Rhodium dendrimer supported catalysts

The same authors recently described the synthesis of similar rhodium-complexed dendrimers supported on a resin having both interior and exterior functional groups. These were tested as catalysts for the hydroformylation of aryl alkenes and vinyl esters (52). The results show that the reactions proceeded with high selectivity for the branched aldehydes, with excellent yields, even up to the tenth cycle. The hydroformylation experiments were carried out with first- and a second-generation rhodium-complexed dendrimers as catalysts, with a mixture of 34.5 bar of CO and 34.5 bar of H2 in dichloromethane at room temperature. Each catalyst was easily recovered by simple filtration and was reusable for at least six more cycles without... 

In a similar vein, a resin-supported rhodium-complexed dendrimer 340 has been shown to promote the carbonylative ring expansion of aziridines to /3-lactams <1988CC710, 2006JHC11>, as illustrated by the conversion of the A-7-butyl aziridine 341 to the corresponding lactam 342 in almost quantitative yield. The supported catalyst, which shows reactivity comparable to the solution-phase variety, is easily recovered by filtration and exhibits no significant loss of activity upon recycling (Scheme 88). 

There have been multiple efforts toward supported catalysts for asymmetric transfer hydrogenation, and the 4 position on the aryl sulfonate group of 26 has proven a convenient site for functionalization. Thus far, this ligand has been supported on dendrimers [181,182], polystyrenes [183], silica gel [184], mesoporous siliceous foam [185], and mesoporous siliceous foam modified with magnetic particles [186]. The resulting modified ligands have been used in combination with ruthenium, rhodium, and iridium to catalyze the asymmetric transfer of imines and, more commonly, ketones. 

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. 

Rhodium-complexed dendrimers supported on a resin were evaluated as catalysts for the hydroformylation of aryl olefins and vinyl esters. Up to 99% yields and an outstanding selectivity for the branched aldehydes (up to 38 1) were obtained at room temperature and 69 bar CO H2 = 1 1. The dendritic catalysts were recycled by simple filtration and reused even up to the 10th cycle without any loss of activity and selectivity ]82]. 

In addition to dendrimers, hyperbranched polymers have been used by several groups as soluble supports for catalysts [7, 17]. These supports are polydisperse and randomly branched, and, since they are prepared in a single reaction step, are generally much cheaper materials. Nevertheless, it has been shown that catalysts immobilized on hyperbranched polymers may possess similar properties as dendritic systems [18]. Therefore, dendritic catalysts serve as ideal model systems for catalysts attached to hyperbranched polymers. We functionalized several hyperbranched polyethyleneimines (PEIs) employing the peptide coupling protocol in reactions with the pyrphos linker system. The pyrphos-rhodium complexes bound to the hyperbranched polymers were also found to be active catalysts for the hydrogenation ofZ-methyl-a-acetamidocinnamate [16]. As observed for the... 

Nixantphos (3S) and its modified derivatives were immobilized on soluble and solid polymer supports, such as dendrimers, polyglycerol, and polyurethanes. The new catalysts were tested in rhodium-catalyzed regioselective hydroformylation of 1-octene and proved to be selective and reusable. Polymer support was found to require spacers between the ligand and the supporting units [92]. 

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