Dendrimers rhodium-based catalysts

The thermal instability of rhodium-based hydroformylation catalysts has already been overcome commercially in the Ruhrchemie/Rhone-Poulenc process for propene hydroformylation in which the sodium salt of a sulfonated triphe-nylphosphine ligand (TPPTS, la) is used to solubilize the catalyst in the aqueous phase. In this process, the second phase is toluene and the reaction is carried out as a batch process with rapid stirring to intimately mix the two immiscible phases. After reaction, the system is allowed to separate and the organic phase is simply decanted from the aqueous catalyst phase. Both water-soluble polymers and PAMAM dendrimers have been reported as supports for rhodium-catalyzed hydroformylation under aqueous biphase conditions, but reactivities and regioselec-tivities were only comparable to or worse than those obtained with the reference TPPTS ligand. The aqueous biphase approach has found limited application for the hydroformylation of longer-chain alkenes, because of their very low solubility in water leading to prohibitively slow reaction rates, but there have been a variety of approaches directed at the solution of this problem. 

Beside these catalytically active metallophosphine dendrimers (see above), preliminary studies on the chemical properties of phoshorus-based dendrimers complexed to metals such as platinum, palladium and rhodium have been described by Majoral, Caminade and Chaudret [21], They showed that these macromolecules (see Scheme 13) could be useful for the (in situ) generation of metallodendrimer catalysts. 

By encapsulation of the trialkylphosphine-modified rhodium catalyst into zeolites, the high chemoselectivity was maintained but the Hb ratio in the product alcohol could be increased by as much as 10 times [65]. Anchoring of trialkylphos-phines to carbosilane dendrimers based on polyhedral silsesquioxane (POSS) produced dendrimeric ligands, which in the hydroformylation-hydrogenation of 1-octene preferentially produced linear alcohols. Regioselectivity exceeded that achieved with the low molecular ligand [66]. 

Also, dendrimer-based rhodium catalysts with a typical size of 2-4nm are soluble in several organic solvents and can be separated after the hydroformylation... 

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