(671a) 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.