Hydrosilylative reduction of aldehydes and other groups by halosilanes using nucleophiles and ionic liquids as promoters

Barclay, Isabelle (2005). Hydrosilylative reduction of aldehydes and other groups by halosilanes using nucleophiles and ionic liquids as promoters. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.000101a4

Abstract

The reduction of aldehydes using hydrosilanes and nucleophiles (TBAX where X = OTf, Br, Cl and I, NMI, polymer amberlysts) gave only symmetric and mixed ethers as products and permitted an alternative to the Williamson synthesis. The yield of the reduction depended on the nature of the salt, the silane and also the aldehyde used. The efficiency of the salts for the reduction is in the following order: OTf > Br > Cl > I. We also proved that the efficiency of the silanes is: Me₂HSiOTf > Me₂HSiCl. For the aldehydes, bulky alkyl groups slowed down the reduction. The attempted reduction of ketones only gave the enol product.

Route b is predominant in the reduction of aldehydes, but route a is predominant in the reduction of ketones. For the aldehydes, proton transfer occurs easily and faster than proton abstraction from CH₂ to give the enol product, which occurred in the ketones due to steric hindrance. However, due to the presence of salts soluble in the organic phase, the separation of products from reactants remained problematic. Ionic liquids are receiving an upsurge of interest as green solvents, primarily as replacements for conventional media in chemical processes. The reduction of aldehydes using ionic liquids as both solvent and source of anions also gave dialkyl ethers in good yields.

Studies of the hydrosilylatian of alkenes and other nucleaphilic reactions, such as bramination and the Peterson reaction, showed that different products and isomer ratios are obtained using conventional organic media compared with ionic liquids. Ionic liquid and the presence of the phenyl group as a substituent on the alkenes encourage the formation of the α-product (Markownikov product). The nature of the silane, (EtO)₃SiH and Et₃SiH, does not greatly affect the α- to β-ratio of the products. The yields of α- and β-products obtained are in the range of 30-50%.

The Peterson reaction was not successful with only starting material being recovered; the ionic liquid is not nucleophilic enough to cause the cleavage of the silicon-carbon bond, which allows the formation of the β-silylcarbanion.