Site-isolation and recycling of PdCl(2) using PDMS thimbles.

Macroscopic thimbles composed of polydimethylsiloxane (PDMS) were used to site-isolate PdCl(2) from the products of Wacker-Tsuji oxidations and Pd-mediated homocouplings. The reactions were completed on the interior of hollow thimbles, and the organic products were isolated by their selective flux to the exterior of the thimbles. Although organic molecules had high flux through the walls of the thimbles, PdCl(2) remained encapsulated as a result of its polar structure that rendered it insoluble in PDMS. In two examples, less than 0.002% of the PdCl(2) added to the interior of the thimble partitioned to the exterior; thus, over 99.998% remained encapsulated on the interior of the thimble. Because it was encapsulated, this catalyst was readily recycled five times for the Wacker-Tsuji oxidation of styrene. A sequential reaction was also completed where p-methylstyrene was oxidized to 4'-methyl-acetophenone by PdCl(2) on the interior of a thimble and then fluxed to the exterior to react with phenylmagnesium bromide to yield 1-phenyl-1-p-tolyl-ethanol. This method site-isolated PdCl(2) catalysts without requiring them to be rendered heterogeneous, the addition of exogenous ligands, or any modifications to the catalyst. The catalyst was site-isolated by affecting its environment rather than by altering its ligand structure.