Strong evidence of solution-phase catalysis associated with palladium leaching from immobilized thiols during Heck and Suzuki coupling of aryl iodides, bromides, and chlorides

A 3-mercaptopropyl-functionalized silica, SH-SBA-15, is used both as a support for palladium acetate, Pd-SH-SBA-15, and as a selective poison of soluble palladium complexes. Pd-SH-SBA-15 is used as a precatalyst for Heck and Suzuki couplings of an aryl iodide, bromide, and chloride under a range of reaction conditions. In all reactions in which metal-free SH-SBA-15 is added, the catalysis associated with either homogeneous palladium acetate or from Pd-SH-SBA-15 ceased. This strongly suggests that (i) SH-SBA-15 can be used as an effective scavenger and poison of soluble palladium and (ii) catalysis with Pd-SH-SBA-15 is solely associated with leached metal. Supporting evidence is given from hot filtration tests, self-quenching behavior of partially metalated Pd-SH-SBA-15, and poisoning by other solid-phase poisons such as poly(4-vinylpyridine), Quadrapure TU, and 3-mercaptopropyl-functionalized silica gel. It is postulated that the SH-SBA-15 poisons by over coordination of soluble monomeric or dimeric palladium, which rules out catalysis by palladium nanoparticle surfaces. SH-SBA-15 is an effective and selective poison of homogeneous palladium species and can be used as a test for heterogeneity of palladium catalysts developed for Heck and Suzuki reactions.