Solid Cocatalysts for Activating Manganese Triazacyclononane Oxidation Catalysts

Abstract

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.

Mechanisms in manganese catalysed oxidation of alkenes with H2O2

“…The recognition that carboxylic acids act as ligands stimulated renewed interest in heterogeneous Mn-TMTACN catalysts first investigated by De Vos and co-workers. 78 In contrast to the early studies in which the complexes were immobilised on mesoporous silica gel via a specifically modified TMTACN derivative, Notestein and co-workers [79][80][81] reported that immobilisation of the catalyst by in situ reduction of [Mn IV,IV 2 -(m-O) 3 (TMTACN) 2 ] 2+ in the presence of carboxylic acid coated silica particles allowed for similar levels of activity to be observed, both for epoxidation and cis-dihydroxylation, as found previously in homogeneous reactions by de Boer et al (vide supra). 69 Although characterisation of species formed in solution is relatively straightforward, when immobilised on surfaces/particles the application of spectroscopic methods becomes more challenging.…”

Iron‐Catalyzed Highly Enantioselective cis‐Dihydroxylation of Trisubstituted Alkenes with Aqueous H2O2

“…Non‐Os‐metal catalysts for the AD reactions of trisubstituted alkenes remain underdeveloped. Previous examples of non‐Os‐metal‐catalyzed cis ‐dihydroxylation reactions are either confined to the AD reactions of mono‐ and/or di‐substituted alkenes using chiral Fe or Mn catalysts or belong to non‐enantioselective cis ‐dihydroxylation reactions (with scarce use of trisubstituted alkene substrates) catalyzed by compounds of transition metals such as Ru, Mn, Fe, Mo, and Pd . A recent report demonstrates enantioselective anti ‐dihydroxylation of alkenes (including trisubstituted ones) catalyzed by Mo‐bishydroxamic acid complex .…”

Synthesis of a family of peracid-silica materials and their use as alkene epoxidation reagents

“…Silanes were grafted onto silica utilizing a previously reported method (Figure 2). [19][20][21] Silica (Selecto) was dehydrated overnight at 120°C under dynamic vacuum. 1-2 g dehydrated silica was added to 50 mL of anhydrous pyridine in a round bottom flask under N 2 with stirring, and 2-(carbomethoxy)-ethyltriethoxysilane was added at 1.0 mmol of silane per g of silica.…”