Preparation of new catalysts by the immobilization of palladium(II) species onto silica: an investigation of their catalytic activity for the cyclization of aminoalkynes.

Silica-immobilized palladium catalysts are readily prepared by treating partially dehydroxylated silica with solutions of the palladium(II) complexes, cis-[PdMeXL2] (X = Me, L2 = dmpe; X = Cl, L2 = dmpe, dppe, phen, bipy, 2PMe3), trans-[PdMeXL2] (X = Cl, NO3, OTf, L = PMe3; X = Cl, L = PPh3), or [PdPh(OH)L]2 (L = PPh3, PCy3), at room temperature. A chemisorption reaction is presumed to occur on the surface Si-OH groups, with elimination of 1 equiv of methane, benzene, or water and the initial formation of a covalent Pd-O bond to the silica surface. The amount of chemisorbed material is strongly dependent on the nature of the complex employed, and the Pd content of the materials, determined by ICP analysis, was found to vary widely (from 1.47 to 0.021 wt %). It appears that the complexes stabilized by more basic ligands undergo a more facile reaction with the surface. The catalytic activity of the materials was first tested in the cyclization of 6-aminohex-1-yne. Higher conversions were found for those catalysts containing more basic ligands, due to the higher loadings, and for those complexes containing more weakly coordinating anions. Silica/trans-[PdMe(NO3)(PMe3)2] was identified as the best catalyst and was used to test the generality of the catalytic cyclization method with two other alkynes, namely, 5-phenyl-4-pentyn-1-amine and 6-phenyl-5-hexyn-1-amine. The catalysts prepared here show rates comparable to, or greater than, those found for homogeneous late transition metal complexes, including their molecular precursors. Furthermore, the supported catalysts are only slightly air-sensitive and can be recycled, after filtration in air, with only moderate loss of activity.