Lipophilic chiral cobalt (III) complexes of hexaamine ligands: Efficacies as enantioselective hydrogen bond donor catalysts

Abstract

Four known chiral enantiopure octahedral cobalt(III) trichloride salts with aliphatic hexaamine ligands are converted to new CH2Cl2 soluble tris(tetraarylborate) or 3BArf− salts (BArf = B(3,5-C6H3(CF3)2)4 in biphasic reactions. These include sepulchrate and sarcophagine complexes in which the hexaamine ligands are bicyclic (Z(CH2NHCH2CH2NHCH2)3Z, Z=N, CX), as well as a truncated sarcophagine in which a missing carbon vertex renders the ligand acyclic and tripodal, with three terminal NH2 groups that define a trigonal C3 symmetric face (CH3C(CH2NHCH2CH2NH2)3). In the presence of tertiary amine bases, these are effective catalysts for two Michael type carbon-carbon bond forming reactions and a related carbon-nitrogen bond forming reaction. However, only the last complex affords significant enantioselectivities (30–57% ee). Rationales, and directions for future catalyst optimization, are proposed.