The phosphorus aspects of green chemistry:: the use of quaternary phosphonium salts and 1,3-dialkylimidazolium hexafluorophosphates in organic synthesis

The search for suitable ionic liquids that are non-volatile, non-toxic, dissolve a variety of reactants and are recyclable is a real challenge for chemists to be able to replace the traditional solvents that are volatile, flammable and toxic. This segment of chemistry has, these days, become a firm part of green chemistry attracting much attention. The present review summarizes and critically discusses the data accumulated in recent years on the synthesis and utilization of phosphorus containing ionic liquids, such as imidazolium hexafluorophosphates and phosphonium salts. As regards the preparation of ionic liquids, the key steps are quaternization and the introduction of the corresponding anion. Physical properties of the ionic liquids can be fine-tuned by selecting suitable cation-anion pairs. The reactions studied in ionic liquids embraces catalytic hydrogenations, transfer hydrogenations, oxidations, hydroformylations, carbonylations, alkylations, acylations, nucleophilic substitutions, halogenations, condensations, Diels-Alder reactions, Michael additions, coupling reactions and transformations in organophosphorus chemistry. Ionic liquids are especially suitable to accommodate catalytic reactions where one of the components is a gas. Stress was laid to highlight the chemo-, regio-, diastereo- and, especially, the enantioselectivity of the reactions performed in ionic liquids. The data on the ecotoxicity of imidazolium hexafluorophosphates and phosphonium salts are briefly discussed.