Carbosilane Dendrons as Solubilizers of Metal Complexes in Supercritical Carbon Dioxide

Palladium complexes of general formula [PdCl₂(L)₂], 3a-c, were synthesized by reaction of [PdCl₂(COD)] (COD = 1,5-cyclooctadiene) with the appropriate functionalized phosphine L = [P(C₆H₅)_n((C₆H₄CH₂CH₂Si(CH₂CH₂CH₂SiEtMe₂)₃)_3-n)] (n = 2, 2a; 1, 2b; 0, 2c, respectively), which integrated carbosilane dendritic wedges attached to the para-carbon atom of the phenyl group. As a result of the inclusion of the carbosilane dendrons, the solubility of the corresponding palladium complexes 3 in supercritical carbon dioxide (scCO₂) was increased in comparison with the palladium complex containing a PPh₃ ligand, which showed negligible solubility. The employment of these complexes as precatalysts in a selected Heck coupling reaction in scCO₂ was investigated. Good activities were found for 3a-c in such a medium due to the acquired solubility, in contrast with the palladium complex containing PPh₃, which showed no activity.