Application of Cryptand/Polystyrene Drift‐Type Phase Transfer Catalysts for Reduction of Ketones

Abstract

AbstractA drift‐type phase transfer catalyst, cryptand‐22, adsorbed on poly(styrene/diviny benzene)‐sulfonic resin was prepared and applied to catalyze the reduction of ketones, e.g., acetophenone, benzophenone and benzaldehyde with NaBH4 as a reducing agent. Before the reaction, cryptand‐22 was adsorbed on the sulfonic resin with ion‐pairing, resin‐SO3− +NH‐cryptand‐22. The ion‐pairs can be destroyed by adjusting the basicity of the reaction solution with NaOH and the cryptand can be released from the resin into the reaction solution as a homogeneous catalyst during the reaction period. After the reaction, the cryptand catalyst can be readsorbed on the resin by adjusting the acidity of the solution with HCl and can be readily recovered by filtration like a heterogeneous catalyst. The draft‐type cryptand catalyst exhibited better catalytic ability than some common crown ethers, e.g., 15‐crown‐5, benzo‐15‐crown‐5, 12‐crown‐4 and dibenzo‐18‐crown‐6 for the reduction of acetophenone with NaBH4. Effects of solvents, pH of solutions, concentration of the catalyst, reducing agents and resin property on the reduction of ketones were investigated and discussed. The reaction mechanism of the cryptand catalyzed reduction was also studied.