The use of soluble polymer supports in catalysis and synthesis.

摘要

Three different approaches have been developed to recover soluble polymer-bound catalysts or substrates in this study: acid-base chemistry, thermal responsive property and thermomorphic behavior. Acid-base chemistry was used to recover polymeric acid derivatives. In this case, an amphoteric soluble polyacrylic acid (Gantrez)-bound hydrogenation catalyst is soluble and active in water at pH >7.5 and insoluble and inactive below that pH. Such catalysts are recoverable and reusable and have, activities that closely resemble those of a low molecular weight analog. An insoluble polymeric acid was also used to simultaneous deprotect and purify BOC-protected amines. This chemistry was shown to be very useful in solution-phase parallel synthesis. In the second approach, catalysts and substrates are bound to poly(N-isopropylacylamide) (PNIPAM) that are in solution and active at low temperatures but precipitate and inactive at high temperatures. Recovery of such catalysts can then be accomplished by heating. Such "smart catalysts" were shown to be active in many reactions such as hydrogenations, allylic substitutions and C-C coupling reactions. In the third case, we used a thermomorphic system that combines attractive features of biphasic and homogeneous catalysis. This chemistry uses a solvent system (90% EtOH/heptane) that changes thermally from biphasic (25°C) to monophasic (70°C) and a polymeric ligand that prefers one phase under biphasic conditions. When a substrate (product) is preferentially soluble in the opposite phase, repetitive reactions proceed in excellent synthetic yield with facile catalyst/product separation at the biphasic stage. In this case, we also discussed some preliminary studies for the air-stable Pd catalysts. Such catalysts are modified with tridentate PCP or SCS ligands. They showed high activity in Heck type reactions in open air without any deactivation. More importantly, these PCP-Pd or SCS-Pd catalysts can be bound to polymer supports which leads to air-stable and recoverable palladium catalysts.