Allylic Substitution using Dendritic Catalysts in a CFMR

Allylic Substitution using Dendritic Catalysts in a CFMR 

Reetz et al. (16) functionalized commercially available DAB-dendrimers with diphenylphosphine groups at the periphery (1) via a double phosphination of the amines with diphenylphosphine and formaldehyde. The transition metal complexes la-le have been prepared in which the dendrimer-N-(CH2PPh2)2 groups act as bidentate ligands. 

Better results were obtained by using in situ prepared palladium complexes of a G4 dendrimer (calculated molecular weight 20 564 Da for 100% palladium loading of the 32 diphosphines). After 100 residence times, the conversion had decreased from 100% to approximately 75% (Fig. 3). A small amount of palladium was leached from the catalyst during this experiment (0.14% per residence time), which only partly explains the decrease in conversion. The formation of inactive PdCl2 was proposed to account for the additional drop in activity. A sound conclusion about the effect of this dendritic catalyst requires more experiments. 

De Groot et al. (18) prepared phosphine-functionalized carbosilane dendrimers of different generations (4, 8, 24, and 36 phosphine end groups) and used their palladium complexes as catalysts for the allylic alkylation of allyl trifluoroacetate with diethyl sodio-2-methylmalonate. 

In a batch process, all dendritic catalysts showed very high activity. When a substrate-to-Pd molar ratio of 2000 was used, the conversions after 5 min were 49, 55, 45, and 47% when dendrimers with 4, 36, 8, and 24 phosphine ligands were used, respectively. These results show that all the active sites located at the periphery of the dendrimer support acted independently as catalysts. 

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A.l. Allylic Substitution using Dendritic Catalysts in a CFMR... 

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