Supported Manganese Triazacyclononane Cis-Dihydroxylation Catalysts

Manganese complexes of 1,4,7-trimethyl-1,4,7-triazacyclononane are highly active and selective for the low temperature oxidation of alkenes and other substrates with environmentally benign oxidants such as H₂O₂.  Previous investigations on the homogeneous catalyst revealed that carboxylate co-catalysts could be employed to raise catalyst productivity by reducing the inherent catalase activity of the materials, as well as tune the reaction selectivity toward either epoxidation or cis-dihydroxylation. Cis-dihydroxylation is a relatively unusual but powerful reaction, primarily catalyzed by expensive Rh complexes or extremely toxic OsO₄. There is thus an incentive for the development of novel catalysts, especially solid catalysts. These Mn complexes use mild reaction conditions (room temperature and below), benign oxidants (H₂O₂) and less toxic metal centers (Mn), demonstrating potential utility for the organic transformations necessary in pharmaceutical synthesis.

We have found that functionalization of common catalyst supports with carboxylate moieties allows the in-situ assembly and activation of the Mn complex onto the support under oxidation reaction conditions. The immobilized catalyst requires orders of magnitude less carboxylate co-catalyst than the soluble analog and requires up to 8x lower residence times for a given conversion. The immobilization technique also removes cumbersome synthetic requirements typically required to create amine ligands that can be covalently attached to a support. For example, we have found that the cis-diol selectivity can be increased from <50% to ~75% by changing only the nature of the supported carboxylate. Supporting characterization by X-ray absorption fine structure and diffuse reflectance UV-visible spectroscopy helps us further understand the nature of this supported catalyst.