Functionalized poly(propylene imine) dendrimers as novel phase transfer catalysts in supercritical carbon dioxide

Perfluoro-functionalized poly(propylene imine) dendrimers have been used as reactive extractants for anionic species and as phase transfer catalysts f or two types of reactions. Different generations of dendrimers have been us ed for applications in carbon dioxide. First, the reactive extraction of th e inorganic ions permanganate and dichromate, from an aqueous phase to a su percritical carbon dioxide phase, has been studied using the functionalized dendrimers. The yield for these extractions is rather law, indicating that the perfluoro-functionalized poly(:propylene imine) dendrimers are not sui ted for oxidation reactions with these ions in carbon dioxide. Second, the poly(propylene imine) dendrimers have been used as a phase transfer catalys t for the halogen exchange reaction of benzyl chloride into benzyl bromide and for the esterification of oxalic acid with pentafluorobenzylbromide. In contrast with the low extraction yield for the inorganic permanganate and dichromate ions, a high reaction rate was obtained for the halogen exchange reaction with the functionalized dendrimers as phase transfer catalysts. i t is found that the rate of reaction for both cases depends on the generati on number of the dendrimer. For the higher-generation dendrimers, it is mor e difficult for the substrate to migrate from the bulk carbon dioxide phase to the interior of the dendrimer. This indicates that the mass transfer of the reagents from the organic phase into the interior of the dendrimer det ermines the reaction rate for the fourth and higher generation dendrimers. A model is proposed to describe the extraction and the phase transfer catal ysis based on the fact that the modified dendrimers combine micellar proper ties with those of standard phase transfer catalysts. From the different ex periments it can be concluded that the perfluoro-functionalized poly(propyl ene imine) dendrimers have great potential as multisite phase transfer cata lysts.