Hydroesterification of styrene derivatives catalyzed by an acidic resin supporting palladium complexes

Pd-TPPTS-OTPPTS (denoted as Pd-P-OP, where TPPTS was the sodium salt of tri(m-sulfophenyl) phosphine and OTPPTS was the oxidized form of TPPTS) complexes supported on acidic resins (denoted as Pd-P-OP/resin) were prepared and employed as versatile heterogeneous catalysts for the hydroesterification of vinyl-aromatics with alcohols. The catalysts were characterized by the methods of FT-IR, XPS, N-2 physisorption, XRD, TGA, SEM and P-31 NMR. According to the P-31 NMR results, there was a weak coordination between OTPPTS and the Pd sites. Consequently, OTPPTS was a weak ligand to the Pd sites and thus dissociated easily, acting as a protective agent of the vacant coordination site of the Pd metal sites, which allowed the substrates more access to the metal sites. In the hydroesterification of styrene, the distribution of the branched and linear esters was remarkably impacted by adding an appropriate amount of OTPPTS and the acidic resin supports. A high yield and excellent selectivity towards the branched ester were obtained when OTPPTS and TPPTS were used in equimolar amounts under optimized reaction conditions. The generality and recyclability of the catalyst for the hydroesterification of vinyl-aromatics were also examined. In addition, the poisoning and hot filtration tests indicated that Pd(0) acted as the active species in a truly heterogeneous way. A Pd-hydride mechanism was proposed for the hydroesterification over the Pd-P-OP/LSI-600 catalyst.