New heterogeneous catalysts for the synthesis of chiral amino acids: Functionalization of organic resins with chiral salen complexes

• First study with heterogeneous salen catalysts to obtain asymmetric amino acids.
• 4 new heterogeneous catalysts with V and Al salen complexes on polystyrene resins.
• V catalysts more efficient than the Al heterogeneous catalysts.
• V heterogeneous catalyst can be used in 3 runs without loss of conversion and ee.

Four new heterogeneous catalysts were synthesized by covalent attachment of vanadium(V) and aluminum(III) salen complexes to polystyrene polymers, namely, Merrifield and JandaJel resins. The solid catalysts were characterized by analytical and spectroscopic techniques and tested in the asymmetric addition of hydrogen cyanide (generated in situ from TMSCN) to N-benzyl benzylimine (Strecker type reaction). Heterogeneous vanadium(V) catalysts are more efficient than the Al(III) catalysts, as expected from comparison with the corresponding homogeneous systems. The activity of the vanadium(V) heterogeneous catalysts is similar to the homogenous counterpart (after 4 h of reaction at −40 °C using 10 mol% of catalysts), while the enantiomeric excess was slightly inferior to the one obtained with the corresponding homogeneous catalysts. The Janda-V(V) heterogeneous catalyst can be reused by simple filtration up to three times without significative loss of conversion and enantioselectivity. This is the first study dedicated to the synthesis of asymmetric amino acids through the Strecker reaction using heterogeneous salen catalysts.

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