Sonogashira coupling: silica supported aqueous phase palladium catalysts versus their homogeneous analogs

Abstract

Two palladium catalysts, Pd(OAc)2/n TPPTS, n = 2, 5 [TPPTS = P(m-C6H4SO3Na)3), immobilised in an aqueous film supported on a mesoporous silica were tested for the coupling of phenylacetylene with phenyliodide, in presence of a catalytic amount of CuI and of a base. The two solids show very different activities, which are both strongly dependent on their initial water content. Their stability toward palladium leaching is such that their recycling cannot be considered on a reasonable scale. The fact that in most cases the leached palladium is very active for the same coupling reaction suggests that these Pd-nP-SAP solids behave essentially as a reservoir of a highly soluble species, which forms during the successive catalytic cycles. A careful study of the related analogous homogeneous catalytic systems shows that most of the phosphine introduced with Pd(OAc)2 stays free: the reactants inhibit considerably the reduction of Pd(II) to Pd(0) by the phosphine. The data when taken all together suggest that Pd(CCPh)2(PR3)n, a lipophilic catalytic intermediate, may be the leaching entity.