Unsymmetrical chiral salen Schiff base ligands - Synthesis and use in metal-based asymmetric epoxidation reactions

Six novel unsymmetrically substituted chiral non-racemic salen ligands were prepared from resolved trans-1,2-diaminocyclohexane and two different salicylaldehydes. These were then used to construct cationic chromium(III) and manganese(III) complexes that were employed in studies relevant to catalytic asymmetric epoxidation of alkenes. In the case of the chromium complexes, the yields and enantioselectivities obtained were compared to those obtained with the analogous symmetrically substituted counterparts. In individual cases improved enantioselection and a very strong beneficial effect of phosphine oxide additive was found, but in general the selectivity varied in an unpredictable manner. The results are rationalised through the formation of a mixture of diastereomeric active oxidants. In the case of the manganese complexes, where one of the salicylaldehydes is the 3,5-di-tert-butyl case, the standard preparative method leads to substantial scrambling of the ligand and the isolation of mainly Jacobsen's catalyst. One of the preparative methods investigated for the ligand synthesis worked for racemic ligand but failed for the non-racemic version. (c) 2005 Elsevier B.V. All rights reserved.