Biomimetic Catalysis with an Immobilised Rhodium(III)-Complex

An organometallic transition state analog for the asym. redn. of acetophenone with a Cp*Rh complex was synthesized and structurally characterized - rac-[(C5Me5)RhOP(O)MePh] (3). This complex has a chiral N,N'-chelate ligand with a styrene side chain to allow its incorporation into org. polymers. The remaining coordination site is occupied by a methylphenylphosphinato ligand. This ligand acts as a pseudo substrate which mimics acetophenone. The conformation and configuration of 3 in the crystal are in excellent agreement with the postulated transition structure. Following the protocol of mol. imprinting, complex 3 was co-polymd. with ethylene glycol dimethacrylate in the presence of a porogen. The resulting polymer P3 was ground and sieved and the phosphinato ligand was substituted with a chloro ligand to generate a shape-selective cavity in proximity to the catalytically active metal center. When tested for its ability to catalyze the redn. of acetophenone and related substrates the imprinted polymer P3 showed a significantly higher activity than a control polymer P2 without a cavity. Excellent enantioselectivities (up to 95% ee) were obtained, with the catalyst P3 being more selective than the resp. control catalyst P2 (Dee = 2-9%). Competition expts. with acetophenone and a 2nd co-substrate revealed that the cavity generated with the phosphinato ligand is specific for acetophenone.