxide to the higher substituted spin center. In principle, the reaction is also amenable to large-scale synthesis. This can be attributed to several reasons: easy accessibility of epoxides, abundance and non-toxicity of titanium and easy synthesis of titanocene catalysts. Unfortunately, the silanes that are currently in use are very expensive and characterized by the formation of by-products, which may be difficult to remove during purification.

Herein, Jonathan Heinrich Schacht, Dr. Shangze Wu, Dr. Sven Klare, Sebastian Höthker, Niklas Schmickler and Professor Andreas Gansäuer from the University of Bonn proposed the use of polymethylhydrosiloxane (PMHS) as a sustainable reductant in the epoxide hydrosilylation via titanocene catalysis. Their research work is currently published in the peer-reviewed journal, ChemCatChem.

The research team showed that the obtained products formally constituted anti-Markovnikov products of water addition to alkenes. Compared to other available silanes like PhSiH₃ and PhSi(CH₃)H₂, PMHS exhibited numerous advantages ranging from improved sustainability, low cost, non-toxicity, ease of handling, and high bench stability. While the costs could vary, it was worth noting that the relative values provided a favorable cost comparison of the terminal reductant. When Cp₂TiCl₂ was used as a precatalyst, high yields and when applicable high diastereoselectivities were obtained. While it is mandatory to use bulkier titanocenes to obtain similar results with PhSiH₃ and PhSi(CH₃)H₂, it was not a requirement in this case.

PMHS has become a highly attractive reagent for hydrosilylation purposes. It can be readily prepared by the hydrolysis of a by-product of the Müller-Rochow process, an indispensable industrial process in silicone industry. The experimental results showed that activating violet [Cp₂TiBn] with PMHS to lime green [Cp₂TiH] was more difficult and challenging than anticipated. Whereas adding PMHS to [Cp₂TiBn] resulted in no color change, the solution turned green when a substoichiometric amount of “kickstarter”-PhSiH₃ was added.

In summary, Professor Andreas Gansäuer and his research team demonstrated the feasibility of using PMHS as a potential stoichiometric reductant for the titanocene-catalyzed hydrosilylation of epoxides. Indeed, it was proven to be an excellent terminal hydrogen atom transfer reagent for this kind of reaction. As a waste product, PMHS is readily available at lower prices and has many benefits over many other silanes. In a statement to Advances in Engineering, Professor Andreas Gansäuer, the lead and corresponding author explained the application of PMHS, would enable sustainable catalytic epoxide hydrosilylation for alcohol production.