Chiral Guanidinium Ionic Liquids for Asymmetric Dihydroxylation of Olefins with Recycling of the Catalytic System by Supercritical CO2

This paper presents the advantages of combining two alternative solvents in asymmetric dihydroxylation (AD) of olefins: chiral ionic liquids (CIL) as chirality-inducing media and supercritical CO2, as extraction media. Initially, the AD reaction was optimized using 1-hexene as the model substrate and as the chiral source, guanidinium ionic liquids ([(di-h)2dmg]) based on chiral anions such as (S)-mandelate, (R)-mandelate, quinic, l-lactic, (S)-camphorsulfonate, BOC-hydroxyproline, and acetylhydroxyproline. In the first approach, chiral ionic liquids [(di-h)2dmg][quinic] and [(di-h)2dmg][lactic] were used as unique reaction and chirality-inducing media, allowing for yields and enantiomeric excess comparable to or higher than the conventional systems (using Sharpless chiral ligands). The use of this method offers the additional advantage of performing the reactions without the need for a slow addition of olefins. Several olefins containing aliphatic and aromatic units were tested using the selected CILs with a good performance in terms of yields and ee’s. In the second approach, the chiral ionic liquid [(di-h)2dmg][quinic] was used as a chiral source in combination with nonchiral imidazolium ionic liquids ([C10mim][BF4] and [C8mim][PF6]). Asymmetric dihydroxylation of 1-hexene in [(di-h)2dmg][quinic] as chiral source and [C10mim][BF4] as reaction media and using N-methylmorpholine-N-oxide as co-oxidant was selected as a model system to evaluate the feasibility of recycling the catalytic system (osmium catalyst + CIL) using either organic solvents or supercritical CO2 (scCO2) as the extraction medium. The reaction media and catalytic system (osmium + CIL) were successfully recycled and reused by scCO2 extraction for at least five successive cycles without significant decrease in yield (90%) or enantioselectivity (77%).