Solid cocatalysts for activating manganese triazacyclononane oxidation catalysts

Immobilizing a homogeneous catalyst provides obvious handling benefits, but ideally can also enhance catalyst productivity or selectivity because of beneficial interactions between the surface and the active site. Here, Mn 1,4,7-trimethyl-1,4,7-triazacyclononane dimers (1) are activated by carboxylate-containing solids for cyclooctene epoxidation/dihydroxylation with H₂O₂ at 0 °C. The productivity (TON, moles per mole 1) and selectivity (to cis-diol) of cyclooctene oxidation by 1 are known to be tuned by choice of soluble carboxylate cocatalyst, and this concept is extended here to solid carboxylate cocatalysts. These solid cocatalysts are synthesized by covalently grafting benzoate and propanoate silanes to SiO₂ or allowing terephthalic acid or dihydroxyhydrocinnamic acid to chemisorb on oxides. Comparing analogous structures, SiO₂-grafted carboxylates outperform soluble benzoic acid (275 vs 25 TON at 2 equiv), hydrocinnamic acid (150 vs 40 TON at 2 equiv), and valeric acid (675 vs 70 TON at 10 equiv). Of the oxides tested as modifiers for carboxylate cocatalysts, TiO₂ leads to the largest improvements in oxidation productivity, boosting productivity to 300 TON when combined with 2 equiv of benzoic acid and to 425 TON when combined with ditopic terephthalic acid. The latter enhancement may be due to both a buffering effect and a high surface concentration of chemisorbed species that encourages formation of the presumed carboxylate-bridged active state of the catalyst. Dihydroxylation selectivities are a function of the carboxylate employed as well as the nature of the other groups on the solid cocatalyst surface. SiO₂ grafted with propionate groups gives ∼50% cis-diol selectivity, but further modification with alkyl or perfluoroalkyl silanes increase cis-diol selectivity up to ∼60%. Dihydroxylation selectivity is also ∼60% for SiO₂ grafted with benzoate or ∼70% for terephthalic acid chemisorbed on TiO₂. These solid cocatalysts introduce a number of additional tunable parameters that lead to enhanced productivity or selectivity for molecular oxidation catalysts like 1 that are activated by carboxylates.