Gold-Nanoparticle-Embedded Poly(N-isopropylacrylamide) Microparticles for Selective Quasi-Homogeneous Catalytic Homocoupling Reactions

Composite particles derived from poly(N-isopropylacrylamide) and nanoscale gold exhibit an extremely high reactivity and selectivity in C–C bond-forming reactions. The light-induced reduction of gold ions in the presence of polymer particles in situ results in the effective formation of composite particles physically loaded with gold nanoparticles. After the removal of the unreacted ions and free gold nanoparticles, the composite particles are fully dried and subsequently redispersed in various organic solvents. Although this purification step efficiently reduces the number of surface-bound reducing/stabilizing agents (i.e., citrate) that can often serve as physical barriers, the resulting composite particles still maintain excellent stability overall due to the polymer particles. The use of nonaqueous solvents readily eliminates the co-nonsolvency behavior and temperature responsiveness of the composite particles, allowing the polymer particles to remain fully swollen above the lower critical solution temperature of 32 °C. Given these features, employing these composite particles as quasi-homogeneous catalysts in the homocoupling reaction of various arylboronic acids results in their unexpectedly high reactivity under aerobic conditions. In addition, the reactions in pure organic solvents allow the composite particles to solely yield targeted products without the formation of any byproducts, even after multiple cycles. Various reaction conditions, including time, temperature, base, and catalyst amount, are fully examined to optimize the reactivity, selectivity, and recyclability of the composite particles as quasi-homogeneous catalysts. Understanding atypical catalytic functions of the composite particles under various reaction conditions will lead to the development of robust and industrially practical catalytic systems.