Recoverable salen-based macrocyclic chiral complexes; catalysts for enantioselective Henry reactions

Cobalt and chromium complexes have been prepared from chiral calix–salen cyclic ligands. The corresponding tetrahydrosalen reduced forms have been used for copper salt complexation. These new chiral catalysts have been tested for their ability to promote asymmetric Henry reactions between various aldehydes and nitromethane under heterogeneous conditions. The best results were obtained by using tetrahydrosalen-based copper macrocycles, in terms of activity, selectivity, and stability during the recycling process. Ten consecutive runs could indeed be performed with the same catalyst batch to produce the target 1-(2-methoxy-phenyl)-2-nitro-ethanol with highly stable values in terms of yield and enantioselectivity (up to 94% ee).

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Calix–tetrahydrosalen–(C34H44N2O2)–n = 2, 3, 4[α]D20=-89 (c 0.005, CHCl3)Absolute configuration: (S,S)(assigned by synthesis from (S,S)-1,2-diaminocyclohexane)