Abstract

DHIMBA, George  and  BALA, Muhammad D.. Recyclable polymeric ionic liquids applied as metal-free transfer hydrogenation catalysts. S.Afr.j.chem. (Online) [online]. 2022, vol.76, pp.120-126. ISSN 1996-840X.  http://dx.doi.org/10.17159/0379-4350/2022/v76a17.

A new methodology for immobilizing a triazolium salt onto polyethylene glycols (PEG) was developed. The immobilization is a two-step reaction, involving the tosylation of PEGs followed by a neat reaction between the PEG tosylates and the triazole. It is a high-yielding reaction conducted under mild conditions with a simple workup procedure. Hence, the salt 1-benzyl-4-phenyl-1H-1,2,3-triazolium iodide was immobilised onto PEGs of varying chain lengths (PEG₃₀₀_₈₀₀₀) to yield various polymeric ionic liquids (PILs). All the PIL compounds were characterised by spectroscopic analysis and utilised as homogeneous metal-free organocatalysts for the transfer hydrogenation of ketones to alcohols in isopropanol as solvent and hydrogen source. The metal-free PIL catalysts showed activity for transforming acetophenone to 1-phenyl alcohol at up to 87% conversion. The representative PEG₆₀₀ system was recycled three times with minimal loss in activity. The mechanism of a possible catalyst leaching was investigated using computational calculations. The results reveal that the promoter, isopropanol, also promotes the leaching of the triazole from the PEG backbone.

Keywords : computational chemistry; metal-free catalysis; polymeric ionic liquids; transfer hydrogenation.