Effect of chiral cavities associated with molecularly imprinted platinum centers on the selectivity of ligand-exchange reactions at platinum

The metallomonomer (vinyl dppe)Pt[(R)-Bu2BINOL] (1) was copolymerized with ethylene glycol dimethacrylate to produce the molecularly imprinted polymer (MIP) P1. The chiral imprinting ligand (R)-Bu2BINOL was removed from P1 by treatment with HCl or an excess of alpha,alpha,alpha-trifluoro-m-cresol, BINOL, or Br2BINOL, giving polymers P2-P5. The amount of imprinting ligand released from P1 varied inversely with the steric bulk of the cleaving agent, indicating that a distribution of Pt sites with different accessibilities exists in the MIP. Exposure of P3 (containing Pt(OAr)(2) centers surrounded by (R)-Bu2BINOL-shaped cavities) to rac-BINOL, an imprinting ligand analogue, led to preferential rebinding of the imprinted enantiomer via ligand exchange. Both aggregate selectivity and extent of reaction (percent of the total Pt sites in the MIP that participated in rebinding) increased with rebinding time and with temperature, up to 69% ee and 58% Pt sites rebound, respectively. A positive linear correlation between selectivity and reactivity was observed. Open, easily accessible Pt sites were proposed to be less selective than unreactive sites because their associated chiral cavities are less well-defined and therefore less capable of shape-based enantiodiscrimination. The kinetic selectivity of the least reactive sites in P3 was assessed by sequential rebinding of rac-BINOL and then rac-Br2BINOL. Rebound BINOL was recovered from 8 to 20% of the least reactive Pt sites accessible in the MIP in 89-94% ee. All data indicate that chiral cavities (outer sphere) associated with the reactive Pt centers in these MIPs are responsible for the selectivities observed for stoichiometric ligand-exchange reactions at the metal center.