Trinuclear C3-Symmetric Extension of Jacobsen’s Catalyst: Synthesis, Characterization, and Catalytic Properties of a Chiral Trinuclear MnIII Triplesalen Complex

The synthesis of a chiral version of a triplesalen ligand has been performed in two steps starting from 2,4,6-triacetyl-1,3,5-trihydroxybenzene (1). Reaction with excess trans-(1R,2R)-1,2-cyclohexanediamine and trans-(1S,2S)-1,2-cyclohexanediamine provided the chiral triplesalen half units 2,4,6-tris[1-((1R,2R)-2-aminocyclohexylimino)ethyl]-1,3,5-trihydroxybenzene (2(RR)) and 2,4,6-tris[1-((1S,2S)-2-aminocyclohexylimino)ethyl]-1,3,5-trihydroxybenzene (2(SS)), respectively. The two enantiomeric pure triplesalen ligands H(6)chand(RR) and H(6)chand(SS) were obtained by reaction of the triplesalen half units with 3,5-di-tert-butylsalicylaldehyde. Reaction with MnCl(2) x 2 H(2)O under basic aerobic conditions afforded the chiral trinuclear triplesalen complexes 3(RR) and 3(SS). Single-crystal X-ray diffraction studies on both enantiomers showed the presence of the two ionization isomers [(chand){Mn(III)Cl(MeOH)}(3)] and [(chand){Mn(III)(MeOH)(2)}(3)]Cl(3) in the solid state, resulting in the formulation of 3 (either 3(RR) or 3(SS)) as [(chand){Mn(III)Cl(MeOH)}(3)][(chand){Mn(III)(MeOH)(2)}(3)]Cl(3). The crystal structures exhibit chiral hydrophobic channels of approximately 8 A diameter decorated with tert-butyl groups. These form left-handed helices in the 3(RR) enantiomer and right-handed helices in the 3(SS) enantiomer. Magnetic measurements are in accord with weak exchange interactions between the Mn(III) S(i) = 2 ions and strong local magnetic anisotropy as has been found in other trinuclear Mn(III)(3) triplesalen complexes. As a proof-of principal, we have investigated the catalytic ability of the two enantiomers in the enantioselective epoxidation of unfunctionalized olefins. The chiral trinuclear Mn(III)(3) triplesalen acts under non-optimized conditions as a catalyst with relatively good yields and moderate enantiomeric excess.