Adsorption salen ligands

Hutchings el al. (102,103) synthesized enantioselective Mn catalysts by adsorption of chiral salen ligands on Mn-exchanged Al-MCM-41 and used the materials in selective epoxidation of dv-stilbcnc with PhlO. Although the simplicity of this approach is appealing, reuse of the material in a second run led to a dramatic decline of the epoxide yield from 69 to 18%, and of the ee of the trans epoxide from 70 to 30%. 

This method relies on the size of the metal complex rather than on a specific adsorptive interaction. There are two different preparation strategies One, often called the ship-in-a-bottle approach, is based on building up catalysts in well defined cages of porous supports. Recently, enantioselective Mn epoxidation catalysts with different salen ligands have been assembled in zeolites. In zeoHte EMT [35] ees up to 88% and in zeolite Y [36] ees up to 58% were obtained with czs-P-methylstyrene. However, both entrapped catalysts were much less active than their homogeneous counterparts. Rh diphosphine complex were entrapped in the interlayers of Smectite [37]. The resulting catalyst was active for the enantioselective hydrogenation of N-acetamidoacrylic acid (ee 75%). 

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