Fluorous Palladacycle Catalysts

Using a fluorous palladacycle catalyst 10 originating from the corresponding fluorous Schiff base and palladium acetate, a fluorous Mizoroki-Heck reaction was achieved with an excellent turnover number (Scheme 12). A homogeneous catalytic reaction system was obtained when DMF was used as the solvent. After the reaction, perfluorooctyl bromide was added to facilitate the separation of DMF (containing the products and amine salts) from the catalyst phase. The resulting lower fluorous layer was condensed under vacuum and the catalyst residue was used in a second run. In this reaction, the palladacycle catalyst appears to act as a source of palladium nanoparticles, which are thought to be the dominant active catalyst. 

The Mizoroki-Heck reaction using a fluorous palladacycle catalyst... 

Ref. 67) A Schlenk tube was sequentially charged with DMF (6mL), iodoben-zene (5.02 mmol), methyl acrylate (6.28 mmol), NEts (1.4 mL) and a solution of the fluorous palladacycle catalyst 10 in CF3C6H5 (0.229 mA/ 0.015 mL, 3.44 x 10 mmol). The tube was connected to a condenser and placed in a 140°C oil bath. The solution was vigorously stirred (14 h), removed from the bath to cool and... 

Rocaboy, C. and Gladysz, J.A. (2002) Highly active thermomorphic fluorous palladacycle catalyst precursors for the Heck reaction evidence for a nanoparticle pathway. Org. Lett, 4, 1993-6. 

As detailed elsewhere, the fluorous palladacycle acetates and hahdes 7 and 8 were synthesized [38,39]. These feature three Rfg ponytails, and were poorly soluble in common organic solvents at room temperature, and insoluble in DMF. However, they were very soluble in DMF at higher temperatures. All were effective catalyst precursors for Heck reactions (100-140 °C), and precipitated (as the halides) upon cooling. However, a number of control experiments established that 7 and 8 served as steady-state sources of colloidal palladium nanoparticles, formed anew with each cycle imtil the palladacycles were exhausted. These, or low-valent Pd(0) species derived therefrom, were the true catalysts. 

Palladacycle catalysts are a class of phosphine-free, highly active, and unstable zero-valent palladium catalysts [16]. Oxime-derived palladacycle was known for its thermal robustness, tolerance to air and moisture, and slow formation of large palladium particles. However, many methods for recycling palladacycle are not successful. In order to recover and reuse the catalyst, a fluorous oxime-derived palladacycle 20 was prepared from 4,4 -dihydroxybenzophenone as a precatalyst for the coupling reactions (Scheme 7.8). 

Gladysz showed that a thermomorphic fluorous paUadacyde acts as a PdNP catalyst precursor for the Heck reaction at 80-140 C in DMF with very high turnover numbers [24a]. Molecular palladium complexes such as palladacycles and other palladium salts have also been used as PdNP precursors upon treatment with CO in DMF or toluene at room temperature, and these PdNPs catalyzed nudeophiUc substitution/carbonylation/amination affording iso-indolinones at room temperature [24bj. PdNPs capped with spedal ligands such as polyoxometal-... 

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