Synthesis and catalytic properties of p-acylthio(phenylacetylene)n substituted chiral manganese salen complexes

Abstract

The syntheses of three new salen ligands that are tethered to a p-acylthio(phenylacetylene)_n linker are described. The two key steps in the syntheses are coupling of the p-acylthio(phenylacetylene)_n linker (n = 0–2) with a 5-iodosalicylaldehyde and the subsequent condensation of the aldehyde moiety of the formed adducts with the monoimine of (S,S)-1,2-diphenylethylenediamine to give the salen ligands. In the catalytic asymmetric epoxidation reactions of (Z)-2-methylstyrene, performed using the Mn–salen complexes of these new ligands, high diastereoselectivities of up to 20 ∶ 1 and enantioselectivities of up to 89% ee are obtained. The results are compared with the analogous reaction using Jacobsen's asymmetric epoxidation catalyst and the results are very similar. The synthesised ligands are promising candidates for the immobilisation of chiral Mn–salen complexes on gold electrodes and surfaces.